| Abstract
| - We have investigated several 1,3-dipolar cycloadditions of a chiral nitrone prepared froml-erythrulose. While cycloadditions to carbon−carbon multiple bonds of dipolarophiles such as ethylacrylate, ethyl propiolate, or dimethyl acetylenedicarboxylate were poorly stereoselective, reactionwith acrylonitrile provided predominantly one diastereomeric adduct. Furthermore, the regioselectivity exhibited by the two structurally similar dipolarophiles ethyl acrylate and ethyl propiolatewas found to be opposite. The molecular mechanisms of these cycloadditions have thus beeninvestigated by means of density functional theory (DFT) methods with the B3LYP functional andthe 6-31G* and 6-31+G* basis sets. A simplified achiral version of nitrone 1 as the dipole, andmethyl propiolate or acrylonitrile as the dipolarophiles, were chosen as computational models. Thecycloadditions have been shown to take place through one-step pathways in which the C−C andC−O σ bonds are formed in a nonsynchronous way. For the reaction with methyl propiolate, DFTcalculations predict the experimentally observed meta regioselectivity. For the reaction withacrylonitrile, however, the predicted regioselectivity has been found to depend on the computationallevel used. The calculations further indicate the exo approach to be energetically favored in thecase of the latter dipolarophile, in agreement with experimental findings. The main reason for thisis the steric repulsion between the nitrile function and one of the methyl groups on the nitronethat progressively develops in the alternative endo approach.
|
