Total Synthesis Of Cryptophycins-1,-3,-4,-24 (Arenastatin A), And-29, Cytotoxic Depsipeptides From Cyanobacteria Of The Nostocaceae

A convergent synthesis of cryptophycins has been developed in which (5S,GR)-5-hydroxy-6-methyl-8-phenylocta-2(E),7(E)-dienoic add (A) is coupled with an amino acid. segment (B). Two stereoselective routes to A are described, the first employing allylation of an alpha-homochiral aldehyde and the second using asymmetric crotylation of an achiral aldehyde to establish the two stereogenic centers present in a. The styryl moiety of A was attached either via Stille coupling or through a Wadsworth-Emmons condensation with diethyl benzylphosphonate. The amino acid subunit B was prepared from benzyl (2S)-2-hydroxyisocaproate by connection first to N-Boc-beta-alanine or its (2R)-methyl-substituted derivative and then to (2R)-N-Boc-O-methyltyrosine or its nt-chloro derivative. Fusion of the A and B subunits was accomplished by initial esterification of the former with the latter, followed by macrocyclization using diphenyl phosphorazidate. In this way, cryptophycin-3, -4, and -29 were obtained along with the nonnatural cyclic depsipeptide 52. Epoxidation of cryptophycin-3 with dimethyldioxirane gave cryptophycin-1; analogous epoxidation of 52 afforded arenastatin A (cryptophycin-24).