On the structure of 3-acetylamino-5-methyl- 1,2,4-oxadiazole and on the fully degenerate rearrangements (FDR) of its anion: a stimulating comparison between the results of 'in-silicon chemistry' and 'laboratory chemistry't

摘要

An accurate crystal structure determination has provided evidence for a planar conformation for 3-acetylamino-5-methyl-1,2,4-oxadiazole (5), in agreement with quantum-mechanical calculations in the gas phase. In the crystal, aseries of strong intermolecular N7H7 '09 hydrogen bonds link the amido groups of different molecules, causing theformation of infinite parallel ordered chains. The effect of the DMSO solvent on the energy and charge distribution ofcompound 5 and on its relevant 5- anion, involved in a fully degenerate rearrangement (FDR), has been deepened byquantum-mechanical DFT calculations. The calculated energy barrier for the FDR increases in going from in vacua toDMSO, in agreement with previsions based on the Hughes and IngoId rules concerning the nucleophilic substitutionof an anionic reagent (the deprotonated amido group in the side chain) on a neutral substrate (the 1,2,4-oxadiazolering).