Chapter 5 - Cycloadditions 2: Stereochemistry of [4 + 2] and [2 + 2] Cycloadditions

Abstract

This chapter describes the stereochemistry of cycloaddition reactions with respect to [4+2] and [2+2] cycloadditions. The [4+2] cycloadditions include Diels–Alder reactions, 1,3-dipolar cycloadditions and cycloadditions with cations and anions. The stereospecificity and stereoselectivity of Diels–Alder reactions have been described wherein the product stereochemistry has been delineated using simple mnemonics. The asymmetric Diels–Alder reactions involving chiral auxiliary and chiral catalyst strategies have been presented and a mnemonic has been devised to draw product stereochemistry. The important features of intramolecular Diels–Alder reactions and examples of photochemical Diels–Alder and thermal antara/antara cycloadditions are also included. The stereochemistry of 1,3-dipolar cycloadditions are described with respect to stereospecificity, stereoselectivity, intramolecularity and asymmetric approach. The [4+2] cycloadditions with allyl cations, pentadienyl cations and allyl anions are presented with their stereochemistry. The descriptions of stereochemistry of [2+2] cycloadditions include both photochemical and thermal cycloadditions. The [2+2] photocycloadditions of alkenes and alpha, beta-unsaturated carbonyl compounds are described. The [2+2] thermal cycloadditions include mainly ketene cycloadditions with reference to regioselectivity, stereospecificity, stereoselectivity and periselectivity, and a brief description of the cycloadditions of related systems such as isocyanates, vinyl cations and allenes. The cycloaddition of singlet vinyl carbenes and cycloaddition of transition metal alkylidene in alkene metathesis are also illustrated.