ACYCLIC STEREOCONTROL IN [3-CYCLOADDITIONS OF AMINO ACID-DERIVED ISOMUNCHNONE DIPOLES(2])

alpha-Diazoimides derived from chiral amines undergo Rh(II)-catalyzed cyclization to give the corresponding chiral isomunchnone dipoles whic h were trapped with a variety of dipolarophiles. The extent and sense of diastereoselectivity in the [3+2]-cycloaddition is a function of th e substitution pattern on the chiral amine. Exo-cycloadducts were form ed in high yield, but the pi-facial selectivity is low with dipoles de rived from 1-phenylethyl amine and 1-(1-naphthalenyl)ethyl amine. Howe ver, very high facial discrimination was observed when amino acid este rs were used as the chiral amine component. The best results (greater than or equal to 95 : 5) were obtained using chiral dipoles derived fr om phenylalanine methyl ester with a variety of dipolarophiles. The ob served syn preference can be rationalized in terms of a stereoelectron ic effect of the ester functional group in the preferred conformation in the transition state of the cycloaddition. Another possibility invo lves interaction of the ester carbonyl with the rhodium metal thereby causing the attack of the amido group to occur in a stereodefined mann er. Effective shielding of one of the pi faces by pi-stacking with the aromatic ring explains the high level of pi-facial selectivity of the phenylalanine derivatives. The present study introduces a new method for efficient acyclic stereocontrol in isomunchnone cycloaddition reac tions. (C) 1998 Elsevier Science Ltd. All rights reserved.