Synthetic Strategies Directed Towards 5a-Carbahexopyranoses and Derivatives Based on 6-endo-trig Radical Cyclizations

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.