Angular Regioselectivity in the Reactions of 2-Thioxopyrimidin-4-ones and Hydrazonoyl Chlorides: Synthesis of Novel Stereoisomeric Octahydro[1,2,4]triazolo[4,3-a]quinazolin-5-ones

The regioselective synthesis of cis and trans stereoisomers of variously functionalized octahydro[1,2,4]triazolo[4,3-a]quinazolin-5-ones was performed. The 2-thioxopyrimidin-4-ones used in the synthesis reacted with hydrazonoyl chlorides in a regioselective manner to produce the angular regioisomers [1,2,4]triazolo[4,3-a]quinazolin-5-ones rather than the linear isomers [1,2,4]triazolo[4,3-a]quinazolin-5-ones. The synthesis process took place with electronic control. The angular regiochemistry of the products was confirmed by X-ray experiments and two-dimensional NMR studies.

Herein, we report the extension of our research for the regioselective synthesis of novel cisand trans-octahydro[1, 2,4]triazolo [4,3-a]quinazolin-5-ones 4a-g and 5a-g via the reaction of cyclohexane-fused cisor trans-2-thioxopyrimidin-4-ones 1 and 2 with hydrazonoyl chlorides 3a-g, taking place under electronic control. Moreover, X-ray and two-dimensional NMR studies were used to prove the stereochemistry of the products.

Results and Discussion
Cyclohexane-fused cis-and trans-2-thioxopyrimidin-4-one 1 and 2 were prepared according to previously described procedures [22]. The thioxopyrimidinone derivatives 1 or 2 thus prepared were reacted with the hydrazonoyl chlorides 3a-g bearing varied functionalities in dioxane in the presence of triethylamine as a base under reflux conditions (Scheme 1). According to the reaction mechanism depicted in Scheme 2, the angular regioisomers [1,2,4]triazolo[4,3-a]quinazolin-5(3H)-one 4a-g and 5a-g and linear regioisomers [1,2,4]triazolo[4,3-a]quinazolin-5(3H)-one 6a-g and 7a-g were expected to be formed. The outcome of the reactions depends on the involvement of the tautomeric structures I or II of the cyclohexane-fused 2-thioxopyrimidin-4-ones 1 and 2. The reactions proceeded through S-alkylation [17][18][19][20][21] to give S-alkylated products A followed by Smiles rearrangement [23], affording intermediates B, which cyclized in situ under the employed reaction conditions via the elimination of hydrogen sulfide gas to give the desired products 4a-g and 5a-g [20]. As evidenced by TLC and NMR spectroscopy, the transformations took place in a regioselective manner, producing the corresponding angular regioisomers as the sole products.

Results and Discussion
Cyclohexane-fused cis-and trans-2-thioxopyrimidin-4-one 1 and 2 were prepared according to previously described procedures [22]. The thioxopyrimidinone derivatives 1 or 2 thus prepared were reacted with the hydrazonoyl chlorides 3a-g bearing varied functionalities in dioxane in the presence of triethylamine as a base under reflux conditions (Scheme 1). According to the reaction mechanism depicted in Scheme 2, the angular regioisomers [1,2,4]triazolo[4,3-a]quinazolin-5(3H)-one 4a-g and 5a-g and linear regioisomers [1,2,4]triazolo[4,3-a]quinazolin-5(3H)-one 6a-g and 7a-g were expected to be formed. The outcome of the reactions depends on the involvement of the tautomeric structures I or II of the cyclohexane-fused 2-thioxopyrimidin-4-ones 1 and 2. The reactions proceeded through S-alkylation [17][18][19][20][21] to give S-alkylated products A followed by Smiles rearrangement [23], affording intermediates B, which cyclized in situ under the employed reaction conditions via the elimination of hydrogen sulfide gas to give the desired products 4a-g and 5a-g [20]. As evidenced by TLC and NMR spectroscopy, the transformations took place in a regioselective manner, producing the corresponding angular regioisomers as the sole products. The steric structure of the angular regioisomers was evidenced with information acquired through various instrumental techniques, namely, 1 H-NMR, 13 C-NMR, and two-dimensional NMR including NOESY (neighboring Overhauser effect spectroscopy correlation), HMBC (heteronuclear multiple bond correlation), and X-ray crystallographic analysis. The 1 H-NMR spectra of the products Scheme 2. Proposed reaction pathways to form angular and linear regioisomers.
The steric structure of the angular regioisomers was evidenced with information acquired through various instrumental techniques, namely, 1 H-NMR, 13 C-NMR, and two-dimensional NMR including NOESY (neighboring Overhauser effect spectroscopy correlation), HMBC (heteronuclear multiple bond correlation), and X-ray crystallographic analysis. The 1 H-NMR spectra of the products formed by the hydrazonoyl chloride ethyl esters 3a-f show a more multiplicated signal pattern corresponding to the CH 2 moiety of the ester functional group (Supplementary Materials), which suggests the steric proximity of the ester group and the cyclohexane skeleton. Moreover, the NOESY spectra exhibit a mutual correlation between the hydrogens of CH 2 and cyclohexane. In addition, the HMBC spectra show a mutual correlation between H-9a and C-1, which are separated by three bonds in the angular regioisomers. However, this correlation cannot exist in the linear regioisomers, because the C-3 and H-9a atoms are separated by five bonds (Figure 1a). Last but not least, the 13 C-NMR spectra reveal the signal of the carbonyl carbon of the pyrimidinone ring residue at nearly 176 ppm. These chemical shift values are similar to those of annelated pyrimidinones of type A rather than those of type B (Figure 1b) [24]. Finally, the X-ray crystallographic analysis of 5b provided conclusive evidence for the angular regiochemistry of the products ( Figure 2).
Molecules 2020, 25, x FOR PEER REVIEW 3 of 8 formed by the hydrazonoyl chloride ethyl esters 3a-f show a more multiplicated signal pattern corresponding to the CH2 moiety of the ester functional group (Supplementary Materials), which suggests the steric proximity of the ester group and the cyclohexane skeleton. Moreover, the NOESY spectra exhibit a mutual correlation between the hydrogens of CH2 and cyclohexane. In addition, the HMBC spectra show a mutual correlation between H-9a and C-1, which are separated by three bonds in the angular regioisomers. However, this correlation cannot exist in the linear regioisomers, because the C-3 and H-9a atoms are separated by five bonds (Figure 1a). Last but not least, the 13 C-NMR spectra reveal the signal of the carbonyl carbon of the pyrimidinone ring residue at nearly 176 ppm. These chemical shift values are similar to those of annelated pyrimidinones of type A rather than those of type B (Figure 1b) [24]. Finally, the X-ray crystallographic analysis of 5b provided conclusive evidence for the angular regiochemistry of the products (Figure 2).  On the basis of the above evidence, the angular structures 4a-g and 5a-g were assigned for the products, and, consequently, the linear structures 6a-g and 7a-g could be rejected.
The regioselectivity of these reactions delivering the angular regioisomers was ascribed to electronic factors rather than steric factors. That is, since the tautomeric form I is electronically and formed by the hydrazonoyl chloride ethyl esters 3a-f show a more multiplicated signal pattern corresponding to the CH2 moiety of the ester functional group (Supplementary Materials), which suggests the steric proximity of the ester group and the cyclohexane skeleton. Moreover, the NOESY spectra exhibit a mutual correlation between the hydrogens of CH2 and cyclohexane. In addition, the HMBC spectra show a mutual correlation between H-9a and C-1, which are separated by three bonds in the angular regioisomers. However, this correlation cannot exist in the linear regioisomers, because the C-3 and H-9a atoms are separated by five bonds (Figure 1a). Last but not least, the 13 C-NMR spectra reveal the signal of the carbonyl carbon of the pyrimidinone ring residue at nearly 176 ppm. These chemical shift values are similar to those of annelated pyrimidinones of type A rather than those of type B (Figure 1b) [24]. Finally, the X-ray crystallographic analysis of 5b provided conclusive evidence for the angular regiochemistry of the products (Figure 2).  On the basis of the above evidence, the angular structures 4a-g and 5a-g were assigned for the products, and, consequently, the linear structures 6a-g and 7a-g could be rejected.
The regioselectivity of these reactions delivering the angular regioisomers was ascribed to electronic factors rather than steric factors. That is, since the tautomeric form I is electronically and On the basis of the above evidence, the angular structures 4a-g and 5a-g were assigned for the products, and, consequently, the linear structures 6a-g and 7a-g could be rejected.
The regioselectivity of these reactions delivering the angular regioisomers was ascribed to electronic factors rather than steric factors. That is, since the tautomeric form I is electronically and energetically predominant, the reaction proceeds through tautomeric form I and leads to the formation of the angular regioisomer (Scheme 2).

General Methods
NMR analyses were performed at 500.20 MHz for 1 H-NMR and at 125.62 MHz for 13 C-NMR in CDCl 3 at room temperature, using a Bruker AV NEO Ascend 500 spectrometer (Bruker Biospin, Karlsruhe, Germany) with a Double Resonance Broad Band Probe (BBO). Tetramethylsilane (TMS) was used as an internal standard. The reactions were monitored by thin-layer chromatography (TLC) using aluminum sheets coated with silica gel (POLYGRAM ® SIL G/UV254, Merck, Kenilworth, NJ, USA). The TLC plates were visualized under UV light. The melting points were measured using a Hinotek-X4 micro melting point apparatus (Hinotek, Ningbo, China).
X-ray diffraction data were collected on a Rigaku Oxford Diffraction Supernova diffractometer using Cu Kα radiation, measured at a temperature of 120 K using a crystal of 5b immersed in cryo-oil and mounted in a loop. The CrysAlisPro [29] software package was used for cell refinement and data reduction. An analytical absorption correction (CrysAlisPro) was applied to the intensities before structure solution. The structure was solved by an intrinsic phasing method (SHELXT [30,31]). Structural refinement was carried out using the SHELXL [30] software with the SHELXLE [31] graphical user interface. Hydrogen atoms were positioned geometrically and constrained to ride on their parent atoms, with C-H = 0.95-1.00 Å and U iso = 1.2-1.5·U eq (parent atom). The crystallographic details are summarized in Table S1.