Diels-Alder reactions of (E)-2-phenyl-4-[(S)-2,2-dimethyl-1,3-dioxolan-4-ylmethylen]-5(4H)-oxazolone with heterogeneous catalysts

Diels-Alder reactions of (E)-2-Phenyl-4-[(S)-2,2-dimethyl-1,3-dioxolan-4-ylmethylen]-5(4H)-oxazolone with 2,3-dimethylbutadiene, cyclopentadiene, and 2-methyl-1,3-butadiene (isoprene) are catalysed by silica gel and aluminium, titanium, and zinc catalyst supported on silica gel. Overall conversion and asymmetric induction are obtained with very low percentages of EZ isomerisation. The best results are obtained with ZnCl2 supported on silica gel, and by carrying out the reaction without a solvent. The stereochemistry of the major cycloadducts has been determined by single crystal X-ray structure determination and AM1 calculations of the corresponding transition structures.This work was made possible by the generous financial support of the Comision Interministerial de Ciencia y Tecnologia (Project MAT96-1053-C02-O1) and of the European Union (Contract CHRX CT93-0276). One of us (E.P.) thanks the Diputacion General de Atagon for a grant.Peer reviewe

Theoretical study on the BF3-catalyzed meinwald rearrangement reaction

The mechanisms of the BF3-catalyzed Meinwald rearrangement reactions of five epoxides in dichloromethane solution have been studied at the M062X/6-311++G(2df,2pd) level. Accordingly, the Lewis acid-epoxide complex can react through several alternative pathways, though three phases (ring opening, C-C bond rotation, and hydrogen or alkyl group migration) are required in any path. In some cases, a concerted pathway (involving all three successive phases) is found. Otherwise, the reaction takes place through a reaction mechanism involving a zwitterion or a BF3 addition compound (formed by fluoride transfer from the BF3 moiety to the incipient carbocationic center generated by C-O bond rupture) or both as reaction intermediate(s). The BF 2-bound fluorohydrin yields the reaction product through a concerted process involving fluoride transfer from the C-F bond to the OBF2 group and hydrogen or alkyl group migration, as first demonstrated in this work. Effects of a number of features (solvent effects, concurrent hydrogen/alkyl group migration, carbocation substitution, benzylic conjugation) are also discussed. © 2014 American Chemical Society.Financial support from the Ministerio de Ciencia e Innovación (MICINN/FEDER) of Spain (Project CTQ2011-28124) and the Gobierno de Aragón (Group E11) is gratefully acknowledged.Peer Reviewe