2- Cyano-3,6-dimethyl-4/7-pyran

Cyano-3,6-dimethyl-4//-pyran was isolated in 60% yield in attempts to generate a./S-unsaturated acyl cyanide by the substitution of H2C=C(Me)COCl with CuCN to give H2C=C(Me)COCN and CuCl, but the structure of the 4//-pyran is not well explained by the postulated (83TL2847 86JOC1199) mechanism. An alternative route involving initial decarbonylation (H2C=C(Me)COCl — H2C=C(Me)Cl + CO) and subsequent cycloaddition leading to intermediate 90 appears to be a more acceptable version. 

The 7-aryl-5-cyanomethyl-l,3-dimethyl-2,4-dioxopyrido[2,3- pyrimidines 458 have been regioselectively synthesized through the nucleophilic attack of the amino groups in 6-amino-l,3-dimethyluracil 452 on the highly electrophilic center at C-6 in 6-aryl-3-cyano-4-methylthio-2//-pyran-2-ones 457. Ring opening of 457 followed by decarboxylation and recyclization with the elimination of methylmercaptan gave 458 with no detection of the 5-aryl-7-cyanomethyl isomers 459 (Equation 38) <20008541 >. 

The reaction of propargylic halides or alcohols with carbon monoxide in the presence of Ni(CN)2 affords 4,6-dimethyl-5-cyano-pyran-2-ones in good yield (Scheme 152) <2000J(P1)1493>. 

Thiocoumarin (127) underwent [2 + 2]-photocycloaddition reactions in better yields than 126. In contrast to coumarin, cis- and trans-fused products are being found, however, for example, in the reaction with 2,3-dimethyl-2-butene, possibly because the thiopyran ring is more flexible than the pyran ring due to the longer C—S bonds. HTproduct is favored with electron donor-substituted olefins [121]. Electron acceptor substitution in 3-position, as in 3-cyano-l-thiocoumarin (128), leads to an improved performance in [2 + 2]-photocycloaddition reactions [122]. 

Amino-4-aryl-3-cyano-7,7-dimethyl-5,6,7,8-tetrahydro-4H-benzo[f>]pyran 281 were reacted with benzylidene malononitrile in the presence of piperidine at 160 °C to afford pyrido[3,4-frJqu inclines 341 (89JPR971) (Scheme 65). 

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