S-Acetyl compounds

Hydrolysis of the high-energy S-acetyl compound formed sets free about 34 kJ/mol this value corresponds approximately to the amount of energy set free in the hydrolysis of ATP. 

Several methods were described for the selective de-S-acetylation of 0-acetyl protected 1-thioglycoses. Sodium methoxide in methanol at low temperature (below -20 °C) was known to afford mainly the de-S-acetylated compound [16] or exclusively this compound when the reaction was quenched at low temperature by adding H-l- resin [17]. Demercuration of tetra-O-acetyl-l-phenylmercury(II)-thio- -D-glucopyranose (12) (Scheme 4) obtained by treatment of (8e) with phenylmercury(II)acetate afforded a convenient synthesis of tetra-0-acetyl-l-thio-/3-D-glucose (8a) [18]. This sequence applied to the a-anomer (10a) (Scheme 3) led to the expected de-S-acetylated compound (10b) [19]. Chemoselective deprotection of thioacetate at the anomeric position of peracetylated 1-thioglycoses was also achieved in good yield by action of cysteamine in acetonitrile or hydrazinium acetate in DMF [20,11]. 

Heterocyclic Thiols.—Representative synthetic methods described in the recent literature cover the synthesis of tetrachloropyridine-2-thiol from pentachloro-pyridine A -oxide and thiourea, the synthesis of 2-phenylthiazole-5-thiols from the S -acetyl compounds, and syntheses of 4-phenyl-l,2-dithiolan-3-... 

S-Formylbenzofuroxan has been prepared, but the 4-acetyl compound rearranges under the conditions used for its jDreparation, to an anthranil derivative (see Section VIII). 

The bicyclic compound 123 is well known. - " It was first prepared by cyclodehydration of (2-pyridylthio)acetic acid. It is noteworthy that 123 is formed directly from S-acetyl-2-mercaptopyridine and chloroacetic acid. ... 

However, two recent papers suggest that DMF is also a good solvent for this type of condensation under appropriate activation conditions of the donor. A simple and efficient method for the preparation of thioglycosides of N-acetyl-neuraminic acid has been developed [30]. This procedure involves the selective in situ S-deacetylation and activation of the 2-S-acetyl NeuSAc (24a) and the displacement of primary bromide of methyl glycosides (28a, 28b). The desired a(2->6) thioglycosides (29a,29b) were obtained in 75 and 82% yield. Condensation of the sodium salt (24b), freshly derived from (24 a) by selective S-deacetylation with sodium methoxide [16], with (28c) in DMF at 45 °C also gave the expected compound (29c) in 76% yield (Scheme 8) [31]. 

The thiol group can also be found in heterocyclic compounds such as 2-mercaptobenzothiazole (9) and echinoclathrine C (10). Compound 9 was isolated from the symbiont bacterium Micrococcus sp., which was obtained from the sponge Tedania ignis [13]. The pyridine alkaloid echinoclathrine C (10) and its S-acetylated derivative, echinoclathrine B (11), were isolated from the Okinawan sponge Echinoclathria sp. [14]. The position of the hydroxyl and acylamino group on the phenyl ring in echinoclathrines (11 and 10) was recently corrected [15]. Compound 11 showed weak immunosuppressive activity in the mixed lymphocyte reaction assay with an IC50 of 9.7 pg/ml [14]. 

Auropolin (140) was isolated from the aerial parts of Teucriumpolium (Lamiaceae).77 Although the naturally occurring compound 140 did not reduce feeding by final stadium larvae of S. littoralis, the semisynthetic acetylated compound, acetyl-auropolin (141), reduced feeding by 57.8% at 100 ppm with an EC50 value (calculated via dose-dependent experiments) of 90 ppm.77 This result shows the importance of the esterification of the hydroxy group at C-20 in auropolin. 

Enantioselective total syntheses of (-)-6-epitrehazolin and (+)-trehazolin were achieved by the synthesis of 275, which began with an asymmetric heterocycloaddition between [(benzyloxy)methyl]cyclopentadiene (263),108 prepared from thallous cyclopentadienide, and the acylnitroso compound arising from in situ oxidation of (,S )-mandelohydroxamic acid (264) with tetrabutylammonium periodate. Cycloaddition led to a mixture of 265 and its diastereomer (Scheme 35).109 The inseparable mixture was reduced to afford cyclopentenes 266 and 268 in 40% and 11 % overall yields, respectively, from thallous cyclopentadienide. Catalytic osmylation of 266 favored syn addition, while the osmylation of diacetate 267 was more selective and nearly quantitative, affording, after acetylation, compounds 270 and 269 in >5 1 ratio. 

Compound 215 may be acetylated with acetic anhydride in pyridine to tetra-O-acetyl-5-thio-a-D-xylofuranose. Acetylation with sodium acetate and acetic anhydride gives, besides the a-form a smaller quantity of the levorotatory /3-D-tetraacetate also. Neither acetate shows an S-acetyl band at 230 nm in the ultraviolet spectrum, showing that the compounds have the thiopyranoid ring. ... 

Acetylation of the pyranose 227 gives crystalline tetra-O-acetyl-5-thio-/3-D-ribopyranose and the syrupy a-D anomer. Both compounds lack S-acetyl bands in their infrared spectra. Treatment of 227 with methanolic hydrogen chloride furnishes a mixture of anomeric methyl 5-thio-D-ribopyranosides which cocrystallize. Pure anomers are obtained on separation of the glycosides by use of a basic ion-exchange resin, or by separation of the triacetates on silica gel. The sulfur-containing, six-membered ring was established by periodate assay. 

Deoxy-6-thio-D-a yfo-hexose forms a 1,6-anhydro compound analogous to 270. S-Acetyl-5-deoxy-l,2-0-isopropylidene-6-thio-a-D-xyZo-hexofuranose is obtained either by nucleophilic displacement on the corresponding 6-p-tolylsulfonyloxy compound with potassium thioacetate or by the photochemical addition of thioacetic acid to... 

Bohlmann has isolated another naturally occurring thymol epoxy-ester (c/. Vol. 1, p. 34 Vol. 3, p. 46), this time from Wedelia forsteriana Compound (137)has been isolated from Pluchea odorata. Pseudomonas fluorescens converts (-)-menthone into cis- and trans-(138). In addition to known (Vol. 2, p. 31) p-menthane-8-thiol-3-ones in Buchu leaf oil, 5-methyl dnd S-acetyl derivatives have been isolated... 

Acetylation. The reagent, neat or in nitromethane solution, acylatcs aromatic compounds in good yield (I). It is an effective reagent for O-acetylation of alcohols (2), for S-acetylation of mercaptans, and for N-acetylation of primary and secondary amides. 

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