AbstractNitro‐substituted 1,3‐dioxanes and pyrans with potential biological activity were synthesized. The relative configurations of thethreoanderythroisomers of 2‐nitro‐1‐phenylpropane‐1,3‐diol (the starting materials) and of 5‐nitro‐2,4‐diphenyl‐1,3‐dioxane, the products formed with benzaldehyde, were determined by1H and13C NMR spectroscopy. Cinnamaldehyde and nitroethanol give a homologue of nitrophenylpropanediol, which exists as a mixture of two diastereomeric cyclic hemiacetals. After dissolving in CDCl3, the ratio of these components changes from approximately 10:1 to 3:2 in a few days. The ethyl acetal of the hemiacetal is stereohomogeneous, and the ethoxy group is axial to the pyran ring owing to the anomeric effect. This configuration was proved by two‐dimensional NMR measurement, permitting separate observation of the originally overlapping multiplets and direct reading of t