TOTAL SYNTHESIS OF CALICHEAMICIN-GAMMA-1(I) .3. THE FINAL STAGES

The first total synthesis of calicheamicin gamma1I (1) has been achieved. The stereoselective glycosidation, joining the appropriately functionalized aglycon 3 with the oligosaccharide fragment 2, was realized using Schmidt's trichloroacetimidate methodology. Segment 4, equipped with the photolabile 2-nitrobenzyl group at the reducing end, was synthesized using similar chemistry to that applied to the synthesis of its methyl glycoside counterpart (see accompanying paper). Stereoselective reduction of oxime 31, obtained from the coupling product, with NaCNBH3 in the presence of BF3.OEt2, late in the synthetic scheme, generated the desired alkoxylamine 32 and its A-4 isomer 32-epi. Installment of the allylic trisulfide and appropriate deprotections allowed transformation of 32 and 32-epi to calicheamicin gamma1I (1) and its A-4 epimer 1-epi, respectively.