The structures of thielavins A, B and C. Prostaglandin synthetase inhibitors from fungi.

Sir: As reported in the previous paper,') thielavins A (1) and B (2) produced by Thielavia terricola SANK 10475 are potent inhibitors of prostaglandin (PG) synthetase. Thielavins A and B were shown to have 0-substituted salicylic acid as evidenced by UV absorption at 310"315nm in basic MeOH and 275 278 nm in acidic MeOH and by IR absorption at 1760, 1610 and 795 cm-1. Thielavins A and B possessed molecular compositions of C2,H300,, and C31H3401o, respectively, and gave the same ester (3), mp 208210'C, C,,H,SO,, (m/z 594), by reaction with diazomethane. The ester (3) gave a monoacetate, (C36H40011, m/z 636) by acetylation with acetic anhydride in pyridine. The structures of thielavins A and B were thus concluded to have the same tridepside skelton with a difference in substitution at the same position, i.e. hydroxyl or methoxyl group in A or B, respectively. Here we report further structure elucidation of thielavins A and B as well as thielavin C, a recently isolated minor component of the same group also produced by the said microorganism. Hydrolysis of 3 with NaOCH, in MeOH gave phenolic compound (4), mp 91-92°C, C11H,404, m/z 210, and didepside (5), mp 168-169'C, C~,H,,O,, m/z 416. The 1H NAM spectrum of 4 showed two methyl groups at 2.08 and 2.51, methoxyl at 3.85, ester methyl at 3.94, aromatic proton at 6.29 and phenolic hydroxyl at 11.8 ppm. Also, in the 13C NMR spectrum the aromatic carbons in 4 indicated signals at 162.2(s), 161.4(s), 140.1(s), 110.9(s), 105.8(d), and 105.5(s) ppm. It is well known that the signals of carbons with phenolic OH at ortho and para positions shifted to upfield, 12.8 ppm and 7.1 ppm, respectively, and that of meta shifted to downfield, 1.6 ppm.Z" Two carbons at 105.8 and 105.5 ppm in the high field were assigned ortho and/or para substituted phenol. From the above results and in consideration of biogenetic pathway, the structure of 4 was assumed to be 2,4-dihydroxy-3,6dimethylbenzoic acid derivative, which was also verified in comparison with the hydrolysis product of diffractic acid in alkali solution. Hydrolysis of thielavin A methyl ester (7), obtained by treatment of thielavin A with one mole diazomethane, with NaOCH, in MeOH gave phenolic compound (6), mp 141 -143'C, C1,H,.,04, m/z 196. This compound (6) was identical with the hydrolysis product of diffractic acid methyl ester and monomethoxy derivative of 6 was also identical with 4. This phenolic compound (4) originated from C-ring, not Aring in thielavin A or B, because deuterated methyl ester (C,,H,,D304, m/z 213) of 4 was obtained by reaction of 3 with NaOCD3 in deuterated methanol. The structure of didepside (5) was determined by X-ray analysis. The crystals obtained from hexane acetone mixture are triclinic, with a=9.491(3), b=13.000(5), c=9.673(5)A, a=79.36(3), /3=68.70(4), r_ 81.03(4)°. Z=2, D°a,e=1.271 g cm-3. Space group PT was assigned during the structure