Pyridine-2-thiol, esters

Additional acceleration of acylation can be obtained by inclusion of cupric salts, which coordinate at the pyridine nitrogen. This modification is useful for the preparation of highly hindered esters.122 Pyridine-2-thiol esters can be prepared by reaction of the carboxylic acid with 2,2 -dipyridyl disulfide and triphenylphosphine123 or directly from the acid and 2-pyridyl thiochloroformate.124... 

The addition of Grignard reagents to aldehydes, ketones, and esters is the basis for the synthesis of a wide variety of alcohols, and several examples are given in Scheme 7.3. Primary alcohols can be made from formaldehyde (Entry 1) or, with addition of two carbons, from ethylene oxide (Entry 2). Secondary alcohols are obtained from aldehydes (Entries 3 to 6) or formate esters (Entry 7). Tertiary alcohols can be made from esters (Entries 8 and 9) or ketones (Entry 10). Lactones give diols (Entry 11). Aldehydes can be prepared from trialkyl orthoformate esters (Entries 12 and 13). Ketones can be made from nitriles (Entries 14 and 15), pyridine-2-thiol esters (Entry 16), N-methoxy-A-methyl carboxamides (Entries 17 and 18), or anhydrides (Entry 19). Carboxylic acids are available by reaction with C02 (Entries 20 to 22). Amines can be prepared from imines (Entry 23). Two-step procedures that involve formation and dehydration of alcohols provide routes to certain alkenes (Entries 24 and 25). 

Cyclisation was achieved using the pyridine-2-thiol esters to... 

The bis-2-pyridyl disulfide 232 is obtained by oxidation of the corresponding thione 234 with I2 in a basic medium. In combination with triphenylphosphane, it activates carboxylic acids in the formation of amides and esters. In this reaction, carboxylic acids are converted into pyridine-2-thiol esters 233 as intermediates ... 

Corey-Nicolaou (pyridine-thiol ester) method to form th e final 371... 

The reduction of a carboxyl group to an aldehyde group can be effected by a reductive desulfurization of the thiol ester with Raney nickel. The thiol esters are prepared by the reaction of the acyl chloride with an excess of ethyl mercaptan in pyridine or by reaction with lead mercaptide in dry ether. The hydrogenolysis is then carried out by refluxing an ethanol ic solution of the thiol ester with Raney nickel for 6 hours. By this new synthesis, propionaldehyde and benzaldehyde have been prepared in 73% and 62% yields, respectively. ... 

NaBH4 has been found to be an effective reducing agent for the reduction of both thiocyanates (in DMF at 25 C) - - and selenocyanates (in pyridine at 25 °C). A procedure for the reduction of thiocyanates using tributyltin hydride (equation 30) has been reported by Ueno et The mechanism of the reduction is probably homolytic and the thiostannanes (15) formed are stable and can be readily isolated. The thiostannanes, in addition to being more pleasant to handle than the parent thiols, can be converted into typical thiol derivatives, such as thiol esters, in excellent yields. 

The reaction depicted was run in THF at 0 C, other solvents having been found to be inferior. The S-(2-pyridyl) thioates may be prepared through reaction of the corresponding acid chloride and 2-pyridine-thiol in the presence of a tertiary amine. They are also available directly from carboxylic acids by reaction with 2,2 -dipyridyl disulfide (Aldrithiol-2) and triphenylphosphine. In the case illustrated above, protection of the ketone would seem unnecessary if Grignard addition was selective for the thiol ester however, the starting material, 5-(2-pyridyl) y-oxopentanethioate, is not stable to the lactonization shown in equation (18). 

Esters and thiol esters. Treatment of acids with diphosgene and pyridine at —40°C and then with alcohols or thiols produces esters and thiol esters. 

Scheme 28, by application of the known thiol ester-thiopyrone phototransformation to selenium-containing systems. 5/f-[l]Benzoselenino[2,3-6]-pyridine, 4.ff-selenolo[2,3-6][l]benzoselenine, and 9/f-seleno[3,2-6][l]benzo-selenine have similarly been obtained by the corresponding selenol ester-seleninone conversion. Phenyl areneselenosulphonates undergo facile photo-induced homolysis of the selenium-sulphur bond in the presence of alkenes, a free-radical chain reaction leads to the formation of -phenylselenosulphones. ... 

Thiohydroxamic esters, such as V-hydrox5q)5nidine-2-thione, were first used as free radical precursors by Barton. " The decomposition of such esters by heat or visible light yields acyloxy radicals and pyridine thiol radicals. However, on irradiation at low-temperature, the chain reaction is essentially suppressed. 

Table 3 gives the corresponding physical properties of some commercially important substituted pyridines having halogen, carboxyHc acid, ester, carboxamide, nitrile, carbiaol, aminomethyl, amino, thiol, and hydroxyl substituents. 

---