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Gewald reaction
The Gewald reaction is an organic reaction involving the condensation of a ketone (or aldehyde when R2 = H) with a α-cyanoester in the presence of elemental sulfur and base to give a poly-substituted 2-amino-thiophene.〔Gewald, K.; Schinke, E.; Böttcher, H. ''Ber.'' 1966, ''99'', 94-100.〕〔Sabnis, R. W. ''Sulfur Rep.'' 1994, ''16'', 1-17. (Review)〕
The reaction is named after the German chemist Karl Gewald (born 1930).〔John A. Joule, Keith Mills: ''Heterocyclic Chemistry'', John Wiley & Sons, 5. Auflage (2010), p. 340, ISBN 978-1-4051-3300-5.〕〔Bradford P. Mundy, Michael G. Ellerd, Frank G. Favaloro, Jr.: ''Name Reactions and Reagents in Organic Synthesis'', John Wiley & Sons, 2. Auflage (2005) p. 306, ISBN 0-471-22854-0.〕〔Christopher Hume: ''Applications of Multicomponent Reactions in Drug Discovery – Lead Generation to Process Development'', p. 311−341, see p. 332−334, In Jieping Zhu, Huges Bienaymé: ''Multicomponent Reactions'', Wiles-VCH Verlag, 2005, ISBN 978-3-527-30806-4.〕
==Reaction mechanism==
The reaction mechanism of the Gewald reaction has only recently been elucidated.〔Sabnis, R. W.; Rangnekar, D. W.; Sonawane, N. D. ''J. Heterocyclic Chem.'' 1999, ''36'', 333.〕 The first step is a Knoevenagel condensation between the ketone (1) and the α-cyanoester (2) to produce the stable intermediate 3. The mechanism of the addition of the elemental sulfur is unknown. It is postulated to proceed through intermediate 4. Cyclization and tautomerization will produce the desired product (6).
Microwave irradiation has been shown beneficial to reaction yields and times.〔Sridhar, M.; Raoa, R. M.; Babaa, N. H. K.; Kumbhare, R. M. ''Tetrahedron Lett.'' 2007, ''48'', 3171-3172. ()〕
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