Synthesis of musafluorone: A naphthoxanthenone isolated from Musa acuminata

Article abstract of DOI:10.1016/j.tetlet.2010.06.128

5-Methoxy-3H-naphtho[2,1,8-mna]xanthen-3-one (musafluorone, 1), the only naphthoxanthenone reported so far from Musaceae, was synthesized starting from 2-naphthol in nine steps and resulted in an overall yield of 3%. Grignard addition of phenylmagnesium b

Full text of DOI:10.1016/j.tetlet.2010.06.128

Tetrahedron Letters 51 (2010) 4640–4643
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Tetrahedron Letters
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Synthesis of musafluorone: a naphthoxanthenone isolated from
Musa acuminata
a,
Luisa Duque a, Catalina Restrepo a, Jairo Sáez a, Jesús Gil b, Bernd Schneider c, , Felipe Otálvaro
*
*
a Instituto de Química-Universidad de Antioquia, Calle 67# 53-108, A.A. 1226, Medellín, Colombia
b Universidad Nacional de Colombia sede Medellín, Autopista Norte, Calle 64 Cra 65, A.A. 1027, Medellín, Colombia
c Max-Planck Institut für Chemische Ökologie, Beutenberg Campus, Hans-Knöll-Strasse 8, 07745 Jena, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
5-Methoxy-3H-naphtho[2,1,8-mna]xanthen-3-one (musafluorone, 1), the only naphthoxanthenone
reported so far from Musaceae, was synthesized starting from 2-naphthol in nine steps and resulted in
an overall yield of 3%. Grignard addition of phenylmagnesium bromide to 4-methoxyperinaphthenone
afforded the corresponding 4-methoxy-9-phenylphenalenone which, after epoxidation and methyl trans-
position, was subjected to a photochemical cyclization procedure to furnish 1.
Received 2 June 2010
Revised 16 June 2010
Accepted 28 June 2010
Available online 3 July 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Grignard addition
Naphthoxanthenones
Phenylphenalenones
Photochemical cyclization
Naphthoxanthenones constitutea rare group of natural pigments
characterized by the formation of intensely fluorescent solutions in
polar solvents.1 Of the five members reported as natural products,1
four were isolated from Haemodoraceae plants with a common
1H-naphtho[2,1,8-mna]xanthen-1-one skeleton.1 Musafluorone
(5-methoxy-3H-naphtho[2,1,8-mna]xanthen-3-one, 1) is the only
naphthoxanthenone isolated from Musa acuminata with an isomeric
3H-naphtho[2,1,8-mna]xanthen-3-one core nucleus.1d There are no
reports concerning the biological activity of naphthoxanthenones;
however, evidence exist that the extracts of Lachnanthes tinctoria
containing among their major constituents 2,5-dihydroxy-1H-
naphtho[2,1,8-mna]xanthen-1-one (lachnanthofluorone) exhibit
photodynamic activity against Staphylococcus epidermidis.2 The pho-
totoxic potential of naphthoxanthenones can be further suspected
by their relationship with perinaphthenone, which due to its nearly
100% yield of 1O2 production2 possesses a high photosensitizing
capacity.2 The phototoxic effects of various perinaphthenones
against Mycosphaerella fijiensis recently have been demonstrated.2
Because of their minute occurrence in only a few phenylphe-
nalenone-producing plants, naphthoxanthenones are almost inac-
cessible from natural sources. At the extreme is musafluorone
(1), of which 0.5 mg was isolated from 80 kg of fresh rhizomes of
M. acuminata after laborious purification procedures.1d Therefore,
it is important to develop synthetic routes for these kinds of
compounds in order to guarantee their accessibility for biological
assays.
In the previous work, we stated that 4-methoxy-1H-phenalen-
1-one (4-methoxyperinaphthenone, 6) could be used as a starting
material for the synthesis of oxabenzochrysenones (naphthoxan-
thenones).3 Here, we report the use of 6 in the preparation of 1,
employing a Grignard addition as a C–C bond-forming reaction
and a photochemical cyclization as the key steps in the generation
of the naphthoxanthenone nucleus.
Biosynthetically, it is presumed that musafluorone (1) and re-
lated naphthoxanthenones are derived from 9-phenylphenalenon-
es by oxidative phenol coupling.1d Interestingly, typical EI-MS
spectra of 9-phenylphenalenones present an [Mꢀ1]+ ion as the
base peak.
This [Mꢀ1]+ ion is supposed to be of naphthoxanthenium struc-
ture.1a Therefore, combining both biomimetic and retro mass spec-
trometric strategies provides an interesting approach to the
naphthoxanthenone nucleus.4 This approach has been applied suc-
cessfully in the case of lachnanthofluorone and structural
analogs.1a,c
The general features of the musafluorone synthesis are outlined
retrosynthetically in Scheme 1.
4-Methoxyperinaphthenone (6) can be conveniently prepared
by a one-pot cyclization procedure of 3-(2-methoxy-1-naph-
thyl)propanoic acid.3 This precursor can be obtained in good yield
through the use of a Heck–Fujiwara coupling between 1-bromo-2-
methoxynaphthalene and ethyl acrylate.3 However, hydrolysis of
3-(2-methoxynaphthalen-1-yl)propanenitrile (8) can provide the
same precursor,6 with the advantage that 8 can be obtained from
* Corresponding authors. Tel.: +49 3641571600; fax: +49 3641571601 (B.S.); tel.:
+57 42195653; fax: +57 42330120 (F.O.).
E-mail addresses: schneider@ice.mpg.de (B. Schneider), pipelion@quimica.
udea.edu.co (F. Otálvaro).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.06.128

L. Duque et al. / Tetrahedron Letters 51 (2010) 4640–4643
4641
biomimetic oxidation
OMe
5
OMe
O
O
9
O
1
O
3
OH
OMe
4
7
1
Friedel-Crafts
1
6
Grignard
Michael
CN
OH
OMe
Scheme 1. Retrosynthetic analysis of musafluorone (1).5
2-naphthol in two steps without the use of palladium and on a
multigram scale.6 Thus, refluxing acrylonitrile with a basic solu-
tion of 2-naphthol in benzene according to the method reported
by Hardman6a (Scheme 2) afforded 3-(2-hydroxynaphthalen-1-
yl)propanenitrile (9) in 43% yield (84% based on recovered
2-naphthol) after flash column chromatography.7 Treatment of 9
with iodomethane produced the methyl ether 8 (80%)8 which,
after basic hydrolysis, afforded the corresponding 3-(2-meth-
oxynaphthalen-1-yl)propanoic acid (7) in 67% yield after acidic
workup and in suitable purity for the next step.9 Friedel–Crafts
cyclization of 7 using our previously reported method3 completed
the synthesis of 4-methoxyperinaphthenone (6, 60%) on the gram
scale.
Adding a phenylmagnesium bromide solution to 4-methoxy-
perinaphthenone (6) resulted in 4-methoxy-9-phenyl-1H-phena-
len-1-one (5) after dehydrogenation of the crude product by the
action of DDQ (72%, Scheme 3).10 The double bond in 5 displayed
a marked resilience to the epoxidation by the action of alkaline
H2O2,11a t-BuOOH/Triton BÒ,11b oxoneÒ,11c and Na2O2.11d This phe-
nomenon was previously observed for a closely related compound
(6-methoxy-9-phenylphenalen-1-one) for which the Jacobsen–
Katsuki epoxidation proved to be a convenient solution.12 In the
present case, the epoxidation of 5 using Ca(OCl)2 as an oxidizing
agent and (salen)Mn as a catalyst furnished 2-hydroxy-4-meth-
oxy-9-phenyl-1H-phenalen-1-one (4) in 50% yield after prepara-
tive TLC.13 The immediate color change from pale yellow to red
that occurred during the application of the crude dichloromethane
extract to TLC plates was indicative of the lability of the epoxide to
acid treatment and therefore no attempt was made to isolate it.
Demethylation of 4 with HBr produced the desired 2,4-dihy-
droxy-9-phenyl-1H-phenalen-1-one (4-hydroxyanigorufone, 3, 84%),
which is identical to the compound isolated from Anigozanthos
flavidus and Monochoria elata.14 Treatment of 3 with an ethereal
solution of diazomethane afforded 4-hydroxy-2-methoxy-9-phe-
nyl-1H-phenalen-1-one (2) in 95% and set the stage for the photo-
chemical key step.15 Transformation of 2 into musafluorone (1)
was achieved in a yield of 56% upon irradiation of a methanolic
solution in an open air atmosphere with an overhead projector
lamp.16
In summary, we have developed a nine-step synthesis of 5-
methoxy-3H-naphtho[2,1,8-mna]xanthen-3-one
(musafluorone,
1) starting from 2-hydroxynaphthalene in a 3% global yield using
photochemical cyclization as a key step. The use of 4-hydroxy-2-
methoxy-9-phenyl-1H-phenalen-1-one (2) in the photochemical
cyclization process supports the hypothesis that 2 is the natural
precursor of 1 in Musa.
CN
CO2H
OH
OR
OMe
a
c
43%
67%
8:
9:
7
R = CH3
R = H
b, 80%
O
d
OMe
60%
6
Scheme 2. Reagents and conditions: (a) acrylonitrile, NaOH, benzene, 3 h reflux then HCl 10% until pH ꢁ2; (b) CH3I, K2CO3, acetone, 4 h reflux; (c) NaOH 20%, 8 h reflux then
HCl 10% until pH ꢁ1; (d) SOCl2, 30 °C until dryness then CH2Cl2, AlCl3 (3 equiv) 10 min, DDQ (1.3 equiv) 15 min.

4642
L. Duque et al. / Tetrahedron Letters 51 (2010) 4640–4643
OH
O
O
O
OMe
OMe
OMe
a, b
c
72%
65%
6
5
4
OR
OMe
O
O
d
f
OH
O
90%
50%
2:
1
R = CH3
R = H
e, 95%
3:
Scheme 3. Reagents and conditions: (a) PhMgBr (1 M in THF), ꢀ70 °C to 25 °C in 30 min, then NH4Cl(aq); (b) DDQ, CH2Cl2, reflux 10 min; (c) (salen)Mn, CH2Cl2, 4-
phenylpyridine-N-oxide, 30% Ca(OCl)2, 0 °C, 4 h, then silica gel; (d) 45% HBr, AcOH, reflux 6 h; (e) 1 equiv CH2N2; (f) MeOH, irradiation (ENX lamp, 82 V, 360 W, 2500 lumens),
8 h.
(C3D6O, 500.13 MHz) d 8.04 (d, J = 8.8 Hz, H-80), 7.84 (d, J = 8.1 Hz, H-50), 7.82
Acknowledgments
(d, J = 8.8 Hz, H-40), 7.51 (dd, J = 8.8, 7.6 Hz, H-70), 7.42 (d, J = 8.8 Hz, H-30), 7.34
(dd, J = 8.1, 7.6 Hz, H-60), 3.98 (s, –OCH3), 3.39 (t, J = 8.3 Hz, H-3), 2.55 (t,
13
´
We thank Sybille Lorenz for mass spectrometric analyzes and
Emily Wheeler for editorial assistance. This research was finan-
cially supported by the Universidad de Antioquia (Grant CODI
IN580CE) and the Max-Planck-Institut für Chemische Ökologie.
J = 8.3 Hz, H-2); C NMR (C3D6O, 125.75 MHz) d 174.2 (C-1 155.5 (C-2), 133.6
(C-8a0), 130.2 (C-4a´), 129.4 (C-50), 129.0 (C-40), 127.3 (C-70), 124.0 (C-60), 123.6
(C-80), 122.3 (C-10), 114.2 (C-30), 56.8 (–OCH3), 34.4 (C-2), 21.3 (C-3). HREIMS
m/z 256.094212 (calcd for C14H14O3, 256.094294).
10. Experimental procedure: To a ꢀ70 °C THF solution of 6 (1.41 g, 6.7 mmol in
10 mL) was slowly added (5 min) 1.3 equiv of phenylmagnesium bromide
(9 mL, 1.0 M in THF) under nitrogen. The reaction mixture was stirred for
10 min at ꢀ70 °C before being warmed up to 25 °C (total stirring time 30 min).
The reaction was quenched with a saturated aqueous solution of NH4Cl and
extracted with CH2Cl2 (20 mL). The organic phase was dried (Na2SO4), filtered,
mixed with 1.5 g of DDQ (6.5 mmol), and boiled for 15 min. The product was
purified by flash column chromatography using CH2Cl2 as an eluent to afford 4-
methoxy-9-phenyl-1H-phenalen-1-one (5, 1.38 g, 72%) as a yellow solid. 1H
NMR, (C3D6O, 500.13 MHz) d 8.27 (d, J = 8.2 Hz, H-7), 8.26 (d, J = 9.1 Hz, H-6),
8.21 (d, J = 10.0 Hz, H-3), 7.62 (d, J = 9.1 Hz, H-5), 7.43 (d, J = 8.2 Hz, H-8), 7.38
(m, H-20/60), 7.33 (m, H-40), 7.32 (m, H-30/50), 6.45 (d, J = 10.0 Hz, H-2), 4.18 (s, –
OCH3); 13C NMR, (C3D6O, 125.75 MHz) d 185.0 (C-1), 160.0 (C-4), 149.0 (C-9),
144.6 (C-10), 135.5 (C-6), 134.6 (C-7), 134.0 (C-3), 130.5 (C-9b), 130.3 (C-8),
129.2 (C-2), 129.0 (C-30/50), 128.7 (C-20/60), 128.3 (C-6a), 127.5 (C-40), 126.5 (C-
9a), 114.7 (C-3a), 114.5 (C-5), 57.1 (–OCH3). HREIMS m/z 286.099380 (calcd for
References and notes
1. (a) Weiss, U.; Edwards, J. M. Tetrahedron Lett. 1969, 10, 4325–4328; (b)
Edwards, J. M.; Weiss, U. Phytochemistry 1974, 13, 1597–1602; (c) Cooke, R. G.;
Dagley, I. J. Aust. J. Chem. 1979, 32, 1841–1847; (d) Opitz, S.; Otálvaro, F.;
Echeverri, F.; Quiñones, W.; Schneider, B. Nat. Prod. Lett. 2002, 16, 335–338.
2. (a) Kornfeld, J. M.; Edwards, J. M. Biochim. Biophys. Acta 1972, 286, 88–90; (b)
Flors, C.; Nonell, S. Acc. Chem. Res. 2006, 39, 293–300; (c) Hidalgo, W.; Duque,
L.; Saez, J.; Arango, R.; Gil, J.; Rojano, B.; Schneider, B.; Otálvaro, F. J. Agric. Food
Chem. 2009, 57, 7417–7421.
3. Nanclares, J.; Gil, J.; Rojano, B.; Saez, J.; Schneider, B.; Otálvaro, F. Tetrahedron
Lett. 2008, 49, 3844–3847.
4. (a) Kametani, T.; Fukumoto, K. Acc. Chem. Res. 1976, 9, 319–325; For a different
synthesis of naphthoxanthenones, see: (b) Cooke, R. G.; Merrett, B. K.;
C
20H14O2, 286.100510).
´
OLoughlin, G. J.; Pietersz, G. A. Aust. J. Chem. 1980, 33, 2317–2324.
11. Experimental procedures were performed as described in the following papers.
No attempts were made for optimization: (a) Forte, G. J.; Zito, J. A.; Edwards, J.
M. Lloydia 1976, 39, 192–196; (b) Yang, N. C.; Finnegan, R. A. J. Am. Chem. Soc.
1958, 80, 5845–5848; (c) Rani, S.; Vankar, Y. D. Tetrahedron Lett. 2003, 44, 907–
909; (d) Reddy, V. K.; Haritha, B.; Yamashita, M. Lett. Org. Chem. 2005, 2, 128–
131.
5. Numbering of musafluorone (1) was taken over from Opitz et al.1d
6. (a) Hardman, A. F. J. Am. Chem. Soc. 1948, 70, 2119–2120; (b) Koelsch, C. F.;
Hood, H. E. J. Org. Chem. 1955, 20, 1282–1287.
7. In our case, recrystallization of the crude product 9 from ethanol as reported by
Hardman6a did not afford satisfactory results. Therefore, flash column
chromatography was conducted using CH2Cl2 as an eluent. 1H NMR (C3D6O,
500.13 MHz) d 8.80 (s, –OH), 8.03 (d, J = 8.5 Hz, H-80), 7.81 (d, J = 8.1 Hz, H-50),
7.72 (d, J = 8.9 Hz, H-40), 7.50 (ddd, J = 8.5, 6.9, 1.2 Hz, H-70), 7.31 (ddd, J = 8.1,
6.9, 1.2 Hz, H-60), 7.24 (d, J = 8.9 Hz, H-30), 3.45 (t, J = 7.7 Hz, H-3), 2.76 (t,
12. Otálvaro, F.; Quiñones, W.; Echeverri, F.; Schneider, B. J. Labeled Compd.
Radiopharm. 2004, 47, 147–159.
13. Experimental procedure: Compound 5 (325 mg, 1.1 mmol), (S,S)-(+)-N,N0-bis-
(3,5-di-tert-butylsalicylidene)-1,2-diamino-cyclohexane
manganese-(III)-
13
J = 7.7 Hz, H-2); C NMR (C3D6O, 125.75 MHz) d 153.6 (C-20), 134.1 (C-8a0),
chloride ((salen)Mn, 30 mg, 0.05 mmol), and 4-phenylpyridine-N-oxide
(47 mg, 0.3 mmol) were dissolved in CH2Cl2 (10 mL) and the resulting
solution was cooled in an ice-water bath. A 30% slurry of Ca(OCl)2 in water
(10 mL) buffered with 5 mL of a sodium phosphate solution (0.05 M) was
added in one portion under vigorous stirring and the reaction was maintained
at 0 °C for 2 h. The mixture was separated and the organic layer dried and
applied directly to a preparative TLC plate. Separation using CH2Cl2 as an
eluent resulted in a red zone (Rf = 0.74), which gave 165 mg (50%) of 2-
hydroxy-4-methoxy-9-phenyl-1H-phenalen-1-one (4). 1H NMR (C3D6O,
130.0 (C-4a0), 129.6 (C-40), 129.5 (C-50), 127.5 (C-70), 123.6 (C-60), 123.2 (C-80),
120.4 (C-1), 118.6 (C-30), 117.4 (C-10), 21.9 (C-3), 17.2 (C-2). HREIMS m/z
197.084343 (calcd for C13H11NO, 197.084064).
8. Experimental procedure: Compound 9 (5.0 g, 25 mmol), K2CO3 (5.2 g), and CH3I
(2.12 mL, 37.5 mmol) were refluxed in acetone (30 mL) for 4 h. The reaction
mixture was filtered, the solvent removed under vacuum, and the crude extract
submitted to flash column chromatography using a CH2Cl2–n-hexane mixture
(2:1) as eluent to give 4.2 g (80%) of 3-(2-methoxynaphthalen-1-
yl)propanenitrile (8) as a yellow oil. 1H NMR, (C3D6O, 500.13 MHz) d 8.06 (d,
J = 8.6 Hz, H-80), 7.88 (d, J = 9.0 Hz, H-40), 7.86 (d, J = 8.1 Hz, H-50), 7.52 (ddd,
J = 8.6, 6.8, 1.3 Hz, H-70), 7.45 (d, J = 9.0 Hz, H-30), 7.36 (ddd, J = 8.1, 6.8, 1.1 Hz,
H-60), 4.01 (s, –OCH3), 3.45 (t, J = 7.7 Hz, H-3), 2.73 (t, J = 7.7 Hz, H-2); 13C NM0R,
500.13 MHz)
d 8.33 (d, J = 8.2 Hz, H-7), 8.15 (d, J = 9.0 Hz, H-6), 7.61 (d,
J = 9.0 Hz, H-5), 7.51 (s, H-3), 7.46 (d, J = 8.2 Hz, H-8), 7.34–7.45 (br m, H-20–H-
60), 4.17 (s, –OCH3); 13C NMR (C3D6O, 125.75 MHz) d 179.7 (C-1), 159.1 (C-4),
150.7 (C-2), 150.2 (C-9), 144.1 (C-10), 136.1 (C-7), 133.1 (C-6), 129.9 (C-8),
129.0 (C-30/50), 128.7 (C-20/60), 128.2 (C-6a), 127.7 (C-40), 126.8 (C-9b), 124.2
(C-9a), 115.2 (C-3a), 114.7 (C-5), 106.9 (C-3), 57.1 (–OCH3). HREIMS m/z
302.093636 (calcd for C20H14O3, 302.094294).
0
0
´
(C3D6O, 125.75 MHz) d 155.9 (C-2), 133.5 (C-8a ), 130.2 (C-4a ), 129.9 (C-4 ),
129.5 (C-50), 127.6 (C-70), 124.2 (C-60), 123.4 (C-80), 120.3 (C-1), 120.2 (C-10),
114.1 (C-30), 56.8 (–OCH3), 21.6 (C-3), 17.4 (C-2). HREIMS m/z 211.098701
(calcd for C14H13NO, 211.099714).
14. (a) Hölscher, D.; Schneider, B. Phytochemistry 1999, 50, 155–161; (b) Hölscher,
D.; Reichert, M.; Görls, H.; Ohlenschläger, O.; Bringmann, G.; Schneider, B. J.
Nat. Prod. 2006, 69, 1614–1617. Experimental procedure: To a refluxing solution
9. Experimental procedure: Compound 8 (5.55 g, 26 mmol) was refluxed with
NaOH (20%, 50 mL) for 8 h until evolution of ammonia ceased. The dropwise
addition of
a 10% HCl solution afforded 3-(2-methoxynaphthalen-1-
of compound 4 (180 mg, 0.6 mmol) in AcOH (18 mL) was added 300
lL of
yl)propanoic acid (7, 6.0 g, 67%) as a white solid after filtration. 1H NMR,
aqueous HBr (48%). This addition was repeated over intervals of 15 min until a

L. Duque et al. / Tetrahedron Letters 51 (2010) 4640–4643
4643
total of 2.4 mL had been added and the reaction was refluxed for a further
20 min. Extraction with CH2Cl2 and purification by preparative TLC (CH2Cl2–
AcOEt 4:1, Rf = 0.83) afforded 146 mg (84%) of 2,4-dihydroxy-9-phenyl-1H-
phenalen-1-one (3), which rapidly decomposes in the presence of light and
oxygen. 1H NMR (C3D6O, 500.13 MHz) d 8.26 (d, J = 8.2 Hz, H-7), 7.99 (d, J = 8.7
Hz, H-6), 7.59 (s, H-3), 7.42 (d, J = 8.2 Hz, H-8), 7.34–7.44 (br m, H-20–H-60),
7.40 (d, J = 8.7 Hz, H-5); 13C NMR (C3D6O, 125.75 MHz) d 179.4 (C-1), 158.4 (C-
4), 150.2 (C-2), 149.6 (C-9), 144.3 (C-10), 135.7 (C-7), 133.0 (C-6), 129.3 (C-8),
129.0 (C-30/50), 128.7 (C-20/60), 128.1 (C-6a), 127.6 (C-40), 127.5 (C-9b), 124.2
(C-9a), 119.8 (C-5), 113.1 (C-3a), 107.2 (C-3). HREIMS m/z 288.078911 (calcd
for C19H12O3, 288.078644)..
by means of preparative TLC (Et2O–n-hexane 7:1, Rf = 0.35) to give 20 mg (95%)
of 4-hydroxy-2-methoxy-9-phenyl-1H-phenalen-1-one (2) as an orange
powder which was used immediately in the next step. Compound 2 is also a
natural product isolated from Musa acuminata cv. ‘Williams’. 1H NMR, (MeOH-
d4, 500.13 MHz) d 7.96 (s, H-3), 7.93 (d, J = 7.8 Hz, H-7), 7.76 (d, J = 9.3 Hz, H-6),
7.33 (m, H-20–H-60), 7.15 (d, J = 7.8 Hz, H-8), 6.97 (d, J = 9.3 Hz, H-5), 3.89 (s, –
OCH3); 13C NMR, (MeOH-d4, 125.75 MHz) d 177.0 (C-1), 175.5 (C-4), 151.2 (C-
2), 148.0 (C-9), 145.4 (C-10), 137.3 (C-6), 135.3 (C-7), 130.8 (C-20/60), 130.1 (C-
40), 130.0 (C-30/50), 128.5 (C-9b), 128.5 (C-8), 128.4 (C-5), 126.2 (C-3a), 126.0
(C-6a), 124.2 (C-9a), 114.5 (C-3), 57.3 (–OCH3). HREIMS m/z 302.0947 (calcd for
C20H14O3, 302.0943).
15. Experimental procedure: An alcohol-containing ethereal diazomethane solution
was prepared using a standard procedure (Aldrich technical bulletin AL-180)
16. Experimental procedure: Compound
2 (10 mg, 0.03 mmol) dissolved in
methanol (1 mL) was irradiated for 8 h (quartz cell) on an overhead projector
(3M™ 1711 Plus) in an open atmosphere to give 5 mg of musafluorone (1) after
TLC purification (CH2Cl2–AcOEt 5:1, Rf = 0.57). Spectroscopic data were
identical with those of the natural compound.1d
and added at a rate of 1
(500 L) at room temperature. The addition continued until the disappearance
of 3 (as judged by TLC, CH2Cl2–AcOEt 4:1, Rf = 0.83). Compound 2 was purified
lL/min to 20 mg of compound 3 suspended in ether
l