Pyrones and Triazines as Dienes

The 2-pyrones can behave as dienes or dienophiles depending on the nature of their reaction partners. 3-Carbomethoxy-2-pyrone (84) underwent inverse Diels-Alder reaction with several vinylethers under lanthanide shift reagent-catalysis [84] (Equation 3.28). The use of strong traditional Lewis acids was precluded because of the sensitivity of the cycloadducts toward decarboxylation. It is noteworthy that whereas Yb(OTf)j does not catalyze the cycloaddition of 84 with enolethers, the addition of (R)-BINOL generates a new active ytterbium catalyst which promotes the reactions with a moderate to good level of enantio selection [85]. 

5-Disubstituted pyridines [86] 87 have been prepared by catalyzed cycloaddition of 4-methyl-1,2,3-triazine 85 with aldehyde enamines 86 (Equation 3.29). The best yields were obtained when ZnBr2 was used as catalyst. 

The /lomo-Diels-Alder reaction is a [2 + 2 + 2] cycloaddition of a 1,4-diene with a dienophile which produces two new bonds and a cyclopropane ring. This reaction is an example of a multi-ring-forming reaction that to date has found few applications in synthesis, since the use of 1,4-dienes has been limited mainly to bridged cyclohexa-1,4-dienes and almost exclusively to norbornadiene. Lewis-acid catalysts accelerate /lowo-Diels-Alder reactions and increase the selectivity for the [2 + 2 + 2] vs. [2 + 2] cycloaddition. 

A cobalt-based catalyst, prepared by reducing Co(acac)3 with diethylalumi-num chloride in the presence of the bidentate ligand l,2-bis(triphenylphosphi-no)ethane, accelerates [87] the cycloadditions of norbornadiene (88) with a variety of acetylenes (Equation 3.30). 

An example that illustrates the influence of the nickel catalyst on the reaction yield is the cycloaddition between tricyclo [5.3.1.0 ]-undeca-2,5-diene(90) and dimethylacetylenedicarboxylate (Equation 3.31). Whereas a thermal process afforded cycloadduct 91 in an unsatisfactory yield (22 %), the catalyzed process 

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