Free radical cycloisomerization of enantiomerically pure alkyne-tethered oxime ethers: A new method for the asymmetric synthesis of aminocyclopentitols

Abstract

We describe for the first time the free radical cyclization of enantiomerically pure alkyne-tethered oxime ethers obtained from carbohydrates. The synthesis of compounds 6 and 7, obtained from 2,3-O-isopropylidene-D-ribose 3 is reported. These radical precursors have been submitted to cyclization with tributyl or triphenyltin hydride plus triethylborane, to yield, after ring closure, the aminocyclopentitols 8–10. These carbocycles have been obtained as mixtures of Z and E vinyltin isomers, but with excellent diastereoselection. After protodestannylation only one diastereoisomer was detected. The absolute configuration at the new stereocenter formed during the ring closure has been established by detailed 1H NMR analysis. The specific transformation of compound 9 (or 10) into aminocyclitol 14 is described. From these results, we can conclude that a new method for the asymmetric synthesis of aminocyclopentitols of biological interest is now available.