Mendeleev Communications Electronic Version, Issue 1, 1999 (pp. 1–44) Synthesis of racemic germicidin Igor P. Lokot,* Felix S. Pashkovsky and Fedor A. Lakhvich Institute of Bioorganic Chemistry, Academy of Sciences of Belarus, 220141 Minsk, Belarus. Fax: +375 172 637 132; e-mail: prostan@ns.iboch.ac.by, lokot@yahoo.com The seven-step synthesis of racemic germicidin in 40% overall yield has been accomplished for the first time.Alkyl derivatives of 4-hydroxy-2-pyrone attract considerable attention because of a broad spectrum of their chemical and biological properties. In recent years, a great number of 3-, 5- and 6-alkyl derivatives of 4-hydroxy-2-pyrone have been isolated from different fungi, plants and molluscs. In the synthesis of complex natural 2-pyrones such as verrucosidin,1 cytreoviridin,2 asteltoxin,3 cytreomontanin4 etc., a wide variety of chemical methods and biosynthetic approaches to the molecular transformations were used.In this paper, we describe the first total synthesis of racemic germicidin 8 (a 3,6-dialkyl derivative of 4-hydroxy-2-pyrone). Germicidin was isolated from Streptomices viridochromogenes NRRL B-1551, and it exhibited an inhibitory effect on the germination of arthrospores of its own producer (at a concentration of 40 pg ml–1).5 Acylation of Meldrum’s acid by 2-methylbutyric acid chloride6 in the presence of pyridine followed by methanolysis of acyl derivative 2 gave rise to methyl ester 3 in 82% overall yield.Dropwise addition of water to a solution of 3 and sodium methylate (1:1 equiv.) in methanol at 5–10 °C and acidification with 1 M HCl led to 4-methyl-3-oxohexanoic acid 4, which was further used for the acylation of Meldrum’s acid in order to obtain tetracarbonyl compound 5, the key precursor for the synthesis of 6-sec-butyl-4-hydroxy-2-pyrone 6.Due to the instability of 3-oxocarboxylic acid chlorides7 we used a modified procedure which consists in acylation of Meldrum’s acid by acid 4 under the action of N,N'-dicyclohexylcarbodiimide (DCC) and a catalytic amount of 4-N,N-dimethylaminopyridine (DMAP).The ring system of 4-hydroxy-2-pyrone 6 was formed by thermal cyclization of tetracarbonyl compound 5 at reflux with toluene. In this case, the sec-butyl substituent at the 6-position was introduced at the stage of pyrone cycle formation.After chromatographic purification, the target 6-sec-butylpyrone 6† was obtained in 82% overall yield on a basis of starting methyl ester 3. The last step in the synthesis of germicidin includes the introduction of an ethyl substituent at the 3-position in the cycle of compound 6. Methods for a,a'-alkylation of cyclic b-dicarbonyl compounds in general and 4-hydroxy-2-pyrones in particular have been developed insufficiently.Direct alkylation of the 4-hydroxy- 6-methyl-2-pyrone anion by methyl iodide8 resulted in the formation of the target product only in 16% yield. The reduction of readily available 3-acetyl-4-hydroxy-6-methyl-2-pyrone (dehydroacetic acid) with a borane–methyl sulfide complex9 resulted in the formation of the 3-ethyl derivative in low yield (23%).Catalytic hydrogenation of 3-acetylpyrones over palladium is also unusable for our purpose, because in this case the D5-bond is primarily reduced.10 This fact results in the formation of 5,6-dihydro-2-pyrone ring. The introduction of the 3-ethyl substituent into 6-sec-butylpyrone 6 was carried out by the previously suggested procedure. 11,12 The procedure includes the preparation of the corresponding b,b'-tricarbonyl compounds followed by the reduction of the oxo-function of acyl substituents by ionic hydrogenation. 3-Acetylpyrone 7‡ was obtained by one-pot acetylation of pyrone 6 by acetic acid in the presence of DCC. The intermediate enolacylate was isomerised in situ under the action of DMAP, and 3-acetyl-6-(2-butyl)-4-hydroxy-2-pyrone 7 was obtained in 91% yield as an oil.Its reduction by triethylsilane in trifluoroacetic acid in the presence of a catalytic amount of LiClO4 gives rise to racemic germicidin 8 in 84% yield as an oily product, which crystallises on standing. Recrystallisation from diethyl ether–hexane resulted in the crystalline product with mp 95–97 °C. Spectral characteristics of the compound obtained§ † Spectroscopic data for 6: 1H NMR, d: 6.00 (d, 1H, J 2 Hz), 5.60 (d, 1H, J 2 Hz), 2.50 (m, 1H), 1.45–1.80 (m, 2H), 1.20 (d, 3H, J 6.5 Hz), 0.90 (t, 3H, J 7.3 Hz).IR (n/cm–1): 1245, 1445, 1575, 1630, 1670, 1700, 2880, 2940, 2970. ‡ Spectroscopic data for 7: 1H NMR, d: 16.70 (s, 1H, OH), 5.93 (s, 1H), 2.70 (s, 3H), 2.53 (m, 1H), 1.50–1.90 (m, 2H), 1.25 (d, 3H, J 7 Hz), 0.92 (t, 3H, J 7.4 Hz).IR (n/cm–1): 1400, 1455, 1580, 1655, 1765, 2890, 2945, 2980. Cl O O O O O O O O O O i ii O OMe O iii O OH O iv O O O O O O OH O C v 1 2 3 4 5 O OH O O O O O vi vii 6 O OH O 7 viii O O OH O 8 rac-germicidin O 1 2 3 4 5 6 Scheme 1 Reagents and conditions: i, 2 equiv. Py, CHCl3, –20 °C, then 5% HCl; ii, MeOH, reflux; iii, MeONa/MeOH, H2O, 5–10 °C, then 1 M HCl; iv, Meldrum’s acid, DCC, 0.3 equiv.DMAP, Et3N, CH2Cl2, then 5% HCl; v, toluene, 6 h, reflux; vi, AcOH, DCC, Et3N, CH2Cl2; vii, 0.3 equiv. DMAP, Et3N, CH2Cl2, then 10% HCl; vii, 3 equiv. Et3SiH, TFA, cat. amount LiClO4.Mendeleev Communications Electronic Version, Issue 1, 1999 (pp. 1–44) are in good agreement with the literature data for the natural product.5 References 1 K. Whang, R.J. Cooke, G. Okay and J. K. Cha, J. Am. Chem. Soc., 1990, 112, 8985. 2 (a) H. Suh and C. S. Wilcox, J. Am. Chem. Soc., 1988, 110, 470; (b) D. R. Williams and F. H. White, J. Org. Chem., 1987, 52, 5067. 3 (a) K. Tadano, H. Yamada, Y. Idogaki, S. Ogawa and T. Suami, Tetrahedron, 1990, 46, 2353; (b) S. L. Schreiber and K. Satake, J. Am. Chem. Soc., 1984, 106, 4186. 4 (a) H. Venkataraman and J.K. Cha, Tetrahedron Lett., 1987, 28, 2455; (b) P. Patel and G. Pattenden, J. Chem. Soc., Perkin Trans. 1, 1991, 1941. 5 F. Petersen, H. Zähner, J. W. Metzger, S. Freund and R.-P. Hummel, J. Antibiot., 1993, 46, 1126. 6 M. Sato, K. Takayama and S. Kobayashi, Chem. Pharm. Bull., 1990, 38, 94. 7 H. Brintzinger and H.-W. Ziegler, Ber., 1948, 81, 381. § Spectroscopic data for racemic germicidin 8: 1H NMR, d: 6.22 (s, 1H), 2.48 (s + q, 2H + 1H, J 7.4 Hz), 1.24–1.75 (m, 2H), 1.20 (d, 3H, J 6.7 Hz), 1.11 (t, 3H, J 7.5 Hz), 0.89 (t, 3H, J 7.5 Hz). 13C NMR and DEPT, d: 169.6 (C), 168.8 (C), 168.0 (C), 105.0 (C), 100.9 (CH, J 169 Hz), 39.8 (CH, J 125 Hz), 27.5 (CH2, J 125 Hz), 17.7 (Me, J 125 Hz), 16.4 (CH2, J 125 Hz), 12.4 (Me, J 125 Hz), 11.6 (Me, J 125 Hz).MS, m/z: 196 [M+]. IR (n/cm–1): 1160, 1285, 1430, 1595, 1680, 2885, 2945, 2980. 8 E. Suzuku, B. Katsuragawa and S. Inoue, Synthesis, 1978, 144. 9 T. Shimizu, S. Hiranuma and T. Watanabe, Heterocycles, 1993, 36, 2445. 10 (a) W. A. Ayer and Y. D. Villar, Can. J. Chem., 1985, 63, 1161; (b) J. N. Walker, J. Am. Chem. Soc., 1956, 78, 3201. 11 (a) A. A. Akhrem, F. A. Lakhvich, S. I. Budai, T. S. Khlebnikova and I. I. Petrusevich, Synthesis, 1978, 12, 925; (b) F. A. Lakhvich, T. S. Khlebnikova and A. A. Akhrem, Synthesis, 1985, 8, 784. 12 (a) A. A. Akhrem, F. A. Lakhvich, L. G. Lis, V. A. Khripach, N. A. Fil’chenkov, V. A. Kozinets and F. S. Pashkovsky, Dokl. Akad. Nauk SSSR, 1990, 311, 1381 [Dokl. Chem. (Engl. Transl.), 1990, 311, 79]; (b) F. S. Pashkovsky, I. P. Lokot and F. A. Lakhvich, Vesti ANB, Ser. Khim. Navuk, 1993, 81 (in Russian). Received: Moscow, 22nd June 1998 Cambridge, 23rd July 1998; Com. 8/05512B