| Abstract: |
An account of the reasoning and reduction to practice of a highly convergent total stereospecific synthesis of calicheamicin γ1I (1) is provided. The key finding was the use of a very mild promoter system (silver triflate and 4 Å molecular sieves in methylene chloride) to allow for coupling of trichloroacetimidate 21 with advanced calicheamicinone-like acceptors (see 17 and 18). Glycosidation with 17 affords a 3:1 ratio of equatorial to axial glycosides. The use of 18 seems to afford only the equatorial β-glycoside. Remarkably, use of the enantiomer of 17 as the acceptor gave an 18:1 ratio of axial to equatorial product. © 1995, American Chemical Society. All rights reserved. |
