Syn-Anti Isomerism in the 1,3-Dipolar Cycloaddition to Cis 3,4-Disubstituted Cyclobutenes. 5. Diastereoselectivity in the Reaction with Diazoalkanes

Article abstract of DOI:10.1021/jo00297a060

The 1,3-dipolar cycloaddition of diazomethane, diazoethane, phenyldiazomethane, and 2-diazopropane with several cyclobutenes, with open and cyclic cis 3,4-disubstitution, has been investigated.The cycloaddition proceeds in good yield to give syn and/or anti adducts.The structure was established by spectroscopic data.The reactions of diazomethane cover the complete range of facial selectivity from 100percent syn diastereoselectivity (e.g., with bis(mesyloxy)cyclobutene) to 100percent anti diastereoselectivity (e.g., with bicyclo<3.2.0>hept-6-ene) through mixtures of various syn:anti ratios .In particular electron-attracting substituents containing heteroatoms favor syn attack whereas in the case of cyclic disubstitution, even with heteroatoms, there is a shift toward anti adducts.The face selectivity data of the reactions of diazoethane and phenyldiazomethane are very similar to those of diazomethane.In the case of the bulky 2-diazopropane there is a significant shift toward anti addition, but the observed diastereoselectivity roughly parallels that of diazomethane.Noneplanarity of the double bond (as disclosed by ab initio calculations) and the related energy asymmetry of the out-of-plane bending of the olefinic hydrogens are advanced as the inherent facial bias of cyclobutenes which tends to govern facial selectivity.