Photochemistry of 2-Alkoxymethyl-5-methylphenacyl Chloride and Benzoate

Abstract

Irradiation of 2-(alkoxymethyl)-5-methyl-alpha-chloroacetophenones (1a-c) and 2-(methoxymethyl)-5-methylphenacyl benzoate (1d) in dry, nonnucleophilic solvents afforded 3-alkoxy-6-methylindan-1-ones (3a-c) in very high chemical yields. 3-Methylisobenzofuran-1(3H)-one (2) was, however, isolated as a major photoproduct in the presence of trace amounts of water. Quenching experiments and laser flash spectroscopy revealed that the indanone derivatives 3 are formed by 1,5-hydrogen migration from the lowest triplet excited state of the acetophenones 1 and cyclization of the resulting photoenols. In contrast, production of the lactone 2 in wet solvents was found to result from two consecutive photochemical transformations. The photoenols produced by photolysis of 1a-c add water as a nucleophile to form 2-acetyl-4-methylbenzaldehyde (4), which is further converted to 2 via a second, singlet state photoenolization process. Exhaustive photolysis of 1a in methanol produced the acetal 2-(dimethoxymethyl)-5-methylacetophenone (7a) as the exclusive product. The remarkable selectivity of these photoreactions may well be useful in synthetic organic chemistry.

Spin‐Center Shift (SCS) – A Versatile Concept in Biological and Synthetic Chemistry

“…Therefore, this system should be suitable for novel photocleavable protecting groups48 for phosphates and alcohols. Indeed, Klán and Wirz and their co‐workers recently demonstrated that 2,5‐dimethylphenacyl esters49a–49d and carbonates 49e are excellent protecting groups for carboxylic acids, phosphates, sulfonates, alcohols, and phenols. The use of 2‐methylphenacyl esters as photocleavable linkers in solid‐phase synthesis has also been reported.…”

The photoformation of a phthalide: a ketene intermediate traced by FSRS

“…6 A ketene intermediate was indeed observed via IR spectroscopy in a matrix isolation experiment. Time resolved spectroscopy on oPA and related compounds 15,17,18 lacked the temporal resolution to trace the ketene formation. Time resolved spectroscopy on oPA and related compounds 15,17,18 lacked the temporal resolution to trace the ketene formation.…”

“…We have opted for oABA instead of oPA since for the latter at least two rotamers (denoted as EZ and EE) coexist. 18 The computations by Blancafort et al on oPA did not take intersystem crossing (ISC) to triplet states into account. 15 For sterical reasons, in oABA the rotamer depicted in Scheme 1 is expected to predominate.…”

Effect of an Extra Chloro Substituent on Photochemistry of o-Alkylphenacyl Chloride

“…The latter reaction has recently been reported by Klan and coworkers, although only the nonnucleophilic solvents provided the cyclization products in their studies. 18 A final feature to note in photolysis of DCMA in methanol is the formation of a solvolysis product, H. A control experiment in the dark clearly showed that the reaction was light-driven, which differs from the classical haloform reaction. Our literature searches found that similar observations had been made in photolysis of α,α,α-trichloroacetophenone 19 and α,α-dichloropropiophenone.…”