π-Facial Diastereoselection in [3 + 2]-Cycloadditions of Isomünchnone Dipoles

The Rh(II)-catalyzed reaction of 1-[diazo(methoxycarbonyl)acetyl]-2-oxopyrrolidine derivatives led to the formation of ring-fused isomünchnone betaines which are trapped with various dipolarophiles to give the corresponding [3 + 2]-cycloadducts in excellent yield. The cycloaddition reaction proceeded with high levels of diastereoselectivity leading to the predominant formation of the exo dipolar cycloadduct. The influence of substituents on the stereoselectivity of the reaction was investigated. Diastereotopic facial selectivity by the dipolarophile on the mesoionic betaine was found to depend upon the nature of the stereogenic center. Inclusion of substituents at any position of the fused five-membered ring was found to enhance exo selectivity. Isomünchnone dipoles derived from pyrrolidinones containing a carboalkoxy substituent in the 5-position of the ring afforded mainly exo−syn cycloadducts. In contrast, when a methyl group was present at the 3-position of the pyrrolidinone ring, the facial selectivity was reversed with the exo−anti adduct being the major product. A computational study was carried out to rationalize the observed product distribution.