Lactones photodeconjugation

The interesting pyranone (1) has been obtained by photodecarboxylation of (2) using low-temperature matrix isolation techniques (Bleasdale et al.). Bart-nik et al. have described a simple photodecarboxylation route to azabicyc-lobutanes such as (3). Attention is drawn to a new photochemical synthesis of p-lactones (Aoyama et al.) and -lactams (see ref. 50 in Part III, Chapter 2). Skinner and Weedon have reported an interesting method for photodeconjugation of a,p-unsaturated esters e.g., (4) (5). Polar solvents are required,... 

The amide (38a) is photochemically inert on irradiation in ether. The related compound (38b) is, however, photochemically reactive and undergoes fission by a Norrish Type II process to yield a mixture of products.The results of a study of the enantioselective photodeconjugation reactions of the lactones (39) have been published. The behaviour of the ketones (40) and (41) in the isotropic and the two solid phases of heneicosane (CcxH ) has been evaluated. The influence of the various phases on the ratio of elimination to cyclization products of the ketones was discussed. The modification of the photoreactivity of ketones (42) in cyclodextrin has been... 

Henin, F., Mortezaei, R., Muzart, J., Pete, J.-P, and Piva, O., Enantioselective photodeconjugation of conjugated esters and lactones in the presence of ephedrine. Tetrahedron, 45, 6171, 1989,... 

Since the early observations of photodeconjugation of conjugated aliphatic carbonyl and carboxyl derivatives [19], the reaction has been generalized to unsaturated esters [20], lactones [21], acids [22], amides [20n], aldehydes [23], aliphatic enones [24], and medium-ring unsaturated enones [25]. 

The percentage of ee does not depend on the conversion yield and the aminoalcohols can be recovered unchanged in the photodeconjugation of esters and lactones. A study of the dependence of the enantioselectivity of the photodeconjugation of esters in nonpolar solvents as a function of concentration of the chiral inductor (Fig. 2), indicated that very small amounts of the inductor are needed. 

Enantioselective photodeconjugation occurs with lactones, esters, and conjugated enones [33]. In principle, as soon as a prochiral photodienol can be produced, an enantioselective protonation is expected in the presence of a chiral -aminoalcohol. However, the corresponding acids and amides are unsuitable starting materials for enantioselective photodeconjugation. 

Scheme 4. Unsaturated lactones, esters, enones and -aminoalcohols considered in enantioselective photodeconjugation reactions.

Asymmetric Photodeconjugation Principle Enantioselective Photodeconjugation of a,P-Unsaturated Esters and Lactones Diastereoselective Photodeconjugations. .. 70-7... 

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