Thioamide groups

The reaction of PAN with hydrogen sulfide was for the first time achieved in DMF solution at 50-70 °C. It has been shown that under these conditions the nitrile groups are partially converted into thioamide groups and polymers corresponding to copolymers of AN with acrylthioamide are formed54 ... 

This reaction has recently been applied with success5 s for the modification of poly-(a-chloroacrylonitrile). As reported by the authors, the level of conversion of the nitrile groups into thioamide groups reaches 90%. 

Fig. 5. Effect of the nature of adjacent functional groups in copolymers on the rate of conversion of the nitrile groups into thioamide groups. (1) PAN (2) AN-VDC (3) AN-AA (4) AN-MA (5) AN-VC (6) AN-ST...

An investigation of the thioamidation reaction on model compounds of PAN under similar conditions and a study of the functional composition of modified copolymers led to the assumption that the most probable course of the two types of side reactions causing the decrease of the number of thioamide groups are hydrolysis and cyclization reactions. 

These copolymers undergo side reactions which are mainly reducible to reactions in which chloride atoms take part. When copolymers of AN and vinylidene chloride are treated with aqueous solutions of ammonium sulfide at 90 °C, nitrile groups have been shown to be almost fully converted into thioamide groups, and practically no hydrolysis of the formed thioamide groups is taking place in this case. At the same time, it was found that in the course of this treatment dehydrochlorination is taking place almost quantitatively ... 

An interesting feature of modified PAN containing thioamide groups (PAN-T) is that the presence of even a small amount ( 5%) of thioamide groups in the macromolecules of PAN initiates the thermal cyclization and lowers the temperature at which its cyclization starts down to 130—150 °CS9). 

In both cases this process is accompanied with the formation of conjugated —C=N— bonds, which is indicative of the participation of the thioamide groups in the cyclization reaction. 

The following scheme can be suggested for the initiation of PAN cyclization by thioamide groups ... 

When PAN-T samples are heated by a gradual increase of temperature, the mercapto groups of the tautomeric form of the thioamide groups, enter into an intramolecular reaction with the nitrile groups in accordance with the following scheme ... 

The activation energy of the reaction of thermal cyclization of PAN containing 5% of thioamide groups amounts to 20 kcal/mol (83,7 kJ/mol). 

Keto amides (or thioamides) could participate in cyclocondensations by elimination of the amino or oxo (thioxo) function from the amide (thioamide) grouping according to conditions. The other synthons should behave reasonably predictably. The following examples illustrate some possibilities. 

The results of ab initio calculations provide evidence that Me2NC(S)-[14+] is stabilized by resonance electron donation from the a-thioamide group (A, Scheme 12) and by covalent bridging of sulfur to the benzylic carbon (B, Scheme 12).96 Direct resonance stabilization of the carbocation will increase the barrier to the nucleophile addition reaction, because of the requirement for the relatively large fractional loss of the stabilizing resonance interaction (A, Scheme 12) at the transition state for nucleophile addition to a-substituted benzyl carbocations.8,13,28 91-93 If the solvent adds exclusively to an open carbocation that is the minor species in a mixture of open and closed ions, then... 

Recrystallization of thioamides 24f-h gave colorless or slightly yellow prismatic crystals [49]. All three thioamides were subjected to X-ray single crystal analysis, and it was revealed that they prefer the same conformation 24-1 in which a benzyl group was placed close to the thiocarbonyl group, the same as the major isomer in solution. The alkenyl double bonds are perpendicular to the thioamide group, and twist in the range of 87.9° to 100.5°. 

Thieno[4,3,2-/g]benz[l]azocine 187 is produced in high yield (79MI397) from oxime 186 by Beckman rearrangement in PPA. Transformations of amide and thioamide groups of azocines 187 and 188, lead to derivatives 189-192, as shown in Scheme 52. 

Oxathiazolidinimine (180) containing the S—C=N thioamidate grouping can enter the cycloaddition-elimination reaction sequence with iso thiocyanates to give 1,2,4-dithiazolidindiimine (181) which in turn adds-eliminates RNCS to produce diimine (184) which isomerizes into thia-diazolidine (183). Another thiadiazolidine (182) is also formed directly from (180) which apparently... 

Scheme 29 Conversion of an Amide into a Thioamide Group...

C-Terminal activation of peptides containing a terminal thioamide group results in thiazo-lone formation this leads to epimerization at the marked chiral centers (see Scheme 35) the thiazolone is only a weak acylating agent 500 The coupling yield may still be satisfactory, if achiral C-terminal amino acids are employed 488,503 ... 

Loss of methanethiol from 132 following nucleophilic attack by a ring nitrogen atom under basic conditions, or thermally, afforded the s-triazolo[4,3-6]-s-triazole (133).135 In a recently reported reaction,136 methylation of the thioamide group in 134 resulted in cyclodeamination to give the quaternary salt 135 with loss of 2,6-dichloroaniline, rather than loss of methanethiol as in 132 - 133. 

A new tandem Michael-aldol reaction of a,ft-unsaturated compounds bearing a chalcogenide or thioamide group with electrophiles has been reviewed.163 The product o -(o -hydroxyalkyl)enones - Morita-Baylis-Hillman (MBH) adducts - can be formed with significant stereocontrol when an optically active thione is used. 

Polyamide A polymer in which the structural units are linked by amide or thioamide groupings. Many polyamides are fibre forming. 

Ozonation of 2-thioxo-2,3-dihydroquinazolin-4(l//)-one in glacial acetic acid affords quin-azoiin-4(3/f)-one in 82% yield, probably by loss of sulfur dioxide from the reactive sulfinate intermediate 1, formed by exhaustive oxidation of the thioamide group. The same reaction carried out in acetic acid/water (1 1 v/v) affords quinazoline-2,4(l//,3/f)-dione in 75% yield.55 ... 

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