The [2+2]-photocycloaddition of aromatic aldehydes and ketones to 3,4-dihydro-2-pyridones: Regioselectivity, diastereoselectivity, and reductive ringopening of the product oxetanes

3,4-Dihydro-2-pyridones [3,4-Dihydropyridin-2(1H)-ones] 6 were evaluated wi th respect to their use as alkene components in stereoselective Paterno-Buc hi reactions. The parent compound 6a was shown to be a versatile synthetic building block that reacted with various photoexcited aromatic carbonyl com pounds (benzaldehyde, benzophenone, acetophenone, methyl phenylglyoxylate, 3-pivaloyloxybenzaldehyde) with high regio- and diastereoselectivity (51-63 % yield). The products can be subjected to hydrogogenolysis, new and effici ent route for the synthesis of 2-arylmethyl-3-piperidinols. As examples, th e oxetanes 7a and 8a were hydrogenolytically cleaved and yielded the produc ts 12 (88%) and 13 (93%). The ability of compound 6a to bind to a chiral la ctam host through two hydrogen bonds was used favorably to differentiate th e enantiotopic faces of its double bond. In the photocycloaddition to the c hiral aldehyde 15, which was conducted at -10 degreesC in toluene, a high f acial diastereoselectivity (> 90% de, 56% yield) was recorded. The stereose lectivity results from a 1:1 association of dihydropyridone 6a to the aldeh yde. The 4-substituted dihydropyridones 6b-6d (R = methyl, isopropyl, pheny l) were found to be less suited for potential use in photochemistry. The yi elds and facial diastereoselectivities recorded in their photocycloaddition to benzophenone remained low.