Photochemically removable silyl protecting groups

Several o-phenol-containing alkoxyvinylsilanes were prepared and their photochemistry was investigated. These materials were prepared via hydrosilylation of the corresponding o-acetoxy arylacetylenes. Two major classes of photochemical processes were identified in these reactants: trans-->cis isomerization, leading to an intramolecular nucleophilic substitution process at silicon, and 1,5-silyl shift, leading to an unsymmetrical dialkoxysilane. The major outcome of this work is a novel class of photochemically removable protecting groups. Two alkyl substitutions on silicon, the dimethyl and diisopropyl, were examined. The latter is more stable and is preferred for protecting groups that must tolerate multiple steps or reagents. Protection of alcohols is generally performed starting with the arylethynyl acetate, which can be subjected to hydrosilylation, alcohol substitution, and acetate deprotection without isolation of intermediates. Two groups were studied in detail, the phenol and 2-naphthol vinyl silane derivatives. A variety of primary and secondary alcohols were protected with these reagents. These groups can be deprotected cleanly and in high yield by irradiation from 250 to 350 nm.