Synthesis and Aqueous Chemistry of α-Acetoxy-N-nitrosomorpholine:  Reactive Intermediates and Products

Abstract

[reaction: see text] Alpha-acetoxy-N-nitrosomorpholine (7) has been synthesized starting by the anodic oxidation of N-acetylmorpholine in methanol. The 55% yield of N-nitrosomorpholinic acid, after cyanide-for-methoxy group exchange and hydrolysis, is an improvement of approximately 10-fold over our original 10-step method, and this is readily converted to 7. A study of the kinetics of decomposition of 7 in aqueous media at 25 degrees C and 1 M ionic strength was conducted over the pH range from 1 to 12. The reaction exhibited good first-order kinetics at all values of pH, and a plot of the log of k0, the buffer-independent rate constant for decomposition, against pH indicated that a pH-independent reaction dominates in the neutral pH region whereas acid- and base-catalyzed reactions dominate in the low and high pH regions, respectively. Reaction at neutral pH in the presence of increasing concentrations of acetate ion results in a decrease in the value of k(obsd), to an apparent limiting value consistent with a common-ion inhibition by the capture, and competing base-catalyzed hydration of, an N-nitrosiminium ion intermediate. The 100-fold smaller reactivity of 7 at neutral pH compared with its carbon analogue, alpha-acetoxy-N-nitrosopiperidine, is also consistent with the electronic effects expected for such a reaction. The dinitrophenylhydrazones derived from pH-independent and acid-catalyzed reactions are identical in kind and quantity, within experimental error, to those observed in the decay of alpha-hydroxy-N-nitrosomorpholine. Decay of 7 in the presence of benzimidazole buffer results in the formation of 2-(2-(1H-benzo[d]imidazol-1-yl)ethoxy)acetaldehyde (12) and 2-(1H-benzo[d]imidazol-1-yl)ethanol (13). Independent synthesis and study of 12 indicates that it is stable at 80 degrees C in 0.1 M DCl, but it slowly decomposes to 13 in neutral and basic media in a reaction that is stimulated by primary and secondary amines, but not by tertiary amines and carbonate buffer. The benzimidazole trapping studies and those of the stability of 12 indicate the possibility that metabolic activation of N-nitrosomorpholine by hydroxylation alpha to the nitroso nitrogen can result in the deposition of a metastable ethoxyacetaldehyde adduct on the heteroatoms of DNA.

Products of the Direct Reaction of the Diazonium Ion of a Metabolite of the Carcinogen N-Nitrosomorpholine with Purines of Nucleosides and DNA

“…Values of k obsd for the EA adduct attached to the exocyclic amino group of Gua, N 2 -EA-Gua, could not be measured due to irregularities in peak shape possibly due to the equilibrium between the hydrated aldehyde/aldehyde/alkene forms that could not be separated. Finally, for comparison, the rate constants were also measured for the EA adduct of benzimidazole, whose chemistry has been previously characterized, and these values are also included in Table .…”

“…While decomposition to the HE-nucleobase was detected by mass spectrometry to the extent of 1−3% during acid hydrolysis of the dimethylacetals, the kinetics of decay studied at neutral and basic pH indicated that the stability of the EA moiety attached to the purines is comparable to that when attached to benzimidazole. In an earlier study, it was noted that the EA-fragment attached to benzimidazole was quite stable at neutral pH, but its decay was accelerated in basic media and in the presence of primary and secondary amines . Thus, the possibility that the pendant amines of the purines might destabilize the EA fragment was anticipated.…”

“…The compounds ( 15 N 2 )-2′-deoxyguanosine , ( 15 N 6 )-2′-deoxyadenosine − , α-hydroperoxy- N -nitrosomorpholine , , 2-(6-amino-7 H -purin-7-yl)ethanol , 2-(9 H -purin-6-ylamino)ethanol , 2-(6-amino-3 H -purin-3-yl)ethanol , 2-(2-amino-9 H -purin-6-yloxy)ethanol , 2-amino-7-(2-hydroxyethyl)-1 H -purin-6(7 H )-one , 9-((2 R ,4 R ,5 R )-4-hydroxy-5-(hydroxymethyl)-tetrahydrofuran-2-yl)-2-(2-hydroxyethylamino)-1 H -purin-6(9 H )-one , and 2-amino-9-((2 S ,4 S ,5 S )-4-hydroxy-5-(hydroxymethyl)-tetrahydrofuran-2-yl)-1-(2-hydroxyethyl)-1 H -purin-6(9 H )-one , were synthesized as described in the references cited and exhibited 1 H NMR and 13 C NMR spectra consistent with either what was previously reported or, in the absence of such, the structure indicated. Hydroxyethylated nucleobases from the last two compounds were liberated by acid hydrolysis and purified by neutralization and recrystallization in water.…”

“…Notwithstanding, it was recently established that the diazonium ion derived from α-hydroxyNMOR deposits an ethoxyacetaldehyde (EA) on the “model nucleobase” benzimidazole presaging actual EA-nucleobase adducts, eq . It was additionally demonstrated that the EA-benzimidazole adduct slowly decays to a hydroxyethyl (HE) adduct and that this decay is accelerated by primary and secondary amines or base (eq ).…”

Reactions of Aldehydes and Ketones and their Derivatives

“…58 It decomposes cleanly, with first-order kinetics, via an N -nitrosoiminium ion intermediate (23), with the pH-rate profile showing acid-and basecatalysed regions, and an extensive pH-independent region between 3 and 9, the latter being ca 100 times slower than its C-analogue, the corresponding piperidine derivative. Implications for the interaction of N -nitrosomorpholine with DNA are discussed.…”