Formation mechanism of triketo cage compounds from reaction of phencyclone with benzoquinones: Cascade reaction of intermolecular [4+2]π and intramolecular [2+2]π cycloadditions

Article abstract of DOI:10.1248/cpb.c13-00240

The [4+2]π cycloadduct (3aa) of phencyclone (1a) and benzoquinone (2a) readily transformed into the corresponding [2+2]π cycloadduct (4aa) during purification. The structure of the triketo cage compound was determined by single crystal X-ray analysis. The

Full text of DOI:10.1248/cpb.c13-00240

October 2013
Note
Chem. Pharm. Bull. 61(10) 1065–1070 (2013)
1065
Formation Mechanism of Triketo Cage Compounds from Reaction of
Phencyclone with Benzoquinones: Cascade Reaction of Intermolecular
[4+2]π and Intramolecular [2+2]π Cycloadditions
Koki Yamaguchi, Masashi Eto,* and Kazunobu Harano
Faculty of Pharmaceutical Sciences, Sojo University; 4–22–1 Ikeda, Kumamoto 860–0082, Japan.
Received March 24, 2013; accepted July 27, 2013; advance publication released online August 3, 2013
The [4+2]π cycloadduct (3aa) of phencyclone (1a) and benzoquinone (2a) readily transformed into the
corresponding [2+2]π cycloadduct (4aa) during purification. The structure of the triketo cage compound
was determined by single crystal X-ray analysis. The structure–reactivity relationships are discussed based
on the PM6-calculated energy profiles for the whole photocycloaddition of the [4+2]π cycloadducts of some
cyclopentadienones and benzoquinones.
Key words photocycloaddition; cage compound; cyclopentadienone; benzoquinone; PM6 calculation
Cyclopentadienones are reactive and versatile dienes, whose treatments led to the formation of a compound 4aa, instead of
cascade reactions with various dienophiles such as non-con- the expected inclusion of a guest solvent molecule. The IR and
jugated dienes,1) conjugated polyenes,2,3) allyl alcohols,4) 2-al- 13C-NMR spectra of 4aa indicated the absence of enedione
kynyl alcohols,5) 2-alkynyl amines,6) and but-3-yn-2-ones,7) moiety. The formation of the 2:1 cycloadduct of 3aa via retro
have been extensively studied. A previous report showed that Diels‒Alder reaction was initially thought to occur on heating.
the endo[4+2]π cycloadduct of 2,5-bis(methoxycarbonyl)-3,4- The structure of the compound was determined by X-ray dif-
diphenylcyclopentadienone (1b) and benzoquinone (2a) in fraction analysis. The single crystal suitable for the analysis
benzene underwent photocyclization to yield an oxetane was obtained from CHCl3 by slow-evaporation. As shown in
derivative (5).8,9) On the other hand, irradiation of UV light Fig. 1, the newly formed compound (4aa) was the intramo-
on the powdered crystals of the [4+2]π cycloadduct gave the
intramolecular [2+2]π cycloadduct (triketo cage compound,
4ba) (Chart 1).
Our recent studies on the [4+2]π cycloadducts of phen-
cyclone (1a) as clathrate host10–13) showed that the [4+2]π
cycloadduct of 1a and 2a readily transformed into the cor-
responding [2+2]π cycloadduct during purification, despite
being thermally forbidden.14) Wooi and White reported that
the same reaction occurred in dull light.15) However, the
mechanistic details of the reaction is still unclear. Therefore,
we explored the reason for the difference in the reactivity of
several cyclopentadienone-[4+2]π cycloadducts to form the
[2+2]π cycloadduct.
In this study, we determine the structure of the cage com-
pound by X-ray crystallographic analysis and discuss a reac-
Chart 1
tion mechanism based on molecular orbital (MO) calculations.
Results and Discussion
The [4+2]π cycloadducts of 1a and quinones (2) were pre-
pared according to a previously reported method16) (Chart 2,
Table 1).
A mixture of 1a and 2a in toluene was heated at 80°C for
24h to give a 1:1 endo[4+2]π cycloadduct (endo-3aa, 99%).
Similar reactions between 1a and compounds 2b, 2c and 2e
afforded the corresponding endo[4+2]π cycloadducts (3ab,
3ac, 3ae) in moderate yield. In contrast, the reaction between
1a and 2d yielded 9,10-dibenzoylphenanthrene as the predom-
inant product, possibly derived from the [4+2]π cycloadduct of
1a and molecular oxygen.17)
Experiments to form an inclusion complex of the cycload-
duct 3aa were carried out with the following solvents: o-xy-
lene, m-xylene, p-xylene, and acetone. Interestingly, the above
The authors declare no conflict of interest.
Chart 2
© 2013 The Pharmaceutical Society of Japan
*To whom correspondence should be addressed. e-mail: meto@ph.sojo-u.ac.jp

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Vol. 61, No. 10
Table 1. [4+2]π Cycloaddition of Phencyclone (1a) with Quinones (2) Table 2. Intramolecular [2+2]π Photocycloaddition of 3 in CDCl3
in Toluene
Cycloadduct, 3
Time (h)
Yielda) of 4
Diene
Quinones
Temp. (°C)
Time (h)
Yield of 3
3aa
0.5
1.5
3
32 (4aa)
76 (4aa)
100 (4aa)
100 (4ab)
100 (4ac)
—
1a
2a
2b
2c
2d
2e
80
60
24
48
48
68
72
99 (3aa)
73 (3ab)
84 (3ac)
60
3ab
3ac
3ae
3ba
3
a)
60
—
4
105
95 (3ae)
4
4
—
a) 9,10-Dibenzoylphenanthrene was obtained (yield 67%).
a) Determined based on integrated intensity of the methine signals in 1H-NMR.
Fig. 2. Potential Energy Profile for the Intramolecular Photocycloaddi-
tion of 3
of 3aa–ac were completed within 4h and resulted in the for-
mation of the corresponding triketo cage compounds (4ab–ac)
(Table 2). The cycloadduct 3ae derived from the reaction of
1a and naphthoquinone (2e) was inert to irradiation by vis-
ible light, presumably owing to the aromaticity of the benzene
Fig. 1. Crystal Structure of 4aa·CHCl3 Determined by X-Ray Diffrac-
tion
Protons are omitted for clarity.
lecular [2+2]π cycloadduct: a triketo cage compound contain- ring.
ing a cyclobutane ring.18) This reaction is thermally inhibited
In stark contrast to 3aa, the [4+2]π cycloadduct derived
and results in the loss of resonance energy of phenanthrene from compounds 1b and 2a (3ba) was photochemically un-
moiety. The cage compound had highly strained σ-bonds reactive under the reaction conditions used.8,9) To understand
between the phenanthrene ring and the α carbon attached to the difference in the reactivity of 3aa and 3ba, we performed
the bridged carbonyl [C(4)–C(5) and C(7)–C(11) bonds]. The PM621) semi-empirical MO calculations on the energy profiles
bond lengths were significantly elongated to 1.608(5)Å and for photocycloaddition of the [4+2]π adducts. It is known that
1.609(6)Å. Density function theory (DFT) calculations19) at the photocycloaddition of enones to alkenes proceed via the
B3LYP/6-31G(d) level also showed bond elongation (1.620Å) triplet excited state of enones.22) The photoexcitation of an
for the bond in question, indicating that there existed steric enone followed by intersystem crossing, results in the lowest
repulsions between the face-to-face oriented aromatic rings.20) energy triplet state, which participates in the [2+2]π cycload-
The biphenyl moiety of dihydrophenanthrene is oriented at a dition reaction. Based on theoretical studies which have looked
dihedral angle [C(16)–C(17)–C(18)–C(19)] of −9.1(7)°, indicat- at the [2+2]π photocycloaddition of acrolein to ethylene,23) and
ing that the ring is considerably twisted. The structure was intramolecular [2+2]π cycloaddition of α,β-unsaturated fu-
found to be non-planar, and the interplanar angle between the ranones,24) we postulate the initial step in the reaction to be
two phenyl rings was 18.5°.
the interaction between a triplet state of enedione moiety and
Close inspection of the 1H-NMR spectrum of the crude a ground-state of alkene moiety. The possible pathway of the
3aa revealed the presence of small amounts of 4aa, indicating intramolecular photocycloaddition of enedione moiety to the
that the [2+2]π cycloaddition took place readily even under alkene moiety is shown in Fig. 2.
very mild reaction conditions and in the absence of artificial
We postulate that the photocycloaddition proceeds via a
1
light. H-NMR was used to monitor the reaction, and the con- singlet biradical intermediate.22) The reaction starts with the
centration of 4aa was found to increase over time. In control attack of the excited triplet state of an enedione (33) on a
experiments where the test tube was covered with aluminum ground-state alkene. The first step corresponds to bond forma-
foil, the cage compound was not formed even on heating. tion between the triplet state of enedione and the double bond
This observation suggested that the reaction was induced by of 3, and gives rise to a transition state (TS1). The biradical
natural light. In fact, the irradiation of 3aa with a fluorescent intermediate formed from TS1 either closes to form the tri-
lamp for indoor use (60W) in CDCl3 through a Pyrex filter keto cage compound (4) via a second transition state (TS2) or
afforded 4aa. To maintain the irradiation conditions of light, goes back to the starting material (3) via a different transition
the cycloaddition reaction of 3 was examined using a transpar- statre (TS3). The PM6-calculated potential energies and TS1
ency overhead projector (160W halogen lamp). The reactions geometries are summarized in Table 3 and Fig. 3, respectively.

October 2013
1067
Fig. 3. PM6-Calculated Structures for First Transition State (TS1)
Table 3. PM6-Calculated Potential Energies (ΔHf, kcal/mol)
Reaction
3
33
TS1
Biradical
86.8
TS2
TS3
4
3aa→4aa
50.2
95.7
(0.0)a)
111.4
(15.7)a)
86.9
93.1
40.6
[0.0]b)
[0.1]b)
[6.3]b)
c)
3ba→4ba
−154.4
−111.9
−92.4
−122.4
—
−110.5
−167.9
(0.0)a)
(19.5)a)
[0.0]b)
[11.9]b)
a) Energy barrier relative to triplet-state of 3 (33). b) Energy barrier relative to biradical. c) Stationary point was not obtained.
The energy barrier associated with the formation of TS2 was
much smaller than the barrier for the biradical reverting back
to starting material (3) via TS3. Therefore, the rate-determin-
ing step is formation of biradical intermediate through TS1.
The TS barrier of 3aa was considerably smaller (ca. 3.8 kcal/
mol) than that of 3ba.25) The difference in the reaction barri-
ers of 3aa and 3ba can be explained by an attractive CH/O
type interaction26–31) between the substituents in the transition
state. The distances between the hydrogen of the phenyl group
and the oxygen atom on the enone group of TS13aa are 2.242Å
and 2.342Å, respectively, indicating the presence of TS stabi-
lizing by CH/O interactions.
Applying frontier molecular orbitals (FMO)32,33) theory to
the [4+2]π cycloadducts provided a clue for understanding
the difference in the reactivity in the first step of intramo-
lecular cyclization. FMO calculations were performed using
the analogues of each reaction site: stilbene (A), phenan-
threne (B), and 1,4-enedione (C). As shown in Fig. 4, the
energy difference between lowest singly occupied molecu-
lar orbital (LSOMO)enedione and highest occupied molecular
orbital (HOMO)alkene was found to be smaller than that of
Fig. 4. PM6-Calculated FMO Energy Levels for Enedione Triplet and
Alkenes
the corresponding highest singly occupied molecular or- an approach of an enedione moiety. As a result, one of the
bital (HSOMO)enedione and lowest unoccupied molecular orbital carbon atoms of stilbene is closer to the oxygen atom of the
(LUMO)alkene, thus making the former an effective frontier enone (C···O, 3.136Å) than the carbon atom of the alkene
orbital interaction. Compared to the stilbene analogue (A), the (C···C, 4.085Å). Thus, the Paterno–Büchi reaction of 3ba,
HOMO of the phenanthrene analogue (B) was raised, bringing which yields an oxetane, is favourable. The distance between
3
it closer in energy to the LSOMO of enedione, resulting in interacting carbon atoms of 3aa is 0.95Å shorter than that of
favourable the orbital interaction for the cycloaddition.
33ba, which gave the [2+2]π cycloadduct only upon UV ir-
3
In addition to this, the degree of orbital overlapping be- radiation. In such the geometry, the orbital overlap of 3aa is
tween the reaction sites contributes to the reactivity. The large enough to undergo stepwise [2+2]π cycloaddition.
PM6-optimized triplet-state structure of 3aa showed that the
The UV-Vis spectrum of 3aa suggests the through-space
cyclohexene-1,4-dione moiety was bent considerably inwards interaction between the two π-systems. The absorption spec-
(below the phenanthrene ring), in comparison to the structure tra in CHCl3 showed a weak and broad absorption band (ca.
of 33ba34) (Fig. 5), where CH/O type interactions between 380–450nm) without a clearly defined absorption maximum
the carbonyl oxygen and 2′-hydrogen of the phenyl group (Fig. 6). On the other hand, 4aa, 2a, and 1a-norbornene
might be present. On the other hand, the two phenyl groups cycloadduct14) of having no enedione moiety did not show
3
(stilbene moiety) in 3ba were no longer planar and interfere any absorption band at the region. These results suggest that

1068
Vol. 61, No. 10
Fig. 5. PM6-Calculated Triplet-State Structures of 3aa and 3ba
Fig. 6. Absorption Spectra of 3aa, 4aa, 2a, and 1a-Norbornene Cycloadduct in CHCl3 (2.0 mmol/L)
the observed broad absorption in 3aa is due to intramolecu-
Table 4. X-Ray and Calculated Bond Lengths (Å) of 4aa
lar charge-transfer band arising from the interaction between
phenanthrene ring and enedione moiety.35)
Bond
X-Ray
B3LYP/6-31G(d)
In conclusion, the [4+2]π cycloadduct of phencyclone (1a)
and benzoquinone (2a) readily transformed into the corre-
sponding [2+2]π cycloadduct. The intramolecular cycloaddi-
tion reactivity depends on the energy of the first transition
state (TS1), and sterically favourable orbital interactions be-
tween the alkenes in the triplet-state.
C1–C2 (C8–C9)
C1–C5 (C8–C7)
C1–C8
1.517 (1.512)
1.547 (1.559)
1.592
1.528
1.556
1.599
1.518
1.570
1.585
1.620
1.576
1.538
C2–C3 (C9–C10)
C3–C4 (C10–C11)
C3–C10
1.508 (1.502)
1.579 (1.564)
1.565
C4–C5 (C11–C7)
C4–C11
1.608 (1.609)
1.563
Experimental
Melting points were uncorrected. The IR spectra were taken
C5–C6 (C7–C6)
1.516 (1.539)
1
with a Hitachi 270-30 spectrophotometer. H- and 13C-NMR
spectra were taken with JEOL JNM-A 500 (500MHz) spec-
trometers for ca.10% solution with tetramethylsilane (TMS) as
3aa: Yield 99%, pale yellow prisms; mp 263–266°C.
an internal standard; chemical shifts are expressed as δ values IR (KBr): 1788 (bridged >C=O), 1676 (enone >C=O)
1
and the coupling constants (J) are expressed in Hz. Mass cm−1. H-NMR (CDCl3) δ (ppm): 4.46 (s, 2H, CH), 5.75 (s,
spectra were obtained with JEOL JMS-T100LC Accu TOF 2H,=CH–), 7.08–7.66 (m, 14H, aromatic H), 8.29 (d, 2H,
and JMS-DX303HF instruments.
Materials Phencyclone (1) was prepared by the previously 13C-NMR (CDCl3) δ (ppm): 48.1 (CH), 65.6 (>C<), 123.2,
reported method.36)
126.0, 126.3, 126.7, 127.1, 128.2, 128.3, 128.9, 131.2, 133.3,
J=7.9Hz, aromatic H), 8.68 (d, 2H, J=8.5Hz, aromatic H);
Reaction of 1a with 2a (General Procedure) A solu- 134.0 (aromatic C), 141.5 (=CH–), 194.7 (enone >C=O), 198.1
tion of 0.38g (1.0mmol) of 1a in 4mL of toluene and 0.16g (bridge >C=O). m/z (electrospray ionization (ESI+)) high res-
(1.5mmol) of 2a was heated at 80°C for 24h under light- olution (HR)-MS Calcd for C35H22O3Na (M++Na): 513.14666.
shielded condition. The mixture was cooled and the solvent Found: 513.14793.
was removed under vacuum. The products separated were
recrystallized from acetone in darkness.
3ab: Yield 73%, pale yellow prisms; mp and decomp.
260–265°C. IR (KBr): 1791 (bridged >C=O), 1669 (enone

October 2013
1069
1
>C=O) cm−1. H-NMR (CDCl3) δ (ppm): 0.75 (s, 3H, CH3), 3H, CH3), 2.80 (d, 1H, J=3.4Hz, CH (cyclobutane)), 3.79 (dd,
4.53 (d, 1H, J=8.6Hz, CH), 4.57 (d, 1H, J=8.6Hz, CH), 5.50 1H, J=3.4, 10.3Hz, CH), 3.84 (d, 1H, J=10.3Hz, CH), 5.38 (d,
(s, 1H,=CH–), 7.12–7.66 (m, 14H, aromatic H), 8.34 (d, 2H, 1H, J=8.0Hz, aromatic H), 5.68 (d, 1H, J=7.5Hz, aromatic
J=6.8Hz, aromatic H), 8.68 (d, 2H, J=8.5Hz, aromatic H); H), 6.66 (t, 1H, J=8.1Hz, aromatic H), 6.69 (d, 1H, J=8.0Hz,
13C-NMR (CDCl3) δ (ppm): 15.5 (CH3), 47.8, 48.8 (CH), 65.7, aromatic H), 7.17–7.30 (m, 12H, aromatic H), 7.88 (d, 1H,
65.9 (>C<), 123.4, 123.5, 126.0, 126.1, 126.5, 126.8, 127.0, J=8.6Hz, aromatic H), 7.94 (d, 1H, J=8.6Hz, aromatic H);
127.3, 128.2, 128.3, 128.5, 128.6, 129.1, 131.2, 131.3, 133.6, 13C-NMR (CDCl3) δ (ppm): 15.1 (CH3), 49.4, 55.3, 56.1 (>C<
133.8, 134.3 (aromatic C), 139.0 (=CH–), 152.3 (=C–CH3), (cyclobutane)), 50.9, 52.0 (CH), 60.4, 61.1 (>C<), 61.2 (CH
194.3, 195.9 (enone >C=O), 198.5 (bridged >C=O). m/z (cyclobutane)), 122.9, 123.1, 126.6, 127.4, 128.0, 128.1, 128.5,
(ESI+) HR-MS Calcd for C36H24O3Na (M++Na): 527.16231. 128.7, 129.5, 130.1, 130.3, 131.5, 131.6, 131.7, 131.8, 132.6
Found: 527.16666.
(aromatic C), 205.3, 208.5, 210.0 (>C=O). m/z (ESI+) HR-MS
3ac: Yield 84%, pale yellow prisms; mp and decomp. Calcd for C37H28O4Na (M++CH3OH+Na): 559.18853. Found:
208–212°C. IR (KBr): 1785 (bridged >C=O), 1647 (enone 559.18839.
1
>C=O) cm−1. H-NMR (CDCl3) δ (ppm): 0.81 (s, 3H, CH3),
4ac: Colorless prisms; mp >280°C (from acetone). IR
1.91 (s, 3H, CH3), 4.11 (s, 1H, CH), 5.45 (s, 1H, =CH–), (KBr): 1775, 1735 (>C=O) cm−1. H-NMR (CDCl3) δ (ppm):
7.02–7.71 (m, 14H, aromatic H), 8.35 (d, 1H, J=7.5Hz, 1.16 (s, 3H, CH3), 1.50 (s, 3H, CH3), 2.80 (d, 1H, J=3.4Hz,
aromatic H), 8.68 (d, 3H, J=8.0Hz, aromatic H); 13C-NMR CH (cyclobutane)), 3.18 (d, 1H, J=3.4Hz, CH), 5.54 (d, 1H,
(CDCl3) δ (ppm): 15.8, 22.8 (CH3), 54.5 (–C–CH3), 59.7 (CH), J=8.0Hz, aromatic H), 5.69 (d, 1H, J=8.1Hz, aromatic H),
65.4, 67.5 (>C<), 123.2, 123.5, 124.9, 126.0, 126.6, 127.0, 6.52 (t, 1H, J=7.8Hz, aromatic H), 6.66 (t, 1H, J=8.1Hz,
127.3, 128.0, 128.3, 128.5, 128.7, 129.2, 129.5, 131.3, 132.6, aromatic H), 7.07–7.26 (m, 12H, aromatic H), 7.86 (d, 2H,
133.9 (aromatic C), 138.8 (=CH–), 152.4 (=C–CH3), 194.7, J=8.6Hz, aromatic H); 13C-NMR (CDCl3) δ (ppm): 15.3, 17.8
198.7 (enone >C=O), 199.0 (bridged >C=O). m/z (ESI+) (CH3), 49.5, 54.6, 55.4 (>C< (cyclobutane)), 54.9 (C–CH3),
HR-MS Calcd for C37H26O3Na (M++Na): 541.17796. Found: 61.1 (CH), 61.1 (CH (cyclobutane)), 61.6, 64.5 (>C<), 122.8,
1
541.18245.
123.0, 126.4, 127.3, 127.8, 128.1, 128.2, 128.4, 129.3, 130.5
3ae: Yield 95%, yellow prisms; mp 270–271°C. IR (KBr): (aromatic C), 205.2, 208.9, 212.3 (>C=O). m/z (ESI+) HR-MS
1794 (bridged >C=O), 1689 (enone >C=O) cm−1. H-NMR Calcd for C38H30O4Na (M++CH3OH+Na): 573.20418. Found:
1
(CDCl3) δ (ppm): 4.80 (s, 2H, CH), 6.66–6.71 (m, 2H, aro- 573.20794.
matic H), 6.73–6.78 (m, 2H, aromatic H), 7.13–7.74 (m, 14H,
Crystal Structure Analysis All measurements were
aromatic H), 8.35 (d, 2H, J=8.3Hz, aromatic H), 8.48 (d, 2H, performed on a RAXIS RAPID imaging plate area detector
J=7.9Hz, aromatic H); 13C-NMR (CDCl3) δ (ppm): 49.1 (CH), with graphite-monochromated MoKα radiation (λ=0.7107Å).
65.6 (>C<), 122.8, 124.4, 125.6, 126.4, 126.7, 126.9, 128.1, The structure was solved by direct method (SIR92), and all
128.2, 128.3, 129.0, 130.7, 131.1, 132.3, 133.5, 134.0, 136.3 hydrogen atoms were located at calculated positions. The
(aromatic C), 194.6 (enone >C=O), 198.5 (bridged >C=O). structure was refined by a full-matrix least-squares technique
Anal. Calcd for C39H24O3: C, 86.65; 4.47. Found: C, 86.66; H, using anisotropic thermal parameters for non-hydrogen atoms
4.49.
and a riding model for hydrogen atoms. All calculations were
1
The H- and 13C-NMR spectral data for 3aa and 13C-NMR performed using the crystallographic software package Crys-
spectral data for 3ae were identical to those reported by Wooi tal Structure.37) These X-ray crystallographic data have been
and White,15) whereas the H-NMR data of the aromatic sig- deposited at the Cambridge Crystallographic Data Centre
1
nals for 3ae appear to be partially different from our data (CCDC).
confirmed by the X-ray analysis.
Crystal data of 4aa: C35H22O3·CHCl3, M=609.93, monoclin-
Photocyclization of 3a (General Procedure) A CDCl3 ic, Space group P21/n, a=15.494 (1), b=10.0396 (7), c=18.422
(5mL) solution of 3aa (2mg) in NMR tube was irradiated (1)Å, β=99.787 (3)°, V=2823.9 (4)3, Dc=1.435gcm−3, Z=4,
with a 160W halogen lamp (using a transparency overhead R=0.049, Rw=0.093, CCDC ref. No. 929648.
projector) at room temperature. The solvent was removed
Molecular Orbital (MO) Calculations Semi-empirical
under reduce pressure and purified by recrystallization. The MO (PM6) and Complete Neglect of Differential Overlap/
yield was determined based on the integrated intensity of the Spectroscopic parameterization (CNDO/S) calculations were
1
methine signals of 3aa and 4aa in H-NMR. UV-Vis spectra run through Winmostar38) interface using MOPAC2009.21)
were taken with a Shimadzu UV-2500 spectrometer.
All transition-states were confirmed by the presence of one
4aa15): Colorless prisms; mp >280°C (from CHCl3). IR imaginary frequency. DFT calculations at B3LYP/6-31G(d)
1
(KBr): 1758, 1740 (>C=O) cm−1. H-NMR (CDCl3) δ (ppm): level were carried out using Gaussian09 program package.19)
3.30 (d, 2H, J=2.4Hz, CH (cyclobutane)), 3.81 (d, 2H, The calculation data (atomic coordinates) are available upon
J=2.4Hz, CH), 5.69 (d, 2H, J=7.3Hz, aromatic H), 6.67 (t, request (e-mail: meto@ph.sojo-u.ac.jp).
2H, J=7.3Hz, aromatic H), 6.92 (s, 4H, aromatic H), 7.17–7.29
(m, 8H, aromatic H), 7.88 (d, 2H, J=7.9Hz, aromatic H);
Acknowledgment We thank Mr. T. Miyagoe and Ms. K.
13C-NMR (CDCl3) δ (ppm): 51.5 (>C< (cyclobutane)), 52.1 Aizawa for experimental assistance.
(CH (cyclobutane)), 54.2 (CH), 61.2 (>C<), 122.9, 127.4,
128.1, 128.6, 129.0, 129.5, 129.7, 130.2, 131.5, 131.8 (aromatic
C), 204.7, 207.9 (bridge >C=O). m/z (ESI+) HR-MS Calcd for
C36H26O4Na (M++CH3OH+Na): 545.17288. Found: 545.17657.
4ab: Colorless prisms; mp >280°C (from CHCl3). IR (KBr):
References and Notes
1) Harano K., Uchida K., Izuma M., Aoki T., Eto M., Hisano T.,
Chem. Pharm. Bull., 36, 2312–2322 (1988).
2) Jikyo T., Eto M., Harano K., J. Chem. Soc., Perkin Trans. 1, 3463–
3470 (1998) and cited therein.
1
1778, 1751 (>C=O) cm−1. H-NMR (CDCl3) δ (ppm): 1.05 (s,

1070
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lengths of 4aa are listed below.
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of 3aa suggested the presence of
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a charge–transfer interaction. The calculation using the PM6-
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HOMO→LUMO transition at 351nm (oscillator strength: 0.03).
Similar calculation of exo-3aa isomer did not show the correspond-
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