USE OF 2-DIMENSIONAL NMR AND MOLECULAR MODELING FOR THE STRUCTURE DETERMINATION OF NOVEL CYCLOPHOSPHAMIDE DERIVATIVES - DIASTEREOMERS OF 1-AZA-2-BIS(2-CHLOROETHYL)AMINO-3-OXA-2-OXO-2-PHOSPHA-7-THIABICYCLO[4.4.0]DECANE AND 1-AZA-2-BIS(2-CHLOROETHYL)AMINO-3-OXA-2-OXO-2-PHOSPHA-7-THIABICYCLO[4.3.0]NONANE

High-field NMR studies at 11.7 T of newly synthesized diastereomeric 1-aza-2-bis(2-chloroethyl)amino-3-oxa-2-oxo-2-phospha-7-thia-bicyclo[4.4.0]decanes (4, 5) and the corresponding [4.3.0] nonanes (6, 7) were carried out by employing a combination of one- and two-dimensional homo- and heteronuclear methods. Unambiguous P-31, H-1 and C-13 chemical shift assignments for these bicyclic derivatives of cyclophosphamide were obtained, and the spin-spin couplings involving H-1, C-13 and P-31 were analysed in detail to determine the stereochemistry. Particularly useful were H-1 J-resolved experiments, which separated phosphorus-proton couplings from proton-proton couplings, and H-1-C-13 shift-correlation experiments, which resolved the carbon-phosphorus couplings and provided information on the relative signs of J(P,H) and J(P,C). In addition, extensive molecular modelling calculations using various force-field (ALCHEMY, DISCOVER) and molecular orbital methods (MNDO, AM1, PM3) were carried out, and improved parameters for cyclophosphamides were developed for ALCHEMY. The diastereomeric pairs 4, 5 and 6, 7 differ in the axial vs. equatorial orientation of the sulphur substituent on the oxazaphosphorinane ring, which is itself exclusively in a chair conformation with axial P=O for all substances.