1,4-dihydropyridine synthesis iminium activation

Omission of the phenolic group from cyclazocine results in a molecule which retains analgesic activity. In a classical application of the Grewe synthesis,15 the methylated pyridinium salt 54 is condensed with benzylmagnesium bromide. There is thus obtained the dihydropyridine 55. Treatment of that intermediate with sodium borohydride results in reduction of the iminium function to afford the tetrahydro derivative 56. Cyclization of 56 on treatment with acid leads to the desired benzomorphan nucleus. The cis compound (57) is separated from the mixture of isomers and demethylated by the cyanogen bromide procedure (58,... 

Cyclopentenes were also activated (with anff-directed methylsulfenylation) by DMTSF in the presence of nucleoside base anions yielding nucleoside analogs that could be converted into the antiviral agent Carbovir and related substances (eq 16). DMTSF-treatment of a dihydropyridine afforded an iminium ion, which was attacked intramolecularly by an indole moiety to form a key intermediate (albeit in a low yield) in a total synthesis of 7V-methylvitsine (eq 17). ... 

In 2013, Yoshikai and Wei developed a copper-catalyzed pyridines synthesis from oximes and enals [65]. Under redox-neutral reaction conditions, with 0-acetyl ketoximes and 0 ,/3-unsaturated aldehydes as the substrates and using copper(I) salt and a secondary ammonium salt (or amine) as the catalyst system, a variety of substituted pyridines were prepared with a broad range of functional groups tolerance (Scheme 3.29). By merging iminium catalysis and copper catalyst, imder the redox activity of the copper catalyst, the reaction started to reduce the oxime N—O bond to generate a nucleophilic copper(II) enamide and later oxidize a dihydropyridine intermediates to the final products. 

The synthesis is initiated by the organocatalyzed cascade that activates a,p-unsaturated aldehyde 8 with the formation of an iminium ion (Scheme 14.2). In this way, the imidazolidinone catalyst allows hydride transfer from the Hantzsch dihydropyridine 9 onto the highly activated conjugated alkene 11, which creates the nucleophilic enamine intermediate 12. Because of the chirality of the organocatalyst, stereoselective Michael addition (97% ee) to the adjacent enone occurs, with minor preference for the cis diastereomer (2 1 dr). Fortunately, this undesired diastereomer slowly epimerizes to the required trans isomer, which produces (-l-)-ricciocarpin A when treated with samarium triisopropoxide. Besides the Cannizzaro-like redox disproportionation, which allows the lactone producing Evans-Tihchenko reaction to occur, samarium(III) also enhances the epimerization to the trans isomer and therefore produces the desired isomer in high selectivity. 

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