Transition-state model thiol

The preference of the (5, .S )-boron cnolatc to attack almost exclusively the Si-face of an aldehyde is rationalized by assuming the Zimmerman-Traxler transition state model. It is postulated that the methyl group of the propyl residue directs the 3-elhylpenlane-3-thiol group towards the borolane moiety, the chirality of which is thus effectively transferred34. 

Beckwith pioneered the use of a Thiol-Oxygen-Co-Oxidation (TOCO) process for the transformation of 1,4-dienes and 1,3,6-trienes to 1,2-dioxolanes [90], As illustrated by the example in Scheme 51 [90a], this process involves phenylthio radical addition to the least substituted double bond, oxygen entrapment, peroxyl radical cyclization, oxygen entrapment and hydrogen atom transfer from the thiol. In accord with the Beckwith-Houk transition state model [91, 92], cyclization provides preferentially the c/s-3,5-disubstituted 1,2-dioxolanes. 

Cysteamine (HSCH2CH2NH2) has been found to add to Michael acceptors, such as thujone, in dimethylsulfoxide (DMSO). By contrast, no reaction occurred in nonpolar solvents. NMR spectroscopy was employed to identify good Michael acceptors and sort them into reversible and irreversible thiol sinks with a view of developing a cellular assay for thiol-trapping agents. In another paper, calculated transition-state energies of the reaction of Michael acceptors with MeSH have been used as model system to asses thiol toxicity in aqueous media. ... 

Dmuchovsky, Vineyard and Zienty observed a quite unusual inverse isotope effect for k jkj of 0-65 for the base catalysed addition of n-butane-thiol-S-dj to maleic anhydride. While inconsistent with any model of a transition state involving S—H bond cleavage, the inverse isotope effect could be accounted for by postulating a pre-reaction equilibrium between butanethiol and triethylamine, much h ke the one in equations (10) and (11). In fact, substitution into equations (16) and (17) of 2566 and 1850 cm for the S—H and S—D stretching frequency, respectively, and 3253 and 2380 cm for the N—H and N—D stretches of the amine-thiol complex, yields an equilibrium isotope effect of 0-68 . 

Yet another full paper from the Cram group details the catalysis of transacylation of the 4-nitrobenzoate esters of several a-amino-acid salts by the thiol-crown (100), and the extent of chiral discrimination shown by the (5)-catalyst in favour of L-amino-acid substrates. These results are rationalized in terms of a model for preferred transition-state complexation cf. 1, 422). 

The previous section demonstrated that chiral macrocyclic polyether hosts discriminate in complexation reactions in chloroform solution between enantiomers of amino ester salt guests. With these results can we go one step further and mimic a catalytic site We will now describe the design of a host that upon complexation with a-amino ester salts produces a transition state intermediate corresponding to a transacylation (thiolysis) reaction between the chiral host catalytic group (thiol) and the enantiomeric guest salts (143). However, it should immediately be realized that these model systems mimic only the acylation step encountered in serine protease catalysis. So far no acceleration in rate has been observed for the deacylation step. 

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