Influences of alpha-substituent in 4,5-dimethoxy-2-nitrobenzyl-protected esters on both photocleavage rate and subsequent photoreaction of the generated 2-nitrosophenyl ketones: A novel photorearrangement of 2-nitrosophenyl ketones

• Alpha-substituted 2-nitrobenzyl esters form multiple by-products upon photodeprotection.
• 2-Nitrosoketones isolated after deprotection reaction formed similar photoproducts.
• Isopropyl substitution gave bicyclic oxazole as a major photorearrangement product.
• Tertiary-butyl substitution gave azoxy and azo compounds as photoirradiation products.
• Nature of α-substituent affects photodeprotection rate and types of photo-products.

Ultraviolet(UV)-induced photodeprotection of 2-nitrobenzyl esters to release the acid with formation of the corresponding 2-nitrosoketone proceeds rapidly when the α-position in the 2-nitrobenzyl group is substituted by a branched alkyl group. The generated 2-nitrosophenyl ketones undergo multiple photoreactions, including a unique photorearrangement, depending upon the nature of the α-substituent. 2-Nitrosoketone bearing an isopropyl substituent mainly undergoes this rearrangement to afford a bicyclic oxazole via a shift of the isopropyl group to the bridgehead position, resulting in loss of aromaticity of the six-membered ring. 2-Nitrosopheyl ketone bearing a tertiary-butyl substituent gives mainly azoxy and azo compounds via intermolecular reaction of nitrosoketones with loss of isobutene. The photorearrangement does not proceed in the case of the phenyl-substituted compound. These findings will be helpful for the selection and/or design of photolabile protecting groups.

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