A tolan substituted optically active spiropyran

Article abstract of DOI:10.1016/S0957-4166(99)00507-8

A substituted optically active spiropyran with a tolan group in the 6'- position is described. Both the parent spiropyran and its derivative were separated by chiral HPLC and characterized by CD spectroscopy. There is a 'diastereomeric switch' at temperat

Full text of DOI:10.1016/S0957-4166(99)00507-8

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 10 (1999) 4485–4494
A tolan substituted optically active spiropyran
Lutz Eggers and Volker Buß ∗
Fachgebiet Theoretische Chemie der Gerhard-Mercator-Universität Duisburg, Lotharstr. 1, D-47048 Duisburg, Germany
Received 15 June 1999; revised 10 November 1999; accepted 12 November 1999
Abstract
A substituted optically active spiropyran with a tolan group in the 60-position is described. Both the parent
spiropyran and its derivative were separated by chiral HPLC and characterized by CD spectroscopy. There is
a ‘diastereomeric switch’ at temperatures below −30°C , but the compound does not show liquid crystalline
properties. © 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction
The development of technologies for the non-destructive readout of photochromic bistable molecu-
les — one of the requirements for their use as rewritable optical memory devices1 — continues to com-
mand attention in materials research. One possible approach is the use of optically active molecules with
switchable optical activity, so-called chiroptical switches,2 another is the use of achiral photochromic
compounds to control the pitch or phase of a liquid crystal.3 Optically active molecules have also
been used to switch between liquid crystalline phases.2 Light of two different wavelengths is used to
write and erase the information, whereas the information is read with light of a wavelength that does
not affect the organic compound photochemically. Recently, we demonstrated that the optically active
spiroindolinopyran 1 is capable of functioning as a chiroptical molecular switch (Scheme 1). The UV/vis
induced photochromic switch between the closed spiro and the open merocyanine form was followed
by CD (circular dichroism) spectroscopy. In addition, we found a second switch probably based on
the temperature-dependent inversion of the diastereomeric composition (‘diastereomeric switch’). The
information is photochemically permanent but can be erased simply by heating. The disadvantage for
this switching is the limitation to temperatures below −40°C.
The combination of photochromic and liquid crystalline properties in one compound is of great
technical interest, e.g. in display technology.4 The synthesis of a copolymer with spiroindolinopyran
and mesogenic side groups gave a light switchable liquid crystal, which, however, operated only
∗
Corresponding author. E-mail: theobuss@uni-duisburg.de
0957-4166/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(99)00507-8
tetasy 3142

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L. Eggers, V. Buß / Tetrahedron: Asymmetry 10 (1999) 4485–4494
Scheme 1.
below −10°C or required temperature change.4,5 Attempts to attach mesogenic groups directly to a
spiroindolinopyran resulted in a material which exhibited some features of liquid crystals but did not
show photochromism (‘quasi-liquid crystals’).3,6
In the following we describe the synthesis of 2, a compound with a mesogenic group attached to the
benzopyran moiety of the spiropyran, as a first attempt to raise the temperature of the diastereomeric
switch to ambient temperature and to improve the photostability of the spiropyran. Also, we describe
the preparation and the chiroptical properties of the diastereomers resulting from the ring closure of the
optically active spiropyrans 1 and 2.
2. Results and discussion
2.1. Synthesis of the optically active spiropyran derivative 2
The tolan group is a simple but convenient mesogenic group. As R3 in 2, the phenylethynyl moiety
becomes part of the benzopyran chromophore of the spiropyran molecule, with possible steric and
electronic effects. The key intermediate for the synthesis of 2 is the tolanaldehyde 6, which, after
condensation with the suitable Fischer base, should give the desired product (Scheme 2). Compound
6 was obtained by starting from the salicylaldehyde 3, which was tosylated and then reacted with
phenylethyne in triethylamine in the presence of CuI and PdCl2(PPh3)2 (Hagihara variant7 of the Heck
reaction8) to give 5 in 86% yield (Scheme 3). Detosylation was effected in low yield (32%) with
potassium hydroxide in boiling aqueous ethanol.
Scheme 2.

L. Eggers, V. Buß / Tetrahedron: Asymmetry 10 (1999) 4485–4494
4487
Scheme 3.
To test the feasability of the condensation reaction 6 was reacted with the commercially available
achiral Fischer base 7. In order to achieve complete conversion, an excess of the base was employed,
1
but the result was, not unexpectedly,4 the bicondensation product 8 (identified by H NMR and mass
spectroscopy). The formation of this product is favored when substituents render the pyran electron-
rich. However, the second Fischer base could be cleaved off and removed by distillation at 150°C under
reduced pressure to yield the simple spiropyran 9 (Scheme 4).
Scheme 4.
Based on this result the successful synthesis of the chiral spiropyran 2 was effected in 76% yield by
condensing a 1.37-fold molar excess of the tolanaldehyde 6 with the chiral Fischer base 10.
2.2. HPLC analysis and spectroscopic (CD) assignments
In the closed spiro form 2, as with the parent compound 1, there is a mixture of diastereomers: with the
configuration at C(3) of the starting Fischer base known to be S (R in the case of 1) the new stereogenic
center C(2) can be either R or S. These diastereomers equilibrate via the open merocyanine form of the
spiropyran, which is accessible photochemically. HPLC separation was achieved using a Chiralpak OT+
column. Irradiation with visible light before separation effected maximum conversion into the closed
spiro form. On the chiral column, rather than two peaks being detected, two pairs of peaks, two intense
ones and two less intense, were observed. The ee of the chiral Fischer base 10 used for the synthesis of 1

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L. Eggers, V. Buß / Tetrahedron: Asymmetry 10 (1999) 4485–4494
and 2 was 80%,9 and these four peaks represent all possible stereoisomers. In n-hexane:i-propanol, 9:1,
the diastereomeric equilibrium at 5°C is close to unity as determined from the relative areas of the two
intense peaks. This equilibrium is strongly solvent dependent: in CDCl3 it is 1:1.6 for 1 and 1:1.7 for 2 at
24°C determined by 1H NMR. The two fractions corresponding to the two intense peaks were collected
and the CD spectra obtained. Fig. 1 shows the spectra of 1, Fig. 2 those of 2. The spectra are very similar,
displaying a prominent band at 255 nm and a second one of lower intensity and opposite sign at 300 nm.
Since the fractions shown in Figs. 1 and 2 are diastereomers rather than enantiomers, their spectra are
not mirror-images, although they very nearly are. The contribution of the different alkyl groups at the
stereogenic center C(3) to the spectra is very small, as was tested for the chiral Fischer bases 10 which
show Cotton effects ∆ε of ±1. On the basis of NOE data, the configuration of 1 has been determined to
be (2R,3R) for the first fraction with the positive CD at 255 nm and (2S,3R) for the second fraction.9 In
accord with these data, the first eluting fraction of 2 has a negative CD at 255 nm and its configuration
should be (2S,3S) while that of the second fraction is positive and the configuration (2R,3S).
Fig. 1. CD spectra of HPLC-separated diastereomers of the optically active spiropyran 1 in hexane:2-propanol, 9:1, at 5°C;
( — ): (2R,3R)-1, c=3.07×10−5 M; (- - -): (2S,3R)-1, c=3.3×10−5
M
With the CD spectra of the diastereomers with 100% de known, the composition of any diastereo-
meric mixture resulting from photochemical interconversion can be calculated if the ee is 100%, and
contamination by the open form does not affect the CD spectra too much.
2.3. Photochemical equilibration
Irradiation of both fractions with UV produces the open merocyanine form in which the stereogenic
spirocenter is lost (Scheme 1). Upon irradiation with visible light the merocyanine is closed back to the
spiro form, and the stereogenic center at C(3) is formed again. In contrast to 110 the open merocyanine
form of 2 cannot be detected at room temperature even after prolonged irradiation with UV light. The

L. Eggers, V. Buß / Tetrahedron: Asymmetry 10 (1999) 4485–4494
4489
Fig. 2. CD spectra of HPLC-separated diastereomers of the optically active spiropyran 2 in hexane:2-propanol, 9:1, at 20°C;
( — ): (2S,3S)-2, c=3.04×10−5 M; (- - -): (2R,3S)-2, c=2.92×10−5
M
spiro form is favored under these conditions which is well known from similar compounds without a nitro
group,4 and equilibration occurs too fast for us to detect (Fig. 3, solid line). The CD spectrum reflects the
diastereomeric composition of 2 after photochemical equilibration at this temperature (Fig. 4, solid line).
The small Cotton effects observed are a measure of the small diastereomeric excess remaining under these
conditions. However, the positive peak at 250 nm followed by a smaller negative one at 255 nm is proof
that the (2R,3S) diastereomer is the preferred one (in contrast, a negative peak was followed by a smaller
positive one in the case of 1, indicating that for that compound the (2S,3R) diastereomer is prevalent).
According to high-quality ab initio calculations at the RHF/6-31-G** level with complete geometry
optimization and second derivatives for the energy minima11 the energy difference between (2S,3R)-1
and the (2R,3R) epimer is 0.494 kcal/mol in favor of the former, which agrees with our configurational
assignment, at least at this temperature.
We have recently described the temperature dependent UV/vis and CD spectra of 1.10 The existence
of an isosbestic point and theoretical calculations lead us to conclude that the change, which is observed
in the temperature dependent CD spectrum after irradiation with UV or visible light, is due to a change
in the diastereomeric composition of 1.
Fig. 3 shows what happens when the sample is irradiated at −30°C. Only a small amount of the
merocyanine is formed after UV irradiation for 45 min (dotted line). The long wavelength maximum
occurs around 590 nm, which is a bathochromic shift of 60 nm compared with 1: a consequence of the
more extended π-system. The spiro form is not completely restored by visible light at this temperature
(dashed line). The CD spectra taken under identical conditions are shown in Fig. 4. The increase of the
negative band at 255 nm at the expense of the positive one at 250 nm (dotted line) indicates that UV light
at −30°C effects an inversion of the diastereomeric excess from (2R,3S) to (2S,3S). After irradiation
with visible light (dashed line) the CD amplitudes decrease indicating that the equilibrium between the
diastereomers is now closer to 1. Repeating the irradiation with UV restores the dotted spectrum with

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L. Eggers, V. Buß / Tetrahedron: Asymmetry 10 (1999) 4485–4494
Fig. 3. UV/vis spectra of 2 in methanol:ethanol, 1:4, c=3.3.10−5 M, cooled in the dark from room temperature to −30°C ( — );
after UV irradiation for 45 min (···) and after vis irradiation for 20 min (- - -)
Fig. 4. CD spectra of 2 in methanol:ethanol, 1:4, c=3.3×10−5 M, cooled in the dark from room temperature to −30°C ( — );
after UV irradiation for 45 min (···) and after vis irradiation for 20 min (- - -)

L. Eggers, V. Buß / Tetrahedron: Asymmetry 10 (1999) 4485–4494
4491
the strong negative amplitude; the effect of visible light on this spectrum is now almost nil and is only a
manifestation of the equilibrium between the closed and the open forms.
For an explanation we assume the existence of two different processes: photochemical interconversion
between the closed and the open spiropyran and thermal isomerization of the open merocyanine, a view
which is supported by the recent ab initio calculations by Robb et al.12 According to these calculations
the photochemically generated cis-configured open spiropyran relaxes via a conical intersection into the
ground state from where it can either return to the closed form or undergo bond isomerization to the
trans-merocyanine. This latter step is the necessary prerequisite for inversion of the configuration at
C(2). For the −30°C equilibrium to be established every molecule must have had a chance to react via
this latter path. The initial observation of an intermediate spectrum at −30°C is thus a direct indication
of these two competing processes. Once the equilibrium between the open cis-configured merocyanines
has been established changes of the diastereomeric composition upon excitation with UV or visible light
are no longer observed.
In a preliminary investigation, no liquid crystalline properties were detected for the chiral spiropyran
2. The introduction of the phenylacetylene group did not improve the suitability of spiropyrans for optical
data storage, although it was possible to raise the temperature necessary for the diastereomeric switch of
2, somewhat, to −30°C.
3. Experimental
3.1. General
Mp: Thermovar (Reichert, Wien). MS: AMD 604 (AMD Intectra, Harpstedt/Germany) at 70 eV (EI).
1
Elemental analysis: Carlo Erba 1106. IR spectra: Perkin–Elmer 1320. H and 13C NMR: Bruker DRX
500 or AM 300 spectrometer, TMS, chemical shifts (δ) in ppm, coupling constants J in hertz, carbon
substitution by DEPT-135° and DEPT-90°. For the non-IUPAC numbering of the spiropyrans — to make
comparison with the parent indole base easier — see the formulas above. UV/vis spectra: Perkin–Elmer
Lambda 5. CD spectra: 62 A DS (Aviv, USA), equipped with a liquid nitrogen Cryostat DN 1714,
static (Oxford Instruments, UK). Solvents: spectrograde (Uvasole, Merck, Darmstadt). Column chro-
matography: silica gel 60 (70–230 mesh; Merck, art. 7734).
3.2. 5-Bromo-2-(p-toluenesulfonyloxy)benzaldehyde 4
A mixture of 5 g (24.8 mmol) 2-hydroxy-5-bromobenzaldehyde 3 and 5.72 g (30 mmol) p-
toluenesulfonyl chloride in 60 mL anhydr. pyridine was heated to 100°C for 2 h. The solution was
stirred overnight at room temperature and then poured into ice/water (200 mL). The aqueous phase
was extracted with CHCl3 (2×50 mL). The organic layer was washed with dilute aq. HCl, NaHCO3
solution, and brine, dried over CaCl2, and evaporated. The residue was purified on silica gel by column
chromatography (cyclohexane:EtOAc, 3:1; Rf=0.38) to give a yellow oil of 4; yield: 5.85 g (66.5%). 1H
NMR (CDCl3, 300 MHz): δ=2.47 (s, 3H, CH3), 7.09 (d, 3J=8.7 Hz, 1H), 7.34–7.37 (m, 2H), 7.66–7.72
(m, 3H), 7.97 (d, 4J=2.5 Hz, 1H), 9.90 (s, 1H, CHO); MS (relative intensity, %): m/z=356 (8.7, M+ 81Br),
354 (8.5, M+ 79Br), 201 (9.3), 155 (76.5), 139 (6.6), 91 (100), 65 (19.6), 39 (7.1).

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L. Eggers, V. Buß / Tetrahedron: Asymmetry 10 (1999) 4485–4494
3.3. 2-(p-Toluenesulfonyloxy)-5-(phenylethynyl)benzaldehyde 5
Aldehyde 4 (1.6 g, 2.2 mmol) and 0.88 mL phenylethyne (816 mg, 4 mmol) were dissolved in 100 mL
anhyd. triethylamine and 40 mL anhydr. THF. To this mixture 20 mg CuI, 20 mg PdCl2(PPh3)2 and 40 mg
PPh3 were added, stirred and heated under reflux for 3 h and at 20°C for 16 h. A precipitate was filtered off
and the solution evaporated under reduced pressure. Chromatography on silica gel (cyclohexane/EtOAC
3:1; Rf=0.49) gave a yellowish viscous oil: yield: 1.42 g (86%). 1H NMR (CDCl3, 500 MHz): δ=2.46 (s,
3H, H-22), 7.20 (dd, 3J=8.5, 5J=0.3 Hz, 1H, 3-H), 7.33–7.37 (m, 5H, H-12, 13, 14, 18, 20), 7.50–7.53
(m, 2H, H-11, 15), 7.68–7.72 (m, 3H, H-4, 17, 21), 7.99 (dd, 4J=2.2, 5J=0.3 Hz, 1H, 6-H), 9.96 (s, 1H,
H-7); 13C NMR (CDCl3, 125 MHz): δ=21.8 ppm (-CH3), 86.9 (C^C), 91.7 (C^ C), 122.3 (q), 123.3
(q), 124.0, 128.5, 128.6 (q), 128.9, 130.2, 131.5 (q), 131.70, 131.73, 137.8, 146.5 (q), 150.4 (q), 186.6
(-CHO); MS (%): m/z=376 (61, M+), 221 (100), 193 (22), 165 (35), 155 (29), 91 (53); anal. calcd for
C22H16O4S: C, 70.20; H, 4.28; found: C, 70.21; H, 4.34.
3.4. 2-Hydroxy-5-(phenylethynyl)benzaldehyde 6
A solution of potassium hydroxide (6 g) in water (100 mL) and ethanol (100 mL) was prepared. The
alkaline solution was added in three 50 mL portions at 15 min intervals to the tosylated tolan 5 and stirred
at 20°C. After refluxing for 1 h, the solution was cooled, neutralized with acetic acid and concentrated to
half the original volume at 50°C under reduced pressure. A yellow precipitate was sucked off and washed
with diethylether. The filtrate was extracted three times with diethylether. The combined organic extracts
were washed with aqueous NaHCO3, dried (Na2SO4), filtered, and the solvent removed. The residue was
recrystallized from ethanol/water and filtered hot to separate insoluble particles. The resulting yellow
precipitate was washed with methanol and dried in vacuo in a desiccator; mp=79–80°C ; yield: 0.52 g
(32%); 1H NMR (CDCl3, 500 MHz): δ=2.87 (s [br], 1H, OH), 7.04 (d, 3J=8.6 Hz, 1H, H-3), 7.40–7.44
3
4
(m, 3H, H-12, 13, 14), 7.53–7.56 (m, 2H, H-11, 15), 7.74 (dd, J=8.6, J=2.2 Hz, 1H, H-4), 7.98 (dd,
4
4
4J=2.2, J=0.4 Hz, 1H, H-6), 10.08 (d, J=0.4 Hz, 1H, CHO); 13C NMR (CDCl3, 125 MHz): δ=88.6
ppm (C-8), 89.3 (C-9), 115.8 (C-5), 118.8 (C-3), 122.1 (C-1), 124.0 (C-10), 129.5 (C-13), 129.6 (C-
12/14), 132.3 (C-11/15), 137.7 (C-6), 140.3 (C-4), 162.2 (C-2), 197.5 (C-7); MS (%): m/z=222 (100,
M+), 221 (29), 178 (10), 165 (26), 163 (11), 88 (9); anal. calcd for C15H10O2: C, 81.07; H, 4.54; found:
C, 81.07; H, 4.54.
3.5. 1,3,3-Trimethyl-60-(phenylethynyl)-spiro-(2H-10-benzopyran-2,20-indoline) 9
To a solution of 200 mg (0.9 mmol) of the salicylaldehyde 6 and 0.4 mL triethylamine in 15 mL
anhyd. methanol, 210 mL (1.2 mmol) Fischer base 7 were added with stirring under N2. The mixture
was heated to reflux for 5 h. After cooling a yellow-green precipitate was filtered off and recrystallized
1
from methanol and a few mL CHCl3 to yield 180 mg. [The H NMR spectrum revealed that this was
not compound 9 but the bicondensation product 8; mp 192–194°C dec.; MS (FD): m/z=505.4 (100).] A
Kugelrohr distillation at 150–200°C under reduced pressure was performed with this product; the Fischer
base was distilled off and 2 remained as a tarry blue residue. Yield: 100 mg (29%); UV (EtOH:MeOH,
4:1): 298 nm (ε=29010), 268 (32060), 259.5 (30300), 248.5 (31600), 222 (29042); 1H NMR (300 MHz,
3
CDCl3): δ=1.16 (s, 3H, -CH3), 1.30 (s, 3H, -CH3), 2.72 (s, 3H, N-CH3), 5.71 (d, J=10.2 Hz, 1H, H-
3
3
30), 6.52 (d, J=7.7 Hz, 1H, H-7), 6.67 (d, J=8.1 Hz, 1H, H-80), 6.80–6.87 (m, 2H, H-5, H-40), 7.07
3
4
3
4
(dd, J=7.2, J=0.7 Hz, 1H, H-4), 7.16 (‘dd’, J=7.7, J=1.2 Hz, 1H, H-6), 7.20–7.45 (m, 5H, H-50,
70, 130, 140, 150), 7.47–7.51 (m, 2H, H-120, H-160); 13C NMR (125 MHz, CDCl3): δ=20.1 ppm (C-9),

L. Eggers, V. Buß / Tetrahedron: Asymmetry 10 (1999) 4485–4494
4493
25.9 (C-10), 28.9 (C-8), 51.8 (C-3), 87.8 (C-90), 89.3 (C-100), 104.7 (C-2), 106.8 (C-7), 114.7 (C-60),
115.3 (C-80), 118.9 (C-4a0), 119.2 (C-5), 120.2 (C-30), 121.5 (C-6), 123.6 (C-110), 127.6 (C-4), 127.9
(C-140), 128.3 (C-120/160), 128.9 (C-40), 130.0 (C-50), 131.4 (C-130/150), 133.2 (C-70), 136.6 (C-3a),
148.1 (C-7a), 154.7 (C-8a0); MS (%): m/z=377 (64, M+), 362 (11), 173 (33, C12H15N), 159 (100), 144
(13), 163 (11), 88 (9).
3.6. (3S)-1,3-Dimethyl-3-propyl-60-(phenylethynyl)-spiro-(2H-10-benzopyran-2,20-indoline) 2
Salicylaldehyde 6 (250 mg, 1.1 mmol) was dissolved in 10 mL of abs. methanol and 0.5 mL of
triethylamine under nitrogen. A solution of (3S)-(−)-1,3-dimethyl-2-methylene-3-propylindoline 10 (180
mg, 0.8 mmol) in 5 mL of anhyd. methanol was prepared. This solution was added in one portion to the
benzaldehyde and heated to reflux for 4 h. After cooling to room temperature the solvent was removed
and the residue recrystallized from ethanol/water. The precipitate was dissolved in ca. 5 mL diethyl ether
and allowed to stand at 4°C for 1 day. The precipitate (educt 6) was filtered off, the filtrate evaporated
and dried under reduced pressure at 65°C. The blue residue was recrystallized from ethanol to yield 250
mg (76%) 2: mp 42–43°C ; UV (n-hexane:2-propanol, 9:1): 313.5 nm (ε=25700), 297.5 (30700), 267.5
(34500), 259 (33100), 248.5 (35200); 1H NMR (500 MHz, CDCl3): δ=0.75 (t, 3H, H-12*), 0.80 (t, 3H,
H-12), 1.19–1.25 (m, 3H, H-10a, 11), 1.21 (s, 3H, H-9*), 1.27 (s, 3H, H-9), 1.41–1.49 (m, 3H, H-10b,
H-11*), 1.73–1.79 (m, 1H, H-10a*), 1.86–1.91 (m, 1H, H-10b*), 2.64 (s, 3H, H-8*), 2.73 (s, 3H, H-8),
5.75 (d, 3J=10.2 Hz, 1H, H-30), 5.77 (d, 3J=10.2 Hz, 1H, H-30*), 6.49 (d, 3J=7.6 Hz, 1H, H-7*), 6.53
(d, 3J=7.6 Hz, 1H, H-7), 6.67 (d, 3J=8.3 Hz, 1H, H-80), 6.70 (d, 3J=8.3 Hz, 1H, H-80*), 6.78–6.85 (m,
4H, H-5, H-40, H-5*, H-40*), 7.02 (dd, 3J=7.2, 4J=1.2 Hz, 1H, H-4), 7.09 (dd, 3J=7.3, 4J=1.2 Hz, 1H,
H-4*), 7.15–7.20 (m, 2H, H-6, H-6*), 7.23–7.36 (m, 10H, H-50, 70, 130, 140, 150, H-50*, 70*, 130*, 140*,
150*), 7.48–7.53 (m, 4H, H-120, 160, H-120*, 160*) [*, minor compound, integral=1; major compound,
integral=1.7]; 13C NMR (125 MHz, CDCl3): δ=14.61 ppm (CH3), 15.13 ppm (CH3), 17.56 (CH2), 17.62
(CH3), 17.71 (CH2), 22.93 (CH3), 28.18 (N-CH3), 29.06 (N-CH3), 36.97 (CH2), 38.94 (CH2), 53.63 (C-
3*), 55.70 (C-3), 87.81 (C^C), 87.86 (C^C), 89.28 (C^C), 89.29 (C^C), 103.48 (C-2*), 105.58 (C-2),
106.81 (C-7*), 106.82 (C-7), 114.61 (C-60), 114.62 (C-60*), 115.22 (C-80*), 115.28 (C-80), 118.65 (C-
4a0*), 118.71 (C-5), 118.94 (C-4a0), 119.15 (C-5*), 120.38, 120.77, 121.74, 123.19, 123.58 (C-110*),
123.61 (C-110), 127.57, 127.62, 127.90, 127.92, 128.30, 128.42, 128.86, 129.22, 130.05, 131.43, 131.52,
133.21, 134.36 (C-3a), 136.87, 137.06 (C-3a*), 139.83, 147.53 (C-7a*), 148.81 (C-7a), 154.25 (C-8a0*),
154.83 (C-8a0) [*, minor compound]; MS (EI): m/z=405 (100, M+), 376 (15), 363 (33), 362 (32), 347
(16), 334 (8), 222 (22), 187 (78), 165 (7), 159 (18), 158 (63), 144 (7); HRMS: calcd for C29H27NO:
405.20927; found: 405.21093.
Acknowledgements
This work was financially supported in part by the DFG (Project Bu 419/16-2) and by the University
of Duisburg. Cand. chem. Marko Schreiber provided valuable assistance.
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