Using new heterodienophiles

Bryce, M. R., Becher, J., Falt-Hansen, B., Heterocyclic Synthesis using new Heterodienophiles, 55, 1. 

An interesting entry to functionalized dihydropyrans has been intensively studied by Tietze in the 1990s using a three-component domino-Knoevenagel Hetero-Diels-Alder sequence. The overall transformation involves the transient formation of an activated heterodienophile by condensation of simple aldehydes with 1,3-dicarbonyls such as barbituric acids [127], Meldrum s acid [128], or activated carbonyls. In situ cycloaddition with electron-rich alkenes furnished the expected functionalized dihydropyrans. Two recent examples concern the reactivity of 1,4-benzoquinones and pyrazolones as 1,3-dicarbonyl equivalents under microwave irradiation. In the first case, a new three-component catalyst-free efficient one-pot transformation was proposed for the synthesis of pyrano-1,4-benzoquinone scaffolds [129]. In this synthetic method, 2,5-dihydroxy-3-undecyl-1,4-benzoquinone, paraformaldehyde, and alkenes were suspended in ethanol and placed under microwave irradiations to lead regioselectively the corresponding pyrano-l,4-benzoquinone derivatives (Scheme 38). The total regioselectivity was... 

Heterodienophiles, new, heterocyclic synthesis using, 55, 1 Heteropentalenes, 69, 271 Hilbert-Johnson reaction of 2,4-dialkoxypyrimidines with halogenoses, 8, 115... 

The hetero-Diels-Alder reaction is amongst the most efficient processes for the synthesis of six-membered heterocyclic ring systems. Solvent-free conditions have been used to improve reactions of heterodienophiles and heterodynes with low reactivities. Cado et al. (1997) have described the hetero-Diels-Alder reaction of ethyl lH-perimidine-2-acetate as heterocyclic ketene aminal with ethyl propiolate nnder solvent-free conditions with focused microwave irradiation. The new fused perimi-dines (23) were obtained in good yields (67-98%). 

It has been shown that the phosphonodifluorodithioacetate (339) acts as a powerful heterodienophile and can be used to prepare a new class of phos-phonodifluoromethyl thiopyrans (340). These adducts can be submitted to a selective dihydroxylation to produce diastereoisomers (341) (an example is given in Scheme 9l)P ... 

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