2- furan, reaction with dimethyl acetylene dicarboxylate

The results of the photochemical reactions of dimethyl acetylene-dicarboxylate with furan,222 thiophene,18 and 1 //-pyrrole11 were found to differ widely. Furan yielded either a 1 1 or a 2 1 adduct, and thiophene yielded a sulfur-free reaction product, viz., dimethyl phthalate. These products were formed by (4 + 2)-cycloadditions, which—in the case of thiophene—was followed by extrusion of sulfur from the initial Diels-Alder adduct. 1//-pyrrole, however, reacted in a different way because a l//-azepine (195) was isolated. This product was most probably generated by (2 + 2)-cycloaddition of the acetylene to the... 

The first naphtho[a]cyclopropene 68d was also obtained by that route. 3H-indazole was proposed as an intermediate in this reaction 78>. The reaction proceeds equally well from the triplet state of 7. However with acetone light capture from the sensitizer may not have been complete. A trapping of the diradical intermediate 67 to give the known indene 69 by photolysing 7 in dimethyl-acetylene-dicarboxylate (ADC) afforded only a trace of 69. However the yield of 68a was reduced to 40%. Either trapping of 67 was not effective enough or ADC may have acted as quencher. Trapping with furan or cyclopentadiene was also not effective 78a). 

The ylide (42) is formed when carbon dioxide and dimethyl acetylene-dicarboxylate react in the presence of a trialkyl phosphite (Scheme 6). The reaction of (42) with an aromatic aldehyde afforded the furan (43). ... 

Jurczak has shown that cycloaddition reactions involving furan could also be catalyzed by transfer RNA under high pressure [13]. Flence, kinetin 9 reacts with maleic anhydride and dimethyl acetylene dicarboxylate (DMAD) at lOkbar showing that tRNA is a true Diels-Alderase (Scheme 2). These products are not formed at high pressure without tRNA. It seems that a conformation of tRNA induced at high pressure meets structural requirements of a transition state of Diels-Alder reaction. It provides complementarity of size and shape of both substrates and is a driving force in molecular recognition. 

An example of an acetylenic furan cycloaddition reaction was reported by Ho and Wong [14]. While in some cases, the reaction of furans with dimethyl acetylenedicarboxylate (DMAD) leads to ring opening and aromatization (see below), the addition of 3,4-bis(trimethylsilyl)furan to DMAD at 75 °C gave furan-3,4-dicarboxylate 13, presumably by extrusion of bis(trimethylsilyl)acetylene from the initially formed cycloadduct 12. Stable [47t-l-27i] furan cycloadducts were also obtained using the highly reactive bis(terf-butylsulfonyl)acetylene [15], as well as nitro(trime thylsilyl)acetylene (Scheme 13.5) [16]. 

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