Amino acid FlAsH approach

In view of the observation that A-o-nitrophenyl derivatives of glycine and other a-amino acids could be converted into benzimidazolones by the action of heat, and the assumption that iV-oxides were intermediates in the thermolysis [156], it was thought tliat flash vacuum pyrolysis with a very short reaction time might allow isolation of the IV-oxides. This approach, however, did not turn out to be as synthetically efficient as the base treatment method [97]. Indeed, heating such compounds in sand at 200° C is probably of more use for making benzimidazoles or benzimidazolones [156]. Benzimidazole iV-oxides can be made from acid-catalysed thermal or photochemical reactions of AyV-dialkyl-o-nitroanilincs, but not from purely thermal reactions. 

In a more recent study, the enamide photocyclization with very similar photosubstrates was examined in the presence of chiral amino alcohols and chiral amines as asymmetric inductors [47]. The achieved enantioselectivities are in the same range as the ones reported by Ninomiya and Naito, but in this approach the asymmetric induction was more effective for the cis products. In cyclopentane at — 40°C, 0.1 equivalents of the most effective inductor, (— )-ephedrine (entity, gave the cis cyclization products with up to 37% ee and the trans products with only 2% ee. The role of the chiral inductor as a Br0nsted acid was supported by flash photolysis experiments. The presence of the chiral amino alcohol led to an increase in the rate of disappearance of a transient that was assigned to the primary cyclization intermediate of type 29, i.e., the chiral inductor accelerates the protonation/deprotonation sequence that reestablishes the aromatic ring. 

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