FREE-RADICAL CYCLOISOMERIZATION OF ENANTIOMERICALLY PURE ALKYNE-TETHERED OXIME ETHERS - A NEW METHOD FOR THE ASYMMETRIC-SYNTHESIS OF AMINOCYCLOPENTITOLS

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 H-1 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.