Stereoselective cyclisation of the 2-allyloxytetrahydropyran-3-yl radical and related species: The influence of anomeric effects

Ring closures of the 2-allyloxytetrahydropyran-3-yl radical 1b, its mono-ox a analog 1d and the all carbon system 1a give mainly the cis-fused syn-subs tituted bicyclononylcarbinyl radicals 2b, 2d and 2a, but the mono-oxa radic al 1c gives mainly the cis-fused anti-substituted radical 3c. Molecular mec hanics calculations show that the unexpected failure of the dioxa radical 1 b to reflect the influence of the anomeric effect on the stereochemistry of cyclisation arises from the loss of one stabilising anomeric interaction i n the conversion of the conformer 17 into either of the transition structur es 16 or 19. (C) 1999 Elsevier Science Ltd. All rights reserved.