A Novel Strategy for the Synthesis of Trifluoroalanine ligopeptides

Chapter 29

Downloaded by UNIV MASSACHUSETTS AMHERST on September 27, 2012 | http://pubs.acs.org Publication Date: January 11, 2007 | doi: 10.1021/bk-2007-0949.ch029

A Novel Strategy for the Synthesis of Trifluoroalanine Oligopeptides Kenji Uneyama, Yong Guo, Kana Fujiwara, and Y u m i Komatsu

Department of Applied Chemistry, Faculty of Engineering, Okayama University, Okayama 700-8630, Japan

2-N-(p-Methoxyphenyl)(trimethylsilyl)aminoperfluoropropene was prepared in 95% yield by the Mg-promoted C-F bond activation in an Mg-TMSCl-DMF system. The fluorinated enamine is very electrophilic so that it reacts with a variety of α-amino esters in a triethylamine-DMF system at room temperature, affording trifluoroalanine dipeptides in 70-90% yields via one-pot process. The reaction proceeds via three steps; addition of amino group of amino esters, subsequent dehydrofluorination leading to the corresponding imidoyl fluoride, then acid-catalyzed hydrolysis of the imidoyl fluoride. The protocol is applicable for alanine tripeptides.

462

© 2007 American Chemical Society

In Current Fluoroorganic Chemistry; Soloshonok, V., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

463 The chemistry and science of peptides and their related molecules has been increasingly developed over several decades due to their potential biological activity, and thus, a number of non-natural peptides and peptidomimetics have been designed and synthesized to gain the advantages of increased bioactivity, biostability, and bioselectivity over those of natural peptides. On this basis, highly efficient peptide-bond fomation has been increasingly needed to enable the synthesis of more complex, newly functionalized, and/or structurally new peptides. So far, peptide bonds have been prepared mostly by condensation of amino acids. The rational activation of the carboxyl group of one amino acid followed by condensation with another aminoester, or ring-opening of β-lactams with amino esters has been used.


1

2

3

Carboxyl

Amino Ester

Action

H N-OC0 R 2

2

O

ft

Z

Zjj-O-C-^D

ZN-QC0 H 2

O

ft 6

C0

jjO -fj"D" 2

R

Activated leaving group Figure 1. Coventional Method for Peptide Bond Formation 4

5

Both enzymatic and chemical activation has been commonly employed for the condensation. One of the enzymatic carboxyl groupR H

2

r

r

2

H

3

H

*

°

H

HF CCH

N

N \ 2

u r

2

C

P

HF CCH 2

50-22%

N O

/

X

Y

H

N

^

A

R

o

3

1 F3CCH3 B

O

3 H

F 3

CPY 4

j

5

-

1

7

Z

%

X

H 3

*

O

4

X

N R

5 2

R = H, C H , CH(CH ) , C H C H ( C H ) 3

3

2

2

3

2

Figure 2. Enzymatic Peptides Formation


2

464 activation is shown in Fig. 2, where carboxypeptidase Y (CPY) was employed for the peptide bond formation of a-trifluoromethyl and difluoromethylalanines. One example of the chemical activation of carboxyl group by D C C is shown in Fig. 3, where a rather higher temperature was needed for the peptide bond formation. 6

7


NH

a:

C0 R 3 :

2

7

NHCbz JL

2

nu

CbzHN

DCC

H

A

V

130-170 °C 67 - 79%

» 6

N .

o

X0 R

2

2

R l

8

NHCbz R 2 - ^ C 0 H 2

NH

F

2

OR

F

3

c

1 +

1

+

1 U

0

DCC "

130-170 ° C

0

9

55 - 71%

-

"

a

0

9 R ^ ^ / k . ^ ^ N H C b z

"~ V O

H

R

2

11

Figure 3. Chemical Activation of Carboxyl Group with DCC for Peptide Bond Formation Meanwhile, Burger demonstrated Rh-catalyzed insertion of oc-ketocarbene to N - H bond of a-aminoamide 13 leading to dipeptide formation via noncondensation route (Fig. 4). But, very few methods for non-condensation type peptide synthesis have been known. A novel peptide synthesis method, in which C - N bond formation mechanism is totally different from the conventional condensation of two kinds of amino acids would open a door into the field of structurally unique and highly functionalized peptides, which would create unique biological activity as yet unknown. In this context, it has been proposed 2-aminoperfluoropropene 15 would be effective synthon for trifluoroalanine which would react smoothly with amino group of amino esters 17, affording dipeptides 16 (Fig. 5). The concept of this novel peptide-bond formation is shown schematically in Fig. 6. 8


465

N

2

Rh (OAc)


2

O

NHCbz

12

13

6

8

4

%

0

f3

0

CbzHN 14

Figure 4. A New Peptide Formation by Ketocarbene Insertion to Amino Group

NH

2

? (TMS)Ar- N F

3

R

°

/ -

I F

C

NHR J < j X / C 0 u

0

2

R

2

DMF

O

2

R

2

-

X

RI 16

15 Figure 5. A Novel Concept for Peptide Formation

NHAr

NHAr H

R I > V

18

F

R

=

R1^

R

20

2

-

—

X

R 1 ^ R 2

21

Y %

>

2

0

F

19

-

V

1

R R «

1

R

^

22

F

F

R

1

2

23° X = O H , O R , NHR

Figure 6. Difluoromethylene Moiety as a Synthon for Carbonyl Group


466 In principle, difluoromethylene moiety is a synthon for a carbonyl group just as do the dichloromethylene and dibromomethylene groups. Thus, the difluoromethylene group in the alkene 22 would be a synthon for a carboxylic acid, ester and amide since it can be transformed to the carbonyl derivatives on reacting with water, alcohols and amines, respectively. This difluoromethylene-carbonyl group interconversion concept makes it possible to design the difluoroenamine 18 as a synthon for oc-aminoacids and peptides 19 as shown in Fig. 6. Downloaded by UNIV MASSACHUSETTS AMHERST on September 27, 2012 | http://pubs.acs.org Publication Date: January 11, 2007 | doi: 10.1021/bk-2007-0949.ch029

9

10

(3-Fluorine stabilized carbanion

Unstable difluoroethene F , H

sp

2

electron deficient carbon

Figure 7. Highly Electrophilic Reactivity o/CF2 Site of Difluoroethene

CF CH OH 3

2

CF CH OCF -CH 3

2

81%

2

3

25

PhMe SiLi/THF 2

PhMe Si-CF=CH 2

56% CF =CH 2

2

26

2

C0 Et

i-PrC0 Et/LDA

2

2

24 58%

(CH ) C-CF=CH 3

2

2

27 Ph PLi / THF 2

Ph P-CF=CH 2

86%

2

28

Figure 8. Electrophilic Reactions of Difluoroethene with nucleophiles



467

Figure 9. Higher Reactivity toward Nucleophiles at Difluoromethylene Site The highly electrophilic reactivity of difluoromethylene site of 1,1difluoroethene is well known. The origin of the high electrophilicity arises from the electron-deficiency at cc-carbon and the favorable transformation of sp -CF carbon to sp ~CF carbon, and the stabilization of (5-carbanion by the electron-withdrawing C F group (Fig. 7). Some examples of electrophilic reactions of difluoroethene 24 are shown in Fig. 8. Hydroxyl group, silyl lithium, enolate, and phosphorus anion, all of these nucleophiles attack exactly on cc-carbon of 24, affording an adduct product 25 or products 26, 27, and 28 formed via addition-elimination process (Fig. 8). 11

2

3

2

2

2

12

13

14

15

Figure 10. Remarkable Reactivity Difference at ^-Carbon between Fluoro andNonfluorO' a$-Unsaturated Esters Figure 9 shows an example of the electrophilicity of C F carbon in the intramolecular reaction. Thus, carbanion 30 attacks exclusively at the difluoromethylene site in [2,3]sigmatropic rearrangement. 2

16


468 Similar preference for nucleophilic attack at the CF -site has been also demonstrated by five-membered ring formation shown in Fig. 10. Nitrogennucleophile attacks preferentially CF -carbon rather than the carbonyl carbon in difluorinated oc,P-unsaturated ester 32. On the other hand, reaction can occur at the carbonyl carbon in nonfluorinated oc,P-unsaturated ester 34, leading to lactam 35. 2

2

17


18

Results and Discussion On this basis, 2-aminoperfluoropropene 15 was designed as a precursor for trifluoroalanine peptides 36 (Fig. 11). The enamine 15 would be highly electrophilic due to the strong electron-withdrawing C F and C F groups even though amino group at 2-position deactivates the difluoromethylene carbon toward nucleophilic attacks. The enamine 15 was prepared in 95% yield by M g promotedC-F bond activation o f i m i n e 3 7 in M g -TMSC1-DMF system at room temperature. 3

19

2

20

(TMS)Arj- N

21

(TMS)Arj-N

Figure 11. 2-Aminoperfluoropropene as a Synthon of Trifluoroalanine

NAr

Mg

/ TMSCI / D M F

(TMS)Ar- N A . . F

F3C

CF

37

3

g o 5

/ o

F C 3

15

F

Figure 12. Mg-Promoted Defluorination of Hexafluoroimine Enamine 15 was subjected to the reaction with the commercially available amino esters 17. The enamine 15 is stable enough to be easily handled and can


469 be stored at - 5 °C, but it is very reactive even with the less nucleophilic amino group of amino esters due to the strong electron-withdrawing effect of gemdifluoromethylene and trifluoromethyl groups. Thus, 15 reacted very smoothly with amino esters in D M F at room temperature within 90 min, affording the desired trifluoroalanine dipeptides 16 as a diastereomeric mixture. The total reaction consists of three steps; addition of amino ester 17 to 15, subsequent dehydrofluorination from the more acidic N - H bond of 38 to give imidoyl fluoride 39, and then acid-catalyzed hydrolysis of the imidoyl fluoride 39 to amide 16 (Fig. 13). In some cases the imidoyl fluoride 39 was isolated as an intermediate and characterized spectroscopically. Table 1 summarizes the results of dipeptide synthesis. Most of the amino esters 17 reacted with 15 smoothly, affording dipeptides 16 in good to excellent yields. Thus, esters of glycine, alanine, phenylalanine and leucine provided the corresponding dipeptide 16 in more than 85% yields. The secondary amino group of proline ester was as reactive to 15 as the primary amino group of glycine, alanine and leucine. O f course, the primary amino group is more reactive than the secondary in tryptophan 41, where a product 43 with an amide 22


23

NH

2

NHR

(TMS)Ar- N R ^ C 0

2

R

H

2

17

F C

X0 R

NL

F C

2

2

3

3

DMF

15 R = p-MeOC H

16a;

6

4

16b; R = H JH 0 2

NHAr

NHAr H NL

F C

X0 R 2

X0 R

2

2

2

F C 3

3

F F

R1 38

F

R

1

39

Figure 13. Reaction Mechanism and Intermediates for Peptide Synthesis of 16



470

Table I. Yields of Dipeptides 16 in the Reaction of 15 with 17

R

Amino ester 17 glycine

H

Alanine

CH

Phenyl alanine Leucine

1

R

3

PhCH

2

(CH ) CHCH 3

2

2

Proline Serine

HOCH

2

Tyrosine

HOCefttCHj

Methionine

CH SCH CH 3

Tryptophan

2

2

2

Yield of 16 (%)

de

a)

Bn

16a

91

Me

16b

86

14

Et

16c

93

31

Et

16d

85

48

Me

16e

69

36

Me

16f

80

8

Me

16g

68

1

Me

16h

88

21

Me

16i

79

19

a) diastereomeric ratios were analyzed by H P L C



471

Figure 14. Reactions of Proline and Tryptophan Esters



472

1) in D M F , R T / 30 min

2) 3N HCI aq., 0 ° C / 1h

Figure 15. Reactions of Serine and Tyrosine Esters bond connected with primary amino group in 41 was formed exclusively (Fig. 14). Interestingly, serine 44 and tyrosine 45 esters reacted exclusively at the amino site rather than the hydroxyl site even when the amino esters were employed without protection of the hydroxyl group (Fig. 15). Both esters provided the desired dipeptides 46 and 47 in 80 and 68% yields, respectively. The addition reaction in the first step is quite chemoselective as shown in 49. The preferred reactivity of the amino group to 15 over the hydroxyl group is also revealed by the fact that the competitive reaction of 15 with a mixture of equimolar amounts of both benzyl amine and benzyl alcohol provided exclusively benzyl amide 50 of trifluoroalanine in 98% isolated yield and none of the corresponding benzyl ester (Fig. 16). In relation to the higher chemoselectivity of amino group over hydroxyl group, it is interesting to determine whether the more nucleophilic thiol group can compete with amino group in the reaction of 15 with cysteine. The reaction of 15 with cysteine 51 bearing an unprotected thiol group resulted in the formation of complex miture which might arise from the nucleophilic attack of thiol group to the activated difluoromethylene site. On the other hand, 5-benzylcysteine 52 provided the desired amide 54 in a reasonable yield (Fig. 17).


473

(TMS)-NAr

NHAr H

2

N

c

,

c

r

15

\

DMF

1

R

0

F

48

49

' HOT


(TMS)-NAr

F

NHAr

c \ ?

3

•

BnNH

•

2

BnOH

F

^

F

°

h

15

3

N

c \

H

B

°

O

50

Figure 16. Chemoselective Reaction of 15 with Amine in the Presence of Alcohol

(TMS)Ar N

NH =

F

F C 3

•

H S H

2

1) D M F 2

RT/1-1.5h

C ^ C 0

2

R *

2

)

3

51

15

NH

F

C

'

|

2

^ ^ " ^ C O P R

3

H

0°C/0.5h

NHAr F C

N

2

+ complex mixture

F

53 (TMS)ArN

NH

1) D M F

2

RT/1-1.5 h F

3

C

T

+

BnSH C^C0 R 2

2

2

F

15

•

2) 3 N H C l 0°C/0.5h

74%

52 ArHN H N.

F C

X0 R 2

3

0

CH SBn 2

54 Figure 17. Reaction of Enamine 15 with Cysteine and S-Bn-Cysteine


2

474 The diastereoselectivity of the dipeptides 16 was poor and no improvement of the diastereoselectivity was found with variation of solvents, bases and reaction temperature. The stereochemistry of 16 seemed to be stable under the acidcatalyzed hydrolysis conditions except for phenyl alanine dipeptide where 7 % epimerization of asymmetric carbon of phenylalanine side was observed. The N-protecting group, p-methoxyphenyl group, was readily removed by the oxidation of 55 with ammonium cerium nitrate in M e C N - H 0 at room 24 temperature to give the deprotected dipeptides in 80% yield. The diastereoisomers of dipeptides 56 and 57 could be easily separated and isolated as an enantiomerically pure form by column chromatography. The absolute stereochemistry of the major diastereomer 56 (R, S) was clarified by X-ray crystallographic analysis of the single crystal of its hydrochloric acid salt of 56. This is the first example of preparation of the enantiomerically pure (R,S) and (5,5) trifluoroalanine-phenylalanine dipeptides (Fig. 18).


2

separable crystals Figure 18. N-Deprotection and Separation of Diastereoisomers of Trifiuoroalanine-Phenylalanine Dipeptide The protocol for trifluoroalanine dipeptide synthesis can be applied to the tripeptide synthesis. Thus, alanine dipeptide 58 was subjected to the peptide formation reaction with 15. Likewise, the reaction proceeded smoothly and provided the desired tripeptide 59 in a reasonable yield although the stereoselectivity was poor (Fig. 19). The reaction rate (2 hours at room temperature) in tripeptide synthesis seems to be fundamentally as same as that in dipeptide synthesis. 25



475

Figure 19.

Trifluoroalanine-Alanine-Alanine-Tripeptide

Conclusion

It is suggested that the present protocol can be promising for the construction of peptide skeletons and applicable to the preparation of a variety of trifluoroalanine dipeptides and oligopeptides in an enantiomerically pure form, although 1,4-asymmetric induction of the chiral center of amine esters to the occarbon atom in trifluoroalanine moiety has remained intractable at this moment. References

1. (a) Barrett, G. C. Eds.; Amino Acid Derivatives, Oxford Univ. Press., Oxford, 1999. (b) Abell, A. Eds.; Advances in Amino Acid Mimetics and

Peptidomimetics, JAI Press Inc., Greenwich, 1997. 2. (a) Jones, J.; Amino Acid and Peptide Synthesis, Oxford Univ. Press., Oxford, 1992. (b) Kusumoto, S. Eds.; Experimental Chemistry 4 ed., No 22, Maruzen, Tokyo, 1990. (c) Hodgson, D. R. W.; Sanderson, J. M. Chem. th

Soc. Rev., 2004, 33, 422.

3. Ojima, I.; Delaloge, F. Chem. Soc. Revs., 1997, 26, 377-380. 4. (a) Schellenberger, V.; Jakubke, H. D.; Fachbereich, B. Angew. Chem. 1991, 103, 1440-1452; (b) Murakami, Y.; Hirata, A. J. Bio Chem., (In Japanese, Seibutsu Kogaku Kaishi ), 1998, 76, 238.

5. Bordusa, F.; Dahl, C ; Jakubke, D.-D.; Burger, K.; Koksch, B. Tetrahedron:Asymmetry 1999, 10, 307. 6. Thust, S.; Koksch, B. Tetrahedron Lett., 2004, 45, 1163.

7. Weygand, F.; Steglich, W.; Oettmeier, W. Chem. Ber., 1970,103,1655. 8. Osipov, S. N.; Sewald, N.; Kolomiets, A. F.; Fokin, Α. V.; Burger, K. Tetrahedron Lett., 1996,37, 615.



476 9. Ishikawa, N.; Takaoka, A . ; Iwakiri, H . ; Kubota, S.; Kagaruki, S. R. F. Chem. Lett., 1980, 1107. 10. (a) Bailey, P. D . ; Boa, A. N.; Crofts, G . A.; Vandiepen, M.; Helliwell, M.; Gamman, R. E . ; Harrison, M. J. Tetrahedron Lett., 1989, 30, 7457; (b) England, D . C.; Melby, L. R.; Dietrich, M . A . ; Lindsey, Jr. R. V . J. Am. Chem. Soc., 1960, 82, 5116. 11. Uneyama, K . "Organofluorine Chemistry", 2006, Blackwell Publishing, Oxford. 12. Murata, J.; Tamura, M.; Sekiya, A .; Green Chemistry 2002, 4, 60. 13. Hanamoto, T.; Harada, S.; Shindo, K . ; Kondo, M .; Chem. Commun., 1999, 2397. 14. Crouse, G.D.; Webster, J.D.; J. Org. Chem., 1992, 57, 6643. Hanamoto, T.; Shindo, K . ; Matsuoka, M .: Kiguchi, Y .; Kondo, M ., J. Chem. Soc., Perkin Trans 1, 2000, 103. 16. Garayt, M. R.; Percy, J. M. Tetrahedron Lett., 2001, 42, 6377. 17. Ichikawa, J.; Wada, Y.; Fujisawa, M.; Sakoda, K . Synthesis, 2002, 1917. 18. Baldwin, J. E., J. Chem. Soc., Chem. Commun., 1976, 734. 19. (a) Mae, M . ; A m i i , H . ; Uneyama, K . Tetrahedron Lett., 2000, 41, 7893. (b) Amii, H . ; Kobayashi, T.; Hatamoto, Y . ; Uneyama, K . Chem. Commun., 1999, 1323. 20. Middleton, W , J.; Krespan, C. G . J. Org. Chem., 1965, 30, 1398. 21. Guo, Y.; Fujiwara, K . ; Uneyama, K . Org. Lett., 2006, 8, 827-829. 22. The absolute stereochemistry (R) or (S) of trifluoroalanine site of both major and minor diastereomers has not been determined except 16c. 23. Imidoyl fluoride 39 was isolated as an intermediate which was easily hydrolyzed to the corresponding amide under the hydrolysis conditions. 24. Sakai, T.; Yan, F.; Uneyama, K . Synlett., 1995, 753. 25. Fujiwara, K . ; Komatsu, Y.; Guo, Y.; Uneyama, K . The 86th Annual Meeting of the Chem. Soc. Japan, 2006 March, Funahasi.


A Novel Strategy for the Synthesis of Trifluoroalanine ligopeptides

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