Photogeneration of amines from α-keto carbamates: design and preparation of photoactive compounds

Abstract

The design and synthesis of substituted desyl (2-oxo-1,2-diphenylethyl) groups has been investigated to create new photolabile protecting groups. The photoreactivity of these chromophores stems from the diverse photochemistry of the desyl group. Several chromophore designs have been explored in which the substitution pattern of the parent desyl chromophore was varied systematically. The required benzoin chromophores are prepared by a variety of synthetic routes, depending on the structure of the benzoin chromophore desired. Symmetrical benzoins are readily available via the benzoin condensation. Unsymmetrical benzoins including 2,2-disubstituted α-hydroxy ketones are generally prepared via trimethylsilyl (TMS) masked cyanohydrins. On reaction with a Grignard reagent, the TMS masked cyanohydrin functions as an α-hydroxycarbonyl equivalent to form α-hydroxy ketones. Alternatively, lithiation of a TMS masked cyanohydrin generates a benzoyl anion equivalent which reacts with aldehydes and ketones to generate substituted benzoins. These desyl chromophores have significant potential as new photolabile protecting moieties for a variety of functional groups and are used to mask primary and secondary amines as photosensitive α-keto carbamates. The substituted benzoin carbamates are readily prepared from the appropriate benzoin by reaction with isocyanates or by activation as a mixed carbonate followed by reaction with the free amine. These α-keto carbamates are interesting for two main reasons. First, the facile synthesis of these materials indicates the ease of introduction of the desyl based photolabile group. Second, these α-keto carbamates may be used for rapid evaluation of novel photoactive desyl based chromophores.