Total synthesis of a cis-syn 2-carbomethoxypsoralen furan-side thymidine monoadduct

Article abstract of DOI:10.1021/ja960511e

The first total chemical synthesis of a cis-syn furan-side photoproduct between a psoralen derivative and thymidine is described. The key step in the synthesis was an intramolecular [2 + 2] photocycloaddition, which directed the stereochemical course of t

Full text of DOI:10.1021/ja960511e

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J. Am. Chem. Soc. 1996, 118, 7101-7107
7101
Total Synthesis of a Cis-Syn 2-Carbomethoxypsoralen
Furan-Side Thymidine Monoadduct
William R. Kobertz and John M. Essigmann*
Contribution from the Department of Chemistry, Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139
ReceiVed February 16, 1996X
Abstract: The first total chemical synthesis of a cis-syn furan-side photoproduct between a psoralen derivative and
thymidine is described. The key step in the synthesis was an intramolecular [2 + 2] photocycloaddition, which
directed the stereochemical course of the reaction to afford a product equivalent to that formed when a psoralen
molecule is allowed to react at a 5′-TpA-3′ site in DNA. A model system consisting of a simple benzofuranyl acid
tethered to the 5′ hydroxyl of thymidine showed that it was possible to bias the stereochemical outcome of the
photochemical reaction in favor of the desired cis-syn product. Further refinement of the model system allowed for
the elaboration of a benzofuran-thymidine photoproduct into a cis-syn 2-carbomethoxypsoralen furan-side thymidine
monoadduct. The stereochemistry of all photoproducts as established by NMR and CD spectroscopy indicated that
the cycloadditions occurred on the 3′ face of thymine, the equivalent of a psoralen monoadduct in a 5′-TpA-3′ site
in DNA. Previously, the inability to program the sequence context of psoralen-DNA adducts has constrained certain
biological studies, such as experiments aimed at deciphering the transcriptional effects of adducts at TATA sites.
This method is a key step toward overcoming that problem.
Introduction
salen (HMT) have been made with the aim of enhancing water
solubility and photoreactivity.7
Psoralens are linear furocoumarins used medicinally for the
treatment of skin disorders1 and cutaneous T-cell lymphoma.2
The double stranded nucleic acids of cells treated with psoralen
react by a [2 + 2] photocycloaddition with pyrimidines. These
bulky psoralen-pyrimidine adducts lead to mutations and have
been shown to be genotoxic.3 Psoralen-DNA adducts have
been used as tools for studying DNA repair,4 for probing
protein-DNA interactions,5 and for arresting transcription.6
Because of the usefulness of psoralens as molecular probes,
many synthetic derivatives such as 2-carbomethoxypsoralen,
3-aminomethyltrioxsalen (AMT), and 3-hydroxymethyltriox-
Immediately before the photochemical reaction of psoralen
with DNA, psoralen intercalates between the bases of a helical
nucleic acid. Upon exposure to long-wave UV light (320-
410 nm), either the pyrone or furan double bonds of psoralen
can react with the 5,6-double bond of a pyrimidine to form an
initial monoadduct. A furan-side adduct can absorb a second
photon and react with an adjacent pyrimidine on the opposite
strand forming an interstrand cross-link. By contrast, pyrone-
side monoadducts cannot absorb long wave UV light and
therefore do not form cross-links. When irradiated in the
presence of model oligonucleotides, psoralens show sequence
selectively as evidenced by the 100-fold preference for reaction
at 5′-TpA-3′ as compared to 5′-ApT-3′ sites.8 All psoralen
photoadducts can be reversed by 254 nm light;9 cross-links and
furan-side adducts can also be reversed by treatment with base.10
Hearst and co-workers have isolated and characterized the
three major psoralen-DNA monoadducts.9b,11 The structure of
* Author to whom correspondence should be addressed.
X Abstract published in AdVance ACS Abstracts, July 1, 1996.
(1) (a) Fitzpatric, J. B.; Stern, R. S.; Parrish, J. A. Proceedings of the
Third International Symposium. In Psoriasis; Grune and Stratton: New
York, 1982; pp 149-156. (b) Parrish, J. A.; Stern, R. S.; Pathak, M. A.;
Fitzpatric, T. B. Photochemotherapy of skin dieases. In The Science of
Photomedicine; Plenum Press: New York, 1982; pp 595-624.
(2) (a) Knobler, R. M.; Ho¨nigsmann, H.; Edelson, R. L. Psoralen
phototherapies. In Psoralen DNA Photobiology; CRC Press: Boca Raton,
FL, 1988; Vol. II, pp 117-134. (b) Edelson, R. L. Sci. Am. 1988, 259, 68.
(3) (a) Piette, J.; Gamper, H. B.; van de Vorst, A.; Hearst, J. E. Nucleic
Acids Res. 1988, 16, 9961. (b) Saffran, W. A.; Cantor, C. R. J. Mol. Biol.
1984, 178, 595. (c) Averbeck, D.; Dardalhon, M.; Magan˜a-Schwencke, N.;
Meira, L. B.; Meniel, V.; Boiteux, S.; Sage, E. J. Photochem. Photobiol. B
1992, 14, 47. (d) Laquerbe, A.; Moustacchi, E.; Papadopoulo, D. Mutat.
Res. 1995, 346, 173. (e) Piette, J.; Decuyper-Debergh, D.; Gamper, H. Proc.
Natl. Acad. Sci. U.S.A. 1985, 82, 7355. (f) Bredberg, A.; Nachmansson, N.
Carcinogenesis 1987, 8, 1923.
(7) (a) Bisagni, E. J. Photochem. Photobiol. B: Biol. 1992, 14, 23. (b)
Isaacs, S. T.; Shen, C. J.; Hearst, J. E.; Rapoport, H. Biochemistry 1977,
16, 1058. (c) Worden, L. R.; Kaufman, K. D.; Weis, J. A.; Schaaf, T. K. J.
Org. Chem. 1969, 34, 2311. (d) Zub´ıa, E.; Luis, F. R.; Massanet, G. M.;
Collado, I. G. Tetrahedron 1992, 48, 4239. (e) Queval, P.; Bisagni, E. Eur.
J. Med. Chem. 1974, 9, 335.
(8) (a) Gamper, H.; Piette, J.; Hearst, J. E. Photochem. Photobiol. 1984,
40, 29. (b) Tessman, J. W.; Isaacs, S. T.; Hearst, J. E. Biochemistry 1985,
24, 1669.
(9) (a) Shim, S. C.; Kim, Y. Z. Photochem. Photobiol. 1983, 38, 265.
(b) Kanne, D.; Straub, K.; Hearst, J. E.; Rapoport, H. J. Am. Chem. Soc.
1982, 104, 6754. (c) Yeung, A. T.; Jones, B. K.; Chu, C. T. Biochemistry
1988, 27, 3204.
(10) (a) Yeung, A. T.; Dinehart, W. J.; Jones, B. K. Biochemistry 1988,
27, 6332. (b) Shi, Y.-B.; Spielmann, H. P.; Hearst, J. E. Biochemistry 1988,
27, 5174.
(11) (a) Straub, K.; Kanne, D.; Hearst J. E.; Rapoport, H. J. Am. Chem.
Soc. 1981, 103, 2347. (b) Kanne, D.; Straub, K.; Rapoport, H.; Hearst, J.
E. Biochemistry 1982, 21, 861. (c) Cimino, G. D.; Gamper, H. B.; Isaacs,
S. T.; Hearst J. E. Ann. ReV. Biochem. 1985, 54, 1151. (d) Hearst, J. E.;
Isaacs, S. T.; Kanne, D.; Rapoport, H.; Straub, K. Q. ReV. Biophys. 1984,
17, 1. (e) Peckler, S.; Graves, B.; Kanne, D.; Rapoport, H.; Hearst, J. E.;
Kim, S.-H. J. Mol. Biol. 1982, 162, 157. (f) Hearst, J. E. Chem. Res. Tox.
1989, 2, 69.
(4) (a) Van Houten, B.; Gamper, H.; Holbrook, S. R.; Hearst, J. E.;
Sancar, A. Proc. Natl. Acad. Sci. U.S.A. 1986, 83, 8077. (b) Cheng, S.;
Sancar, A.; Hearst, J. E. Nucleic Acids Res. 1991, 19, 657. (c) Islas, A. L.;
Baker, F. J.; Hanawalt, P. C. Biochemistry 1994, 33, 10794.
(5) (a) Sinden, R. R.; Carlson, J. O.; Pettijohn, D. E. Cell 1980, 21, 773.
(b) Courey, A. J.; Plon, S. E.; Wang, J. C. Cell 1986, 45, 567. (c) Cheng,
S.; Van Houten, B.; Gamper, H. B.; Sancar, A.; Hearst, J. E. J. Biol. Chem.
1988, 263, 15110. (d) Umlauf, S. W.; Cox, M. M.; Inman, R. B. J. Biol.
Chem. 1990, 265, 16898.
(6) (a) Shi, Y.-B.; Gamper, H.; Van Houten, B.; Hearst, J. E. J. Mol.
Biol. 1988, 199, 277. (b) Shi, Y.-B.; Gamper, H.; Hearst, J. E. J. Biol.
Chem. 1988, 263, 527. (c) Sastry, S. S.; Hearst, J. E. J. Mol. Biol. 1991,
221, 1091. (d) Sastry, S. S.; Hearst, J. E. J. Mol. Biol. 1991, 221, 1111.
S0002-7863(96)00511-2 CCC: $12 00 © 1996 American Chemical Society

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7102 J. Am. Chem. Soc., Vol. 118, No. 30, 1996
Kobertz and Essigmann
Scheme 1
Figure 1. Structure of psoralen 1 and the structures of the major mono-
adducts between psoralen and thymidine: cis-syn pyrone-side photo-
product 2 and cis-syn furan-side photoproduct 3. dR ) deoxyribose.
psoralen 1 and the structures and stereochemistries of the
isolated adducts are shown in Figure 1.12 The adducts consist
of a psoralen-thymidine cis-syn pyrone-side adduct 2 and both
diastereomers of a cis-syn furan-side adduct 3. The two furan-
side adducts arise from reaction of the psoralen on either the 5′
or the 3′ face of thymidine. The stereoselectivity of the
photoreaction is exceptional given that 64 monoadducts are
theoretically possible between thymidine and psoralen; 48 of
these adducts however, do not form apparently because they
would contain a highly strained and sterically hindered trans
fusion about the pyrimidine or psoralen rings. Elimination of
these strained isomers still leaves eight possible isomers for each
monoadduct. Attempts to synthesize these adducts in the
absence of DNA have met with modest success. DNA helps
to control the stereochemistry of the photochemical reaction
because the hydrophobic psoralen molecule intercalates between
the nucleobases resulting in primarily cis photoproducts; in
addition, DNA also inhibits psoralen photodimerization.13 Two
similar approaches to mimic the intercalation step involve
irradiation of frozen aqueous solutions or evaporated thin films
containing psoralen and thymidine.14 Unfortunately, numerous
photoproducts were isolated and in very poor yields. A more
promising strategy by Lhomme and co-workers utilized succinic
acid to attach a hydroxy psoralen derivative to the 5′-hydroxyl
of thymidine to control stereochemistry and eliminate psoralen
photodimerization.15 Unfortunately, this method provides only
the pyrone cis-anti isomer and not the desired cis-syn isomer.
In addition, the removal of the succinyl linker was not reported.
more reactive pyrone ring of psoralen, thereby minimizing
dimerization.15 Interestingly, the furan double bond has been
shown to be the more reactive double bond when intercalated
in DNA owing to its juxtaposition to the 5,6 thymidine double
bond in a 5′-TpA-3′ site.16 After setting the stereochemistry
with the photochemical step, the benzofuran-thymidine adduct
could be elaborated into the desired 2-carbomethoxypsoralen-
thymidine adduct. In this paper we describe the details of our
model study using benzofuran as a psoralen synthon and report
its successful application to the first total synthesis of a cis-syn
furan-side monoadduct between a psoralen derivative and
thymidine.
Results and Discussion
The retrosynthetic route to cis-syn 2-carbomethoxypsoralen-
thymidine furan-side adduct 4 is shown in Scheme 1. Removal
of the pyrone ring would provide benzofuran photoproduct 5.
Photoproduct 5 could be formed in a stereospecific manner from
a benzofuranyl acid 7 linked to the 5′-hydroxyl of thymidine
6.15 Benzofuranyl acid 7 is readily obtained from salicylalde-
hyde 8. The key step in the total synthesis would be the
photochemical reaction with benzofuran. The photochemistry
of benzofuranyl esters was not known, and we believed it was
necessary to test our strategy in a model system. The primary
goal of the model system was to direct the stereochemical
outcome of the photochemical reaction to obtain the desired
cis-syn stereochemistry. The successful application of this
model is shown in Scheme 2.
Commercially available salicylaldehyde 9 was allowed to
react with diethyl bromomalonate to give a benzofuran ester,
which was saponified to afford benzofuranyl acid 10. Esteri-
fication of 10 with a 3′-protected thymidine, 6, provided
photoprecursor 11 in 75% yield. Irradiation of an acetone
sensitized, argon degassed, dilute (<0.002 M) solution of 11
in acetonitrile afforded two diastereomers with a 4:1 ratio in
71% overall yield. To our surprise, attempts to purify photo-
products 12a and 12b by using standard silica gel chromatog-
raphy with methylene chloride and methanol as an eluent,
transesterified the nine-membered lactone of the major isomer
12a to give ring opened photoproduct 13 containing a methyl
ester. The minor isomer 12b did not transesterify with methanol
and silica gel and could be purified as the lactone using methanol
as the eluent. We have tested a few alcohols in an attempt to
transesterify the major isomer 12a, and so far only methanol
As a continuation of our biochemical and genetic studies on
damaged nucleosides caused by carcinogens, ionizing radiation,
or chemotherapeutic agents we desired to synthesize a psoralen
furan-side monoadduct. Our strategy involved linking a pso-
ralen retron to thymidine in order to control the stereochemistry
of the photoreaction. Based on some earlier results with tethered
thymidine-psoralen derivatives, we determined that attachment
at the 2 position of psoralen could afford the cis-syn stereoi-
somer. Because psoralen dimerization is a severe problem in
the absence of double stranded DNA, we chose benzofuran as
a psoralen synthon. Benzofuran is missing the photochemically
(12) The usual numbering system of furocoumarins is based upon the
psoralen. We have used, however, the Ring Index nomenclature because
the numbering system is the same for benzofuran and psoralen.
(13) (a) Gervais, J.; Boens, N.; de Schryver, F. C. NouV. J. Chim 1979,
3, 163. (b) Gervais, J.; de Schryver, F. C. Photochem. Photobiol. 1975, 21,
71.
(14) (a) Land, E. J.; Rushton, F. A. P.; Beddoes, R. L.; Bruce, J. M.;
Cernik, R. J.; Dawson, S. C.; Mills, O. S. J. Chem. Soc., Chem. Commun.
1982, 22. (b) Cadet, J.; Voituriez, L.; Gaboriau, F.; Vigny, P.; Negra, S. D.
Photochem. Photobiol. 1983, 37, 363.
(16) (a) Spielmann, H. P.; Dwyer, T. J.; Sastry, S. S.; Hearst, J. E.;
Wemmer, D. E. Proc. Natl. Acad. Sci. U.S.A. 1995, 92, 2345. (b) Spielmann,
H. P.; Dwyer, T. J.; Hearst, J. E.; Wemmer, D. E. Biochemistry 1995, 34,
12937.
(15) De´cout, J.-L.; Lekchiri, Y.; Lhomme, J. Photochem. Photobiol. 1992,
55, 639.

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Cis-Syn Furan-Side Photoproduct Synthesis
J. Am. Chem. Soc., Vol. 118, No. 30, 1996 7103
Scheme 2
and water effect this transformation. In fact, a 50:50 solution
of methanol and ethanol led only to methyl ester photoproduct
13. It is possible that ethanol is too sterically hindered to
transesterify the major isomer because the nucleophilicity
between methanol and ethanol are minimal.
We used NMR to determine both the regio- and stereochem-
istry of the isolated photoproducts 12b and 13. The regio-
chemistry of both photoproducts was determined by observing
the coupling constant between the H6 proton of thymidine and
the H3 proton of benzofuran. Only long range coupling (2 Hz)
was observed for both photoproducts, which is indicative of
syn stereochemistry.17 To determine if the thymidine methyl
group were on the same side of the four-membered ring as the
H3 and H6 protons as desired, an NOE difference experiment
was performed (Figure 2). Irradiation of the thymidine methyl
group of major isomer 13 gave a positive NOE for both ring
protons, which confirmed that the major photoproduct possessed
the desired cis-stereochemistry. Irradiation of the thymidine
methyl group of minor isomer 12b gave only one positive NOE
for the H6 proton of thymidine (Figure 2c). Hence, the minor
isomer is the trans-syn isomer. Attempts to crystallize the major
photoproduct, to assign on which face of the thymidine ring
the [2 + 2] cycloaddition occurred, have yet to yield a crystal
of X-ray diffraction quality. However, Hearst et al. have shown
that the CD spectrum of a 5′-TpA-3′ psoralen adduct has a
negative ellipticity from 270 to 360 nm, whereas the 5′-ApT-3′
adduct has a positive ellipticity in the same range.8a It appears
that the contribution of the deoxyribose ring affects the CD
spectrum minimally, and the two diastereomers behave as
enantiomers in the presence of polarized light. Although the
model compound did not contain the full coumarin ring, we
believed that the facial selectivity could be ascertained from a
CD spectrum. The CD spectrum of the major isomer 13, shown
in Figure 2, has a negative ellipticity from 260 to 290 nm
suggesting the cycloaddition had occurred on the 3′ face of
thymidine.
Figure 2. (Above) Structures of the benzofuran-thymidine photoprod-
ucts 13 and 12b, where dR ) 2′-deoxy-3′-O-acetylribose. (Middle)
Nuclear Overhauser enhancements for photoproducts 13 and 12b. (A)
difference spectrum between the major isomer 13 irradiated at the
thymidine methyl resonance (1.78 ppm) and (B); (B) proton NMR
spectrum of the major isomer 13 (4.0-5.0 ppm). (C) difference
spectrum between the minor isomer 12b irradiated at the thymidine
methyl resonance (1.28 ppm) and (D); (D) proton NMR spectrum of
the minor isomer 12b (4.0-5.0 ppm). (Below) Circular dichroism
spectrum of the benzofuran-thymidine major photoproduct 13.
Having firmly established a methodology to construct a cis-
syn photoproduct by using benzofuran as a psoralen guise, we
were confident that a 2-carbomethoxypsoralen-thymidine ad-
duct was achievable by total synthesis. Two minor modifica-
tions of the model system were necessary. First, a simple
change of a protecting group on the phenol would allow for a
mild deprotection when coupled to the nucleoside. The second
(17) (a) Pretsch, E.; Clerc, T.; Seibl, J.; Simon, W. In Tables of Spectral
Data for Structure Determination of Organic Compounds; Springer-
Verlag: New York, 1989; p H185. (b) Silverstein, R. M.; Bassler, G. C.;
Morrill, T. C. In Spectrometric Identification of Organic Compounds; John
Wiley & Sons: New York, 1981; p 209.
modification was to incorporate an aldehyde ortho to the
methoxymethyl (MOM) protected phenol. Addition of a formyl

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7104 J. Am. Chem. Soc., Vol. 118, No. 30, 1996
Kobertz and Essigmann
Scheme 3
mild conditions.18 Treatment of aldehyde 23 with N ′,N ′-
dimethylacetamide dimethylacetal in the presence of 4 Å sieves
at room temperature gave coumarin 24 in 38% yield. Although
the yield for this step is moderate, the chemoselectivity of the
reagent to form a carbon-carbon double bond in the presence
of a free primary hydroxyl and imide is noteworthy. The
protected 2-carbomethoxypsoralen-thymidine adduct was con-
verted into the desired photoproduct 4 using DBU in methanol.
Determination of the stereochemistry of photoproduct 4 was
identical to the model system. The coupling constant between
the H6 thymidine proton and the H3 benzofuran proton of
psoralen was small, 1.7 Hz, confirming the syn regioisomer.
Figure 3a shows the difference NOE experiment where irradia-
tion of the thymidine methyl shows an enhancement of both
H6 and H3 protons confirming the stereochemistry as cis. The
final stereochemical determination was whether the 2-car-
bomethoxypsoralen was situated on the 5′ or 3′ face of
thymidine. The CD spectrum of the synthesized adduct 4, was
nearly identical with the CD spectra of an isolated psoralen-
thymidine adduct even though the psoralen moieties are slightly
different (Figure 3).8a,11f The synthesized 2-carbomethoxypso-
ralen-thymidine adduct is the adduct equivalent to the reaction
occurring at a 5′-TpA-3′ site in DNA; this is the 100-fold most
prevalent adduct under normal conditions of DNA damage.
group at this position would allow for a mild conversion of the
benzofuran photoproduct into the desired psoralen-thymidine
adduct.18 Earlier formylations of 6-alkoxybenzofurans utilized
DMF-POCl3 to formylate directly the benzofuran. However,
these conditions resulted in primarily 7-formylbenzofurans and
afforded little 5-formyl product.7e Scheme 3 depicts a two step
approach utilizing halogenation followed by a palladium(0)
catalyzed formylation19 to attain a 5-formylbenzofuran. Treat-
ment of selectively protected 1420 with sodium iodide and
chloramine T gave primarily para iodinated 15.21 Cyclization
of 15 to benzofuran 16 was accomplished with diethyl bromo-
malonate and potassium carbonate. The desired aldehyde was
synthesized using a Stille coupling.19 Palladium(0) coupling
with 16, carbon monoxide, and slow addition of tributyltin
hydride gave 5-formylbenzofuran 17. The overall two step yield
for incorporation of the 5-aldehyde was 53%. Saponification
of 17 yielded benzofuranyl acid 18.22
With the benzofuran portion of the molecule in hand, the
next task was to couple it to thymidine and produce a
photoprecursor. The synthesis began with carbodiimide cou-
pling of 18 with 3′-O-actetylthymidine 6 to give esterified 19,
as shown in Scheme 4. Removal of the protecting group using
trityl cation afforded phenol 20 in 90% yield.23 Efforts to use
HCl/methanol to remove the MOM group led to side products
and lower yields. All attempts at photochemistry with aldehyde
20 resulted in no reaction presumably owing to fluorescence
quenching by the benzaldehyde moiety. To circumvent this
problem, aldehyde 20 was protected by reduction NaBH4 to the
readily oxidizable benzylic alcohol 21. A dilute solution of
photoprecursor 21 in 20:1 acetonitrile/acetone was irradiated
with 300 nm light and afforded only one diastereomer, which
was transesterified with silica gel and methanol to afford the
ring opened photoproduct 22 in 58% yield. Selective oxidation
of 22 with oxygen in the presence of CuCl and TEMPO afforded
aldehyde 23.24 Typically harsh, basic conditions are needed to
construct the pyrone ring from ortho formyl phenols.25 Barton
and others have used enediamines or commercially available
acetamide acetals to effect this transformation under relatively
Summary
We have described the first total synthesis of a cis-syn furan-
side monoadduct between thymidine and a psoralen derivative.
Linking a benzofuranyl acid derivative to the 5′ hydroxyl of
thymidine was crucial in the control of the regio- and stereo-
chemistry to give a cis-syn adduct. The key [2 + 2] cycload-
dition occurs with high facial selectivity giving primarily or
only the cis-syn 3′ face adduct, which is the most prominent
adduct found upon DNA-psoralen irradiation or a medicinal
treatment. The benefit of this total synthetic approach is that it
affords the opportunity to place psoralen adducts into DNA
sequence contexts containing multiple sites of adduction. Efforts
to incorporate these adducts into DNA for biological studies
are underway.
Experimental Section
General Procedures. Unless otherwise specified, materials were
obtained from commercial suppliers and were used without further
purification. Tetrahydrofuran (THF) was distilled from potassium/
benzophenone ketyl. Methanol (MeOH) was distilled from sodium
metal and used immediately after distillation. Anhydrous solvents were
otherwise obtained from Aldrich. Column chromatography was
performed on Merck silica gel (230-400 mesh). 1H NMR spectra were
recorded at 500 or 300 MHz on superconducting FT spectrometers.
13C NMR spectra were proton decoupled and were recorded at 125 or
75 MHz. Coupling constants are measured in hertz. Melting points
(pyrex capillary) are uncorrected.
6-Methoxy-2-benzofuran Carboxylic Acid (10). To a solution of
5.74 g (37.7 mmol) of 4-methoxysalicylaldehyde 9 in 400 mL of dry
2-butanone were added 9.70 mL (56.6 mmol) of diethyl bromomalonate
and 15.6 g (113 mmol) of oven dried K2CO3. The solution was stirred
vigorously and heated to reflux for 7 h, cooled, and filtered. The
solvents were removed and the solids were diluted in 100 mL of
methanol and 200 mL of 1 N NaOH solution. The solution was then
heated to reflux for 1 h. After cooling to room temperature, the reaction
mixture was extracted once with ethyl acetate and then acidified to pH
1. The tan precipitate that formed was collected, rinsed with 1 N HCl,
and dried to afford 5.53 g (76%) for two steps: 1H NMR (500 MHz,
DMSO-d6) δ 3.82 (s, 3 H), 6.96 (dd, 1 H, J ) 2.0, 8.3), 7.28 (d, 1 H,
J ) 1.5), 7.58 (s, 1 H), 7.63 (d, 1 H, J ) 8.3); 13C NMR δ 55.62,
95.81, 101.70, 113.60, 119.89, 123.18, 145.24, 156.37, 159.86, 159.95;
HRMS (EI) calcd for C10H8O4 (M)+ 192.0423, found 192.0424.
5′-O-(6-Methoxy-2-benzofuranyl)-3′-O-acetylthymidine (11). To
a solution of 0.38 g (1.52 mmol) of compound 10 in 5 mL of dry
(18) (a) Barton, D. H. R.; Hewitt, G.; Sammes, P. G. J. Chem. Soc. (C)
1969, 16. (b) Begley, M. J.; Crombie, L.; Slack, D. A.; Whiting, D. A. J.
Chem. Soc., Perkin Trans. 1 1977, 2402. (c) Effenberger, F.; Maier, R.
Liebigs Ann. Chem. 1969, 729, 246.
(19) Baillargeon, V. P.; Stille, J. K. J. Am. Chem. Soc. 1986, 108, 452.
(20) Su¨sse, M.; Johne, S.; Hesse, M. HelV. Chim. Acta 1992, 75, 457.
(21) Kometani, T.; Watt, D. S.; Ji, T. Tetrahedron Lett. 1985, 26, 2043.
(22) Miknis, G. F.; Williams, R. M. J. Am. Chem. Soc. 1993, 115, 536.
(23) Nakata, T.; Schmid, G.; Vranesic, B.; Okigawa, M.; Smith-Palmer,
T.; Kishi, Y. J. Am. Chem. Soc. 1978, 100, 2933.
(24) Semmelhack, M. F.; Schmid, C. R.; Corte´s, D. A.; Chou, C. S. J.
Am. Chem. Soc. 1984, 106, 3374.
(25) Murray R. D. H.; Me´ndez, J.; Brown, S. A. In The Natural
Coumarins: Occurrence, Chemistry and Biochemistry; Wiley: New York,
1982; pp 131-161.

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Cis-Syn Furan-Side Photoproduct Synthesis
J. Am. Chem. Soc., Vol. 118, No. 30, 1996 7105
Scheme 4
95.48, 111.52, 114.52, 115.44, 119.98, 123.21, 134.76, 143.80, 150.49,
157.21, 158.61, 161.11, 163.53, 170.35; UVmax (CH3CN) 312, 268, 246
nm; HRMS (FAB+, 3-NBA) calcd for C22H22O19N2 (M + H)+
459.1404, found 459.1404.
5′-O-(6-Methoxy-2-benzofuranyl)-3′-O-acetylthymidine Photo-
product. A solution of 75 mg (0.16 mmol) of photoprecursor 11 in
85 mL of dry CH3CN was deaerated with argon bubbling for 30 min.
Acetone (4.25 mL) was added and the solution was irradiated with
300 nm light in a 16 bulb Rayonet photoreactor for 5 h at room
temperature. The solvent was removed in Vacuo, and the crude material
was dissolved in 25 mL of methanol with 150 mg of silica gel and
stirred at room temperature for 30 min to effect transesterification. The
methanol was removed, and the absorbed material was directly added
to the top of a silica gel column. Elution with a gradient of 100:1 to
50:1 dichloromethane/methanol afforded 45 mg (57%) of a major
isomer 13 and 10 mg (14%) of a minor isomer 12b.
Major isomer cis-syn (13): 1H NMR (500 MHz, CDCl3) δ 1.78 (s,
3 H), 2.07-2.12 (m, 1 H), 2.08 (s, 3 H), 2.37 (ddd, 1 H, J ) 2.0, 5.4,
14), 3.76 (s, 3 H), 3.79-3.83 (m, 2 H), 3.86-3.88 (m, 1 H), 3.87 (s,
3 H), 3.95 (d, 1 H, J ) 2.0), 4.73 (d, 1 H, J ) 2.0), 5.19-5.22 (m, 1
H), 5.95 (dd, 1 H, J ) 5.4, 9.3), 6.46-6.48 (m, 2 H), 6.99 (d, 1 H, J
) 7.8), 7.06 (br s, 1 H); 13C NMR δ 21.00, 22.71, 36.12, 46.34, 53.44,
55.52, 56.05, 57.85, 62.58, 74.48, 84.25, 85.42, 86.72, 96.49, 108.75,
115.30, 126.81, 150.69, 161.70, 162.33, 169.67, 170.65, 170.69; UVmax
(CH3CN) 284 nm; HRMS (FAB+, 3-NBA) calcd for C23H26O10N2 (M
+ H)+ 491.1666, found 491.1664.
Minor isomer trans-syn (12b): 1H NMR (500 MHz, CDCl3) δ 1.28
(s, 3 H), 2.10 (s, 3 H), 2.24 (app dd, 1 H, J ) 7.3, 15), 2.84 (ddd, 1 H,
J ) 5.9, 8.8, 15), 3.80 (s, 3 H), 3.89 (s, 1 H), 4.18 (app d, 1 H, J )
12), 4.25 (app d, 1 H, J ) 2.0), 4.66 (s, 1 H), 5.19 (dd, 1 H, J ) 2.0,
12), 5.48 (app d, 1 H, J ) 5.9), 6.35 (app t, 1 H, J ) 8.8), 6.55-6.58
(m, 2H), 7.15 (d, 1 H, J ) 8.3), 7.50 (br s, 1 H); 13C NMR δ 20.98,
22.49, 32.25, 48.00, 55.61, 65.59, 67.57, 77.20, 77.85, 82.60, 87.86,
91.55, 97.13, 108.35, 114.93, 126.97, 151.24, 161.12, 161.73, 165.55,
170.58, 171.77; HRMS (FAB+, 3-NBA) calcd for C22H22O9N2 (M +
H)+ 459.1404 found 459.1398.
Figure 3. (Above) Nuclear Overhauser enhancements for the 2-car-
bomethoxypsoralen-thymidine furan-side mono-adduct 4. (A) difference
spectrum between sample irradiated at the thymidine methyl resonance
(1.80 ppm) and (B); (B) 4.0-5.0 ppm region of the proton NMR
spectrum. (Below) Circular dichroism spectrum of the 2-carbomethox-
ypsoralen-thymidine furan-side mono-adduct 4.
pyridine were added 0.46 g (1.60 mmol) of 3′-O-acetylthymidine 6,
0.35 g (1.82 mmol) of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide
hydrochloride (EDC), and 20 mg (0.15 mmol) of DMAP. The solution
was stirred at room temperature under an argon atmosphere for 16 h.
The reaction mixture was concentrated, dissolved in CH2Cl2, washed
with 1 N HCl and saturated NaHCO3, and then dried over Na2SO4.
The crude product was purified by silica gel chromatography eluting
with 100:1 dichloromethane/methanol to afford 0.54 g (75%) of a white
foam: 1H NMR (300 MHz, CDCl3) δ 1.63 (app s, 3 H), 2.12 (s, 3 H),
2.38 (ddd, 1 H, J ) 6.2, 8.8, 14), 2.50 (app dd, 1 H, J ) 5.6, 14), 3.84
(s, 3 H), 4.36 (app d, 1 H, J ) 1.6), 4.59 (dd, 1 H, J ) 2.5, 12), 4.66
(dd, 1 H, J ) 2.9, 12), 5.35 (app d, 1 H, J ) 6.2), 6.44 (dd, 1 H, J )
5.6, 8.8), 6.92-6.96 (m, 2 H), 7.26-7.54 (m, 3 H), 9.54 (br s, 1 H);
13C NMR δ 12.18, 20.78, 37.52, 55.71, 64.55, 74.89, 82.35, 84.89,
2-Hydroxy-5-iodo-4-(methoxymethyl)benzaldehyde (15). Chloram-
ine T (4.54 g, 20.6 mmol) in 15 mL of DMF was added over a 10 min
period to a solution of 3.13 g (17.2 mmol) of 14 and 3.09 g (20.6
mmol) of sodium iodide in 50 mL of DMF. The solution was stirred
for an additional 30 min, diluted with ethyl acetate, and washed with
1 N HCl and 5% sodium thiosulfate. The organic layer was then
extracted twice with 1 N NaOH, and the aqueous extractions were

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7106 J. Am. Chem. Soc., Vol. 118, No. 30, 1996
Kobertz and Essigmann
separated. The aqueous layers were acidified with 1 N HCl in the
presence of ethyl acetate. The organic layer was dried over MgSO4.
Purification of the crude product by silica gel chromatography eluting
with 15:1 hexanes/ethyl acetate afforded 3.92 g (74%) of a white flaky
solid: 1H NMR (300 MHz, CDCl3) δ 3.34 (s, 3 H), 5.12 (s, 2 H), 6.48
(s, 1 H), 7.72 (s, 1H), 9.52 (s, 1 H), 11.13 (s, 1 H); 13C NMR δ 56.74,
74.47, 94.81, 102.55, 117.70, 143.81, 162.15, 164.08, 193.61; HRMS
(EI) calcd for C9H9O4I (M)+ 307.9546, found 307.9545. Anal. Calcd
for C9H9O4I: C, 35.09; H, 2.94; I, 41.19. Found: C, 35.24; H, 3.04;
I, 41.52; mp 77-79 °C.
2-Carboethoxy-5-iodo-6-(methoxymethyl)benzofuran (16). To a
solution of 3.89 g (12.6 mmol) of compound 15 in 200 mL of dry
2-butanone were added 3.25 mL (18.9 mmol) of diethyl bromomalonate
and 5.25 g (37.8 mmol) of oven dried K2CO3. The solution was stirred
vigorously and heated to reflux for 24 h, cooled, and filtered. Removal
of the solvents in Vacuo and purification by silica gel chromatography
eluting with 10:1 hexanes/ethyl acetate afforded 2.76 g (58%) of a fluffy
white solid: 1H NMR (300 MHz, CDCl3) δ 1.39 (t, 3 H, J ) 7.1),
3.49 (s, 3 H), 4.39 (q, 2 H, J ) 7.1), 5.26 (s, 2 H), 7.31 (s, 1 H), 7.36
(s, 1 H), 8.05 (s, 1 H); 13C NMR δ 14.23, 56.39, 61.39, 82.87, 95.30,
98.59, 112.47, 123.26, 132.27, 145.81, 155.31, 156.51, 159.09; HRMS
(EI) calcd for C13H13O5I (M)+ 375.9808, found 375.9807. Anal. Calcd
for C13H13O5I: C, 41.51; H, 3.48; I, 33.74. Found: C, 41.42; H, 3.48;
I, 33.93; mp 90-91 °C.
2-Carboethoxy-5-formyl-6-(methoxymethyl)benzofuran (17). A
solution of 1.00 g (2.66 mmol) of compound 16 and 231 mg (0.20
mmol) of Pd(Ph3P)4 in 10 mL of dry THF was heated to 50 °C under
an CO atmosphere (1 atm). A solution of 0.79 mL (2.93 mmol) of
tributyltin hydride in 20 mL of dry THF was added to the solution
over a 6 h period. The solution was cooled to room temperature and
the solvents were removed in Vacuo. The crude material was purified
by a short silica gel column eluting with chloroform to separate the tin
impurities from the product. Purification of the tin free product eluting
with a gradient of 10:1 hexanes/ethyl acetate to 5:1 hexanes/ethyl acetate
afforded 0.53 g (72%) of yellow-white solid: 1H NMR (300 MHz,
CDCl3) δ 1.35 (t, 3 H, J ) 7.2), 3.48 (s, 3 H), 4.36 (q, 2 H, J ) 7.2),
5.30 (s, 2 H), 7.33 (s, 1 H), 7.42 (s, 1 H), 8.09 (s, 1 H), 10.43 (s, 1 H);
13C NMR δ 14.10, 56.39, 61.43, 95.04, 98.36, 113.99, 121.14, 123.38,
123.42, 146.62, 158.73, 159.47, 159.49, 188.76; HRMS (EI) calcd for
C14H14O6 (M)+ 278.0790, found 278.0792. Anal. Calcd for
C14H14O6: C, 60.43; H, 5.07. Found: C, 60.34; H, 5.07; mp 70-71
°C.
95.14, 98.31, 111.51, 115.75, 120.98, 123.85, 123.94, 134.71, 145.64,
150.45, 158.13, 159.59, 160.00, 163.52, 170.43, 188.76; HRMS (FAB+,
3-NBA) calcd for C24H24O11N2 (M + H)+ 517.1458, found 517.1458.
5′-O-(5-Formyl-6-hydroxy-2-benzofuranyl)-3′-O-acetylthymi-
dine (20). To a solution of 0.44 g (0.86 mmol) of compound 19 in 20
mL of dry CH2Cl2 was added 0.32 g (0.95 mmol) of trityl tetrafluo-
roborate. The solution was stirred at room temperature under an argon
atmosphere for 30 min. Completion of the reaction was monitored by
the product’s green fluorescence under long wave UV light (366 nm).
The solution was diluted with CH2Cl2, washed with water, and dried
over Na2SO4. Purification of the crude product by silica gel chroma-
tography eluting with a gradient of 100:1 to 50:1 dichloromethane/
methanol afforded 0.50 g (90%) of a white foam: 1H NMR (300 MHz,
CDCl3) δ 1.68 (d, 3 H, J ) 1.0), 2.12 (s, 3 H), 2.37 (ddd, 1 H, J ) 6.4,
8.8, 14), 2.53 (ddd, 1 H, J ) 1.3, 5.5, 14), 4.35-4.38 (m, 1 H), 4.60
(dd, 1 H, J ) 3.0, 12), 4.67 (dd, 1 H, J ) 3.4, 12), 5.34 (app d, 1 H,
J ) 6.4), 6.39 (dd, 1 H, J ) 5.5, 8.8), 7.02 (s, 1 H), 7.39 (d, 1 H, J )
1.0), 7.61 (s, 1 H), 7.93 (s, 1 H), 9.58 (br s, 1 H), 9.95 (s, 1 H), 11.23
(s, 1 H); 13C NMR δ 12.33, 20.83, 37.39, 64.93, 74.63, 82.16, 85.04,
99.88, 111.45, 115.20, 119.51, 120.08, 130.23, 134.66, 145.80, 150.45,
158.08, 160.13, 162.20, 163.60, 170.45, 195.79; HRMS (FAB+, 3-NBA)
calcd for C22H20O10N2 (M + H)+ 473.1196, found 473.1202.
5′-O-[6-Hydroxy-5-(hydroxymethyl)-2-benzofuranyl]-3′-O-acetylth-
ymidine (21). To a solution of 0.32 g (0.69 mmol) of compound 20
in 8 mL of a 1:1 mixture of ethanol/dioxane was added 10 mg (0.26
mmol) of NaBH4. The solution was stirred at room temperature for
15 min, neutralized with saturated NH4Cl, diluted with CH2Cl2, washed
with water, and dried over MgSO4. The crude product was purified
by silica gel chromatography eluting with a gradient of 20:1 to 10:1
dichloromethane/methanol to afford 0.29 g (88%) of a white solid: 1H
NMR (300 MHz, acetone-d6) δ 1.63 (d, 3 H, J ) 1.8), 2.10 (s, 3 H),
2.47 (ddd, 1 H, J ) 6.4, 8.7, 14), 2.55 (ddd, 1 H, J ) 2.0, 5.9, 14),
2.87 (br s, 1 H), 4.40-4.42 (m, 1 H), 4.59 (dd, 1 H, J ) 3.7, 12), 4.67
(dd, 1 H, J ) 4.0, 12), 4.79 (d, 2 H, J ) 2.0), 5.42-5.45 (m, 1 H),
6.38 (dd, 1 H, J ) 5.9, 8.7), 7.03 (s, 1 H), 7.63 (d, 1 H, J ) 2.6), 7.71
(app s, 2 H); 13C NMR (DMSO-d6) δ 11.97, 20.74, 35.88, 58.37, 64.33,
74.15, 81.14, 84.03, 96.62, 109.97, 115.65, 118.32, 120.62, 128.04,
135.50, 142.75, 150.39, 155.53, 155.91, 158.36, 163.55, 170.02; UVmax
(CH3CN) 314, 270, 249 nm; HRMS (FAB+, glycerol) calcd for
C22H22O10N2 (M + H)+ 475.1353, found 475.1355. Anal. Calcd for
C22H22O10N2: C, 55.70; H, 4.67; N, 5.91. Found: C, 55.48; H, 4.76;
N, 5.63.
[2-Carbomethoxy-6-hydroxy-(5-methylalcohol)benzofuran]-3′-O-
acetylthymidine Cis-Syn Photoproduct (22). A solution of 0.25 g
(0.52 mmol) of compound 21 in 350 mL of dry CH3CN was deaerated
with argon bubbling for 60 min. Acetone (17.5 mL) was added, and
the solution was irradiated with 300 nm light in a 16 bulb Rayonet
photoreactor for 3 h at room temperature. Removal of the solvents
and absorption of the crude material onto silica gel using methanol as
the solvent effected the ring opening. The absorbed compound was
loaded on top of a silica gel column and purified by eluting with a
gradient of 30:1 to 15:1 dichloromethane/methanol to afford 0.15 g
(58%) of a clear foam: 1H NMR (300 MHz, acetone-d6) δ 1.70 (s, 3
H), 2.00-2.11 (m, 2 H), 2.06 (s, 3 H), 3.69-3.71 (m, 2 H), 3.85 (s, 3
H), 3.88-3.90 (m, 1 H), 4.00 (d, 1 H, J ) 1.0), 4.04 (t, 1 H, J ) 5.4,
(-OH)), 4.55 (d, 1 H, J ) 13), 4.63 (d, 1 H, J ) 13), 4.84 (d, 1 H, J
) 2.0), 5.17-5.19 (m, 1 H), 6.18 (dd, 1 H, J ) 5.4, 9.3), 6.37 (s, 1 H),
6.94 (s, 1 H), 8.71 (br s, 1 H), 8.93 (br s, 1 H); 13C NMR δ 21.07,
22.95, 35.80, 47.35, 53.43, 56.76, 57.56, 61.68, 63.06, 75.56, 84.42,
84.84, 88.11, 98.32, 116.04, 122.28, 126.57, 152.55, 157.85, 162.83,
170.87, 170.93; UVmax (CH3CN) 287 nm; HRMS (FAB+, 3-NBA) calcd
for C23H26O11N2 (M + H)+ 507.1615, found 507.1611.
(2-Carbomethoxy-5-formyl-6-hydroxybenzofuran)-3′-O-acetylth-
ymidine Cis-Syn Photoproduct (23). To a solution of 0.16 g (0.31
mmol) of compound 22 in 2 mL of DMF were added 13 mg (0.08
mmol) of TEMPO and 8 mg (0.08 mmol) of CuCl. Oxygen was
bubbled through the solution for 2 h after which another 13 mg of
TEMPO and 8 mg of CuCl were added. After stirring under an oxygen
atmosphere for 6 h, the reaction was diluted with ethyl acetate and
washed with 1 N HCl, saturated NaHCO3, and brine, and then dried
over MgSO4. Purification of the crude product by silica gel chroma-
tography eluting with 50:1 dichloromethane/methanol afforded 0.11 g
(70%) of a white foam: 1H NMR (300 MHz, acetone-d6) δ 1.75 (s, 3
5-Formyl-6-(methoxymethyl)-2-benzofuran Carboxylic Acid (18).
A solution of 0.49 g (1.77 mmol) of compound 17 and 7.5 mL of
saturated aqueous K2CO3 in 30 mL of ethanol was heated to reflux for
30 min. The solvents were removed, the crude product was dissolved
in water, and the aqueous layer was washed twice with ethyl acetate.
The aqueous layer was acidified to pH 3 with 0.1 M H2SO4. The
resulting precipitate was filtered, washed with cold pH 3 water, and
dried overnight to afford 0.35 g (79%) of a white solid: 1H NMR (300
MHz, DMSO-d6) δ 3.46 (s, 3 H), 5.43 (s, 2H), 7.52 (s, 1 H), 7.67 (s,
1 H), 8.13 (s, 1 H), 10.39 (s, 1 H); 13C NMR δ 56.31, 94.91, 98.64,
114.11, 121.14, 123.17, 123.44, 147.23, 158.84, 159.02, 159.62, 188.74;
HRMS (FAB+, glycerol) calcd for C12H10O6 (M + H)+ 251.0556, found
251.0552.
5′-O-[5-Formyl-6-(methoxymethyl)-2-benzofuranyl]-3′-O-acetylth-
ymidine (19). To a solution of 0.30 g (1.18 mmol) of compound 18
in 4 mL of dry pyridine were added 0.36 g (1.25 mmol) of
3′-O-acetylthymidine 6, 0.29 g (1.50 mmol) of 1-(3-dimethylamino-
propyl)-3-ethylcarbodiimide hydrochloride (EDC), and a catalytic
amount of DMAP. The solution was stirred at room temperature under
an argon atmosphere for 24 h. The solvent was concentrated, dissolved
in CH2Cl2, washed with 1 N HCl and saturated NaHCO3, and then
dried over MgSO4. The crude product was purified by silica gel
chromatography eluting with 100:1 dichloromethane/methanol to afford
0.50 g (90%) of a white foam: 1H NMR (300 MHz, CDCl3) δ 1.67 (d,
3 H, J ) 1.2), 2.12 (s, 3 H), 2.37 (ddd, 1 H, J ) 6.4, 9.0, 14), 2.51
(ddd, 1 H, J ) 1.3, 5.5, 14), 3.52 (s, 3 H), 4.34-4.37 (m, 1 H), 4.59
(dd, 1 H, J ) 2.7, 12), 4.67 (dd, J ) 3.1, 12), 5.32-5.37 (m, 1 H),
5.34 (s, 2 H), 6.42 (dd, 1 H, J ) 5.5, 9.0), 7.32 (s, 1 H), 7.44 (d, 1 H,
J ) 1.2), 7.62 (s, 1 H), 8.19 (s, 1 H), 9.46 (br s, 1 H), 10.48 (s, 1 H);
13C NMR δ 12.31, 20.83, 37.43, 56.58, 64.91, 74.70, 82.13, 84.87,

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Cis-Syn Furan-Side Photoproduct Synthesis
J. Am. Chem. Soc., Vol. 118, No. 30, 1996 7107
H), 1.96-2.11 (m, 2 H), 2.06 (s, 3 H), 3.70-3.73 (m, 2 H), 3.88 (s, 3
H), 3.89-3.91 (m, 1 H), 4.16 (app s, 1 H), 4.95 (d, 1 H, J ) 1.8),
5.18-5.21 (m, 1 H), 6.19 (dd, 1 H, J ) 5.6, 9.3), 6.49 (s, 1 H), 7.47
(d, 1 H, J ) 0.7), 9.80 (d, 1 H, J ) 0.7); 13C NMR δ 21.07, 22.78,
35.84, 47.50, 53.70, 55.04, 57.55, 63.04, 75.59, 84.53, 84.77, 89.53,
98.98, 117.57, 119.14, 133.49, 152.30, 166.11, 169.34, 169.73, 170.59,
170.87, 196.10; HRMS (FAB+, 3-NBA) calcd for C23H24O11N2 (M +
H)+ 505.1458, found 505.1455.
(2-Carbomethoxypsoralen)-3′-O-acetylthymidine Cis-Syn Pho-
toproduct (24). To a solution of 43 mg (0.09 mmol) of compound 23
in 0.4 mL of dry THF were added 4 Å sieves and 125 mL (0.85 mmol)
of N,N-dimethylacetamide dimethylacetal. The solution stirred for 2
h at room temperature under an argon atmosphere. The reaction was
diluted with ethyl acetate, washed with saturated NH4Cl, saturated
NaHCO3, and brine, and then dried over MgSO4. The crude product
was purified by silica gel chromatography eluting with a gradient of
50:1 to 33:1 dichloromethane/methanol to afford 17 mg (38%) of a
white foam: 1H NMR (300 MHz, acetone-d6) δ 1.76 (s, 3 H), 2.01-
2.05 (m, 1 H), 2.07 (s, 3 H), 2.13 (ddd, 1 H, J ) 2.0, 5.9, 14), 3.71-
3.73 (m, 2 H), 3.88 (s, 3 H), 3.90-3.92 (m, 1 H), 4.11 (t, 1 H, J ) 5.4,
(-OH)), 4.24 (s, 1 H), 4.97 (d, 1 H, J ) 2.0), 5.20-5.22 (m, 1 H), 6.20
(dd, 1 H, J ) 5.9, 9.8), 6.24 (d, 1 H, J ) 9.3), 6.90 (s, 1 H), 7.39 (s,
1 H), 7.90 (d, 1 H, J ) 9.3); 13C NMR (CDCl3) δ 21.00, 22.85, 35.80,
46.60, 53.68, 54.97, 57.28, 62.53, 74.27, 84.10, 84.94, 87.77, 99.35,
113.81, 114.41, 121.48, 125.81, 143.23, 150.43, 156.32, 160.47, 163.98,
169.20, 169.70, 170.69; UV (MeOH) 324, 293; HRMS (FAB+, 3-NBA)
calcd for C25H24O11N2 (M + H)+ 529.1458, found 529.1456.
2-Carbomethoxypsoralen Thymidine Cis-Syn Photoproduct (4).
To a solution of 9 mg (17 mmol) of compound 24 in 0.5 mL of freshly
distilled methanol was added 50 mL of DBU. After stirring for 10
min at room temperature the reaction was diluted with ethyl acetate,
washed with saturated NH4Cl, saturated NaHCO3, and brine, and then
dried over Na2SO4. The crude product was purified by silica gel
chromatography eluting with 25:1 dichloromethane/methanol to afford
8 mg (97%) of a white foam: 1H NMR (300 MHz, CDCl3) δ 1.80 (s,
3 H), 1.87 (br s, 2 H), 2.02-2.11 (m, 1 H), 2.27 (ddd, 1 H, J ) 3.1,
5.9, 14), 3.70 (dd, 1 H, J ) 3.8, 12), 3.80-3.89 (m, 5 H), 3.89 (s, 3
H), 4.02 (s, 1 H), 4.39-4.43 (m, 1 H), 4.77 (d, 1 H, J ) 1.7), 5.96 (dd,
1 H, J ) 5.9, 8.1), 6.27 (d, 1 H, J ) 9.5), 6.91 (s, 1 H), 7.24 (s, 1 H),
7.40 (br s, 1 H), 7.60 (d, 1 H, J ) 9.5); 13C NMR (methanol-d4) δ
22.92, 38.64, 48.44, 54.01, 56.16, 57.97, 63.36, 72.41, 85.16, 87.35,
89.97, 99.67, 113.97, 115.85, 124.40, 127.84, 145.90, 153.72, 157.68,
162.91, 166.21, 170.73, 172.26; UV (1:1 methanol:water) 332, 294;
HRMS (FAB+, 3-NBA) calcd for C23H22O10N2 (M + H)+ 487.1353,
found 487.1349.
Acknowledgment. We thank D. Nauman for technical
assistance. This work was supported by NIH Grants CA52127
and ES07020.
JA960511E