1,3-dipolar cycloaddition of diazomethane to chiral azlactones. Experimental and theoretical studies

The reaction of (E)-2-phenyl-4-[(S)-2,2-dimethyl-1,3-dioxolan-4-ylmethylen]-5(4H)-oxazolon with diazomethane has been studied under a variety of reaction conditions, and the results compared with those obtained with the corresponding (Z)-isomer. The origin of the high diastereofacial selectivity in the cyclopropane products, observed with both oxazolones, is discussed in the light of theoretical calculations of the first reaction step, i.e., the 1,3-dipolar cycloaddition of diazomethane, carried out at the semiempirical (AM1) and ab initio (RHF/3-21G) levels. Both theoretical levels correctly predict the direction of the asymmetric induction in the concerted reaction pathway leading to the Delta(1)-pyrazoline. AM1 calculations predict also the existence of a stepwise reaction pathway leading to an open-chain reaction intermediate, which is nor supported by the ab initio results. (C) 1997 Elsevier Science Ltd.