Synthesis of novel precursors of calicene and 7-methylcalicene

Article abstract of DOI:10.1002/(SICI)1522-2675(20000315)83:3<571::AID-HLCA571>3.0.CO;2-M

Cyclopent-2-en-1-one is a versatile electrophilic cyclopentadiene equivalent in reactions with 1-bromo-1-lithiocyclopropanes. The synthetic sequence (outlined in Scheme 1) has been applied to the synthesis of functionalized 7-X-7,8-dihydrocalicenes 13c (Scheme 3) and 13d (Scheme 4). 7- Bromo-7,8-dihydrocalicene (13d) is considered to be a promising precursor of the so far unknown parent calicene (2). A similar sequence has been realized for 7-(chloromethyl)-7,8-dihydrocalicene (21a, Scheme 5) which, under appropriate conditions, could give 7-methylcalicene (16).

Full text of DOI:10.1002/(SICI)1522-2675(20000315)83:3<571::AID-HLCA571>3.0.CO;2-M

Helvetica Chimica Acta ± Vol. 83 (2000)
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Synthesis of Novel Precursors of Calicene and 7-Methylcalicene1)
by Ahmad Al-Dulayymi2), Xiaoming Li3), and Markus Neuenschwander*
Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern
Cyclopent-2-en-1-one is a versatile electrophilic cyclopentadiene equivalent in reactions with 1-bromo-1-
lithiocyclopropanes. The synthetic sequence (outlined in Scheme 1) has been applied to the synthesis of
functionalized 7-X-7,8-dihydrocalicenes 13c (Scheme 3) and 13d (Scheme 4). 7-Bromo-7,8-dihydrocalicene
(13d) is considered to be a promising precursor of the so far unknown parent calicene (2). A similar sequence
has been realized for 7-(chloromethyl)-7,8-dihydrocalicene (21a, Scheme 5) which, under appropriate
conditions, could give 7-methylcalicene (16).
1. Introduction. ± Triafulvene (1) and calicene (2 ˆ pentatriafulvalene) have
fascinated experimental as well as physical organic chemists for more than three
decades. During that time, the parent triafulvene has been generated in solution [2][3],
as well as in the gas phase [4][5], so that its chemistry, and essential spectroscopic
properties are well known today [4][5]. On the other hand, parent calicene 2 has not
been isolated, trapped, or spectroscopically identified so far. Considering the fact that 2
contains two fully conjugated rings, which are expected to support each other
electronically and hence to increase p-delocalization4)5), this is at first sight quite
surprising but could be explained with the higher reactivity of 2 due to the increased
dipolar character4).
Most synthetic sequences for substituted calicenes start with highly substituted
cyclopropenylium salts of the type 4 ± 66)7) or cyclopropenes such as 7, which are
reacted with (substituted) cyclopentadienides to give cyclopropenyl-cyclopentadienes.
Provided that these intermediates contain additional leaving groups X in the three-
membered ring (which is the case starting with 4, 6, and 7), base-induced HX-
elimination is usually straightforward and leads to substituted calicenes6). On the other
hand, cyclopropenyl-cyclopentadienes prepared from 5 lack an additional leaving
group and have to be subjected to hydride abstraction (followed by deprotonation)
1
)
)
)
)
Fulvenes, Fulvalenes, Part 73. Part 72: [1].
Postdoctoral fellow 1997 ± 1999.
Postdoctoral fellow 1996 ± 1999.
2
3
4
Ab initio calculations [6][7] show that bond lengths of formal single and double bonds of 2 are less
alternating (and m ꢀ 4.5 D is increased) compared with 1 (m ˆ 1.9 D [8]) or pentafulvene (3) (m ˆ 0.424 D
[9]).
According to our aromaticity plot for pentafulvenes and pentafulvalenes derived from 3J(H,H) values [10],
an aromaticity of ca. 30% may be estimated for calicene. This is perfectly in accord with the results of ab
initio calculations [6], suggesting a 31% contribution of dipolar 2Æ.
For typical examples, see [11 ± 16] (from 4), [17][18] (from 5), [19][20] (from 6), and [21] (from 7).
Cyclopropenylium salts 4 (X ˆ AcO) are supposed to be formed as intermediates in reactions of
cyclopropenones with Ac2O [22].
5
)
6
7
)
)

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[17][18]. All these classical pathways fail in attempts to synthesize parent calicene 2:
while in reactions of unsubstituted cyclopropenylium salts 4 (R1 ˆ R2 ˆ H) with
cyclopentadienide the leaving groups end up in vinylic position and cannot be
eliminated later on [23], hydride abstractions from cyclopropenylcyclopentadienes
(prepared from 5) may be tedious even for relatively stable, substituted intermediates
[17][18] and have always failed in cases with unsubstituted cyclopentadiene rings
[13][24][25]. Furthermore, attempts to replace the amino groups of 7,8-diaminocali-
cenes (prepared from 6 [19][20]) or the alkylthio substituents of 7,8-bis(alkylthio)-
calicenes (available from 7 [21]) have been undertaken [21][26] and did not result in for-
mation of the parent calicene. Finally, it is interesting to note that 8 still represents the
simplest alkyl-substituted calicene [17][18]. However, it is so highly substituted that it does
not give much insight into spectroscopic and chemical properties of naked calicene (2)8).
Since the parent calicene (2) is expected to be an extremely reactive compound, it
has to be generated either at very low temperature in dilute and O2-free solutions [27],
or by gas-phase pyrolysis followed by trapping 2 at low temperature. During the last ten
years, we have made several attempts in both directions [28][29]. As far as low-
temperature reactions in solutions are concerned, we planned to make use of the easy
conversion of 1,1-dibromocyclopropanes 9 to 1-bromo-1-lithiocyclopropanes 10 [30]
and the well-known reactivity of these carbenoids towards various electrophiles [31 ±
34] and in reactions with cyclopent-2-en-1-one (10 ! 11), which proceed at 958 with
reasonable yields [28]. The most tricky step of the sequence 10 ! 13 (Scheme 1) is the
acid-catalyzed dehydration of cyclopentenol 11 to give cyclopentadiene 12, which, in
some cases (with electron-donating substituents X), induced a ring-opening of the
cyclopropyl unit of 11. Although a small number of 7,8-dihydrocalicenes 13 has been
prepared [28][29], no compounds 13 with good leaving groups X (e.g., Br, Cl) nor with
easily functionalizable substituents9) have been available so far, which would be a
perequisite in view of a low-temperature synthesis of calicene (2) in solution.
For instance, 3J values are not available from the 1H-NMR spectra of 8 [18], which would be important for
deriving the extent of bond-length alternation in the five-membered ring of 8.
For instance, substituents X such as COOR could be transformed into X ˆ Br by the Hunsdiecker reaction
[35].
8
)
)
9

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573
Scheme 1
In this paper, we report the synthesis of the first promising precursors of type 13 of
calicene (2) and of 7-methylcalicene.
2. Synthesis of 1,1-Dibromocyclopropanes (9). ± An easy access to functionalized
1,1-dibromocyclopropanes 9 is crucial in view of promising calicene precursors of type
13 (Scheme 1). Some years ago, we were thinking of allylic alcohol 14 as a versatile
starting material but had to learn that the seemingly simple step 14 ! 9a (Scheme 2)
cannot be directly realized by means of the available procedures [36 ± 39] for
dibromocarbene additions10). This problem can be solved by ketalization 14 ! 15 [43],
followed by dibromocarbene addition [37] and hydrolysis (15 ! 9a). Permanganate
oxidation of the alcohol 9a gives the carboxylic acid 9b, from which various carboxylic
esters, such as 9c, are prepared [44]. On the other hand, Hunsdiecker decarboxylation
of 9b provides a good access to 1,1,2-tribromocyclopropane (9d)11).
Scheme 2. Synthesis of 1,1-Dibromocyclopropanes 9
10
)
)
There exist, however, several multistep sequences for 9a in the literature [40 ± 42].
Direct dibromocarbene addition to bromoethene (according to phase-transfer procedures) gives only a 5%
yield of 9d [45].
11

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3. Synthesis of Calicene Precursors 13c and 13d. ± Our experiments show that the
trickiest step of the synthesis of functionalized calicene precursors of type 13 is the
alkylation of 1-bromo-1-lithio-2-X-cyclopropanes (10 ! 11) with cyclopentenone.
Although this step is normally fast enough at temperatures around 1008 [28],
alkylation 10 ! 11 has to compete with intramolecular LiX elimination, which leads to
the corresponding 1-bromocyclopropene [46]. On the other hand, BuLi is able to attack
carbonyl groups of esters [47], so that the step 9c ! 11c can turn out to be problematic
as well. The best way to circumvent most of the problems is to slowly add BuLi to a
mixture containing the 1,1-dibromocyclopropane 9 and cyclopentenone at very low
temperature, so that the hereby generated carbenoid 10 is immediately trapped by
cyclopentenone. Since in the course of the sequence 9 ! 11 three centers of chirality are
generated, diastereoisomeric mixtures of cyclopropylcyclopentenols 11 are isolated12),
in which normally one or two diastereoisomers are dominating.
In fact, methyl-2,2-dibromocyclopropanecarboxylate (9c) reacts easily in Et2O
solution with cyclopentenone and BuLi at 958 to give a 60 :40% mixture of two
diastereoisomeric esters 11c with moderate yields (Scheme 3). Acid-catalyzed dehy-
dration of esters 11c proceeds easily by reacting them with small amounts of TsOH in
dry benzene to give two isomeric esters 12c (which turn out to be tautomers, see later).
Finally, base-induced HBr elimination 12c ! 13c (77%) makes methyl 7,7-dihydroca-
licene-7-carboxylate (13c) available.
Scheme 3
To transform the methoxycarbonyl group of 13c into a good leaving group, we
planned to carefully hydrolyze the ester 13c and to replace COOH with Br through the
well-known Hunsdiecker decarboxylation [35]. So far, however, hydrolysis of 13c
proved to be problematic due to the reactivity of the fulvene moiety of 13c13).
Therefore, we tried to realize the tricky ꢀcyclopentadienylationꢁ of 1,1,2-tribromo-
cyclopropane (9d) which is very critical because the intermediary 1,2-dibromo-1-
lithiocyclopropane 10d will easily form 1-bromocyclopropene even at low temperature
[46]. In fact, when BuLi is slowly added to a THF solution containing 9d and
cyclopentenone under routine conditions ( 958), the desired product 11d cannot be
detected. This prompted us to lower reaction temperatures as much as possible and to
replace THF by Et2O in order to still enable a reaction in solution at 1108. Under
12
)
This is not as dramatic as it seems, since in the course of the sequence 11 ! 12 ! 13 ! 2, the chirality centers
will be eliminated again. Furthermore, cyclopropyl protons of compounds 11, 12, and 13 display three sets
of well-resolved dds in the H-NMR spectrum (see Figs. 1 and 2).
Pentafulvenes are known to be sensitive towards traces of strong acids, which induce polymerization
[48][49]. On the other hand, they can be attacked by strong nucleophiles at C(6) [50][51].
1
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)

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575
these conditions, a diastereoisomeric mixture of cyclopropylcyclopentenol 11d could be
isolated in moderate yields (18%), which allowed us to complete the planned sequence
(Scheme 4) and to generate 7-bromo-7,8-dihydrocalicene (13d). To our knowledge, 13d
is the first precursor of parent calicene (2) to contain a good leaving group14).
Scheme 4
The structures of the new compounds (Schemes 3 and 4) are consistent with the
spectroscopic data, of which 1H-NMR and 13C-NMR data are very conclusive even for
1
isomeric mixtures. This is demonstrated by the H-NMR spectrum of the ester 12c
(Fig. 1), where two isomers (in a ratio of 2 :1) are clearly identified. Each isomer
produces three sets of dds in the cyclopropane range at 2.17, 1.98, and 1.68 ppm (major
isomer), and 2.13, 1.99, and 1.65 ppm (minor isomer) with typical J values. The narrow
multiplets of the cyclopentadiene CH2 units are localized at 3.07 and 3.13 ppm. In the
vinylic range, which seems to be very complex at first sight, both ABX subspectra15) of
the cyclopentadiene protons are visible, and the estimated coupling constants are
typical for cyclopentadienes [52]15).
4. Synthesis of a Precursor of 7-Methylcalicene (16). ± Compared to calicene (2), 7-
methylcalicene (16) is expected to display very similar electronic and spectroscopic
properties. Its main advantage over 2 (besides minor stabilization) is the fact that, due
to its reduced symmetry, the pairs of ring-H-atoms and ring-C-atoms are not equivalent
1
anymore, which should allow a first-order analysis of the high-field H-NMR spectra.
On the other hand, the full set of coupling constants of the five-membered ring would be
3
still available so that the extent of bond-length alternation could be derived from J
values5).
Making use of the same type of cyclopentadienylation reactions of 1-bromo-1-
lithiocyclopropanes as explored previously (Schemes 3 and 4), a quite simple approach
to 7-methylcalicene (16) can be conceived (Scheme 5): 1,1-dibromo-2-(halomethyl)cy-
clopropanes 18a (XˆCl) and 18b (XˆBr) can be prepared from 3-halopropenes 17 by
dibromocarbene addition, according to phase-transfer procedures [37]. Reaction with
cyclopentenone and BuLi gives cyclopropylcyclopentenols 19 in good yields, and the
synthesis of 7-(chloromethyl)-7,8-dihydrocalicene (21a) has been realized by acid-
‡
14
)
First tentative experiments show that 13d eliminates HBr in the presence of bases, such as Et4N Br at
room temperature, or sodium cyclopentadienide (formation of NaBr at 08). So far, however, neither parent
2 nor a cycloaddition product of 2 could be spectroscopically identified. These experiments will be
continued at low temperature and under carefully controlled conditions.
They are labelled ABX (major isomer) and A'B'X' (minor isomer), the A'B' part being degenerate. For the
major isomer with H C(A) at 6.54, H C(B) at 6.50, and H C(X) at 6.31 ppm, JAB ˆ 5.2 Hz is typical for
adjacent vinylic H-atoms of cyclopentadienes. H C(A) displays a dq, because all its long-range couplings
are ca. 1.5 Hz.
15
)

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Fig. 1. Sections of the 1H-NMR Spectrum (300 MHz, CDCl3) of 12c (tautomeric mixture)
Scheme 5
catalyzed dehydration of 19a ! 20a followed by base-induced dehydrobromination
20a ! 21a.
Spectroscopic data are consistent with the structures of 19a, 19b, 20a, and 21a16).
1
Considerable structural information stems from H-NMR spectra, which very often
show a quite spectacular splitting pattern. In the 1H-NMR spectrum of the predominant
diastereoisomer of substituted cyclopropylcyclopentenol 19b (Fig. 2), the dxt of both
16
)
Despite the presence of three centers of chirality, diastereoisomeric mixtures of 19a and 19b contain only
two isomers in a ratio of ca. 3 :1. In the case of 20a, one single isomer is predominant.

Helvetica Chimica Acta ± Vol. 83 (2000)
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vinylic protons are found at 6.08 (H C(2)) and 5.53 ppm (H C(3)) with approximate
coupling-constant values17) typical for cyclopentene moieties, while the diastereotopic
protons of CH2(4) and CH2(5) produce complex multiplets at 2.6 ± 2.3 and 2.00 ppm. On
the other hand, all the H-atoms of the (bromomethyl)cyclopropane unit are not
equivalent and display a first-order spectrum with the typical dd splittings of H-atoms
of the exocyclic CH2 unit at 3.75 and 3.50 ppm (with JAB ˆ 10.3 Hz), the dd of the
cyclopropane ring at 1.40 and 0.99 ppm (with JAB ˆ 6.6 Hz), and the complex m of the
cyclopropane-CH at 1.61 ppm. The only s of the spectrum, at 2.20 ppm, is generated by
the OH group.
Fig. 2. 1H-NMR Spectrum (300 MHz, CDCl3) of 19b
5. Discussion. ± Our results show that cyclopent-2-en-1-one can be applied as an
electrophilic cyclopentadiene equivalent in reactions with 1-bromo-1-lithiocyclopro-
panes 10 at low temperature. Because nucleophiles such as 10 nearly exclusively attack
the carbonyl group of cyclopentenone, no Michael-addition products have been
isolated in the reactions outlined in Schemes 3, 4, and 518). It is interesting to note that
trapping of 1,2-dibromo-1-lithiocyclopropane 10d with cyclopentenone can compete
with intramolecular LiBr elimination at very low temperature, albeit with reduced
yields (Scheme 4). The hereby formed compounds 11c (Scheme 3), 11d (Scheme 4),
and 19a (Scheme 5) can be transformed into functionalized 7,8-dihydrocalicenes 13c,
13d, and 21a under appropriate conditions. As typical pentafulvenes, these compounds
are reasonably stable in O2-free solutions and allow experiments in the temperature
range up to 0 ± 208. To our knowledge, 7-bromo-7,8-dihydrocalicene (13d) is the first
promising precursor for parent calicene (2) containing a good leaving group in the
three-membered ring.
The authors thank the Swiss National Science Foundation (project No. 20-50331.97) for financial support.
3
17
18
)
)
J(2,3) ꢀ 5.75; J(2,4) ꢀ 2.2; J(3,4) ꢀ 2.0 Hz. J(2,3) is very typical for J couplings of localized cyclopentene
double bonds. The splittings are only approximate, because three of the four H-atoms of CH2(4) and
CH2(5) at 2.6 ± 2.3 and 2.00 ppm are close together, hence producing a high-order subspectrum.
Traces of Michael-addition products have been detected in other cases [28][29].

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Experimental Part
General. Unless otherwise stated, all the reactions were performed under N2 or Ar in two- or three-necked
round-bottomed flasks equipped with a dropping funnel (or a septum), a magnetic stirrer, an N2-inlet and,
where needed, a thermometer with H2O-free solvents and reagents. Prior to the introduction of reagents, the
vessels were thoroughly flame-dried and flushed with N2 or Ar. Small amounts of sensitive liquids or solns. were
injected into the reaction vessel through the septum with a syringe. Temp. of 958 ( 1108) were reached by
freezing toluene (EtOH/MeOH 5 :1) with liq. N2. Spectra were recorded on the following instruments: UV:
1
Perkin-Elmer 554 and Hewlett-Packard HP-8452 A; lmax (e) in nm. IR: Perkin-Elmer 399 B and 1600; nÄ in cm
NMR: Bruker AC-300; d in ppm rel. to TMS, J in Hz. MS: Varian-MAT CH-7A, m/z (rel. %).
.
1. Synthesis of Dibromocyclopropanes 9 and 18. ± 1.1. 2,2-Dibromocyclopropane-1-methanol (9a): [44].
1.2. 2,2-Dibromocyclopropanecarboxylic Acid (9b). In a 750-ml flask, a soln. of 37.8 g (0.24 mol) of KMnO4
‡
in 300 ml of H2O was vigorously stirred at 08. 3.08 g (9.5 mmol) of Bu4N Br and a soln. of 11.0 g (47.8 mmol) of
9a in 20 ml of benzene were added, and the reaction mixture was vigorously stirred for 16 h at 08. Then, the
mixture was carefully treated with sat. NaHSO3/H2O until the brown color disappeared, acidified with 10%
H2SO4, and then extracted with Et2O (3 Â 100 ml). The combined org. layers were dried (MgSO4) and the
solvent removed i.v. (14 mmHg) to give 10.6 g (91%) of 9b as a colorless solid, which was recrystallized from
petroleum ether.
1.3. Methyl 2,2-Dibromocyclopropanecarboxylate (9c): [44].
1.4. 1,1,2-Tribromocyclopropane (9d). To a suspension of 4.8 g (19.8 mmol) of 9b and 4.26 g (19.7 mmol) of
red HgO in 20 ml of CCl4, a soln. of 3.7 g (23.1 mmol) of Br2 in 10 ml of CCl4 was added dropwise under stirring
at 808. The mixture was then refluxed for 3 h, stirred at r.t. for 4 h and, after adding 30 ml of petroleum ether,
filtered through ꢀflashꢁ silica gel. The filtrate was evaporated i.v. (14 mmHg) and purified by column
chromatography (CC) over silica gel with petroleum ether to give, after evaporation, 3 g (54%) of 9d as a
colorless oil [45]11).
1.5. 1,1-Dibromo-2-(chloromethyl)cyclopropane (18a). In a 250-ml flask, 50 ml of 50% aq. NaOH soln.
were slowly added at 08 to a vigorously stirred mixture of 14.0 g (183 mmol) of 3-chloropropene (17a), 16.8 ml
(192 mmol) of CHBr3, and 1.5 g (4.1 mmol) of cetrimide in 100 ml of CH2Cl2. The reaction mixture was
vigorously stirred for 2 h at 58 and overnight at r.t. (TLC showed complete reaction). The mixture was then
transferred into a separatory funnel containing 200 ml of H2O, the org. layer separated, washed with H2O (2 Â
100 ml), and concentrated i.v. The residue was diluted with 150 ml of Et2O, dried (MgSO4), filtered, and
evaporated to give 34.0 g (75%) of practically pure 18a as a colorless oil. Analytically pure 18a was obtained by
distillation (7.6 Torr). 1H-NMR (300 MHz, CDCl3): 3.65 (m, 2 H); 2.04 (m, 1 H); 1.94 (m, 1 H); 1.49 (m, 1 H).
13C-NMR (75 MHz, CDCl3): 46.1 (t); 32.2 (d); 28.9 (t); 25.7 (s).
1.6. 1,1-Dibromo-2-(bromomethyl)cyclopropane (18b). In a 250-ml flask, 50 ml of 50% aq. NaOH soln.
were slowly added to a vigorously stirred mixture of 15.0 g (124 mmol) of 3-bromopropene (17b), 12.0 ml
(137 mmol) of CHBr3, and 1.5 g (4.1 mmol) of cetrimide ((hexadecyl)trimethylammonium bromide) in 50 ml of
CHCl2. The reaction mixture was vigorously stirred at 58 for 2 h, then overnight at r.t. (TLC showed incomplete
reaction). After addition of another mol-equiv. of CHBr3, stirring was continued for 10 h at r.t. Workup
according to procedure 1.5 gave 23.6 g (65%) of practically pure 18b as an orange-colored oil. Anal. pure 18b
was obtained by distillation (7.6 Torr). 1H-NMR (300 MHz, CDCl3): 3.47 (m, 2 H); 2.07 (m, 1 H); 1.95
(m, 1 H); 1.45 (m, 1 H). 13C-NMR (75 MHz, CDCl3): 33.6 (t); 32.4 (d); 30.3 (t); 27.4 (s).
2. Synthesis of Calicene ( ˆ 5-(Cycloprop-2-enylidene)cyclopenta-1,3-diene) Precursors 13c (Scheme 3) and
13d (Scheme 4). ± 2.1. Methyl 2-Bromo-2-(1-hydroxycyclopent-2-enyl)cyclopropanecarboxylate (11c). 1.33 ml of
1.6m BuLi in hexane (2.1 mmol) were added dropwise to a stirred soln. containing 0.5 g (1.94 mmol) of 9c and
205 mg (2.5 mmol) of cyclopent-2-en-1-one in 15 ml of Et2O under N2 at 958. The mixture was stirred for 1 h at
958, allowed to warm to 308, and stirred for another 30 min at 308. After quenching the reaction by adding
3 ml of H2O at 308, the mixture was extracted with Et2O (3 Â 15 ml). The combined org. phases were
evaporated (12 Torr) and the crude product purified by CC on ca. 30 g of silica gel with pentane/Et2O 5 :2 to give
0.16 g (32%) of a colorless oil (2 :1 mixture of two isomers) of 11c. IR (CDCl3)19): 3476m, 2953m, 1732s, 1441s,
1384m ± s, 1252m ± s, 1202m ± s, 1176s, 1089m ± s, 1066m ± s, 912s, 728s, 648m ± s. 1H-NMR (300 MHz, CDCl3):
major isomer: 6.10 (m, 1 H); 5.52 (m, 1 H); 3.76 (s, 3 H); 2.6 ± 2.3 (m, 4 H); 2.05 (dd, J ˆ 9.2, 7.0, 1 H); 1.73
(t, J ˆ 7.0, 1 H); 1.58 (dd, J ˆ 9.2, 7.0, 1 H): signals of cyclopropane protons of minor isomer19): 2.21 (dd, J ˆ 9.2,
7.0, 1 H); 1.70 (dd, J ˆ 7.0, 6.6, 1 H); 1.45 (dd, J ˆ 9.0, 6.6, 1 H). 13C-NMR (75 MHz, CDCl3; 2 isomers): 169.9 (s);
19
)
Other signals of the minor isomer are obscured by those of the major isomer.

Helvetica Chimica Acta ± Vol. 83 (2000)
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169.6 (s); 137.9 (d); 137.6 (d); 131.5 (d); 131.3 (d); 88.4 (s); 88.3 (s); 52.18 (q); 52.17 (q); 46.2 (s); 46.0 (s); 37.58
.
.
‡
‡
(t); 37.57 (t); 31.20 (t); 31.15 (t); 24.9 (d); 24.6 (d); 18.6 (t); 18.4 (t). MS20): 262 (1, M ), 260 (1, M ), 230 (67),
228 (70), 181 (31), 180 (33), 178 (32), 176 (100), 174 (99), 163 (46), 149 (58), 121 (56), 95 (46), 83 (59), 54
(29)21).
2.2. Methyl 2-Bromo-2-(cyclopenta-1,4-dienyl)cyclopropanecarboxylate (12c)22).
A soln. of 1.0 g
(3.8 mmol) of 11c in 10 ml of dry benzene was stirred at r.t., and 140 mg (0.74 mmol) of TsOH were added.
After 3 h of stirring at r.t. (TLC showed complete reaction), the solvent was removed i.v. (14 Torr), and the
crude product was separated by CC over ca. 30 g of silica gel at 108 with pentane/Et2O 5 :2 to give 0.50 g (54%)
of a mixture of 2 tautomers of 12c. IR (CDCl3)20): 2951m ± s, 1738s, 1440m ± s, 1385m ± s, 1354m, 1196m ± s,
1171s, 921m, 896m, 668m, 606m. 1H-NMR (300 MHz, CDCl3): major tautomer: 6.54 (dm, J ˆ 5.2, 1 H); 6.50
(dm, J ˆ 5.2, 1 H); 6.31 (m, 1 H); 3.81 (s, 3 H); 3.07 (m, 2 H); 2.17 (dd, J ˆ 8.8, 7.2, 1 H); 1.98 (dd, J ˆ 7.2; 6.4,
1 H); 1.68 (dd, J ˆ 8.8, 6.4, 1 H); minor tautomer: 6.45 (m, 1 H); 6.40 (m, 2 H); 3.81 (s, 3 H); 3.13 (m, 2 H); 2.13
(dd, J ˆ 8.8, 7.2, 1 H); 1.99 (dd, J ˆ 7.2, 6.4, 1 H); 1.65 (dd, J ˆ 8.8, 6.4, 1 H). 13C-NMR (75 MHz, CDCl3)22):
169.3 (s); 169.2 (s); 148.8 (s); 147.8 (s); 135.2 (d); 133.4 (d); 132.0 (d); 131.6 (d); 130.0 (d); 128.8 (d); 52.4 (q);
.
.
‡
‡
42.2 (t); 41.1 (t); 33.5 (s); 32.5 (s); 29.8 (d); 28.5 (d); 22.9 (t); 21.9 (t). MS20): 244 (6, M ), 242 (5, M ), 185 (14),
183 (13), 163 (100), 135 (17), 131 (16), 104 (32), 103 (58), 77 (11)21).
2.3. Methyl 7,8-Dihydrocalicene-7-carboxylate (13c). A soln. of 0.20 g (0.82 mmol) of 12c (tautomeric
mixture) in 3 ml of Et2O was transferred to a cooled ( 158) chromatography column containing 50 g of Al2O3
(basic with 3 drops of Et3N) and eluted with Et2O at 158. The hereby formed intensely yellow fraction was
collected. The solvent was evaporated i.v. to give 0.10 g (75%) of 13c as a yellow oil. UV (hexane): 269 (21000),
358 (ca. 350). IR20): 3084w, 2961w, 2256w, 1734s, 1438m, 1200m, 1177m, 911s, 731s, 646m. 1H-NMR (300 MHz,
CDCl3): 6.55 ± 6.42 (m, 2 H); 6.33 (dm, J ˆ 5.2, 1 H); 6.24 (dm, J ˆ 5.2, 1 H); 3.72 (s, 3 H); 2.57 (dd, J ˆ 9.0, 5.3,
1 H); 2.15 (dd, J ˆ 11.2, 5.3, 1 H); 1.95 (dd, J ˆ 11.2, 9.0, 1 H). 13C-NMR (75 MHz, CDCl3): 171.1 (s); 133.4 (d);
.
‡
132.9 (d); 123.3 (d); 122.9 (d); 52.2 (q); 18.2 (d); 11.7 (t). MS20): 162 (85, M ); 147 (36), 119 (44), 91 (100), 90
.
‡
(14), 89 (17), 65 (31), 63 (16), 39 (17)21). HR-MS: 162.0682 (M , C10H10O2; calc. 162.0680).
2.4. 1-(1,2-Dibromocyclopropyl)cyclopent-2-en-1-ol (11d). In a 50-ml flask, a soln. containing 1.0 g
(3.5 mmol) of 9d and 380 mg (4.6 mmol) of cyclopent-2-enone in 12 ml of dry Et2O was stirred at 1108. Then,
2.46 ml of 1.6m BuLi in hexane (3.94 mmol) were dropwise added through the septum with a syringe. The
mixture was stirred at 1108 for 2 h, then quenched with 3 ml of H2O, allowed to warm to r.t., and extracted
with Et2O (3 Â 10 ml). The combined org. layers were evaporated i.v. (108/14 Torr) to give a crude product
whose 1H-NMR spectrum shows the presence of three diastereoisomers 11d in a ratio of 1:0.5 :1. Careful
chromatography on ca. 50 g of silica gel with pentane/Et2O 5 :1 (containing a few drops of Et3N) allowed
separation of two isomers while the third remained impure. Total yield of all the fractions of 11d: 180 mg (18%)
as a colorless oil.
Data of Isomer 1: IR (CDCl3)20): 3548m, 3460m, 3056m, 2936m, 2850m, 1361m, 1322m, 1252m, 1120m,
1060s, 1030m ± s, 961m ± s, 751m, 694m, 594m. 1H-NMR (300 MHz, CDCl3): 5.99 (dm, J ˆ 5.5, 1 H); 5.83
(dm, J ˆ 5.5, 1 H); 3.55 (dd, J ˆ 8.8, 6.2, 1 H); 2.6 ± 2.3 (m, 3 H); 2.27 (s, 1 H); 2.12 (m, 1 H); 1.73 (dd, J ˆ 8.5,
6.2, 1 H); 1.67 (dd, J ˆ 8.8, 8.5, 1 H). 13C-NMR (75 MHz, CDCl3): 136.2 (d); 133.7 (d); 88.4 (s); 42.4 (s); 38.4 (t);
.
.
.
‡
‡
‡
30.9 (t); 26.8 (d); 23.0 (t). MS20): 266 (11, [M
18]), 264 (22, [M
18]), 262 (11, [M
18]), 185 (23), 183
(22), 104 (100), 103 (48), 90 (11), 78 (12), 77 (24), 63 (11), 51 (22), 50 (11), 39 (10).
Data of Isomer 2: 1H-NMR (300 MHz, CDCl3): 6.05 (dm, J ˆ 5.5, 1 H); 5.38 (dm, J ˆ 5.5, 1 H); 3.20
(dd, J ˆ 9.2, 5.9, 1 H); 2.55± 2.20 (m, 3 H); 2.19 (s, 1 H); 1.91 (m, 1 H); 1.64 (dd, J ˆ 9.2, 8.1, 1 H); 1.20 (dd, J ˆ 8.1,
5.9, 1 H). 13C-NMR (75 MHz, CDCl3): 138.4 (d); 131.5 (d); 88.1 (s); 48.2 (s); 37.9 (t); 31.2 (t); 23.34 (t); 23.23 (d)21).
2.5. 1-(1,2-Dibromocyclopropyl)cyclopenta-1,3- and -1,4-diene (12d)22). In a 50-ml flask, 67 mg (0.35 mmol)
of TsOH ´ H2O was added to a stirred soln. of 0.50 g (1.77 mmol) of 11d in 10 ml of dry benzene at r.t. The
mixture was stirred for 3 h (TLC showed complete reaction). Then, the solvent was removed i.v. (14 Torr) and
the oily residue purified by CC on ca. 30 g of silica gel with pentane/Et2O 5 :1. The pale-yellow fraction 1 was
collected and gave, after evaporation i.v., 0.20 g (43%) of 12d as a yellow oil (1.5 :1 mixture of two tautomers).
IR (CDCl3)20): 3061w, 2958m, 2928m, 1420m, 1372m, 1357s, 1178w ± m, 898m ± s, 680m ± s. 1H-NMR (300 MHz,
CDCl3): major tautomer22): 6.7 ± 6.3 (m, 3 H); 3.56 (dd, J ˆ 8.5, 5.5, 1 H); 3.08 (m, 2 H); 1.90 (t, J ˆ 8.5); 1.61
23
)
)
Only important IR absorptions and MS fragments are given.
For more spectroscopic data and illustrations of spectra, see [53], which is available on request from
21
M.N.
22
)
Mixture of two tautomers.

580
Helvetica Chimica Acta ± Vol. 83 (2000)
(dd, J ˆ 8.5, 5.5). 13C-NMR (75 MHz, CDCl3)22): 145.6 (s); 144.8 (s); 134.6 (d); 134.0 (d); 133.2 (d); 133.0 (d);
131.7 (d); 131.2 (d); 42.4 (t); 41.1 (t); 30.0 (s); 28.7 (d); 27.6 (d); 25.5 (t); 24.9 (t)21).
2.6. 7-Bromo-7,8-dihydrocalicene (13d). A soln. of 200 mg (0.75 mmol) 12d in 2 ml of Et2O was transferred
to a cooled ( 208) chromatography column containing 50 g of Al2O3 (basic, with 3 drops of Et3N) and eluted
with Et2O at 208. The hereby formed intensely yellow zone was collected. The solvent was evaporated i.v.
(14 Torr, 258) to give 60 mg (44%) of 13d as an intensely yellow oil. IR (CDCl3)20): 3071w, 2953m, 2922m ± s,
1
2851m ± s, 1214m ± s, 910vs, 762s, 730w, 615w. H-NMR (300 MHz, CDCl3): 6.53 (m, 2 H); 6.40 (m, 1 H); 6.29
(m, 1 H); 3.86 (dd, J ˆ 8.5, 4.4, 1 H); 2.24 (dd, J ˆ 13.1, 8.5, 1 H); 1.85 (dd, J ˆ 13.1, 4.4, 1 H). 13C-NMR
.
(75 MHz, CDCl3): 134.3 (d); 133.2 (d); 123.1 (d); 122.3 (d); 15.8 (t); 12.1 (d). MS20): 184 (12, M ), 182 (12,
‡
.
.
‡
‡
M
), 103 (100), 102 (23), 77 (67), 63 (12), 51 (25), 50 (14)21). HR-MS: 181.973 (M , C8H7Br; calc. 181.988).
3. Synthesis of a Precursor 21a of 7-Methylcalicene (Scheme 5). ± 3.1. 1-[1-Bromo-2-(chloromethyl)cy-
clopropyl]cyclopent-2-en-1-ol (19a). A 100-ml flask was charged with 3.0 g (12 mmol) of 18a, 1.4 g (18.3 mmol)
of cyclopent-2-en-1-one, and 50 ml of THF. The soln. was cooled to 958, and 15.0 ml of 1.6m BuLi in hexane
(24 mmol) were added dropwise under stirring. After 2 h of stirring at 958, warm-up, and 1 h of stirring at r.t.,
the mixture was quenched at 08 by adding 50 ml Et2O and 50 ml H2O and extracted with Et2O (3 Â 100 ml). The
combined extracts were dried (MgSO4) and evaporated i.v. The crude product was purified by low-temp. ( 208)
CC on ca. 50 g of silica gel with pentane/Et2O 10 :4 to give colorless oils of two isomers 19a23): 0.5 g (16.5%) of
isomer 1 (Rf 0.44) and 1.8 g (60%) of isomer 2 (Rf 0.25). Data of Isomer 124): 1H-NMR (300 MHz, CDCl3): 6.15
(m, 1 H); 5.57 (m, 1 H); 3.94 (m, 1 H); 3.69 (m, 1 H); 2.65 ± 2.4 (m, 3 H); 1.95 (m, 1 H); 1.84 (m, 1 H); 1.30
(m, 1 H); 1.20 (m, 1 H). 13C-NMR (75 MHz, CDCl3): 137.9 (d); 129.1 (d); 96.4 (s); 66.5 (t); 40.1 (s); 33.2 (t);
32.1 (t); 26.4 (d); 17.8 (t)25).
3.2. 1-[1-Bromo-2-(bromomethyl)cyclopropyl]cyclopent-2-en-1-ol (19b). According to procedure 3.1, with
3.0 g (10.2 mmol) of 18b, 1.3 g (15.8 mmol) of cyclopent-2-en-1-one, 50 ml THF, and 14 ml of 1.6m BuLi in
hexane (22.4 mmol). After low-temp. ( 208) CC of the crude product on ca. 50 g of silica gel with pentane/Et2O
10 :1, colorless oils of two isomers 19b23) were obtained: 0.76 g (25%) of isomer 1 (Rf 0.40) and 1.7 g (56%) of
isomer 2 (Rf 0.24). Data of Isomer 224): 6.08 (m, 1 H); 5.53 (m, 1 H); 3.75 (dd, J ˆ 10.3, 6.2, 1 H); 3.50 (dd, J ˆ
10.3, 8.8, 1 H); 2.6 ± 2.3 (m, 3 H); 2.20 (s, 1 H); 2.00 (m, 1 H); 1.61 (m, 1 H); 1.40 (dd, J ˆ 9.9, 6.6, 1 H); 0.99
(dd, J ˆ 7.0, 6.6, 1 H). 13C-NMR (75 MHz, CDCl3): 137.6 (d); 131.7 (d); 88.6 (s); 51.8 (s); 37.7 (t); 36.0 (t); 31.2
(t); 23.4 (d); 21.3 (t)25).
3.3. 2-[1-Bromo-2-(chloromethyl)cyclopropyl]cyclopenta-1,3-diene (20a)26). A 50-ml flask was charged
with 1.0 g (4 mmol) of 19a, 20 ml of dry benzene, and 140 mg (0.8 mmol, cat. amount) of TsOH, the mixture was
stirred for 3 h at r.t., and then evaporated i.v. The crude product was purified by low-temp. ( 208) CC over ca.
1
30 g of silica gel with pentane/Et2O 20 :1 to give 0.85 g (91%) of 20a26) as a colorless oil. (Rf 0.85). H-NMR
(300 MHz, CDCl3): 6.56 (m, 1 H); 6.50 (m, 1 H); 6.26 (m, 1 H); 3.92 (dd, J ˆ 11.4, 6.2, 1 H); 3.76 (dd, J ˆ 11.4,
8.1, 1 H); 3.07 (m, 2 H); 1.61 (dd, J ˆ 9.6, 6.6, 1 H); 1.53 (m, 1 H); 1.24 (dd, J ˆ 6.6, 6.2, 1 H). 13C-NMR
(75 MHz, CDCl3): 148.4 (s); 134.9 (d); 132.5 (d); 128.2 (d); 47.6 (t); 41.1 (t); 36.3 (s); 27.0 (d); 22.9 (t)25).
3.4. 7-(Chloromethyl)-7,8-dihydrocalicene (21a). The crude product obtained above (see 3.3) was dissolved
in ca. 2 ml of benzene, the soln. was transferred to a cooled ( 208) chromatography column containing 10 g of
Al2O3 (basic) and eluted with pentane/Et2O 20 :1 at 208. The hereby formed intensely yellow zone was
collected. The solvent was evaporated i.v. to give 0.44 g (80% from 20a) of 21a as an orange oil, which is
reasonably stable below 108. UV (hexane): 270 (23200), 350 (430). IR20): 3060w, 2920w, 1627m ± s, 1465m,
1370m, 1085w ± m, 1007w ± m, 875w ± m, 800w ± m, 760s, 595m. 1H-NMR (300 MHz, CDCl3): 6.59 (m, 2 H); 6.42
(m, 2 H); 3.62 (m, 2 H); 2.32 (m, 1 H); 1.89 (dd, J ˆ 11.2, 9.6, 1 H); 1.48 (dd, J ˆ 11.2, 5.5, 1 H). 13C-NMR
(75 MHz, CDCl3): 139.6 (s); 137.3 (s); 133.3 (d); 132.5 (d); 123.5 (d); 123.2 (d); 47.0 (t); 18.5 (d); 11.0 (t). MS20):
.
.
.
‡
‡
‡
154 (35, M ), 152 (70, M ), 117 (100), 104 (40), 86 (30), 74 (25). HR-MS: 152.0392 (M , C9H9Cl; calc.
152.0393)25).
24
)
)
)
Mixture of two diastereoisomers.
NMR Spectra of isomer 2 are very similar.
For more spectroscopic data and illustrations of spectra, see [54], which is available on request from
25
26
M.N.
27
)
Quite surprisingly, only one isomer 20a is isolated.

Helvetica Chimica Acta ± Vol. 83 (2000)
581
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Received October 11, 1999