A radical version of the bromine cyclization of alkenols

The scope of an alkoxyl radical version of the classical bromine cyclizatio n was explored. Oxygen-centered radicals were generated in photochemically initiated radical chain reactions from AT alkenoxy-4-(p-chlorophenyl)thiazo le-2(3H)-thiones 5a-c, N-alkenoxy-4-methylthiazole-2(3H)-thiones 13, 14, 21 , rac-23, and N-alkenoxypyridine-2(1H)-thiones 6d-f, 16, rac-25. Thus, 2-(2 -bromopropyl)-substituted tetrahydrofurans 4a-c, which are minor compounds (< 10 %) in NBS-mediated bromine cyclizations of corresponding alkenols 1a- c, were prepared in 87-90 % yield and with good to excellent diastereoselec tivities. Further, photochemical conversions of O-alkyl thiohydroxamates 14 , 16 and 21 in benzene and BrCCl3 afforded beta -oxy-functionalized bromome thyl substituted tetra hydrofurans 29, 34, and 36. The results of this stud y indicate, that efficient 5-exo-trig cyclizations of beta -oxy-substituted 4-penten-1-oxyl radical require an electronwithdrawing substituent at the beta -heteroatom substituent. In the third part of the study a syntheticall y useful new access to oxabicyclo[4.3.0]nonanes rac-38 and rac-40 is report ed which takes profit from highly diastereoselective 5-exo-trig-ring closur es and, in case of the formation of rac-40, stereoselective bromine atom tr ansfer. (C) 2001 Academie des sciences / Editions scientifiques et medicale s Elsevier SAS.