Asymmetric total synthesis of (-)-callystatin A and (-)-20-epi-callystatin A employing chemical and biological methods

Article abstract of DOI:10.1002/1521-3765(20020916)8:18<4272::AID-CHEM4272>3.0.CO;2-K

An efficient asymmetric total synthesis of the potent cytotoxic marine natural product (-)-callystatin A and its 20-epi analogue has been achieved. The synthetic pathway involved the preparation of three fragments to be coupled with each other at the end of the route. The first fragment 3 was obtained using a biocatalytic enantioselective reduction of a 3,5-dioxocarboxylate as the key step. For the second intermediate 4 the asymmetric α-alkylation of an O-protected derivative of 4-hydroxybutanal was performed exploiting the SAMP/ RAMP hydrazone alkylation methodology, and followed by a highly Z-selective Homer-Wadsworth-Emmons reaction under modified conditions. For the synthesis of the polypropionate fragment 5 a diastereoselective syn-aldol reaction was employed between a chiral ethyl ketone and an α-substituted chiral aldehyde, both prepared in enantiopure form again by means of the asymmetric alkylation of their corresponding RAMP hydrazones. Finally, these three building blocks were coupled using highly E-selective Wittig reactions via allyltributylphosphonium ylides to afford the target compounds after a final oxidation/deprotection sequence.