Lawesson type reagents

Thiophenes of type 31 (X-Y = CH) were generated via Lawesson s reagent-mediated cyclization of 1,4-dicarbonyl compounds 30 under microwave irradiation in the absence of solvent [37]. The reaction was carried by mixing the two solid reagents in a glass tube inserted inside a household microwave apparatus and irradiating until the evolution of H2S ceased. An interesting application of this method is the preparation of liquid crystals and other ferro- and antiferroelectric material such as compound 33 (Scheme 10). 

Another example of microwave-assisted PSR chemistry involves the rapid conversion of amides to thioamides by use of a polystyrene-supported Lawesson-type thio-nating reagent. By use of microwave irradiation at 200 °C in sealed vessels (monomode reactor), a range of secondary and tertiary amides was converted within... 

Modest yields of bicyclic 1,2,4-trithiolanes (156) have been obtained from the sulfurization of nonenolizable diketones of type (157) using Lawesson s reagent (Scheme 44) <90JOC2421>. Although the reaction is limited, a similar procedure has provided one of the few routes to 1,3,4-oxadithiolanes (158) from the corresponding keto hydrazone (159) (Scheme 44) <92CC7,93BCJ1714>. 

The typical Wittig-type intermediates, that is, 1,3,2-oxathiaphosphetane derivatives, were isolated from the thionation reaction of .r-bicyclo[3.3.0 octane-3,7-dioncs with Lawesson s reagent <1998EJ02647>. However, under similar conditions both ketones and the corresponding thioketones formed a mixture of trans- and as-1,3,2-dithiaphosphetanes 26a and 26b and 66a and 66b (Scheme 34), <2004TL1331>. 

Synthesis of ly3-Thiazole-5(4/ ft-thiones. The preferential thionation of /V,/V-disubstituted carboxamides can be used for a convenient synthesis of 2,4,4-trisubstituted l,3-thiazole-5(4/ -thiones of type 11. Treatment of Wacylated a,a-disubstituted a-amino acid amide 10 with 1 in toluene/pyridine at 100 °C gives 11 as the sole product. In contrast, the analogous reaction with Lawesson s reagent leads exclusively to the corresponding 1,3-thiazol-5(4/0-one 12 (eq 5). ... 

Thioamides are used as more reactive precursors than amides for amidine synthesis. Lawesson reagent is normally used for conversion of amide to thioamide. Preparation of an amidine-type mannosidase inhibitor 6 is shown as a representative example [14] (Scheme 3.13). 

Thioketones. Thioketones of different types have been obtained from the corresponding ketones. Lawesson, who discovered LR to be a powerful reagent for the conversion of C=0 into C=S groups, described the preparation of diaryl thioketones. Substituents such as Me, Br, NO2, orNMe2 do not affect the good yields of up to 98% (eq 1). 

---