Stereoselectivities and Regioselectivities of (4 + 3) Cycloadditions Between Allenamide-Derived Chiral Oxazolidinone-Stabilized Oxyallyls and Furans: Experiment and Theory

摘要

A systematic investigation of the regioselectivities and stereoselectivities of (4 + 3) cycloadditions between unsymmetrical furans and a chiral oxazolidinone-substituted oxyallyl is presented. Cycloadditions were performed using an oxyallyl containing a (R)-4-phenyl-2-oxazolidinone auxiliary (2pb), under either thermal or ZnCl2-catalyzed conditions. Reactions of 2_(Ph) with 2-substituted furans gave syn cydoadducts selectively, while cycloadditions with 3-substituted furans gave selectively anti cydoadducts. The stereoselectivities were in favor of a single diastereoisomer (I) in all but one case (2-CO_2R). Density functional theory calculations were performed to explain the selectivities. The results support a mechanism in which all cydoadducts are formed from the E isomer of the oxyallyl (in which the oxazolidinone C=O and oxyallyl oxygen are anti to each other) or the corresponding (£)-ZnCl_2 complex. The major diastereomer is derived from addition of the furan to the more crowded face of the oxyallyl. Crowded transition states are favored because they possess a stabilizing CH-π interaction between the furan and the Ph group.