Abstract
AbstractThe utility of 2‐ketohexosyl (“ulosyl”) bromides has been extended to the straightforward generation of cis‐fused pyrano[2,3‐b]dioxanes (1,4,5‐trioxadecalins) of type 11 and 13 by β‐specific and α‐selective glycosidation with glycol, and subsequent intramolecular hemiketalization. Mild basic conditions elicit highly stereoselective rearrangements to trioxadecalinones 22−25, representing the structurally and configurationally correct framework of steroidal natural products in which the sugar portion is doubly glycosidically linked to the aglycon. Exposure of 11 or 13 to mineral acid results in an essentially stereospecific contraction of the pyranoid ring to yield trioxaspiro[4,5]decanes. Mechanistic considerations of the unusual rearrangements observed are discussed in light of available experimental facts and the scarce existing analogies. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)
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