Cyclosporin A: Regioselective Ring Opening and Fragmentation Reactions via Thioamides. A Route to Semisynthetic Cyclosporins
作者:Marcel K. Eberle、Anne-Marie Jutzi-Eme、Francois Nuninger
DOI:10.1021/jo00103a015
日期:1994.12
Cyclosporin A (1a) served as the starting material for the semisynthetic preparation of a variety of novel cyclosporins. Acetylcyclosporin A (2) was treated with Lawesson's reagent. From the reaction mixture, three novel acetylated thioamides were isolated: the 4,7-bis(thioamide) 3a, the 7-thioamide 3b, and the 4-thioamide 3c. The acetylated products 3a-c were hydrolyzed to the known thioamides 4a-c, respectively. The 7-thioamide 3b was alkylated to give the S-benzyl thioamidate 5b. A regioselective ring opening reaction at the activated site was induced by treating 5b under acidic conditions giving the 7,8-seco-cyclosporin 7a. The (R)-alanine moiety was replaced by (R)-phenylalanine via the Edman degradation product 7e giving 7f. Removal of the protecting groups led to 7h. This was cyclized to [(R)-phenylalanine](8)-cyclosporin (1b). The N-protected 7,8-seco-cyclosporin 7i was reduced to the aldehyde 7j, homologated (7k), and deprotected to give 7m. This was cyclized to the vinylogous cyclosporin 8a. Similarly,:the 4,5-seco-cyclosporin 9c was prepared and converted via several steps (9d-h) to the vinylogous cyclosporin 8b. Finally, under acidic conditions, the dibenzyl bis(thioamidate) 5a underwent a fragmentation reaction to give the octapeptide 10a and the tripeptide 11a. The octapeptide 10a was coupled with a different tripeptide (11d) to 9i and cyclized via 9j-k to the [(S)-phenylalanine](7)-cyclosporin (1c).