The influence of 4-alkyl substituents on the formation and reactivity of 2-methoxy-quinone methides: evidence that extended pi-conjugation dramatically stabilizes the quinone methide formed from eugenol.
The effects of para-alkyl substituents on both the cytochrome P450-catalyzed oxidation of phenols to quinone methides (QMs; 4-methylene-2,5-cyclohexadien-1-ones), and on the rates of nucleophilic additions to the QMs were investigated. The derivatives of 4-alkyl-2-methoxyphenol studied were 4-methyl (creosol), 4-ethyl, 4-propyl, 4-isopropyl, and 4-allyl (eugenol). The relative reactivities of QMs derived from these phenols with water were 4-methyl > 4-ethyl = 4-propyl > 4-isopropyl > 4-allyl. These variations in rate were rationalized by differences in stabilization of positive charge density at the site of nucleophilic attack. In particular, saturation of the vinyl substituent on eugenol-QM increases the solvolysis rate 100-fold. This effect is presumably due to the loss of the contribution of an additional aromatic resonance structure to the overall resonance hybrid of the QM from 2-methoxy-4-propylphenol. Finally, the kinetic results show that there is a 472-fold difference in reactivity within this series of QMs. The QM glutathione conjugates were synthesized and characterized by 1H-NMR and electrospray mass spectrometry and a HPLC assay was developed to quantify QM formation in rat liver microsomes. The general trend is increasing alkyl substitution at the para position results in more QM; however, in contrast to the large range of reactivities of the QMs observed in the kinetic experiments, the amounts of P450-derived QM GSH adducts varied only by a factor of 3. In particular, similar amounts of the QMs from eugenol and 2-methoxy-4-propylphenol were produced which suggests that the lack of reported hepatotoxicity for the latter phenol in mice depleted of GSH, may be due to the extreme reactivity of 4-propyl-QM that would be rapidly detoxified by hydrolysis. These data suggest that there may be a threshold cytotoxicity level for QMs related to their reactivity which may affect the relative toxicities of 4-alkyl-2-methoxyphenols.
Bolton
J L
JL
Department of Chemistry, Queen's University, Kingston, Ontario, Canada.
Comeau
E
E
Vukomanovic
V
V
D000478:Alkylation; D000818:Animals; D002851:Chromatography, High Pressure Liquid; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D005054:Eugenol / Q000737:chemistry / Q000378:metabolism*; D005978:Glutathione / Q000378:metabolism; D066298:In Vitro Techniques; D045563:Indolequinones; D007211:Indoles / Q000737:chemistry / Q000378:metabolism / Q000633:toxicity*; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D008297:Male; D013058:Mass Spectrometry; D008862:Microsomes, Liver / Q000187:drug effects / Q000201:enzymology*; D010084:Oxidation-Reduction; D011809:Quinones / Q000737:chemistry / Q000378:metabolism / Q000633:toxicity*; D051381:Rats; D017207:Rats, Sprague-Dawley; D013056:Spectrophotometry, Ultraviolet; D013329:Structure-Activity Relationship
D045563:Indolequinones; D007211:Indoles; D011809:Quinones; C068040:quinone methide; D005054:Eugenol; D003577:Cytochrome P-450 Enzyme System; D005978:Glutathione