Amidinium salts reduction

In an approach toward a synthesis of tetraponerine 37, Gevorgyan first synthesized the fully aromatic tricyclic system 49 and then reduced it over two steps, first via hydrogenation under pressure (50 psi) to give 36 followed by a second reduction by lithium aluminium hydride of the amidinium salt (Scheme 1) <2002OL4697, 2004JOC5638>. 

Palladium on charcoal (Pd/C) is commonly used in the catalytic hydrogenation of pyrimidines in acidic media which halts the reduction at the 1,2,4,5-tetrahydro level because these, as amidinium salts, are stabilized <1962JOC2170, 1965JCS1406>. Platinum effects hydrogenation of the 5,6-double bond of uracils, for example, in the addition of deuterium to produce [S,6-2H2]S,6-dihydrouracil. The addition of hydrogen to the 5,6-double bond of thymidine and other S-substituted uridines is stereospecific with rhodium-on-alumina as catalyst. 

Fitting the current response for each individual H-bond donor revealed the stoichiometry of binding to be 2 1 for the monocationic and neutral catalysts, and 1 1 for the bisamidinium catalyst. To further support this proposed mechanism, additional kinetic smdies revealed a second-order dependence on the rate of quinone reduction with monocationic catalysts, and a first-order dependence on the rate of reduction using the biscationic amidinium salt. 

N-condensed, ring opening, reductive 26, 37 Amidinium salts... 

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