Triarylbismuth(III) Compounds as

Reagents for C-terminus Peptide Ester Synthesis

Basilia Pires 10107800

Bachelor thesis Van t’Hoff Institute for Molecular Science Synthetic Organic Chemistry Group Prof. dr. Henk Hiemstra First reviewer: dr. Jan van Maarseveen Second reviewer: prof. dr. Bas de Bruin Daily Supervisor: Stanimir Popovic MSc.

Amsterdam, July 2013

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Abstract Bismuth is not a common element to use in the chemical world. It is present in a small amount in nature, but can be easy obtained with the refining process of metals. Bismuth has some interesting features, like non-toxicity and good stability. This makes bismuth interesting for green chemistry. There are already some organobismuth compounds known like alkylbismuth compounds and arylbismuth compounds. For this project the focus laid on the use of triarylbismuth compounds. Triarylbismuth compounds come in two ways: triarylbismuth(III) and triaryl(V)bismuth compounds. In the last two decade these compounds were used for some important synthetic organic reactions, like arylation and cross-coupling reactions. Peptides are the building block of proteins. Proteins have important roles in living organism, but also in the medicinal chemistry. They can be used to make medicines. To create a peptide chain, several aminoacids are coupled together. This process comes with a problem: epimerisation. This problem was solved with the Cu(II) catalysed Chan-Lam reaction. The aim of this project was to apply triaryl(III)bismuth compounds as reagents in the Cu(II) catalysed Chan-Lam reaction for esterification of peptide esters with the main focus on avoiding epimerisation. The first part was to synthesize different triarylbismuth(III) compounds. Only one had succeeded: triphenylbismuth with a yield of 69%. This synthesis was done by doing a transmetallation of phenylmagnesium bromide with bismuthchloride. The other triarylbismuth(III) compounds were tris(4-nitrophenyl)bismuth and tris(pentafluorophenyl)bismuth. These compounds couldn’t be synthesized. Tris(4-nitrophenyl)bismuth was tried with two different routes. The first one was, making 4-nitrophenylmagnesium bromide and then transmetallation with bismuth chloride. The second, was lithiation of 4-nitrophenyl bromide with n-BuLi and then transmetallation with bismuthchloride. There was only one attempt to make tris(pentafluorophenyl)bismuth: transmetallation of pentafluoromagnesium bromide with bismuthchloride. The second part of the project was to apply triphenylbismuth as reagent in the Cu(II) catalysed Chan-Lam reaction. This reaction was a success. Five different dipeptide phenylesters could be made and without epimerisation. The yield were between 58%-73%. The conclusion of this project was: triarylbismuth(III) compounds can be used as reagents in the Cu(II) catalyzed Chan-Lam for esterification of peptides esters. These reactions could be done without epimerisation. For further research, the Cu(II) catalyzed Chan-Lam reaction must be further optimized for triyarylbismuth(III) compounds as reagents. An another try must be done to synthesize tris(4-nitrophenyl)bismuth and tris(pentafluorophenyl)bismuth. It is also interesting to make other triaryl(III)bismuth compounds. For the synthetic organic chemistry, it is maybe interesting to apply triaryl(III)bismuth compounds for other reactions, like the Ullman reaction.

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Abstract – Dutch Version Bismuth is de 83e element in het periodieke systeem. Dit element behoort tot de stikstofgroep en is een metaal. Over dit metaal is niet veel bekend en wordt weinig gebruikt in de scheikunde. Mede doordat het een schaars metaal is: Op basis van hoeveelheid is het de 64e element die aanwezig is in de aardkorst. Maar bismuth is makkelijk te krijgen als bijproduct bij het raffineren van metalen zoals lood en koper. In de afgelopen twintig jaar is de interesse naar toepassingen van bismuth zeer gestegen. Bismuth heeft ook een aantal interessante eigenschappen zoals: zwaarste van de stabiele metalen, niet giftig of kankerverwekkend en niet radioactief. Deze eigenschappen maakt bismuth interessant voor de schone en duurzame chemie. Er zijn al verschillende bismuth verbindingen bekend die worden gebruikt voor chemische reacties zoals anorganische en organometallische verbindingen. Voor dit project waren de organometallische verbindingen interessant, met de nadruk op triarylbismuth verbindingen. Triarylbismuth verbindingen zijn moleculen met bismuth als het centraal atoom en drie benzeenringen aangekoppeld. Bismuth komt hierin voor in twee oxidatie toestanden: +3 en +5. Bij een oxidatietoestand van +5 zijn er nog twee liganden verbonden aan het bismuthatoom. Tegenwoordig zijn er al belangrijke organische reacties bekend waarbij triarylbismuth verbindingen worden gebruikt zoals, arylering en koppelingsreacties. Eiwitten zijn essentiële moleculen voor de natuur. Zij hebben belangrijke rollen in levende organismen zoals bouwstenen en energiedragers, maar ook voor de farmaceutische industrie zijn zij belangrijk. Sommige eiwitten kunnen worden gebruikt om medicijnen te maken zoals vaccins. Eiwitten zijn opgebouwd uit peptidenketens. De bouwstenen voor deze ketens zijn aminozuren. De aminozuren worden aan elkaar gekoppeld. Er zijn al verschillende methodes ontwikkeld om deze koppelingsreactie uit te voeren. Alleen is er telkens een probleem. Het stereocentrum naast de zuurgroep is gevoelig voor epmirisatie. Dit houdt in: dat er twee diastereomeren gevormd kunnen worden. De groep van prof.dr. Henk Hiemstra heeft een oplossing hiervoor verzonnen: de Chan-Lam koppelingsreactie met koper(II) als katalysator. Het doel van dit project was om triarylbismuth(III) verbindingen toe te passen in de Chan-Lam koppelingsreactie om peptidenesters te maken. Het project was verdeeld in twee delen. Het eerste gedeelte was het maken van triarylbismuth(III) verbindingen. Het was gelukt om triphenylbismuth te maken. Hierbij was bismuth gekoppeld aan drie benzeenringen. Het idee was ook om tri(4-nitrophenyl)bismuth te maken (bismuth gekoppeld aan drie nitrobenzeen ringen). Maar deze is niet gelukt. Het tweede gedeelte was het aanpassen van de koper(II) gekatalyseerde Chan-Lam koppelings reactie om triphenylbismuth te gebruiken als reagens. Deze reactie was gelukt en hiermee zijn er vijf verschillende dipeptides phenylesters gemaakt. Uit dit onderzoek kon geconcludeerd worden, dat triarylbismuth(III) verbindingen gebruikt kunnen worden om peptidenesters te maken m.b.v. de koper(II) gekatalyseerde Chan-Lam koppelings reactie. Voor verdere onderzoek is het interessant om andere triaryl(III)bismuth verbindingen te maken, waarmee andere peptidenesters gemaakt kunnen worden.

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Abbrevations PE Petroleum ether EtOAc Ethyl acetate BOC-L-Phe-OSu BOC protected N-terminus of the succimide ester of phenylalanine with the

L-configuration LC-MS Liquid chromatography-mass spectrometry THF Tetrahydrofuran TLC Thin Layer Chromatography Cu(OTf)2 Copper(II) trifluoromethanesulfonate Mp Melting point NMR Nuclear Magnetic Resonance IR Infrared spectroscopy TEA Triethylamine EtOH Ethanol NaHCO3 Sodium bicarbonate (CH3)2CO Ethanol Nu Nucleophile Cu(NO3)2 H2O Semi-penta copper nitrate Phg Phenylglycine Val- Valine Phe Phenylalanine

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Table of content Abstract ................................................................................................................................................... 2

Abstract – Dutch Version ........................................................................................................................ 3

Abbrevations ........................................................................................................................................... 4

Table of content ...................................................................................................................................... 5

1. Introduction .................................................................................................................................... 6

1.1 What are triarylbismuth compounds? .................................................................................... 6

1.2 The use of triarylbismuth compounds .................................................................................... 6

1.2.1 Arylation with triarylbismuth(V) and triarylbismuth(III) compounds ............................. 6

1.2.2 Cross-couplings reactions with triarylbismuths .............................................................. 8

1.3 The essence of peptide esters ................................................................................................ 8

1.4 The Cu(II)-catalyzed Chan-Lam reaction ................................................................................. 9

1.5 The aim of the project ........................................................................................................... 10

2. Results and Discussion .................................................................................................................. 11

2.1 Synthesis of triarylbismuth compounds ............................................................................... 11

2.1.1 Synthesis of triphenylbismuth ...................................................................................... 11

2.1.2 Synthesis of tris(4-nitrophenyl)bismuth ....................................................................... 12

2.1.3 Synthesis of tris(pentafluorophenyl)bismuth ............................................................... 13

2.2 Synthesis of the dipeptide free acid ..................................................................................... 13

2.3 Chan-Lam coupling reactions ................................................................................................ 14

2.4 1H-NMR analysis: epimerisation ........................................................................................... 15

2.4.1. Boc-L-Phe-L-Phe-OPh and Boc-L-Phe-D-Phe-OPh diastereomers ....................................... 15

2.4.1. Boc-L-Phe-L-Phg-OPh and Boc-L-Phe-D-Phg-OPh diastereomers ................................. 16

3. Conclusion ..................................................................................................................................... 18

4. Future Prospects ........................................................................................................................... 19

4.1 Further synthesis of triarylbismuth(III) compounds ............................................................. 19

4.2 Further optimization of the Chan-Lam coupling reaction with BiAr3 ................................... 20

4.3 Further applications of triarylbismuth(III) compounds......................................................... 20

5. Acknowledgement ............................................................................................................................ 21

6. References ........................................................................................................................................ 22

7.Experimental Section ......................................................................................................................... 23

7.1 Procedures ............................................................................................................................ 23

7.2 Spectra .................................................................................................................................. 26

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1. Introduction

1.1 What are triarylbismuth compounds? The element bismuth is a rare element, the 64th compound in the Earth’s crust3 and therefore not very common to use. While bismuth have some interesting properties1-3:it is the most diamagnetic of all metals, has the lowest thermal conductivity (in exception of mercury), got a high electrical resistance, is the heaviest stable element, is not toxic or carcinogen in contradiction to the other heavy metals like arsenic and lead, not radioactive and easily available. Bismuth comes as a side product at the process of refining lead, copper, tin, silver and gold ores and this is a low cost process.1 This makes bismuth interesting to use for green chemistry.3 In this century the need of green chemistry is a must, because of the environmental principles that the field of research and industry are dealing with. In bismuth compounds, bismuth is mostly present in an oxidation state of +3 or +5.2.3 Bismuth(III) compounds become easily bismuth(V)compounds, because this oxidation state is the most stable one. There are some cases known of bismuth(I) and bismuth(II) compounds.4-5 These compounds are mostly metal complexes. When a bismuth compound has an oxidation state of +3, it has some Lewis acid properties. This is due of the Lanthanide contraction. The electron configuration of bismuth is [Xe] 4f145d106s26p3, so the 4f electrons are weakly shielded. The most of the known bismuth compounds are not toxic, what is another important feature for green chemistry. Triarylbismuth compounds are organometallic compounds with the oxidation state of +3 or +5 in the form of BiAr3 or BiAr3L2.2 The organic derivatives of bismuth compounds were first used for medicinal preparations.1 In the 90s came forward that organobismuth compounds can be used as reagents in organic synthesis and catalysis.2 Due to the subject of this project, the focus will be only on the use of triarylbismuth compounds in organic chemistry.

1.2 The use of triarylbismuth compounds In the last two decades, the use of organobismuth compounds grew fast.1 The most research about these compounds was done in this period. There are several applications and in this section the most common are being discussed.

1.2.1 Arylation with triarylbismuth(V) and triarylbismuth(III) compounds Some arylation reaction with different compounds were developed using triarylbismuth(V) and triarylbismuth(III) compounds as reagents.2 The core behind these reactions was the discovery of the oxidizing powers of the pentavalent and trivalent states of the organobismuth compounds. In some types of arylation reactions the oxidation states determines the selectivity of the reaction. And the choise of ligand in the case of triarylbismuth(V) compounds determines the chemoselectivity. In Scheme 1 are the arylation reactions presented with triarylbismuth(V) compounds. These compounds are efficient in mild conditions and gives high yield (>90%).

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BiAr3L2R1

R2

HO

R1R2

O

Ar

O

OAr

R-Z-H

R-Z-Ar

NN H

H

Boc

Boc

NN Ar

H

Boc

BocL = Cl, OAcZ = O, NH

Scheme 1: Arylation reaction with triarylbismuth(V) compounds2

In most arylation reactions triarylbismuth(V) compounds is favored above triarylbismuth(III) compounds.2 This is because of the reaction conditions and selectivity. There is one arylation reaction that triarylbismuth(III) compounds were favored: selective N-arylation of 3-aminobenzanilides (compound 1A, Scheme 2).

NH2

O NH

R2

R1 HN

O NH

R2

R1

R3Bi R3

3

TEA, CH2Cl2Cu(OAc)2

1A 1 Scheme 2: Selective N-arylation of 3-aminobenzanilides with triarylbismuth(III) compounds2

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1.2.2 Cross-couplings reactions with triarylbismuths The use of organometallic compounds for couplings reactions is one of the important methods in the organic chemistry to form C-C bonds.2 These reactions are always metal-catalyzed (likely a palladium catalyst) and uses organic electrophilic reagents. The use of triarylbismuth compounds as organometallic reagents for this type of reaction was a solution for the search for organometallic compounds, that can serve in a sub-stoichiometric way. Triarylbismuth can react with more than 1 equivalent of the organic electrophilic reagent. This makes triarylbismuth an atom-efficient organometallic coupling reagent, an important feature for the industry. The presence of organometallics in a reaction mixture will be reduced. The cross-coupling reactions with triarylbismuth compounds are being developed since 2008. These reactions are done in mild conditions and gives good yields. In Table 1 are shown, the known cross-coupling reactions with triarylbismuth compounds. Table 1: Overview cross-coupling reaction with triarylbismuth

Type reaction Catalyst Highest Yield Reference Cross-coupling with aryl halides and triflates PdCl2/PPh3 96% [2] Cross-coupling with α,β-unsaturated acyl chlorides PdCl2/PPh3 91% [2] Cross-coupling with allylic carbonates PdCl2/(PPh3)2 90% [2] Cross-coupling with aryl bromides or iodides Polystyrene-

supported PdII 94% [2]

Multi-coupling with vinylic iodides PdCl2/(PPh3)2 85% [2] Domino coupling with 1,1-dibromo-1-alkenes Pd(PPh3)2 88% [2] Multi-coupling with bromide and chloride derivatives of Baylis-Hillman adducts

Pd2dba3 91% [2]

Cross-coupling of allylic carbonates Pd(PPh3)2 90% [6] Cross-coupling of 2-bromo- and 2-iodothiophenes Several 92% [7] Cross-coupling with benzylic bromides Pd(PPh3)4 95% [8]

1.3 The essence of peptide esters Proteins are long organic molecules, that are build up from peptides.9 Proteins and peptides play several and important roles in the physiological and biological processes of living organism.10 They can function as building blocks, signal transmitters, energy carriers etc. Proteins has also another important application: they are being used for making medicine, like HIV-vaccins.11

To end up with a protein molecule, the peptide chain is first to be synthesized. Peptides are made from amino acids (Figure 1). There are 500 amino acids known , of which 20 are the proteogenic aminoacids.9 To make a peptide chain, several amino acids and short peptides are coupled together.12 The core behind this process is protection group strategy and C-terminus activation. When two amino

acids are coupled together, they form a dipeptide. Usually the N-terminus is protected against side reactions and the C-terminus stays as a free acid. By doing a esterification of the C-terminus, this side becomes active for further coupling

reactions. So a peptide ester is made to create the possibility to elongate the chain. Only this process comes with a problem. The stereocenter of the peptide is sensitive for epimerization. When this occurs, the diastereomer of the peptide is formed. To solve this problem, a coupling reaction method is developed: the cu(II) catalyzed Chan-Lam reaction.

H2NOH

O

R

2

∗

Figure 1: General structure of an amino acid

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1.4 The Cu(II)-catalyzed Chan-Lam reaction The Cu(II)-catalyzed Chan-Lam reaction is a coupling reaction to make peptide esters.13 In this reaction, boroxins are being used as reagents to provide the phenyl group for the esterification (Scheme 3) and epimirisation is taking place during the reaction.

HN

O

Peptide ∗OH

R1

OBO

BOB

O

R2 R2

R2

+Chan-Lam conditions H

N

O

Peptide ∗O

R1

OR2

3A 3B 3

Scheme 3: The general Chan-Lam couplings reaction with boronic acids13

In this reaction, copper changes several times of oxidation state to complete the catalytic cycle (Scheme 4).14

Cu(I)OTf

Cu(II)(OTf)2

Ar-Cu(II)OTf

Ar-Cu(III)(OTf)2

A B

CDPeptide

Peptide phenylester

Cu(II)(OTf)2

Cu(I)(OTf)Ar-B(OAr)2

TfO-B(OAr)2

1/2 O2 + Cu(I)OTfTfOH + TfO-B(OAr)2

HO-B(OAr)2 + Cu(II)(OTf)2

A = TransmetalationB = CoordinationC = Reductive eliminationD = Oxidation

Scheme 4: Proposed mechanism of the Cu(II)-catalyzed Chan-Lam reaction14

The copper species uses the boroxin to mediate and therefore form an esterbond. The phenylgroup is coupled to the oxygen atom by reductive elimination. Another advantage of the Chan-Lam reaction: the reaction can proceed in mild conditions.

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1.5 The aim of the project Section 1.2 described the most used and important organic reactions using triarylbismuth compounds as reagents. Section 1.4 described the advantages of using the Cu(II)-catalyzed Chan-Lam method to synthesize peptide esters. The core of this project is to combine these two aspects. This leads to the aim of the project: to apply triarylbismuth(III) compounds as reagents for the synthesis of dipeptide esters (compound 3, Scheme 5).

HN *

OH

O

R2O

*NH

R1PG Chan-Lam conditions H

N *O

O

R2O

*NH

R1PG

Ar

BiAr3

4A 4

Scheme 5: General synthetic route of a dipeptide ester with a triarylbismuth(II) compound

The project is divided in two part: The first part, is to synthesize different triarylbismuth(III) compounds and second, to apply these compounds as reagents in esterification of dipeptide acids under modified Chan-Lam conditions. After succeeding this synthesis, the focus will be on optimizing the yield and retaining the stereointegrity.

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2. Results and Discussion

2.1 Synthesis of triarylbismuth compounds In this project several attempts were done to synthesize three triarylbismuth(III) compounds: triphenylbismuth, tris(4-nitrophenyl)bismuth and tris(pentafluorophenyl)bismuth. The procedures were done in dry conditions.

2.1.1 Synthesis of triphenylbismuth First was to synthesize triphenylbismuth. This triarylbismuth(III) compound is the simplest derivative. The first route to try, was a transmetallation of phenyl lithium with bismuth chloride (Scheme 6).15 This synthetic route was done three times (Table 2).

Li

BiBiCl3

THF

5

5a

Scheme 6: Synthetic route to BiPh3 starting with phenyllithium

In all three cases TLC showed, that there was a side product present in a big amount. The aromatic region in the 1H-NMR showed several peaks and this couldn’t be interpreted. It is expected for triphenylbismuth to have two triplet and one doublet in the aromatic region. The side product was biphenyl and it was impossible to separate this compound from the product. This was tried with several columns with different eluents. It is a possibility, that n-butyl lithium is too reactive toward bismuth chloride and triphenylbismuth. The formation of biphenyl can occur easily. It is known, that the Bi-C bond is weak and easily can be broken2. In this case, reductive elimination occurs easily. After the method above didn’t work, another method was used to synthesize triphenylbismuth: transmetallation of bismuth chloride with phenylmagnesiumchloride (Scheme 7).16

MgCl

BiBiCl3 (1 equiv.)

THF, 0°C, overnight69% Yield

5b 5

(3 equiv.)

Scheme 7: Synthetic route to BiPh3 starting with phenylmagnesiumchloride

Attempt Time interval Addition reagents

Temp. Time Yield

1 5 min -78°C 4h 56% 2 1.25h -10°C -RT Overnight 74% 3 1h -78°C 1h Not determined

Table 2: Reaction conditions for scheme 4

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This reaction with the Grignard reagent proceeded very efficiently. There was no side product formed and the yield was reasonable (69% yield). To optimize the reaction, another procedure was done to synthesize triphenylbismuth: first making the Grignard reagent and followed by transmetallation with bismuth chloride (scheme 6).

Br

THF, Reflux, 3h

Mg (3 equiv.)

MgBr

BiBiCl3 (1 equiv.)

THF, 0°C, overnight9% Yield(3 equiv.)

5c5d

5 Scheme 8: Synthetic route to BiPh3 starting with bromobenzene

The yield was very low (9%) and this time biphenyl was present as side product. It is possible, that the environment wasn’t dry enough and reductive elimination could occur. But this time the sideproduct was removed from the product and gave pure triphenylbismuth. Triphenylbismuth was crystallized a few times with an EtOAc/PE mixture. Unfortunately, this process had an negative effect on the overall yield of the reaction.

2.1.2 Synthesis of tris(4-nitrophenyl)bismuth Tris(4-nitrophenyl)bismuth was the next triarylbismuth compound to be made. Triphenylbismuth can only give peptide phenylesters. These type of peptides are inactive and cannot be used efficiently for further couplings reactions. In contrary, dipeptide 4-nitrophenylesters are active peptide esters and can be used for further couplings reactions. For the synthesis of tris(4-nitrophenyl)bismuth a procedure, shown in Scheme 9, was attempted.15 First the lithiation of 4-nitrophenylbromide and then transmetallation with bismuth chloride. This procedure was done two times (Table 3).

NO2

Br

NO2

Li

Bi

NO2

O2N NO2

n-BuLi BiCl3

THF, 0°CTHF, -78°C

6a 6b 6

(1 equiv.)

Scheme 9: Synthetic route to tris(4-nitrophenyl)bismuth

Attempt Equiv. n-Buli Equiv. BiCl3 Reaction times Yield 1 3 1.01 2h, overnight 21% 2 3.3 1.1 Overnight, overnight Not determined Table 3: Reaction conditions for scheme 7

In the first attempt, there were 3 side products next to the unreacted material (compound 5a). TLC showed, that there were two apolar compounds and two polar compounds. One of the apolar compound was the starting compound. The focus was on the compounds, that gives two doublets in the aromatic region of the 1H-NMR spectrum. None of the compound in the reaction mixture, except for the 4-nitrophenylbromide, had two doublets. So the reaction was redone with other equivalents and reaction time (Table 3). TLC of the reaction mixture showed one compound. To make sure, that there were no other compounds, TLC was taken several times with diffrent eluents. Still there was

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shown only one compound (one dot shown), but 1H-NMR said differently. The spectrum had signals between 3 and 1 ppm and several in the aromatic region. The peaks in the aromatic region couldn’t be interpreted. Even so, there were two doublets between all those signals. It is a possibility, that tris(4-nitrophenyl)bismuth had formed. The signals between 3 and 1 ppm could point out to the butylbismuth compound. Another synthetic route was tried to form tris(4-nitrophenyl)bismuth. The idea was to make first the Grignard reagent of 4-nitrophenylbromide and then followed by the transmetallation with bismuth chloride. The first part didn’t worked: the Grignard reaction would not start. This was attempted for three times. It is possible, that the Grignard reagent of 4-nitrophenylbromide isn’t stable or cannot form. Magnesium can coordinate with the nitro group and favors this form against oxidative insertion.

2.1.3 Synthesis of tris(pentafluorophenyl)bismuth After the synthesis of tris(4-nitrophenyl)bismuth didn’t work, tris(pentafluorophenyl)bismuth was tried to synthesize. This triarylbismuth(III) compound can also give active peptide esters. For this synthesis, a transmetallation reaction was done between pentafluorophenylmagnesium bromide and bismuth chloride (Scheme 10)16

FF

FMgBr

F

F

FF

F

Bi

F

F

FF

F

FF

F

FF

FF

BiCl3 (1equiv.)

Diethyl/THF,0°C, overnight

(3 equiv.)

17a 17 Scheme 10: Synthetic route to tris(pentafluorophenyl)bismuth

After the work up process, a black oil was obtained. TLC showed one dots with different eluens. So, a 19F-NMR and a 13C-NMR was taken to make clear what the compound was. 19F-NMR showed multiplets instead of singlets. Even with NMR help of the technician, this spectrum couldn’t made better. The 13C-NMR showed four peaks in the aromatic region, as expected, but it couldn’t determination that the product was formed. There is no literature spectra properties available of this compound and possible side products. So a Chan-Lam reaction was done with this compound and Boc-L-Phe-D-Phe-OH and no ester was formed. With these results, no conclusion can be made if the ester didn’t formed, because it was tris(pentafluorophenyl)bismuth or not.

2.2 Synthesis of the dipeptide free acid For the second part of the project: a dipeptide free acid was synthesized. It was determined to synthesize Boc-L-Phe-L-Phe-OH (compound 6). This was done by coupling Boc-L-Phe-OSu (compound 6a) with L-phenylalanine(compound 6b, Scheme 10).13

13

NH

HNBOC

OOH

O

NH

Boc

O

OSu +H2N

OH

O

(CH3)2CO, H2O, EtOH, rt78% Yield

NaHCO3 (2.5 equiv.)

77b7a

(1 equiv.) (1.1 equiv.)

Scheme 11: Reaction scheme of Boc-L-Phe-L-Phe-OH

The reaction gave a reasonable yield(78%). LC-MS showed, that all of the starting compounds (compounds 6a and 6b) was gone. This dipeptide acid was chosen, because it is known to be a stable dipeptide acid and epimerisation doesn’t occur easily. The rest of the dipeptide free acids was already made by other group members.

2.3 Chan-Lam coupling reactions The final step is to apply the triphenylbismuth in the Cu(II) catalysed Chan-Lam reaction to make dipeptide phenyl esters (Scheme 11). This was the only triarylbismuth(III) compound to be made. The first test reaction was done with Boc-L-Phe-L-Phe-OH (Table 4, entry 1) and this was a success, but the yield wasn’t high (44% yield). The reaction was repeated for Boc-L-Phe-D-Phe-OH (Table 4, entry 2) and this one gave a higher yield. Also other dipeptide acids were used for the esterification process (Table 4, entry 3-5).

NH

HN *Boc

OOH

O

NH

HN *Boc

OO

O

BiCl3 (1.11 equiv.)

THF, O2, 65°C, 8.5h-overnight

Cu(OTf)2 (1 equiv.)1,3-diethylurea (2.06 equiv.)

TEA (0.49 equiv.)R1

R1

(1 equiv.)

8a8

Scheme 12: Reaction scheme of the dipeptide phenyl ester

Entry R1 Configuration R1 Highest yield

1

L

58%

2

D

71%

3

L

73%

4

L

57%

14

5

D

73%

Table 4: The 5 dipeptide phenyl esters

Then the Chan-Lam reaction was done in different reaction conditions to optimize the yield of Boc-L-Phe-L-Phe-OPh, so the developed procedure would be applied for other dipeptide acids. Only one change gave the highest yield for Boc-L-Phe-L-Phe-OPh. The original procedure13 says to divided the reagents in two batches. Boc-L-Phe-L-Phe-OPh had the highest yield by simply using one batch: so add all of the reagents at once and stir for 9h at 65 °C. The yields of the synthesized dipeptide esters (Table 4) were different (ranging from 58%-73%). These results can’t say anything about the influence of the side chain functionalities.

2.4 1H-NMR analysis: epimerisation Like said in section 1.3: epimersation of the stereocenter in peptides is a problem. In this project the NMR spectra of dipeptide esters and their diastereomers were used to determine if epimerisation had occurred during the Chan-Lam coupling reaction with triphenylbismuth.

2.4.1. Boc-L-Phe-L-Phe-OPh and Boc-L-Phe-D-Phe-OPh diastereomers In Figure 2 are shown the 1H-NMR spectra of Boc-L-Phe-L-OPh(red) and Boc-L-Phe-D-Phe-OPh(blue) and enhanced parts in Figure 3.

Figure 2: Overlapped 1H-NMR spectra of Boc-L-Phe-L-OPh(red) and Boc-L-Phe-D-Phe-OPh(blue)

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Figure 3: Enhanced speaks of figure 1

The two diastereomers gave different spectra. They are different for N-H (peak C), N-H(peak A) and –CH2(peak B). This is an indication, that there was no epimerization during the reaction.

2.4.1. Boc-L-Phe-L-Phg-OPh and Boc-L-Phe-D-Phg-OPh diastereomers In Figure 4 is shown the 1H-NMR spectra of Boc-L-Phg-L-OPh(red) and Boc-L-Phe-D-Phg-OPh(blue) and enhanced parts in Figure 5.

Figure 4: Overlapped 1H-NMR spectra of Boc-L-Phg-L-OPh(red) and Boc-L-Phe-D-Phg-OPh(blue)

16

Figure 5: Enhanced peaks of figure 3

These Boc-L-Phe-L-Phg-OH and Boc-L-Phe-D-Phg-OPh diastereomers are different on two places: peak D(C-H) and peak E(-CH2). In the aromatic region one peak of Boc-L-Phe-D-Phg-OPh is shifted, in comparison of Boc-L-Phe-L-Phg-OPh. In this reaction, there was also no epimerisation of the dipeptides.

17

3. Conclusion The synthesis of just one triarylbismuth(III) compound was a success: triphenylbismuth. The route to this compound was difficult, even so that this is the most simple triarylbismuth(III) compound. But after several attempts, an efficient method was developed. The synthesis of tris(4-nitrophenyl)bismuth and tris(pentafluorophenyl)bismuth were not a success. These compounds were never made. Maybe it is possible to make these organometallic compounds with other synthetic routes. The application of a triarylbismuth(III) compound as a reagent for the synthesis of dipeptide esters was a success (Scheme 13). This was possible with triphenylbismuth. The Chan-Lam was modified in small ways for the epimerisation-free esterification of dipeptide acids.

HN * OH

O

R1ONH

Boc Chan-Lam conditions HN * O

O

R1ONH

Boc

BiPh3

8a 8b

Scheme 13: Succeded route to dipeptide esters

18

4. Future Prospects

4.1 Further synthesis of triarylbismuth(III) compounds The synthesis of tris(4-nitrophenyl)bismuth failed with the used methods, described in section 2.1.2. So two methods are proposed to try to synthesize this triarylbismuth(III) compound. The first method is a nitration method. Triphenylbismuth (compound 4) is nitrated with Cu(NO3)2 2.5H2O (Scheme 12). This method is based on the nitration of triphenylamine.17

Bi Bi

NO2

O2N NO2

5 6

Cu(NO3)2 2.5H2O

Ac2O, rt, overnight

Scheme 14: Propesed route to tris(4-phenyl)bismuth (1)

The second method is a substitution route. P-nitrophenyldiazonium tetrafluroborate is added to tri(p-toly)bismuth (compound 13) to substitute the phenyl rings (Scheme 13). This method is based on the method of the synthesis of triarylbismuth(V) compounds.16

Bi Bi

NO2

O2N NO2

Cu

O2N

N2 BF4

DMF, rt, 18h

14 6

Scheme 15: Proposed route to tris(4-nitrophenyl)bismuth (2)

For tris(pentafluorophenyl)bismuth: the synthetic route described in section 2.1.3 should be redone. If this doesn’t work, then other synthetic routes should be searched. It is also interesting to make another triarylbismuth(III) compound to get active peptide esters. A group member is making the boroxin of compound 14a in Scheme 14. Due to time, there was no chance to make the bismuth compound of it. In Scheme 14 are two prosoped route to make tris(2,3-dihydrobenzo[b][1,4]dithiin-6-yl)bismuth (compound 14). These methods are based on the methods, that were used to make triphenylbismuth, tris(4-nitrophenyl)bismuth and tris(pentafluorophenyl)bismuth.

19

S

S

Br

S

S

Bi

SS

SS

15a

15

1) Mg2) BiCl3

1) n-BuLi2) BiCl3

Scheme 16: Two proposed route to tris(2,3-dihydrobenzo[b][1,4]dithiin-6-yl)bismuth

4.2 Further optimization of the Chan-Lam coupling reaction with BiAr3 Even so, that triarylbismuth(III) compounds can be used as reagents in the Chan-Lam reaction for peptide ester synthesis: the reaction is not fully optimized. Some peptide esters have a low yield (50-60%). The reaction conditions can be further explored: longer reaction time, other base or solvent etc. Never the less, a simplified method to synthesize triarylbismuth(III) compounds can save time in the process, compared to commonly used boroxines.

4.3 Further applications of triarylbismuth(III) compounds Like already said in section 1.2: triarylbismuth compounds are being used in important organic reactions and now it can be used in esterification of peptides. During the first phase of the project, the Ullman reaction came forward. This method is used to make phenyl ethers. Triarylbismuth compounds can be used for phenylation and maybe it is possible to make phenyl ethers with these compounds. In Scheme 15 are two proposed routes to phenyl ethers. One is based on a method of the Ullmann reaction in mild conditions.18 This method was chosen, because triarylbismuth can react in mild conditions. The other method is a metal-ligand free Ullmann-type reaction.19 This method is chosen, because it is known, that triarylbismuth can react without a catalyst.

Bi R1

3+

HOR2

O

R1 R2

Cu2OLigandCs2CO3

t-BuOK

DMSO

MeCN

16a 16b 16

Scheme 17: Proposed route to phenylesters with BiAr3 as reagent

20

5. Acknowledgement First of all, I want to thank prof.dr. Henk Hiemstra for giving me the opportunity to do a bachelor project in his group. I really wanted to do an synthetic organic chemistry project, because this is my direction what I am heading in my master and I did projects at other different groups. Then I want to thank dr. Jan van Maarseveen for introducing me in to the project and sharing his enthusiasm with me about peptide synthesis. Peptides are one of the large molecules, that are being used in medicinal chemistry and in the future I want to make new medicine. Also a thank to dr. Steen Ingemann for extra support for finalizing the report and prepare the presentation. Dr. Bas de Bruin, thank you for being my second reviewer. A another thanks goes to Jelmer Koole BSc. He was really a partner in the lab. We had all lot of discussion about the chemistry what we are doing and I got all lot of tips and ideas from these discussions. This helped me in the process of optimizing the reactions and solving synthetic problems. For last I want to thank Stanimir Popovic Msc. for taking me as his bachelor student. I was happy with the project I got. It was really a challenge for me to take the Chan-Lam reaction to another level. And I was satisfied with him as my supervisor, because we both are workaholics. And of course I want to thank the rest of the Synthetic Organic Chemistry group for taking me in the group as family.

21

6. References 1. Weast, R.C. Handbook of Chemistry and Physics, 49th ed.; The Chemical Rubber Co: Ohio, 1968,

B102-B103. 2. Luan, J.; Zhang, L.; Hu, Z. Synthesis, Properties Characterization and Applications of Various

Organobismuth Compounds. Molecules 2011, 16, 4192-4230. 3. Leonard, N.M.; Wieland, L.C.; Mohan, R.S. Applications of bismuth (III) compounds in organic

synthesis. Tetrahedron, 2002, 58, 8373-8397. 4. Dikarev, E.V.; Li, B. Rational Syntheses, Structure, and Properties of the First Bismuth(II)

Carboxylate. Inorg. Chem. 2004, 43, 3461-3466. 5. Davies, S.J.; Compton, N.A.; Huttner, G.; Zsolnai, L.; Garner, S.E. Synthesis and Reactivity of

“Bismuthinidene” Compounds and the formation of Bi(I) Chelate Complexes. Eur. J. Inorg. Chem. 1991, 124, 2731-2738.

6. Rao, M.L.N.; Banerjee, D.; Giri, S. Palladium-Catalyzed Cross-Couplings of Allylic Carbonates with Triarylbismuths as Mulit-Coupling Atom-Efficient Organometallic Nucleophiles. J. Organomet. Chem. 2010, 695, 1518-1525.

7. Rao. M.L.N.; Banerjee, D.; Dhanokar. R.J. Synthesis of Functionalized 2-Arylthiophenes with Triarylbismuths as Atom-Efficient Multicoupling Organometallic Nucleophiles under Palladium Catalysis. Synlett, 2011, 9, 1324-1330.

8. Rao, M. L. N.; Dhanokar, R. J. Pd-Catalyzed Chemoselective Threefold Cross-Coupling of Triarylbismuths with Benzylic Bromides. RSC Advances, 2013, 3, 6974-6798.

9. Berg, J.M.; Tymoczko, J.L.; Stryer, L. Biochemistry, 6th ed.; W.H. Freeman&Co, 2007. 10. Alberts, B.; Johnson, A.; Lewis, J.; Raff, M.; Roberts, K.; Walter, P. Molecular Biology of the Cell,

5th ed.; Garland Science, 2008. 11. Bray, B.L. Large-Scale Manufacture of Peptide Therapeutics by Chemical Synthesis, Nat. Rev,

2003, 2, 587-593. 12. El-Faham, A.; Albericio, F. Peptide Coupling Reagents, More than a Letter Soup. Chem. Rev. 2011,

111, 6557-6602. 13. J.H. van Maarseveen et al. Unpublished manuscript. 14. Rao, K.S.; Wu, T. Chan-Lam coupling reactions: synthesis of heterocycles, Tetrahedron, 2012, 68,

7735-7754. 15. Parrish, J.P.; Hughes, T.V.; Hwang, I.; Boger, D.L. Establishing the Parabolic Relationship between

Reactivity and Activity for Derivatives and Analogues of the Duocarmycin and CC-1065 Alkylation Subunits. J. Am. Chem. Soc. 2004, 126, 80-81.

16. Barton, D.H.R.; Bhatnagar, N.Y.; Finet, J.; Motherwell, W.B. Pentavalent Organobismuth Reagents. Part VI. Comparative Migratory Aptidues of Aryl Groups in the Arylation of Phenols and Enols by Pentavalent Bismuth Reagents. Tetrahedron 1986, 42, 3111-3122.

17. Wu, X.; Dube, M.A.; Fry, A.J. Electrophilic Nitration of Triphenylamine as a Route to High Oxidation Potential Electrocatalyst. Polynitraiotn, Nitrodebromination, and Bromine Dance. Tetrahedron Lett. 2006, 47, 7667-7669.

18. Cristau, H.; Cellier, P.P.; Hamada, S.; Spindler, J.; Taillefer, M. A General and Mild Ullman Type Synthesis of Diaryl Ethers. Org. Lett. 2004, 6, 913-916.

19. Yang, S.; Wu, C.; Ruan, M.; Yang, Y.; Zhao, Y.; Niu, J.; Yang, W.; Xu, J. Metal- and Ligand-Free Ullamn-Type C-O and C-N Coupling Reactions Promoted by Potassium tert-Butoxide. Tetrahedron Lett. 2012, 4288-4292.

22

7.Experimental Section

7.1 Procedures Triphenylbismuth

This procedure was performed under dry conditions BiCl3 (1.48 g, 4.68 mmol, 1 equiv..) was dissolved in 10 ml THF at a temperature of 0°C. Then 9.4 ml of a 25% wt phenylmagnesiumchloride solution in THF was added dropwise with a time interval of 2h. After stirring overnight, there a grey suspension was formed. The reaction mixture was quenched with 5 ml H2O:THF=1:5 and 4 ml H2O. Here developed a white solid on the bottom. After extraction (three times with EtOAc) the organic layer was dried over MgSO4. The filtrate gave 1 spot on TLC (PE:EtOAc=9:1). Silica was added to the solution and

the product was purified with column chromatography. The product (compound 4) was a white solid with a yield of 69% (1.37 g, 3.11 mmol). Mp 76°C IR υ 3056, 3044, 3034, 3015, 2976, 1980, 1645, 1876, 1812, 1759, 1692, 1659, 1631, 1581,1566, 1502, 1472, 1424, 1326, 1299, 1259, 1182, 1153, 1055, 1013, 995, 965, 900, 851, 721, 691, 644, 615, 448, 435. 1H NMR (400 MHz, CD2Cl2) δ 7.83-7.81 (t, 6H), 7.48-7.44 (t, 6H), 7.40-7.36 (d, 3H). 13C NMR (300 MHz, CD2Cl2) 137.52, 130.48, 127.75. BOC-L-Phe-L-Phe-OH

BOC-L-Phe-OSu (4.39 g, 12.12 mmol, 1 equiv.) was dissolved in 20 ml ethanol and 80 ml acetone. A solution of L-phenylalanine (2.55 g, 13.35 mmol, 1.1 equiv.) and NaHCO3 (2.55 g, 30.32 mmol, 2.5 equiv.) in 100 ml water was added at once. This reaction mixture was stirred for 26h at room temperature. All of the volatiles, except water, were evaporated and 1M KHSO4 was added to the solution to acidify the mixture to pH 3. Then 400 ml of EtOAc was added to the formed suspension. The organic layer was washed three times with 100 ml 1M KHSO4, 100 ml water (quick) and 100 ml brine and dried over MgSO4.

After evaporation, the white solid was precipitated in PE and then crystallised out of a PE/EtOAc mixture to purify compound 6 further. This resulted in a yield of 78% (1.56 g, 3.78 mmol). Mp 139 °C. IR υ 3335, 2924, 2854, 1757, 1722, 1680, 1662, 1592, 1519, 1492, 1454, 1391, 1367, 13616, 1294, 1273, 1243, 1197, 1164, 1111, 1079, 1043, 1021, 993, 959, 916, 862, 796, 779, 746, 697, 627, 595, 553, 527, 46, 434, 410. 1H NMR (400 MHz, CDCl3) δ 9.36 (s, 1H), 7.30-7.19 (m, 8H), 7.09-7.07 (d, 2H), 6.67-6.65 (d, 1H), 5.18 (s, 1H), 4.78 (s, 1H), 4.45 (s, 1H), 3.16-3.13 (d, 2H), 3.01-2.98 (m, 2H), 1.40 (s, 9H). 13C NMR (400 MHz, CDCl3) 173.68, 171.71, 155.71, 136.46, 135.84, 129.43, 129.33, 128.56, 128.48, 127.03, 126.90, 80.41, 55.64, 53.41, 38.25, 37.52, 28.25. LC-MS m/z 413 (M+). BOC-L-Phe-L-Phe-OPh

BOC-L-Phe-L-Phe-OH (75.1 mg, 0.18 mmol, 1 equiv.), BiPh3 (88.5 mg, 0.20 mmol, 1.1 equiv.), Cu(OTf)2 (65.1 mg, 0.18 mmol, 1 equiv.) and 1,3-diethylurea (43 mg, 0.37 mmol, 2.06 equiv.) was dissolved in 5 ml of THF. 1 ml of 8,8x10-2 M triethylamine solution in THF (0.088 mmol, 0.49 equiv.) was added to the reaction mixture, resulting in a dark purple mixture. A balloon filled with compressed air was put on the flask. The reaction mixture was stirred for 9h at 65 °C and got a green colour. TLC (PE:EtOAc = 8:2) showed 4 spots. The product (compound 8) was isolated with

Bi

5

NH

HNBOC

OOH

O

7

NH

HNBOC

OO

O

9

23

column chromatography (PE:EtOAc=8:2 PE:EtOAc=7:3). Yield=58% (51.9 mg, 0.11 mmol). Mp 142 °C IR υ 3335, 3063, 3029, 2971, 2929, 2361, 2146, 2112, 2050, 2034, 1981, 1945, 1758, 1682, 1662, 1592, 1519, 1492, 1454, 1390, 1367, 1318, 1294, 1274, 1243, 1197, 1163, 1079, 1043, 1022, 993, 960, 916, 863, 825, 796, 719, 748, 737, 697, 621, 594, 561, 526, 494, 473, 413. 1H NMR (400 MHz, CDCl3) δ 7.40-7.23 (m, 11H), 7.13-7.11 (d, 2H), 6.98-6.96 (d, 2H), 6.37-6.35 (s, 1H), 5.02-5.00 (s, 2H), 4.39 (s, 1H), 3.22-3.20 (d, 2H), 3.09-3.07 (d, 2H), 1.41 (s, 9H). 13C NMR (400 MHz, CDCl3) 170.84, 169.49, 150.03, 136.31, 135.23, 129.30, 129.23, 128.55, 126.00, 121.08, 53.36, 38.12, 29.55, 28.08. LC-MS m/z 489 (M+). BOC-L -Phe-D-Phe-OPh

BOC-L-Phe-D-Phe-OH (148.1 mg, 0.36 mmol, 1 equiv.), BiPh3 (87.6 mg, 0.20 mmol, 0.56 equiv.), Cu(OTf)2 (65.2 mg, 0.18 mmol, 0.50 equiv.), 1,3-diethylurea (118.5 mg, 1.02 mmol, 2.84 equiv.) and triethylamine (10 mg, 0.10 mmol, 0.55 equiv.) were dissolved in 12 ml THF. This resulted in a dark green mixture. The reaction mixture was stirred for 2.5h at 65 °C under a balloon filled with compressed air. Then a second batch was added of BiPh3 (93.5 mg, 0.21 mmol, 0.58 equiv.), Cu(OTf)2 (70.4 mg, 0.19 mmol, 0.53 equiv.) en triethylamine (10 mg, 0.10 mmol, 0.55 equiv.) in 3 ml

THF. Then the reaction was stirred further for another 6h at 65 °C under a balloon filled with compressed air. This resulted in a green mixture. TLC (PE:EtOAc = 8:2) showed 4 spots. The product (compound 10) was isolated with column chromatography (PE:EtOAc=8:2 PE:EtOAc=7:3), resulting in a yield of 71% (105.1 mg, 0.25 mmol). Mp 129 °C IR υ 337, 3063, 3030, 2968, 2929, 2870, 1756, 1684, 1655, 1593, 1516, 1492, 1453, 1390, 1367, 1322, 1291, 1241, 1196, 1162, 1070, 1043, 1021, 993, 936, 916, 864, 824, 195, 778, 150, 138, 697, 625, 594, 525, 493, 463, 436. 1H NMR (400 MHz, CDCl3) δ 7.37-7.21 (m, 11H), 7.09 (d, 2H), 6.98-6,95 (d, 2H), 6.48 (s, 1H), 5.09-5.04 (q, 1H), 4.94 (s, 1H), 4.41 (s, 1H), 3.27-3.20 (q, 1H), 3.15-3.05 (m, 3H), 1.40 (s, 9H). 13C NMR (400 MHz, CDCl3) 171.05, 169.71, 155.26, 150.13, 136.46, 135.26, 129.26, 129.23, 129.19, 128.59, 121.12, 99.81, 55.52, 53.20, 37.73, 28.09. LC-MS m/z 489 (M+). BOC-L-Phe-L-Val-OPh

BOC-L-Phe-L-Val-OH (131.5 mg, 0.36 mmol, 1 equiv.), BiPh3 (88.2 mg, 0.20 mmol, 0.56 equiv.), Cu(OTf)2 (65.1 mg, 0.18 mmol, 0.51 equiv.) and 1,3-diethylurea (43.5 mg, 0.37 mmol, 1.04 equiv.) in 10 ml THF. Then 1 ml of 8.8x10-2M triethylamines solution in THF was added. This resulted in a dark blue mixture. The reaction mixture was stirred for 3h at 65 °C under a balloon filled with compressed air. Then a second batch was added of BiPh3 (93.1 mg, 0.21 mmol, 0.55 equiv.), Cu(OTf)2 (64.1 mg, 0.18 mmol, 0.50

equiv.) and 1,3-diethylurea (41.8 mg, 0.36 mmol, 1 equiv.) in 2 ml THF with 1 ml of 8.8x10-2M triethylamine. The reaction was stirring overnight at 65 °C under a balloon filled with compressed air. This resulted in a green mixture. TLC (PE:EtOAc = 8:2) showed 3 spots. The product (compound 11) was isolated with column chromatography (PE:EtOAc=8:2 PE:EtOAc=7:3), resulting in a white solid with a yield of 73% (115.9 mg, 0.26 mmol). Mp 133°C. IR υ 3336, 2989, 2965, 2928, 2870, 1760, 1681, 1658, 1593, 1519, 1494, 1454, 1391, 1367, 1324, 1296, 1278, 1242, 1200, 1183, 1163, 1155, 1123, 1072, 1049, 1021, 993, 975, 918, 885, 867, 815, 795, 778, 750, 737, 698, 652, 621, 593, 551, 524, 494, 461, 435, 412. 1H NMR (400 MHz, CDCl3) δ 7.41 (t, 2H), 7.33-7.22 (tt, 6H), 7.08 (d, 2H), 6.46 (s, 1H), 5.07 (s, 1H), 4.73-4.70 (q, 1H) 4.41 (s, 1H), 3.13-3.11 (d, 2H), 2.33 (q, 1H), 1.43 (s, 9H), 0.90-0.85 (q,

NH

HNBOC

OO

O

10

NH

HNBOC

OO

O

11

24

6H) 13C NMR (400 MHz, CDCl3) 171.40, 169.85, 155.38, 150.20, 136.50, 129.29, 129.17, 128.46, 126.74, 125.92, 121.17, 57.30, 31.07, 28.10, 18.80, 17.70. LC-MS m/z 441 (M+). BOC-L-Phe-L-Phg-OPh

BOC-L-Phe-L-Phg-OH (71.5 mg, 0.17 mmol, 1 equiv.), BiPh3 (46 mg, 0.10 mmol, 0.61 equiv.), Cu(OTf)2 (32.1 mg, 0.09 mmol, 0.52 equiv.) and 1,3-diethylurea (22.7 mg, 0.20 mmol, 1.15 equiv.) in 5 ml THF. Then 1 ml of 8.8x10-2M triethylamine solution in THF was added. This results in a blue mixture. The reaction mixture was stirred for 3h at 65°C under a balloon filled with compressed air. Then a second batch was added of BiPh3 (44.9 mg, 0.10 mmol, 0.60 equiv.), Cu(OTf)2 (31.8 mg, 0.09 mmol, 0.52 equiv.) and 1,3-diethylurea (24.5 mg, 0.0.21 mmol, 1.24 equiv.) in 2 ml THF with 1

ml of 8.8x10-2M triethylamine. The reaction was stirred for 6h at 65 °C under a balloon filled with compressed air. This resulted in a green mixture. TLC (PE:EtOAc = 8:2) showed 3 spots. The product (compound 12) was isolated with column chromatography (PE:EtOAc=8:2 PE:EtOAc=7:3), resulting in a white/yellowish solid with a yield of 57% (45.9 mg, 0.097 mmol). Mp 108°C IR υ 3338, 2976, 2926, 1758, 1744, 1685, 1661, 1650, 1591, 1514, 1492, 1454, 1391, 1368, 1317, 1291, 1267, 1232, 1208, 1189, 1164, 1080, 1068, 1047, 1023, 1003, 962, 938, 918, 905, 892, 870, 855, 819, 795, 779, 745, 735, 696, 663, 648, 620, 590, 578, 565, 541, 505, 496, 471, 459, 443, 409. 1H NMR (400 MHz, CDCl3) δ 7.40-7.34 (m, 8H), 7.27-7.22 (m, 5H), 7.01-6.98 (d, 2H), 6.90 (s, 1H), 7.73-7.71 (d, 1H), 5.04 (s, 1H), 4.47 (s, 1H), 3.20-3.15 (q, 1H), 3.10-3.05 (q, 1H), 1.42 (s, 9H). 13C NMR (400 MHz, CDCl3) 171.00, 169.90, 150.53, 135.54, 129.59, 129.42, 129.29, 128.88, 127.56, 121.28. LC-MS m/z 375 (M+). BOC-L-Phe-D-Phg-OPh

BOC-L-Phe-D-Phg-OH (72.0 mg, 0.18 mmol, 1 equiv.), BiPh3 (44.3 mg, 0.10 mmol, 0.56 equiv.), Cu(OTf)2 (32.3 mg, 0.09 mmol, 0.50 equiv.) and 1,3-diethylurea (22 mg, 0.19 mmol, 1.05 equiv.) in 5 ml THF. Then 1 ml of 8.8x10-2M triethylamine solution in THF was added. This resulted in a purple mixture. The reaction mixture was stirred for 3h at 65 °C under a balloon filled with compressed air. Then a second batch was added of BiPh3 (44.8 mg, 0.10 mmol, 0.57 equiv.), Cu(OTf)2 (21 mg, 0.09 mmol, 0.51 equiv.) and 1,3-diethylurea (21.0 mg, 0.18 mmol, 1 equiv.) in 2 ml THF with 1 ml

of 8.8x10-2M triethylamine. The reaction was stirred for 6h at 65 °C under a balloon filled with compressed air. This resulted in a green mixture. TLC (PE:EtOAc = 8:2) showed 3 spots. The product (compound 13) was isolated with column chromatography (PE:EtOAc=8:2 PE:EtOAc=7:3), resulting in a yield of 73% (115.9 mg, 0.26 mmol). Mp 146°C. IR υ 3337, 3063, 3035, 2978, 2926, 1758, 1744, 1686, 1660, 1651, 1592, 1563, 1515, 1493, 1454, 1391, 1368, 137, 1291, 1267, 1233, 1207, 1190, 1164, 1069, 1046, 1023, 1003, 961, 938, 918, 905, 893, 870, 854, 819, 795, 779, 746, 735, 969, 648, 622, 591, 578, 564, 541, 505, 496, 471, 459, 439. 1H NMR (400 MHz, CDCl3) δ 7.39-7.17 (m, 13H), 7.01-6.99 (d, 2H), 6.90 (s, 1H), 5.74 (s, 1H), 5.07 (s, 1H), 4.46 (s, 1H), 3.10 (s, 2H), 1.41 (s, 9H). 13C NMR (400 MHz, CDCl3) 170.66, 168.96, 150.20, 135.21, 129.26, 129.08, 128.96, 128.54, 127.23, 120.94. LC-MS m/z 375 (M+).

NH

HNBOC

OO

O

12

NH

HNBOC

OO

O

13

25

7.2 Spectra Compound 4: Triphenylbismuth

26

Compound 6: Boc-L-Phe-L-Phe-OH

27

28

Compound 8: Boc-L-Phe-L-Phe-OPh

29

30

Compound 9: Boc-L-Phe-D-Phe-OPh

31

32

Compound 10: Boc-L-Phe-L-Val-OPh

33

34

Compound 11: Boc-L-Phe-L-Phg-OPh

35

36

Compound 12: Boc-L-Phe-D-Phg-OPh

37

38

39