Total Synthesis and Stereochemical Revision of (+)-Aeruginosin 298-A Peter Wipf* and Joey-Lee Methot Department of Chemistry, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15260 pwipf+@pitt.edu Received October 20, 2000 ABSTRACT Novel routes toward both enantiomers of the bicyclic proline surrogate 2-carboxy-6-hydroxyoctahydroindole, i.e., Choi, were developed on the basis of the oxidative cyclization of L-tyrosine. Synthesis of the proposed sequence of (+)-aeruginosin 298-A did not provide the natural product. Incorporation of a D-leucine residue, in contrast, led to the total synthesis of this thrombin inhibitor. The discovery of novel anticoagulant agents for the treatment of thrombosis continues to receive significant attention. 1 Thrombin, a key enzyme in the blood coagulation cascade, catalyzes the conversion of fibrinogen to fibrin which then polymerizes to form a haemostatic plug. 2 Current therapeutic agents, such as heparins and coumarins, require careful monitoring of the patient to avoid excessive haemorrhage and are not orally active. 3 In 1994, Murakami and co-workers isolated the thrombin inhibitor (IC 50 of 0.5 µM) aeruginosin 298-A (1) from the blue-green freshwater algae Microcystis aeruginosa. 4 At the time, only the leucine stereochemistry was assigned by chiral GC analysis of the acid hydrolysate. The configurations of the hydroxyphenyllactic acid and argininol fragments were later established by Marfey analysis. 5 In 1998, Tulinsky and co-workers reported an X-ray crystallographic structure of the ternary complex of 1 bound to hirugen-thrombin. 6 Surprisingly, the binding mode closely resembled that of D-Phe-Pro-Arg chloromethyl ketone with the L-Leu residue occupying the D-S3 subsite. Their work also confirmed the absolute stereochemistry of the novel hydroindole core 2 (L-Choi). Conformationally restricted proline derivatives such as 2 project peptide chains into defined regions of space, promoting specific turns in peptide folding and conferring a bioactive conformation. 7 Further- more, the hydroxyl group in 2 can participate in hydrogen bonding and increase water solubility or can be functionalized (e.g., as a sulfate) as found for certain aeruginosins. Our approach toward 2 highlights the utility of our tyrosine oxidation-rearrangement methodology to prepare function- (1) Vacca, J. Annual Reports in Medicinal Chemistry; Bristol, J. A., Ed.; Academic Press: San Diego, 1998; Vol. 33, pp 81-90. (2) Buchanan, M. R.; Brister, S. J.; Ofosu, F. A. Thrombin: Its Key Role in Thromogenesis: Implications For Its Inhibition Clinically; CRC Press: Boca Raton, FL, 1995. (3) Das, J.; Kimball, S. D. Bioorg. Med. Chem. Lett. 1995, 3, 999- 1007. (4) Murakami, M.; Okita, Y.; Matsuda, H.; Okino, T.; Yamaguchi, K. Tetrahedron Lett. 1994, 35, 3129-3132. Aeruginosin 298-A exhibits modest selectivity against trypsin (IC 50 of 1.7 µM) but does not inhibit papain, elastase, chymotrypsin, or plasmin. One dozen aeruginosins have since been isolated with thrombin IC50 values ranging from 0.04 to 15 µM, all sharing the same hydroindole core 2. 5 (5) Ishida, K.; Okita, Y.; Matsuda, H.; Okino, T.; Murakami, M. Tetrahedron 1999, 55, 10971-10988. (6) Steiner, J. L. R.; Murakami, M.; Tulinsky, A. J. Am. Chem. Soc. 1998, 120, 597-598. (7) See, for example: Zhang, R.; Brownewell, F.; Madalengoita, J. S. J. Am. Chem. Soc. 1998, 120, 3894-3902. Blanco, M. J.; Paleo M. R.; Penide, C.; Sardina, F. J. J. Org. Chem. 1999, 64, 8786-8793. Tam, J. P.; Miao, Z. J. Am. Chem. Soc. 1999, 121, 9013-9022. ORGANIC LETTERS 2000 Vol. 2, No. 26 4213-4216 10.1021/ol006759x CCC: $19.00 © 2000 American Chemical Society Published on Web 11/28/2000