- Internal amine-assisted attack of alcoholic hydroxy group on esters: Serine esterase models
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o-(N,N-Dimethylaminomethyl)benzyl alcohol (o-DBA) shows extensive internal hydrogen bonding in aqueous solvent as evidenced by the following observations: (i) The ionization constant of protonated o-DBA is more than two times larger than that of protonated o-(N,N-dimethylaminomethyl)benzyl methyl ether (o-DBMEH+), (ii) The second-order rate constant for the cleavage of p-nitrophenyl acetate (PNPA) catalysed by o-DBA is more than 150 times larger than that catalysed by o-DBME.Similar observations are obtained in the cleavage of phenyl acetate (PA) also.The estimated contributions to the catalytic cleavage of PNPA and PA by the internally hydrogen bonded form of o-DBA are ca. 45 and 48 times, respectively, larger than the respective contributions by nonhydrogen bonded zwitterionic form of o-DBA.The deuterium oxide solvent isotope effect on the reactivity of o-DBA toward PNPA reveals that proton transfer is not important in the rate-determining step.The expulsion of leaving group (ArO-) from the zwitterionic tetrahedral addition intermediate formed due to the attack of nucleophile at ester carbonyl group is considered to be the rate-determining step.Both the pK and rate studies on the reactions of a few other amino alcohols and their methyl esters with PNPA do not show the presence of internal hydrogen bonding in these amino alcohols.Second-order rate constants for the reactions of PNPA with six tertiary amines of comparable structural features reveal a Bronsted plot of slope (βnuc) of 0.70 +/- 0.26.
- Hine, Jack,Khan, Mohammad Niyaz
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p. 427 - 435
(2007/10/02)
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- Modulation of cholinergic profile of acetylcholine through its cyclovinylogues
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The o-. m- and p-acetoxymethyl-N,N,N-trimethylbenzenemethanaminium iodide (2a-c) as cyclovinilogues of acetylcholine are described. This structural modification allows to modulate the cholinergic activity of the natural neurotransmitter along the sequence
- Gaion,Grion,Montanari,Valenti,Da Re
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p. 1371 - 1380
(2007/10/02)
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- Pyridyl-substituted imidazoles, compositions containing same and methods of use thereof
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Novel imidazole of the formula: STR1 wherein Ring A is pyridyl group or a substituted pyridyl group; Ring B is phenyl group or a substituted phenyl group; R1 and R2 are hydrogen atom or are combined together to form a group of the formula: --(CH2)q --; m is 1 or 2; n is 0, 1 or 2; and q is 3 or 4, or a salt thereof are disclosed. Said derivative (I) and a salt thereof are useful as anti-ulcer agents.
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