Identification of new aqueous chemical degradation products of isophosphoramide mustard

NMR ((31)p, H-1 and C-13) spectroscopy was used to study the products of the degradation of isophosphoramide mustard (IPM) in buffered solutions at pH ranging from 1 to 13. At pH Il, the only degradation compounds detected were phosphate ion (Pi) and chloroethylammonium chloride (CEA-HCl), resulting from the breakdown of the two P-N bonds (Pathway Ia). At pH 9.3 and 13, only the products of 1,3-cyclization of the N-chloroethyl group (monoaziridinylIPM (monoAzIPM) and a very low level of bisaziridinylIPM (bisAzIPM)) were found after approximate to 15 h of reaction (pathway II). At intermediate pH, the two pathways coexist. At pH 3.5 and 5.0, the P-N bond hydrolysis is the major pathway, but two final phosphorylated products were detected, Pi which represented 67% (pH 3.5) and 17% (pH 5.0) of all the IPM phosphorylated degradation products after approximate to 15 h of reaction, and phosphorylethanolamine (PEA) which represented 16% (pH 3.5) and 46% (pH 5.0) of the same sum. PEA formation can be explained by the 1,5-cyclization of a transient compound giving a 1,3,2-oxazaphospholidine intermediate whose P-N bond is exclusively cleaved in acidic medium. The presence of monohydroxyIPM (monoOHIPM) (whose percentage increases with pH from 5% (pH 3.5) to approximate to 28% (pH 5.0) of all the IPM phosphorylated degradation compounds), probably coming from the alkylation by water of an aziridine/aziridinium intermediate, demonstrates the occurrence of pathway II. At pH 7.0 and 7.4, the pathway II is initiated first, leading to 1,3-cyclization(s), followed by water alkylation of the aziridines formed. The sequences are IPM 1 --> monoAzIPM 5 --> bisAzIPM 9; IPM 1 --> monoAzIPM 5 --> monoOHIPM 6 --> monoAzIPM with a N-hydroxyethylchain (presumed structure) 7 --> dihydroxyIPM 8. Nevertheless, PEA and Pi are the final products observed, which implies the P-N bond hydrolysis of products 5-9 as demonstrated by the presence in the medium of CEA, aziridine and ethanolamine. (C) 2001 Elsevier Science B.V. Al rights reserved.