Highly functionalised cyclopentanes by radical cyclisation of unsaturated bromolactones III. Preparation of carbaaldohexofuranoses. Determination of the relative configuration at C-4/C-5 of 2,3-unsaturated heptono-1,4-lactones by means of H-1 NMR spectroscopy

Two new carbaaldohexofuranoses, carba-beta-D-glucofuranose and carba-alpha- L-mannofuranose, have been prepared using 5,6-O-isopropylidene-D-glycero-L- galacto-heptono-1,4-lactone (6) as the starting material. The key step was a highly stereoselective intramolecular 5-exo-trig radical cyclisation of C -2 substituted 2,3-unsaturated 7-bromo-7-deoxyheptono-1,4-lactones promoted by tributyltin hydride. Assignment of the configuration of the unsaturated lactones was based upon NMR data of related compounds. The starting materi al, compound 6, was obtained by chain elongation of D-gulose, and a facile method for separation of the epimers from the chain elongation has been dev eloped. Thus 5,6-O-isopropylidene-D-glycero-L-galacto-heptono-1,4-lactone ( 6) and D-glycero-L-talo-heptono-1,4-lactone (5) were isolated crystalline i n ca. 30% and 10% yield, respectively.