Tetrahedron report number 635 Cycloaddition reactions of vinyl oxocarbenium ions Michael Harmata * and Paitoon Rashatasakhon Department of Chemistry, University of Missouri-Columbia, Columbia, MO 65211, USA Received 3 January 2003 Contents 1. Introduction 2371 2. 4þ2 (Diels – Alder) cycloaddition reactions 2371 3. 4þ3 Cycloaddition reactions 2379 4. 2þ2 Cycloaddition reactions 2387 5. 3þ2 Cycloaddition reactions 2388 6. 5þ2 Cycloaddition reactions 2390 7. Conclusion 2392 1. Introduction Vinyl oxocarbenium ions can be described as allylic cations that are substituted at one terminus by at least one oxygen atom. In the broadest sense, the literature is replete with examples of the cycloaddition chemistry of such species, particularly their behavior as dienophiles in the Diels – Alder reaction. This is the case because any a,b-unsaturated carbonyl compound complexed to a Lewis acid can be represented in one of its resonance forms as a vinyl oxocarbenium ion (Fig. 1). With a few exceptions in the area of 4þ3 cycloaddition chemistry, the cycloaddition chemistry of such intermediates will not be discussed in this review. Instead, we will focus on systems represented by 1, in which the ‘R’ group is some alkyl substituent and generation of the cation involves loss of a leaving group, not coordination to a carbonyl group oxygen. This review is intended to be comprehensive and we would like to be informed of any omissions that have occurred as an oversight on our part. Data will be presented loosely in chronological order. 2. 412 (Diels – Alder) cycloaddition reactions The 4þ2 cycloaddition or Diels–Alder reaction is perhaps the most effective way of constructing six-membered ring systems. The reaction has been well studied since it was discovered in 1928. 1 Extensive research in the area of stereo- and regiocontrol, as well as application to the synthesis of natural products, has been conducted during the last few decades. 2 In an ordinary or ‘normal’ Diels–Alder reaction, the interaction between the HOMO of an electron- rich diene and the LUMO of an electron-poor dienophile is a major factor in determining the rate and regiochemical outcome of the reaction. Since there is no other carbon- based electron-withdrawing group better than a carbocation, an olefin bearing an adjacent carbocation would be expected to act as an excellent dienophile. Diels –Alder reactions involving allylic cations as dienophiles are referred to as ionic Diels – Alder reactions. Many variations on this theme are possible, but we will mention only those cation ‘substituents’ which are further stabilized by one or more alkoxy groups. 3 What appears to be the first example of an oxocarbenium ion in a 4þ2 cycloaddition reaction came from the laboratories of Sasaki and co-workers. 4 They showed that methyl vinyl ketone underwent a 4þ2 cycloaddition 0040–4020/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0040-4020(03)00253-9 Tetrahedron 59 (2003) 2371–2395 Figure 1. * Corresponding author. Tel.: þ1-573-882-1097; fax: þ1-573-882-2754; e-mail: harmatam@missouri.edu Keywords: Diels–Alder reaction; oxocarbenium ion; Lewis acid; cycloaddition.