Synthetic strategies directed towards 5a-carbahexopyranoses and derivatives based on 6-endo-trig radical cyclizations

Abstract

Several synthetic strategies directed towards 5a-carbahexopyranoses and based on 6-endo-trig radical cyclization of unsaturated carbohydrate derivatives have been devised. Three elements for regiocontrol to optimize the 6-endo/5-exo ratio have been incorporated, and their efficiencies in directing 6-endo cyclizations have been evaluated. These elements - namely: i) the incorporation of a substituent at C-5 (radical numbering), ii) the use of a vinyl (rather than alkyl) radical, and iii) the inclusion of ring strain in the system - have proved useful when used in combination. The simultaneous presence of two of them also results in 6-endo selectivity. On another topic, the ozonation of the ensuing alkenylstananes to afford diols seems to be based on a tin-oxygen rearrangement, similar to that reported for related vinylsilanes, rather than on remarkable stabilities of tin-containing primary ozonides as we had previously suggested. © 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.