Evidence of the exo-Anomeric Stereoelectronic Effect in Orthoester Aminals from X-Ray Structural Data. Crystal Structures of Three 2-Amino-1,3-oxazolin-4-one derivatives

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 structures has shown that exo-cyclic 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.