Caulerpenyne (l), the most abundant of the ecotoxicologically relevant sesquiterpenoids of the Mediterranean-adapted tropical green seaweed Caulerpa taxzfolia, was found to react with Et,N or pyridine in MeOH by initial deprotection o f C(1)HO to give oxytoxin 1 (Za), previously isolated from the sacoglossan mollusc Oxynoe olivacea. With BuNH,, without any precaution to exclude light, 1 gave the series of racemic 3 and 4, and achirdl (4E,6E)-5, (4&62)-5, (4Z,6E)-5, and (42,62)-5 pyrrole compounds, corresponding to formal C(4) substitution, 4S-D -elimination, and (E/Z)-isomerization at the C(4)=C(5) and C(6)=C(7) bonds. Changing to CDCl, as solvent in the dark, 1 gave cleanly, via 2a as an intermediate, 3 and (4E,6E)-5. The latter proved to be prone to (E/Z)-photoisomerization. Under standard acetylation conditions, 3 gave (4E,6E)-5 via acetamide 7 as an intermediate. Particular notice is warranted by selective deprotection of 1 at C(1), mimicking enzyme reactions, and unprecedented formation of pyrrole compounds from freely-rotating, protected 1,Cdialdehyde systems.