Pyranyl heterocycles from inverse electron demand hetero [4+2] cycloaddition reactions of chiral allenamides as a new chiral template for constructing C-glycoside substrates

摘要

A useful sequence involving stereoselective functionalization of the two olefins in pyranyl heterocycles derived from inverse electron demand hetero [4+2] cycloadditions of chiral allenamides is described here. This sequence constitutes stereo-selectively dihydroxylation or hydroboration-oxidation of the sterically accessible C5 exocyclic olefin followed by hydroboration-oxidation of the endocyclic olefin at C2/C3. The ultimate success in the removal of the C6 chiral auxiliary completes the demonstration of the concept of employing these unique hetero cycloadducts as chiral templates for constructing highly functionalized pyrans or C-glycosides. [References: 44]