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Synthesis of endo-fused 5-unsubstituted Hexahydro-2H-pyrano[3,2-c]quinolinesvia Sequential Sc(OTf)3-catalyzed Cationic Imino-Diels-Alder Reaction/N-debenzylation using N-benzylanilines, 3,4-dihydro-2H-pyran and Paraformaldehyde under MW Irradiation

Author(s):

Arturo René Mendoza Salgado, Carlos Eduardo Puerto Galvis, Vladimir V. Kouznetsov and Carlos Mario Meléndez*   Pages 1 - 12 ( 12 )

Abstract:


Background: Hexahydro-2H-pyrano[3,2-c]quinolines are known to have antibacterial, antifungal, and antitumor properties. Great efforts have been made to develop new synthetic methods that lead to the synthesis of valuable libraries. Extensive methodologies, low yields, excessive amounts of catalyst and expensive reactants are some of the limitations of current methodologies.

Aim and Objective: Developing a useful and efficient method to construct diversely substituted hexahydro-2H-pyrano[3,2- c]quinolines into good to excellent yields through a cationic imino-Diels-Alder/N-debenzylation methodology.

Method: The cationic imino-Diels-Alder/N-debenzylation methodology was used for the preparation of substituted hexahydro-2H-pyrano[3,2-c]quinolines. It involves the use of Sc(OTf)3 for activation of cationic imino-Diels-Alder cycloaddition reaction of N-benzylanilines, 3,4-dihydro-2H-pyran and paraformaldehyde in MeCN; and microwave irradiation to shorten reaction time to afford new 6-benzyl-hexahydro-2H-pyrano[3,2-c]quinolines whose catalytic transfer debenzylation reactions with HCO2NH4 in the presence of Pd/C (10%) and methanol give the new 5-unsubstituted pyrano[3,2- c]quinolines in excellent yields.

Results: We found that optimal conditions for the preparation of hexahydro-2H-pyrano[3,2-c]quinolines were Sc(OTf)3 0,5 % and acetonitrile at 160 ° C for 15 min; and using paraformaldehyde obtained the 6-benzyl-hexahydro-2H-pyrano [3,2- c]quinolines with excellent yields, while the N-debenzylation process using ammonium formate in the presence of Pd/C and methanol resulted in the synthesis of hexahydro-2H-pyrano [3,2-c] quinolines with quantitative yields (95-98%).

Conclusion: We describe an efficient method to synthesize hexahydro-2H-pyrano[3,2-c]quinolines via the cationic iminoDiels-Alder/N-debenzylation methodology using Sc(OTf)3 0,5 % as Lewis Acid catalyst. Excellent yields of the products, using of MW irradiation, short times of reactions and an efficient and highly diversified method are some of the main advantages of this new protocol.

Keywords:

Cationic imine Diels-Alder, Lewis acid catalysis, scandium triflate, microwave irradiation, N-debenzylation, pyrano[3, 2-c]quinolones.

Affiliation:

Departamento de Química, Facultad de Ciencias Básicas, Universidad del Atlántico, Barranquilla, Escuela de Química, Facultad de Ciencias, Universidad Industrial de Santander, Bucaramanga, Escuela de Química, Facultad de Ciencias, Universidad Industrial de Santander, Bucaramanga, Departamento de Química, Facultad de Ciencias Básicas, Universidad del Atlántico, Barranquilla



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