Thiocarbonates, synthesis

Vinylene sulfate — see 2,2-dioxide under 1,3,2-Dioxathiole Vinylene thiocarbonate synthesis, 6, 780 Vinylene trithiocarbonates photolysis, 7, 181 Violapterin... 

Thiocarbonates, synthesis of, 17, 3 Thiocyanation of aromatic amines, phenols, and polynuclear hydrocarbons, 3, 6 Thiophenes, synthesis of, 6, 9 Thorpe-Ziegler condensation, 15, 1 31 Tiemann reaction, 3, 9 Tiffeneau-Demjanov reaction, 11, 2 Tin(n) enolates, 46, 1 Tin hydride method to prepare radicals,... 

A radical carboxyarylation approach was introduced as the key step in the total synthesis of several biologically important natural products (Scheme 27). Treatment of thiocarbonate derivatives 112 (R = Me or TBS) with 1.1 equiv of (TMS)3SiH in refluxing benzene and in the presence of AIBN (0.4 equiv added over 6h) as radical initiator, produced compound 113 in 44% yield. This remarkable transformation resulted from a radical cascade, involving (TMSlsSi radical addition to a thiocarbonyl function (112 114), 5-era cyclization (114->115) and intramolecular 1,5-ipso substitution (115 116) with the final ejection of (TMSlsSiS radical. 

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. 

Nitro-aldols, which are readily available (see Henry reaction Section 3.1), are converted into olefins via conversion of the hydroxyl group to the corresponding phenyl thiocarbonate ester and treatment with tin radical.158 The yield was not reported. Because the radical deoxygenation via thiocarbonate (Barton reaction) proceeds in good yield, the elimination of Eq. 7.115 might be good choice for olefin synthesis.159... 

Common synthesis of disulfonium dications involves formation of a S-S bond by oxidative coupling of two sulfide moieties. Involving for oxidative generation of S-S bond, thiocarbonyl compounds can lead to new-type dications containing disulfide moieties between two positively charged fragments. There are several disulfide dications of this type derived from thioureas, thiocarbon-ates and thioketones as well as selenium derivatives.137 148... 

A fused OZT was obtained in the course of the reduction of vicinal azido-thiocarbonates via formation of the intermediate amine, which attacks the thiocarbonyl group (Scheme 37). The condensation reaction proved faster than the deoxygenation process for the synthesis of 2 -amino-2, 3 -dideoxyuridine50 or methyl 3-amino-4,6-0-benzylidene-3-deoxy-2-0-phenoxythiocarbonyl-a-L-talopyranoside.51... 

The reduction of thiocarbonyl derivatives by EtsSiH can be described as a chain process under forced conditions (Reaction 4.50) [89,90]. Indeed, in Reaction (4.51) for example, the reduction of phenyl thiocarbonate in EtsSiD as the solvent needed 1 equiv of dibenzoyl peroxide as initiator at 110 °C, and afforded the desired product in 91 % yield, where the deuterium incorporation was only 48% [90]. Nevertheless, there are some interesting applications for these less reactive silanes in radical chain reactions. For example, this method was used as an efficient deoxygenation step (Reaction 4.52) in the synthesis of 4,4-difluoroglutamine [91]. 1,2-Diols can also be transformed into olefins using the Barton-McCombie methodology. Reaction (4.53) shows the olefination procedure of a bis-xanthate using EtsSiH [89]. 

Because vicinal dibromides are usually made by bromination of alkenes, their utility for synthesis is limited, except for temporary masking of a double bond. Much more frequently, it is desirable to convert a diol to an alkene. Several useful procedures have been developed. The reductive deoxygenation of diols via thiocarbonates was developed by Corey and co-workers.210 Triethyl phosphite is useful for many cases, but the more reactive reductant l,3-dimethyl-2-phenyl-l,3,2-diazaphosphohdine can be used when milder conditions are required.211 The reaction presumably occurs by initial P—S bonding... 

Research Focus Synthesis of poly(thiocarbonate-cothiourethane) derivatives having a high refractive index and Abbe number. 

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... 

For many years, the carbohydrate esters of carbonic acid and thiocar-bonic acid have found important application as intermediates in the synthesis of otherwise difficultly accessible derivatives. A review of these esters is particularly opportune, because both their preparation and properties are considerably influenced by stereochemical factors which require interpretation in the light of modem concepts of reaction mechanisms and conformational analysis. Although the historical development of the chemistry of the carbonic and thiocarbonic esters of carbohydrates has been independent, it is appropriate to give a comparative treatment of their closely related structures and properties. 

Synthesis of 7-deoxy-6a-hydroxypaclitaxel was realized through the regiospeci-fic reduction of 6,7-ot-thiocarbonate 83 as the key step. 2 -TES-7-deoxy-6oc-hydro-... 

The same authors have shown that xanthates 206 (Scheme 61) may also be used for thiirane synthesis as in the case of the adduct 207 which fragmented under the reaction conditions to give 184. Attempted use [78] of a chiral auxiliary failed—reaction of 0-menthyl thiocarbonate 210 with various aldehydes and ketones provided the respective thiiranes in good yields but with a negligible enantiomeric excess. 

Synthesis af okfins from stericaUy hindered akohols, Gerlach el al. have reported a successful variation of theChugaev reaction applicable to the preparation oftemperature-sensilive olefins from sterically hindered alcohols. Thus /ron.t,/ran.s-spiro[4.4jnonane-1,6-diol (2) is converted into the thiocarbonate 0-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-methyIxanthate failed to give the diene. 

Historically, the first practicable reagent for the synthesis of N -Boc amino acids, tert-butyl 4-nitrophenyl carbonate (52), was developed by Anderson and McGregorb (Scheme 25). Due to the low reactivity of this mixed carbonate, drastic reaction conditions are required and yields remain unsatisfactory because of the difficult and laborious separation of the byproduct 4-nitrophenol. Alternative mixed carbonates have been proposed such as tert-butyl 2,4,5-trichlorophenyl carbonate (53),b32] lert-butyl pentachlorophenyl carbonate (54),b33] tert-butyl 2-pyridyl carbonate (55)0-tert-hvXy 5-4,6-dimethylpyrimidin-2-yl thiocarbonate (56).b3 1 With the latter two reagents the byproducts, pyridin-2-ol and 4,6-dimethylpyrimidine-2-thiol, respectively, are acid soluble and, thus, readily removed. 

Scheme 25 Mixed Carbonates and Thiocarbonates Used for the Synthesis of A -Boc Amino Acids...

Monodeoxygenation of 1,2- and 1,3-diols was achieved via their cyclic thiocarbonates, prepared from the diol and A/,A/ -thiocarbonyldiimidazole, by reaction with BusSnH-AIBN followed by alkaline hydrolysis (presumably, F would also be effective for the cleavage step). Equation (14) shows this process applied to synthesis of a derivative (73) of 5-deoxyglucose. Exclusive secondary deoxygenation is expected on the basis of radical stability in contrast, the derivative (72) was readily converted by an ionic process to an intermediate suitable for 6-deoxygenadon, since treatment with KI gave (74) quantitatively. 

Isopropenyl carbonates 236 were introduced by Sinay [363] and were used with TMSOTf as a promoter. Reactions of imidazole carbonate derivatives 237 were assisted by ZnBr2 [522], whereas for imidazole thiocarbonate derivatives 238, AgC104 was used as the promoter [523, 524]. These methods were used in the total synthesis of avermectin Bla 245 by Ley [523,524] (Scheme 4.47). 

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