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Cycloisomerization is any isomerization in which the cyclic isomer of the substrate is produced in the reaction coordinate. The greatest advantage of cycloisomerization reactions is its atom economical nature, by design nothing is wasted, as every atom in the starting material is present in the product. In most cases these reactions are mediated by a transition metal catalyst, in few cases organocatalysts and rarely do they occur under thermal conditions. These cyclizations are able to be performed with excellent levels of selectivity in numerous cases and have transformed cycloisomerization into a powerful tool for unique and complex molecular construction.〔For a review on cycloisomerization reactions see: ''Enantioselective, transition metal catalyzed cycloisomerizations'' Angela Marinetti, Hélène Jullien and Arnaud Voituriez Chem. Soc. Rev., 2012,41, 4884-4908 , Critical Review .〕 Cyclosiomerization is a very broad topic in organic synthesis and many reactions that would be categorized as such exist. Two basic classes of these reactions are intramolecular Michael addition and Intramolecular Diels–Alder reactions. Under the umbrella of cycloisomerization, enyne and related olefin cycloisomerizations are the most widely used and studied reactions.〔Watson, I. D. G.; Toste, F. D. Chem. Sci. 2012, 3, 2899–2919.〕 ==Intramolecular Michael Addition== A rather intuitive route to cyclic isomers is the intramolecular conjugate addition to α,β–unsaturated carbonyls (intramolecular Michael addition or IMA). Competent Michael acceptors include conjugated enones, enals or nitroalkene derivatives and examples of other acceptors are sparse.〔Nodes, W. J.; Nutt, D. R.; Chippindale, A. M.; Cobb, A. J. A. J. Am. Chem. Soc. 2009, 131, 16016–16017.〕 Despite IMA reactions being ubiquitous in synthesis, very few examples of asymmetric IMA transformations exist.〔Watson, I. D. G.; Toste, F. D. Chem. Sci. 2012, 3, 2899–2919.〕 Thiourea catalysts with pendant chiral backbones have shown to activate systems with tethered nitroalkane and ester motifs to induce asymmetric IMA.〔Nodes, W. J.; Nutt, D. R.; Chippindale, A. M.; Cobb, A. J. A. J. Am. Chem. Soc. 2009, 131, 16016–16017.〕〔For a review on thiourea catalysis see: Zhang, Z.; Schreiner, P. R. Chem. Soc. Rev. 2009, 38, 1187–1198.〕 The utility of this transformation was demonstrated in the synthesis of cyclic γ– amino acid precursors (figure 1).〔Nodes, W. J.; Nutt, D. R.; Chippindale, A. M.; Cobb, A. J. A. J. Am. Chem. Soc. 2009, 131, 16016–16017.〕 It is proposed that activation occurs via H–bonding of both the nitronate and the ester to the thiourea catalyst and explains the interesting selectivity for the ''E''–ester.〔Nodes, W. J.; Nutt, D. R.; Chippindale, A. M.; Cobb, A. J. A. J. Am. Chem. Soc. 2009, 131, 16016–16017.〕 A functional stereodivergent organocatalyzed IMA/lactonization transformation in the synthesis of substituted dihydrofurans and tetrahydrofurans has been studied for its ability to construct important structural motifs in numerous natural products (figure 2).〔Belmessieri, D.; Houpliere, A.; Calder, E. D. D.; Taylor, J. E.; Smith, A. D. Chem. Eur. J. 2014, 20, 9762–9769.〕 When ethers such as 3 are subject to (S)–(–)–tetramisole hydrochloride (4) catalyst the result is the ''syn''–2,3–substituted THF while the complimentary ''anti''–product is easily accessible via a Cinchona alkaloid catalyst such as 7.〔Belmessieri, D.; Houpliere, A.; Calder, E. D. D.; Taylor, J. E.; Smith, A. D. Chem. Eur. J. 2014, 20, 9762–9769.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Cycloisomerization」の詳細全文を読む スポンサード リンク
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