Regiochemistry in radical cyclizations (5-endo versus 4-exo) of N-(2-phenylthio- and 2-phenylcyclohex-1-enyl)-α-halo amides

Bu3SnH-mediated radical cyclization of N-(2-phenylthiocyclohex-1-enyl)-α-halo amides was examined. Bromoacetamide 9a having no substituent α to the halogen atom cyclized exclusively in a 4-exo-trig manner, whereas the fully substituted haloamides 9c and 9e gave 5-endo-trig cyclization products. The mono-substituted haloamides 9b and 9d showed an intermediate behavior to give a mixture of 4-exo and 5-endo cyclization products. The results of experiments on the effect of reaction temperature indicated that at a low temperature, i.e. under kinetically controlled conditions, 4-exo-trig cyclization predominated. On the other hand, the 2-phenylcyclohex-1-enyl congeners 22b and 22c gave exclusively 5-endo cyclization products.