Total Synthesis of (±)--Methyldibromoisophakellin and -Methylugibohlin via Net [3+2] Cycloguanidinylations Employing 2-Amido-1,3-Diamino-Allyl Cations.

A concise total synthesis of (±)--methyldibromoisophakellin, a member of the monomeric pyrrole-aminoimidazole alkaloid family isolated from the marine sponge , 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 -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, -methylugibohlin, achieved via ring opening of cyclic guanidine under acidic conditions suggests that ugibohlin may be an artifact of isolation.