By cyclization of N-unsubstituted, and N1- or N3-methyl-substituted cis- and trans-2-(aminomethyl)cyclohexylamines with phenylphosphonic dichloride, phenyl dichlorophosphate and bis(2-chloroethyl)phosphoramidic dichloride, P epimeric diastereomers a and b of the corresponding 1,3-unsubstituted and 1- or 3-methyl-substituted 2-phenyl-, 2-phenoxy- and 2-[bis(2-chloroethyl)amino]decahydro-1,3,2-benzodiazaphosphinine 2-oxides have been synthesized. The stereochemistry and conformations of the prepared saturated 1,3,2-benzodiazaphosphinine 2-oxides were determined by 1H, 13C and 31P NMR spectroscopy aided by DFT geometry optimizations and J-coupling-constant calculations for selected structures. The 2-phenoxy-substituted trans-fused derivatives with an equatorial phenoxy group in a hypothetical double-chair conformation (14a, 16a, 18a) were observed to adopt nonchair heteroring conformations with a pseudoaxial P–OPh bond instead. The corresponding b epimers, as well as the trans-fused 2-phenyl and 2-[bis(2-chloroethyl)amino] derivatives (both P epimers) were found to retain double-chair conformations. In the cis-fused series, the position of the N-in/N-out equilibrium was dependent on the steric and stereoelectronic requirements of the phosphorus substituents (and thus on the phosphorus configuration) as well as on the substituents at the ring nitrogen atoms. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)