First Synthesis of a Digitalis Saponin. Demonstration of the Scope and Limitations of a Convergent Scheme for Branched Oligosaccharide Synthesis by the Logic of Glycal Assembly
摘要:
The synthesis of complex glycosides, with branching at C-2, is demonstrated. The key element involves the use of a 1,2-oxirane donor. Upon glycosylation, a C-2 hydroxyl is exposed to serve as the acceptor in the next glycosylation. Branching at C-2 with a beta-linked glycoside at C-1 was not achievable with epoxy 23 donor, but was accomplished with fluoro donor 25, in turn derived from 23. (See 19 + 18 --> 20; 20 + 25 --> 26. Compound 26 was deprotected to complete the first total synthesis of a natural saponin, desgalactotigonin (3)). A limitation in stereospecifity in the use of donor 23 and monoprotected galactal acceptor 28 was also encountered.