45 BIOSYNTHESIS OF MEDIUM-SIZED BROMO ETHERS ISOLATED FROM THE RED ALGA, LAURENCIA NIPPONICA
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A number of marine cyclic bromo ethers were isolated from red algae, especially from Laurencia nipponica, whose components were precisely and continuously investigated by Irie. Kurosawa, and their co-workers since late 1960s. These bromo ethers are characterized by the fact that they have terminal enyne or bromo allene moieties, medium-sized cyclic ether skeletons, and a straight carbon chain with (6R,7R)- or (6S,7S)-configurations. It must be emphasized that each population of this single species metabolites completely-different major bromo ethers depending on growth locality in the case of Oshoro Ray. We have proved that a typical mono-cyclic bromo ether, deacetyl-laurencin (2) with (6R,7R)-configurations arises biosynthetically from its precursor (2E,6R,7R)-laurediol (11) by using lactoperoxidase (LPO) and bromopproxidase (BPO) and that 2 is successively converted to laurefucin (3) and laureoxanyne (4) in the same enzymatic reaction conditions.8) In order to prove the biosynthesis of bromo ethers having (6S,7S)-configurations, prelaureatin (13) was proposed to be a key intermediate of laureatin (5). isolaureatin (6), and laurallene (7). Actually the intermediate 13 was converted by LPO (or crude BPO extracted from the alga) into 5, 6, and 7 in the presence of hydrogen peroxide and bromide ion. Then, (3Z,6S,7S) laurediol (12) was subjected to the same enzymatic reaction using LPO to afford prclaureatin (13) in 3% yield. Thus, we tried to isolate the proposed intermediate 13 from the neutral extracts (31g) of the alga (wet weight 15kg) and really found 2mg of the compound, which clarified the whole biosynthetic pathway of the (6S,7S)-group bromo ethers. The results mentioned above indicate that bromoperoxidase in the alga cyclizes laurediols (11 and 12) into two regio-isomers 2 and 13, respectively, without substrate selectivity and that this single enzyme catalizes the multi-steps of bromo etherification reactions in vivo.
