Bromacetic ester

The key step of this MCR according to the opinion of the authors is an interaction of bromacetic ester with aminoazole via ring nitrogen atom to the 3-carboxymethyl-2-iminothiazoline 105, also isolated as a product in the steps-sequence reaction (Scheme 49). Further cyclization of 105 leads to the formation of thiazolo[3,2-a] imidazol-6-one 106, which then reacts with aldehyde affording the desired ylidene in moderate-to-good yields. 

Scheme 49 Mechanism of MCRs involving bromacetic ester...

Oxalyl-acetic ester (20 gms.) is mixed with mono-bromacetic ester (17 gms.) in a flask of 300 c.c, capacity, fitted with an air-condenser. Enough zinc turnings to cover the end of a spatula are added and the mixture becomes brown, while the temperature quickly reaches 50° and the zinc dissolves. Addition of a second portion of zinc is attended by boiling and the reaction is moderated by cooling the metal is added in excess, and the mixture is then heated on the water-bath for a short time. After cooling, the mixture is treated with cold dilute sulphuric acid and ether, well shaken to get a clear solution, and the ethereal layer removed, washed with dilute sodium carbonate and dried over calcium chloride. The ether is next distilled and the remaining oil fractionated under reduced pressure most of it passes over below 200° at 35 mm. That portion taken as citric ester (b.p, 212°—216°) is hydrolysed... 

The interaction of diaryl tellurides and the esters of halogenated fatty acids, e.g. methyl bromacetate, gives diarvl-telluretin halide alkyl esters, e.g. (C6H5)2Te(Br).CH2.COOCH3. The halogen present may be chlorine, bromine or iodine, moist silver oxide replacing the bromine by the hydroxyl group. 

Diphenyl-telluretin bromide methyl ester,2 (C6Hs)2Te(Br). CH2.COOCH3, is obtained by the interaction of diphenyl telluride and methyl bromacetate. It is a white powder, M.pt. 99° to 100° C., very soluble in hot water and separating on cooling as prismatic crystals, M.pt. 105° to 106° C. Boiling alcohol splits it up into its components. The corresponding ethyl ester melts at 63° to 64° C. 

I.G. Farben produced only the warning substance, bromacetic acid methyl ester, as well as the stabilizer, chlor carbonic acid methyl ether. 

---