| Abstract
| - The epoxidation of three stereolabeled methyl-substituted chiral allylic alcohols with 1,2A and/or1,3A allylic strain, namely 3-methylbut-3-en-2-ol (1a), pent-3-en-2-ol (1b), and 3-methylpent-3-en-2-ol (1c), have been studied by the density-functional theory method, B3LYP/6-31+G(d,p). For eachsubstrate we calculated the two prereaction complexes with Ti(OH)4/MeOOH (the oxidant modelfor Ti(O-i-Pr)4/t-BuOOH), their threo and erythro transition states for oxygen transfer, and thecorresponding product complexes. For substrate 1a, the erythro transition state is 0.91 kcal/mol oflower energy than the threo one; for substrates 1b and 1c, the threo compared to the erythrotransition states are by 1.05 and 0.21 kcal/mol more favorable, respectively. The threo/erythroproduct ratios have been estimated from the computed free energies for the competing threo anderythro transition states 3a−c in CH2Cl2 solution to be 12:88 (1a), 92:8 (1b), and 77:23 (1c), whichare in good accordance with the experimental values 22:78 (1a), 91:9 (1b), and 83:17 (1c). Thediastereoselectivity of this diastereoselective oxyfunctionalization is rationalized in terms of thecompetition between 1,3A and 1,2A strain and the electronic advantage for the spiro transition state.In addition, solvent effects are also play a role for the diastereoselectivity at the same time.
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