“…Unfortunately, oxidation of porphyrins with excess OsO, leads primarily to oxidation of diagonally opposite rings, giving tetrahydroxybacteriochlorins but not isobacteriochlorins. This is because the hydrogen atoms at the centre of a chlorin are localized on the two opposite pyrrolic rings, as in (145), and this means that the double-bonds of these two rings are part of the eighteenelectron aromatic system whereas the double-bond in the third pyrrolic ring is not. By contrast, under the strongly acidic conditions of H 2 0 2 in H2S0,, all four nitrogen atoms of the chlorin are protonated and isobacteriochlorins predominate over bacteriochlorins.…”