Synthetic microbial chemistry, XXXII[≠] synthesis and absolute configuration of hongoquercin A, an antibacterial sesquiterpene-substituted orsellinic acid isolated as a fungal metabolite

Article abstract of DOI:10.1002/(sici)1099-0690(200001)2000:2<297::aid-ejoc297>3.0.co;2-b

(±)-Hongoquercin A (1), the racemate of an antibacterial fungal metabolite, has been synthesized starting from geranylacetone (2) and ethyl orsellinate (ethyl 2,4-dihydroxy-6-methylbenzoate, 5). The structure (±)-1 has been confirmed by X-ray analysis of its ethyl ester (±)-10. Synthesis of the naturally occurring (+)-hongoquercin A from (-)-sclareol (11) established its configuration as depicted in 1.

Full text of DOI:10.1002/(sici)1099-0690(200001)2000:2<297::aid-ejoc297>3.0.co;2-b

FULL PAPER
Synthetic Microbial Chemistry, XXXII[°]
Synthesis and Absolute Configuration of Hongoquercin A, an Antibacterial
Sesquiterpene-Substituted Orsellinic Acid Isolated as a Fungal Metabolite
Hisayuki Tsujimori,[a][°°] Masahiko Bando,[b] and Kenji Mori*[a]
Keywords: Antibiotics / Configuration determination / Heterocycles / Hongoquercin A / Terpenoids / Total synthesis
(±)-Hongoquercin A (1), the racemate of an antibacterial
been confirmed by X-ray analysis of its ethyl ester (±)-10.
fungal metabolite, has been synthesized starting from
Synthesis of the naturally occurring (+)-hongoquercin A from
geranylacetone (2) and ethyl orsellinate (ethyl 2,4- (–)-sclareol (11) established its configuration as depicted in 1.
dihydroxy-6-methylbenzoate, 5). The structure (±)-1 has
In 1998, Roll et al. isolated hongoquercin A (1) from an
unidentified terrestrial fungus as an antibacterial antibiotic
against methicillin-resistant Staphylococcus aureus and van-
comycin-resistant Enterococcus faecium.[1] They proposed
the structure 1, i.e. a sesquiterpene coupled with orsellinic
acid, for hongoquercin A, including its relative configura-
tion, mainly on the basis of its NMR analysis.[1] Its absolute
configuration, however, remained unknown. We became
interested in establishing the absolute configuration of the
naturally occurring (ϩ)-hongoquercin A (1) through its
synthesis. This paper describes (i) the synthesis of (±)-1, the
structure of which was unequivocally established by X-ray
analysis of the corresponding ethyl ester (±)-10, and (ii) the
synthesis of (ϩ)-1 from (Ϫ)-sclareol (11), the absolute con-
figuration of which is known. The latter work clarified the
absolute configuration of (ϩ)-1 as depicted.
Our retrosynthetic analysis of 1 is shown in Scheme 1.
As in the case of the synthesis of (±)-K-76[2][3] and natural
(Ϫ)-K-76,[4] the target molecule 1 can be dissected into the
sesquiterpene part A and the orsellinic acid part B, coupling
of which would afford the precursor for the closure of the
tetrahydropyran C-ring of 1. Building block A is a known
compound,[5] while B may readily be prepared from the
Scheme 1. Retrosynthetic analysis of hongoquercin A
commercially available ethyl orsellinate (ethyl 2,4-di-
hydroxy-6-methylbenzoate, 5).
Scheme 2 summarizes our synthesis of (±)-hongoquercin
(5). Iodination of 5 with benzyltrimethylammonium di-
chloroiodate[6] furnished the 3-iodo derivative 6, the phe-
A (1). Geranylacetone (2) was converted into the known
nolic hydroxy groups of which were protected as bis(2-tri-
methylsilylethyloxy)methoxy (bisSEM) ether functionalities
to give 7 ( ϭ B).
Prior to the successful coupling of (±)-4 with 7, various
alcohol (±)-3 according to Mori and Koga[5] in 15% overall
yield (5 steps). Treatment of (±)-3 with phosphorus tribro-
mide afforded the corresponding bromide (±)-4,[5] the ses-
quiterpene part (A) of hongoquercin A (1). Preparation of
the phenolic acid part B ( ϭ 7) started from ethyl orsellinate
attempts were made to find suitable conditions for coupling
of the sesquiterpene part with the phenolic acid part. We
first attempted to use a Stille reaction[7] between the allyl-
[°]
Part XXXI: M. Seki, K. Mori, Eur. J. Org. Chem. 1999,
stannane C and 7. The stannane C, however, could not be
synthesized. We then tried to generate the anion of D in
analogy to the McMurry synthesis of (±)-K-76,[3] but un-
fortunately the protected acetal D could not be obtained.
Our next attempt was to alkylate the dianion derived from
E with (±)-4. Accordingly, the ethyl ester corresponding to
E was prepared and then subjected to various hydrolytic
2965Ϫ2967
[a]
Department of Chemistry, Faculty of Science, Science Univer-
sity of Tokyo,
Kagurazaka 1Ϫ3, Shinjuku-ku, Tokyo 162-8601, Japan
Fax: (internat.) ϩ81-3/3235-2214
Otsuka Pharmaceutical Co., Ltd.,
Kawauchi, Tokushima 771-0192, Japan
Research fellow on leave from Otsuka Pharmaceutical Co.
(1998Ϫ2000)
[b]
[°°]
Eur. J. Org. Chem. 2000, 297Ϫ302
WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000
1434Ϫ193X/00/0102Ϫ297 $ 17.50ϩ.50/0
297

H. Tsujimori, M. Bando, K. Mori
FULL PAPER
of tert-butyllithium at the ester group. The metal-halogen
exchange reaction using 7 was therefore the only solution
we found for the present coupling problem.
Before formation of the ring C, the SEM protective
groups were removed from (±)-8 by treatment with sulfuric
acid in ethanol and THF to give the dihydroxy ester (±)-9.
Ring-closure of (±)-9 was accomplished by treatment with
boron trifluorideϪdiethyl ether in dichloromethane to give
the desired tetracyclic product (±)-10 in 58% yield. Fortu-
nately, unlike in the case of the synthesis of (Ϫ)-K-76,[4] no
spiro product with a benzofuran ring system could be de-
tected on examination of the 1H-NMR spectrum of the
crude products. The product (±)-10 was nicely crystalline
and so its structure could be solved by X-ray crystallo-
graphic analysis. Its perspective view, as shown in Figure
1(a), was in perfect accord with the structure proposed for
hongoquercin A ethyl ester (10). The structure (±)-10 was
1
further supported by the NOE correlations seen in its H-
NMR spectrum, as indicated in Figure 1(b). Alkaline hy-
drolysis of (±)-10 furnished (±)-hongoquercin A (1). The
overall yield of (±)-1 was 6% based on geranylacetone (2,
10 steps) or 20% based on the commercially available ethyl
orsellinate (6, 6 steps).
Scheme 2. Synthesis of (±)-hongoquercin A; reagents: (a) PBr3,
Et2O (quant.); (b) BnMe3NICl2, KHCO3, CH2Cl2/MeOH (73%);
(c) NaH, SEMCl, DMF (87%); (d) BuLi, CuCN, THF, (±)-4 (85%);
(e) H2SO4, THF/EtOH (87%); (f)(i) BF3·OEt2, CH2Cl2; (ii) recry-
stallization from EtOH (58%); (g)(i) KOH, THF/MeOH/H2O; (ii)
H3Oϩ (96%)
Figure 1. (a) Perspective view of (±)-10 and (b) NOE correlations
between the protons of (±)-10
conditions in the hope of hydrolyzing it to E. However, all
Synthesis of the naturally occurring (ϩ)-enantiomer of
attempts were unsuccessful, and most of the starting mate- hongoquercin A (ϩ)-1 is summarized in Scheme 3. Com-
rial was recovered unchanged. This, we thought, must have mercially available (Ϫ)-sclareol (11) was oxidatively de-
been due to steric hindrance caused by the neighboring graded to the known aldehyde (ϩ)-12 according to Barrero
methoxymethyloxy (MOMoxy) and methyl groups. The et al.[8] Reduction of (ϩ)-12 with diisobutylaluminum hy-
above findings led us to the idea of converting 7 to the dride gave the known alcohol (ϩ)-3,[9] which was treated
corresponding lithiate by metal-halogen exchange with bu- with phosphorus tribromide to furnish the bromide (ϩ)-4.
tyllithium, considering that neither butyllithium nor the re- Its conversion to hongoquercin A (1) was accomplished via
sulting lithiate would be able to attack the severely hindered the ethyl ester (ϩ)-10 in the same manner as for the syn-
ester group of 7.
thesis of (±)-1. It must be noted that the crystals of (ϩ)-10
Lithiation of 7 with 1 equiv. of butyllithium at Ϫ78°C in [the ethyl ester of (ϩ)-1] were needle-shaped but unsuitable
THF smoothly generated the 3-lithiate. This was treated for X-ray analysis. The synthesis of (±)-10 therefore pro-
with copper(I) cyanide to give the corresponding cuprate, vided all the desired information, having made it possible
treatment of which with (±)-4 furnished the desired coup- to rigorously assign its configuration by X-ray analysis. The
ling product (±)-8 in 85% yield. It should be added that optically active product 1 was found to be dextrorotatory,
19
direct lithiation of the ethyl ester of E with tert-butyllithium [α]D ϭ ϩ148.6 (MeOH). Accordingly, the absolute con-
25
in the presence of N,N,NЈ,NЈ-tetramethylethylenediamine figuration of the natural (ϩ)-hongoquercin A (1), [α]D
ϭ
(TMEDA) in THF[3] gave a product resulting from attack ϩ140.7 (MeOH), was established as depicted for (ϩ)-1. The
298
Eur. J. Org. Chem. 2000, 297Ϫ302

Synthetic Microbial Chemistry, XXXII
FULL PAPER
acid and extracted with dichloromethane. The combined extracts
were washed with 5% aqueous sodium thiosulfate and brine, dried
with magnesium sulfate, and concentrated under reduced pressure.
The residue was chromatographed on silica gel (150 g) eluting with
benzene/acetone (60:1), and recrystallized from n-hexane to afford
4.00 g (73%) of 6 as colorless needles; m.p. 111Ϫ133°C (decomp.).
Ϫ IR (KBr): ν˜max ϭ 3430 cmϪ1 (vs, OH), 3340 (br. s, OH), 1620
(s, CϭO), 1500 (m, Ar), 1260 (s, CϪO), 800 (s). Ϫ 1H NMR
(400 MHz, CDCl3): δ ϭ 1.42 (t, J ϭ 7.2 Hz, 3 H, CO2CH2CH3),
2.51 (d, J ϭ 0.7 Hz, 3 H, ArMe), 4.42 (q, J ϭ 7.2 Hz, 2 H,
CO2CH2), 5.79 (s, 1 H, 4-OH), 6.47 (d, J ϭ 0.7 Hz, 1 H, 5-H),
12.87 (s, 1 H, 2-OH). Ϫ 13C NMR (100 MHz, CDCl3): δ ϭ 14.2,
24.2, 61.9, 74.2, 105.8, 110.2, 144.0, 159.4, 163.4, 171.3. Ϫ
C10H11O4 (322.1): calcd. C 37.29, H 3.44; found C 37.28, H 3.51.
overall yield of (ϩ)-1 was 6% based on (Ϫ)-sclareol (11,
9 steps).
Ethyl 3-Iodo-6-methyl-2,4-bis[2-(trimethylsilyl)ethyloxymethoxy]-
benzoate (7): A 60% mineral oil suspension of sodium hydride
(1.54 g, 38.5 mmol) was rinsed three times with 10 mL portions of
n-pentane to remove the mineral oil. Dry N,N-dimethylformamide
(DMF, 20 mL) was then added, and to this suspension a solution
of 6 (4.00 g, 12.4 mmol) in dry DMF (20 mL) was added dropwise
at 0°C under argon. The resulting mixture was stirred for 30 min.,
and then 2-(trimethylsilyl)ethyloxymethyl chloride (4.62 mL,
26.1 mmol) was added dropwise at 0°C. After stirring for 30 min.,
the pale-yellow mixture was poured into water (120 mL) and ex-
tracted with diethyl ether. The combined extracts were washed with
water and brine, dried with sodium sulfate, and concentrated under
reduced pressure. The residue was chromatographed on silica gel
Scheme 3. Synthesis of (ϩ)-hongoquercin A; reagents: (a) iBu2AlH,
toluene (89%); (b) PBr3, Et2O (77%)
A brief comment regarding comparison of our synthetic
hongoquercin A with the natural product would seem to be
necessary. The IR, 1H- and 13C-NMR spectra of 1 are easily
affected by the pH of the solution under examination; the
salt of 1 shows spectra different from those of free 1. This
fact complicated our identification. However, direct com-
parison of the synthetic sample with the natural product
(kindly provided by Dr. D. M. Roll and Mrs. S. Kirkwood
of Wyeth-Ayerst Research, U.S.A.) fully proved that they
(80 g) eluting with n-hexane/diethyl ether (20:1) to afford 6.26 g
25
(87%) of 7 as a colorless oil; nD ϭ 1.5123. Ϫ IR (film): ν˜max
ϭ
1730 cmϪ1 (CϭO), 1590 (s, Ar), 1250 (s), 1200 (s), 1160 (s), 1090
(s), 1060 (s), 1020 (s, CϪO), 920 (s), 860 (s), 840 (s). Ϫ H NMR
1
(400 MHz, CDCl3): δ ϭ 0.00 (s, 9 H, SiCH3), 0.03 (s, 9 H, SiCH3),
0.95 (m, 2 H), 1.00 (m, 2 H), 1.37 (t, J ϭ 7.2 Hz, 3 H,
CO2CH2CH3), 2.30 (s, 3 H, 6-Me), 3.77 (m, 2 H, OCH2), 3.87 (m,
2 H, OCH2), 4.36 (q, 2 H, J ϭ 7.2 Hz, CO2CH2), 5.13 (s, 2 H,
OCH2O), 5.28 (s, 2 H, OCH2O), 6.73 (s, 1 H, 5-H). Ϫ 13C NMR
(100 MHz, CDCl3): δ ϭ Ϫ1.43, Ϫ1.40, 14.2, 17.9, 18.0, 19.9, 61.3,
66.8, 68.0, 82.6, 93.2, 98.7, 112.3, 123.5, 138.3, 155.2, 157.9, 167.0.
Ϫ C22H39O6ISi2 (582.6): calcd. C 45.35, H 6.75; found C 45.18,
H 6.51.
1
were identical on the basis of IR (KBr disc), H-NMR (in
CDCl3 or CD3OD), 13C-NMR (CD3OD), and CD (CHCl3)
comparisons, as well as the mixed melting-point determi-
nation.
In conclusion, both (±)- and (ϩ)-hongoquercin A have
been synthesized, and the absolute configuration of the lat-
ter has been established as 5R,8R,9R,10S.
(4aS,8aS)-3,4,4a,5,6,7,8,8a-Octahydro-2,5,5,8a-tetramethyl-1-naph-
thalenemethanol [(؉)-3]: To
a
solution of (ϩ)-12[8] (120 mg,
0.5 mmol) in dry toluene (1.2 mL), DIBAL (1.01  in toluene,
0.8 mL, 0.8 mmol) was added dropwise at Ϫ78°C under argon.
After stirring at Ϫ78°C for 0.5 h, the reaction was quenched with
methanol. The mixture was poured into 1  hydrochloric acid and
extracted with toluene. The combined extracts were washed with
1  hydrochloric acid and brine, dried with magnesium sulfate, and
concentrated under reduced pressure. The residue was chromato-
graphed on silica gel (1.0 g) eluting with n-hexane/ethyl acetate
(10:1), and subsequently recrystallized from n-pentane to afford
108 mg (89%) of (ϩ)-3 as a colorless powder; m.p. 115Ϫ116°C,
Experimental Section
General: Melting points: Yanaco MP-S3, uncorrected values. Ϫ
1
MS: Jeol JMS-SX102A. Ϫ IR: Hitachi PerkinϪElmer 1640. Ϫ H
NMR: Jeol JNMϪEX400 (400 MHz) and Jeol JNM-LA500
(500 MHz, CHCl3 at δ ϭ 7.26 or CH3OD at δ ϭ 3.30 as internal
standards). Ϫ 13C NMR: Jeol JNMϪLA400 (100 MHz) and Jeol
JNM-LA500 (125 MHz, CDCl3 at δ ϭ 77.0 or [D4]methanol at
δ ϭ 49.0 as internal standards). Ϫ MS: Shimadzu GCMS-QP
2000A and Jeol JMS-SX 102A. Ϫ Optical rotation: Jasco DIP-
1000. Ϫ CD spectrum: Jasco J-725. Ϫ CC: Merck Kieselgel 60
Art 1.07734.
18
ref.[9] 89Ϫ90°C. Ϫ [α]D ϭ ϩ113 (c ϭ 0.14, CHCl3), ref.[9]
15
[α]D ϭ ϩ105.4 (c ϭ 0.13, CHCl3). Thus, our (ϩ)-3 showed a
slightly higher m.p. and a slightly larger specific rotation value; this
might be due to the higher purity of our material. Ϫ IR (KBr):
ν˜max ϭ 3370 cmϪ1 (s, OH), 1660 (w, CϭC), 1000 (s, CϭC), 980 (s,
Ethyl 2,4-Dihydroxy-3-iodo-6-methylbenzoate (6): To a solution of
5 (3.34 g, 17.0 mmol) in dichloromethane (100 mL) and methanol
(80 mL), benzyltrimethylammonium dichloroiodate (5.93 g,
17.0 mmol) was added followed by potassium hydrogen carbonate
(5.11 g, 51.0 mmol). The mixture was stirred for 21 h at room tem-
1
CϭC). Ϫ H NMR (400 MHz, CDCl3): δ ϭ 0.84 (s, 3 H, 5-Me),
0.89 (s, 3 H, 5-Me), 0.96 (s, 3 H, 8a-Me), 1.72 (s, 3 H, 2-Me), 4.04
(d, J ϭ 11.5 Hz, 1 H, CHHOH), 4.20 (d, J ϭ 11.5 Hz, 1 H,
CHHOH). Ϫ 13C NMR (125 MHz, CDCl3): δ ϭ 18.8, 18.9, 19.3,
perature. It was then acidified to pH 2Ϫ3 with 1  hydrochloric 20.7, 21.6, 33.22, 33.24, 33.7, 36.8, 38.0, 41.6, 51.7, 58.3, 132.5,
Eur. J. Org. Chem. 2000, 297Ϫ302 299

H. Tsujimori, M. Bando, K. Mori
FULL PAPER
1
140.9. Ϫ C15H26O (222.4): calcd. C 81.02, H 11.79; found C 80.81, Ar), 1580 (s, Ar), 1250 (s, CϪO), 1190 (s), 1020 (m, CϪO). Ϫ H
H 11.75.
NMR (500 MHz, CDCl3): δ ϭ 0.85 (s, 3 H, 5-Me), 0.91 (s, 3 H, 5-
Me), 1.05 (s, 3 H, 8a-H), 1.11 (ddd, J ϭ 13.5, 13.5, 2.5 Hz, 1 H,
8ax-H), 1.13 (ddd, J ϭ 13.5, 13.5, 4.7 Hz, 1 H, 6ax-H), 1.18 (dd,
J ϭ 12.8, 1.9 Hz, 1 H, 4a-H), 1.36 (m, 1 H, 6eq-H), 1.38 (m, 1 H,
7ax-H), 1.41 (t, J ϭ 7.0 Hz, 3 H, CO2CH2CH3), 1.51 (m, 1 H, 4ax-
H), 1.54 (m, 1 H, 7eq-H), 1.66 (m, 1 H, 8eq-H), 1.72 (s, 3 H, 2-
Me), 1.75 (m, 1 H, 4eq-H), 2.15 (br. dd, J ϭ 9.2, 4.0 Hz, 2 H, 3-
H), 2.46 (s, 3 H, ArMe), 3.39 (d, J ϭ 17.7 Hz, 1 H, ArCH2), 3.50
(d, J ϭ 17.7 Hz, 1 H, ArCH2), 4.39 (q, J ϭ 7.0 Hz, 2 H, CO2CH2),
6.15 (s, 1 H, ArH), 7.83 (s, 1 H, ArOH), 12.38 (s, 1 H, ArOH). Ϫ
13C NMR (125 MHz, CDCl3): δ ϭ 14.3, 18.69, 18.71, 19.9, 20.4,
21.7, 24.12, 24.14, 33.1, 33.4, 33.5, 35.7, 39.5, 41.4, 51.5, 61.1,
104.5, 109.5, 112.1, 133.5, 139.9, 140.1, 160.6, 162.7, 172.4. Ϫ
C25H36O4 (400.6): calcd. C 74.96, H 9.06; found C 75.06, H 8.99.
Ethyl (4aS*,8aS*)-2,4-Dihydroxy-6-methyl-3-[(3,4,4a,5,6,7,8,8a-oc-
tahydro-2,5,5,8a-tetramethyl-1-naphthyl)methyl]benzoate
[(±)-9]:
The known (±)-3 was first prepared from geranylacetone. Ϫ (i) Bro-
mide (±)-4: Phosphorus tribromide (0.27 mL, 2.8 mmol) was added
dropwise to a suspension of (±)-3 (1.79 g, 8.0 mmol) in dry diethyl
ether (18 mL) at Ϫ20°C under argon. After stirring for a further
15 min., methanol (0.3 mL) and water (2.0 mL) were added drop-
wise to the colorless solution and the resulting mixture was ex-
tracted with diethyl ether. The combined extracts were washed with
5% aqueous sodium hydrogen carbonate solution, water, and brine,
dried with magnesium sulfate, and concentrated under reduced
pressure to give 2.30 g of crude (±)-4, which could only be charac-
terized by IR due to its instability. Ϫ IR (film): ν˜max ϭ 1650 cmϪ1
(w, CϭC), 1220 (s, CϪO), 1050 (m, CϪO). This unstable colorless
oil (±)-4 was used immediately in the next step without further
purification. Ϫ (ii) Coupling Product (±)-8: Butyllithium (1.53  in
hexane, 7.0 mL, 10.7 mmol) was added dropwise to a cooled solu-
tion of 7 (6.26 g, 10.7 mmol) in dry tetrahydrofuran (63 mL) at
Ϫ78°C under argon. After 30 min., the pale-yellow solution was
treated with copper(I) cyanide (1.92 g, 21.5 mmol, dried at 120°C
for 15 h under reduced pressure) in one portion. The suspension
was slowly allowed to warm to Ϫ10°C until almost all of the cop-
per(I) cyanide had dissolved and was then cooled to Ϫ78°C once
more, whereupon a solution of crude (±)-4 (2.30 g) in dry tetra-
hydrofuran (18 mL) was added dropwise. The resulting mixture was
stirred for 5 h and was then poured into water and extracted with
diethyl ether. The combined extracts were washed with water and
brine, dried with sodium sulfate, and concentrated under reduced
pressure. The residue was chromatographed on silica gel (180 g)
eluting with n-hexane/ethyl acetate (30:1) to give 4.50 g (85%) of
(±)-8 as a colorless, viscous oil; nD25 ϭ 1.5072. Ϫ IR (film): ν˜max ϭ
1730 cmϪ1 (vs, CϭO), 1600 (m, Ar), 1570 (w, Ar), 1270 (s,
SiϪCH3), 1250 (s, CϪO), 1060 (vs, CϪO), 860 (vs, Ar) 840 (vs,
Ar). Ϫ 1H NMR (500 MHz, CDCl3): δ ϭ 0.01 (s, 9 H, SiMe), 0.02
(s, 9 H, SiMe), 0.81 (s, 3 H, 5-Me), 0.86 (s, 3 H, 5-Me), 0.89 (s, 3
H, 8a-Me), 0.94 (m, 2 H, SiCH2), 0.99 (m, 2 H, SiCH2), 1.08 (m,
1 H, 8ax-H), 1.13 (dd, J ϭ 12.7, 1.7 Hz, 1 H, 4a-H), 1.37 (t, J ϭ
7.0 Hz, 3 H, CO2CH2CH3), 1.32Ϫ1.54 (m, 5 H), 1.50 (s, 3 H, 2-
Me), 1.63 (br. dd, J ϭ 12.7, 8.5 Hz, 1 H, 4eq-H), 1.88 (br. d, J ϭ
12.5 Hz, 1 H, 8eq-H), 2.00 (m, 2 H, 3-H), 2.27 (s, 3 H, ArMe), 3.40
(d, J ϭ 15.5 Hz, 1 H, ArCH2), 3.49 (d, J ϭ 15.5 Hz, 1 H, ArCH2),
3.69 (pseudo t, J ϭ 7.6 Hz, 2 H, OCH2), 3.80 (pseudo t, J ϭ 8.6 Hz,
2 H, OCH2), 4.35 (q, J ϭ 7.0 Hz, 2 H, CO2CH2), 4.98 (d, J ϭ
5.8 Hz, 1 H, OCH2O), 5.01 (d, J ϭ 5.8 Hz, 1 H, OCH2O), 5.10 (d,
J ϭ 7.8 Hz, 1 H, OCH2O), 5.11 (d, J ϭ 7.8 Hz, 1 H, OCH2O),
6.71 (s, 1 H, ArH). Ϫ 13C NMR (125 MHz, CDCl3): δ ϭ Ϫ1.42,
Ϫ1.37, 14.3, 18.0, 18.2, 19.1, 19.2, 19.8, 20.3, 20.6, 21.7, 24.1, 33.4,
33.5, 34.6, 37.0, 39.4, 41.8, 51.9, 60.9, 66.1, 67.4, 92.7, 99.0, 111.6,
122.7, 123.2, 126.0, 134.6, 137.9, 153.5, 158.0, 168.5. Ϫ (iii) Di-
hydroxy Ester (±)-9: The colorless oil (±)-8 (4.48 g) was dissolved
in a mixture of ethanol (25 mL) and tetrahydrofuran (25 mL), and
then a solution of sulfuric acid (0.7 mL) in ethanol (5 mL) was
added at room temperature. The resulting mixture was stirred for
20 h, then neutralized to pH 7Ϫ8 with 5% aqueous sodium hydro-
gen carbonate solution, and extracted with dichloromethane. The
combined extracts were washed with brine, dried with sodium sul-
fate, and concentrated under reduced pressure. The residue was
Ethyl
(4aS,8aS)-2,4-Dihydroxy-6-methyl-3-[(3,4,4a,5,6,7,8,8a-oc-
tahydro-2,5,5,8a-tetramethylnaphthyl)methyl]benzoate [(؉)-9]: Ac-
cording to the preparation of (±)-9, (ϩ)-3 (211 mg) was converted
to the bromide, which was coupled with 6 to give 448 mg (77%) of
23
18
(ϩ)-8 as a colorless, viscous oil; nD ϭ 1.5058; [α]D ϭ ϩ39.5
(c ϭ 0.63, CHCl3). (ϩ)-8 (340 mg) was then deprotected to give
18
170 mg (83%) of (ϩ)-9 as a colorless, amorphous solid; [α]D
ϭ
ϩ108 (c ϭ 0.60, CHCl3). Ϫ IR (CHCl3): ν˜max ϭ 3320 cmϪ1 (vs,
OH), 3020 (s, OH), 1640 (s, CϭO), 1620 (s, Ar), 1580 (s, Ar), 1190
(s), 1020 (m, CϪO). Ϫ C25H36O4 (400.6): calcd. C 74.96, H 9.06;
found C 74.84, H 9.03.
(±)-Hongoquercin A Ethyl Ester [(±)-10]: Boron trifluorideϪdiethyl
ether (0.2 mL, 1.5 mmol) was added dropwise to a solution of (±)-
9 (2.32 g, 5.8 mmol) in dry dichloromethane (23 mL) at room tem-
perature under argon. After stirring for a further 5 h, the resulting
pale-brown solution was diluted with water (10 mL) and extracted
with dichloromethane. The combined extracts were washed with
water and brine, dried with magnesium sulfate, and concentrated
under reduced pressure. The residue was recrystallized from etha-
nol to afford 1.34 g (58%) of (±)-10 as a colorless powder, m.p.
139Ϫ141°C. Ϫ IR (KBr): ν˜max ϭ 1650 cmϪ1 (s, CϭO), 1620 (m,
1
Ar), 1580 (s, Ar), 1280 (s, CϪO). Ϫ H NMR (500 MHz, CDCl3):
δ ϭ 0.84 (s, 3 H, 12-Me), 0.90 (s, 3 H, 11-Me), 0.91 (s, 3 H, 14-
Me), 0.95 (ddd, J ϭ 13.1, 13.1, 3.7 Hz, 1 H, 1ax-H), 1.02 (dd, J ϭ
12.2, 1.9 Hz, 1 H, 5-H), 1.16 (ddd, J ϭ 13.5, 13.2, 3.4 Hz, 1 H,
3ax-H), 1.19 (s, 3 H, 13-Me), 1.35 (ddd, J ϭ 13.5, 13.2, 3.4 Hz, 1
H, 3eq-H), 1.40 (t, J ϭ 7.1 Hz, 3 H, CO2CH2CH3), 1.44 (m, 1 H,
6eq-H), 1.47 (m, 1 H, 2ax-H), 1.54 (dd, J ϭ 13.2, 4.9 Hz, 1 H, 9-
H), 1.62 (m, 1 H, 7ax-H), 1.66 (m, 1 H, 2eq-H), 1.76 (m, 1 H, 6ax-
H), 1.81 (m, 1 H, 1eq-H), 2.06 (ddd, J ϭ 12.2, 3.1, 3.1 Hz, 1 H,
7eq-H), 2.29 (dd, J ϭ 16.8, 13.2 Hz, 1 H, 15ax-H), 2.46 (s, 3 H, 23-
H), 2.68 (dd, J ϭ 16.8, 4.9 Hz, 1 H, 15eq-H), 4.38 (q, J ϭ 7.1 Hz, 2
H, CO2CH2), 6.17 (s, 1 H, 18-H), 12.18 (s, 1 H, 21-OH). Ϫ 13C
NMR (125 MHz, CDCl3): δ ϭ 14.3, 14.9, 16.7, 18.5, 19.7, 20.7,
21.6, 24.2, 33.2, 33.4, 37.0, 39.2, 40.9, 41.8, 51.6, 56.1, 61.0, 78.1,
104.0, 108.0, 112.1, 140.0, 157.7, 162.9, 172.3. Ϫ C25H36O4 (400.6):
calcd. C 74.96, H 9.06; found C 74.67, H 9.13. A sample for X-ray
analysis was recrystallized from diethyl ether and dichloromethane
to afford colorless plates, m.p. 140Ϫ141°C.
X-ray Analysis of (±)-10: C25H36O4, Mr ϭ 400.56, triclinic, space
˚
˚
¯
group P1bar a ϭ 12.634(2), b ϭ 13.325(3), c ϭ 7.241(4) A, α ϭ
3
95.94(3)°, β ϭ 100.52(3)°, γ ϭ 109.74(1)°, V ϭ 1110.0(6) A , Z ϭ
2, Dc ϭ 1.198 MgmϪ3, F(000) ϭ 436, µ (Mo-Kα) ϭ 0.792 cmϪ1
.
chromatographed on silica gel (100 g) eluting with n-hexane/diethyl The crystal used for data collection was a colorless plate of approxi-
ether (10:1), and subsequently recrystallized from n-hexane to af- mate dimensions 0.80 ϫ 0.50 ϫ 0.15 mm. All data were obtained
ford 2.35 g (87%) of (±)-9 as colorless needles, m.p. 130Ϫ131°C. Ϫ with a Rigaku AFC-5S automated four-circle diffractometer using
IR (KBr): ν˜max ϭ 3330 cmϪ1 (vs, OH), 1650 (s, CϭO), 1630 (s, graphite-monochromated Mo-Kα radiation. Unit cell parameters
300
Eur. J. Org. Chem. 2000, 297Ϫ302

Synthetic Microbial Chemistry, XXXII
FULL PAPER
19
were determined by least-squares refinement of the optimized set-
ting angles of 25 reflections in the range 11.7 < θ < 18.03°. The
intensities were measured using ω/2θ scans up to 50°. Three stand-
product: m.p. 135Ϫ148°C; mixture m.p. 135Ϫ150°C]. Ϫ [α]D
ϭ
ϩ149 (c ϭ 0.57, MeOH). Ϫ CD (c ϭ 1.34 ϫ 10Ϫ4, CHCl3): ∆ε (λ,
nm) ϭ ϩ30.1 (277), ϩ15.7 (304) [natural product: ϩ26.2 (276),
ard reflections were monitored every 150 measurements. The data ϩ14.2 (304)]. Ϫ IR (KBr): ν˜max ϭ 2940 cmϪ1 (s), 2860 (s), 2570
were corrected for Lorentz and polarization factors and an absorp-
(m), 1620 (s, CϭO), 1580 (s, Ar), 1500 (m), 1450 (m), 1370 (s), 1260
tion correction was applied. Of the 4133 independent reflections (s, CϪO), 1220 (m), 1180 (m), 1130 (s), 1080 (m), 1000 (m), 930
1
collected, 2375 reflections with I > 2.0 σ(I ) were used for the struc-
(m), 900 (w), 850 (m), 810 (m), 760 (m), 720 (m), 660 (m). Ϫ H
ture determination and refinement. The structure was solved by NMR (500 MHz, CDCl3): δ ϭ 0.85 (s, 3 H, 12-Me), 0.91 (s, 3 H,
direct methods using the teXsan crystallographic software pack-
age.[10] All non-H atoms were located on a Fourier map. The
atomic parameters were refined by full-matrix least-squares meth-
ods, using anisotropic temperature factors for all non-H atoms. H
atoms were placed geometrical positions and were not refined. The
final refinement converged with R ϭ 0.057 and Rw ϭ 0.069 for 266
parameters. Atomic scattering factors were taken from the ЉInter-
11-Me), 0.92 (s, 3 H, 14-Me), 0.96 (ddd, J ϭ 13.1, 13.1, 4.0 Hz, 1
H, 1ax-H), 1.03 (dd, J ϭ 12.2, 1.9 Hz, 1 H, 5-H), 1.17 (ddd, J ϭ
13.5, 13.5, 4.0 Hz, 1 H, 3ax-H), 1.20 (s, 3 H, 13-Me), 1.36 (ddd,
J ϭ 13.5, 13.5, 3.0 Hz, 1 H, 3eq-H), 1.40 (m, 1 H, 6eq-H), 1.49 (m,
1 H, 2ax-H), 1.55 (dd, J ϭ 13.3, 4.9 Hz, 1 H, 9-H), 1.64 (m, 1 H,
7ax-H), 1.67 (ddd, J ϭ 13.1, 13.1, 4.6 Hz, 1 H, 2eq-H), 1.78 (m, 1
H, 6ax-H), 1.81 (m, 1 H, 1eq-H), 2.07 (ddd, J ϭ 12.5, 3.1, 3.1 Hz,
national Tables for X-ray CrystallographyЉ.[11] Supplementary ma- 1 H, 7eq-H), 2.29 (dd, J ϭ 16.8, 13.3 Hz, 1 H, 15ax-H), 2.51 (s, 3
terial available includes lists of atomic coordinates for the non-H
atoms and the bond lengths and angles in (±)-10 with their e.s.d.Јs
in parentheses.[12]
H, 23-Me), 2.69 (dd, J ϭ 16.8, 4.9 Hz, 1 H, 15eq-H), 6.21 (s, 1 H,
18-H), 11.86 (s, 1 H, 21-OH). Ϫ 1H NMR (500 MHz, CD3OD):
δ ϭ 0.88 (s, 3 H, 12-Me), 0.91 (s, 3 H, 11-Me), 0.95 (s, 3 H, 14-
Me), 1.00 (ddd, J ϭ 12.9, 12.9, 3.7 Hz, 1 H, 1ax-H), 1.06 (dd, J ϭ
12.2, 1.8 Hz, 1 H, 5-H), 1.18 (s, 3 H, 13-Me), 1.21 (ddd, J ϭ 13.5,
13.5, 3.8 Hz, 1 H, 3ax-H), 1.41 (m, 1 H, 3eq-H), 1.44 (m, 1 H, 6eq-
H), 1.46 (m, 1 H, 2ax-H), 1.51 (dd, J ϭ 13.2, 5.1 Hz, 1 H, 9-H),
1.64 (ddd, J ϭ 13.2, 12.5, 3.4 Hz, 1 H, 7ax-H), 1.69 (m, 1 H, 2eq-
H), 1.71 (m, 1 H, 6ax-H), 1.77 (m, 1 H, 1eq-H), 2.04 (ddd, J ϭ
12.5, 3.1, 3.1 Hz, 1 H, 7eq-H), 2.26 (dd, J ϭ 16.8, 13.2 Hz, 1 H,
15ax-H), 2.45 (s, 3 H, 23-Me), 2.63 (dd, J ϭ 16.8, 5.1 Hz, 1 H,
15eq-H), 6.10 (s, 1 H, 18-H). Ϫ 13C NMR (125 MHz, CD3OD):
δ ϭ 15.4, 17.7, 19.6, 20.8, 21.1, 22.0, 24.2, 33.9, 34.1, 38.1, 40.4,
42.1, 43.0, 53.1, 57.5, 78.9, 104.9 (w), 108.8, 112.8, 141.7, 158.8,
164.5, 175.6 (w). These IR and NMR data are identical to those
of the authentic sample of (ϩ)-1. Ϫ C23H32O4 (372.5): calcd. C
74.16, H 8.66; found C 73.87, H 8.62.
(؉)-Hongoquercin A Ethyl Ester [(؉)-10]: According to the prep-
aration of (±)-10, (ϩ)-9 (156 mg) was treated with boron trifluori-
deϪdiethyl ether to give 82 mg (53%) of (ϩ)-10 as a colorless pow-
18
der; m.p. 169Ϫ171°C; [α]D ϭ ϩ131 (c ϭ 0.51, CHCl3). Ϫ IR
(KBr): ν˜max ϭ 1640 cmϪ1 (s, CϭO), 1580 (s, Ar), 1280 (s, CϪO),
1270 (s), 1250 (s), 1180 (s), 1130 (s), 1010 (s). Ϫ C25H36O4 (400.6):
calcd. C 74.96, H 9.06; found C 74.83, H 9.07.
(±)-Hongoquercin A [(±)-1]: The colorless crystalline (±)-10 (57 mg,
0.14 mmol) was dissolved in a mixture of methanol (1 mL) and
tetrahydrofuran (1.5 mL), and then a potassium hydroxide solution
(23 mg, 85%, 0.35 mmol, in 1 mL of water) was added and the mix-
ture was stirred for 3 h under reflux. The reaction mixture was
subsequently cooled and acidified to pH 2Ϫ3, whereupon it was
extracted with dichloromethane. The combined extracts were
washed with water and brine, dried with sodium sulfate, and con-
centrated under reduced pressure. The residue was chromato-
graphed on silica gel (1.0 g) eluting with n-hexane/ethyl acetate
(4:1) and subsequently recrystallized from methanol to afford
51 mg (96%) of (±)-1 as a colorless powder; m.p. 134Ϫ138°C. Ϫ
IR (KBr): ν˜max ϭ 2940 cmϪ1 (s), 2860 (s), 2840 (s), 2690 (m), 2640
(m), 2570 (m), 2520 (m), 1620 (s, CϭO), 1580 (s, Ar), 1500 (m),
1460 (m), 1390 (s), 1380 (s), 1320 (m), 1260 (s, CϪO), 1180 (s),
1150 (m), 1130 (s), 1080 (m), 1060 (m), 1010 (m), 980 (m), 930 (m),
900 (w), 860 (m), 840 (m), 800 (m), 760 (m), 720 (m), 660 (m), 580
(؉)-Hongoquercin A Monopotassium Salt: Potassium carbonate
(5 mg, 0.04 mmol) was added to a solution of (ϩ)-1 (26 mg,
0.07 mmol) in methanol (1 mL) at room temperature. After stirring
for a further 0.5 h, the mixture was filtered and the filtrate was
concentrated under reduced pressure to afford 29 mg (quant.) of
the salt as a colorless powder. Ϫ [α]D20 ϭ ϩ110 (c ϭ 0.57, MeOH).
Ϫ IR (KBr): ν˜max ϭ 3441 cmϪ1 (br. s), 2937 (s), 2865 (m), 1624
(sh, CϭO), 1587 (s, Ar). Ϫ 1H NMR (500 MHz, CD3OD): δ ϭ
0.87 (s, 3 H, 12-Me), 0.91 (s, 3 H, 11-Me), 0.94 (s, 3 H, 14-Me),
1.00 (ddd, J ϭ 13.4, 13.4, 4.0 Hz, 1 H, 1ax-H), 1.05 (dd, J ϭ 12.2,
1.9 Hz, 1 H, 5-H), 1.16 (s, 3 H, 13-Me), 1.20 (ddd, J ϭ 13.5, 13.5,
3.5 Hz, 1 H, 3ax-H), 1.40 (m, 1 H, 3eq-H), 1.43 (m, 1 H, 6eq-H),
1.47 (m, 1 H, 2ax-H), 1.51 (dd, J ϭ 13.4, 5.0 Hz, 1 H, 9-H), 1.62
(ddd, J ϭ 13.1, 12.4, 4.0 Hz, 1 H, 7ax-H), 1.69 (m, 1 H, 2eq-H),
1.75 (m, 1 H, 6ax-H), 1.78 (m, 1 H, 1eq-H), 2.01 (ddd, J ϭ 12.4,
3.0, 3.0 Hz, 1 H, 7eq-H), 2.25 (dd, J ϭ 16.5, 13.4 Hz, 1 H, 15ax-
H), 2.50 (s, 3 H, 23-Me), 2.61 (dd, J ϭ 16.5, 5.0 Hz, 1 H, 15eq-H),
5.97 (s, 1 H, 18-H). Ϫ 13C NMR (125 MHz, CD3OD): δ ϭ 15.4,
18.0, 19.6, 20.8, 21.1, 22.1, 23.7, 33.9, 34.1, 38.1, 40.5, 42.3, 43.1,
53.6, 57.6, 78.0, 108.0, 111.2, 111.4, 141.2, 156.1, 162.9, 177.3. Ϫ
C23H31O4K (410.6): calcd. K 9.52; found K 9.22. Ϫ MS: FABϩ:
found 410.1868 (C23H31O4K, Mϩ), calcd. 410.1859.
1
(w), 490 (w), 470 (w). Ϫ H NMR (500 MHz, CD3OD): δ ϭ 0.88
(s, 3 H, 12-Me), 0.91 (s, 3 H, 11-Me), 0.95 (s, 3 H, 14-Me), 1.00
(ddd, J ϭ 12.9, 12.9, 3.7 Hz, 1 H, 1ax-H), 1.07 (dd, J ϭ 12.2,
1.8 Hz, 1 H, 5-H), 1.18 (s, 3 H, 13-Me), 1.21 (ddd, J ϭ 13.5, 13.5,
3.8 Hz, 1 H, 3ax-H), 1.41 (m, 1 H, 3eq-H), 1.44 (m, 1 H, 6eq-H),
1.46 (m, 1 H, 2ax-H), 1.51 (dd, J ϭ 13.2, 4.9 Hz, 1 H, 9-H), 1.64
(ddd, J ϭ 13.2, 12.5, 3.4 Hz, 1 H, 7ax-H), 1.69 (m, 1 H, 2eq-H),
1.71 (m, 1 H, 6ax-H), 1.77 (m, 1 H, 1eq-H), 2.04 (ddd, J ϭ 12.5,
3.1, 3.1 Hz, 1 H, 7eq-H), 2.26 (dd, J ϭ 16.8, 13.2 Hz, 1 H, 15ax-
H), 2.45 (s, 3 H, 23-Me), 2.63 (dd, J ϭ 16.8, 4.9 Hz, 1 H, 15eq-H),
6.10 (s, 1 H, 18-H). Ϫ 13C NMR (125 MHz, CD3OD): δ ϭ 15.4,
17.7, 19.6, 20.8, 21.1, 22.0, 24.2, 33.9, 34.2, 38.1, 40.4, 42.2, 43.0,
53.2, 57.5, 78.9, 108.8, 112.7, 141.7, 158.8, 164.5. Ϫ C23H32O4
(372.5): calcd. C 74.16, H 8.66; found C 73.86, H 8.71. Ϫ MS:
FABϪ found 371.2219 (C23H31O4, Mϩ Ϫ H), calcd. 371.2222.
Acknowledgments
(؉)-Hongoquercin A [(؉)-1]: According to the preparation of (±)-
1, (ϩ)-10 (65 mg) was hydrolyzed and chromatographed on silica
We thank Dr. Deborah M. Roll and Mrs. Susan Kirkwood (Wyeth-
Ayerst Research, Pearl River, N.Y., U.S.A.) for their kind gift of
gel (1.0 g) eluting with n-hexane/ethyl acetate (4:1) to give 56 mg natural product (ϩ)-1. This work was financially supported by Ot-
(93%) of (ϩ)-1 as a colorless powder; m.p. 147Ϫ150°C [natural
Eur. J. Org. Chem. 2000, 297Ϫ302
suka Pharmaceutical Co., Ltd.
301

H. Tsujimori, M. Bando, K. Mori
FULL PAPER
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Further details of the crystal structure investigation have been
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Copies of the data can be obtained free of charge on
application to the CCDC, 12 Union Road, Cambridge
CB2 1EZ, U.K. [Fax: (internat.) ϩ44 (0)1223 336033;
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302
Eur. J. Org. Chem. 2000, 297Ϫ302