ChemInform Abstract: Development of a New Lewis Acid-Catalyzed Claisen Rearrangement.

Abstract

Department of Chemistry, UniVersity of California, Berkeley, California 94720 ReceiVed June 14, 1999 Since its discovery in 1912,1 the Claisen rearrangement has become one of the most powerful tools for carbon-carbon bond formation in chemical synthesis.2 This [3,3]-sigmatropic rearrangement, which involves the conversion of an allyl vinyl ether to an R,â-disubstituted γ,δ-unsaturated carbonyl, generally proceeds with high levels of stereocontrol, securing its widespread use in natural product and medicinal agent synthesis.2 Activation of this rearrangement has traditionally been accomplished under thermal control; however, significant rate acceleration can be achieved through the incorporation of cationic3 or anionic4 charge in the bond reorganization event. Despite its prolific use in chemical synthesis and disposition toward charge acceleration, only two examples of catalytic Claisen variants have been reported.5 In this paper we describe a broadly useful Lewis acidcatalyzed [3,3]-sigmatropic rearrangement that we expect will provide a new avenue for the development of an enantioselective catalytic Claisen process.6 In 1978, Bellus and Malherbe reported the conceptually novel ketene-Claisen reaction.7 In an attempt to perform a [2 + 2] cycloaddition, the authors discovered that treatment of an allyl ether with dichloroketene resulted instead in the formation of a 1,3-dipolar allyl vinyl ether, which subsequently underwent [3,3]bond reorganization (eq 1). Although the scope of this reaction