Spiro thiadiazoles

The cycloaddition reactions of [(thioacyl)methylene]thiadiazoles 83 with dimethyl acetylenedicarboxylate (DMAD) under UV irradiation at room temperature gave the spiro[3/7-l,3,4-thiadiazoline-2,4 -477-thiopyrans] 84 in 50-60% yields (Equation 23) <2003EJ02480>. 

The second453 has shown that ynamines of the general type 510 (NR2 = pyrrolidino, piperidino, morpholino) combine with dehydro-dithizone (413, R1 = R2 = Ph) in chloroform at room temperature giving the red crystalline spiro 1/Z-indene-1,2 (3 //)-1,3,4-thiadiazoles (511). The mode of formation of 511 is not clear but a reasonable mechanism would appear to require participation by the acyclic valence tautomer (415). 

A study of the 1,3-dipolar cycloaddition of pyrazines, pyrimidines and l//-pyrimidinthiones with nitrilimines (80), generated in situ by dehydrohalogenation of the corresponding hydrazonoyl chlorides (79), was carried out. Reaction of pyrimidine-2( l//)-thiones (81) and -4(l//)-thiones with nitrilimines in benzene at reflux gave spiro[pyrimidine-2(l//), 2 (3 f/)-[ 1,3,4]thiadiazoles... 

Thioxanthione and thiobenzophenone 3 -methylide, obtained by the elimination of N2 from 2,5-dihydro-2,2-diphenyl-l,3,4-thiadiazole at —45°, give the spiro[thioxanthene-9,4 -[l,3]dithiolane] 355 through a 1,3-dipolar cycloaddition (Equation 78) <2000EJ01695>. 

The exocyclic C=S bond of 5-thioxo-2-thiazolines (203) can act as a dipolarophile towards several compounds. For instance, cycloaddition reactions have been reported with nitrilium betaines (204) to give heterocyclic spiro compounds <84HCA534>, with thiocarbonyl ylides (205) to give spirocyclic 1,3-dithiolanes <9IHCA1386>, and with 2-diazopropane (206) to give thiadiazoles <92HCA1825>. On the other hand, the reaction of (203) with diazomethane leads to a mixture of five products (Scheme 49) <93hcai715>. 

Some interesting spiro jS-lactams (22) are obtained from the reaction of 2-imino-2,5-dihydro-l,3,4-thiadiazoles and ketones. The products presumably arise via a [27t + 27t] cycloaddition followed by elimination of a molecule of nitrogen. 

Kagi et al., 1993). The thiirane 6.44 loses sulfur easily, as found earlier in the similar reaction sequence with 2-diazopropane (6-24). With di( e/ butyl)diazo-methane the spiro compound corresponding to the 2,5-dihydro-l,3,4-thiadiazole 6.39 is sufficiently stable to be analyzed by NMR spectroscopy and by X-ray diffraction (Mlostoh et al., 1994). 

Diazo Compounds. Diazomethane reacts immediately with 1 dissolved in diethyl ether in a regioselective manner to give 2,5-dihydro- 1,3,4-thiadiazole (17) as a relatively stable sohd. This confound is a superior precursor of the reactive thiocarhony 1 5-methylide (8, eq 9) as it smoothly eliminates nitrogen at 45 °C. In the absence of an appropriate interceptor, thiirane 9 is formed. In the presence of electron-deficient dipolarophiles, diverse five-membered spiro-heterocycles (18) are formed via 1,3-dipolar cycloaddition (eq 9).22.23 Representative examples of dipolarophiles leading to 18 in high yield are DMAD, TV phenyl maleinimide, chloral, dimethyl azodicarboxylate, adamantanethione, as well as 1.21... 

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