Photoaddition Reactions of Acetylpyridines with Silyl Ketene Acetals: SET vs [2 + 2]-Cycloaddition Pathways

摘要

Photoaddition reactions of silyl ketene acetalsnwith 2-, 3- and 4-acetylpyridine have been explored. Thenresults show that the acetylpyridines react with an electronnrich, dimethyl-substituted silyl ketene acetal via a pathway innwhich excited state single electron transfer (SET) takes placento produce β-hydroxyesters in high yields. In contrast,nphotochemical reactions of the acetylpyridines with annelectron deficient, nonmethyl-substituted silyl ketene acetalngenerate oxetanes as major products, which arise via a routeninvolving excited state [2 + 2]-cycloaddition. In addition, annincrease in solvent polarity significantly enhances the relativenefficiencies of the SET processes versus [2 + 2]-cycloaddition reactions. Importantly, the carbonyl groups rather than thenpyridine moieties in the acetylpyridine substrates participate in both types of addition reactions. Finally, the results demonstratenthat photoinduced electron transfer (PET)-promoted chemical reactions between acetylpyridines and electron rich silyl ketenenacetals in polar solvent serve as useful methods to promote β-hydroxyester forming, Claisen or Mukaiyama condensationnreactions under mild conditions.