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-β-L-gulo- (8), 5a-carba-D-glycero-β-D-ido- (22) and 5a-carba-L-glycero-α-L-galacto-heptopyranose (25), have been prepared from 8-bromo-8-deoxy-2,3-unsaturated octono-1,4-lactones with L-galacto-, D-gluco- and D-manno-configuration, respectively. The key step was a regio- and stereoselective 6-exo-trig radical-induced carbocyclisation of the unsaturated octonolactones to give bicyclic cyclohexane-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.