Evidence for the exo-anomeric stereoelectronic effect in cyclic orthoester aminals from X-ray structural data. Crystal structures of three 2-amino-1,3-oxazolidin-4-one derivatives

Abstract

The crystal structures of three 2-amino-1,3-oxazolidin-4-one derivatives are reported. 5,5-Dimethyl-2-dimethylamino-3-phenyloxazolidin-4-one crystallizes in the space group P2₁,/c with a= 15.540(3), b= 7.016(2), c= 12.491(4)Å, β= 105.6(2)° 1 342 reflections with I 2.5 σ(I) were used in the refinement. 4-Benzyl-2,2-dimethyl-1-oxa-4,6-diazaspiro[4,5]decan-3-one crystallizes in the space group P2₁/c with a= 15.188(6), b= 12.074(2), c= 8.468(1)Å, β= 100.03(2)° 2 068 reflections with I 3σ(I) were -used in the refinement. 4-Benzyl-2,2-dimethyl-1-oxa-4,6-diazaspiro[4,6]undecan-3-one crystallizes in the space group P2₁,/c with a= 9.460(5), b= 20.192(6), c= 9.012(3)Å, β= 110.66(4)° 1 873 reflections with I 3σ(I) were used in the refinement. The analysis of bond distances of the three structure has shown that exo-cylic C–N bonds (exo with respect to the oxazolidinone ring) are significantly shorter than the standard C(sp³)–N(sp³) distance by 0.050 Å on average. This could be the main factor determining the stability of the three cyclic orthoester aminals, the stabilization being attributable to an exo-anomeric effect involving the interaction between the nitrogen sp³-like lone pair, in an antiperiplanar orientation, and the σ*orbital of the endo C–O bond.