The concentration dependence of the diastereoselectivity of the Paternò- Büchi reaction of a series of cyclic enolethers, cyclooctene, allylic alcohols and acetates, respectively, with aromatic as well as aliphatic aldehydes was studied. For most aliphatic aldehydes, a sharp transition from low to high diastereoselectivity was observed, indicating a distinct switch from singlet to triplet photocycloaddition with different selectivity controlling mechanisms. Furthermore, the effect of solvent viscosity and temperature on the spin- directed stereoselectivity of the carbonyl-ene photocycloaddition was investigated. The variation of the solvent viscosity over a large range resulted in a weak but significant increase in the endo-selectivity of triplet benzaldehyde cycloaddition to 2,3-dihydrofuran from 82 to 91 %. For aliphatic aldehydes, the diastereoselectivity strongly increased with increasing solvent viscosity. The temperature dependence of the endo/exo- selectivity with aliphatic aldehydes RCHO (R = Me, Et, i-Bu) showed characteristic non-linear behavoir with inversion points from which activation parameters for singlet as well as the triplet photocycloaddition were determined. 5-Methoxyoxazole derivatives were prepared and evaluated with respect to their use as diene components in stereoselective Paternò-Büchi reaction. These oxazoles were versatile synthetic building blocks that reacted with various photoexcited aliphatic as well as aromatic carbonyl compounds with high regioselectvity and excellent exo-diastereoselectvity. Hydrolysis of the primary photoadducts resulted in twofold ring-opening and provided a convenient and high yielding access to erythro (S*,S*) (-amino-(-hydroxy carboxylic acid derivatives
