Tandem radical mediated 5-exo-trig, 6-endo-dig cyclization. Stereoselectivity, alpha-trimethylsilyl effect and the product spread .4.

Stereochemistry and mechanism of the title reaction have been investigated. The reaction proceeds well in the (S) series. In the epimeric (R) series a product spread is usually observed. In either series olefin configuration is lost (7 examples), consistent with a freely rotating exocyclic radical intermediate. An alpha-trimethylsilyl group at the olefinic terminus enhances the selectivity of the radical cascade Spectroscopic evidence and MMX calculations suggest that the tandem annulation products of the (S) series are favoured thermodynamically, being more stable than those of the (R) series. Pyranoside (R)G2a is frozen in a half chair conformation. (C) 1997 Elsevier Science Ltd.