STRUCTURE AND BIOSYNTHESIS OF BOROPHYCIN, A NEW BOESEKEN COMPLEX OF BORIC-ACID FROM A MARINE STRAIN OF THE BLUE-GREEN-ALGA NOSTOC LINCKIA

Borophycin (1) is the potent cytotoxin in the lipophilic extract of a marine strain of the blue-green alga (cyanobacterium) Nostoc linckia (Roth) Bornet ex Bornet and Flahault (UH isolate GA-5-23). The gross structure of this boron-containing compound was determined by spectral methods and its relative stereochemistry established by X-ray crystallography. Borophycin is made up of two identical halves with an overall structure reminiscent of the ionophoric antibiotics boromycin (2) and aplasmomycin (3). The biosynthesis of 1 differs from the biosynthesis of 2 and 3. All three compounds are acetate-derived polyketides that utilize a C-3 precursor for the starter unit and methionine for the methyl branches on the polyketide chain. Whereas phosphoglycerate or phosphoenolpyruvate has been suggested to be the C-3 starter unit in the biosynthesis of 2 and 3, the C-3 starter unit for the biosynthesis of I is derived from acetate and methionine, but not propionate.