Premium
Synthesis and structural elucidation of pyrimido‐[1,2‐ a ]benzimidazoles and fused derivatives. i. dihydropyrimido[1,2‐ a ]benzimidazoles
Author(s) -
El Ella Dalai Abou,
Gößnitzer Edith,
Wendelin Winfried
Publication year - 1996
Publication title -
journal of heterocyclic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.321
H-Index - 59
eISSN - 1943-5193
pISSN - 0022-152X
DOI - 10.1002/jhet.5570330228
Subject(s) - tautomer , chemistry , trifluoroacetic acid , hydrochloric acid , chalcone , yield (engineering) , hydrochloride , condensation , medicinal chemistry , stereochemistry , ionone , organic chemistry , materials science , physics , metallurgy , thermodynamics
The condensation of 2‐benzimidazolarnine (4, BIA ) with α,β‐unsaturated ketones 1 affords, according to Desenko, Orlov et al . [12,13], 1,4‐dihydropyrimido[1,2‐ a ]benzimidazoles (1,4 ‐DHPBI, 5I ). However, the described ring closure reactions could a priori also yield isomeric 1,2 ‐DHPBI 8I or tautomers of DHPBIs 5I and 8I . An unequivocal proof for the postulated structures 5I was not presented. We prepared, as described [12,13], BIA ‐chalcone‐ and BIA ‐benzalacetone‐condensate X and Y, 5a, e or 8a, e , and additionally, their hydrochlorides. One and two dimensional high resolution nmr analyses showed that only isomers 5a, e and salts 5a, e ·HCl were generated. In DMSO‐d 6 these isomers exclusively exist as 1,4‐dihydro tautomers 5a, eI and 5a, eI ·HCl. In trifluoroacetic acid 3,4‐dihydro tautomers 5a, eIII ·CF 3 COOH besides of tautomers 5a, eI·CF 3 COOH (≈1:4) were ascertained. Action of ethanolic hydrochloric acid on base 5eI afforded, besides of 5eI ·HCl as main product, a small amount of 2‐methyl‐4‐phenyltetrahydropyrim‐ido[1,2‐ a ]benzimidazol‐2‐ol hydrochloride 9e ·HCl. The reaction of BIA 4 with α,β‐unsaturated ketones 1b‐d and f yielded 1,4 ‐DHPBI 5b‐dI and 5fI and their hydrochlorides, respectively. The stereochemistry of bases 5a, eI , of salts 5a‐eI ·HCl and 5a, eIII ·CF 3 COOH, and of addition product 9e ·HCl was elucidated by nmr (stereoformulae in Figures 1–3 and Scheme 5). The accomplished nmr‐analyses are documented in detail and discussed.