Oxetanes chiral induction

Koch H. Chirale Induction bei photochemischen Reaktionen Untersuchungen zur chiralen Oxetan- und CyclobutanbildungRWTH Aachen, Aachen, 1984. 

A patent has been lodged dealing with the photochemical synthesis of the thiolactones (82) from the thioacids (83). The cycloadduct (84) is the sole product from the irradiation of 1,1-diethoxyethene with biacetyl in non-polar solvents. The authors suggest that an electron transfer process is operative and that the cycloaddition therefore proceeds via the zwitterion (85). A study of chiral induction in the photochemical synthesis of oxetanes using the chiral... 

Isatine derivatives gave the corresponding cycloadducts with high stereoselectivity when irradiated in the presence of furan and benzofuran [72]. The reaction of furan with acyl cyanides yields the corresponding oxetanes, but both diastereoisomeric endo- and exo-oxetanes are formed (Scheme 3.36). When chiral acyl cyanides are used, low asymmetric induction is observed [73]. Furan also reacts with chiral ketones. In this case, an ot-cleavage reaction before the 2+2 cycloaddition modifies the expected products (Scheme 3.37). When (—)-menthone was used as a substrate, a chiral product was obtained as a 2 1 diastereoisomeric mixture the most abundant product has the (1/ , 3R) configuration [74]. When the reaction was performed on carbohydrate 36, a complex reaction mixture was obtained (Scheme 3.38) [74, 75]. 

A preliminary examination of asymmetric induction in photochemical [2 + 2] cycloaddition reaction of an enone to an alkene employed a chiral auxiliary attached to the alkene component74. The photocycloaddition of 2-cyclopentenone to the optically active ketene acetal 1 led both to oxetanes and cyclobutanes in the ratio 6.5 3.5 with a total yield of 60%. The m-[2C + 2C] addition of the chiral alkene I to 2-cyclopentenone was completely regioselective and gave four diastereomeric head-to-tail cycloadducts 2-4 in the ratio 6 29 33 32. 

The importance of asymmetric synthesis in organic technology was emphasized in Chapter 9. It is also possible to introduce chirality through a photochemical reaction by transferring it from a chiral auxiliary attached to the reacting molecule. If a bimolecular photoreaction is involved, the chiral auxiliary may be attached to any one of the reactants. Examples of such asymmetric induction are cyclobutanes, oxetanes, and cyclohexenes by [4 + 2] photocycloaddition (Pfoertner, 1990). 

Regiospecific Lewis acid catalyzed l2- -2]-cycloaddition with asym. induction Chiral oxetanes... 

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