The Preparation of Thiocarbonates

The only other example of this type of ester was made accidentally in 1935 by Lieser and Leckzyck whilst seeking a means to remove a dithio-carbonyl group specifically. The treatment of methyl 6-0-benzoyl-2-0-[(methylthio)thiocarbonyl]-a-D-glucopyranoside, the anomer of HI (see p. 96), with aqueous silver nitrate caused partial desulfurization to the corresponding mono-0-[(methylthio)carbonyl] ester in 64% yield. 

Foster and Wolfrom, in 1956, encountered an anomalous reaction of methyl 3,4-0-isopropylidene-2-0-(sodium thiolthiocarbonyl)- S-D-arabino-pyranoside (LIX) with certain alkyl halides, namely ethyl iodide, propyl bromide, isopropyl bromide, and ter -butyl chloride. The expected 2-0-[(alkylthio)thiocarbonyl] esters LX were not formed, but, in each case, bis(methyl 3,4-0-isopropylidene-j8-D-arabinopyranoside) 2-thionocarbonate (VI) was isolated. A probable explanation of this behavior was suggested 

It is remarkable that neither of these workers appears to have investigated aqueous preparative methods, despite the fact that Cross and Bevan and their colleagues, from 1892 onward, had been developing the Viscose process for cellulose, and had, in 1907, applied similar reaction-conditions to starch. The formation of a starch viscose imder aqueous conditions was confirmed later by Ost and coworkers, who found that the solution decreased in viscosity on storage ( ripened ), like cellulose Viscose, but with no accompanying separation of starch. Various industrial uses for starch xanthate have since been suggested for example, as a frothing 

In 1926, Berl and Bitter studied the action of sodium hydroxide and carbon disulfide upon ethylene glycol, glyceritol, and their methyl ethers, and concluded that, whilst one vacant hydroxyl group is essential for occurrence of esterification, the presence of additional free hydroxyl groups inhibits reactivity. They failed to achieve xanthation of even the mono-hydric compounds in the presence of an equimolar proportion of water, unless activated carbon was also added. By using various 0-methylated celluloses, they were able to demonstrate that, for maximum esterification, two free hydoxyl groups for each D-glucopyranose residue are necessary. 

The first preparation of an 0-(metal thiolthiocarbonyl) monosaccharide was made the following year, when Freudenberg and Wolf caused carbon disulfide to react with 1,2 5,6-di-0-isopropylidene-3-0-sodio-D-glucofuran-ose to give the 3-0-(sodium thiolthiocarbonyl) derivative, which could be converted to the S-methyl ester by treatment with methyl iodide. The application of these reactions to 2,3 5,6-di-0-isopropylidene-n-manno-furanose and 1,2 3,4-di-O-isopropylidene-D-galactopyranose gave analogous products. 

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Carbon disulfide is a useful synthon for the preparation of thiocarbonic acid derivatives (see Chapter 8, p. 135). Carbon disulfide will react with alkynes (18) under forcing conditions the reaction is facilitated by the presence of electron-... 

Methods for the preparation of organic sulfur compounds by alteration of sulfur-containing groups already present include reduction, oxidation, and cleavage. The preparation of thiocarbonic acid and its derivatives is treated at the end of the present Chapter. For the rearrangement of sulfur compounds see the Section on p. 1074. 

For the preparation of thiocarbonic half esters, alkoxides or thiolates are added to carbon disulfide or carbonyl sulfide to the following scheme (where M = a metal) ... 

S -Alkyl thiocarbamates (Table 4.24) have been synthesized [I] by a procedure (Scheme 4.19) which is closely analogous to that employed for the preparation of S-alkyl thiocarbonates (see Section 4.1), S-Glycosyl dithiocarbamates [2], which are useful precursors for thiosugars, have been prepared by simple nucleophilic displacement of the tosyloxy group by the N,N-diethyldithiocarbamate anion (cf preparation of S-glycosyl dithiocarbonates, 4.1.13). 

The preparation of carbonates is mechanistically closely related to the synthesis of carbamates, and similar reagents can be used for this purpose (Table 14.10). Resin-bound alcohols can be directly converted into carbonates by treatment with a chloro-formate (see Experimental Procedure 14.2), in two steps by activation with phosgene or a synthetic equivalent thereof followed by reaction with an alcohol in the presence of a base, or by treatment of a resin-bound alcohol with carbon dioxide and an alkyl halide under basic reaction conditions [125]. Thiocarbonates can be prepared from... 

Of the many possible types of carbohydrate thiocarbonates, only a few have been synthesized. The essential difference between the preparations of carbonates and thiocarbonates is that, in the former case, one method may introduce different types of substituents, whereas, with the thiocarbonates, this complication has not so far arisen. The methods are thus classified according to the different types of thiocarbonate. 

Finally, a method for the preparation of (S-(alkyloxythiocarbonyl) derivatives has been introduced involving the reaction of ethyl potassium di-thiocarbonate with a glycosyl bromide. 

Synthesis of olefins from stericaUy hindered alcohols. Gerlach et al. have reported a successful variation of the Chugaev reaction applicable to the preparation of temperature-sensitive olefins from sterically hindered alcohols. Thus /ran.v./ran.v-spiro[4.4]nonane-1,6-diol (2) is converted into the thiocarbonate O-ester (3) by reaction with (1) in pyridine for 4 hr. at room temperature (80% yield). This derivative decomposes smoothly into the diene, 1,6-spiro[4.4]nonadiene (4), at a temperature of > 135°. In this case pyrolysis of the diacetate or of the di-S-methylxanthate failed to give the diene. 

The preparation of the CD building block was continued as follows. After reductive opening of the epoxide 91 with sodium cyanoborohydride, the resulting diol was converted into the bis-acetyl ester 92. Selective hydrolysis of the less sterically hindered ester was followed by conversion of the unprotected alcohol into the thiocarbonate. After deoxygenation via a Barton-McCombie reaction, the ensuing product 93 is set up for deprotection and oxidation to a key chiral CD intermediate for vitamin D synthesis. 

The preparation of N-benzyloxyureas without phosgene can be accomplished using several coupling agents, including 2(S),3-pyridinediyl thiocarbonate (PTC) [101], JV,JV -carbonyldiimidazole (GDI) [102, 103], triphosgene, (p-nitrophenoxy)carbonyl chloride [104], and ethyl chloroformate [105] (see also Section 4.3 on carbony-lation). Experimental studies revealed the imidazolylcarbonyl synthon 162 to be a superior synthetic equivalent [106],... 

The Schiff bases (18) are metallated under kinetically controlled conditions to give mostly the enamines (19), whereas under thermodynamic conditions the corresponding Z-isomers predominate. The thiocarbonic acid derivatives (20, R = CHaPh, R = Et) react with electrophiles at the a-position, after metalla-tion however, for (20, R=H or Ph) metallation and alkylation take place exclusively at the R group. This latter sequence has been used for the preparation of the oxathiolanes (21) from ketones [R R C(O)] and (20, R = R = Me). ... 

Shimizu M, Sodeoka M (2007) Convenient method for the preparation of carbamates, carbonates, and thiocarbonates. Org Lett 9(25) 5231-5234... 

The thiocarbonate enolate (100) is a useful reagent for the preparation of a-alkylidenebutyrolactones by condensation with aldehydes followed by elimination. " Similarly, the very reactive dianion (101) derived from a-mercapto-butyrolactone has been used in a one-pot procedure for the elaboration of... 

In 1863 Husemann prepared an intermediate, to which he assigned the formula C2H4S, by the action of sodium sulfide on ethylene bromide. From it he obtained the cyclic dimer, dithiane, by distillation. Mans-feld (1886) reinvestigated the intermediate and concluded that it is a polymer. As a reminder of the significance of the term polymer at that time it is to be noted, however, that Mansfeld suggested the cyclic trimeric formula for the intermediate, which is now known to be a linear polymer. Other polymers prepared similarly by Husemann (1863) include methylene sulfide (—CH2—S—)a and methylene tri-thiocarbonate (—CH2—S—CS—S—) . Neither was recognized as a polymer, and neither has since been investigated from this standpoint. 

The asymmetric reduction of the benzoxathiin is very appealing because of its simplicity (Scheme 5.3). It was envisioned that intermediate 16 could be prepared from thiol-phenol 7 and bro moke tone 17. Scheme 5.8 summarized the synthesis for 16. The l,3-benzoxathiol-2-one 35 was prepared from 1,4-benzoquinone and thiourea following a literature procedure with minor modifications. Benzylation of 35 with benzyl bromide in the presence of KI gave benzyl ether 36 as a crystalline solid. It was observed that the benzylation gave better results when the reaction was run under anaerobic conditions. Hydrolysis of thiocarbonate 36 gave free thiophenol 7 which was used directly in the next reaction. 

Sodium perthiocarbonate, Na2CS4, is formed when an alcoholic solution of sodium sulphide is converted into the disulphide by the addition of sulphur and subsequently treated with carbon disulphide.2 Perthiocarbonates may also be formed by the direct combination of thiocarbonates with sulphur.3 There is no analogy to this latter method of preparation in the case of percarbonates this fact is usually regarded as illustrating the superior chain-forming power of sulphur atoms as compared with oxygen atoms. 

Thiocarbonates.—The sulphocarbonates, or thiocar-bonates (from the Greek tbeion, sulphur) form a class of salts analogous to the carbonates, both in their formulae and in the method of their preparation. Carbon disulphide, a volatile liquid, boiling at 46°, possessing a disagreeable smell, is produced when sulphur vapour is led over charcoal... 

Radical deoxygenation of alcohols is important, and the reduction of xanthates prepared from alcohols, with Bu3SnH in the presence of AIBN is called the Barton-McCombie reaction (eq. 2.13) [37-51]. The driving force for the reaction is the formation of a strong C=0 bond from the C=S bond, approximately 10 kcal/mol stronger. This reaction can be used for various types of substrates such as nucleosides and sugars. Though methyl xanthates, prepared from alcohols with carbon disulfide and methyl iodide under basic conditions are very frequently used, other thiocarbonates, as shown in eq. 2.14, can also be employed. 

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