GLYCONOTHIO-O-LACTONES .3. THERMOLYSIS OF A 4,5-DIHYDRO-1,2,3-THIADIAZOLE AND A 2,5-DIHYDRO-1,3,4-THIADIAZOLE

Addition of CH2N2 to 2,3:5,6-di-O-isopropylidene-1-thio-mannono-(1) ga ve the 2,5-dihydro-1,3,4-thiadiazole 1 and the 4,5-dihydro-1,2,3-thiad iazole 3. First-order kinetics were observed for the thermolysis of 3 (Scheme 2) at 80-110 degrees in C6D5Cl solution and of 2 (Scheme 3) at 20-35 degrees in CDCl3, respectively. The 1,2,3-thiadiazole 3 led to mixtures of the thiirane 9, the starting thionolactone 1, the thiono-1 ,5-lactone 8, and the enol ether 7, while the isomeric 1,3,4-thiadiazo le 2 led to mixtures of the anomeric thiiranes 9 and 12, the O-hydroge n S,O,O-ortholactone alpha-D-14; the S-methyl thioester 15, the S,S,O- ortholactone 13, and the 2,3:5,6-di-O-isopropylidene-mannono-1,4-lacto ne (16). Pure products of the thermolysis were isolated by semiprepara tive supercritical fluid chromatography (SFC), whereas preparative HPL C led to partial or complete decomposition. Thus, the beta-D-mannofura nosyl beta-D-mannofuranoside 10, contaminated by an unknown S species, was isolated by preparative HPLC of the crude product of thermolysis of 3 at 115-120 degrees and partially transformed in CD,OD solution in to the symmetric di(alpha-D-mannofuranosyl) tetrasulfide 11. Its struc ture was evidenced by X-ray analysis. Similarly, HPLC of the thermolys is product of 2 gave the enethiol 17, the sulfide 19, and the mercapto alcohol 18 as secondary products. Thermolysis of the thiirane 9 at 11 0-120 degrees (Scheme 4) led to the anomeric thiirane 12 which was tra nsformed into mixtures of the enethiol 17 and the enol ether 7. Additi on of H2O to 17 and 7 gave the corresponding hemiacetals 18 and 20. Th e mechanism of the thermolysis of the dihydrothiadiazoles 2 and 3, and the thiiranes 9 and 12 is discussed.