Efficient photochemical synthesis of 2-vinylcyclopropanecarbaldehydes, precursors of cyclopropane components present in pyrethroids, by using the oxa-di-π-methane rearrangement

A comparative study on the triplet photoreactivity of a series of β,γ,δ,ɛ-unsaturated aldehydes 14a–f and 15, using 3-methoxyacetophenone and 4-phenylbenzophenone as sensitizers, has been carried out. When 3-methoxyacetophenone is used as the triplet sensitizer, the aldehydes undergo oxa-di-π-methane rearrangement (ODPM) to afford the corresponding cyclopropanecarbaldehydes 16a–f and 17 in yields ranging from 14% to 62% in addition in most cases to products resulting from decarbonylation. However, when 4-phenylbenzophenone is used as the photosensitizer ODPM products are obtained exclusively in almost quantitative yield. These are additional examples of the recently described, unexpected influence of the nature of the triplet sensitizer on the photoreactivity of organic compounds.