Diastereoselective radical cyclization reactions;: the synthesis of O-methylcorytenchirine

Highly diastereoselective cyclization of radicals such as 4 provides a model for the synthesis of 8-substituted berbines. Thus the reaction of 6,7-dimethoxyisoquinoline 21 with the acid chloride 19 affords the key intermediate 22, which undergoes free radical cyclization on treatment with tributylstannane to give (+/-)-23 as the sole product. Reduction of 23 affords (+/-)-O-methylcorytenchirine 14. The carbamate 24 does not undergo radical cyclization when treated with tributylstannane, but the acetyl pyridine 33 affords the cyclized products 37 and 38 in reasonable yield and with good diastereoselectivity.