Pechmann-Type Reactions

The condensation of 4-chloro-2-methylphenol with sodium salt of hydroxyl diethyl ester in the presence of a dehydrating agent such as concentrated sulfuric acid gives spirodilactone. A plausible mechanism comprises a transesterification of the oxaloacetate ester and 4-chloro-2-methylphenol to afford the ketoester 44. Then intermediate 36 submits to an intramolecular condensation/dehydration to give 45, which could, 

A three-component reaction of phenols, dimethyl acetylenedicarboxyl-ate (DMAD), and triphenylphosphine yields the corresponding coumarins 

Gunnewegh et al. researched on the synthesis of coumarin derivatives using solid acid catalysts, such as zeolite H-Beta or Amberlyst-15, in toluene. Zeolite H-Beta is the favored catalyst above Amberlyst-15 because of its excellent regenerability.The reaction of phenols and 3-sub-stituted phenols with acryhc acid gave the library of possible reaction products. The results showed that resorcinol and 3-methoxyphenol are 

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T. Wang et al. reported the synthesis of several coumarin derivatives via Pechmann-type reaction and then linked them with a cyclopentanon... 

Reaction of lithium trimethylsilyldiazomethane (TMSC(Li)N2) with thiocarbonyl compounds has proved to be a convenient method for the preparation of 5-substituted 1,2,3-thiadiazoles. This reaction is very similar to the Pechmann-Nold reaction but probably does not proceed through a dipolar cycloaddition pathway. A number of examples of this type of reaction were described in CHEC-II(1996). More recently, it was reported that TMSCN2Li also reacts with diethylaminothiocarbonyl chloride to afford a mixture of 1,2,3-thiadiazoles 66 and 67 (Equation 19) <1997BSB533>. 

Diazomothane, which was first prepared in 1894 by von Pechmann/ undergoes numerous reactions, In the heterocyclic series, its reactions can be divided into the following types ... 

Diazoalkanes, like azides, are 1,3-dipoles of the propargyl-allenyl type (Scheme 87)15 and their reaction with imines provides a route for building the triazoline framework from the C—N—N and C—N fragments. Although diazomethane addition to the carbon-carbon double bond was achieved by von Pechmann in 1898,325 its reaction toward the carbon-nitrogen double bond was investigated only 50 years later. 

Reaction of diazoalkanes with thiocarbonyl compounds, which may be considered an extension of Pechmann s synthesis, forms 1,2,3-thiadiazole derivatives (75SST(3)67o). For example, methyl dithioacetate reacts with diazomethane to form a mixture of thiadiazole isomers and thiirane the latter often is the main product of this type of reaction (equation 26). O-Ethyl thionoacetate reacts with ethyl diazoacetate to form a substituted thiadiazole (equation 27). However, an aryl thionoester reacts with diazomethane to yield a 1,2,3-thiadiazole derivative which is only formed as an intermediate (31) and then rapidly decomposes to alkene (Scheme 5). 

In the synthesis of coumarins by the von Pechmann reaction, a spiro-compound (251) was simultaneously formed from 4-chloro-2-methylphenol (and from 2,3,5-trimethylphenol). A possible mechanism for this reaction involves the formation of the lactone (252), which reacts with another molecule of the phenol. The condensation of the dicyano-ester (253) [obtained from malononitrile and ethyl cyanoacetate] with substituted o-hydroxybenzaldehydes yields substituted coumarinimines, e.g. (254).A series of 4,6,7-substituted coumarins have been prepared and assessed for their suitability in fluorescence labelling of polymers. Esters of the type PhCH CRCOaAr react with AICI3 to give coumarins, which are probably formed via dihydrocoumarins (255) by dearylation. ... 

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