THE MECHANISM OF LEWIS ACID CATALYZED ENE REACTIONS. LEWIS ACID CATALYZED INTER- AND INTRAMOLECULAR (2+2) CYCLOADDITIONS OF CONJUGATED ALLENIC ESTERS TO ALKENES. THE REGIOCHEMISTRY OF TRIETHYLAMINE DEPROTONATION OF ALPHA, BETA-UNSATURATED ACID CHLORIDES T

This dissertation is divided into three chapters. The first chapter establishes that the Lewis acid catalyzed ene reactions of methyl propiolate, formaldehyde and diethyl oxomalonate proceed through a stepwise reaction with rate determining formation of any of the following (1) a three-membered ring intermediate lacking the geometrical rigidity of perepoxides and related intermediates, (2) a pair of rapidly equilibrating zwitterions, or (3) a (pi)-complex between the ene component and enophile-Lewis acid complex.;The third chapter examines the regiochemistry of vinylketene generation. Kinetically controlled deprotonation of (alpha),(beta)-unsaturated acid chlorides can occur at two alternative (gamma)-sites. We have found that the resulting vinylketenes were almost exclusively derived from deprotonation of the less substituted (gamma)-carbon due to the well known kinetic acidity of alkyl groups which follows the order CH(,3) > CH(,2) > CH. Consequently, the stereochemistry of the (alpha),(beta)-unsaturated acid chloride has no effect on the reaction, allowing mixtures of isomers to be used without affecting product distribution.;The Barton-McCombie method for the deoxygenation of secondary alcohols with tributylstannane appeared to offer an attractive method to synthesize (alpha)-bourbonene from the bicyclo- 3.2.0 heptanone (64). Unfortunately, attempted cyclizations of (66) gave no (alpha)-bourbonene. Analysis of the NMR spectrum suggested that cyclization of the radical formed by addition of Bu(,3)Sn to the thiocarbonyl group occurred prior to fragmentation.;The second chapter discusses the 2+2 cycloadditions of the substituted allenic esters. Cycloaddition of methyl 2,3-pentadienoate (2), methyl 4-methyl 2,3-pentadienoate (3) and methyl 2-methyl-2,3-butadienoate (4) with alkenes occur at carbons 3 and 4 as with methyl 2,3-butadienoate (1) to give similar mixtures of products. The stereo- and regioselectivity of these Lewis acid catalyzed reactions are remarkably similar to the thermal cycloaddition reactions of the corresponding ketenes, which indicates that the mechanistic models used to rationalize ketene cycloaddition reactions are probably also applicable to the allenic ester cycloadditions.