Ru(II)–arene complexes with N-substituted 3,4-dihydroquinazoline ligands and catalytic activity for transfer hydrogenation reaction

Triflic anhydride mediated synthesis of 3,4-dihydroquinazolines: a three-component one-pot tandem procedure

A one-pot three-component tandem reaction involving a key Pictet-Spengler-like annulation step has been developed, providing an efficient method for the synthesis of 3,4-dihydroquinazolines in moderate to good yields from amides, aldehydes, and amines. The multicomponent triflic anhydride mediated reaction tolerates the installation of numerous functional groups, affording extensive diversity about the heterocyclic scaffold.

Synthesis of dihydroquinazolines from 2-aminobenzylamine: N 3 -aryl derivatives with electron-withdrawing groups

The sequential N-functionalization of 2-aminobenzylamine (2-ABA) followed by cyclodehydration allowed for a straightforward and efficient synthesis of 3,4-dihydroquinazolines with N-aryl substituents bearing electron-withdrawing groups. The sequence involves an initial S_NAr displacement, N-acylation and MW-assisted ring closure. Remarkably, the uncatalyzed N-arylation of 2-ABA led to the monosubstitution product using equimolar amounts of both reagents. The individual steps were optimized achieving good to excellent overall yields of the desired heterocycles, avoiding additional protection and deprotection steps. A mechanistic interpretation for the cyclodehydration reaction promoted by trimethylsilyl polyphosphate (PPSE) is also proposed on the basis of literature data and our experimental observations.

Syntheses of 3,4- and 1,4-dihydroquinazolines from 2-aminobenzylamine

A straightforward strategy for the synthesis of dihydroquinazolines is presented, which allows for the preparation of 3,4- and 1,4-dihydroquinazolines with different substitution patterns from 2-aminobenzylamine (2-ABA) as common precursor. The required functionalization of both amino groups present in 2-ABA was achieved by different routes involving selective N-acylation and cesium carbonate-mediated N-alkylation reactions, avoiding protection/deprotection steps. The heterocycles were efficiently synthesized in short reaction times by microwave-assisted ring closure of the corresponding aminoamides promoted by ethyl polyphosphate (PPE).

Experimental and theoretical investigations into the stability of cyclic aminals

Background: Cyclic aminals are core features of natural products, drug molecules and important synthetic intermediates. Despite their relevance, systematic investigations into their stability towards hydrolysis depending on the pH value are lacking. Results: A set of cyclic aminals was synthesized and their stability quantified by kinetic measurements. Steric and electronic effects were investigated by choosing appropriate groups. Both molecular mechanics (MM) and density functional theory (DFT) based studies were applied to support and explain the results obtained. Rapid decomposition is observed in acidic aqueous media for all cyclic aminals which occurs as a reversible reaction. Electronic effects do not seem relevant with regard to stability, but the magnitude of the conformational energy of the ring system and pKa values of the N-3 nitrogen atom. Conclusion: Cyclic aminals are stable compounds when not exposed to acidic media and their stability is mainly dependent on the conformational energy of the ring system. Therefore, for the preparation and work-up of these valuable synthetic intermediates and natural products, appropriate conditions have to be chosen and for application as drug molecules their sensitivity towards hydrolysis has to be taken into account.