An Elusive Nonaromatic Goal behind the Centropolyindanes: Aufbau of Veratrolo-Annelated Centropolyquinanes and Ozonolytic Abbau

Article abstract of DOI:10.1002/cplu.201700090

This study presents a potential experimental approach to the still elusive topologically nonplanar (K₅) parent hydrocarbon, centrohexaquinane (1), by a construction-dismantling (aufbau–abbau) strategy via electron-rich aromatic centropolyindanes. A series of veratrole-based centropolyindanes are synthesized and subjected to ozonolytic degradation. These include the 2,2′-spirobiindanes 30–32, fuso-diindane 33, triptindanes 34–36, tribenzotriquinacene 37, and tetramethoxycentrohexaindane 9. Spirane 30 and propellane 36 are characterized by X-ray structure analysis. Ozonolysis of 32 and 33 gives a keto ester (59) and a dimethyl muconate (60), respectively. Dimethoxytriptindane 34 gives a [3.3.3]propellane-cis,cis-muconate (61) in good yield, the stereochemistry of which is determined by X-ray structure analysis. Tetramethoxytriptindane 35 gives the [3.3.3]propellane-bis-muconate 62 along with a [3.3.3]propellane-dialdehyde-muconate (63). Hexamethoxytriptindane 36 furnishes three products of mainly intra-dimethoxy cleavage, with the [3.3.3]propellane-tris-muconate 64 as the major component. X-ray structure analysis of 64 reveals molecular C₃ symmetry and all-cis,cis stereochemistry of the three muconate groups. Hexamethoxytribenzotriquinacene 37 gives the triquinacene-tris-muconate 68, albeit in very low yield. Ozonolysis of tetramethoxycentrohexaindane 9 affords the bis-muconate 10 in moderate yield, along with two further centrohexacyclic products of single-wing degradation.

Full text of DOI:10.1002/cplu.201700090

Accepted Article
Title: An Elusive Nonaromatic Goal behind the Centropolyindanes:
Aufbau of Veratrolo-annelated Centropolyquinanes and
Ozonolytic Abbau
Authors: Marco Harig, Beate Neumann, Hans-Georg Stammler, and
Dietmar Kuck
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10.1002/cplu.201700090
ChemPlusChem
An Elusive Nonaromatic Goal behind the Centropolyindanes: Aufbau
of Veratrolo-annelated Centropolyquinanes and Ozonolytic Abbau
Marco Harig,[a] Beate Neumann,[a] Hans-Georg Stammler[a] and
Dietmar Kuck[a,b]
*
[a]Department of Chemistry and [b]Center of Molecular Materials (CM2),
Bielefeld University, Universitätsstraße 25, 33615 Bielefeld, Germany
*E-mail: dietmar.kuck@uni-bielefeld.de
Tel.: +0049 521 106 2060
Fax: +0049 521 106 6146
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Abstract. This study presents a potential experimental approach to the still elusive
topologically nonplanar (K5) parent hydrocarbon, centrohexaquinane (1), by a
construction-dismantling (aufbau-abbau) strategy via electron-rich aromatic
centropolyindanes. A series of veratrole-based centropolyindanes were synthesized
and subjected to ozonolytic degradation. These include the 2,2’-spirobiindanes 3032,
fuso-diindane 33, the triptindanes 3436, tribenzotriquinacene 37, and
tetramethoxycentrohexaindane 9. Spirane 30 and propellane 36 were characterized by
X-ray structure analysis. Ozonolysis of 32 and 33 gave a keto ester (59) and a
dimethyl muconate (60), respectively. Dimethoxytriptindane 34 gave
a
[3.3.3]propellane-cis,cis-muconate (61) in good yield, the stereochemistry of which
was determined by X-ray structure analysis. Tetramethoxytriptindane 35 gave the
[3.3.3]propellane-bis-muconate 62 along with a [3.3.3]propellane-dialdehyde-
muconate (63). Hexamethoxytriptindane 36 furnished three products of mainly intra-
dimethoxy cleavage with the [3.3.3]propellane-tris-muconate 64 as the major
component. X-ray structure analysis of 64 revealed molecular C3-symmetry and all-
cis,cis-stereochemistry
of
the
three
muconate
groups.
Hexamethoxytribenzotriquinacene 37 gave the triquinacene-tris-muconate 68, albeit in
minute yield. Ozonolysis of tetramethoxycentrohexaindane 9 afforded the bis-
muconate 10 in moderate yield, along with two further centrohexacyclic products of
single-wing degradation.
Keywords: polycyclic aromatic compounds; centropolyindanes; muconic acid
derivatives; ozonolysis; topology
Introduction
The family of centropolyindane hydrocarbons[1,2] has been developed as an extension
of polyquinane chemistry since 1984.[25] A strong and special inspiration came from
the challenge to synthesize centrohexaquinane, the polycyclic hydrocarbon 1 (Figure
1), reports about which were published simultaneously by Simmons III and by
Paquette in 1981.[6,7] The structure of 1 consists of six three-dimensionally fused
cyclopentane rings and represents the simplest normal-ring organic polycycle that
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corresponds to the topologically nonplanar graph K5.[8,9] Thus, the synthesis of
centrohexaindane (2), the six-fold benzo-annelated congener of 1, as the highest
member of the centropolyindane family, was a breakthrough in several respects: The
first K5-type hydrocarbon, a perfectly Td-symmetrical aromatic structure and a
molecule in which the six indane wings are stretched out perfectly along the three
axes of the Cartesian coordinate system.[10,11] Centrohexaindane comprises a massif
C17 core with octahedral orientation of its benzene units and thus represents a
complementary parent unit to buckminsterfullerene, a hollow C60 core that offers the
same octahedral extension in the 3-space.[1216]
Figure 1. Structures of the elusive centrohexaquinane (1) and the known centrohexaindane (2), both
representing graph-theoretically nonplanar hydrocarbons with K5 topology. Symmetries given for 1 and
2 refer to their constitutional (Td) and conformational ground-state (T and Td) structures, respectively.
In another mathematical view, centrohexaindane (1) represents the highest member of
even two families of centropolycyclanes.[17,18] One is that of the centropolyindanes,
which bear several indane units fused to each other at the central carbon.[1a]
Centropentaindane[19] is the next lower congener but tribenzotriquinacene
(TBTQ),[1,2,2028] triptindane,[29,30] and fenestrindane[31] are better known. The other
family is that of the partially benzoannelated centropolyquinanes, the members of
which bear the centrohexaquinane core of 1 with one to six annelated benzene
units.[1a,9,32,33] Pentabenzocentrohexaquinane (3) and the corresponding 1,2-diketone
are 4 known,[18,32] as are the C3v-symmetrical tribenzocentrohexaquinane 5 and its C3-
symmetrical, and thus chiral, triketone 6 (Figure 2).[32,33] We have also reported the
preparation of a dibenzo- (7) and the monobenzocentrohexaquinane (8), the latter of
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which was obtained in minute amounts by two-fold oxidative degradation/reduction of
5 via 7.[32] Notably, the parent nonaromatic hydrocarbon 1 has remained elusive since
Simmons’ and Paquette’s papers of 1981 on this topic.[6,7]
Will it possibly remain so for quite a while?
O
O
3
4
(C2v)
(C2v)
O
O
O
5
6 (rac)
(C3)
(C3v)
7
8
(Cs)
(C2v)
Figure 2. Structures of known partially benzoannelated centrohexaquinanes 3, 5, 7 and 8 and two
known derivatives, the achiral diketone 4 and the chiral triketone 6. Symmetries given refer to the
constitutional structures only.
Before embarking to the experimental efforts undertaken in the present investigation,
we present an overall view on the potential of a construction-dismanteling (aufbau-
abbau) strategy that may lead to centrohexaquinane (1) and a number of novel
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partially benzoannelated derivatives. The high variability and the symmetry features of
such unusual three-dimensional centrohexacyclanes are displayed in Scheme 1,
which comprises the hypothetical access to three parent hydrocarbons, viz., the
tetrabenzo, tribenzo and dibenzo derivatives 12, 16 and 20, respectively, as well as
some variants that could lead to the parent hydrocarbon 1.
Among the starting veratrole-based centrohexaindanes, the bis-veratrole 9,[34] and the
hexakis-veratrole 27[35] are known, while the tris-veratroles 13, 21 and 24 and tetrakis-
veratrole 17 are yet unknown but should be synthetically accessible along the
published synthesis strategies of the so-called propellane and fenestrane routes to the
centrohexaindanes.[1a,10b] As will be shown in the present work, the two electron-rich
veratrole entities of the D2d-symmetrical centrohexaindane 9 can be converted to the
corresponding bis(dimethyl muconate) 10 by ozonolysis (Scheme 1a). Further
dismanteling by oxidative cleavage of the four double bonds of the ester groups would
lead to the centrohexacyclic tetraketone 11 and subsequent reduction would yield
tetrabenzocentrohexaquinane 12. While the molecular symmetry would be maintained
during degradation, the conformational ground-state symmetry of hydrocarbon 12 is
predicted to be S4, in analogy to that of fenestrindane (73, see Figure 11)[31] and the
(calculated) conformation of 1.[36] Analogous stepwise degradation of the hypothetical
C3v-symmetrical centrohexaindane 13 would lead to the tris-muconate 14, for which a
C3-symmetrical conformation can be assumed, and to the hexaketone 15 (Scheme
1b). It would be interesting to elucidate experimentally whether 15 and the related
hydrocarbon 16 would also exhibit C3- rather than C3v-symmetry in their ground-state
conformations, the latter being characteristic for centropolyindanes bearing the
tribenzotriquinacene core.[1a] It should be noted that the structure of the unknown
hydrocarbon 16 is complementary to that of the known isomer 5. The likewise
unknown centrohexaindane 17, bearing four veratrole units, would be complementary
to bis-veratrole 9. As a consequence, the putative tetrakis-muconate 18, octaketone
19, and the unknown dibenzocentrohexaindane 20, an isomer of hydrocarbon 7, would
have D2d-symmetrical constitution but very likely also adopt S4-symmetrical
conformational ground states (Scheme 1c).
Tribenzocentrohexaquinane 21 and the corresponding triketone 24 would represent
the tris-veratrolo analogs of the known parent centrohexacyclanes 5 and 6,
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(a)
OMe OMe
CO2Me
MeO2C
O
O
ozonolysis
(this work)
O
MeO2C
MeO2C
MeO
O
MeO
11
9
10
12
(D2d / S4)
(D2d
)
(D2d / S4)
(D2d / S4)
(b)
O
O
O
COOMe
OMe
MeO2C
CO2Me
MeO
OMe
O
O
O
MeO2C
CO2Me
CO2Me
13
15
16
MeO
OMe
14
OMe
(C3v
)
(C3v / C3)
(C3v / C3)
(C3v / C3)
(c)
OMe OMe
MeO
MeO
MeO C
2
CO2Me
MeO2C
MeO2C
O
O
O
O
O
O
MeO2C
MeO2C
CO2Me
CO2Me
MeO
OMe
O
O
MeO
19
20
OMe
17
18
(D2d / S4)
(D2d
)
(D2d / S4)
(D2d / S4)
Scheme 1. Conceptual approaches to the partially benzoannelated centrohexaquinanes 12, 16 and 20
and centrohexaquinane (1), all yet being unknown, by stepwise oxidative degradation of veratrole-
based centrohexacyclic precursors, among which bis-veratrole 9 and hexakis-veratrole 27 are known.
The conversion of 9 to 10 is reported in the present work, whilst all other conversions are hypothetical.
Alternative symmetries given refer to the constitutional and (assumed) conformational structures.
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respectively, and should be accessible in established steps along the propellane
route.[33]
Oxidative dismanteling of 21 (Scheme 1d) and 24 (Scheme 1e) via the corresponding
tris-muconates 22 and 25 should give rise to the respective hexa- and nonaketones 23
and 26, the former of which having C3v-symmetrical constitution – but probably C3-
symmetrical ground-state conformations – and thus being achiral, whereas the latter
being chiral owing to its C3-symmetrical constitution. Finally, the known
dodecamethoxycentrohexaindane 27, for which a Td-symmetrical ground state
conformation can be safely assumed,[11] once being accessible in suitable amounts,[35]
should be subjected to oxidative degradation giving the hexakis-muconate 28, which is
assumed to exist in two equivalent T-symmetrical ground states, again in analogy to
the parent centrohexaquinane 1. Even more exciting (and exotic) would be the
centrohexacyclic dodecaketone 29, a hypothetical C17O12 carbon suboxide. Besides its
general chemical properties, this constitutionally Td-symmetrical centrohexacyclane
can also be assumed to exist in two equivalent T-symmetrical ground-state
conformations. Conceptually, at least, the oligoketones 23, 26 and 29 can be seen as
suitable precursors of centrohexaquinane (1) and a large number of highly unusual
derivatives, including unsaturated congeners (e.g., centrohexaquinacene 71, see
Figure 11) should become into experimental reach. Of course, all this will be a long
way to go. However, the concept appears to be valid and it may include a lot of highly
interesting intermediate goals. As will be shown in the present paper, further steps on
the way to centrohexaquinane (1) are challenging but also promising.
This report is arranged not only at the borderline between chemistry and mathematical
chemistry, but much more so at the borderline between aromatic and nonaromatic
(centro)polycyclic chemistry and, in the same time, at the borderline between the
construction (synthesis) and dismantling (degradation) of complicated polycyclic
architecture. The German terms “Aufbau” and “Abbau” describe this antagonism in an
(untypically) short and precise manner. We report a part of our extended efforts to use
electron-rich centropolyindanes as precursor substrates in an abbau approach to the
ultimate goal, centrohexaquinane (1). We performed this work as an orienting study to
explore the suitability of ozonolytic degradation of the arene units of electron-rich
centropolyindanes bearing veratrole units instead of the simple benzene rings (3037
and 9, Figure 3)[22,35,37,38] We will describe the synthesis of these veratrolo-annelated
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substrates and the variety of the ozonolysis products, only some of which represent
nonaromatic polycyclic compounds, while the others can be regarded as new
derivatives
of
the
partially
benzoannelated
centropolyquinanes
or
centropolyquinacenes. This work may inspire researchers working on indane[39] and
propellane chemistry,[40] in particular, and those being interested at the boundaries
between different fields of chemistry to contribute alternative approaches to elusive
centrohexaquinane (1) and its derivatives.
H
O
MeO
MeO
OMe
OMe
MeO
MeO
OMe
OMe
MeO
OMe
H
X
MeO
OMe
30
31
33
(X = CH2)
32 (X = CO)
OMe
MeO
OMe
MeO
OMe
OMe
OMe
MeO
MeO
OMe
OMe
OMe
34
35
36
MeO
OMe
MeO
OMe
OMe
OMe
Me
Me
MeO
OMe
MeO
OMe
MeO
OMe
MeO
MeO
OMe
MeO
37
9
Figure 3. The veratrolo-annelated centropolyindanes studied in this work: 2,2’-Spirobiindane 30, 2,2’-
spirobiindanone 31, 2,2’-spirobiindanedione 32, fuso-diindane 33, the triptindanes 34, 35 and 36,
tribenzotriquinacene 37, and centrohexaindane 9.
Results and Discussion
1. Construction (Aufbau) of centropolyindanes bearing veratrole units.
Cyclization of electron-rich precursors such as benzylic alcohols and phenones
bearing methoxy-substituted benzene rings are known to occur readily.[41] This has
been confirmed in the field of the centropolyindanes in numerous
cases.[1a,3,22,24,25,29,35,37,38,42,43] Therefore, many of the centropolyindanes 3037 and 9
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(Figure 3) that are the subject of ozonolytic degradation in the present work were
synthesized in good to excellent yields. The syntheses of some of them were reported
earlier and will be commented only briefly here.
5,5’,6,6’-Tetramethoxy-2,2’-spirobiindane (30) and its monoketone 31 and diketone 32
were synthesized on the basis of established procedures (Scheme 2). 5,6-
dimethoxyindan-1-one (38) was spiroalkylated with 4,5-bis(chloromethyl)veratrole 41,
which in turn was obtained in improved yield (48%) by two-fold chloromethylation of
veratrole (40).[44] The yield of the spiroannelation product 31 was relatively low (38%),
but acceptable in view of the fact that repeated recrystallization was necessary to
remove substantial amounts of by-products. Notably, no isomeric cyclization product
was observed in this case. Ionic hydrogenation of the electron-rich spiroketone 31
using sodium borohydride/trifluoroacetic acid in methylene chloride was found to be
the method of choice,[45,46] in contrast to catalytic hydrogenolysis, and gave the
tetramethoxyspirane 30 in good yield (66%).
Scheme 2. Synthesis of spirobiindane 30, spirobiindanone 31 and spirobiindanedione 32.
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5,5’,6,6’-Tetramethoxy-2,2’-spirobiindane (30) crystallizes from methanol in the
monoclinic space group C2/c with four molecules per asymmetric unit, and the spiro
carbon atom possesses a crystallographic two-fold axis. The five-membered rings
both adopt envelope conformations (Figure 4a) with an angle of inclination of 25.3(1)°.
All non-hydrogen atoms of the dimethoxyindane moieties except the spiro carbon
atom lie almost perfectly within the same plane; the mean deviation is 0.04 Å and the
maximum deviation shows C10 with 0.083(1) Å. This plane and its symmetry
equivalent within the same molecule enclose an angle of 77.3(1)°. Within the crystal,
the molecules are arranged in sheets that consist of parallel ribbons (Figure 4b). The
methoxy groups appear to play a dominant role for the molecular packing in the crystal
by forming a nonclassical hydrogen bond between O1 and H11B generated by 1/2-
X,1/2+Y,3/2-Z symmetry with a distance of 2.47(1) Å.[47]
(a)
(b)
Figure 4. (a) Molecular structure of 2,2’-spirobiindane 30 in the solid state, determined by single crystal
X-ray diffraction; (b) view onto the layered orientation of the molecules along the b axis. Oxygen atoms
are shown in red, hydrogen atoms are omitted. Thermal ellipsoids are drawn at 50% level.
The synthesis of the corresponding spirodiketone 32 turned out to be less
straightforward. In contrast to the fact that the literature on 1,3-indanediones is very
extended,[1a,39,42,4851] we required an improved access to larger amounts of the
literature-known 5,6-dimethoxy-1,3-indanedione (39).[52] Based on extended
experiments, we found that diketone 39 can be readily obtained by oxidation of the
corresponding monoketone 38 with chromium trioxide in yields up to 54%, a method
that is known to be suitable for electron-rich para-alkylanisoles.[53] Whereas a large
excess of the reagent is required,[54] no products of over-oxidation were observed.
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Recently, McKeown et al. reported the same conversion of 38 into 39 with a similar
yield Spiroalkylation of diketone 39 using again 4,5-
(51%).[38]
bis(chloromethyl)veratrole (41) was tried under various conditions and turned out to be
difficult. Sodium hydride gave mainly the product of C,O-dialkylation, enol ether 42,
while phase transfer catalysis brought about mainly the product of single C-alkylation,
which, after recrystallization from ethanol, furnished the benzylic ether 43
contaminated with enol ether 42 and the desired spirodiketone 32. In contrast, use of
potassium fluoride on celite 545 in acetonitrile turned out to be superior to other
bases.[55] This alkylation method was found to largely suppress O-alkylation.[2a,31b] In
the present case, reaction of diketone 39 with dichloride 41 for only 6 h gave again
compound 43 as the major product after recrystallization. In contrast, reaction of 39
and 41 for 24 h afforded a 2 : 1 mixture of enolether 42 and diketone 32. Separation of
the isomers by gravity column chromatography was unsuccessful and recrystallization
from ethanol gave rise to heavy loss of material. As a viable alternative, we found that
thermolysis of the crude product mixture under argon at 260270 °C for 30 min gave
rise to clean isomerization of enol ether 42 to spirodiketone 32. Subsequent
chromatography and recrystallization from ethanol afforded the pure diketone in 39%
yield. The efficient thermally induced 1,3-shift in 42 parallels similar isomerization
reactions reported in the literature, including the formation of spirocyclic
ketones.[33,56,57]
Scheme 3. Synthesis of the 2,3,6,7-tetramethoxy-substituted fuso-diindane 33.
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The synthesis of 2,3,6,7-tetramethoxy-4b,9,9a,10-tetrahydroindeno1,2-aindene 33, a
fuso-diindane,[17]
is outlined in Scheme 3. Condensation of 3,4-
dimethoxybenzaldehyde (44) with 5,6-dimethoxyindan-1-one (38) afforded the
benzylideneindanone 45 in 92% yield.[58] Subsequent reduction was carried out by
both transfer hydrogenolysis[59] with tris(triphenylphosphine)ruthenium(II) chloride and
ethylene glycol (65% yield) and by heterogeneous catalytic hydrogenation,[29,60] which
proved to be superior. Over-reduction was avoided by use of acid-free ethyl acetate
(passed through basic Al2O3) as a solvent and pyridine as an additive to suppress
hydrogenolysis of the carbonyl functionality. In this way, the benzylindanone 46 was
obtained in virtually quantitive yield and in 88% yield after recrystallization.
Subsequent reduction of 46 with sodium borohydride in ethanol gave a 1 : 1 mixture of
the corresponding benzylindanol and the respective benzylindene, as determined by
1H NMR spectroscopy. It appears that one of the diastereomeric indanols[61] is
extremely sensitive to traces of acidic impurities. The crude product mixture was
subjected to cyclization using polyphosphoric acid as a catalyst.[62] The cyclization
process was found to be complete within 1 h, in stark contrast to the analogous
conversion of unsubstituted 2-benzyl-1-indanol, leading to the parent diindane, which
requires more than 1 d.[62] Once again, this clearly reflects the strongly increased
reactivity of the veratrole derivatives towards electrophilic reactants. The fuso-diindane
33 was isolated in good yield (66%) over two steps.
Scheme 4. Synthesis of the 2,3-dimethoxy-substituted triptindane 34.
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2,3-Dimethoxytriptindane 34 was synthesized by starting from 1,3-indanedione (52)
and employing the well-established stepwise benzylation strategy. The
monobenzylindanedione 47, which is obtainable in two steps in good yield (55%),[35,38]
was reacted with benzyl bromide using the potassium fluoride/Celite method, which
afforded the dimethoxy-substituted dibenzylindanedione 48 in 86% yield (Scheme 4).
Two-fold cyclodehydration of 48 was achieved by use of Amberlyst 15 as a catalyst in
refluxing toluene, giving the interesting (unsymmetrical and thus chiral) triptindan-9-
one 49 in only moderate yield (42%) after 16 h. Notably, extended reaction times (e.g.
5 d) gave rise to ether cleavage, yielding the monophenols 50 and 51 along with 49 as
still the major product. All three products were separable by gravity column
chromatography and characterized. In the final step, triptindanone 49 was subjected to
remarkably slow catalytic hydrogenolysis with palladium-on-charcoal under medium
pressure conditions to give 2,3-dimethoxytriptindane 34 in excellent yield (92%).
Scheme 5. Synthesis of the 2,3,6,7-tetramethoxy-substituted triptindane 35 (see also ref. [38]).
2,3,6,7-Tetramethoxytriptindane 35 was synthesized following various paths (Scheme
5).[35] This compound was also synthesized recently by McKeown et al.,[38] using our
general methodology, as a precursor of triptindane- and other centrotriindane-based
microporous polymers.[22,38,63] Our synthesis sequence employs slightly deviating
techniques and is briefly outlined here.[35] KF-Celite assisted two-fold benzylation of
1,3-indanedione
(52)
with
dimethoxybenzyl
bromide
53[64]
gave
the
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dibenzylindanedione 55 in only moderate yield (42%). This method turned out to be
much less practical than the alternative secondary benzylation of
monobenzylindanedione 47, which was carried out with either 53 or, more favorably,
with dimethoxybenzyl chloride 54[65] under similar conditions and afforded diketone 55
in significantly better yields (69% and 87%, respectively). In fact, use of
dimethoxybenzyl chloride 54 gave rise to less side products, in line with literature
reports.[66] Two-fold cyclization of indanedione 55 using Amberlyst 15 in toluene turned
out to be quite inefficient; the desired tetramethoxytriptindanone 56 was obtained in
surprisingly low yield (17%). It appears that ether cleavage processes intervene here
even more heavily than in the synthesis of the dimethoxytriptindanone 49 from
indanedione 48 (see above). Both intermediates 55 and 56 were also reported
recently; compound 55 was obtained from 52 and 53 in lower yields (15%) using
KF/Celite and compound 56 was obtained in higher yields (76%) using
orthophosphoric acid in toluene.[38] Catalytic hydrogenolysis of triptindanone 56 under
medium pressure conditions and in slightly acidic solution occurred smoothly and
furnished the desired tetramethoxytriptindane 35 in good yield (55%).
Scheme 6. Synthesis of the 2,3,6,7,13,14-hexamethoxy-substituted triptindane 36 (see also ref. [38]).
The C3v-symmetrical 2,3,6,7,13,14-hexamethoxytriptindane 36 is also accessible via
both one-step two-fold benzylation and the stepwise benzylation variant.[35] McKeown
et al. recently repeated the synthesis using the former approach.[38] The sequence
starts from 5,6-dimethoxy-1,3-indanedione (39) and leads to the six-fold methoxy-
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substituted 2,2-dibenzyl-1,3-indanedione 57 as the key intermediate (Scheme 6). Two-
fold cyclization (“bicyclodehydration”[30a,42a]) of this compound using orthophosphoric
acid in refluxing toluene gave the hexamethoxytriptindanone 58 in good yield, which
was then subjected to ionic hydrogenation with sodium borohydride in trifluoroacetic
acid. The overall yield of hexamethoxytriptindane 36 was satisfactory (50%).[35]
Crystals of hexamethoxytriptindane 36 obtained from methanol were suitable for X-ray
structure analysis, which revealed the monoclinic crystal system with space group P
21/c.[47] Unfortunately, disordering effects are rather large and the R values were too
high to allow detailed structural analysis. Nevertheless, it is obvious that the
propellane wings of structure 36 adopt envelope conformations with some torsion
about the propellane axis (Figure 5).
Figure 5. Molecular structure of triptindane 36 in the solid state, determined by single crystal X-ray
diffraction. Oxygen atoms shown in red, hydrogen atoms are omitted, thermal ellipsoids are drawn at
50% level.
Prior to our report on triptindane 36, we had also disclosed the synthesis of
2,3,6,7,10,11-hexamethoxytribenzotriquinacene 37, which also employs 5,6-
dimethoxy-1,3-indanedione (39) as a starting point.[37] The access to the tetramethoxy-
substituted centrohexaindane 9 is much lengthier. It involves the multistep synthesis of
fenestrindane (73, see Figure 11 below), a tetrafuso-centrotetraindane and thus a
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higher member of the centropolyindane family.[1a] Two re-functionalization steps of this
hydrocarbon are required to enable the incorporation of two veratrole units “spiro-wise”
on both sides of the fenestrane framework.[34,67] In fact, these final conversions in the
multistep synthesis of tetramethoxycentrohexaindane 9 are remarkably efficient and,
besides the bis-veratrole 9, even the corresponding centrohexaindane-based bis-
resorcinol and bis-hydroquinone dimethyl ethers are accessible in this way.[34] This
points to the fact that a number of further electron-rich (multiply methoxylated)
centropolyindane-type polycyclic aromatic compounds may become accessible in the
not-too-far future. A very recent example is noteworthy in this context.[2c,2d]
2. Ozonolytic degradation (Abbau) of centropolyindanes bearing veratrole units.
Ozonolytic degradation of the veratrole-type centropolyindanes described above was
carried out under standard conditions in most cases.[6873] Thus, the substrate was
dissolved in a mixture of anhydrous dichloromethane and anhydrous methanol (1:1)
and the solution was cooled to  78 °C. Under standard conditions, the oxidation was
performed by bubbling the O3/O2 gas mixture through the stirred solution ( 10 L/h)
until its color turned persistingly deep-blue. Then argon was bubbled through the
solution for 30 min to remove the excess of ozone, dimethyl sulfide was added and the
mixture was stirred for 24 h. In special instances, the ozonolysis conditions were
varied.
In several cases, we did not obtain a product that could be isolated in pure form and
fully characterized. Such experiments will not be described in detail below since
information about the products relies mainly on electron ionization (EI) mass
spectrometry and remains partially speculative.[35b] For example, we failed in our
attempts to prepare a defined ozonolysis product from 5,6-dimethoxyindane. Likewise,
ozonolysis of tetramethoxy-2,2’-spirobiindane 30 and the corresponding monoketone
31 were unsuccessful. Variation of the reaction conditions, for example, by adding
boron trifluoride etherate in the case of 30, gave rise to some changes in the EI mass
spectra but did not allow us to isolate a defined product.[35b] Whereas compounds 30
and 31 appeared to be too reactive towards ozone, the spirodiketone 32 was found to
be almost unreactive at  78 °C. We assumed that the limited solubility of 32 at low
temperatures could be a reason for these observations. In fact, working at  40 °C led
to a defined product, namely, the unsaturated triketoester 59, which was isolated after
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chromatography in moderate yield (28%, Scheme 7). It was identified and
1
characterized by EI mass spectrometry (M+ at m/z 344) and H and 13C NMR
spectroscopy (see Supporting Information for selected NMR and mass spectra). For
1
example, the H NMR spectrum of 59 showed only one single olefinic triplet ( 6.62
4
13
with J = 2.9 Hz) and the C NMR spectrum exhibited two resonances for the three
ketone carbonyl groups ( 202.1 and  200.0) and one for the ester functionality (
166.3), in line with the molecular Cs symmetry. It appears that the first oxidative
cleavage leading to the corresponding muconic acid diester is much slower than the
attack of ozone in the second degradation step giving 59, quite different from the
ozonolysis reactions described below. Overall, the ozonolysis of the 2,2’-
spirobiindanes turned out to be rather disappointing.
1. O3
MeOH, CH2Cl2
40 °C
O
O
MeO
MeO
OMe
OMe
MeO
MeO
2. DMS
CO2Me
28%
O
O
O
32
59
1. O3 (1 eq)
MeOH, CH2Cl2
78 °C
2. DMS
CO2Me
CO2Me
MeO
OMe
9%
MeO
MeO
OMe
MeO
33
60
Scheme 7. Ozonolysis of 2,2’-spirobiindane-1,3-dione 32 and fuso-diindane 33.
Ozonolysis of the tetramethoxy-substituted fuso-diindane 33, a close congener of
2,2’-spirobiindane 30, under standard conditions was found to be similarly difficult
(Scheme 7). Obviously, the reaction with excess of ozone at  78 °C did not stop at
defined stages of the degradation. Mass spectrometric analysis under chemical
ionization (CI) conditions indicated that the tertiary bridgehead C-H bonds also reacted
with the reagent.[35b,73] Even dimethyl oxalate was identified as a product of the
ozonolysis of compound 33. This indicated that the oxidative cleavage occurs both
between the methoxy groups of a given veratrole unit (“intra-dimethoxy cleavage”) and
at the adjacent C-C bonds of the ring (“extra-dimethoxy cleavage”). The only defined
degradation product of 33 was isolated when the amount of ozone was restricted to
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one equivalent. Chromatography of the crude product mixture, which contained some
starting material (7%), furnished the product of the single intra-dimethoxy cleavage,
dimethyl cis,cis-muconate 60, in low yield (9%). The EI mass spectrum of 60 exhibited
an intense molecular ion peak at m/z 358 (69%) and the base peak for the
characteristic loss of 59 u ([M  MeO  CO]+ and/or [M  MeOCO]+) at m/z 299
(100%). This fragmentation can be assigned to a stepwise cyclization-cleavage
process ionized muconic esters leading to particularly stable pyrylium ion structures.[74]
1
The H NMR spectrum confirmed the structure of 60 by the resonances of the
unreacted veratrole ring at  6.76 and  6.60 (1 H each) and those of the two olefinic
protons of the muconic acid moiety at  6.10 and  5.80, besides other characteristic
signals. The results show that, here again, ozonolytic dismantling of a simple electron-
rich diindane, such as 33, is no easy task. Nevertheless, two “typical” product sub-
structures resulting from the degradation of a single veratrole ring in such compounds
have been identified.
Scheme 8. Ozonolysis of dimethoxytriptindane 34 and tetramethoxytriptindane 35.
2,3-Dimethoxytriptindane (34) represents the first of the higher centropolyindanes that
were subjected to ozonolysis (Scheme 8). Treatment of this compound under standard
conditions followed by reductive work-up with dimethyl sulfide gave dimethyl cis,cis-
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muconate 61 in remarkably high yield (61%). Thus, as expected, only the electron-rich
veratrole nucleus was attacked under these conditions, leaving the remaining
dibenzo[3.3.3]propellane framework unaffected. EI mass spectrometry and 1H and 13C
NMR spectroscopy of 61 unambiguously revealed its structural identity. The EI mass
spectrum showed the molecular ion peak at m/z 386 with moderate intensity (10%)
and again the base peak for the characteristic loss of 59 u for the loss of a
1
methoxycarbonyl residue at m/z 327. The H NMR spectrum again exhibited the
resonances of the two olefinic protons of the muconic ester moiety at  6.11 (s) and 
5.76 (dt), besides the characteristic low-field doublet at  7.43 indicating the two ortho-
protons at the inner positions of the remaining fuso-diindane unit, and two methoxy
resonances  3.61 and  3.56. An extremely narrow AB spectrum at  3.13 and  3.10
for four protons of two equivalent methylene groups and a 2-H singlet for the unique
methylene group reflect the molecular Cs-symmetry of 61. The 13C NMR spectrum also
confirmed the structure of 61, exhibiting two characteristic resonances for the two
quaternary carbons of the propellane axis at  76.3 and  58.3.
Figure 6. Molecular structure of dibenzo[3.3.3]propellane dimethyl muconate 61 in the solid state,
determined by single crystal X-ray diffraction. Oxygen atoms shown in red, hydrogen atoms are omitted.
Thermal ellipsoids are drawn at 50% level.
Crystals suitable for X-ray diffraction of propellane 61 were grown from methanol and
single crystal structural analysis confirmed the molecular structure and, in particular,
the 2(Z),3(E)-configuration of the dimethyl “cis,cis”-muconate moiety (Figure 6).[47] The
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unit cell of 61 contains four molecules. The two indane wings are distorted by 5.8(2)°
and 7.8(2)° along the propellane axis and the unique cyclopentane ring adopts a half-
chair conformation with a slight torsion of 7.9(2)° along the propellane axis. In contrast,
the torsion about the central C-C single bond of the muconate unit is 47.7(3)°. We note
a close structural relationship between the structure of compound 61 and a dimethyl
muconate with fixed cis,cis-stereochemistry described by Baran et al.[75]
Interestingly, ozonolysis of the next-higher congener of the series of triptindanes
bearing veratrole nuclei, 2,3,6,7-tetramethoxytriptindane (35), occurred with similarly
high efficiency (Scheme 8). Working again under standard conditions at  78 °C, we
found the product of two-fold intra-dimethoxy cleavage, the [3.3.3]propellane
bis(dimethyl cis,cis-muconate) 62, to be the most abundant compound formed. In
addition, a minor product was isolated which turned out to be the combined maleic
dialdehyde dimethyl cis,cis-muconate 63. Gravity column chromatography furnished
the two ozonolysis products in pure form and in yields of 47% and 12%, respectively,
thus giving almost the same combined yield as that found for the ozonolysis of
dimethoxytriptindane 34. Thus, in the case of tetramethoxytriptindane 35, the electron-
rich veratrole units were again converted predominantly into the corresponding cis,cis-
muconate units, that is, in the same way as found for the lower congener 61 by two-
fold intra-dimethoxy cleavage in this case. In addition, however, extra-dimethoxy
ozonolysis of one of the veratrole units of 35 competes significantly, giving rise to
dialdehyde 63.
Both the major and the minor product were obtained as colorless oils. The EI mass
spectrum of the former one, tetraester 62, showed the molecular ion with even lower
relative abundance by the peak at m/z 478 (6%) and the abundant and characteristic
[M  C2H3O2]+ peak at m/z 419 (100%), indicating again the presence of the muconate
units. The lower stability of the molecular ions of 62, as compared to those of 61, is
1
certainly due to the further decreased number of aromatic rings. The H NMR
spectrum of 62 exhibits a singlet for the single remaining benzylic methylene group at
 3.17 and a very narrow AB spectrum system for the two equivalent allylic methylene
groups at  2.73 and  2.70. The two olefinic protons within the molecular cavity of 62
resonate at  6.16 and the two outer ones at  5.84, all as singlets in this case, which
is very similar to the resonances of the respective inner and outer olefinic protons of
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monomuconate 61 (see above). Again, the NMR spectrum reflects the molecular Cs-
13
symmetry of this [3.3.3]propellane tetraester 62. Beyond that, its C NMR spectrum
shows the characteristic resonances of two quaternary carbon atoms of the propellane
axis at  74.0 and  51.7, comparable to the corresponding lines in the 13C NMR
spectrum of [3.3.3]propellane 61 discussed above.
In contrast, the spectroscopic identification of the chiral ozonolysis product 63 bearing,
notably, three different propellane wings is more complicated. In the EI mass
spectrum, the molecular ion peak expected at m/z 392 is absent; however, the
corresponding [M  C2H3O2]+ ion peak at m/z 333 (100%) again clearly indicates the
presence of a muconate unit. Also, in place of the molecular ion, the corresponding
[M + H]+ ion peak (probably formed by self ionization) was observed at m/z 393. The
1H NMR spectrum of 63 shows the four resonances of the remaining intact benzene
ring, two distinct aldehyde proton resonances at  10.54 and  10.49, and only one
pair of olefinic proton resonances at  6.49 (s) and  5.90 (m) for the single muconate
unit. The three nonequivalent methylene groups of 63 give rise to two distinct AB
partial spectra and a singlet at  3.14 due to isochronous proton resonances. Also
owing to the lack of molecular symmetry of 63, its 13C NMR spectrum shows three
distinct methylene resonances at  47.3,  45.9 and  44.5. Besides the two pairs of
aldehyde and ester carbonyl resonances at  188.1 and  186.8 and at  165.9 and 
165.5, respectively, again the two quaternary carbons of the propellane axis
consistently appear at  78.3 and  54.8.
Ozonolysis of 2,3,6,7,13,14-hexamethoxytriptindane (36) under standard conditions
gave an even more complex product mixture (Scheme 9). Among the four fractions
obtained by gravity column chromatography, the first one was the mono-
dimethylacetal 67, which was recognized by EI mass spectrometry (e.g., [M]+ at m/z
530 and [M  C2H3O2]+ at m/z 471) but not fully characterized. Further
chromatography of the second fraction gave three well-defined compounds which
eluted in the following order: The [3.3.3]propellane bis(dimethyl muconate) 65,
representing a further product of incomplete ozonolysis, was isolated in 13% yield; the
[3.3.3]propellane tris(dimethyl muconate) 64, the desired product of three-fold intra-
dimethoxy cleavage, was obtained in pure form as the major product in 25% yield;
and, finally, the [3.3.3]propellane dialdehyde bis(dimethyl muconate) 66, another
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product of three-fold dismantling of the veratrole rings, was isolated in 10%. Thus,
several features of the product variety found with the lower congeners of triptindane 36
appeared here again. Besides the fact that the dimethyl acetal 67 represents the same
progress of ozonolysis as dialdehyde 66 does, it remains unclear why the veratrole
unit could survive the ozonolysis conditions in spite of the substantial excess of the
reagent in these experiments.
Scheme 9. Ozonolysis of hexamethoxytriptindane 36.
The finding that the C3v-symmetrical tris(dimethyl muconate) 64 was isolated as the
major product and in 25% yield, after all, was encouraging. However, ozonolysis
experiments carried out under quite a number of varied reaction conditions in the case
of 2,3,6,7,13,14-hexamethoxytriptindane (36) did not improve the yield of 64 (Scheme
9). As expected, pronounced temperature dependence was observed; however, it was
found to be in part contra-intuitive. Working at lower temperature ( 90 °C) led to the
same set of products but with increased amounts of the dialdehyde 66 (21%) and
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decreased amounts of the incompletely cleaved bis(dimethyl muconate) 65 (6%). The
tris(dimethyl muconate) 64 was isolated in 17% yield only. Thus, surprisingly, the
farther proceeded extra-dimethoxy cleavage appears to prevail under milder
conditions. In contrast, working at elevated temperature ( 25 °C) furnished the tris-
muconate in decreased yield (18%) and a relative large amount of the dialdehyde
dialdehyde monoketal 67 (22%). Expectedly, in this case the product of partial
degradation, veratrolo[3.3.3]propellane 65, was not found. Interestingly, ozonolysis in
the absence of methanol as a co-solvent gave rise to very low yield (9%) of the
tris(dimethyl muconate) 64 as the sole product. Replacement of methanol by other
additives, such as boron trifluoride di-etherate or pyridine, also gave unsatisfying
results: Tris(dimethyl muconate) 64 was isolated in very low yields only.
The three products 64, 65 and 66 of the ozonolysis of tris-veratrolo[3.3.3]propellane 36
were fully identified and characterized by mass spectrometry and NMR spectroscopy.
In line with the findings discussed above, the EI mass spectrum of
veratrolo[3.3.3]propellane 65, being the only aromatic compound among the three
products, exhibits a relative intense molecular ion peak (m/z 538, 34%) and, once
1
again, the characteristic [M  C2H3O2]+ signal (m/z 479) as the base peak. The H
NMR spectrum clearly shows the singlet resonances of the remaining veratrole ring at
 6.75 and  6.61. The equivalent muconate units are indicated by the resonances two
pairs of equivalent olefinic protons at  6.61 and  6.08 which, in this case, undergo
pronounced 4J coupling with the protons of the two equivalent methylene groups. The
1H NMR spectrum of 65 recorded in benzene-d6 solution shows an ASIS effect, which
is particularly pronounced for the methoxy proton resonances. The molecular Cs-
symmetry of propellane 65 is also confirmed by the 13C NMR spectrum. For example,
two methoxy carbon resonances are found for the single veratrole unit at  56.3 and 
55.8 and two methoxy carbon resonances of the pairwise equivalent ester methoxy
groups appear at  51.4 and  51.1 with about two-fold relative intensity.
The [3.3.3]propellane dialdehyde bis(dimethyl muconate) 66 was also identified
unequivocally. The CI(NH3) mass spectrum displays predominant peaks for the
molecular ions [M + NH4]+ (m/z 502) and [M + H]+ (m/z 485) together for those of the
corresponding characteristic fragment ions, such as [M + NH4  MeOH]+ (m/z 470)
and [M + H  MeOH  CO]+ (m/z 425, 100%), the latter peak being probably also due
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to the fragmentation of the molecular radical cation, [M]+ (m/z 484), and hence to the
[M  C2H3O2]+. The 1H NMR spectrum of 66 exhibits two distinct aldehyde resonances
at  10.43 and  10.40 as well as the olefinic and ester resonances of two equivalent
dimethyl muconate groupings. The 13C NMR spectrum also confirms the molecular Cs-
symmetry of 66.
Figure 7. 13C NMR spectrum (126 MHz, CDCl3) of [3.3.3]propellane tris(dimethyl muconate) 64.
Finally, the spectroscopic identification and structural characterization of the
[3.3.3]propellane tris(dimethyl muconate) 64 deserves some more detailed comments.
The EI mass spectrum exhibits a relative intense molecular ion peak at m/z 570 and
the characteristic loss of 59 u, again generating the [M  C2H3O2]+ ions (m/z 511) and
giving rise to the base peak. Other peaks as well as the CI(NH3) mass spectrum
further corroborate the structure. The NMR spectra of compound 64 clearly reflect the
1
molecular C3v-symmetry. In the H NMR spectrum, the signal at  5.88 shows two
5
long-range couplings (4J = 1.6 Hz and J = 0.6 Hz) and can therefore be assigned to
the three equivalent protons of the Z-configuated double bonds adjacent to the three
methylene groups. Similar to the muconate congeners presented above, the
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resonance of the three olefinic protons of the E-configuated double bonds close to the
molecular cavity appear at significantly lower field ( 6.60). As shown in Figure 7, the
13C NMR spectrum of 64 exhibits pairs of three identical resonances for all of the
carbon atoms with exception of the two quaternary carbon atoms of the propellane
axis, which appear at  69.3 and  46.7, and the three equivalent methylene carbon
atoms ( 46.3). The quaternary olefinic carbons of the tris-muconate 64 resonate at
 149.3 and  148.4, whereas the signals of the tertiary olefinic carbons are found at 
120.4 and  119.4.
Figure 8. Molecular structure of propellane 64 in the solid state, determined by single crystal X-ray
diffraction. Oxygen atoms shown in red, hydrogen atoms are omitted. Thermal ellipsoids are drawn at
50% level.
The solid-state molecular and crystal structures of 64 were obtained with single
crystals grown from methanol (Figure 8).[47] The structure is monoclinic (P 21/c) and
the asymmetric unit comprises one molecule of 64 and 26% of one additional
molecule of methanol introducing a disordering with ratio 74:26 of one ester
functionality. The three propellane wings of 64 adopt two different conformations by
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rotation of one of the carboxylate groups. Thus, the three 1,3-diene units exhibit
different torsional angles about their central C-C bonds [52.6(3)°, 57.7(2)° and
59.8(2)°] – all of them exceeding the corresponding angle in mono-muconate 61 (see
above) – whereas the torsion of 64 within the cyclopentane rings along the propellane
axis is almost negligible [2.3(1)4.1(1)°]. All of the three cyclopentene rings exist in the
envelope conformation, with the tip of the envelope bearing the quaternary carbon
atom of the Z-configurated double bond. The bond distance of the propellane C-C
bond was found to be 1.590(2) Å, similar to most of the cases discussed above and to
the central C-C bond of triptindane [1.572(2)][76] and the ozonolysis precursor of 64,
hexamethoxytriptindane 36.
Scheme 10. Ozonolysis of hexamethoxytribenzotriquinacene 37.
In contrast to the methoxylated triptindanes, ozonolytic degradation of 2,3,6,7,10,11-
hexamethoxytribenzotriquinacene 37 turned out to be very disappointing (Scheme 10).
Working under standard conditions at  78 °C gave an extremely complex mixture of
1
products. The H NMR spectra of the mixture exhibited several aldehyde resonances
in the range  10.0010.50 and a number of signals close to  6.00 indicating the
presence of non-equivalent muconate groupings. The EI mass spectra of the crude
product showed the molecular ion peak of the triquinane derivative 68 (M+ at m/z 570)
and the corresponding fragment ion peak for the loss of 59 u ([M  C2H3O2]+) at m/z
511 with relative high intensities and along with numerous other signals. Gravity
column chromatography led to improved mass spectra, and the aldehydic components
of the mixture were removed by treatment with aqueous sodium bisulfite, as shown by
1H NMR spectroscopy. Preparative thin layer chromatography allowed us to further
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enrich compound 68. Unfortunately, recrystallization failed due to the small amounts of
1
material. The H NMR spectrum of the hexakis(dimethyl muconate) 68 reveals the
presence of six olefinic protons ( 5.98), each of which undergoes long-range coupling
(4J  1.5 Hz) with one of the three diallylic methyne protons at the outer bridgeheads (
3.23). The presence of six methoxy groups was confirmed by a singlet at  3.62 and
13
the central methyl group appears at  1.36. Although a C NMR spectrum was not
recorded in this case, the molecular C3v-symmetry was clearly documented by the
simplicity of the 1H NMR spectrum. The EI mass spectrum also supports the identity of
compound 68, as already mentioned above.
We also studied the ozonolysis of the fully bridgehead-methylated analog of
compound
37,
2,3,6,7,10,11-hexamethoxy-4b,8b,12b,12d-
tetramethyltribenzotriquinacene (not shown),[37] in which the three bridgehead C-H
bonds were blocked and thus protected against oxidation under the reactions
conditions. This compound was found to react extremely slowly at  78 °C. Although
the reaction mixture turned to permanent deep-blue color, work-up gave mainly the
starting material. Chromatography followed by mass spectrometric analysis indicated
the presence of three cleavage products, among which was the tetramethyl analog of
68. Further experiments at elevated temperatures appear to be reasonable but were
not carried out so far.
Scheme 11. Ozonolysis of tetramethoxycentrohexaindane 9.
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As a final substrate, the tetramethoxycentrohexaindane 9 was subjected to ozonolysis
(Scheme 11). This compound is relatively easily accessible in larger amounts along
the fenestrane route to the centrohexaindanes[1a,10b,34] – much more easily than the
(otherwise most interesting) dodecacentrohexaindane 27 (Scheme 1f) bearing twelve
methoxy groups at the molecular periphery.[35] Similar to the bridgehead-
tetramethylated tribenzotriquinacene mentioned above, compound 9 was found to
react very sluggishly under standard conditions at  78 °C. Considerable amounts of
the starting material (22%) were recovered by gravity column chromatography but,
after all, three ozonolysis products were identified in 47% combined yield, similar to
the case of 36. The major, and last-eluting, product among them was the most
interesting one, bis(dimethyl muconate) 10, which was isolated in 26% yield and fully
characterized (see below). In line with the recovery of the starting material, the product
of single intra-dimethoxy cleavage, the mono-(dimethyl muconate) 69, still containing
one unreacted veratrole ring, was isolated in 19% yield and also fully characterized.
Furthermore, small amounts (2%) of the corresponding dialdehyde 70 were obtained
in a fast-eluting fraction, albeit not in completely pure form. Thus, the extra-dimethoxy
cleavage of one veratrole unit of 9 again was found to compete to a small extent with
the intra-dimethoxy cleavage. When the ozonolysis reaction of centrohexaindane 9
was carried out at elevated temperature ( 40 °C), the product of two-fold intra-
dimethoxy cleavage, 10, was the only obtainable product. Notably, it was isolated in
even increased yield (34%) after simple recrystallization from methanol; isolation of 10
by chromatography was not carried out in this case.
1
The H NMR spectrum of the diformyl-tetrabenzoveratrolo-centrohexaquinane 70
exhibits the characteristic aldehyde resonance at  10.67. The multiplets at 
8.008.04 and  7.737.78 reflect the fenestrindane core of the structure, which bears
different C2-bridges on either side. The two singlet resonances at  7.17 and  3.90
clearly indicate the presence of the veratrole ring that survived the oxidation reaction.
1
Overall, the simple H NMR spectrum reflects the molecular C2v-symmetry of
compound 70. Some impurities were found in this minor ozonolysis product. The other
product of partial ozonolysis of the veratrole nuclei, bis(methoxycarbonylmethylene)-
tetrabenzoveratrolocentrohexaquinane 69, was obtained as a colorless solid and fully
characterized by EI mass spectrometry and NMR spectroscopy. The EI mass
spectrum shows the molecular ion peak at m/z 668 with moderate intensity (17%) and,
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10.1002/cplu.201700090
ChemPlusChem
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as found for the other dimethyl muconates above, the pronounced [M  C2H3O2]+ peak
at m/z 609 (100%), indicating the facile and characteristic loss of one methoxycarbonyl
1
group. The H NMR spectrum of 69 again exhibits two singlets at  7.15 and  3.88,
originating from the veratrole ring, and another two singlets at  6.31 and  3.55 due to
the dimethyl muconate grouping. Moreover, the presence of the differently C2-bridged
fenestrindane core is evident from the two multiplets at  7.757.73 and  7.587.56,
in analogy to the spectrum of dialdehyde 70. The degeneracy of resonances in both
1
the H and the 13C NMR spectra of 69 confirms its molecular C2v-symmetry. As an
interesting detail, and in line with symmetry, the latter spectrum exhibits three
quaternary aliphatic resonances at  92.8,  72.5 and  70.3 for the central carbon
atom and, respectively, the four pairwise equivalent bridgehead carbons of the
neopentane core.
The
major
product
of
ozonolysis,
tetrakis(methoxycarbonylmethylene)-
tetrabenzocentrohexaquinane 10, was also isolated as a colorless solid. EI mass
spectrometry and NMR spectroscopy proved its structural identity. Moreover, the NMR
spectra revealed an interesting feature of the conformational flexibility of this
Figure 9. 1H NMR spectrum (500 MHz, CDCl3, 20 °C) of tetrabenzocentrohexaquinane
bis(dimethylmuconate) 10. The inserts show the aromatic region at 100 °C (C2D2Cl4) and  60 °C
(CD2Cl2).
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