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The first total synthesis of the calicheamicin'oligosaccharide fragment in the formof its methyl glycoside (62) has been achieved. The synthetic challenge of the B-ring was recognized and studied initially, resulting in a novel and unique solution to the stereochemical problems posed involving a [3, 3]-sigmatropic rearrangement of an allylic thionoimidazolide (111). This chemistry was initially worked out on a model for the ABC-ring system (47) and then successfully applied to the real system. The success of thissynthesis has enabled the completion of the first synthesis of the natural product itself, calicheamicin 711 (1), as will be describedin the following papers in this issue.