1.4.2- Oxathiazoles, thermal reactions

Thermal reactions of 1,4,2-dioxazoles, 1,4,2-oxathiazoles and 1,4,2-dithiazoles are summarized in Scheme 3. The reactive intermediates generated in these thermolyses can often be trapped, e.g. the nitrile sulfide dipole with DMAD. 

Synthesis of Isothiazoles from l,3,2-Oxathiazol-5-ones (Type B S-N—C + C-C). The thermal reaction of meso-ionic l,3,2-oxathiazol-5-one [4 R =2,3,4-(MeO)3CeH2] with dimethyl acetylenedicarboxylate (DMAD) at 80 C with CO2 evolution gives 80% of isothiazole (5 R the same) (Scheme 1). ... 

A novel method is reported to convert 1,3,5-oxathiazine-A-oxides 116 into 1,2,4-oxathiazoles 117 under thermal conditions. Lewis acid-promoted reaction of compounds 117 furnishes 1,2,4-thiadiazoles 118 and the final step is a type A synthesis (see Section 5.08.9.2) (Scheme 15) <2004HAC175>. 

The exploration of novel preparative routes commands much attention in isothiazole chemistry. Especially interesting are the reactions of 3-phenyl-l,2,4-oxathiazol-5-one with acetylenic esters which afford isothiazoles in high yield (72, 7). The reaction is envisaged as a 1,3-dipolar addition of benzonitrile 7V-sulphide, produced as a transient intermediate, to the dipolarophilic dimethyl acetylenedicarboxylate or propiolic ester. Isothiazoles are also produced by photoisomerization of thiazoles (72, 19). Nitrenes may be implicated in the thermal decomposition of 2-azidoaryl thioketones into benzoisothiazoles (72, 51). 

Relatively few papers have appeared concerning heterocycles which fall into this category, and only a small number of them merit discussion. Nitrile sulphides can be generated by thermal decarboxylation of l,3,4-oxathiazol-2-ones, and if this is done in the presence of dipolarophiles new heterocycles are formed. Reactions of this type are known to occur with alkenes, alkynes, and nitriles, and they have now been shown to occur with activated aldehydes and ketones. This provides a new route to 1,3,4-oxathiazolines (312) which were previously rather inaccessible. Silylated thioketones (313) are unstable but may be prepared and reacted in situ to provide routes to a variety of new heterocycles, depending upon the dipolar species used. ... 

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