Thiones esters

Reductive decarbonylation and decarboxylation can be carried out by (TMSlsSiH using acyl chlorides, phenylseleno esters, or N-hydroxypyridine-2-thione esters. Examples are shown in Reactions (17)-(19). Hydrolysis of the methyl ester followed by decarbonylation at the C2 position of hexahydropyrro-loindole (+)-17 afforded the desired tricycle (+)-18 in 84% yield and >99% ee. ... 

The acyl selenide 19 affords the decarbonylated )S-lactam in good yield. A N-hydroxypyridine-2-thione ester 20 is used in the key step to construct the chiral cis-cyclopropane structure in compounds designed as antidopaminergic agents. The observed high cis selectivity is due to the hydrogen abstraction from the sterically demanding (TMSlsSiH, which occurs from the less-hindered side of the intermediate cyclopropyl radical. 

SCHEME 10. Preparation of hydroperoxides via oxygenation of carbon radicals generated from iV-hydroxypyridine-2-thione esters as radical precursors... 

Recent work by Newcomb has resulted in the emergence of a second method for the cyclizations of amminium radical cations that shows good synthetic potential.175-176 As illustrated in Scheme 42, AMiy-droxypyridine-2-thione carbamates can be generated and cyclized by analogy to the Barton method (which uses thione esters). In Scheme 42, the preparation of the precursors and two variants on the cyclization, reductive trapping and thiopyridyl trapping are illustrated. 

Newcomb and coworkers have very elegantly demonstrated that N-hydroxypyridine-2-thione carbamates lc (PTOC carbamates, X = NR2) are directly analogous to the A-hydroxy-pyridine-2-thione esters la. They can be prepared readily from primary and secondary amines. Reaction of these carbamates in a radical chain process gives carbamoyl radicals that decarboxylate to give aminyl radicals, Scheme 40.66 71 In the... 

Perhaloalkanes serve as bromination or iodination agents in the radical decarbox-ylative halogenation of carboxylic acids. In an interesting modification of the Hunsdiecker-Bodin reaction Barton and coworkers have applied iV-hydroxypyridine-2-thione esters as nonelectrophilic intermediates for the decarboxylative bromination and iodination of primary, secondary and tertiary aliphatic and alicyclic592, as well as aroma-... 

The N-hydroxypyridine-2-thione esters of Boc-protected amino acids undergo decarboxylation on photolysis in the presence of a hydrogen-atom-transfer reagent such as t-butyl mercaptan in 80-95% yield. Decarboxylation of the side-chain carboxy groups of suitably protected aspartic and glutamic acids can also be effected, but yields are lower (50-80%). ... 

This protocol has been successfully applied to the reactions of carboxylic acid derivatives such as thioamides and thione esters (cqs 3 and 4). 3-Acetylthiazolidine-2-thiones are quite suitable substrates for the tin(ll) enolate mediated asymmetric aldol reaction and various optically active p-hydroxy 3-acetylthiazolidine-2-thiones are obtained by using chiral diamine 1 (eq 5). ... 

Decarboxylation of acids by photolysis of their N-hydroxy-pyridine-2-thione esters has been the subject of much study. The reaction proceeds by way of a radical chain pathway and involves... 

Soft metal ions such as Hg and Ag have long been known to react readily with organo sulfur compounds, and a review of the metal ion-promoted reactions which can occur is available. The hydrolysis of thiol esters (equation 25), thione esters (equation 26) and thiolbenzimidate esters (equations 27 and 28) are all strongly promoted by Hg" in aqueous solution at pH <3... 

A cyclopropyl to allyl radical rearrangement could also be induced in 20 % yield by the thermal decomposition (in carbon tetrachloride at 32 °C with sonication) of the A-hy-droxypyridine-2-thione ester of l-fluoro-2,2-diphenylcyclopropane-l-carboxylic acid (Barton decarboxylation reaction). ... 

A new decarboxylative route to free radicals, which has proved particularly successful in preparative work, embodies the thermal (or photochemical) decomposition reaction of 1-hy-droxypyridine-2(l/f)-thione esters 23 with tributyltin hydride, /er/-butanethiol, or a similar hydrogen donor.These esters can be easily prepared from acyl halides and the sodium salt of l-hydroxypyridine-2(l//)-thione, or from the carboxylic acid, dicyclohexylcarbodiimide and l-hydroxypyridine-2(l/f)-thione. The intermediate radicals were readily reduced to the corresponding hydrocarbons 24 in efficient chain reactions with organotin hydrides or thiols as reaction partners, and the proportion of rearranged to unrearranged products could be controlled by the choice of hydrogen donor, its concentration and the temperature. This system was sufficiently quantitative and well behaved for use in kinetic studies, and the rate constants of the (S-scission reactions of the listed cyclopropylmethyl species were determined. 

The generation and synthetic uses of carbon radicals obtained by photochemically initiated elimination of carbon dioxide from thiohydroxamic esters, and in particular from esters of W-hydroxy-2-thiopyridone, have again been widely reported. Exploratory studies of these W-hydroxypyridine-2-thione esters using laser flash photolysis techniques have been described,and improved... 

In terms of their properties, poly (thiol esters) lie between polyesters and polyamides. In polyesters, oxygen atoms can be replaced formally by sulfur in three different ways to give poly (thiol esters) (I), poly (thion esters) (II), and poly (dithio esters) (III). 

Barton has devised such a pathway for converting carboxylic acids to hydroperoxides via A -hydroxypyridine-2-thione esters [16a,b] (Scheme 5). The yields of hydroperoxides are 45-89% [16]. Reduction of the crude products with trimethyl phosphite (rt) or dimethyl sulfide (80 °C) readily provides the corresponding alcohols. On the other hand, tosylation and ensuing Kornblum-De La Mare type fragmentation [17] leads to carbonyl compounds [16b] (Scheme 5). 

Bohne, C., Boch, R., and Scaiano, J. C., Exploratory studies of the photochemistry of N-hydroxy-pyridine-2-thione esters generation of excited radicals by laser flash photolysis and in a conventional fluorescence spectrometer, /. Org. Chem., 55, 5414,1990. 

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