The novel heptachloro-1'-methyl-1,2'-bipyrrole ("Q1") (1) is a natural organohalogen that has been detected in more than 100 worldwide environmental samples. In recent years mixed halogenated 1,2'-bipyrroles have also been detected in several marine sources. For example, heptabromo-1'-methyl-1,2'-bipyrrole (2) was reported in 2006, but this structure has remained unconfirmed until our present work. Formal syntheses of Q1 (1) and heptabromo-1'-methyl-1,2'-bipyrrole (2) were accomplished in 23% and 31% yields, respectively, from 1-methyl-2-nitropyrrole. In the course of the syntheses, an efficient synthesis of pyrrolylamides and pyrrolylimides was developed using indium or tin in the presence of acetic acid and anhydrides. An improved version of this reductive acylation, without using acetic acid, was also developed using catalytic hydrogenation. A new general synthesis of 1,3'- and 1,2'-bipyrroles was realized as well using both methodologies. A new two-step synthesis of 2,2'-bipyrroles was accomplished using a Paal–Knorr pyrrolylation. A Fischer Indole pathway for the synthesis of 2,2'- and 2,3'-biindoles was also attempted.
Lycogarubin C, a heterocycle containing one pyrrole and two indole moieties, is a metabolite isolated from the fruit bodies of Lycogala epidendrum in 1994. Our successful synthesis of bisindoleacetylene 160 prompted us to explore a concise synthesis of these two compounds. Diels-Alder reaction between bisindolealkene 175 and tetrazine 161 followed with reductive ring contraction reaction completed the synthesis of Lycogarubin C in 49% from 175.
cis-Trikentrin A, a novel indole natural product, was isolated from the marine sponge Trikentrion flabelliforme in 1986, and herbindole B, which only differs at the C-4 and C-5 positions (indole numbering) from cis-trikentrin A, was isolated from an orange-colored sponge, Axinella sp. in 1990. Their unusual cyclopent[g]indole ring system and potential biological activities prompted us to explore an enantioselective synthesis. A model study successfully furnished the cyclopent[g]indole skeleton in 54% yield, but aldehyde 310, which differs from 5-hexen-1-al (294) only with two extra methyl groups, failed in the same reaction probably due to the steric hindrance. Other smaller protecting groups, such as –CO2Et, –Boc, and –SO2CF3 were introduced to hopefully finish the syntheses of cis-trikentrin A and herbindole B.