Ring closure reactions of disubstituted 4-penten-1-oxyl radicals - Towardsa stereoselective synthesis of allo-muscarine

The trisubstituted functionalized tetrahydrofurans 10, 11, 16, 18, and 19 w ere photochemically prepared from 2,3-syn- and 2,3-anti-configured N-(3-ben zoyloxy-5-hexen-2-oxy)thiazole-2(3H)-thione anti-6, pyridinethiones 7, anti -8, and BrCCl3. The formation of tetrahydrofurans was achieved by an effici ent and highly regioselective alkoxyl radical cyclization (5-exo-trig), The 2,3-anti substituted intermediates 9 and 12 cyclize stereoselectively wher eas a 2,3-syn-coniigured O-radical affords both possible diastereomeric add ition products in equal amounts. The cyclized tetrahydrofuryl methyl radica ls were trapped with the bromine atom donor BrCCl3, to afford the bromometh yl-substituted cyclic ethers 10, 11, 18, and 19 in excellent yields. The ut ility of this reaction was stressed by conversion of one of the newly prepa red tetrahydrofurans in a two-step synthesis into (+)-allo-muscarine (+)-20 .