Tertiary alkoxyl radicals from 3-alkoxythiazole-2(3 H)-thiones

Abstract

This study deals with the synthesis of tert- O-alkyl thiohydroxamates and their use as tert-alkoxyl radical precursors. tert-Alkoxyl radicals were applied in mechanistic studies to determine rate constants of (i) p-chlorocumyloxyl radical addition to bicyclo[2.2.1]heptene ( k=1×10 7 M −1 s −1), (ii) 2-phenylhex-5-en-2-oxyl radical 5- exo- trig-cyclization ( k cis=3×10 9 s −1, k trans=1×10 9 s −1), and (iii) 2-methyl-5-phenylpent-2-oxyl to 2-hydroxy-2-methyl-5-phenylpent-5-yl radical isomerization (1,5-H-atom shift; k=0.4–1.5×10 8 s −1). The reactions pose key steps in synthesis of 2,2,5-substituted tetrahydrofurans and 2-bromo-3-alkoxybicyclo[2.2.1]heptanes. Stereoselectivity in 5- exo- trig cyclization (2,5-cis) and intermolecular addition ( exo/endo>99:1), originates from torsional strain in transition structures of alkoxyl radical reactions.