Intramolecular asymmetric photoreaction

In comparison to the variety of intramolecular asymmetric photoreactions, inter-molecular asymmetric photoreactions have been scarcely reported. However, the reaction types do include [2 4- 2] photodimerization, photoaddition, photodecar-boxylative condensation, and photooxygenation. Herein the intermolecular reactions are more precisely described, including their reaction mechanisms and reaction paths in the crystals. 

The host-guest and ionic chiral auxiliary approaches have been most intensively applied for solid-state asymmetric induction. A number of achiral organic compounds could be converted into chiral compounds in high enantioselectivities. However, all the photoreactions in themselves are well-known intramolecular photoreactions photocyclization, [2 + 2] photocyclization, Norrish type II photo-cyclization, di-ir-methane photorearrangement and photoisomerization. New types of asymmetric photoreactions have never been reported. 

Since the concept of topochemically controlled reactions was established, various approaches to asymmetric synthesis using a solid-state reaction have been attempted, most actively by the research group at the Weismann Institute. Their studies have been concerned with the bimolecular reactions of chiral crystals in the solid state. In these studies, successful absolute asymmetric synthesis has been performed by using topochemically controlled four-centered photocyclodimerizations of a series of unsymmetrically substituted diolefin crystals. Research on reactivity in the crystalline state has been extended in recent years to a variety of new systems, and many absolute asymmetric syntheses have been provided. Successful examples of absolute asymmetric synthesis using chiral crystals are listed in Tables 2 to 4, which are divided into three categories intermolecular photoreaction in the solid state (Table 2), intramolecular photoreaction in the solid state (Table 3, A-D), and asymmetric induction in the solid-gas and homogeneous reactions (Table 4). 

Hydrogen abstraction by an excited carbonyl group is the most typical photoreaction in both solution and solid states. A number of intramolecular Norrish type II hydrogen abstraction reactions in the crystalline state are already known, and the geometric requirements have been precisely discussed by Scheffer [58]. Solid-state asymmetric induction in the Norrish type II photocyclization of carbonyl compounds using supramolecular approaches has been also intensively studied. 

When the stereogenic center is a chiral allene, intramolecular 12 + 2 photocycloaddition produces optically active adducts [117]. A recent reinvestigation of the photoreactivity of optically active allenic enones (156) has shown that the adducts 157a and 158a are obtained with 100% asymmetric induction [146] (Scheme 24). 

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