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

3,4-Dihydro-2-pyridones [3,4-Dihydropyridin-2(1H)-ones] 6 were evaluated with respect to their use as alkene components in stereoselective Paternò-Büchi reactions. The parent compound 6a was shown to be a versatile synthetic building block that reacted with various photoexcited aromatic carbonyl compounds (benzaldehyde, benzophenone, acetophenone, methyl phenylglyoxylate, 3-pivaloyloxybenzaldehyde) with high regio- and diastereoselectivity (51-63% yield). The products can be subjected to hydrogenolysis, opening a new and efficient route for the synthesis of 2-arylmethyl-3-piperidinols. As examples, the oxetanes 7a and 8a were hydrogenolytically cleaved and yielded the products 12 (88%) and 13 (93%). The ability of compound 6a to bind to a chiral lactam host through two hydrogen bonds was used favorably to differentiate the enantiotopic faces of its double bond. In the photocycloaddition to the chiral aldehyde 15, which was conducted at - 10°C in toluene, a high facial diastereoselectivity (> 90% de, 56% yield) was recorded. The stereoselectivity results from a 1:1 association of dihydropyridone 6a to the aldehyde. The 4-substituted dihydropyridones 6b-6d (R = methyl, isopropyl, phenyl) were found to be less suited for potential use in photochemistry. The yields and facial diastereoselectivities recorded in their photocycloaddition to benzophenone remained low.