SYNTHESIS OF SPECIFICALLY N-15-LABELED AND C-13-LABELED ANTI-TUMOR (2-HALOETHYL)NITROSOUREAS - THE STUDY OF THEIR CONFORMATIONS IN SOLUTION BY N-15 AND C-13 NUCLEAR MAGNETIC-RESONANCE AND EVIDENCE FOR STEREOELECTRONIC CONTROL IN THEIR AQUEOUS DECOMPOSITION

The synthesis of certain specifically 15N, 13C and 2H isotope-labeled 1-(2-chloroethyl)-3-alkyl-1-nitrosoureas (CENU) is described. Spectroscopic examination of CENU and their isotope-labeled counterparts by 1H, 15N and 13C NMR and IR spectra indicates that they adopt preferred conformations in nonpolar aprotic solvents in which the NO group is aligned toward the 2-chloroethyl group. The result is in accord with the conformation of MeCCNU in the crystalline state derived from X-ray diffraction. The chemical shifts and coupling constants in the CENU change with solvent polarity and basicity. In aqueous phosphate buffer there is evidence for the formation of tetrahedral intermediate, the conformation of which alters according to the reaction conditions and ultimately controls the formation of the aqueous decomposition products of CENUs. This is revealed most clearly by 13C NMR of carbonyl-13C- and nitroso-15N-labeled BCNU and CCNU where 2 distinct 15N-coupled 13C doublets with different chemical shifts are observed. The rate of conformational change is comparable with the rate of decomposition of CENUs (vis the 2nd conformer) and may therefore represent the critical initial step of the latter process in vivo. The intermediacy of the postulated tetrahedral intermediates for CENUs is supported by observed 18O exchange into the carbonyl group in 18O-enriched water. Consideration of the conformations of the intermediates and of the alignment of the heteroatom lone pairs provides a satisfactory interpretation of the reactions of CENUs in aqueous solution as well as their pH dependence in terms of strict stereoelectronic control and accounts for the formation of the observed products. The latter analysis requires that available lone pairs on the 2 heteroatoms in a particular tetrahedral intermediate be aligned antiperiplanar to the bond which is cleaved.