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Photoaddition Reactions of Acetylpyridines with Silyl Ketene Acetals: SET vs [2 + 2]-Cycloaddition Pathways
Photoaddition reactions of silyl ketene acetals with
2-, 3- and
4-acetylpyridine have been explored. The results show that the acetylpyridines
react with an electron rich, dimethyl-substituted silyl ketene acetal
via a pathway in which excited state single electron transfer (SET)
takes place to produce β-hydroxyesters in high yields. In contrast,
photochemical reactions of the acetylpyridines with an electron deficient,
nonmethyl-substituted silyl ketene acetal generate oxetanes as major
products, which arise via a route involving excited state [2 + 2]-cycloaddition.
In addition, an increase in solvent polarity significantly enhances
the relative efficiencies of the SET processes versus [2 + 2]-cycloaddition
reactions. Importantly, the carbonyl groups rather than the pyridine
moieties in the acetylpyridine substrates participate in both types
of addition reactions. Finally, the results demonstrate that photoinduced
electron transfer (PET)-promoted chemical reactions between acetylpyridines
and electron rich silyl ketene acetals in polar solvent serve as useful
methods to promote β-hydroxyester forming, Claisen or Mukaiyama
condensation reactions under mild conditions