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-oxa analog 1d and the all carbon system 1a give mainly the cis-fused syn-substituted bicyclononylcarbinyl radicals 2b, 2d and 2a, but the mono-oxa radical 1c gives mainly the cis-fused anti-substituted radical 3c. Molecular mechanics calculations show that the unexpected failure of the dioxa radical 1b to reflect the influence of the anomeric effect on the stereochemistry of cyclisation arises from the loss of one stabilising anomeric interaction in the conversion of the conformer 17 into either of the transition structures 16 or 19. (C) 1999 Elsevier Science Ltd. All rights reserved.