Goniodomic Acid, a Transient Oxirane Intermediate in the Conversion of the Macrolide Algal Toxin Goniodomin A to Seco Acids

The algal macrolide goniodomin A (GDA) undergoes ring-cleavage under unusually mild, alkaline conditions to form mixtures of stereoisomers of seco acids GDA-sa and iso-GDA-sa. In the primary fragmentation pathway, opening of the macrolide ring occurs by displacement of the carboxyl group by a base-catalyzed attack of the C32 hemiketal hydroxy group on C31, yielding an oxirane-carboxylic acid, named goniodomic acid. The oxirane ring is unstable, undergoing solvolytic opening to form mainly GDA-sa. Experimental support for this pathway obtained by carrying out the ring-opening reaction in H₂¹⁸O resulted in incorporation of the isotopic label at C32 of the seco acid. Collision-induced dissociation (CID) mass spectrometry of Na⁺ and NH₄⁺ ion adducts was employed to establish that ring-opening of the macrolide ring occurred by alkyl-O cleavage. Fragmentation was dominated by Grob-Wharton decarboxylation and retro-Diels-Alder reactions of the labeled seco acids. Direct observation of goniodomic acid was achieved when the ring-opening reaction was carried out under anhydrous conditions. A minor alkyl-O cleavage pathway gives rise to iso-GDA-sa by allylic attack at C29 of GDA or of the oxirane. In the formation of both GDA-sa and iso-GDA-sa, ring-opening is likely to be catalyzed by Na⁺ and NH₄⁺. Reversal of GDA-sa formation can occur in the mass spectrometer. CID fragmentation of the ¹⁸O-labeled GDA-sa restores the oxirane ring but causes preferential loss of the ¹⁸O label from C32.