The Nicholas-Type Activation of Cyclopropanes toward Reactions with ­Nitrones in the Homo-[3+2]-Dipolar Cycloaddition

2-Ethynyl-1,1-carboethoxyclopropane, when treated with Co2(CO)8, results in the formation of the Co2(CO)6 adduct which in turn undergoes smooth cycloaddition in the presence of Sc(OTf)3 to form the corresponding tetrahdyro-1,2-oxazines. The adducts can either be decomplexed to yield the parent acetylenes or made to undergo Pauson-Khand reactions to form 6cyclo­pentenyltetrahydro-1,2-oxazines.