Special reactions of α,β-unsaturated ketones III [1]. Formation and structure elucidation of dimers of 2-aryl-methyleneketones: (5E)-5-benzylidene-9β-phenyl-trans-4a-1,2,3,4,4a,5,6,7,8,9a- decahydroxanthen-4aα-ol and 3β,5β-bis-(4-methoxyphenyl)-2α,4β,6α-t

Article abstract of DOI:10.1007/s007060170049

The reaction of 2-benzylidenecyclohexanone and 1-(4-methoxyphenyl)-2-methyl-1-penten-3-one with guanidine did not yield the expected 4-phenylhexahydro-2-quinazolinamine and 4-(p-methoxyphenyl)-dihydro-2-pyrimidinamine, respectively, but nitrogen-free prod

Full text of DOI:10.1007/s007060170049

Monatshefte fuÈr Chemie 132, 1081±1093 (2001)
Special Reactions of a,b-Unsaturated
Ketones III [1]. Formation and Structure
Elucidation of Dimers of 2-Aryl-
methyleneketones: (5E)-5-Benzylidene-
9b-phenyl-trans-4a-1,2,3,4,4a,5,6,7,8,9a-
decahydroxanthen-4aa-ol and 3b,5b-Bis-
(4-methoxyphenyl)-2a,4b,6a-trimethyl-
4a-propionyl-cyclohexanone [2]
È
Edith Goûnitzer*, Winfried Wendelin, Karl Schermanz,
and Karin Frieberger
Institute of Pharmaceutical Chemistry and Pharmaceutical Technology, Karl-Franzens-University
Graz, A-8010 Graz, Austria
Summary. The reaction of 2-benzylidenecyclohexanone and 1-(4-methoxyphenyl)-2-methyl-1-
penten-3-one with guanidine did not yield the expected 4-phenylhexahydro-2-quinazolinamine and
4-(p-methoxyphenyl)-dihydro-2-pyrimidinamine, respectively, but nitrogen-free products which
turned out as [3‡3]- and [4‡2]-cycloadducts of two molecules of the applied vinylogous ketone
each. According to elemental analyses, mass spectra, and, in particular, NMR analyses (1H and
13C NMR, HH-COSY, gs-HSQC, gs-HMBC, 1D TOCSY, NOESY, and 1D NOE difference spectra),
the prepared dimers were identi®ed as racemic (5E)-5-benzylidene-9ꢀ-phenyl-trans-4a-1,2,3,4,4a,5,
6,7,8,9a-decahydroxanthen-4aꢁ-ol and 3ꢀ,5ꢀ-bis-(4-methoxyphenyl)-2ꢁ,4ꢀ,6ꢁ-trimethyl-4ꢁ-
propionylcyclohexanone, respectively. Structure and stereochemistry of the dimers are elucidated,
and mechanisms for their formation are proposed.
Keywords. Decahydroxanthen-4a-ol, 5-benzylidene-9-phenyl-trans-4a-; Cyclohexanone, 3,5-bis-(4-
methoxyphenyl)-2,4,6-trimethyl-4-propionyl; Dimers of 2-benzylidene ketones; NMR spectroscopy;
Structure elucidation.
Introduction
As previously reported, dihydropyrimidinamines 3 and fused derivatives, e.g. hexa-
hydroquinazolinamines 4, tetrahydropyrimidopyrimidines 5, and pyrimidobenz-
Ã
Corresponding author. E-mail: edith.goessnitzer@uni-graz.ac.at

È
E. Goûnitzer et al.
1082
Scheme 1
imidazoles 6, can be prepared by reaction of acyclic and bridged ꢁ,ꢀ-unsaturated
ketones 1 with guanidines and bridged guanidines 2, respectively [3±8].
In some special cases, the vinylogous ketones 1 did not react with the
guanidines 2 but with a second molecule of enone 1 to yield nitrogen-free products
which turned out to be dimers of the employed ketones 1 [1, 8]. In this paper
we report on the dimerization reactions of 2-benzylidenecyclohexanone (1a) and
1-(4-methoxyphenyl)-2-methyl-1-penten-3-one (1b), which afforded 5-benzyli-
dene-9-phenyldecahydroxanthen-4a-ol (7a) and 3,5-bis-(4-methoxy-phenyl)-2,4,6-
trimethyl-4-propionylcyclohexanone (12b). The structure elucidations of 7a and
12b were accomplished mainly by means of one- and two-dimensional NMR
spectroscopy.
Results and Discussion
Formation and structure elucidation of decahydroxanthenol 7a
Heating of 2-benzylidenecyclohexanone (1a) with an excess of guanidine in
ethanol yielded 4-phenylhexahydro-2-quinazolinamine 4a [4]. In contrast, in
boiling methanol under the in¯uence of guanidine enone 1a was transformed into a
nitrogen-free product in a yield of 41%. According to the following structure
analysis, this product represents 5-benzylidene-9-phenyldecahydroxanthen-4a-ol
(7a).
Scheme 2
Elemental analysis and the mass spectrum of condensate 7a revealed a
molecular formula of C26H28O2 (m/z ˆ 372), pointing towards a dimer of the starting
material generated by reaction of two molecules of 1a without loss of water.
Inspection of the IR spectrum of 7a showed no absorption bands for CO
1
groups, but OH-stretching frequencies at ꢂ ˆ 3480 cm . Thus, the primarily

Dimers of 2-Arylmethyleneketones
1083
Scheme 3
expected structures of Diels-Alder dimers 8, 9, both spiro compounds, and also of a
Michael adduct 10 (Scheme 3) had to be ruled out. However, an absorption band at
1
ꢂ ˆ 1630 cm pointed towards the existence of a conjugated enol ether, which
guided from dione 10 to xanthenol 7a and was not consistent with the isomeric
structure 11 (the IR absorption of the non-conjugated enol ether moiety of 11 is
1
expected at ꢂ > 1650 cm [9]).
The ®nal proof of the structure of 7a was obtained from NMR spectroscopic
analysis. The complete assignment of all proton and carbon signals was carried out
on the basis of one- and two-dimensional techniques (1H and 13C, HH-COSY,
NOESY, gs-HSQC, gs-HMBC, 1D TOCSY, 1D NOE).
The 13C spectrum of the dimer 7a showed only 19 signals instead of 22 which
were expected due to the symmetry of the two monosubstituted phenyl groups.
This resulted from the accidental isochrony of additional two carbon atoms of the
decahydroxanthene moiety (C-2 and C-7) and of the para and meta carbons of
the two phenyl groups (C-14/16 and C-15; C-20/22 and C-21) each (Scheme 4).
The evaluation of the gs-HSQC contour plots allowed the assignment of all proton-
bearing carbons as listed in Table 1. Additionally, it con®rmed the ole®nic nature
1
of the proton appearing as a broad singlet at ꢃ ˆ 6.94 in the H NMR spectrum,
relating it to the methine carbon atom at 120.89 ppm with the typical chemical shift
of ole®nic carbons [10].
The ®nal decision between the dimers 7a and 11 was made straightforward on
the basis of the gs-HMBC experiments optimized for C±H coupling constants of 4,
8, and 10 Hz. The evaluation of the non-overlapping 13C±1H long-range
correlations of the benzyl proton H-9 (ꢃ ˆ 3.12) with C-8a, C-10a, C-5, and C-
11, as well as of the benzylidene proton H-11 (ꢃ ˆ 6.94) with the carbon atoms C-5
and C-10a as well as with C-14/16 of the phenyl group at C-11, and, over six
bonds, with C-18 of the phenyl residue at C-9 (see Table 1) resulted in the
establishment of the diphenylpentadiene moiety with two conjugated C=C double
bonds (highlighted in Scheme 4), which applies only to xanthenol 7a (Scheme 3).

È
E. Goûnitzer et al.
1084
Scheme 4
1
By inspection of the H spectrum together with the HH-COSY contour plots
the resonances of the methylene group in position 6 of the cyclohexene moiety
were identi®ed by the transoid allylic long-range correlation of H-6ax with the
6
ole®nic proton H-11. Moreover, an observed JHH correlation between H-11 and
H-9 indicated a zig-zag arrangement of the C±H and C±C/C=C bonds
connecting these protons. This fact was in accordance with the above deduced
conjugation of double bonds found in IR and HMBC spectra and con®rmed
structure 7a.
The connection of the second phenyl group with C-9 followed from the long-
range correlation between C-9 and the o-protons H-19/23 as well as from the
observed NOE between these o-protons and H-9 (Fig. 1). Further heteronuclear
correlations starting from H-6ax and H-8ax as described in Table 1 enabled to
complete the cyclohexene ring. Analogously, CH2-1 and CH2-4 were found as the
terminal links of the tetramethylene bridge furnishing the cyclohexane ring. The
1D TOCSY experiment described below with irradiation of H-9 (Fig. 2) allowed to
assign the protons of CH2-2 and CH2-3.
The carbon atom C-4a at 96.39 ppm, with a chemical shift characteristic of
hemi-acetal carbons [10], bears the hydroxylic group and is the starting point for
the oxygen bridge to the enol carbon C-10a, which completes the central
dihydropyrane ring.
Stereochemistry of xanthenol 7a
The orientation of the phenyl group in the benzylidene substituent at C-5, i.e.
5E (trans to the central pyrane ring) as shown in Scheme 5, became evident by the
dipolar interactions of both H-6ax and H-6eq with the o-phenyl protons H-13/17
on the one hand, and the NOE between H-11 and H-4eq on the other. The lack of
deshielded ortho protons H-13/17 suggested that the coplanar arrangement of the
phenyl group at C-11 and the pentadiene moiety of 7a is probably lost.
3
The magnitude of JꢀH-9;H-9a† of the H-9 doublet (11.6 Hz) is signi®cant for its
anti conformation with respect to the bridgehead proton H-9a and reveals at the
same time the equatorial position of the phenyl group at C-9 (see Scheme 5) [11].

Dimers of 2-Arylmethyleneketones
1085
Fig. 1. 1D NOE difference spectrum of 7a (irradiation of the o-protons of the phenyl group at C-9)
Fig. 2. 1D TOCSY spectrum of 7a (irradiation of H-9; mixing time: 0.2 s)
Connected with the question of the orientation of the hydroxy group at C-4a
was the type of junction, cis or trans, of the dihydropyrane and cyclohexane ring
of 7a. As can be seen from Scheme 5, the signal pattern of the bridgehead proton
H-9a is indicative of the relevant type of ring junction. In the case of trans fusion,
the resonance of H-9a should appear as a doublet of a doublet or a pseudo-triplet
3
with a JH-9a=H-1ax;H-9 of ca. 10±12 Hz, whereas in the cis-type compound the
signal should show only one large antiperiplanar (3JH-9a=H-9) and two smaller
3
synclinal couplings with JH-9a=H-1ax;H-1eq ꢁ 3±4 Hz.
The partly overlapped signal of H-9a (ꢃ ˆ 1.80 ppm) became visible in a 1D
NOE difference experiment with irradiation of the o-protons H-19/23 (ꢃ ˆ
7.18 ppm) of the phenyl ring at C-9. The signal of H-9a appeared as a triplet of
doublets (J ˆ 11.6 and 3.2 Hz; see Fig. 1).
The same signal pattern of H-9a could be observed in the 1D TOCSY
experiment with irradiation of H-9, in which the connectivities from H-9 through
the cyclohexane ring to both H-4ax and H-4eq could be derived (Fig. 2).
Consequently, the trans fusion of the dihydropyrane and cyclohexane rings
as well as the axial orientation of the hydroxy group in position 4a (trans to
H-9a) were proved (Scheme 5). Additional and independent con®rmation of
the axially oriented HO-4a could be obtained from the vice versa NOE be-
tween H-9 and the OH group, showing their location on the same side of the
molecule.

È
E. Goûnitzer et al.
1086
Scheme 5
Conclusions concerning preferred conformations of the three fused rings of
xanthenol 7a could be drawn from further NOE spectra and the typical signal
patterns of particular ±CH2±CH2± ring moieties. The signal multiplicities and
coupling constants of H-1ax/H-1eq and H-4eq, which are not overlapped,
are in agreement with the chair form of the fused cyclohexane ring. H-1ax ex-
hibits a quartet of doublets arising from the geminal and two axial couplings
2
ꢀ JH-1ax;H-1eq ˆ 3JH-1ax;H-2ax ˆ 3JH-1ax;H-9a ˆ12:4 Hz† and one equatorial coupling
3
ꢀ JH-1ax;H-2eq ˆ 2:4 Hz†. Furthermore, signi®cant NOEs between H-1ax and H-3ax
as well as H-2ax and H-9a/H-4ax show their pairwise arrangement above and
below the plane of the chair, respectively.
On account of the three sp2 carbon atoms C-5, C-10a, and C-8a, the fused
benzylidene-substituted cyclohexene ring adopts predominantly a sofa conforma-
tion with CH2-7 as out-of-plane group [12]. The methylene protons H-6ax/H-6eq
obviously experience deshielding effects of both the neighbouring 5(11) double
bond and the spatially near phenyl group, resulting in down®eld shifts of their
signals to 2.39 and 2.82 ppm.
As follows from considerations using Dreiding models and according to
Ref. [12] and literature cited therein, the central dihydropyrane ring prefers a half-
chair form, which is slightly distorted towards a sofa. Weak NOEs between H-11
and HO-4a/H-4eq indicate a certain steric proximity and are consistent with the
proposed intermediate half-chair/sofa conformation of the dihydropyrane ring
(Scheme 5).
The above mentioned ®ndings led ®nally to the correct structure of the isolated
dimer 7a, racemic (5E)-5-benzylidene-9ꢀ-phenyl-trans-4a-1,2,3,4,4a,5,6,7,8,9a-
decahydroxanthen-4aꢁ-ol [13]. The stereoformula of 7a shown in Scheme 5 is in
good accordance with the observed spectroscopic data.
Mechanism of formation of xanthenol 7a
Dimer 7a could easily be generated via partly base-induced transformation of
benzylidenecyclohexanone 1a into 1a-enolate and nucleophilic [3‡3]-cycloaddi-
tion of this enolate to enone 1a in one (Scheme 6, route A) or two steps. In case of
a two step reaction, the already mentioned Michael adduct 10 might be generated

Dimers of 2-Arylmethyleneketones
1087

È
E. Goûnitzer et al.
1088
Scheme 6
as intermediate. Transformation of the benzylidene-substituted cyclohexanone ring
of 10 into an enolate and ring closure between the enolate oxygen and the carbonyl
group affords again xanthenol 7a (Scheme 6, route B1,2).
A literature search showed that there are many reports on dimerization
reactions of ꢁ,ꢀ-unsaturated ketones occurring in the course of the preparation of
enones from aldehydes and ketones [14,15]. In particular, Oszbach et al. [16] have
prepared the dimer 7a from enone 1a by treatment with ethanolic sodium
hydroxide solution and postulated the correct constitution on the basis of UV, IR,
1
and some H NMR data. However, an unequivocal proof of the structure and
stereochemistry of xanthenol 7a has not been published so far.
Structure elucidation of propionylcyclohexanone 12b
The reaction of 1-(p-methoxyphenyl)-2-methyl-1-penten-3-one (1b) with guani-
dine in boiling butanol yields, probably via 1,4-dihydropyrimidinamine 3b as
intermediate, 6-ethyl-5-methyl-4-(p-methoxyphenyl)-2-pyrimidinamine (11b)
[17]. In contrast, heating of the low-melting pentenone 1b (m.p.: 50ꢂC) with
guanidine without solvent under a nitrogen atmosphere furnished exclusively a
nitrogen-free product of m.p. 205ꢂC. Together with the very lipophilic behaviour in
TLC, the elemental analysis of compound 12b led to the assumption that a dimer-
ization of enone 1b in the presence of guanidine had occurred. The evaluation of
the IR and NMR spectra revealed structure and stereochemistry of 3,5-bis-
(4-methoxyphenyl)-2,4,6-trimethyl-4-propionylcyclohexanone 12b (Scheme 7). So
far, 12b as well as its analogues are unknown in literature.
A ®rst glance at the IR spectrum proved that the conjugated keto function of
educt 1b at 1661 cm 1 is no longer existent in the isolated compound 12b, whereas
two intense absorption bands at ꢂ ˆ 1709 and 1697 cm 1 indicated the presence of
two carbonyl groups.
1
The H NMR spectrum (Fig. 3) showed nine different types of protons in two
sets of fragments with an intensity ratio of 1:2, indicating symmetry in the cyclo-
hexanone moiety. The HH-COSY revealed the ABX3 system of the two equivalent

Dimers of 2-Arylmethyleneketones
1089
Scheme 7
-CH(Ar)±CH±CH3 fragments of the cyclohexane ring, i.e. a doublet at ꢃ ˆ
3.30 ppm, a doublet of a quartet at ꢃ ˆ 2.97 ppm, and a doublet for the methyl
protons at ꢃ ˆ 0.83 ppm.
The evaluation of the gs-HSQC spectrum afforded the assignment of all C±H
resonances (Table 2). The heteronuclear long range shift correlation (gs-HMBC)
con®rmed the assignment and linkage of the quarternary carbons. In detail, the
connectivities of the carbonyl C-1 (211.87 ppm) with H-2/6 and H-3/5 and
the protons of CH3-11/12 as well as of C-4 (57.19 ppm) with H-3/5 led to the
establishment of the cyclohexanone ring. Correlations of the second carbonyl C-7
(216.02 ppm) with the methylene and methyl protons of CH2-8 and CH3-9 allowed
to complete the propionyl residue. The location of the propionyl residue and of
CH3-10 at C-4 as well as the linkage of the two 4-methoxyphenyl substituents with
C-3/5 followed from the long-range correlations described in Table 2, which also
provides further details for the structural analysis of 12b (Scheme 9).
The knowledge of the correct structure of dimer 12b allowed to discuss a
[4‡2]-cycloaddition of two molecules of the phenylmethylpentenone 1b as a
possible way of formation. In the probably occurring Diels-Alder reaction the
tautomeric 1b-enolate served as diene and added at postions 1,4 to the second
vinylogous ketone 1b (Scheme 8, route A). However, 12b could also be formed by
means of two consecutive Michael additions via the acyclic 1-nonen-3,7-dione 13b
as intermediate (route B1,2).
1
Fig. 3. 400 MHz H NMR spectrum of dimer 12b in CDCl3 (I: intensity in units of protons)

È
E. Goûnitzer et al.
1090
Table 2. NMR data of propionylcyclohexanone 12b (400 MHz 1H/100 MHz 13C, CDCl3, 300 K; numbering of 12b: see
Scheme 9)
1
ꢃ(C)/ppm ꢃ(H)/ppmmultiplicity JHH/Hz
Long-range CH correlations Relevant H,1H-NOE
interactions
1
C=O 211.87
±
2/6
3/5
CH
CH
43.89
57.26
2.97dq
3.30d
13.2/6.0 11.09; 57.26; 211.87
13.2
11.09; 12.65; 43.89; 57.19; CH3-10; H-14/18,H-20/24
130.27; 211.87; 216.02
4
C
57.19
±
7
C=O 216.02
±
8
CH2
CH3
CH3
34.72
6.88
1.21q
0.49t
1.43s
7.2
7.2
6.88; 216.02
34.72; 216.02
9
10
11.09
57.26; 216.02
H-2/6; CH2-8;
H-14/18,H-20/24
H-3/5; H-14/18,H-20/24
11/12
13/19
14/18
20/24
15/17
21/23
16/22
25/26
CH3
C
12.65
130.58
130.27
0.83d
±
6.0
8.8
8.8
43.89; 57.26; 211.87
57.26; 130.27; 158.27
113.34; 130.27; 158.27
CH
CH
CH
CH
C
6.92d
H-2/6; H-3/5; CH3-10
113.34
6.75d
158.27
55.11
±
CH3
3.74s
158.27
Scheme 8
Stereochemistry of dimer 12b
Statements concerning the stereochemistry of dimer 12b could be made by means
of the H signal analysis and signi®cant NOESY correlations. Thus, the chair
1

Dimers of 2-Arylmethyleneketones
1091
Scheme 9
conformation of the cyclohexanone with both equatorially oriented methyl groups at
C-2/6 and p-methoxyphenyl substituents at C-3/5 was evidenced by the vicinal
coupling constants of the equivalent proton pairs H-2/6 and H-3/5. The large value of
3JHH ˆ 13.2 Hz is signi®cant for their pairwise antiperiplanar arrangement. The
NOESY spectra proved the axial position of CH3-10 via the dipolar interactions with
the axially oriented H-2/6. Moreover, the NOESY cross peaks between CH2-8 and
CH3-10 argued for a time average gauche conformation of the methylene protons of
CH2-8 with respect to CH3-10 (Scheme 9). The carbonyl C-7 might be directed
parallely to H-3/5. On account of steric hindrance by the vicinal methyl groups
at C-2/6 and C-4, the methoxyphenyl substituents in position 3 and 5 should be
approximately perpendicularly arranged with respect to the plane of the
cyclohexanone ring.
In accordance with these ®ndings, the remarkable high®eld shift of about
1.2 ppm for the signals of the methyl and methylene protons (ꢃ ˆ 0.49 and
1.21 ppm) of the propionyl residue can be clearly attributed to the anisotropic
effects of the facing phenyl groups. Studies on Dreiding molecular models showed
that no anisotropic effect of the phenyl groups towards CH2-8 could be expected if
the methylene protons would be oriented parallely to H-3/5. The 13C resonance of
CH3-10 experienced a signi®cant up®eld shift of about 8 ppm from 19.80 to
11.09 ppm caused by the ꢄ-effect also due its axial position [10].
Because of the mirror image equivalence of the substitutents in positions 2/6
and 3/5, the dimer 12b has a plane of symmetry in the molecule and represents
meso-3ꢀ,5ꢀ-bis-(4-methoxyphenyl)-2ꢁ,4ꢀ,6ꢁ-trimethyl-4ꢁ-propionylcyclohexa-
none.
Experimental
Melting points were determined on a Ko¯er melting point apparatus and are uncorrected. Thin-layer
chromatograms (TLC) were run on TLC plastic sheets silica gel 60 F254 (E. Merck, Darmstadt);
elution systems: C6H6:MeOH ˆ 90:10 (ES 1), toluene-Et2O ˆ 90:10 (ES 2). The spots were detected
by visual examination under UV light (254 and 366 nm). Infrared spectra were recorded with Perkin-

È
E. Goûnitzer et al.
1092
1
Elmer 881 and 2000 FTIR spectrophotometers in KBr disks; frequencies are reported in cm
(s ˆ strong, m ˆ middle, w ˆ weak). NMR spectra were acquired on a Varian 400 MHz Unity Inova
NMR spectrometer equipped with a Sun Sparc 5 computer system and operating at an observation
frequency of 399.98 MHz for H and 100.59 MHz for 13C. 1D and 2D NMR experiments were
1
performed using a reverse geometry 5 mm broad-band probehead and a pulsed ®eld gradient unit.
The HH-COSY [18], gs-HSQC [19], gs-HMBC [20], NOESY [21], 1D NOE difference [22], and 1D
TOCSY [23] experiments were performed using the pulse programs supplied by the manufacturer.
The gs-HMBC experiments were optimized for 4, 8, and 10 Hz (delays of 125.0, 62.5, and 50.0 ms,
respectively). All NOEs were measured in degassed samples. For the 1D NOE experiments, 3 s pre-
irradiation times were used. FIDs were exponentially multiplied prior to Fourier transformation. The
mixing time in the NOESY experiments was 0.8 s. 1D TOCSY spectra were achieved in series with
arrayed mixing times (0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.16, 0.20, 0.24 s). 15±25 mg of the
substances were dissolved in 0.5 cm3 of deuterated solvents and measured at 300 K. All chemical
shifts are reported in ꢃ units (ppm) with TMS as internal standard. Mass spectra were taken with a
Finnigan Mat 212 spectrometer (EI, 120 EV, R ˆ 1000) by Dr. R. Saf, Institute of Chemical
Technology of Organic Materials, Technical University Graz. Elemental analyses were performed by
J. Theiner, Institute of Physical Chemistry, University of Vienna; they agreed favourably with the
calculated values.
Racemic (5E)-5-benzylidene-9ꢀ-phenyl-trans-4a-1,2,3,4,4a,5,6,7,8,9a-decahydro-
xanthen-4aꢁ-ol (7a; C26H28O2)
A mixture of 9.30 g benzylidenecyclohexanone 1a (50 mmol) and 2.95 g guanidine (50 mmol) in
20 cm3 MeOH was heated to re¯ux for 2.5 h. Then, the reaction mixture was evaporated to dryness,
and the residue was dissolved in benzene. After neutralization of the benzene solution with 2 N HCl
and dilution with H2O the benzene layer was separated, dried over Na2SO4, and evaporated to
dryness. The oily residue was triturated with MeOH and recrystallized from 2-PrOH to give
colourless needles of 7a.
Yield: 3.81 g (41%); m.p.: 121ꢂC (Ref. [16]: 135±136ꢂC); TLC (ES 1): Rf ˆ 0.65; MS: m/z
‡
‡
‡
‡
(%) ˆ 372 (M , 30), 354 (M -18, 5), 275 (M -97, 100), 186 (M -186, 58), 117 (27), 115 (28), 91
(43); IR (KBr): ꢂ ˆ 3480s, 2930s, 2860/2830m/w, 1630m, 1592m, 1490m, 1125s, 1065s, 955/948s/
s cm 1. H (CDCl3, ꢃ, 400 MHz) and 13C NMR (CDCl3, ꢃ, 100 MHz): see Table 1.
1
meso-3ꢀ,5ꢀ-Bis-(4-methoxyphenyl)-2ꢁ,4ꢀ,6ꢁ-trimethyl-4ꢁ-propionylcyclohexanone
(12b; C26H32O4)
Pentenone 1b (8.17 g, 40 mmol) was melted under an N2-atmosphere, and 2.36 g guanidine
(40 mmol) were added. Within 1 h the reaction temperature was elevated to 100ꢂC under stirring and
held for 10 h till the melt became solid. After cooling, the reaction mixture was distributed between
CHCl3 and H2O. The organic layer was separated, neutralized with 6% HBr, dried over Na2SO4, and
evaporated to dryness. The residue was triturated with MeOH to give colourless needles of 12b.
Yield: 3.27 g (40%); m.p.: 205ꢂC; TLC (ES 2): Rf ˆ 0.67; IR (KBr): ꢂ ˆ 2986s, 2940s, 2838m,
1
1709/1697m/s, 1611s, 1579m, 1458/1444m/m, 1297s, 1187/1180s/s, 858m, 816s cm 1. H (CDCl3,
ꢃ, 400 MHz) and 13C NMR (CDCl3, ꢃ, 100 MHz): see Table 2.
Acknowledgments
Â
The authors express their thanks to Prof. Dr. E. Breitmaier, Kekule Institute of Organic Chemistry
È
and Biochemistry, Rheinische Friedrich-Wilhelms Universitat Bonn, Germany, for recording some
NMR spectra and for helpful discussions on the structure elucidation of xanthenol 7a.

Dimers of 2-Arylmethyleneketones
1093
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Received April 11, 2001. Accepted (revised) May 18, 2001