Thiols, -hydroxy

This type of tautomerism pertains to azole systems which contain exo-cyclic amino, hydroxy, thiol, and other proton donor groups. 

Cleavage of the oxathiane moiety can be carried out with iV-chlorosuccinimide/silver nitrate and leads to the a-hydroxy aldehyde 21 along with a diastereomeric mixture of sultines 20. The sultines can be reduced to the hydroxy thiol 22 which can be reconverted to the chiral auxiliary 16 in ail overall recovery of about 70%39. 

Another method involves treatment with Lawesson s reagent (see 16-10). When epoxides are substrates, the products are 3-hydroxy thiols. Tertiary nitro compounds give thiols (RNO2 RSH) when treated with sulfur and sodium sulfide, followed by amalgamated aluminum. 

The addition of H2S to an aldehyde or ketone can result in a variety of products. The most usual product is the trithiane (8). ° ot-Hydroxy thiols (5) can be prepared from polychloro and polyfluoro aldehydes and ketones.Apparently, compounds 5 are stable only when prepared from these compounds, and not even for all of them. Thioketones (6) can be prepared from certain ketones, such as diaryl ketones, by treatment with H2S and an acid catalyst, usually HCl. They are often unstable and tend to trimerize (to 8) or to react with air. Thioaldehydes are even less stable and simple ones apparently have never been isolated, though t-BuCHS has been... 

Advances have been achieved in recent years, such as the use of CL reagents as labels to derivatize and sensitively determine analytes containing amine, carboxyl, hydroxy, thiol, and other functional groups and their application in HPLC and CE [35, 36], the synthesis and application of new acridinium esters [37], the development of enhanced CL detection of horseradish peroxidase (HRP) labels [38], the use of immobilization techniques for developing CL-based sensors [39-42], some developments of luminol-based CL in relation to its application to time-resolved or solid-surface analysis [43], and the analytical application of electrogenerated CL (ECL) [44-47], among others. 

Aggarwal et al.108 reported excellent results with the catalytic asymmetric epoxidation of aldehydes. As shown in Scheme 4-52, a series of thioacetals 137 was prepared from hydroxy thiol 136 and the corresponding carbonyl compound. Among them, compound 138, derived from 136 and acetaldehyde, proved to be the best catalyst for asymmetric epoxidation of aldehydes. 

Hydroxy, thiol, and amino groups in pyrimidine exist in tautomeric equilibria with their oxo, thioxo, and imino forms. An amino group in an electrophilic position exists predominantly as such, and the compound is named as an amine. Pyrimidines with a hydroxy or thiol group in an electrophilic position are dominated by the oxo or thioxo forms and are named as such, or with -one or -thione suffixes, if these are the principal groups. In the benzenoid 5-position, these derivatives are mainly present in the hydroxy, thiol, or amino forms and are named as such. Similar considerations apply to the nomenclature of quinazolines and perimidines <1996CHEC-II(6)93>. 

Miller, C., Cuendet, R, and Gratzel, M. (1991). Adsorbed omega-hydroxy thiol... 

Substituents can enhance or reduce reactivity, but the usual substituent effects may not always be apparent in cases where the substituent can exist in a tautomeric modification, e.g., hydroxy, thiol, amino. Thus,... 

Hydroxy Thiols and Thioethers 0-35 Reaction between epoxides and NaSH... 

As discussed in the General Chapter (Section 4.01.5), imidazoles with potential hydroxy, thiol and amino substituents can exist in a variety of tautomeric forms. In contrast to the hydroxypyrazoles which have been studied in detail, comparatively little is known about the corresponding 1,3-diazoles. Many of the imidazoles are not easily accessible synthetically, and they may not be particularly stable e.g. 4-hydroxyimidazoles). Amino derivatives... 

Hydroxy thiol (4) likewise has been used to resolve dimethyl 4-oxocyclopentane-l,2-dicarboxylate by crystallization of the mixture of the derived oxathianes. This provided the (R,R) enantiomer in >99% purity (eq 9). ... 

Reduction and dehydration. The ketone carbonyl group in acetoacetyl ACP is next reduced to an alcohol by NADPH (nicotinamide adenine dinucleotide phosphate), a reducing coenzyme closely related to NADH. Subsequent dehydration of the resulting /3-hydroxy thiol ester (E2 reaction) in step 7 yields crotonyl ACP, and the carbon-carbon double bond of crotonyl ACP is further reduced by NADPH in step 8 to yield butyryl ACP. 

Activation by addition of a carboxylic acid to a triple bond occurs with ethyl ethynyl ether,which forms amides via reactive enol esters. The reaction is catalyzed by mercury(II) oxide under almost neutral conditions. Push-pull alkynes exert higher reactivityThe intermediate enol esters (Scheme 4) rearrange and react with the amino function of a second amino acid. Hydroxy, thiol and imidazole functional groups do not have to be protected. The degree of racemization is low, and yields are good in the case of small peptides. 

Becka A. M. and Miller C. J. (1992), Electrochemistry at co-hydroxy thiol coated electrodes. 3. Voltage independence of the electron tunneling barrier and measurements of redox kinetics at large overpotentials , J. Phys. Chem. 96, 2657-2668. 

Amino acids have a much more diverse set of functional groups than nucleotides. The functional groups of the side chains of the amino acids include carboxylate, carboxamide, imide, hydroxy, thiol, and primary and secondary amines. These functional groups are responsible for the numerous interactions in peptides and proteins. When they are part of large proteins, they display three-dimensional structures. The repertoire may be even larger by forming special active sites. 

The molar fractions of oxo (thiono) and hydroxy (thiol) tautomers for a series of monohydroxy- and monomercapto-substituted pyridines, quinolines, and acridines were calculated from the acid-base constants of these compounds at the isoelectric points in an amphiphilic medium (80ZOR1499). 

Both AMI and MNDO methods correctly predict the predominance of the oxo-thione form for A-monosubstituted 2-thiouracils in the gas phase (89JCS(P2)1507). The gas phase calculated relative stabilities, proton affinities and aqueous phase calculated acidity constants predicted the higher stability of the oxo-thione form of 6-propyl-2-thiouracil compared to the hydroxy(thiol) tautomer (04JST(679)33). 

Rearrangement of sulfonium ylides constructed on a chiral oxathianone template provides an effective asymmetric entry to optically active 4-pentenoic acids118. The requisite ylides 17 are obtained by intramolecular alkylation of diazoketones 16, which are in turn prepared from the valine derived hydroxy thiol 15. Ylide formation can be accomplished directly by treatment of diazoketones 16 with rhodium(II) acetate or by a two-step sequence involving acid-catalyzed formation of the sulfonium salt and subsequent treatment with base at low temperature, a procedure which affords superior yields and diastereoselectivity. 

In a three-step synthesis of a chiral, conformationally locked 1,3-hydroxy thiol from ( + )-pule-gone, 2-(l-benzylthio-l-methylethyl)-5-methyl-l-cyclohexanone was stereoselectively reduced by sodium/ammonia/methanol to the corresponding cyclohexanol9. The diastereomerie ratio (equatorial/axial 78 22) was determined by H NMR. 

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