A concise total synthesis of (±)-N-methyldibromoisophakellin, a member of the monomeric pyrrole-aminoimidazole alkaloid family isolated from the marine sponge Stylissa carbica, was achieved via a net [3+2] cycloaddition to install the cyclic guanidine. This ring annulation employs a 2-amido-1,3-aminoallyl cation obtained under oxidative conditions from variously N-substituted guanidines which in one instance led to isolation of a tetracycle bearing a carbinolamine center through subsequent benzylic oxidation. Finally, the serendipitous formation of a unique, related alkenyl guanidine, N-methylugibohlin, achieved via ring opening of cyclic guanidine under acidic conditions suggests that ugibohlin may be an artifact of isolation.
Keywords: carbinolamine; guanidine; isolation artifact; pyrrole–aminoimidazole alkaloid; total synthesis.