Aust. J. Chem., 1984. 37. 2153-7

Synthesis of the Lichen Depsides Pseudocyphellarin A and B

John A . Elix and Labunmi Lajide

Department of Chemistry, The Faculties, Australian National University, P.O. Box 4, Canberra, A.C.T. 2601.

Abstvact

The total synthesis of the fully substituted lichen depsides, pseudocyphellarin A (3-hydroxy-4-methoxy- carbonyl-2,5,6-trimethylphenyl 3-formyl-2,4-dihydroxy-5,6-dimethylbenzoate) (1) and pseudo- cyphellarin B (3-hydroxy-4-methoxycarbonyl-2,5,6-trimethylphenyl 2,4-dihydroxy-3-hydroxymethyl- 5,6-dimethylbenzoate) (2) is described.

Huneckl recently reported the isolation of pseudocyphellarin A (1) and pseudo- cyphellarin B (2) from the Antarctic lichen Pseudocyphellaria endochvysea (Del.) Vain. These fully substituted depsides are the third and fourth representatives of this class of lichen compound, previously exemplified by nephroarctin (3) and phenarctin (4), metabolites of the lichen Nephvonu avcticum (L.) Torss.'

R ' -- R2 R' R~ - OR' (1) CHO Me C02Me H

(2) CHzOH Me C02Me H

HO OH Me R3 (3) CHO CHO H Me

(4) CHO CHO C02Me H Me

Huneck' elucidated the structure of (1) and (2) from spectroscopic data and chemical degradation by alcoholysis (ButOH) of the depside-ester linkage. These results established the substitution of the individual aromatic nuclei and structures (1) and (2) followed from biogenetic considerations (i.e. assuming these are typical para-depsides, where linkage occurs through the oxygen of ring Bpara to the methoxy- carbonyl). We now report the total synthesis of these two depsides and in so doing, confirmation of the accepted structures (1) and (2) for pseudocyphellarin A and B respectively.

The key intermediates for the synthesis gf these metabolites were 3-formyl- 2,4-dihydroxy-5,6-dimethylbenzoic acid (10) and methyl 2,4-dihydroxy-3,5,6-trimethyl- benzoate (12) (Scheme I ) . Methyl isohaematommate (7) was prepared by formylation

Huneck, S., Phytochemistvy, 1984, 23, 431. Bruun, T., Acta Chem. Scand., 1971, 25, 2831.

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of methyl 2,4-dimethoxy-6-methylbenzoate3 (5) and subsequent demethylation of the product, methyl 5-formyl-2,4-dinlethoxy-6-methylbenzoate (6), with boron trichloride. The Clemmensen reduction of methyl isohaematommate (7) gave methyl 2,4-dihydroxy- 5,6-dimethylbenzoate (8) and formylation of this compound with titanium(1v) chloride and dichloromethyl methyl ether gave the aldehyde (9). Hydrolysis of this ester (9) with concentrated sulfuric acid gave the key benzoic acid (lo), while catalytic reduction of (9) gave the key intermediate ester (12). The latter compound was also obtained by catalytic reduction of methyl 5-formyl-2,4-dihydroxy-3,6-dimethyl- benzoate (1 1) as reported previ~usly.~

SnCl OHC,&COzMe -, oHC&l:Me 4 0 1 C12CHOMe

Me0 Me0 OMe

HCI Zn-Hg 1

CHO

(9)

Scheme 1

The condensation of the benzoic acid (10) with the phenol (12) in the presence of trifluoroacetic anhydride afforded pseudocyphellarin A (1). Acetylation of (1) with acetic anhydride and a trace of sulfuric acid gave the corresponding pentaacetate (13). The synthetic depside (1) was identical in all respects with the natural material (m.p., m.m.p., t.l.c., 'H n.m.r. and mass spectrum). Similarly, the synthetic pentaacetate (13) proved identical with that derived from the natural product.

Cresp, T. M., Sargent, M. V., Elix, J. A., and Murphy, D. P. H., J. Chem. Soc., Perkin Trans. I , 1973, 340.

Hamilton, R. J., and Sargent, M. V., J. Chem. Soc., Perkin Trans. 1, 1976, 943.

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Pseudocyphellarin B (2) was synthesized by reduction of pseudocyphellarin A (I) with sodium borohydride in diglymelether and a trace of ethanol. Acetylation of (2) in the usual manner gave pseudocyphellarin B tetraacetate (14). The physical and spectroscopic properties of (2) and (14) so obtained were identical with those reported1 for the natural material and the corresponding acetate.

k k w o $ o A c mew^$:;;^ AcO C02Me AcO

OAc Me OAc Me

CH(OAC)~ Me CHzOAc Me

Experimental The general experimental details have been described previou~ly.~

Methyl 5-Formyl-2,4-dimethoxy-6-methylbenzoate (6)

This compound was prepared by a modification of the method we reported previ~usly.~ Dichloromethyl methyl ether (23 g) was added rapidly to a solution of tin(1v) chloride (51 g) in

anhydrous dichloromethane (100 ml) cooled to - 10'. Then methyl 2,4-dimethoxy-6-methylbenzoate (5) (32 g) was added to this solution over a period of 1 h such that the temperature remained below 5". The reaction mixture was warmed to room temperature and then boiled under reflux for 1.5 h. After cooling, the reaction mixture was poured into ice-water and extracted with ether. The combined organic layer was washed with 3 M hydrochloric acid and brine, and dried (MgSO,). The solvent was evaporated and the crude product was recrystallized from light petroleum/dichloromethane to give the first crop of product (26 g). The mother liquor was concentrated and the residue chromato- graphed on a short silica gel column, with 30% ethyl acetatelhexane as eluent. The slower moving band yielded a further quantity of the aldehyde (6) (total yield 35 g, 96%) which formed colourless needles from light petroleum/dichloromethane, m.p. 124-125" ( lk3 125-126") (Found: C, 60.4; H, 6.1. Calc. for C,,Hl,O,: C, 60.5; H, 5.9%).

Methyl 5-Formyl-2,4-dihydroxy-6-methylb~nzoate (Methyl Isohaematommate) (7)

A solution of boron trichloride (50 g) in anhydrous dichloromethane (75 ml) was added over a period of 30 min to a stirred solution of methyl 5-formyl-2,4-dimethoxy-6methylbenzoate (6) (12 g) in anhydrous dichloromethane (200 ml) cooled to - 78". The mixture was then stirred at - 78" for a further 1 h and then at room temperature for 3 h. The reaction mixture was then poured into an ice-water mixture and was extracted into chloroform. The combined organic layer was washed with saturated sodium hydrogen carbonate solution and brine, and dried (MgSO,). Evaporation of the solvent gave the crude product which on crystallization from dichloromethane/hexane afforded the aldehyde (7) (9 g, 85 %)as colourless needles, m.p. 129-130" ( lk6 130") (Found: C, 57.1; H, 4.8. Calc, for Cl1H1,O5: C, 57.2; H, 4.7%).

Methyl 2,4-Dihyduoxy-5,6-dimethylbenzoate (8)

Zinc wool (25 g) was treated with mercury(11) chloride (2 g) and water (30 ml). After 5 min, the water was decanted and replaced by concentrated hydrochloric acid (75 ml) and water (60 ml). The mixture was then heated to reflux and a solution of the aldehyde (7) (10 g) in hot ethanol (60 ml) was added at such a rate as to prevent crystallization in the funnel. The reaction mixture was then boiled under reflux for 40 min, cooled, diluted with water and extracted with ether. The ethereal

Sargent, M. V., Vogel, P., Elix, J. A., and Ferguson, B. A., Aust. J. Chem., 1976,29,2263. Curd, F. H., Robertson, A., and Stephenson, R. J., J. Chem. Soc., 1937, 130.

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extract was washed with water, dried (MgSO,) and evaporated. The residue so obtained was crystal- lized from hexane to give the ester (8) (8 g, 86 %) as colourless prisms, m.p. 120-122" (Found: C, 61.4; H, 6.2. CloH120, requires C, 61.2; H, 6.1 %). 'H n.m.r. (CDC1,) 6 2.18, 2.50, 2s, ArCH3; 3,99,s ,OMe; 6.40,s ,ArHand6.56,s ,OH.

Methyl 3-Formyl-2,4-dihydroxy-5,6-dimethylbenzoate (9)

A stirred solution of the ester (8) (3 g) in dichloromethyl methyl ether (3 ml) and anhydrous dichloromethane (20 ml) was cooled to -7" while a solution of titanium(1v) chloride (6 ml) in anhydrous dichloromethane (10 ml) was added dropwise. The reaction mixture was then stirred at 0" for 1 h and room temperature for 16 h, and then poured into an ice-water mixture and extracted with ether. The organic layer was washed with dilute hydrochloric acid solution, saturated brine solution, dried (MgSO,) and evaporated. The residue obtained was applied to a short silica gel column and eluted with 20% ethyl acetatelhexane. The major band yielded the aldehyde (9) (2.5 g, 73 %) which crystallized from dichloromethane/hexane as colourless needles, m.p. 87-88" (Found: C, 58.8; H, 5.5. C1,Hl2O6 requires C, 58.9; H, 5.4%). 'H n.m.r. (CDCI,) 6 2.12, 2.50, 2s, &Me; 3.95, s, OMe; 10.33, s, CHO; 12.41, 12.86, 2s, OH.

3-Forn~l-2,4-dihydroxy-5,6-dimetlzylbenzoic Acid (10)

The ester (9) (2.3 g) was dissolved in concentrated sulfuric acid (80 ml) at O0, and stored at room temperature for 16 h. The reaction mixture was then diluted by addition of crushed ice and was extracted with ethyl acetate. The ethyl acetate solution was washed with water and saturated brine solution, and then dried (MgSO,). The solvent was then evaporated under reduced pressure and the residue recrystallized from ethyl acetatelhexane to give the benzoic acid (10) (1.8 g, 83 %) as colourless needles, m.p. 182-180" (dec.) (Found: C, 57.1; H, 4.9. C1,3H1005 requires C, 57.1; H, 4.8%). 'H n.m.r. (CDCl,/CD,COCD,) 6 2.12, 2.60, 2s, ArMe; 10.30, s, CHO; mlz 210 (M, lo%), 166 (100).

Methyl 2,4-Dihydroxy-3,5,6-trimethylbenzoate (12)

A solution of methyl 5-formyl-2,4-dihydroxy-3,6-dimethylbenzoate (ll), (3 . O g) in ethyl acetate containing 5 % palladium on carbon (30 mg) was stirred in an atmosphere of hydrogen for 16 h. The catalyst was then filtered off, the solvent evaporated and the residue crystallized from dichloro- methanejhexane to give the ester (12) (2.5 g, 89%) as colourless needles, m.p. 96"-97' (lit? 97-98") (Found: C, 62.7; H, 6.7. Calc. for CllH1404: C, 62.8; H, 6.7%).

Synthesis of 3-Hydroxy-4-methoxycarbonyl-2,5,6-trimethyheny 3-Formyl-2,4-dihydroxy- 5,6-dimethylbenzoate (Pseudocyphellarin A) (I)

Trifluoroacetic anhydride (1 ml) was added to a solution of 3-formyl-2,4-dihydroxy-5,6-dimethyl- benzoicacid (10) (0.21 g) and methyl 2,4-dihydroxy-3,5,6-trimethylbenzoate (12) (0.21 g) in anhydrous benzene (5 ml), thoroughly mixed and left at room temperature for 16 h. The solvent was then evaporated and the residue applied to a silica gel column and eluted with cycl~hexane/chloroform/ acetone (10 : 8 : 1). The faster moving band yielded the depside (1) (0.30 g, 75 %) which crystallized from cyclohexane in colourless needles, m.p. 173-174" (lit.' m.p. 173-174") (Found: M*', 402.1311. Calc. for 1ZC211H22'60,; Mt., 402.1315). 'H n.m.r. (CDCI,) S 2.07,2.18,2.47,2.70,4s, ArMe; 3.98, s, OMe; 10.38, s, CHO; 11.10, 12.38, 13.07, 3s, bonded OH; mlz 402 (M, 5%), 211 (9), 210 (791, 194 (8), 193 (73), 192 (6), 190 (8), 179 (221, 178 (loo), 164 (17), 150 (39), 149 (6), 136 (5), 107 (5). This synthetic product was identical in all respects with the natural material (t.1.c. in three independent solvent systems, m.m.p., 'H n.m.r., mass spectrum).

Pseudocyphellarin A Pentaacetate (13)

A solution of depside (1) (0.15 g) in freshly distilled acetic anhydride (5 ml) containing 2 drops of concentrated sulfuric acid was stirred at room temperature for 16 h. Water (30 ml) was then added and stirring continued for 0.5 h. The solution was then extracted with ethyl acetate and the organic layer was washed with saturated sodium hydrogen carbonate solution and then dried (MgSO,). Evaporation of the solvent and crystallization of the residue from acetonelhexane gave

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the pentaacetate (13) (0.16 g, 68%) as colourless prisms, m.p. 201-202" (lit.' 198-200") (Found: C, 59.1 ; H, 5.4. Calc. for C31H3404: C, 59.1 ; H, 5.7%). 'H n.m.r. (CDC13) 6 2.08, 2.12,2.16, 2.26, 2.27, 2.41, 2.49, 7s, CMe; 3.89, s, OMe; 7.95, s, CH(OAc),; mlz 630 (M, 2%). The synthetic material was identical with that derived from the natural product (t.l.c., m.m.p., 'H n.m.r. and mass spectrum).

Synthesis of 3-Hydroxy-4-methoxycavbonyl-2,5,6-tvimethylphenyl2,4-Dihydvoxy-3-hydvoxymethyl- 5,6-dimethylbenzoate (Pseudocyphellavin B) (2)

A solution of pseudocyphellarin A (1) (0.25 g) in diglyme (5 ml) and ether (20 ml) was stirred while sodium borohydride (100 mg) was added followed by several drops of ethanol. Stirring was continued until a white precipitate appeared. The reaction mixture was then poured into cold dilute hydrochloric acid and the ether layer separated, washed with water and saturated sodium hydrogen carbonate solution, and then dried (MgSO,). The solvent was evaporated and the residue applied to a small column of silica gel and eluted with cyclohexane~chloroform/acetone (10 : 8 : 1). The major slower-moving band yielded pseudocyphellarin B (2) (0.20 g, 80 %) which crystallized from dichloro- methane/cyclohexane in colourless needles, m.p. 168-170" (1it.l 168-169") (Found: C, 62.4; H, 6.2. Calc. for CZ1H2408: C, 62.4; H, 6.0%). l H n.m.r. (CDCl,/(CD,),SO) 6 2.05, 2.15, 2.43, 2.61, 4s, ArMe; 3.61, s, OH; 3.97, s, OMe; 4.95, s, CH,; 10.50, 10.51, 11.51, 3s, OH; miz 210 (38), 178 (85), 150 (loo), 122 (8), 106 (15). This synthetic material was identical in all respects with the natural product (t.l.c., m.m.p., 'H n.m.r., mass spectrum).

Pseudocyphellavin B Tetvaacetate (14)

The depside (2) (100 mg) was acetylated in the manner described above. The usual workup yielded pseudocyphellarin B tetraacetate (14) (0.10 g, 71 %) which crystallized from acetoneicyclo- hexane in colourless crystals, m.p. 144-146" (lit.' 145-146') (Found: C, 61.1 ; H, 5.8. Calc. for C29H32012: C, 60.8; H, 5.6%). l H n.m.r. (CDCl,) 8 2.00, 2.08, 2.17, 2.20, 2.28, 2 ~ 3 2 , 2.40, 2.51, 8s, CMe; 3.92, s, OMe and 5.05, s, CH,O. This synthetic sample was identical with that derived from the natural material (t.l.c., m.m.p., 'H n.m.r.).

Manuscript received 9 April 1984