Concerning the diastereomerization of stilbenoid hypericin derivatives

Article abstract of DOI:10.1007/s007060200043

Experiments on a newly prepared (E)-configured ω-benzal-hypericin derivative using TLC and ¹H NMR together with quantum chemical calculations revealed that in stilbenoid hypericin derivatives photodiastereomerization between the (E)- and (Z)-diastereomers occurs in principle. However, due to its low diastereomerization quantum yield and photo and thermal equilibria, which reside mostly on the side of the (E)-diastereomer, this photoreaction is only of marginal importance to the photochemistry of stilbenoid hypericin derivatives. Thus, photodiastereomerization does not appreciably interfere with the photoreactions important for photodynamic therapy. This was demonstrated by comparing the sensitized bilirubin photodestruction of hypericin and the ω-benzal-hypericin derivative.

Full text of DOI:10.1007/s007060200043

Monatshefte fur Chemie 133, 717±721 ꢀ2002)
Concerning the Diastereomerization
of Stilbenoid Hypericin Derivativesa
Ã
Bernd Lackner and Heinz Falk
Institute of Chemistry, Johannes Kepler University, A-4040 Linz, Austria
Summary. Experiments on a newly prepared ꢀE)-con®gured !-benzal-hypericin derivative using TLC
and 1H NMR together with quantum chemical calculations revealed that in stilbenoid hypericin
derivatives photodiastereomerization between the ꢀE)- and ꢀZ)-diastereomers occurs in principle. How-
ever, due to its low diastereomerization quantum yield and photo and thermal equilibria, which reside
mostly on the side of the ꢀE)-diastereomer, this photoreaction is only of marginal importance to the
photochemistry of stilbenoid hypericin derivatives. Thus, photodiastereomerization does not apprecia-
bly interfere with the photoreactions important for photodynamic therapy. This was demonstrated by
comparing the sensitized bilirubin photodestruction of hypericin and the !-benzal-hypericin derivative.
Keywords. Photodynamic therapy; Bilirubin photooxidation; ꢀZ)=ꢀE)-Diastereomers; !-Benzal-
hypericin.
Introduction
In continuation of our quest for novel hypericin derivatives useful in photodynamic
therapy [1] we have recently prepared the promising molecule 1 [2]. It nicely
displayed most of the properties desirable for that purpose, like a bathochromic
shift of its long wavelength band and a very low ¯uorescence quantum yield.
However, it failed to sensitize the production of singlet oxygen or corresponding
reactive oxidative species, which are inter alia important for its use in photody-
namic therapy [3].
a
Dedicated to Prof. Silvia Braslavsky on the occasion of her 60th birthday
Ã
Corresponding author. E-mail: heinz.falk@jku.at

718
B. Lackner and H. Falk
One reason for this failure was suspected to be based in a photodiastereomer-
ization reaction involving the stilbenoid double bond of 1, but no indication of such
a process has been obtained so far. This could have been due to either a rapid
thermal equilibration reaction or the absence of such a photodiastereomerization
reaction. To clarify this problem, we now report on investigations of the unsub-
stituted parent compound, the !-benzal-hypericin derivative 2.
Results and Discussions
Synthesis
To prepare 2, we followed the path established for the synthesis of 1 [2]. When
emodin anthrone ꢀ4) was reacted in six-fold molar excess with the stilbene ana-
logue 3 under the conditions of protohypericin synthesis [4], the heterodimer 5 was
formed in optimal relative yield with respect to the dimer of 3 and protohypericin.
Photocyclization of the reaction mixture gave a mixture of the target com-
pound 2 with the corresponding cyclized symmetrical dimer of 3 described earlier
[1a] and the dimer of 4, i.e. hypericin ꢀ6). A 9% yield of 2 could be isolated by
chromatography. From the rather large coupling constant observed for the two
protons of the ethylene fragment ꢀJ  16 Hz) this diastereomer was assigned the
ꢀE)-con®guration.
Photochemical and thermal properties of 2
A ®rst indication that photodiastereomerization of ꢀE)-2 is indeed an option of
stilbenoid hypericin photochemistry came from a TLC experiment. Using the
solvent system petrol ether:methanol ˆ 1:1 on silica, ꢀE)-2 moved with an Rf-value
of 0.66. Upon irradiation of a spot of 2 on silica a second, much smaller spot was
observed to develop with irradiation time at Rf ˆ 0.57. This phenomenon could

Diastereomerization of Stilbenoid Hypericin Derivatives
719
also be substantiated for irradiation of homogeneous solutions of 2 in ethanol, and
1
further by using H NMR spectroscopy. Thus, ꢀE)-2 was dissolved in ethanol-d6,
and its 1H NMR spectrum was recorded. Upon irradiation, the intensity of the AX
system of the ethylene group was rather slowly reduced ꢀas compared with the
photodiastereomerization of the ꢀE)-diastereomer of stilbene conducted under
comparable conditions), and concomitantly a new signal set of low intensity over-
lapping severely with the signals of the aromatic moiety developed. The relative
intensity of the signals did not further change after about 40 minutes irradiation
time. Due to severe signal overlap neither the relative intensities ꢀan estimated
10% of ꢀZ)-2) nor the coupling constant could be measured ꢀan estimated value of
about 12 Hz). However, the nature of this new isomer was corroborated to be the
photodiastereomer ꢀZ)-2 by its thermal reversion: after warming the photostation-
ary reaction mixture to 55ꢀC for 15 minutes, virtually all of the signals pertaining
to the new material were reverted to those of the educt ꢀE)-2. It should be noted
that ES-MS controls attested that the molecular mass signal of 2 remained
unchanged in the course of these photochemical and thermal transformations.
Taken together these observations we concluded that the diastereomer ꢀE)-2
undergoes photodiastereomerization to ꢀZ)-2 in rather low quantum yield, resulting
in a photoequilibrium mixture which is mostly on the side of the ꢀE)-diastereomer.
The photodiastereomer ꢀZ)-2 easily reverts back thermally to the educt diastereo-
mer ꢀE)-2, which thus is obviously strongly favoured from the thermodynamical
point of view.
To account for the observed relative thermodynamic stabilities, the two diaste-
reomers of 2 were investigated using the AM1 method [5, 6]. This method has
proven to yield results with respect to the steric situation of phenanthroperylene
quinones which favourably agreed with experimental ®ndings [7]. Thus, it was
calculated that the ꢀE)-con®gured diastereomer of 2 is more stable by
27 kJ Á molÀ 1 than the corresponding ꢀZ)-diastereomer, thereby nicely corroborat-
ing the experimental results. The destabilization of the ꢀZ)-diastereomer was
derived to originate from a higher torsion of the single bonds joining the ethylene
double bond ꢀꢀhyp, ˆ ˆ 51ꢀ, ꢀbenz, ˆ ˆ 47ꢀ) due to a higher steric congestion as
compared to the ꢀE)-diastereomer ꢀꢀhyp, ˆ ˆ 55ꢀ, ꢀbenz, ˆ ˆ 56ꢀ).
The stilbenoid hypericin derivative ꢀE)-2 was quite effective in the hypericin
sensitized destruction of bilirubin, which has been established as a rapid means
to assess sensitized production of singlet oxygen or reactive oxygen species [8].
As can be derived from Fig. 1, the stilbenoid derivative 2 is nearly as effective
as hypericin ꢀ6) itself. Thus, the low rate of photodiastereomerization only
marginally interferes with the singlet oxygen sensitization due to the rather low
quantum yields of this photodiastereomerization process. These observations
are also in accordance with the ¯uorescence quantum yield of 2, which was
found to be in the same order of magnitude as that observed for hypericin ꢀ6).
In conclusion, it turned out that in stilbenoid hypericin derivatives photo-
diastereomerization is an optional photoreaction. However, due to its low dia-
stereomerization quantum yield and a photo and thermal equilibrium, which
resides mostly on the side of the ꢀE)-diastereomer, this photoreaction is only
of marginal importance in the photochemistry of stilbenoid hypericin derivatives
and thus does not appreciably interfere with the photoreactions important for

720
B. Lackner and H. Falk
Fig. 1. Hypericin derivative sensitized photooxidation of bilirubin IXꢁ: normalized absorption
ꢀA=A0) vs. time curves of solutions of disodium bilirubinate IXꢁ together with sodium hypericinate
ꢀ
ꢀ3À)6) and the sodium salt of the stilbenoid hypericin derivative ꢀ3À)2 in aereated 80% EtOH upon
irradiation at ꢂ>570 nm
photodynamic therapy. Accordingly, stilbenoid or vinylogous homologization
would still constitute a proper lead in search of improved hypericin derived
phototherapeutic agents. With respect to the dimethylaminobenzal derivative 1
mentioned in the introduction we might conclude that, besides a rather small
contribution from photodiastereomerization, a very effective process like intra-
molecular charge transfer in the excited state prohibits singlet oxygen sensitiza-
tion in this case.
Experimental
1
Solvents were of p.a. quality. H NMR, UV=Vis, ¯uorescence, and ES mass spectra were recorded
using Bruker DRX 500, Hewlett Packard 8453, Hitachi 4010F, and Hewlett Packard 59987 quadrupole
instruments. The ¯uorescence quantum yield was determined as described previously [9]. Hypericin
sensitized photooxidation of bilirubinate IXꢁ was executed as described in Ref. [8]. AM1 calculations
[5] were performed at the SGI Origin 2000 of the LIZENS using the MOPAC package [6]. The
anthrone derivatives 3 and 4 were prepared according to Refs. [1a] and [10].
ꢀE)-10-ꢀ2-phenylethen-1-yl)-1,3,4,6,8,13-hexahydroxy-11-methyl-phenanthro[1,10,9,8-
opqra]perylen-7,14-dione ꢀ2; C37H20O8)
A solution of 103 mg 3 ꢀ0.3mmol), 461 mg 4 ꢀ1.8 mmol), 30mg FeꢀSO4)2 Á 7H2O ꢀ0.1mmol), and
1.076 g pyridine-N-oxide ꢀ11.3 mmol) in 10.5cm3 absolute pyridine was stirred under Ar at 105ꢀC for
1 h. After cooling to room temperature the violet reaction mixture was poured on 75cm3 2M HCl and
stirred for 30 min. After centrifugation the residue was washed twice with 3% HCl, three times with
distilled H2O, and dried overnight in vacuo over P4O10 resulting in 976 mg of a mixture of the
protodimers and unidenti®ed contaminants. This mixture was dissolved under sonication in 12 dm3
acetone and irradiated for 15min by means of a 700W Hg high pressure lamp with ¯uorescence

Diastereomerization of Stilbenoid Hypericin Derivatives
721
screen ꢀPhilips) under stirring and air admission. After evaporation of the solvent the residue was
chromatographed on preparative silica TLC plates ꢀTHF:petrol ether:glacial acetic acidˆ 7:1:1) and
then on a Sephadex LH 20 column ꢀ35 Â 3 cm) using MeOH as the eluent.
1
Yield: 16mg ꢀ9%); m.p.: > 310ꢀC; H NMR ꢀ500 MHz, ꢃ, DMSO-d6): 18.47 ꢀs, OH-3=4), 14.78
ꢀs, OH-1,6 or OH-8,13), 14.17 ꢀs, OH-8,13 or OH-1,6), 8.02 ꢀs, H-9), 7.75 ꢀd, J ˆ 16.5Hz, CˆCH),
7.47 ꢀs, H-12), 7.44 ꢀt, J ˆ 7.3 Hz, MM0 part of AA0MM0X, Hph-3,5), 7.35 ꢀd, J ˆ 7.3 Hz, X-part of
AA0MM0X, Hph-4), 7.32 ꢀd, J ˆ 16.5 Hz, HCˆC), 7.32 ꢀd, J ˆ 7.6 Hz, AA0-part of AA0MM0X, Hph-
2,6), 6.60 ꢀs, H-2 or H-5), 6.59 AA0MM0X ꢀs, H-5 or H-2), 2.74 ꢀs, CH3) ppm; 1H NMR ꢀ500 MHz, ꢃ,
EtOH-d6): 7.82 ꢀs, H-9), 7.58 ꢀd, J ˆ 7.7 Hz, AA0-part of AA0MM0X, Hph-2,6), 7.55 ꢀd, J ˆ 16.3Hz,
CˆCH), 7.42 ꢀt, J ˆ 7.7 Hz, MM0-part of AA0MM0X, Hph-3,5), 7.33 ꢀd, J ˆ 8.6Hz, X-part of
AA0MM0X, Hph-4), 7.32 ꢀd, J ˆ 16.3 Hz, HCˆC), 7.32 ꢀs, H-12), 6.77 ꢀs, H-2 or H-5), 6.76 ꢀs, H-5
or H-2), 2.69 ꢀs, CH3) ppm; UV=Vis ꢀ80% EtOH): ꢂmax ˆ 609 ꢀ6500), 598 ꢀ6600), 565 ꢀ4000), 329
ꢀ7100), 287 ꢀ7490) nm ꢀ"); ¯uorescence ꢀ80% EtOH, ꢂexc ˆ 550 nm): ꢂem ˆ 617 nm, Ff ˆ 0.32; ES-
MS ꢀMeOH:H2O ˆ 4:1 ‡ 1% NH3, negativ e ion mode):m=z ˆ 591 ꢀ[M±H]À ).
Acknowledgments
We gratefully acknowledge the recording of mass spectra by Doz. Dr. C. Klamp¯.
References

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Received December 6, 2001. Accepted December 21, 2001