Benzo[6,7]cyclohepta[1,2-d]pyrazolo[1,5-a]pyrimidines: Regioselective synthesis of a Novel Ring System, DFT-based NMR prediction, local reactivity indexes, and MEP analysis

Abstract

Novel benzo[6,7]cyclohepta[1,2-d]pyrazolo[1,5-a]pyrimidines were prepared as a new ring system utilizing a regioselective protocol. A mixture of the appropriate 3,5-diamino-1H-pyrazoles and benzosuberone-based enones in ethanolic potassium hydroxide solution was heated at reflux for 5 h to produce the novel benzosuberone-fused pyrazolo[1,5-a]pyrimidines in 87–94 % yields. Both spectral data as well as elemental analyses were used to establish the structure of the novel products. Theoretical calculation of chemical shifts of NMR spectra is a powerful tool in analyzing experimental spectra for known substrates as well as elucidating the chemical structure for unknown ones. Using the DFT-GIAO approach under the B3LYP and mPW1PW91 functions, as well as a variety of basis sets, including 6–31G(d,p), 6–31+G(d), 6–311G(d), 6–311+G(d,p), 6–311++G-(d,p), and 6–311+G(2d,p), the chemical shifts of ¹H- and ¹³C NMR in DMSO were calculated. The results that are most similar to the experimental observations are obtained from the computation of the chemical shift values under the 6–31G(d,p) or 6–31+G(d,p) basis sets and mPW1PW91 when using the multistranded approach. By identifying the preferred regions for electrophilic and nucleophilic attack, DFT-based FMO, global descriptors, local reactivity indexes, and MEP mapping were applied at the B3LYP function using the 6–31+G(d,p) basis set to provide additional insight into the regioselectivity in the desired reaction as well as the plausible mechanism for the formation of new products.