Deoxy-nitrosugars. 6th communication. Stereoelectronic control in the reductive denitration of tertiary nitro ethers. A synthesis of ‘C-glycosides’

The separate, radical denitration with Bu₃SnH of the pyranose derivatives 3, 4, 9 , and 10 gave in good yields exclusively the ‘C-glycosides’ 5 and 11 , respectively (Scheme 1). Similar reduction of the cyclohexyl derivatives 15, 16, 19 and 20 gave 4:1 mixtures of 17, 18, 21 and 22 , respectively, always with predominant formation of an axial C,H-bond. In the furanose series a divergent behaviour was observed for the D -mannose-derived nitro ethers 25 and 27 and the D -ribose-derived nitro ethers 30 and 31 , respectively, in that the former two gave isomerically homogeneous reduction products ( 26 and 28 , respectively; Scheme 3) and the latter a 1:1 mixture of the diastereoisomers 32 and 33 (Scheme 4). The stereochemical results were explained on the basis of the stereoelectronic effect of the ring O-atom, the preferred conformation of the intermediate, pyramidal alkoxyalkyl radicals and steric effects in the trioxabicyclo [3.3.0]octane ring system.