A convenient synthesis of psoralens

Article abstract of DOI:10.1016/S0040-4020(02)00441-6

An efficient synthesis (yields >70%) of linear 7H-furo[3,2-g]chromen-7-one derivatives (psoralens or furocoumarins) has been carried out starting from ring-substituted 2-(coumarin-7-yl)oxyaldehydes; moreover, the phototoxicity of these compounds has been tested on a cultured cell line of murine fibroblast.

Full text of DOI:10.1016/S0040-4020(02)00441-6

TETRAHEDRON
Pergamon
Tetrahedron 58 *2002) 4859±4863
A convenient synthesis of psoralens
Stefano Chimichi,a,p Marco Boccalini,a Barbara Cosimelli,b Giampietro Viola,c Daniela Vedaldic
and Francesco Dall'Acquac
aDipartimento di Chimica Organica, ªU. Schiffº, Via della Lastruccia 13, I-50019 Sesto F.no, Firenze, Italy
bDipartimento di Chimica Organica, ªA. Manginiº, Via S. Donato 15, I-40127 Bologna, Italy
cDipartimento di Scienze Farmaceutiche, Via Marzolo 5, I-35131 Padova, Italy
Received 3August 2001; revised 20 March 2002; accepted 18 April 2002
AbstractÐAn ef®cient synthesis *yields .70%) of linear 7H-furo[3,2-g]chromen-7-one derivatives *psoralens or furocoumarins) has been
carried out starting from ring-substituted 2-*coumarin-7-yl)oxyaldehydes; moreover, the phototoxicity of these compounds has been tested
on a cultured cell line of murine ®broblast. q 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction
by arti®cial ultraviolet illumination *UVA) of patients'
skin was applied for the ®rst time to cure psoriasis, a disease
characterized by hyperproliferation of skin cells. In 1987, an
extracorporeal form of 8-MOP photochemotherapy *photo-
pheresis) was developed for the treatment of cutaneous
T-cell lymphoma, a CD4 positive T-cell malignancy.7
Photopheresis was also found to be effective in a number
of other T-cell mediated diseases: clinical trials have shown
bene®cial effects in Pemphigo vulgaris,8 severe atopic
dermatitis,9 AIDS-related complex,10 rheumatoid arthritis11
and systemic lupus erythematosus.12
Psoralens, also known as furocoumarins, are naturally
occurring or synthetic tricyclic aromatic compounds. They
exhibit interesting photobiological activities such as skin
photosensitization, characterized by the onset of erythema
followed by dark pigmentation. The related angular
isomers, namely angelicins, are also present in plants and
have been chemically synthesized.1 Psoralens are also of
interest because they are used as a probe in molecular
biology and nucleic acid chemistry.2 The photochemistry
of furocoumarins has been extensively studied.3 In the
dark, they are able to form a complex with base pairs of
DNA in an intercalative manner and upon exposure to near
UV light *300±380 nm) they give a covalent addition to the
5,6 double bond of pyrimidine bases of DNA. The photo-
reaction leads to the formation of monoadducts or cross-
links between the two strands of the nucleic acid due to
the presence of two reactive sites in the furocoumarin
moiety.
A recent application of photoexcited psoralens is the
sterilization of blood products, mainly platelet concentrate.
Different psoralen derivatives such as 8-MOP and 40-amino-
methyl-4,50,8-trimethylpsoralen *AMT) combined with
UVA were found to be useful in the sterilization of platelet
concentrate, because of their capacity to target nucleic
acids.13 Lipid-enveloped viruses such as the model-virus
vesicular stomatitis virus14 as well as immunode®ciency
virus14,15 and duck hepatitis B virus16 are sensitive to this
photochemical treatment. On the other hand, platelets are
well preserved especially in the presence of rutin that
prevents damage to the platelet membrane by reactive
oxygen species.14
Although photochemotherapy has important therapeutic
effectiveness, it is also accompanied by undesired side
effects, both short *erythema, hyperpigmentation, geno-
toxicity)4 and long term *cataract, risk of skin cancer).5
Cross-link formation is believed to be mainly responsible
for the latter phenomena.
Starting from 1934, many syntheses of furocoumarins have
been reported; however, the yields were poor. The ®rst
approaches to psoralen synthesis started from the con-
version of 6-hydroxycoumaran to 2,3-dihydro-7H-
furo[3,2-g]chromen-7-one followed by dehydrogenation to
the desired psoralen.17±19 Subsequently, the synthetic
methods were based on cyclocondensation reactions of
7-hydroxycoumarin derivatives to form the furan ring.20±23
For example, Ray's method22 accomplished the synthesis of
3-methyl- *2) and 3-phenyl-7H-furo[3,2-g]chromen-7-one
An important clinical application of psoralens and UVA
light *PUVA therapy) was introduced into clinical practice
by Parrish et al.6 in 1974. A treatment consisting of oral
administration of 8-methoxypsoralen *8-MOP), followed
Keywords: psoralens; furocoumarins; coumarin derivatives; phototoxicity.
Corresponding author. Tel.: 139-055-457-3537; fax: 139-055-457-
3568; e-mail: stefano.chimichi@uni®.it
p
0040±4020/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020*02)00441-6

4860
S. Chimichi et al. / Tetrahedron 58 52002) 4859±4863
On this basis, alternative syntheses were undertaken with
the aim to prepare psoralens and 3-unsubstituted psoralens
in higher yields, for example, via intramolecular Diels±
Alder reaction26 *overall yield 1.5%, 12 steps) or anionic
[412] cycloaddition27 *overall yield 5%, 7 steps).
We wish to report here a more general, simple and ef®cient
methodology *yields .70%) to obtain linear furocoumarins
by alkaline treatment of ring-substituted 2-*coumarin-7-
yl)oxyaldehydes.
2. Results and discussion
Scheme 1.
Recently,28 we prepared the ring-substituted 2-*coumarin-7-
yl)oxyaldehydes 5, 13±16 *Scheme 2) in excellent overall
yields *,97%) starting from the easily available 7-hydroxy-
coumarins 9±12.
Whereas cyclisation of 2-*coumarin-7-yl)oxyaldehydes 5
and 7 was reported to be unsatisfactory,24,25 in our hands,
treatment of compounds 5, 13±15 in alkaline aqueous solu-
tion gave, after acidi®cation, with average overall yields of
80±90%, a mixture in which psoralens 6, 18, 20, and 22
were largely predominant with respect to angelicins 17, 19,
21, and 23, respectively *see Scheme 3).
Scheme 2. *i) BrCHR3CO2Et, acetone, K2CO3. *ii) NaOH 1.25 M. *iii) HCl
6 M. *iv) SOCl2, CHCl3. *v) H2, Pd/BaSO4, toluene.
Thus, we observed that the 2-*coumarin-7-yl)oxyaldehydes
5, 13±15 give rise to psoralens under basic but not acidic
conditions *acidic conditions giving in this case only tarry
materials). It is noteworthy that to improve the yield of this
cyclisation it is strictly necessary to add the aldehyde to a
NaOH solution at re¯ux.
*4) by cyclisation of 7-*2-oxopropoxy)- *1) and 7-*2-oxo-2-
phenylethoxy)-2H-chromen-2-one *3) in ethanolic medium
using sodium ethoxide as the condensing agent *Scheme 1).
Although the exact mechanism of the reaction is still not
clear and no intermediates have been isolated, it is best
described as shown in Scheme 4. The aldehyde ®rst under-
goes lactone ring opening29 to generate the intermediate
dianion which in turn gives rise to a nucleophilic addi-
tion±elimination reaction on the aldehyde carbon atom
with furan ring formation. Subsequent acidi®cation leads
to lactone ring closure with formation of the target
compounds.
In addition to low overall yields, the method gives only
3-substituted furocoumarins.24 Moreover, it is also
claimed24 that the corresponding 5-methyl-7H-furo[3,2-
g]chromen-7-one *6) cannot be obtained by cyclisation of
4-methyl-7-*2-oxoethoxy)coumarin *5) using both acidic
and basic catalysts. Various attempts led only to recovery
of starting material, uncharacterizable tar, or ether cleavage
with recovery of 4-methyl-7-hydroxycoumarin. More
recently, Egyptian scientists investigated the possibility of
obtaining psoralen derivatives by cyclisation of benzo-
pyranyloxy ketones or aldehydes in a 2% ethanolic solution
of NaOEt at re¯ux; once again, whereas the corresponding
psoralens can be obtained when R3±H, several attempts
starting from the aldehyde 7 failed.25
We wish to point out that, for example, compound 20 was
obtained for the ®rst time in 1972 with a yield lower than
2%30 and, more recently in 1997, by caesium-mediated
Claisen rearrangement of phenyl propargyl ether *overall
yield ,1%).31
Scheme 3.

S. Chimichi et al. / Tetrahedron 58 52002) 4859±4863
4861
Scheme 4.
Finally, with the aim to obtain the angular furocoumarin
sphondin *24), a natural product useful for its antifungal
activity,32 we carried out the same reaction starting from
the newly synthesised aldehyde 16.28 Various attempts
under the reported conditions gave only small amounts
*4±5%) of 24.
pattern, and a remarkable dose UVA-dependence. More-
over, the IC50 values show a signi®cant correlation with
the partition coef®cients of the compounds indicating that
the activity may be related to their active incorporation in
the cellular membrane.33
2.1. Cellular phototoxicity
3. Conclusion
The phototoxicity of test compounds 6, 18, 20, and 22 was
investigated on a cultured cell line of murine ®broblast
*Balb/c 3T3). Table 1 shows the extent of cell survival
expressed as IC50 that is the concentration expressed in
mM inducing 50% inhibition of cell growth, after UVA
irradiation, at different doses. Control experiments with
UVA light or drugs alone were carried out without
observing any cytotoxic effects *data not shown).
In summary, a convenient route to psoralens is described;
whereas it was reported that 2-*coumarin-7-yl)oxyalde-
hydes do not cyclise in alkaline alcoholic medium,
psoralens can be ef®ciently obtained when the reaction is
carried out in alkaline aqueous solution. Phototoxicity
studies of the target compounds on a cultured cell line of
murine ®broblast showed lower IC50 values with respect to
psoralen itself.
It can be noted that the four investigated compounds exhibit
different values of IC50 depending on the substitution
Taking into account that research on the photobiological
and pharmacological activities of psoralen is under
Table 1. Values of IC50 *mM) obtained for the test compounds 18, 6, 20, and 22 at two different UVA doses
Compounds
log Pa
1.41 *J cm22) UVA dose
4.22 *J cm22) UVA dose
18 *Psoralen)
6
20
22
1.97
2.27
2.48
2.77
25.5^3.0
21.12^0.56
18.89^1.61
13.71^0.69
19.42^2.42
8.2^1.61
4.12^0.07
4.63^0.03
Data expressed as mean^S.D. of two independent experiments performed in quadruplicate.
Data taken from Ref. 34.
a

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S. Chimichi et al. / Tetrahedron 58 52002) 4859±4863
development,35 the possibility to obtain psoralen derivatives
in large amounts by a simple synthesis endowed with a high
yield may be an important improvement.
4.1.3. 2-Methyl-7H-furo[3,2-g]chromen-7-one 20. Cycli-
sation of 2-[*2-oxo-2H-chromen-7-yl)oxy]propanal28 *14)
afforded compound 20 *174 mg, 87%) as colourless crys-
tals: mp 149±1508C [lit.31 148±1498C]; 1H NMR
*200 MHz, CDCl3) d 7.77 *1H, d, Jˆ9.7 Hz, H-5), 7.53
*1H, s, H-4), 7.38 *1H, d, Jˆ1.0 Hz, H-9), 6.42 *1H, dq,
Jˆ1.0 Hz, H-3), 6.3 *1H, d, Jˆ9.7 Hz, H-6), 2.48 *3H, d,
Jˆ1.0 Hz, 2-Me).
4. Experimental
Melting points were measured using BuÈchi apparatus and
are uncorrected. NMR spectra were recorded on a Varian
Gemini 200 Instrument in the Fourier transform mode at
A very small sample of 8-methyl-2H-furo[2,3-h]chromen-
2-one *21) *6 mg, 3%), colourless solid, was then recovered
from the fastest running band: mp 153±1548C [lit.31 155±
1568C]; 1H NMR *200 MHz, CDCl3) d 7.77 *1H, d,
Jˆ9.7 Hz, H-4), 7.32±7.26 *2H, AB part of ABX system,
H-5 and H-6), 6.71 *1H, qd appears as quintet, Jˆ1.0 Hz,
H-9), 6.35 *1H, d, Jˆ9.7 Hz, H-3), 2.50 *3H, d, Jˆ1.0 Hz,
8-Me).
1
21^0.58C in CDCl3. H *200 MHz) chemical shifts *d)
are in ppm relative to TMS as secondary reference standard;
coupling constants are in Hz. Silica gel plates *Merck F254
)
and silica gel 60 *Merck 230±400 mesh) were used for
analytical tlc and for ¯ash chromatography, respectively.
Solvents were removed under reduced pressure. All experi-
ments were carried out under nitrogen atmosphere.
4.1.4. 5,6-Dimethyl-7H-furo[3,2-g]chromen-7-one 22.
Cyclisation of 2-[*3,4-dimethyl-2-oxo-2H-chromen-7-
yl)oxy]acetaldehyde28 *15) afforded mainly compound 22
*152 mg, 71%) as colourless crystals: mp 141±1428C
4.1. General procedure for synthesis of the psoralens
A suspension of 2-*coumarin-7-yl)oxyaldehyde *1 mmol) in
H2O±Dioxane *9:1) *10 mL) was added very slowly
*30 min) and under stirring to a re¯uxing solution of
aqueous NaOH 1.25 M *20 mL) and the solution was
re¯uxed for 4 h. After cooling to room temperature, the
solution was acidi®ed with 85% phosphoric acid and the
precipitate was collected by vacuum ®ltration and
separated by ¯ash chromatography *petroleum ether±ethyl
acetateˆ3:1, as eluent).
1
[lit.37 140±1418C]; H NMR *200 MHz, CDCl3) d 7.80
*1H, s, H-4), 7.67 *1H, d, Jˆ2.3Hz, H-2), 7.44 *1H, d,
Jˆ1.0 Hz, H-9), 6.83*1H, dd, Jˆ2.3, 1.0 Hz, H-3), 2.47
*3H, q, Jˆ0.6 Hz, 5-Me), 2.24 *3H, q, Jˆ0.6 Hz, 6-Me).
The fastest running band was then identi®ed as 3,4-
dimethyl-2H-furo[2,3-h]chromen-2-one *23) *19 mg, 9%),
1
colourless crystals: mp 178±1798C [lit.38 179±1808C]; H
NMR *200 MHz, CDCl3) d 7.72 *1H, d, Jˆ2.4 Hz, H-8),
7.60±7.40 *2H, AB part of ABX system, H-5 and H-6), 7.12
*1H, dd, Jˆ2.4, 0.8 Hz, H-9), 2.48 *3H, q, Jˆ0.7 Hz, 4-Me),
2.27 *3H, q, Jˆ0.7 Hz, 3-Me).
4.1.1. 5-Methyl-7H-furo[3,2-g]chromen-7-one 6. Cyclisa-
tion of 2-[*4-methyl-2-oxo-2H-chromen-7-yl)oxy]acetalde-
hyde28 *5) afforded mainly compound 6 *142 mg, 71%) as
1
colourless crystals: mp 177±1788C [lit.21 178±1798C]; H
NMR *200 MHz, CDCl3) d 7.82 *1H, s, H-4), 7.69 *1H, d,
Jˆ2.2 Hz, H-2), 7.48 *1H, d, Jˆ0.9 Hz, H-9), 6.85 *1H, dd,
Jˆ2.2, 0.9 Hz, H-3), 6.28 *1H, q, Jˆ1.5 Hz, H-6), 2.51 *3H,
d, Jˆ1.5 Hz, 5-Me).
4.1.5. 6-Methoxy-2H-furo[2,3-h]chromen-2-one 24.
Cyclisation of 2-[*6-methoxy-2-oxo-2H-chromen-7-yl)-
oxyacetaldehyde28 *16) afforded compound 24 *9 mg, 4%)
1
as colourless crystals: mp 187±1898C [lit.1 188.58C]; H
NMR *200 MHz, CDCl3) d 7.76 *1H, d, Jˆ9.5 Hz, H-4),
7.71 *1H, d, Jˆ2.0 Hz, H-8), 7.14 *1H, dd, Jˆ2.0 Hz, H-9),
6.79 *1H, s, H-5), 6.41 *1H, d, Jˆ9.5 Hz, H-3), 4.05 *3H, s,
OMe).
The fastest running band was then identi®ed as 4-methyl-
2H-furo[2,3-h]chromen-2-one *17) *18 mg, 9%), colourless
crystals: mp 191±1928C [lit.36 1948C]; 1H NMR *200 MHz,
CDCl3) d 7.69 *1H, d, Jˆ2.2 Hz, H-8), 7.58±7.40 *2H, AB
part of ABX system, H-5 and H-6), 7.15 *1H, dd, Jˆ2.2,
0.7 Hz, H-9), 6.28 *1H, q, Jˆ1.1 Hz, H-3), 2.51 *3H, d,
Jˆ1.1 Hz, 4-Me).
4.2. Irradiation procedure
Two HPW 125 Philips lamps, mainly emitting at 365 nm,
were used for irradiation experiments. The total energy was
4.1.2. 7H-Furo[3,2-g]chromen-7-one 18. Cyclisation of 2-
[*2-oxo-2H-chromen-7-yl)oxyacetaldehyde28 *13) afforded
mainly compound 18 *132 mg, 71%) as colourless crystals:
detected by
a
radiometer *Cole-Parmer Instrument
Company, Niles, IL), equipped with a CX-365 sensor.
1
mp 161±1628C [lit.26 160±1618C]; H NMR *200 MHz,
CDCl3) d 7.80 *1H, d, Jˆ9.5 Hz, H-5), 7.70 *1H, d,
Jˆ2.3Hz, H-2), 7.69 *1H, s, H-4), 7.48 *1H, d, Jˆ1.1 Hz,
H-9), 6.83*1H, dd, Jˆ2.3, 1.1 Hz, H-3), 6.38 *1H, d,
Jˆ9.5 Hz, H-6).
4.3. Cell cultures
Cultures of Balb/c mouse 3T3 ®broblast were grown in
DMEM medium *Dulbecco's Modi®ed Eagle Medium
SIGMA Co.) supplemented with 115 units/mL of penicillin
G *GIBCO Laboratories) 115 mg/mL streptomycin *GIBCO
Laboratories) and 10% foetal calf serum *GIBCO Labora-
tories).
The fastest running band was identi®ed as 2H-furo[2,3-
h]chromen-2-one *19) *17 mg, 9%), colourless crystals:
1
mp 136±1378C [lit.1 137±137.58C]; H NMR *200 MHz,
CDCl3) d 7.81 *1H, d, Jˆ9.7 Hz, H-4), 7.70 *1H, d,
Jˆ2.2 Hz, H-8), 7.44±7.20 *2H, AB part of ABX system,
H-5 and H-6), 7.15 *1H, dd, Jˆ2.2, 0.7 Hz, H-9), 6.40 *1H,
d, Jˆ9.7 Hz, H-3).
Individual wells of 96-well tissue culture microtiter plate
*IWAKI Japan) were inoculated with 100 mL of DMEM
containing 5£103 mouse 3T3 cells. The plate was incubated

S. Chimichi et al. / Tetrahedron 58 52002) 4859±4863
4863
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