Photosensitized Intramolecular [2+2] Cycloaddition of 1H-Pyrrolo[2,3-b]pyridines Enabled by the Assistance of Lewis Acids
2020
Arai, Noriyoshi | Ohkuma, Takeshi
The [2+2] photocycloaddition of alkenyl-tethered 1H-pyrrolo[2,3-b]pyridine derivatives sensitized with 3′,4′-dimethoxyacetophenone under irradiation by a high-pressure mercury lamp through Pyrex glass was dramatically accelerated by the addition of Lewis acids, preferably Mg(OTf)₂, to give the products stereoselectively in high yields. The reaction without a Lewis acid gave only small amounts of the [2+2] cycloaddition products. Conformational fixation of the substrates by coordination with a Lewis acid was presumed to facilitate the cycloaddition.
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