[3+2] cycloadditions between α,β-unsaturated esters or nitroalkenes and camphor-derived oxazoline N-oxides:: Experimental and theoretical study

Oxazoline N-oxide 2 in the presence of alpha,beta-unsaturated esters or nitroalkenes afforded the corresponding adducts with excellent regio- and stereoselectivities depending of the substitution pattern on the dipolarophile. Especially, inversion of regioselectivity was observed by switching from a 1,2-disubstituted to a 1,1-disubstituted alpha,beta-unsaturated ester. This was not observed when nitroalkenes are used as dipolarophiles. An attempt to rationalize the observed selectivities by semiempirical calculations at RHF AM1 level is described. The cycloadduct 5e has been used as the starting material in a total synthesis of the pheromone (-)-frontalin 12.