Thionoester

A new synthesis of 4-cyanothiazole derivatives via a-metailated isocyanides and thionoesters have been reported by Hartman and Wein-stock (Scheme 144) (812), with R2 = H, Me, n-C3H7, Ph, PhCHj. 4-Cyanothiazole (in 50% yield) was also formed by condensing thio-fonnamide with Q2C=C(NH2)CN (803). 

Table 6.12 Transfer Constants for Thionoester and Related Transfer Agents...

Certain 1,3,4-oxadiazole and 1,2,4-triazole glyphosate derivatives have been conveniently prepared in a faster, more efficient manner by heating the thionoester intermediates 73 with the appropriate acid hydrazide (61). These versatile thionoesters 73 have been synthesized in nearly quantitative yield from the readily available nitrile 31a, described previously, through the intermediate imidate ester 72. The oxadiazole products such as 70 obtained using this procedure were identical to those obtained from the HHT approach. 

A very similar reaction to that of Pechmann and Nold but which probably does not proceed through a dipolar cycloaddition manifold is the formation of 1,2,3-thiadiazole (6) via a thionoester and lithium trimethylsilyldiazomethane (Equation (17)) <86H(24)589>. Lithium trimethylsilyl-diazomethane also reacts with thioketones to produce 1,2,3-thiadiazoles <87H(26)1467>. 

RCOOR -> RCHiOR. A general method for this conversion1 involves thio-nation of an ester to form a thionoester, which is then reduced with Raney nickel (equation 1). Esters of primary and secondary alcohols are reduced in this way in... 

Thionolactones.1 Lactones can be converted into thionolactones by O-alkylation with Meerwein s salt followed by reaction with sodium hydrosulfide in acetonitrile. Hydroxy thionoesters are usually formed as by-products. 

Notes A combination of phosphorus pentasulfide and hexamethyldisiloxane can be used to prepare thionoesters and thionolactones in comparable yields and with a simplified workup.1 A solvent-free conversion of ketones, lactones, esters and amides, to the corresponding thio analogs with Lawesson s Reagent.2 See also Belleau s and Daw ,v Reagents. 

The thioester group (r )[C(=0)-S]) (Scheme 10) can be distinguished from the thionoester (i )[C(=S)—O]) by the carbonyl infrared stretching frequency (-1700 cm-1) and by the absence of the C=S bond (1200-1050 cm-1). In general, thioesters are both more readily synthesized and are more vulnerable to nucleophilic cleavage by virtue of a sulfur leaving group. 

Extensive studies of enzyme-substrate complexes by resonance Raman spectroscopy (RR) have prompted the synthesis of new peptide bond modifications such as thionoesters and dithioesters (Scheme l7)t82-83l within simple model substrates. The resulting acyl-enzyme complexes are especially amenable to RR analysis with cysteine proteases such as papain due to formation of the transient dithioester intermediates. 

The synthesis of thionoselenoestcrs has been reported [154-156]. For instance, treatment of thionoesters by dimethylmethylselenoaluminium smoothly yielded the methyl derivatives [155],... 

Only a few reports deal with the reaction of thionoesters with organometallic reagents, and carbophilic addition was observed as the usual mode. The reaction of O-ethyl thioesters with alkyllithiums afforded, after methylation, good yields of monothioacetals, which can be hydrolysed to ketones [353]. 

A few aldol reactions have been reported for thionoesters [359], but apparently none for thioketones so far. 

Synthesis of thionoesters from esters and Lawesson s reagent... 

The lithium enolates of thionoesters [134, 367] and thioketones [134] also exhibited a soft nucleophilic character. 

Keywords carbonyl compound, Lawesson s reagent, microwave irradiation, thio-ketone, tliiolactone, thioamide, thionoester, thioflavonoid... 

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