2-azabicyclo[2.1.1]hexanes. 2. Substituent effects on the bromine-mediatedrearrangement of 2-azabicyclo[2.2.0]hex-5-enes

Methyl- and phenyl-substituted N-(ethoxycarbonyl)-2-azabicyclo [2.2.0]hex-5 -enes 6 have been prepared by photoirradiation of appropriately substituted 1,2-dihydropyridines. Torquoselectivity is observed in the synthesis of th e 3-endo-methyl- and 3-endo-phenyl-2-azabicyclo [2.2.0]hexenes 6c-e from 2- methyl- and 2-phenyl-1,2-dihydropyridines 5c-e. Products formed upon additi on of bromine to S-endo-, 4-, and 5-methyl- and 3-endo-phenyl-substituted N -(ethoxycarbonyl)-2-azabicyclo [2.2.0]hex-5-enes 6a-f were substituent depe ndent. For 6a,b, which lack substituents at Ca or Cg, mixtures of unrearran ged dibromides 8a,b and rearranged dibromides 9a,b were obtained. With the 3-endo-substituents in 6c-e, only rearranged dibromides 9c-e were formed; 5 -methyl substitution afforded mainly unrearranged dibromide 8f and some all ylic bromide 10. Both unrearranged 5-endo,6-exo-dibromb-2-azabicyclo [2.2.0 ]hexanes 8 and rearranged 5-anti-6-anti-dibromo-2-azabicyclo [2.1.1]hexanes 9 are formed stereoselectively. The dibromoazabicyclo [2.1.1]hexanes 9 hav e been reductively debrominated to afford the first reported Bazabicyclo [2 .1.1]hexanes 11 with alkyl or aryl substituents at C-3.