Influence of HMPA on the stereochemical outcome of the addition of a racemic allenylzinc onto enantiopure N-tert-butanesulfinimines:: Stereoselective access to enantiopure cis-ethynylaziridines

In the presence of 60 equivalents of HMPA, the condensation of the racemic allenylzinc derived from 1-chloro-3-trimethylsilylpropyne onto enantiopure non-a-branched N-tert-butanesulfinimines was proven to give access to the corresponding cis-ethynylaziriclines as the major products. The good cis selectivity observed presumably resulted from a high kinetic resolution with the allenylzinc being partially configurationally labile with respect to the time scale defined by the rate of the reaction. Both single crystal X-ray analysis and semiempirical calculations conducted at the MM2 and AM1 levels of theory proved that the reaction certainly occurred through a preferred synclinal transition state in a supra- or antarafacial S,2' process. In all cases, chromatographic purification over silica gel allowed the cis-ethynyl-N-tertbutanesulfinylaziridines to be obtained as diastereo- and enantiomerically pure compounds (dr > 98:2 and ee > 99%).