Isolation and Synthesis of Methyl Bovinate, an Unusual Pulvinic Acid Derivative from Suillus bovinus (Basidiomycetes) Helmut Besla, Andreas Bresinskya, Claus Kilpertb, Wolfgang Marschnerb, Helga M. Schmidtb, and Wolfgang Steglichc a Institut f¨ur Botanik der Universit¨at Regensburg, D-93040 Regensburg, Germany b Kekul´e-Institut f¨ur Organische Chemie und Biochemie der Universit¨at Bonn, Gerhard-Domagk-Straße 1, D-53121 Bonn, Germany c Department Chemie und Biochemie, Ludwig-Maximilians-Universit¨at M¨unchen, Butenandtstraße 5 – 13, D-81377 M¨unchen, Germany Reprint requests to Prof. Dr. W. Steglich. E-mail: [email protected] Z. Naturforsch. 2008, 63b, 887 – 893; received February 1, 2008 Cultures and fruit bodies of Suillus bovinus produce the pulvinic acid derivative methyl bovinate (4), which contains an extra carbonyl group that bridges ring A of methyl variegatate with the hydroxy group at the central butenolide ring. This unprecedented structure was deduced from the spectroscopic data and confirmed by total synthesis via a grevillin intermediate. In this synthesis, the menthyl group was used for carboxyl protection. Key words: Suillus, Boletales, Pulvinic Acids, Mushroom Pigments, Synthesis Introduction Suillus bovinus (L.: Fr.) Roussel (Kuhr¨ohrling) is a common bolete found in sandy pine forests in au- tumn. The fruit bodies contain boviquinone-4 as the main pigment [1], in addition to smaller quantities of bovilactone-4,4 [2], concentrated in the pink mycelium at the base of the fruit bodies. Atromentin [1] and the characteristic Boletales pigments atromentic acid (1), xerocomic acid (2), variegatic acid (3), and variega- Fig. 1. Pulvinic acids from mycelial cultures of S. bovinus. torubin represent a second set of coloured metabolites found in S. bovinus [3, 4] (Fig. 1). These common pul- of the organic phase on a polyamide column with vinic acids are also produced in mycelial cultures [5], acetone as eluent yielded an orange-yellow pigment in addition to an orange pigment, methyl bovinate fraction, which was further purified by preparative 4 (4) [3], which can be detected on TLC plates by its TLC on silica gel. Methyl bovinate ( ) was ob- colour change to red-violet on exposure to gaseous am- tained pure after column chromatography on acety- monia. In this communication we report on the isola- lated polyamide followed by repeated chromatography tion, structural elucidation and synthesis of this unique on Sephadex LH-20. The compound was also detected fungal metabolite. in the methanol extract of the fruit bodies by analytical HPLC. Results and Discussion Structural elucidation Isolation Methyl bovinate was obtained as an orange pow- The new pigment was isolated from mycelial cul- der, which exhibited UV/Vis maxima at λmax = 206, tures of S. bovinus, grown on Moser B fluid medium 246, 270, and 408 nm. The high-resolution mass spec- for 25 d. The cultures were extracted with acidi- trum shows a strong molecular ion at m/z = 412.0434 fied acetone, and the concentrated extracts were par- corresponding to the molecular formula C20H12O10. titioned between water and EtOAc. Chromatography In the 1H NMR spectrum a methyl ester signal at 0932–0776 / 08 / 0700–0887 $ 06.00 c 2008 Verlag der Zeitschrift f¨ur Naturforschung, T¨ubingen · http://znaturforsch.com 888 H. Besl et al. · Methyl Bovinate Scheme 1. Attempted synthesis of methyl bovinate (4). Table 1. 1H-coupled 13C NMR spectrum of compound 4a. b c Position δc Multiplicity , J (Hz), coupling partner 1 124.0 d, 7.6 (5-H) 2 108.1 D, 164 (2-H) 3 154.2 dd, 7.6 (5-H), 2.4 (2-H) 4 148.2 dd, 6.4 (2-H), 2.4 (5-H) 5 115.5 D, 164 (5-H) 6 110.9 dd, 8.8 (2-H), 1.4 (5-H) 7 158.6 dd, 4.8 (5-H), 1.4 (2-H) 1 121.3 d, 9.0 (3-H) 2 116.4 Dd, 160 (2-H), 6.8 (6-H) Fig. 2a. Methyl bovinate (4) (pulvinic acid numbering). 3 145.6 ddd, 6.6 (5 -H), 3.8 (2 -H), 1.2 (6 -H) 4 148.1 ddd, 7.0 (2-H), 7.0 (6-H), 3.0 (5-H) 5 116.1 D, 162 (5-H) 6 121.9 Dd, 164 (6-H), 7.6 (2-H) 1 162.7d s 2 102.0 dd, 4.6 (2-H), 1.4 (5-H) 3 157.1d s 4 135.4d s 5 117.7 dd, 4.8 (2-H), 4.8 (6-H) 6 165.9 q, 3.8 (OMe) OMe 53.0 Q, 149 (OMe) a b 1 100.6 MHz, in [D6]DMSO; multiplets due to J(C,H) couplings are indicated by capital letters; 3J (C,H) and other couplings are 3J , 13 in small letters; c confirmed by selective 1H decoupling; d inter- Fig. 2b. Important H C couplings (Hz) in the CNMR spectrum of 4. changeable. The height of the singlet at δC = 135.4 is half of that at 157.1 ppm. show the corresponding signals in the range of 154 – δH = 3.94 is present in addition to two aromatic sin- 167 ppm [7]. glets at δH = 7.45 and 7.52 and typical signals for In order to confirm this structure, we set out to un- a 3,4-dihydroxyphenyl residue, matching those ob- dertake the total synthesis of methyl bovinate. served for ring C of variegatic acid (3) [6]. The 1H- Total synthesis coupled 13C NMR spectrum (Table 1) contains 20 sig- nals, which can be ascribed to 19 signals in the aro- Our first synthetic approach to methyl bovinate (4) matic region and that of a methoxy group (δC = 53.0). started from the known acetal 5, which is easily avail- An analysis of this spectrum, assisted by selective de- able in two steps from veratraldehyde [8] (Scheme 1). couplings, allowed us to propose the 5H-furo[3,4-c] Lithium-halogen exchange with n-butyllithium fol- benzopyran structure 4 for methyl bovinate (Fig. 2a). lowed by addition of dimethyl carbonate afforded Of special significance for this assignment are the methyl ester 6a, which was then converted into alde- 3 JH,C-couplings depicted in Fig. 2b. Unusual is the hyde 7a by acidic cleavage of the acetal group. Con- high-field position of the singlet at δc = 135.4, which densation of aldehyde 7a with pyrandione 8 [9] in must belong to one of the oxygen-carrying carbon acetic acid containing a catalytic amount of HCl gave atoms in the butenolide ring. Simple pulvinic acids grevillin carboxylic acid 9a in high yield. Clearly, the H. Besl et al. · Methyl Bovinate 889 Scheme 2. Synthesis of methyl bovinate (4). methyl ester group was not stable enough to survive arated on a silica gel column. The structural assign- the condensation conditions. Attempts to carry out this ment is based on the chemical shifts of the ortho pro- reaction under milder conditions (e. g., AcOH, ammo- tons at the 3,4-dimethoxyphenyl ring, those of iso- nium acetate, 20 ◦C) were unsuccessful, again yield- mer 13a appearing at lower field due to the deshield- ing the free acid 9a. The alcoholate-catalysed rear- ing effect of the neighbouring butenolide ring [6]. The rangement of the latter into the corresponding ter- desired diester 13b was obtained in 85 % yield, and phenylquinone [10], followed by treatment with min- we were pleased to note that treatment of diester lac- eral acid, proceeded with concomitant lactone forma- tone 13b with BBr3 in CH2Cl2 resulted in cleavage of tion and delivered quinone 10 in excellent yield. the menthyl ester [12] with concomitant lactone forma- In order to avoid the instability problems of the tion to give methyl bovinate (4), in every respect iden- methyl ester, we used the menthyl group for car- tical with the natural product. This confirmed the struc- boxyl protection. The desired (–)-menthyl ester 6b ture of this unusual fungal pulvinic acid derivative and could be easily prepared from bromo acetal 5 and demonstrated the advantageous use of a menthyl group (–)-menthyl chloroformate. After acidic cleavage of for carboxyl protection. Methyl bovinate (4) was ob- the acetal group, the resulting m-opianic acid men- tained from bromo acetal 5 in 7 steps with 11 % overall thyl ester 7b was condensed with pyrandione 8 to yield. yield grevillin carboxylic ester 9b (Scheme 2). We The co-occurrence of methyl bovinate (4) with var- were pleased to note that this reaction and the sub- iegatic acid (3) in fruit bodies and cultures of S. bovi- sequent methoxide-catalysed rearrangement into ter- nus points to a close biosynthetic relationship. Our at- phenylquinone 11 proceeded with retention of the tempts to introduce the extra carbonyl group by treat- ester group. Oxidative cleavage of dihydroxybenzo- ment of methyl variegatate with phosgene were unsuc- quinone 11 with Ac2O/DMSO according to Moore and cessful. 12 Wikholm [11] then afforded the dilactone in high Experimental Section yield. Treatment of the latter with dilute KOH in methanol General led to the formation of two regioisomeric monolac- Melting points (uncorrected): Reichert Thermovar hot tone diesters 13a and 13b, which could be easily sep- stage microscope. UV/Vis: Hewlett-Packard 8452 diode ar- 890 H. Besl et al. · Methyl Bovinate ray spectrophotometer. IR: Perkin-Elmer 1420 Ratio Record- coupled 13C NMR see Table 1. – MS (DI, 250 ◦C): m/z (%) = ing Infrared Spectrometer. Intensity of the bands: ss (very 412 (100) [M]+, 366 (50), 354 (60), 353 (85), 326 (35), 279 strong), s (strong), m (medium), and w (weak). NMR: (38), 235 (30), 120 (70), 44 (75). – HRMS: m/z = 412.0434 + Varian EM 390 with TMS as internal standard, Bruker (calcd.