Absolute asymmetric reaction 2 + 2 photocycloaddition

In addition to bimolecular [2-1-2] photocycloaddition reactions similar to the one discussed above [11], absolute asymmetric syntheses employing spontaneously grown chiral crystals have been studied for a number of unimolecular photorearrangement reactions as well. The first report of such an investigation dealt with the photoisomerization of dibenzobarrelene diester 6 to the corresponding dibenzosemibullvalene compound 7 [12], an example of the well-known di-7i-methane (Zimmerman) photorearrangement. Achiral diester 6 crystallizes spontaneously from cyclohexane in the now familiar chiral space... 

Despite acridine and diphenylacetic acid being achiral molecules, it was found that these two compounds formed a chiral cocrystal on spontaneous crystallization, and further irradiation of this caused photodecarboxylation and then enantio-selective radical coupling to give a chiral condensation product (Scheme 35) [31]. This kind of absolute asymmetric photodecarboxylation is not a topochemical reaction but is accompanied by gradual decomposition of the initial crystal structure as the reaction proceeds. This pathway is different from the four examples of [2 + 2] photocycloadditions described above. 

Sakamoto reported the absolute asymmetric cyclobutane formation via intramolecular [2 + 2] photocycloaddition of iV, Af-diallyl-4-methyl-l-pro-pyl-2-quinolone-3-carboxamide (50) in chiral crystalline state. He also found the two-step reaction involving hydrogenation and intermolecular photocycloaddition of (50) with alkenes (53) afforded chiral cyclobutanes (54) at a low temperature (Scheme 17). ... 

Until now, only few attempts were made to control the absolute configuration of the chiral polycyclic products. When salicylic esters of type 77 (Sch. 14) possessing an asymmetric alcohol moiety as chiral auxiliary instead of the n-butyl group are irradiated, the corresponding products can be isolated with 17% ee [50]. Better results can be obtained with the additionally acylated substrates 79a-c (Sch. 15). Compounds 80a-c were isolated with diastereomeric excesses up to 90% [29,56]. These compounds were formed via [2+2] photocycloaddition yielding the intermediates X. A thermal rearrangement leads to 80a-c. Compounds 81a-c are produced in the same time via a further photochemical rearrangement but due to thermal reversibility of the last step, 80a-d could be isolated with yields up to 90% when the reaction was completed. 

Control of absolute asymmetry is a relatively untouched area for [2 + 2] photochemical cycloaddition reactions despite the recent advances in the field of asymmetric synthesis. The first example of the use of a removable chiral auxiliary was reported by Tolbert, who obtained impressive enantioselectivity in the photocycloaddition of bomyl fumarate to stilbenes (equation 37). More recently, Lange has shown that menthyl cyclohexenonecarboxylates are useful in control of absolute stereochemistry (equation 38). Baldwin and Meyers have also obtained excellent facial selectivity in systems where the stereogenic center which controls the diastereoselectivity can be excised to afford products of high enantiomeric purity (equations 39,40). 

Schreiber, S. L. and Satake, K., Studies of the furan-carbonyl photocycloaddition reaction the determination of the absolute stereostructure of asteltoxin. Tetrahedron Lett., 27, 2575,1986. Jarosz, S. and Zamojski, A., Asymmetric photocycloaddition between furan and optically active ketones. Tetrahedron, 38,1453,1982. 

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