Treatment of the oxime of octaethyloxochlorin 4, available from octaethylporphyrin 3, under Beckmann conditions provided not the expected lactam, but octaethyl-1,3-oxazinochlorin 8, in which a pyrrole moiety of the parent oxochlorin was expanded by an oxygen atom to an 1,3-oxazinone moiety. Its mechanism of formation was demonstrated to occur along an "abnormal Beckmann" pathway, followed by intramolecular ring closure and hydrolysis. The work expands the methodologies known to convert octaethylporphyrin to pyrrole-modified porphyrin analogues.