Furans Diels-Alder cycloadditions, trimethyl

Di- and trisubstituted furans may also be prepared by reacting TMSCHN2 with an acyl isocyanate. The intermediate 4-trimethyl-silyloxy oxazole readily undergoes Diels-Alder cycloaddition in situ with a suitable dienophile to construct the product (eq 67). ... 

Density functional theory computational studies have been used to determine the importance of secondary orbital interactions for the stability of transition-state structures for the 4 + 2-cycloaddition of furan with cyclopropene.175 Kinetic studies of the 2 + 4-cycloaddition of 2-cyclopropylidene acetates with furan and dimethylful-vene suggest a mechanism involving diradicals or zwitterions as intermediates.176 Cyclopropene, produced by the reaction of allyl chloride with sodium bis(trimethyl-silyl)amide, reacts with 1,3-diphenylisobenzofuran to produce both endo- and exo-Diels-Alder cycloadducts isolated for the first time.177... 

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