Additions to the Thiazole System

In contrast to oxazole, thiazole does not undergo the Diels-Alder cycloaddition reaction (331). This behavior can be correlated with the more dienic character of oxazole, relative to thiazole, as shown by quantochemical calculations (184). 

Like pyridines (334), thiazoles undergo addition reactions with dimethyl acetylenedicarboxylate leading to 2 1 molar adducts, the structure of which has been a matter of controversy (335-339). 

Thiazole and DMA (335) react at room temperature in dimethylfor-mamide (DMF) to afford a major product (yield of 20%) forming orange prisms (Amj,j = 430 nm, loge 3.63). On the basis of the NMR spectrum (12H at 53.72-3.92 = 4 COOMe two doublets of an AB system centered at 56.02 and 6.52 with J —4.75cps a singlet (IH) at 58.13), Reid et al. (335) proposed structures 84 or 85 for this adduct (E = COOMe). 

The same reaction performed in ether at 0°C (336) gives the same major adduct, but the structure proposed by Acheson et al. corresponds to 86, although such a structure is hardly compatible with the presence of an isolated low-field proton. Very recently, in a reinvestigation of these cyclo-additions of DMA to azoles (338, 339), Acheson et al. were able to establish the correct structure of the adducts on the base of CNMR spectra and X-ray diffraction studies. The adduct of thiazole is represented by formula 87, and it results from the rearrangement of the 

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