Photocyclization, absolute

It would usually be assumed that abstraction/recombination reactions such as those illustrated in this section would proceed with racemization at the reacting centers. It has been reported22, however, that photocyclization of amide 1 proceeds with complete retention of absolute configuration. Racemization at the site of abstraction requires orbital rehybridization, passing through a planar intermediate. In this case rehybridization appears to be markedly slowed. This may be an electronic effect due to the heteroatom substituent on the intermediate radical, or simply a steric effect. The structure of product 2 was established by X-ray crystallographic analysis. 

In the case of the formation of the [3-lactam, it was shown that the ability of a compound to crystallize in a chiral space group was increased by using compounds with meta-substituted aryl groups such as 50a-d instead of their ortho- and para-substituted analogues the absolute asymmetric photocyclization of the pyridones resulted in the formation of 51a-d with good ees.[31]... 

Irie et al. reported on the absolute reversible photocyclization reactions of achiral diarylethene 52 in chiral crystal leading to 53 in up to 99% ee. The asymmetric photochromic material has potential application in nonlinear optics, for example, in switchable second harmonic generation devices. [32]... 

Yamamoto, S., Matsuda, K., and Irie, M. (2003) Absolute asymmetric photocyclization of a photochromic diarylethene derivative in single crystals, Angew. Chem. Int. Ed., 42, 1636-1639. 

Scheme 24 Absolute asymmetric photocyclization of a succinimide derivative involving a 1,5-hydrogen shift.

Scheme 25 Absolute asymmetric photocyclization involving phenyl migration and radical pair recombination.

Scheme 27 Absolute asymmetric Norrish/Yang photocyclization of adamantane derivative 114.

Scheme 29 Absolute asymmetric photocyclization in a salt crystal formed from two achiral components.

Virantmycin is a tetrahydroquinoline alkaloid that has inhibitory activity against DNA and RNA viruses. A total synthesis of virantmycin making use of a key type II aziridine has elucidated the absolute stereochemistry at C-2 and C-3 <1996T10609>. An intramolecular photocyclization of an azide onto a Z-alkene produces type II aziridine 351 in excellent yield. A three-step reduction/selective reoxidation procees yields key aziridine alcohol 352 in 76% overall yield (Scheme 71). The alcohol is methylated and the ester hydrolyzed without harm to the azirdine. A TFA-induced ring opening of the aziridine by chloride provides the natural product virantmycin in good yield. This entire process was also carried out with the -alkene to produce /)(-virantmycin, thus proving the stereochemistry at C-2 and C-3. 

The chiral crystals were spontaneously obtained from the solutions of 1 and 2,4-dichlorobenzylamine 3 (Scheme 27.1b). This kind of chiral crystallization of achiral molecules leads neccessarily to left- and right-handed crystals. The enantiomeric crystals of M- and P-1-3 were selectively prepared by seeding. Irradiation of the crystals of M-l-3 at around 350 nm through a UV filter gave a cyclopentenol (R,R)-5 as almost the sole product, achieving absolute asymmetric photocyclization [25]. In contrast, irradiation at >290 nm afforded (R,R)-5, (P)-4, and (P)-hydrol in a 6 3 1 molar ratio the enantiomeric excesses of the three products were higher than... 

Scheme 27.1 (a) Enantiospecific and (b) absolute asymmetric photocyclization via single-crystal-to-single-crystal transformation. 

Koshima, H., Kawanishi, H., Nagano, M., Yu, H., Shiro, M., Hosoya, T., Uekusa, H. and Ohashi, Y. (2005) Absolute asymmetric photocyclization of isopropylbenzophenone derivatives using cocrystal approach involving single-crystal-to-single-crystal transformation. J. Org. Chem., 70, 4490-4497. 

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