Type D Syntheses

Type D (C—C—S + C—N) Syntheses.—Extensions of the well-known reactions between Schiff bases and 2-mercaptoacetic acid (see Vol. 3, pp. 602, 603) have 

2-Acetamido-alkenethiolates MeC(0)NHCR =CR S condense with isothiocyanates R NCS to give dithiocarbamates (98). In the presence of a strong base these are converted into thiazolidine-2-thiones (99). When compound (99 R = H, R = R = Me) is treated with trifluoroacetic acid, elimination of acetamide occurs, with formation of 3,4-dimethyl-AMhiazoline-2-thione.  

Type D (C—C—S-t-C—N) Syntheses.—The interaction of thiocyanoacetic ester (90) with aldehyde in the presence of potassium acetate-acetic acid unexpectedly yields 5-arylidene-2-ethoxycarbonylmethylthio-A -thiazolin-4-ones (92). Reaction in acetic acid alone gives only poor yields of 

5-arylidenethiazolidine-2,4-dione (93). The formation of (92) is visualized to involve the preliminary conversion of the thiocyanoacetic ester (90) into mercaptoacetate (91) by the potassium acetate, followed by its successive condensation with more (90) and aldehyde to give the final product (92).  

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Type D synthesis, where the 3,4-CN bond is being formed, involves the reaction of thiosemicarbazones of aldehydes, ketones, and esters 97 with 3,3-pentamethyleneoxaziridine 98 to afford 5-imino-3,3-pentamethylene-l,2,4-thiadi-azolidines 99 (Equation 27). yV-Acylthioureas also undergo this transformation but, whereas this reaction is fairly general for thiosemicarbazones, only acetyl and benzoyl thioureas give 1,2,4-thiadiazolidines <1996CHEC-II(4)307>. There have been no new reports of type D syntheses since the publication of CHEC-II(1996). 

Type D Syntheses [C — C—S + C—N). Further examples of the synthesis of thiazolidin-4-ones, by allowing 2-mercaptoacetic acid to react with C=N-containing compounds, have been reported with azines, only one of the C=N groups reacted with carbodi-imides, 2-iminothiazolidin-4-ones were obtained with cyclic ketones and amines, 2-spirothiazolidin-4-ones were prepared. The reaction between fluorenylidene-anilines (A) and 2-oxothiiran 1,1-dioxide afforded the spiro-thiazolidin-4-one 1,1-dioxides (78), which could also be obtained by oxidation of the products that were formed by starting from (A) and 2-mercaptoacetic acid. ... 

Type D Syntheses (5—A—C). A novel ring-contraction of benzo-1,2,4-thiadiazines (102), which extends the scope of a similar benzoxazine reaction, takes place either thermally, in the presence of tervalent phosphorus compounds, or photochemically (but in lower yields) to give the benzothiazoles (103). ... 

Type D Syntheses (S—CeH —N—C).—Two similar methods, starting from the 2-methylthio-substituted anilides, have been described, in one of which the cyclizing agent was pyridinium chloride, whilst the second utilized phosgene. In the latter case, substituted imidoyl chlorides are probably intermediates. 

Type D Syntheses [C—N—C—S—N], The synthesis of A -l,2,4-thiadiazolines from potassium methyl cyanoiminodithiocarbonate, MeS C(=NCN)SK, and A -chloro-amidines (see Vol. 3, p. 682) has been extended by the inclusion of other A -chloro-cyanamide derivatives, e.g. NH2C(=NCl)OR, as one of the reactants. In this case, condensation at 5 °C produces 2-alkoxyimidoyl-3-imino-5-methylthio-A -l,2,4-thiadiazolines in 41—79% yield. They are dealkylated to the amides by methanolic hydrogen chloride and readily undergo aminolysis to the amidines, iV-Chloro-guanidines and A -chloroguanylurethane may be used similarly in this general synthesis. ... 

Type D Syntheses [CNCSN].— The condensation of N-chloro-amidines (76) and potassium methyl cyanoiminodithiocarbonate (77) in chloroform below 5 C yields 2-imidoyl-3-imino-5-methylthio-A -l,2,4-thiadiazolines (78) in one stage. The reductive and hydrolytic ring cleavage of the products was briefly outlined. ... 

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