Synthesis, Structure and Stereochemistry of Dispirocompounds Based on Imidazothiazolotriazine and Pyrrolidineoxindole

Methods for the synthesis of two types of isomeric dispirocompounds based on imidazothiazolotriazine and pyrrolidineoxindole, differing in the structure of imidazothiazolotriazine fragment, namely, linear dispiro[imidazo[4,5-e]thiazolo[3,2-b][1,2,4]triazine-6,3′-pyrrolidine- 4′,3″-indolines] and angular dispiro[imidazo[4,5-e]thiazolo[2,3-c][1,2,4]triazine-7,3′-pyrrolidine-4′,3″-indolines] were proposed. The first method relies on a 1,3-dipolar cycloaddition of azomethine ylides generated in situ from paraformaldehyde and N-alkylglycine derivatives to the corresponding oxindolylidene derivatives of imidazothiazolotriazine. The cycloaddition leads to a mixture of two diastereomers resulted from anti- and syn-approaches of azomethine ylide in approximately a 1:1 ratio, which were separated by column chromatography. Another method consists in rearrangement of linear dispiro[imidazo[4,5-e]thiazolo[3,2-b][1,2,4]triazine-6,3′-pyrrolidine-4′,3″-indolines] into hitherto unavailable angular dispiro[imidazo[4,5-e]thiazolo[2,3-c]-[1,2,4]triazine-7,3′-pyrrolidine-4′,3″-indolines] upon treatment with KOH. It was found that the anti-diastereomer of linear type underwent rearrangement into the isomeric angular syn-diastereomer, while the rearrangement of the linear syn-diastereomer gave the angular anti-diastereomer.

The stereoselectivity of the cycloaddition can be explained by the possible approach of sterically unhindered "small" azomethine ylide to the plane of the double bond of dipolarophile both from the opposite side (anti-) and from the same side (syn-) to which the imidazolidine cycle deviates (Scheme 2). Scheme 1. Synthesis of racemic spiropyrrolidineoxindoles 3a-d, 4a-d via 1,3-dipolar cyclo using paraformaldehyde and sarcosine as azomethine ylide precursors.
The stereoselectivity of the cycloaddition can be explained by the possible ap of sterically unhindered "small" azomethine ylide to the plane of the double bon polarophile both from the opposite side (anti-) and from the same side (syn-) to w imidazolidine cycle deviates (Scheme 2).
The presumable rearrangement mechanism is depicted in Scheme 6. We a the rearrangement occurs as transamidation reaction upon treatment with KO anol [27]. The nucleophilic attack of the methoxide anion onto the carbon atom in the cleavage of the C(7)-N(8) bond followed by rotation of the spiropyrro dole moiety around the C-S bond and the recyclization involving the nitrogen of the triazine ring (Scheme 6). Scheme 6. The presumable reaction mechanism of the rearrangement of compounds 3 isomers 7a and 6a.

Structural Determination of Compounds 3,4,6,7
The structures of the compounds obtained were elucidated by spectral m Scheme 5. Synthesis of isomeric racemic spiropyrrolidineoxindoles 6a and 7a via rearrangement of spiropyrrolidineoxindoles 4a and 3a.
The presumable rearrangement mechanism is depicted in Scheme 6. We assume that the rearrangement occurs as transamidation reaction upon treatment with KOH in methanol [27]. The nucleophilic attack of the methoxide anion onto the carbon atom C (7) results in the cleavage of the C(7)-N(8) bond followed by rotation of the spiropyrrolidinyloxindole moiety around the C-S bond and the recyclization involving the nitrogen atom N(4) of the triazine ring (Scheme 6). nt. J. Mol. Sci. 2022, 23, x FOR PEER REVIEW 5 of 17 diastereomer 6a (3aR*,4′R*,7S*,9aS*). The yields of the rearrangement products 6a and 7a were 80 and 92%, respectively (Scheme 5).
The presumable rearrangement mechanism is depicted in Scheme 6. We assume tha the rearrangement occurs as transamidation reaction upon treatment with KOH in meth anol [27]. The nucleophilic attack of the methoxide anion onto the carbon atom C (7) results in the cleavage of the C(7)-N(8) bond followed by rotation of the spiropyrrolidinyloxin dole moiety around the C-S bond and the recyclization involving the nitrogen atom N(4 of the triazine ring (Scheme 6). Scheme 6. The presumable reaction mechanism of the rearrangement of compounds 3a and 4a into isomers 7a and 6a.

Structural Determination of Compounds 3,4,6,7
The structures of the compounds obtained were elucidated by spectral methods; sin gle crystal X-ray diffraction of 3a, 4e and 7c was also carried out.
Compounds 3a and 7c crystalize in centrosymmetric space groups P21/c and P1 , re spectively, while 4e structure crystalizes in an acentric orthorhombic space group P21212 as an inversion twin. Thus, these compounds crystalize as racemic mixtures of two enan tiomers. The relative configurations of chiral centers of independent part of compounds Scheme 6. The presumable reaction mechanism of the rearrangement of compounds 3a and 4a into isomers 7a and 6a.

Structural Determination of Compounds 3,4,6,7
The structures of the compounds obtained were elucidated by spectral methods; single crystal X-ray diffraction of 3a, 4e and 7c was also carried out.

Presence of H(N) atom allows H-bonding for three solids
The structures of compounds 3a, 3b, 6a and 7b were also confirmed by NMR data in DMSO solutions using NOESY 2D correlation technique. Hydrogen atoms of alkyl group at the nitrogen atom N(1) and hydrogen atoms H-4 , H-5 of oxindole fragment in compounds 3 are spatially close. The presence of cross-peaks of these protons in the NOESY spectra is characteristic of the anti-diastereomers 3a, 3b. In the NOESY spectra of anti-diastereomer 6a, correlations between protons of N(1)Me group and H-4 of oxindole fragment, between protons of N(3)Me and N(4)H groups, as well as between protons H-7 and N(1 )Me of oxindole fragment were observed. In the NOESY spectrum of syndiastereomer 7b cross-peaks of NMe group and oxindole fragment protons are absent (see Figure 6 and Supplementary Information). perpendicular to each other. The nitrogen atoms N(3) and N(6) are slightly pyramidalized with the sum of bond angles at these atoms less than 340° (Table S2).
Presence of H(N) atom allows H-bonding for three solids (Figures 3-5). In 3a centrosymmetric dimers are formed by two bifurcated contacts N(3)…O(2), N(3)…N (6). In 4e and 7c infinite chains are observed. For 4e a solvent methanol acts as a bridge between two molecules.
The structures of compounds 3a, 3b, 6a and 7b were also confirmed by NMR data in DMSO solutions using NOESY 2D correlation technique. Hydrogen atoms of alkyl group at the nitrogen atom N(1) and hydrogen atoms Н-4″, Н-5″ of oxindole fragment in compounds 3 are spatially close. The presence of cross-peaks of these protons in the NOESY spectra is characteristic of the anti-diastereomers 3a, 3b. In the NOESY spectra of antidiastereomer 6a, correlations between protons of N(1)Me group and Н-4″ of oxindole fragment, between protons of N(3)Me and N(4)H groups, as well as between protons Н-7″ and N(1′)Me of oxindole fragment were observed. In the NOESY spectrum of syn-diastereomer 7b cross-peaks of NMe group and oxindole fragment protons are absent (see Figure 6 and Supplementary Information). In the spectra of angular structures 4 and 7, downfield shifts of the proton signal of the NH group from 6.73-7.05 to 7.43-7.66 ppm were observed in comparison with the spectra of the linear structures 3 and 6. Downfield shifts from 4.59-4.87 to 5. 35-5.85 ppm were also revealed for the signal of one of the bridging protons H(3a) or H(9a) (Figure 7). In the spectra of angular structures 4 and 7, downfield shifts of the proton signal of the NH group from 6.73-7.05 to 7.43-7.66 ppm were observed in comparison with the spectra of the linear structures 3 and 6. Downfield shifts from 4.59-4.87 to 5. 35-5.85 ppm were also revealed for the signal of one of the bridging protons H(3a) or H(9a) (Figure 7). Thus, we obtained two diastereomers of each type of dispirocompounds of linear and angular structure in individual form and proved their structures and stereochemistry by spectral methods and single crystal X-ray diffraction.

General Information
Melting points were determined in open glass capillaries on a Gallenkamp (Sanyo) melting point apparatus. IR spectra were recorded on a Bruker ALPHA instrument in KBr pellets. High resolution mass spectra (HRMS) were measured on a Bruker micrOTOF II instrument using electrospray ionization (ESI). The measurements were done in a positive ion mode (interface capillary voltage-4500 V) or in a negative ion mode (3200 V); mass range from m/z 50 to m/z 3000 Da; external or internal calibration was done with electrospray calibrant solution (Fluka). A syringe injection was used for solutions in acetonitrile or methanol (flow rate 3 mL min −1 ). Nitrogen was applied as a dry gas; the interface temperature was set at 180 °C. 1 H NMR and 13 C NMR spectra were recorded on a Bruker AM300 spectrometer operating at 300.13 MHz and 75.5 MHz,respectively,Bruker DRX 500 spectrometer (500.13 MHz and 125.76 MHz,respectively) and Bruker DRX 600 spectrometer (Bruker,USA;600.13 MHz and 150.90 MHz, respectively) using DMSO-d6 as a solvent. Chemical shifts (δ) are given in ppm from TMS as an internal standard. All reactions were run in air.
Column chromatography was carried out with silica gel (Acros Silica gel, for column chromatography, 60 Angstrom pore size, 0.040-0.063 mm) and to monitor the preparative separations, analytical thin layer chromatography (TLC) was performed at room Thus, we obtained two diastereomers of each type of dispirocompounds of linear and angular structure in individual form and proved their structures and stereochemistry by spectral methods and single crystal X-ray diffraction.

General Information
Melting points were determined in open glass capillaries on a Gallenkamp (Sanyo) melting point apparatus. IR spectra were recorded on a Bruker ALPHA instrument in KBr pellets. High resolution mass spectra (HRMS) were measured on a Bruker micrOTOF II instrument using electrospray ionization (ESI). The measurements were done in a positive ion mode (interface capillary voltage-4500 V) or in a negative ion mode (3200 V); mass range from m/z 50 to m/z 3000 Da; external or internal calibration was done with electrospray calibrant solution (Fluka). A syringe injection was used for solutions in acetonitrile or methanol (flow rate 3 mL min −1 ). Nitrogen was applied as a dry gas; the interface temperature was set at 180 • C. 1 H NMR and 13 C NMR spectra were recorded on a Bruker AM300 spectrometer operating at 300.13 MHz and 75.5 MHz,respectively,Bruker DRX 500 spectrometer (500.13 MHz and 125.76 MHz,respectively) and Bruker DRX 600 spectrometer (Bruker,USA;600.13 MHz and 150.90 MHz, respectively) using DMSO-d 6 as a solvent. Chemical shifts (δ) are given in ppm from TMS as an internal standard. All reactions were run in air.
Column chromatography was carried out with silica gel (Acros Silica gel, for column chromatography, 60 Angstrom pore size, 0.040-0.063 mm) and to monitor the preparative separations, analytical thin layer chromatography (TLC) was performed at room temperature on pre-coated 0.25 mm thick silica gel 60 F 254 aluminum plates 20 × 20 cm Merck, Darmstadt, Germany. Propanol-2 was used as eluent without preliminary purification.
The X-ray data collection for samples 3a and 7c were performed on a Bruker APEX DUO diffractometer equipped with an Apex II CCD detector (graphite monochromator, ω-scans), and for sample 4e on a Bruker Quest D8 diffractometer equipped with a Photon-III area-detector (graphite monochromator, shutterless ϕand ω-scan technique), using Mo Kα-radiation (λ = 0.71073 Å).
The intensity data were integrated by the Bruker SAINT software package [31] and were corrected for absorption and decay using the SADABS program [32]. The structures were solved using the SHELXT program [33] and refined by the full-matrix least-squares technique against F 2 hkl in anisotropic approximation for non-hydrogen atoms with the SHELXL program [34]. Hydrogen atoms connected to heteroatoms were found from difference Fourier synthesis and refined isotropically. Other hydrogen atoms were placed in calculated positions and refined in the riding model with U iso (H) = 1.5U eq (C m ) for methyl groups and 1.2U eq (C i ) for other carbon atoms to which corresponding H atoms are bonded. The structure 4e was refined as an inversion twin with calculated BASF parameter equal to 0.58. The cyclohexyl fragment in 4e was found to be disordered by two positions with refined relative occupancies 0.68:0.32, ISOR instruction was used for the disordered atoms. For 4e a SQUEEZE method implemented in the PLATON program [35] was applied to model the disordered methanol molecule.