Communications Enzymatic Oxidative Phenolic Coupling Zhi-wei Guo, Grzegorz M. Salamonczyk, Kang Han, Koji Machiya, and Charles J. Sih* School of Pharmacy, University of Wisconsin, 425 N. Charter St., Madison, Wisconsin 53706-1515 Received June 6, 1997 Oxidative phenolic coupling, either by a homolytic or heterolytic mechanism, is of great importance in natural products chemistry. 1 Many heterocyclic products, includ- ing alkaloids, cyclic peptides, and glycopeptide antibiot- ics, are biosynthesized via enzyme-catalyzed C-O or C-C bond formation. 2 The diaryl ether linkage, formed by phenolic coupling of two tyrosine units is present in a diverse array of biologically-active natural products ranging from K-13, 3 OF4949, 4 and bouvardin 3,5 to the structurally complex glycopeptide antibiotics such as the vancomycin family. 6 Recently, many synthetic efforts have been directed to developing improved methods for the construction of the basic isodityrosine skeleton, which has been achieved either by the Ullmann 7 ether synthesis or by thallium- (III)-promoted oxidative coupling 8 of tyrosine derivatives. However, the drastic reaction conditions used in these procedures required extensive protection and deprotec- tion of the sensitive functionalities present in the parent molecule and only modest yields of desired product(s) were obtained. Consequently, the development of an alternative mild method for the synthesis of the diaryl ether linkage in high yields is still very much warranted. Oxidative enzymes such as peroxidases, laccases, and -tyrosinases are known to catalyze oxidative phenolic coupling of many aromatic substrates, 1,2 but the yield of the desired products are generally very low. Kametani and co-workers 9 had reported the oxidation of N-coclau- rine with homogenized potato peels in the presence of hydrogen peroxide to give a mixture of dimer and trimer with C-O-C head to tail coupling but in only 1.6% yield. Similar yields were obtained with isoquinoline deriva- tives. 10 Although Zenk 11 reported the isolation of a unique cytochrome P-450 enzyme that mediates regio- and stereoselective intermolecular C-O bond formation to furnish natural dimeric alkaloids. The extremely low levels of this plant enzyme and its detergent instability during solubilization of the complex out of the membrane make it unsuitable for synthetic use. Here, we report a novel efficient synthesis of the isodityrosine skeleton using peroxidase to catalyze the C-O coupling of diha- logenated tyrosine derivatives. Since horseradish peroxidase (HRP) [EC 1.11.1.7] catalyzed the C-C coupling of tyrosine to form dityro- sine 12 and nonhomogeneous oxidized polymerized deriva- tives, we decided to examine the action of this commer- cially available enzyme on dihalogenated tyrosine de- rivatives. When N-acetyl-3,5-dichloro-L-tyrosine, 1, was incubated with HRP at pH 6.0 in the presence of H 2 O 2 at 24 °C for 20 min, products 2 and 3 were isolated by silica gel chromatography in 7% and 55% yield, respec- tively. In turn, 3 was further chromatographed to give two diastereomeric lactones, 3a and 3b. Reduction of 2 and 3 separately with Zn/TFA at 0 °C afforded 1 and 4, respectively, each in over 90% yield. Since the product profile and yield were found to depend critically on the experimental conditions used, we list below an improved representative procedure, developed after much experi- mentation, that furnished 4 directly in 76% isolated yield along with 12% of recovered 1. To a clear solution of 1 (292 mg) in 45 mL of 0.2 M phosphate buffer pH 6.0 and 5 mL of acetonitrile at 24 °C was added, under stirring, 2000 units of horseradish peroxidase (HRP, Sigma), followed by 1.2 mL of 1 M H 2 O 2 . The reaction mixture was stirred for 10 min, quenched with 3 mL of 1 M NaHSO 3 , and the pH of the mixture was adjusted to 7.5 with 1 M NaOH (6 mL). This mild reduction procedure was found to give higher product yields than Zn/TFA or CrCl 2 . After stirring for 10 min, the mixture was acidified to pH 3.0 with 2 M KHSO 4 (10 mL) and then extracted with ethyl acetate (3 × 60 mL). The combined organic extract was washed with brine (40 mL), dried over MgSO 4 , and concentrated to dryness under reduced pressure. The residue was chromatographed over a silica gel column, which was eluted with a solvent mixture consisting of EtOAc- acetone-HOAc (1/0/0 to 100/10/1) to give recovered 1 (35 mg, 12%) and the desired product, 4 (208 mg, 76%), as a white solid (Scheme 1). N-Acetyl-3,5-dibromo-L-tyrosine (5) and the unpro- (1) McDonald, P. D.; Hamilton, G. A. Mechanisms of Phenolic Oxidative Coupling Reactions. In Oxidation in Organic Chemistry; Trahanovsky, W. S., Ed.; Academic Press: New York, 1973; Vol. 5b, pp 97-134. (2) Dhingra, O. M. Intramolecular Oxidative Coupling of Aromatic Substrates. In Oxidation in Organic Chemistry; Trahanovsky, W. S., Ed; Academic Press: New York, 1982; Vol. 5d, pp 207-278. (3) Yasuzawa, T.; Shirahata, K.; Sano, H. J. Antibiot. 1987, 40, 455- 458. (4) (a) Tamai, S.; Kaneda, M.; Nakamura, S. J. Antibiot. 1982, 35, 1130-1136. (b) Nishiyama, K.; Suzuki, S.; Yamura, S. Tetrahedron Lett. 1986, 27, 4481-4484. (5) Kase, H.; Kaneko, M.; Yamada, K. J. Antibiot. 1987, 40, 450- 454. (6) For a review on vancomycin and related antibiotics, see: Wil- liams, D. H.; Rajananda, V.; Williamson, M. P.; Bojesen, G. In Topics in Antibiotic Chemistry; Sammes, P. G., Ed.; John Wiley & Sons Inc.: New York, 1980; Vol. 5, pp 118-158. (7) (a) Tomita, M.; Fujitani, K.; Aoyagi, Y. Chem. Pharm. Bull. 1965, 13, 1341-1345. (b) Boger, D. L.; Yohannes, D. J. Org. Chem. 1989, 54, 2502; 1990, 54, 2498-2502; 1990, 55, 6000-6017. (8) (a) Suzuki, Y.; Nishiyama, S.; Yamamura, S. Tetrahedron Lett. 1989, 30, 6043-6046. (b) Evans, D. A.; Ellman, J. A.; DeVries, K. M. J. Am. Chem. Soc. 1989, 111, 8912-8914. Scheme 1 6700 J. Org. Chem. 1997, 62, 6700-6701 S0022-3263(97)00995-X CCC: $14.00 © 1997 American Chemical Society