Enamides. enantioselective photocyclization

Attempts have been made for the enantioselective synthesis of (—)-yohim-bane. Kametani et al. (216) reported the total synthesis of key intermediate (-)-368 starting from L-tryptophan, using enamide photocyclization. However, the optical purity of (—)-17-methoxyhexadehydroyohimbane obtained was only 17% owing to the partical racemization of intermediates throughout the reaction sequence. 

Ninomiya and Naito established an enantioselective variant of their enamide photocyclization based on chiral proton donors. The enamide photocyclization... 

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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