(2R,3R)-(Z)-cyclo-Phenylalanine

(.HCl)

[110716-95-5]  · C10H12ClNO2  · (2R,3R)-(Z)-cyclo-Phenylalanine  · (MW 213.68)

(reagent for syntheses of conformationally constrained peptidomimetics)

Physical Data: (HCl salt) mp 201 °C (dec); [a]25D +105° (c 0.69, H2O).

Solubility: the compounds of this series closely resemble the parent amino acids; they are sol water, moderately sol lower alcohols, and insol apolar solvents.

Form Supplied in: colorless solid; not currently commercially available on a routine basis.

Analysis of Reagent Purity: chemical purities are accessed by mp and NMR; optical purities are deduced by forming diastereomeric derivatives, or by derivatization and analysis in the presence of chiral NMR shift reagents.

Preparative Methods: substituted 2,3-methanoamino acids are difficult to prepare. Unfortunately, most of the reported syntheses give racemic materials whereas stereochemically pure compounds are required for studies of cyclopropane-based peptidomimetics. The only 2,3-methanologs of protein amino acids prepared in optically active form are (E)- and (Z)-cyclo-Phe1-4 and -Tyr,5 all four stereoisomers of cyclo-Met,6 (Z)-cyclo-Arg7 and (2S,3S)-(Z)-cyclo-Trp,8 although several routes to enantio-enriched 2,3-methanologs of simple nonproteogenic amino acids have been reported.9-12 The most practical synthesis of the title compound is that based on a diastereoselective, rhodium-catalyzed cyclopropanation reaction.3

Handling, Storage, and Precautions: some compounds in this series are slightly hygroscopic, but they are otherwise quite stable, and indefinitely so under an inert atmosphere in a freezer. For good results in peptide syntheses these compounds must be used with the same precautions taken for any common amino acid derivative.

Background.13

One of the least drastic perturbations of amino acid structure is to link the a- and b-carbons of the side chain with a methylene group, giving 2,3-methanoamino acid analogs (or methanologs, e.g. the cyclopropyl analogs of phenylalanine, cyclo-Phe, and of methionine, cyclo-Met). Incorporation of a cyclopropane ring locks the side chain substituent cis or trans to the amino functionality. Designation of the absolute configurations of the two chiral centers completely defines the stereochemistry, whereas (Z) and (E) nomenclature specifies the diastereomeric, but not the enantiomeric, form. Both systems are shown on the examples in Figure 1, even though it is redundant to specify (Z) or (E) stereochemistry if the absolute configuration is marked.

These amino acid surrogates have side chains locked cis or trans to the amino functionality and the cyclopropane ring also restricts rotations about the N-Ca and Ca-CO bonds (i.e. j and y, respectively).14 -16 A Ramachandran plot17 for a related compound indicates these conformational restrictions are severe.14 -16,18,19 Consequently, systematic variations of stereoisomeric 2,3-methanolog substitutions facilitate controlled restrictions on the conformational freedom of peptidomimetics,20,21 and it is possible to constrain them into molecular orientations that resemble bioactive conformations of the parent peptide. Substitution of a protein amino acid with its 2,3-methanolog therefore can enforce or preclude structural shapes required for various bioactivities. They can also impart considerably enhanced resistance to proteolytic degradation.22-27 These are the main applications of this class of compounds.

At the present time, 2,3-methanoamino acid analogs can only be viewed as reagents in the context of syntheses of peptidomimetics. Consequently, this entry describes only that chemistry related to incorporation of protein amino acid methanologs into peptide sequences.

Incorporation into Peptidomimetics.

Solution phase methods have been used to incorporate cyclo-Phe stereoisomers into enkephalin analogs. The mixed anhydride method (i-BuOCOCl, Isobutyl Chloroformate) was used as illustrated in Figure 2, which depicts the deprotection/coupling sequence used in the preparation of Tyr-D-Ala-Gly-(cyclo-Phe)-Leu-OH. Thus the first step was N-Cbz protected cyclo-Phe being coupled with Leu-OMe via the mixed anhydride method; the dipeptide so formed was coupled with Cbz-Tyr-D-Ala-Gly-OH after Trifluoroacetic Acid deprotection.22 Methyl ester protecting groups were used at the C-terminus, and benzyloxycarbonyl (Cbz) protection was employed at the N-termini. The Cbz groups were removed by acid deprotection and not hydrogenolysis, since the latter conditions cause extremely facile ring-opening of methanologs with aromatic substituents connected to the strained ring.2

Peptide syntheses have undergone extensive improvements in the decade since the preparations described above were performed.28,29 A more contemporary approach to syntheses of peptidomimetics using the solid phase Fmoc (9-fluorenylmethoxycarbonyl) approach is illustrated in Figure 3.23,30 The couplings were performed using Castro's reagent (BOP, Benzotriazol-1-yloxytris(dimethylamino)phosphonium Hexafluorophosphate) in the presence of 1-Hydroxybenzotriazole (HOBt).31

Other peptides in the F{cyclo-M}RF-NH2 series have been prepared using a more acid stable resin (MBHA, methoxybenzhydrylamine) and Boc (t-butoxycarbonyl) protecting groups, but the Fmoc approach above gives superior yields.23,30


1. King, S. W.; Riordan, J. M.; Holt, E. M.; Stammer, C. H. JOC 1982, 47, 3270.
2. Kimura, H.; Stammer, C. H. JOC 1983, 48, 2440.
3. Davies, H. M. L.; Cantrell, W. R., Jr. TL 1991, 32, 6509.
4. Fernández, M. D.; Frutos, M. P. D.; Marco, J. L.; Fernández-Alverez, E.; Bernabé, M. TL 1989, 30, 3101.
5. Ahmad, S.; Phillips, R. S.; Stammer, C. H. JMC 1992, 35, 1410.
6. Burgess, K.; Ho, K.-K. JOC 1992, 57, 5931.
7. Burgess, K.; Ho, K.-K. TL 1992, 33, 5677.
8. Bruncko, M.; Crich, D. TL 1992, 33, 6251.
9. Baldwin, J. E.; Adlington, R. M.; Rawlings, B. J.; Jones, R. H. TL 1985, 26, 485.
10. Pirrung, M. C.; Dunlap, S. E.; Trinks, U. P. HCA 1989, 72, 1301.
11. Alami, A.; Calmes, M.; Daunis, J.; Escale, F.; Jacquier, R.; Roumestant, M-L.; Viallefont, P. TA 1991, 2, 175.
12. Williams, R. M.; Fegley, G. J. JACS 1991, 113, 8796.
13. Stammer, C. H. T 1990, 46, 2231.
14. Varughese, K. I.; Srinivasan, A. R.; Stammer, C. H. Int. J. Pept. Protein Res. 1985, 26, 242.
15. Varughese, K. I.; Wang, C. H.; Kimura, H.; Stammer, C. H. Int. J. Pept. Protein Res. 1988, 31, 299.
16. Taylor, E. W.; Wilson, S.; Stammer, C. H. ACS Symp. Ser. 1991, 450, 162.
17. Ramachandran, G. N.; Sasisekharan, V. Adv. Protein. Chem. 1968, 23, 283.
18. Barone, V.; Fraternali, F.; Cristinziano, P. L.; Lelj, F.; Rosa, A. Biopolymers 1988, 27, 1673.
19. Nitz, T. J.; Shimohigashi, Y.; Costa, T.; Chen, H. C.; Stammer, C. H. Int. J. Pept. Protein Res. 1986, 27, 522.
20. Mapelli, C.; Elrod, L. F.; Switzer, F. L.; Stammer, C. H.; Holt, E. M. Biopolymers 1989, 28, 123.
21. Mapelli, C.; Van Halbeck, H.; Stammer, C. H. Biopolymers 1990, 29, 407.
22. Kimura, H.; Stammer, C. H.; Shimohigashi, Y.; Cui, R. L.; Stewart, J. Biochem. Biophys. Res. Commun. 1983, 115, 112.
23. Malin, D. H.; Lake, J. R.; Ho, K.-K.; Corriere, L. S.; Garber, T. M.; Waller, M.; Benson, T.; Smith, D. A.; Luu, T.-A.; Burgess, K. Peptides 1993, in the press.
24. Ogawa, T.; Shimohigashi, Y.; Yoshitomi, H.; Sakamoto, H.; Kodama, H.; Waki, M.; Stammer, C. H. Pept. Chem. 1988, 26, 25.
25. Ogawa, T.; Shimohigashi, Y.; Shiota, M.; Waki, M.; Stammer, C. H.; Ohno, M. Pept. Chem. 1989, 27, 379.
26. Ogawa, T.; Yoshitomi, H.; Kodama, H.; Waki, M.; Stammer, C. H.; Shimohigashi, Y. FEBS Lett. 1989, 250, 227.
27. Breckenridge, R. J.; Suckling, C. J. T 1986, 42, 5665.
28. Atherton, E.; Sheppard, R. C. In Solid Phase Peptide Synthesis: A Practical Approach; IRL: Oxford, 1989.
29. Fields, G. B.; Noble, R. L. Int. J. Pept. Protein Res. 1990, 35, 161.
30. Malin, D. H.; Payza, K.; Lake, J. R.; Corriere, L. S.; Benson, T. M.; Smith, D. A.; Kelley, R. S.; Ho, K. K.; Burgess, K. Peptides 1993, 14, 47.
31. Knorr, R.; Trzeciak, A.; Bannwarth, W.; Gillessen, D. TL 1989, 30, 1927.

Kevin Burgess & Kwok-Kan Ho

Texas A & M University, College Station, TX, USA



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