Enantioselective synthesis of stephacidin B.
We describe an enantioselective synthetic route to the antiproliferative alkaloid stephacidin B (1) proceeding in 18 steps and 4.0% yield from 4,4-(ethylenedioxy)-2,2-dimethylcyclohexanone (3). Key features of the synthetic sequence include the use of the Corey-Bakshi-Shibata (CBS) reduction to introduce asymmetry early in the synthetic route, use of the novel electrophile N-(tert-butoxycarbonyl)-5-(isopropylsulfonyloxymethyl)-2,3-dihydropyrrole in a stereoselective enolate alkylation, a diastereoselective Strecker-type addition of hydrogen cyanide to an N-Boc enamine substrate in the solvent hexafluoroisopropanol, platinum-catalyzed nitrile hydrolysis under neutral conditions, cyclization of an acylamino radical intermediate to form the diketopiperazine core of stephacidin B, and implementation of a convergent procedure for introduction of the key 3-alkylidene-3H-indole 1-oxide functional group in the final stage of the route to prepare the structure 2, previously proposed to be the fungal metabolite avrainvillamide (17 steps, 4.2% yield). We observed that synthetic (-)-2 dimerized in the presence of triethylamine to form (+)-stephacidin B (>95%). We also obtained evidence that 2 can form 1 under mild conditions, and that 2 reacts with nucleophiles, such as methanol, by conjugate addition.