Synthesis of carba sugars from aldonolactones. Part IV. Stereospecific synthesis of carbaheptopyranoses by radical-induced carbocyclisation of 2,3-unsaturated octonolactones

Three new carbaheptopyranoses, 6-deoxy-5a-carba-beta -L-gulo- (8), 5a-carba -D-glycero-beta -D-ido- (22) and 5a-carba-L-glycero-alpha -L-galacto-heptop yranose (25), have been prepared from 8-bromo-8-deoxy-2,3-unsaturated octon o-1,4-lactones with L-galacto-, D-gluco- and D-manno-configuration, respect ively. The key step was a regio- and stereoselective 6-exo-trig radical-ind uced carbocyclisation of the unsaturated octonolactones to give bicyclic cy clohexane-lactone derivatives. Reduction of the lactone moiety using Ca(BH4 )(2) gave the said carbaheptopyranoses. The 8-bromo-8-deoxy-2,3-unsaturated octonolactones were prepared from the inexpensive, commercially available D-glycero-D-gulo-heptonolactone and L-tartaric acid.