title: Stereoselectivities and regioselectivities of (4 + 3) cycloadditions between allenamide-derived chiral oxazolidinone-stabilized oxyallyls and furans: experiment and theory.
abstract: 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 (2(Ph)), under either thermal or ZnCl(2)-catalyzed conditions. Reactions of 2(Ph) with 2-substituted furans gave syn cycloadducts selectively, while cycloadditions with 3-substituted furans gave selectively anti cycloadducts. The stereoselectivities were in favor of a single diastereoisomer (I) in all but one case (2-CO(2)R). Density functional theory calculations were performed to explain the selectivities. The results support a mechanism in which all cycloadducts 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 (E)-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.
authors.affiliation: Division of Pharmaceutical Sciences and Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53705, USA.
authors.forename: Jennifer E
authors.initials: JE
authors.lastname: Antoline
authors.forename: Elizabeth H
authors.initials: EH
authors.lastname: Krenske
authors.forename: Andrew G
authors.initials: AG
authors.lastname: Lohse
authors.forename: K N
authors.initials: KN
authors.lastname: Houk
authors.forename: Richard P
authors.initials: RP
authors.lastname: Hsung
mesh_terms: D000498:Allyl Compounds / Q000737:chemistry*; D002384:Catalysis; D002712:Chlorides / Q000737:chemistry; D018360:Crystallography, X-Ray; D003500:Cyclization; D005663:Furans / Q000737:chemistry*; D015394:Molecular Structure; D023303:Oxazolidinones / Q000737:chemistry*; D010100:Oxygen / Q000737:chemistry; D013237:Stereoisomerism; D017967:Zinc Compounds / Q000737:chemistry
chemical_list: D000498:Allyl Compounds; D002712:Chlorides; D005663:Furans; D023303:Oxazolidinones; D017967:Zinc Compounds; C016837:zinc chloride; D010100:Oxygen