Pyrroles, silver-mediated oxidation

Silver(I)-catalyzed cyclizations of substituted allenes to heterocydic ring systems induding 2,5-dihydropyrroles have been described previously [4,10]. Moreover, sil-ver(I) salts are known to form stable rr-complexes with terminal acetylenes [11]. On the other hand, on treatment with silver nitrate silylacetylenes were reported to afford silver acetylides [12]. Based on these considerations and additional experimental evidence [5,13], the following mechanism has been proposed forthe sUver(I)-mediated oxidative cydization of homopropargylamines to pyrroles ]5] (Scheme 15.3). 

We used the silver(I)-mediated oxidative qfdization of homopropargylamines to pyrroles for the synthesis of crispine A (Scheme 15.4) [7]. 

Reaction of 3,4-dihydro-6,7-dimethoxyisoquinoline with the propargyl Grignard reagent leads to the cyclic homopropargylamine, which on silver(I)-mediated oxidative cyclization affords 5,6-dihydro-8,9-dimethoxypyrrolo[2,l-a]isoquinoline. Che-moselective hydrogenation of the pyrrole ring provides racemic crispine A (three steps, 24 % overall yield). 

Silver enolates are proposed as reactive intermediates in the reaction of sUyl enolates with AgaO in dmso. An important feature of the reaction is the regiospecific formation of l,4-diketones °j. Sessler and coworkers reported that the key step in the preparation of / -substituted tetra- and hexaalkylterpyrrols is the copper(II) triflate-mediated oxidative coupling of the Ida-derived enolates of a-keto pyrrols. The coupling reaction shown in equation 24 produces a mixture of distereoisomers which does not require separation and can be directly converted to the corresponding terpyrroles. 

Scheme 1 Synthesis of pyrroles 3 from Schiff bases 1 by silver(I)-mediated oxidative cyclization of the homopropargylamines 2...

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