For our systematic investigation of the sulfamide-based Biginelli reaction, we chose a set of cyclization partners listed in Figure . Among sulfamides, the simplest derivative 1 and Me-and Phsubstituted analogues 2 and 3 were used. As a "CH-acidic" counterpart the classical ethyl acetoacetate 4a and its amide derivatives 4b-e, which were previously successfully applied by us , were explored. As an aldehyde component, the substituted benzaldehydes 5a-e bearing in para position substituents with different electronic properties and sterically hindered 5f, 5g were chosen as well. Additionally, a poorly enolizable aliphatic aldehyde 5h and diverse heterocyclic ones 5i-l were added to the training set. We assumed that this combination on the one hand allows us to investigate the scope of the reaction and on the other -helps to expect the diversity of future libraries based on the reaction. We started the investigation of the reaction from the straightforward sulfamide 1. Firstly, we applied the most efficient protocol in our parallel synthesis department for Biginelli-type heterocyclization, based on using TMSCl as condensing reagent and water scavenger . The protocol was elaborated in our group more than 15 years ago and was employed all this time in the REAL database program . It was shown ca. 86% success rate for the substituted urea/thiourea and amidine-like Biginelli heterocyclizations on more than 10000 examples . According to protocol the equimolar amounts (2 mmol) of reaction counterparts were placed in a 15 mL pressure tube and dissolved in DMF (2-5 mL). Then slight excess (10 mmol) of trimethylsilylchloride (TMSCl) was added dropwise to the solution. It should be noted that by stoichiometry 2 equivalents of TMSCl are necessary for scavenging 1 equivalent of water. The tube was thoroughly sealed and allowed to stand at rt in an ultrasound bath for 1 h and then at rt for 24-48 h. Further, the reaction mixture was poured onto water (10mL) and allowed to stand at rt in an ultrasound bath for 1-2 h. The precipitate formed was filtered and analyzed. In the vast majority of cases, we obtained a final compound with a good yield and more than 90% purity. Unexpectedly, our attempts to isolate something according to the standard protocol failed in the case of classical ethyl acetoacetate 4a and a set of all aldehydes. Moreover, we did not observe any signals with the molecular weight of expected products on the HPLC MS of the reaction mixtures. Based on these negative seminal results the project was very close to interrupting. Fortunately, before doing this, we tried acetoacetic acid amide 4c in the reaction by the standard protocol in combination with an aldehyde 5c. The crude product analysis showed ca. 60% homogeneity with expected molecular ion, according to HPLC MS data. However, all the attempts to purify the product failed. Moreover, it was quickly decomposed in DMSO solution and the 1 H NMR pattern of signals in the crude mixture was far from the expected structure type 6 (Scheme 1). The reproducibility of the results was also unpredictable. Based on the results, we guessed that the product is intrinsically unstable or decomposed during isolation or in the presence of impurities. The 1 H NMR spectrum of the product was also untypical. A more detailed analysis showed two characteristic signals at ca. 3.9 and 4.7 ppm in spin-spin interaction (see SI). These signals could be assigned to the protons at C(4) and C(5) in structure type 7, which is a tautomer of 6. The compounds type 7 bears a cyclic C=N bond, activated by acceptor SO2 moiety, which could easily hydrolysis by water, especially in acidic conditions. Considering the formation of HCl in our standard protocol, and, therefore, appearing the acidic condition, the modification of the latter became critical. Previously we met a similar problem in using of TMSCl-based methods, like upon isolation of hydrolytically unstable azomethines . In such cases to avoid the acidic media during isolation the DMF was successfully switched to pyridine as solvent . But in the currently investigated reaction such changing of the reaction conditions did not lead to success. Therefore, we decided to keep the synthesis protocol intact and optimize the isolation procedure. Thus, instead of water for treating the reaction mixture the solutions of NaHCO3, hexamethylenetetramine and N-methylmorpholine were explored. Among the above-mentioned bases' solutions hexamethylenetetramine, as well as Nmethylmorpholine, gave expected improved results, but in the case of N-methylmorpholine using the solubility of the final products in the formed mixture was greater, which made the purification step more complicated. Therefore, hexamethylenetetramine was chosen. During the slow-addition of reaction mixture into hexamethylenetetramine solution reaction with vigorous stirring at cooling in the ice bath. pH of the obtained in such a manner solution was ca. 5. The hexamethylenetetramine isolation protocol gives reproducible results affording the crude product with purity > 70% by 1 H NMR. The crystallization of the crude product from the acetone-hexane mixture leads to pure product 5cc, which structure was unambiguously proved by an X-ray study. The hypothesis of the hydrolysis of this product during isolation was confirmed by prolonged mixing of the water-treated reaction mixture without the base. A similar result was obtained by hydrolysis of compound 7cc in acidic media leading to compound 8cc, which structure also was proved by the X-ray (Scheme 1). Treatment of compound 8 by TMSCl in DMF according to the initial protocol gives cyclic compound 7 in significantly higher yield in comparison with three component procedure. The attempt to "isomerize" compound 7 to more usual form 6 via treatment with a strong base (DBU in DMF) and subsequent acidification did not succeed. The starting compound was isolated intact. The increase of the basicity (t-BuOK in DMF) leads to the decomposition of the substrates. Considering the hydrolytic instability of the compounds type 7 its purity analysis by standard HPLC MS protocols (H2O -MeCN with TFA used as eluent) becomes impossible. Also, the preparative HPLC methods do not applicable and the pure compound could be obtained only via crystallization of the crude products. Scheme 1. Three component cyclization leading to thiadiazines using parent sulfamide 1.