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‐d6 these isomers exclusively exist as 1,4‐dihydro tautomers 5a, eI and 5a,eI·HCl. In trifluoroacetic acid 3,4‐dihydro tautomers 5a, eIII·CF3COOH besides of tautomers 5a, eI·CF3COOH (≈︁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·CF3COOH, 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.
