NMR-Verified Dearomatization of 5,7-Substituted Pyrazolo[1,5-a]pyrimidines

Abstract

Tetrahydropyrazolo[1,5-a]pyrimidine (THPP) is an attractive scaffold for designing biologically active compounds. The most obvious way to obtain such compounds is to reduce pyrazolopyrimidines with complex hydrides, because the pyrimidine ring is reduced in the preference over the pyrazole ring. The presence of substituents at positions five and seven of pyrazolo[1,5-a]pyrimidines complicates the set of reaction products but makes it more attractive for medicinal chemistry because four possible stereoisomers can be formed during reduction. However, the formation of only syn-isomers has been described in the literature. This article is the first report on the formation of anti-configured isomers along with syn-isomers in the reduction of model 5,7-dimethylpyrazolo[1,5-a]pyrimidine, which was confirmed by NMR. The bicyclic core in the syn-configuration was shown to be conformationally stable, which was used to estimate the long-range interproton distances using NOESY data. At the same time, long-range dipole-dipole interactions corresponding to a distance between protons of more than 6 Å were first registered and quantified. In turn, the bicyclic core in the trans-configuration represents a conformationally labile system. For these structures, an analysis of conformations observed in solutions was carried out. Our results indicate the significant potential of trans-configured tetrahydropyrazolo[1,5-a]pyrimidines for the development of active small molecules. While possessing structural lability due to the low energy of the conformational transition, they have the ability to adjust to the active site of the desired target.

Keywords: NOESY; conformational analysis; dearomatization; long-range interproton distance; proton–proton vicinal constants; pyrazolo[1,5-a]pyrimidine; t1 noise; t2 noise.

Figure 1

Structures of the most active THPP representatives of CaSR antagonists (a), antituberculous agents (b), and Bruton’s tyrosine kinase inhibitors (c).

Scheme 1

Synthesis of ethyl 5,7-dimethylpyrazolo[1,5-a]pyrimidine-3-carboxylate. Reagents and conditions: (a) (MeO)₂CHNMe₂, 70 °C, 6 h, 90%; (b) NH₂NH₂∙H₂O, EtOH/H₂O, 90 °C, 4 h, 82%; (c) CH₃COCH₂COCH₃, AcOH/EtOH, 100 °C, 6 h, 80%.

Scheme 2

Dearomatization of 5,7-dimethylpyrazolo[1,5-a]pyrimidine-3-carboxylate. Reagents and conditions: (a) NaBH₄, ROH, RONa, R = CH₃, and C₂H₅.

Figure 2

Superposition of main SYN conformations (a) and two possible conformations of the pyrazolopyrimidine core (b,c). Legend: carbon, hydrogen, nitrogen, and oxygen atoms are given in green, white, blue, and red, respectively.

Figure 3

Two possible conformations of the pyrazolopyrimidine core for the anti-configuration of substituents: 5-CH₃ at equatorial position, 7-CH₃ at axial position (a); 5-CH₃ at axial position, 7-CH₃ at equatorial position (b); and the transition between the most energetically favorable ANTI conformations (c). Legend: carbon, hydrogen, nitrogen, and oxygen atoms are given in green, white, blue and red, respectively.

Figure 4

Newman projections along C⁶–C⁵ (a) and C⁶–C⁷ (b) bonds for ANTI1 and ANTI2 conformers. The direction of view is marked with arrows next to letters (a, b). Legend: carbon, hydrogen, and nitrogen atoms are given in green, white, and blue, respectively.

Figure 5

Calculated dependences of vicinal constants on the ANTI1 conformer population under fast two-position exchange: for the pair of constants ³J_H6α–H7 and ³J_H6β–H5 (a) and ³J_H6α–H5 and ³J_H6β–H7 (b). Blue and green full lines correspond to calculated dependences of corresponding constants; blue and green dashed lines indicate their errors. Red full lines are used to determine the conformer population from experimental values of corresponding constants; red dashed lines indicate errors of this value.

Figure 6

Intensified NOESY spectrum of SYN. Legend: negative and positive NOE signals are indicated in blue and red, respectively.

Figure 7

Volume integration of H⁴/H⁶_β (a) and H²/H⁶_β cross-peaks (b) in the NOESY spectrum of SYN. Legend: negative and positive NOE signals are indicated in blue and red, respectively.

Figure 8

Projections of the SYN molecule along its principal axes (a–c) and determination of polar angles for radius vectors of interproton distances. Legend: carbon, hydrogen, nitrogen, and oxygen atoms are given in green, white, blue and red, respectively.

Figure 9

Correlation of the calculated and experimental distances (indicated as curcles) in the case of using r(H⁵–H⁷) (a,c) and r(H⁶_α–H⁶_β) (b) as reference distances (marked in red). Green full lines represent regression lines; green dashed lines represent x = y function.

Figure 10

The structure of multiplets for protons H⁶_α and H⁶_β (a) and the fragment of the NOESY spectrum of ANTI containing cross-peaks H⁴/H⁶_α and H⁴/H⁶_β (b). Legend: negative and positive NOE signals are indicated in blue and red, respectively.

Figure 11

Calculated dependence of the integral intensity ratio of H⁴/H⁶_α and H⁴/H⁶_β cross-peaks on the ANTI1 conformer population under conditions of fast two-position exchange. Green full line represent the dependence. Red full lines are used to determine the conformer population from the experimental ratio; red dashed lines indicate errors of this value.