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6+4 cycloaddition
() Cycloaddition is a type of cycloaddition between a six-atom pi system and a four-atom pi system, leading to a ten-membered ring. Because this is a higher-order cycloaddition, issues of periselectivity arise in addition to the usual concerns about regio- and stereoselectivity. Six-atom pi systems that have been employed in the reaction include tropone and tropone derivatives, fulvenes, and cycloheptatriene cobalt complexes.〔Rigby, J. H. ''Org. React.'' 1997, ''49'', 331. 〕
==Introduction==
() Cycloaddition is a thermally allowed, higher-order cycloaddition process leading to ten-membered rings. Although most linear, acyclic trienes do not give () products selectively, cyclic trienes give high yields of () products in many cases. Both cycloheptatrienes and fulvenes can be employed in this reaction, and electron-deficient tropones in particular work well. The pericyclic and transition-metal-mediated versions of the reaction are stereocomplementary: the former gives ''exo'' products, and the latter ''endo'' products, with essentially complete selectivity in nearly all cases. The possibility of building complex carbocyclic frameworks efficiently has made this reaction particularly attractive synthetically.〔Rigby, J. H. in ''Comprehensive Organic Synthesis'', Trost, B. M.; Fleming, I., Eds., Vol. 5, Pergamon, Oxford, 1991, pp. 617–643.〕
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