Generation of and diastereoselective cycloaddition reactions to ortho-quinodimethane tricarbonylchromium intermediates

Reaction of n-BuLi with either syn- or anti-(1-acetoxycyclobutabenzene)Cr(CO)3 or with syn-(1-hydroxycyclobutabenzene) generates a planar chiral o-quinodimethane complex intermediate at temperatures below 0 °C. This reacts with reactive dienophiles (methyl acrylate, acrylonitrile, phenyl vinyl sulfone, 1-phenyl-2-(phenylsulfonyl)ethene, methyl crotonate, dimethyl fumarate and dimethyl maleate) to give cycloaddition products. X-ray structures of three complexes show that (anti-1-tetrahydronaphthol)Cr(CO)3 complexes are formed selectively. With ester and nitrile substituted olefins, the major diastereoisomer is the cis product (endo-addition) whereas the trans product (exo addition) is largely preferred for vinyl sulfones. The results are compared with those obtained with the free cyclobutabenzenes. After decomplexation, the ß-hydroxy ester products can be alkyl- or allylated highly diastereoselectively. First examples of diastereoselective complexation of enantiomerically enriched cyclobutabenzenes are reported