環拡大反応

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Syntheses of hardly available ring structures through ring enlargement reactions are illustrated with the emphasis on the methods of high utility from the synthetic point of view. I One-Carbon Ring Enlargement. The molecular rearrangements involve active species such as carbonyl-diazomethane adducts, β-hydroxycarbenium ions, β-oxidocarbenoids, and β-bromo alkoxides respectively. Oxidative rearrangement is performed by means of cyanogen azide or thallium (III) nitrate. Bicyclo [n. 1.0] alkanes also are key intermediates for the one-carbon homologation. Photolysis of bicyclo [n. 2.1] alkanones having a carbonyl bridge undergoes Norrish Type I reaction resulting in ring expansion. II Two-Carbon Ring Enlargement. This methodology consists in the reaction of enamines of cyclic ketones with ketenes or acetylenes, acyloin condensation of cycloalkane-1, 2-dicarboxylic acid esters followed by thermolysis, and [1, 3] sigmatropic rearrangement. III Three-Carbon Ring Enlargement. Photolysis of α-cyclopropyl ketones and β, γ-unsaturated ketones, and fragmentation of bicyclo [n. 3.0] alkenones are discussed. IV Four-Carbon Ring Enlargement. Typical examples are the oxy-Cope rearrangement and fragmentation of bicyclo [n. 4.0] alkanes. V Miscellanea
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