Synthesis and characterization of new heptalenes with extended π-systems attached to them

Article abstract of DOI:10.1002/hlca.201300137

Methyl heptalenecarboxylates of type A and B with π(1) and π(2) substituents in 1,4-relation (Scheme 1) were synthetized starting with dimethyl 1-methylheptalene-4,5-dicarboxylates 5b and 6b derived from 7-isopropyl-1,4- dimethylazulene (=guaiazulene) and 1,4,6,8-tetramethylazulene by thermal reaction with dimethyl acetylenedicarboxylate. The further general way of proceeding for the introduction of the π(1) and π(2) substituents is displayed in Scheme 3, and the thus obtained methyl heptalene-5-carboxylates of type A and B are listed in Table 1. The C=C bonds of the 2-arylethenyl and 4-arylbuta-1,3-dien-1-yl groups of π(1) and π(2) were in all cases (E)-configured and showed s-trans conformation at the C-C bonds (X-ray and ¹H-NOE evidence) in the B-type as well as in the A-type heptalenes (cf. Figs. 5-12). All B-type heptalenes showed a strongly enhanced heptalene band I in the wavelength region 440-490 nm in hexane/CH₂Cl ₂ 9: 1 (cf. Table 4 and Figs. 13-20). The A-type heptalenes showed in this region only weak absorption, recognizable as shoulders or simply tailing of the dominating heptalene bands II/III (Table 5). Absorption band I of the B-type heptalenes appeared almost at the same wavelength as the longest wavelength absorption band of comparable open-chain α,ω- diarylpolyenes (cf. Fig. 21). The cyclic double bond shift (DBS) of the A- and B-type heptalenes could be photochemically steered in one or the other direction by selective irradiation (cf. Fig. 22). Copyright

Full text of DOI:10.1002/hlca.201300137

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Synthesis and Characterization of New Heptalenes with Extended p-Systems
Attached to Them
1
by Sarah Maillefer-El Houar ), Peter Uebelhart, Anthony Linden, and Hans-Jꢀrgen Hansen*
Organisch-chemisches Institut der Universitꢀt Zꢁrich, Winterthurerstrasse 190, CH-8057 Zꢁrich
(
phone: þ41-44-6354231; fax: þ41-44-6354233; e-mail: hjhansen@oci.uzh.ch)
Methyl heptalenecarboxylates of type A and B with p(1) and p(2) substituents in 1,4-relation
Scheme 1) were synthetized starting with dimethyl 1-methylheptalene-4,5-dicarboxylates 5b and 6b
(
derived from 7-isopropyl-1,4-dimethylazulene (¼guaiazulene) and 1,4,6,8-tetramethylazulene by
thermal reaction with dimethyl acetylenedicarboxylate. The further general way of proceeding for the
introduction of the p(1) and p(2) substituents is displayed in Scheme 3, and the thus obtained methyl
heptalene-5-carboxylates of type A and B are listed in Table 1. The C¼C bonds of the 2-arylethenyl and 4-
arylbuta-1,3-dien-1-yl groups of p(1) and p(2) were in all cases (E)-configured and showed s-trans
1
conformation at the CꢀC bonds (X-ray and H-NOE evidence) in the B-type as well as in the A-type
heptalenes (cf. Figs. 5 – 12).
All B-type heptalenes showed a strongly enhanced heptalene band I in the wavelength region 440 –
4
90 nm in hexane/CH Cl 9 :1 (cf. Table 4 and Figs. 13 – 20). The A-type heptalenes showed in this region
2 2
only weak absorption, recognizable as shoulders or simply tailing of the dominating heptalene bands II/
III (Table 5). Absorption band I of the B-type heptalenes appeared almost at the same wavelength as the
longest wavelength absorption band of comparable open-chain a,w-diarylpolyenes (cf. Fig. 21). The
cyclic double bond shift (DBS) of the A- and B-type heptalenes could be photochemically steered in one
or the other direction by selective irradiation (cf. Fig. 22).
Introduction. – In a preliminary report, we have described new heptalenes of type A
and B, wherein p(1) and p(2) represent conjugative substituents such as the 4-
phenylbuta-1,3-dien-1-yl or the 4-methoxy- and 4-nitrostyryl group, which are located
in 1,4-relation at the bicyclic [12]annulene core (Scheme 1) [1]. In heptalenes of type A,
the C¼C bonds are arranged in a way that p(1) and p(2) are not connected directly with
each other via the twisted p-system of the heptalene skeleton. In this case, both p-
substituents interact largely independently with the [12]annulene system. On the other
hand, in the constitutionally isomeric heptalenes of type B, which can be generated
from A by thermally or photochemically induced cyclic double-bond shifts (DBS), the
Scheme 1
1)
Part of the Ph. D. Thesis of S. M.-E., University of Zurich, 1998.
ꢂ 2013 Verlag Helvetica Chimica Acta AG, Zꢁrich

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C¼C bonds are located in a manner that p(1) and p(2) are now linked with each other
via an s-cis-buta-1,3-diene substructure, which allows strong cooperative interactions of
p(1) and p(2) with the annulene p-core. These different types of interactions of p-
substituents with respect to the bonding situation in [4n]annulenes influence strongly
the UV/VIS behavior of heptalenes of type A and B, i.e., the A and B state of
heptalenes represent thermo- and/or photochromic DBS isomers, which may serve as
new types of molecular switches and/or data storage devices (see also [2]). Fig. 1 shows,
as an example, the UV/VIS spectra of the 2,5- and 1,4-bis[(1E,3E)-4-phenylbuta-1,3-
dien-1-yl]-substituted heptalenes (all-E)-1a and (all-E)-1b, respectively (cf. [1]). One
recognizes in the spectrum of (all-E)-1b (Fig. 1,b), in which both p-substituents are
situated at the termini of
(
(
a
local s-cis-buta-1,3-diene substructure
C(1)¼C(2)ꢀC(3)¼C(4)), a strong enhancement of the heptalene band I at 439 nm
for band assignment, see [2]). The analogous band I arises in (all-E)-1a at the same
wavelength (cf. Fig. 1,a), however, only as a just recognizable weak absorption. In this
way, the thermo- and/or photochromic behavior of heptalenes are based on the
respective hypo- and hyperchromisms of the UV/VIS absorption of the same basal
chromogenic system of constitutionally isomeric structures such as (all-E)-1a and (all-
E)-1b. A similar situation is found in thermally and/or photochemically induced (E)/
(
Z)-isomerizations of double-bond structures such as those in substituted ethylenes,
azomethines, and azo compounds (cf. [3]) which may also lead to distinctly different
UV/VIS spectra of the same basal chromophor of the diastereoisomeric (E)- and (Z)-
isomers. As an example, the UV/VIS spectrum of the (E)- and (Z)-isomer of dimethyl
1
-[6-(tert-butyl)azulen-1-yl]ethene-1,2-dicarboxylate ((E)- and (Z)-2) is reproduced in
2
Fig. 2 [4] ). Whereas the typical azulene bands above 500 nm are almost not influenced
by the substituted ethene chromophor attached to C(1) of the aromatic azulene core,
i
Fig. 1. UV/VIS Spectra (4% PrOH/hexane) of (all-E)-1a and (all-E)-1b [1]. E ¼COOMe.
2)
According to AM1 calculations and in agreement with a number of X-ray crystal-structure analyses,
E)-configured dimethyl 1-(azulen-1-yl)ethene-1,2-dicarboxylates exhibit markedly larger torsion
(
angles than the corresponding (Z)-configured dicarboxylates at the s-bond, linking both
chromogenic systems, especially when the azulenyl moieties carry additional Me substituents at
C(2) and/or C(8). We will report on these ꢃgreen azulenesꢄ later in this journal.

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Fig. 2. UV/VIS Spectra (hexane) of dimethyl 1-[(6-(tert-butyl)azulen-1-yl]fumarate ((E)-2) and
dimethyl 1-[6-(tert-butyl)azulen-1-yl]maleate ((Z)-2) (bottom, azulene bands) [4]. E ¼COOMe.
one observes significant differences in the spectra at 350 – 400 nm, caused by distinct
changes in the conjugative interaction of both chromophoric systems in (E)- and (Z)-2
due to altered torsion angles at the central s-bond in the isomers, which are determined
by the steric interactions of the azulenyl substituent at C(1) with the COOMe
substituent at C(2) in (E)-2 or with HꢀC(2) in (Z)-2.
Thermo- and photochromism, linked to UV/VIS absorption differences of two
interconvertible structures with the same basal chromophoric system, are in contrast to
the well-established chromism of two thermally and/or photochemically interrelated
structures, which represent differing bonding states and, therefore, possess their
individual excited states. A typical example is depicted in Scheme 2. The fulgide 3 with
its longest-wavelength absorption at 345 nm is transformed photochemically into the
dihydrobenzofuran 4 with an intense and broad absorption maximum at 494 nm due to
the presence of a merocyanine chromophor [5]. In turn, irradiation of 4 at 494 nm or
heating restores the fulgide structure of 3. This photo- and thermochromic behavior of
3
3
and 4 has been amply applied in chemical actinometry [5] ).
Since the described types of thermo- and photochromism are of fundamentally
different origin, we call the former one, which is characterized by no overall change in
bonding, i.e., A(np) !B(np), and is therefore based on hypo- and hyperchromic
3)
The fulgide 3 has been sold under the trade name Aberchrome 540 by Aberchromics Ltd.,
University of Wales, Cardiff; dissolution, June 2000.

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Scheme 2
effects, type-I thermo- and photochromism, and the latter one, which is characterized
by changes in bonding, i.e., A(np) !B([n ꢀm]p þms), and is thus based on hypso-
and bathochromic effects, type II thermo- and photochromism.
The following chapters are dealing with the synthesis and characterization of p-
substituted heptalenes of type A and B and their interconversion by thermally and
photochemically induced DBS, and in this way, represent typical examples of
compounds showing type-I thermo- and photochromic behavior.
Syntheses. – Preamble. Starting materials for our syntheses of heptalenes of type A
and B (Scheme 1) were the heptalene-4,5-dicarboxylates 5b and 6b which are easily
available from the corresponding azulenes by thermal reaction with an excess of
dimethyl acetylenedicarboxylate (¼dimethyl but-2-ynedioate) in toluene or MeCN (cf.
[
1][6][7]). Heptalene 5b possesses three substituents in peri-position, and heptalenes
derived from it undergo thermal DBS already at room temperature (vide infra as well
as [8]). The other heptalene, 6b, carries four substituents in peri-position, and its
derivatives are set in thermal equilibrium with their DBS isomers at temperatures >
608 (vide infra as well as [2]).
Our general synthetic approach to heptalenes of types A and B is depicted in
Scheme 3. In a first step, MeꢀC(1) is transformed into the desired p(1) substituent by
taking advantage of the enhanced acidity of the H-atoms of this group, caused by
MeOOCꢀC(4), which stands in conjugative interaction with MeꢀC(1) via the involved
4
heptalene p-bonds ) [1]. The p(1) substituted heptalenes C(1) can then selectively be
reduced by way of the corresponding pseudo-esters D(1) to the 4-formylheptalene-5-
carboxylates E(1). This procedure allows the introduction of the p(2) substituents at
C(4) by established synthetic methodologies (Wittig and/or Heck reactions or variants
of them; vide infra). We also synthesized the p(2)-substituted heptalenes C(2) by using
the same procedures via 7b and 8b (cf. Scheme 3). The characterization of the
spectroscopic properties of the synthesized heptalenes was also of interest in the frame
of this work (see later). We will discuss in the following parts some characteristic
features of the performed syntheses.
Introduction of the p(1) Substituents. In our first experiments with the heptalene-
dicarboxylate 5b as model compound, we tried to transform MeꢀC(1), on account of its
4)
Indeed, X-ray analyses (cf., e.g., [2]) as well as AM1 calculations of heptalene-4,5-dicarboxylates of
type 5b and 6b show the C¼O group of MeOOCꢀC(4) mostly in almost ideal s-cis (sometimes s-
trans) relation to C(3)¼C(4) of the heptalene core with V(C(3)¼C(4)ꢀC¼O) < 308 or > 1508,
respectively.

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Scheme 3
allylic position, directly into a formyl group which would be useful for further coupling
reactions of the Wittig type. However, the oxidation of 5b with SeO in boiling xylene
2
led to a complex mixture from which only the intensely colored formyl-heptalenedi-
carboxylates 9b and 10b were isolated chromatographically in yields of 3 and 4%,
5
respectively (Scheme 4) ) [9]. Other oxidative agents such as MnO or Pb(OAc) gave
2
4
no better results. The allylic bromination of 5b with N-bromosuccinimide (NBS) under
established conditions gave a 1:3 mixture of the (bromomethyl)heptalenedicarbox-
ylates 11b and 12b in moderate yield and, moreover, in favor of the ꢃwrongꢄ 6-
(
bromomethyl) derivative 12b (Scheme 4). These preliminary experiments showed us
that we had to search for other, more chemoselective procedures for the functional-
ization of MeꢀC(1).
Since 5b (as well as 6b) carries MeꢀC(1), in contrast to MeꢀC(6), in conjugative
interaction with MeOOCꢀC(4), we investigated the base-induced halogenation of
MeꢀC(1). The excellent results that we had observed in the low-temperature
t
chlorination of alkyl phenyl sulfones with C Cl in THF in the presence of BuOK
2
6
[
10], led us to apply these conditions also to 5b. They turned out to become a full
success since only the 1-(chloromethyl) derivative 13b of 5b was formed and isolated in
average yields of 90% after optimization of the reaction conditions (Scheme 5).
The heptalenedicarboxylate 6b, in comparison to 5b, behaved much more reluctant
in the chlorination reaction due to the presence of MeꢀC(10) which hinders the
deprotonation of MeꢀC(1) and – as we suppose – also the chlorination of the
5)
In later experiments, Song found that heptalenedicarboxylates, which carry only one Me group, can
be transformed into the corresponding formyl-heptalenedicarboxylates with SeO in yields up to
2
60% [9].

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1493
Scheme 4
a) SeO , xylene, reflux, 1.5 h. b) NBS, dibenzoyl peroxide, CCl , reflux, 2 h.
2
4
corresponding anion by C Cl . The reaction time had, therefore, to be prolonged, and
2
6
the yield of the 1-(chlormethyl) derivative 14b from 6b dropped to 77% (Scheme 5).
The selective introduction of a chloro substituent at MeꢀC(1) of 5b and 6b opened
the way for the construction of conjugative p-substituents at C(1) by Wittig and
HornerꢀWadsworthꢀEmmons reactions. Compound 13b served as test substrate.
Reaction of 13b with Ph P in the presence of an excess of NaI in acetone at room
3
temperature led just to the formation of the corresponding phosphonium iodide 15b in
excellent yield (Scheme 6). Its reaction with cinnamaldehyde in a two-phase system (2n
aq. NaOH/CH Cl ) at room temperature resulted in the formation of (1Z,3E)- and
2
2
(
1E,3E)-16b in 59% yield with the former stereoisomer in excess. On standing in Et O
2
solution, both isomers 16b cocrystallized from the mixture in dark orange to orange
crystals that could be separated mechanically by hand (see Exper. Part). The 1-
(
(
iodomethyl)heptalene 17b was obtained in good yield by the Finkelstein procedure
Scheme 6). MichaelisꢀArbusov reaction of 17b with triethyl phosphite (P(OEt) )
3
formed the diethyl phosphonate 18b, which was treated without further purification
with cinnamaldehyde in THF at ꢀ788 in the presence of sodium bis(trimethylsilyl)a-
mide (NaHMDSA) as base. This reaction led exclusively to the formation of (1E,3E)-
Scheme 5

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Helvetica Chimica Acta – Vol. 96 (2013)
Scheme 6
a) NaI, PPh , acetone, 258; 88%. b) 2n NaOH, CH Cl , PhCH¼CHCHO, 258; 59%. c) NaI, acetone, 258;
3
2
2
7
6%. d) 1. P(OEt) , 1008; 2. 2m NaHMDSA, THF, aldehyde, ꢀ788.
3
16b in an overall yield of 51%. In the same manner, we synthesized 1-[4-(4-
nitrophenyl)buta-1,3-dien-1-yl]-, 1-[4-(4-methoxyphenyl)buta-1,3-dien-1-yl]-, 1-[2-(4-
nitrophenyl)ethenyl]-, 1-[2-(4-methoxyphenyl)ethenyl]-, and 1-{2-[4-(dimethylamino)-
phenyl]ethenyl}heptalenes (1E,3E)-19b, (1E,3E)-20b, (E)-21b, (E)-22b, and (E)-23b,
respectively, in good to moderate yields (Scheme 6).
In comparison with 13b, the reluctant S 2 reactivity of 1-(chloromethyl)heptalene
N
14b required harsher reaction conditions for the formation of the corresponding diethyl
phosphonate (Scheme 7). Nevertheless, treatment of 14b with P(OEt) in the presence
3
of NaI at 1308 led to the phosphonate, which was not isolated but just treated with the
aromatic aldehyde and sodium bis(trimethylsilyl)amide as base in THF. By this
procedure, we synthesized the configurationally uniform 1-(4-phenylbuta-1,3-dien-1-
yl)-, 1-[4-(4-methoxyphenyl)buta-1,3-dien-1-yl]-, and 1-[2-(4-methoxyphenyl)ethenyl]-
heptalenes (1E,3E)-24b, (1E,3E)-25b, and (E)-26b, respectively, in acceptable overall
yields (Scheme 7).
Introduction of the p(2) Substituents. The key step was here the successful reduction
of the heptalene pseudoester 7b [11] with diisobutylaluminium hydride (DIBAH) in

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Scheme 7
a) 1. NaI, P(OEt) , 1308, 3 h; 2. 2m NaHMDSA, THF, aldehyde, r.t.
3
Scheme 8
a) 1m DIBAH, toluene, ꢀ788; 78%. b) 2n NaOH, CH Cl , (RPPh )Br, r.t.; 31%/16%.
2
2
3
toluene to the corresponding 4-formylheptalene-5-carboxylate 8b (Scheme 8). How-
ever, Wittig reactions of this aldehyde with resonance-stabilized (triphenylphospho-
nio)methanides in the described two-phase system were not very encouraging,
especially, when electron-acceptor-substituted phosphoniomethanides were used. The
formed p(2)-substituted heptalenes 27 and 28 were obtained as thermodynamically
controlled mixtures of their two DBS isomers. On the other hand, following a
procedure of Corey and Fuchs [12], the formyl group of 8b could be transformed via 29
into an ethynyl (!30) substituent, which could then be coupled in good yield to (E)-31
in a Heck reaction with (E)-2-iodostyrene (Scheme 9) (cf. [13] and lit. cit. therein). The
coupling product (E)-31 was again obtained as a mixture of its two DBS isomers.
Treatment of 8b with Tebbeꢄs reagent [14] in THF at ꢀ78 to 08 gave the
corresponding ethenylheptalenecarboxylate 32 as a mixture of its two DBS forms, but
6
unfortunately only in a yield of 19% (Scheme 10) ) [15]. Nevertheless, the Heck
reaction of this mixture with (E)-2-iodostyrene proceeded smoothly and furnished
6)
The reaction of heptalene-4,5-dicarboxylates of type C(1) with Tebbeꢄs or Takaiꢄs reagent leads
selectively, after hydrolysis, in yields > 70% to the corresponding 4-acetylheptalene-5-carboxylates,
which can easily and in high yields be transformed into 4-ethynylheptalene-5-carboxylates via their
enol phosphates [15].

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Scheme 9
a) CBr , Ph P, CH Cl , 1 h, 08; 60%. b) BuLi, THF, ꢀ788, 10 min; 48%. c) CuI, cat. [Pd(Ph P) ],
4
3
2
2
3
4
i
BuNH , PhCH¼CHI, DMF, r.t., 20 h; 76%.
2
Scheme 10
a) 1m Tebbeꢄs reagent in toluene, THF, ꢀ788 to 08; 19%. b) PhCH¼CHI, Ag CO , cat. [Pd(OAc) ], DMF, r.t.,
2
3
2
1
2 h; 71%. c) CrCl , CHI , THF, 30 min, r.t.; 60%. d) Styrene, Ag CO , cat. [Pd(OAc) ], DMF, r.t., 12 h; 54%.
2
3
2
3
2
(1E,3E)-27a and -27b in good yield. Finally, the reversion of the two coupling steps
turned out to be the procedure of choice for the transformation of the formyl group at
C(4) into a 4-phenylbuta-1,3-dien-1-yl substituent. Following a procedure of Takai and
co-workers [16], 8b was treated with CHI in the presence of CrCl in THF to transform
3
2

Helvetica Chimica Acta – Vol. 96 (2013)
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the formyl group into an (E)-2-iodoethenyl substituent. The thus formed mixture (E)-
3
2
3a/(E)-33b was then subjected to the Heck reaction with styrene to deliver (1E,3E)-
7a/(1E,3E)-27b in an overall yield of 32.4% with respect to 8b as starting material.
Combined Introduction of the p(1) and p(2) Substituents. The reactivity of MeꢀC(1)
of the heptalene-4,5-dicarboxylates 5b and 6b under basic conditions determined the
sequence of the introduction of the p(1) and p(2) substituents (cf. Scheme 3). After
formation of the p(1)-substituted heptalene-4,5-dicarboxylates of type C(1),
MeOOCꢀC(4) has to be reduced to a formyl group via formation of the corresponding
pseudoesters of type D(1). The formyl group can then be modified to an (E)-2-
iodoethenyl substituent as a reactive residue for the final Heck coupling step, by which
the p(2) substituents in the preferred (E)-configuration are generated.
We realized in our former experiments [11] the selective saponification of
MeOOCꢀC(4) of heptalene-4,5-dicarboxylates of type C(1) with KOH in EtOH/
H O. However, this procedure led in the case of the p(1)-substituted heptalenedicar-
2
boxylates to an appreciable extent of decomposition. We finally succeeded in selective
saponification without decomposition by using LiOH instead of KOH and exchanging
EtOH by MeOH. The sequence of transformations for the heptalene-4,5-dicarboxylate
(1E,3E)-16b is displayed in Scheme 11.
The selective saponification gave the acid (1E,3E)-34b in 88% yield. The following
formation of the pseudoester (1E,3E)-35b could be realized under standard conditions
Scheme 11
a) LiOH, MeOH/H O, 708; 88%. b) 1. (COCl) , DMF, MeCN, 08; 2. MeOH, r.t.; 81%. c) 1m DIBAH,
2
2
toluene, ꢀ788; 72%. d) 1. CrCl , CHI , THF; 2. styrene, cat. [Pd(OAc) ], Ag CO , DMF, r.t.; 43%.
2
3
2
2
3

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Helvetica Chimica Acta – Vol. 96 (2013)
in 81% yield. Reduction of the latter compound with DIBAH in toluene led to the 4-
formylheptalene-5-carboxylate (1E,3E)-36b in 72% yield. The iodoethenylation and
Heck coupling with styrene was performed as a ꢃone-potꢄ reaction without isolation and
characterization of the intermediate 4-[(E)-2-iodoethenyl]heptalene-5-carboxylate.
The bis[(1E,3E)-4-phenylbuta-1,3-dien-1-yl]-substituted heptalene-5-carboxylate (all-
E)-1 was thus obtained in 43% yield with respect to the combined last two steps or in a
22% overall yield with respect to heptalene-4,5-dicarboxylate (1E,3E)-16b. Heptalene
(all-E)-1 represented a thermodynamically controlled 1:3.6 mixture of the two DBS
isomers (all-E)-1a and -1b, respectively.
We synthesized in the same manner a number of further p(1),p(2)-substituted
heptalene-5- or -1-carboxylates. All data of their synthesis are collected in Table 1. The
heptalenecarboxylates 40, 68, 44, 48, and 52 derived from the heptalene-4,5-
dicarboxylate 5b (isopropyl series; see 19b !37b !38b !39b !40; 20b !65b !
6
4
6b !67b !68; 25b !41b !42b !43b !44; 22b !45b !46b !47b !48; 21b !
9b !50b !51b !52) were isolated, as we expected, as thermodynamically con-
trolled mixtures of their two DBS isomers A and B (or a and b), since thermal
equilibration of the individual forms took place already at room temperature, and it
was rapid at 408 (cf. [1]). Both isomers were characterized by their UV/VIS spectra,
recorded in the course of their anal. HPLC separations at room temperature, and by the
1
H-NMR spectra of their mixtures, which allowed a full analysis only of the spectra of
the major A forms (see also later). The heptalenecarboxylates 56, 60, and 64 derived
form the heptalene-4,5-dicarboxylate 6b (methyl series) were also obtained as mixtures
of their DBS isomers A and B (or a and b) (see Table 1; 26b !53b !54b !55b !56;
2
3b !57b !58b !59b !60; 24b !61b !63b !64). However, the DBS process
started here slowly at temperatures > 608, and it became rapid only at temperatures >
808. Therefore, we were able to isolate the major B forms after chromatography (silica
gel) in pure crystalline state, whereas the minor A forms were obtained as oils. For our
model compounds of this series, i.e., (all-E)-60a and (all-E)-60b, (p(1) ¼p(2) ¼
(
1E,3E)-PhCH¼CHꢀCH¼CH) we demonstrated that (all-E)-60a is also a crystalline
compound in the pure state, since irradiation of (all-E)-60b in 5% CH Cl /hexane at
2
2
room temperature with a high-pressure Hg lamp through a Pyrex filter and 2n aqueous
Cu(NH ) ]SO solution with a transparency window between 350 and 600 nm led to a
[
3
4
4
mixture of (all-E)-60a and (all-E)-60b, which contained > 60% of (all-E)-60a.
Chromatography (silica gel) gave then pure crystalline (all-E)-60a. No (E)/(Z)-
isomerizations in the (1E,3E)-configured 4-phenylbuta-1,3-dien-1-yl side chains were
observed in the course of the photochemically induced DBS process. Pictures of the
crystal powder of (all-E)-60a and (all-E)-60b are reproduced in Fig. 3. The interruption
of the conjugative interaction between the two p-substituents in (all-E)-60a is nicely
demonstrated by the yellow color of its crystals, in contrast to the deep red color of the
crystals of (all-E)-60b, where the two p-substituents, possessing an optimal planar s-
trans arrangement with respect to C(1)¼C(2) and C(3)¼C(4) of the heptalene skeleton,
interact conjugatively (see later).
Structural Assignment. – X-Ray Crystal-Structure Analyses. To get more insight
into the spatial arrangements of the substituents at the twisted heptalene core, a
number of analyses were performed under standard quality conditions (see Exper. Part,

Helvetica Chimica Acta – Vol. 96 (2013)
1499
Fig. 3. Picture of microcrystalline powder of (all-E)-60a and (all-E)-60b
Table 7). Characteristic geometric parameters of the X-ray structures are listed in
Table 2.
We mentioned already the remarkable different behavior of the two 1-(chlorome-
thyl)heptalene-4,5-dicarbarboxylates 13b and 14b in nucleophilic substitution reactions
(
Schemes 6 and 7). For a better understanding of the marked reluctance of 14b to
undergo S 2 reactions at the ClCH group, we performed an X-ray crystal-structure
N
2
analysis of a proper crystal of 14b (Fig. 4,a). The structure reveals an interesting detail
in that the chloromethyl substituent is turned inward with respect to the heptalene core.
The torsion angle V of the ClꢀCH ꢀC(1)¼C(2) part amounts to ꢀ1048. This
2
conformation is stabilized by a weak intramolecular H-bond between the Cl-atom
and one of the H-atoms of MeꢀC(6) (d ¼297 pm). A second H-atom of MeꢀC(6) is
involved in through-space bonding with the ether O-atom of MeOOCꢀC(5) (d ¼
237 pm). AM1 Calculations reflect the crystal structure of 14b quite well, specifically
with respect to the orientation of ClCH ꢀC(1) (V(ClꢀCH ꢀC(1)¼C(2)) ¼ꢀ858) and
2
2
the above mentioned H-bonding (d(Cl ··· HꢀCH C(6)) ¼311 pm). The calculations
2
also demonstrated that the ClCH substituent at C(1) may occupy another conforma-
2
0
7
tion of V¼478 with respect to C(1)¼C(2) and with an almost equal DH value ).
f
Nevertheless, a linear nucleophilic attack at the C-atom of the CH Cl group is strongly
2
hindered in both conformations. It is MeꢀC(10) of the actual structure and MeꢀC(6) of
the second calculated structure of 14b, which are expected to exert strong sterical
hindrance on the nucleophilic substitution. Since it needed a temperature of 1308 to
realize the nucleophilic exchange, it seems that the temperature is needed to keep the
8
thermal DBS equilibrium 14b !14a alive ) [17], and it is indeed 14a, which undergoes
the nucleophilic exchange reaction. The AM1 calculated structure of 14a (Fig. 4,b)
shows ClCH ꢀC(5) in an orientation almost parallel to MeꢀC(6) and with a torsion
2
angle of ꢀ1108 for V(ClꢀCH ꢀC(5)¼C(5a)). Moreover, one of the H-atoms of
2
MeꢀC(6) is engaged in a H-bridge (d ¼280 pm) with the neighboring Cl-atom, and
MeꢀC(8) is too distant to block sterically effectively the nucleophilic exchange at
ClCH ꢀC(5). Therefore, it is reasonable assuming that 14a is the actual reactant in the
2
7)
8)
AM1: DH ¼99.21 kcal · mol for V¼ꢀ858 and 99.18 kcal · mol for V¼478 of 14b. The sign of all
0
ꢀ1
ꢀ1
f
discussed V values refer to the (M)-configuration of the heptalene core (cf. Table 2).
The equilibrium ratio of the comparable 6b/6a amounts to 7.8:1 (1008, tetralin) [17].

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Helvetica Chimica Acta – Vol. 96 (2013)
Table 1. Syntheses of p(1),p(2)-Substituted Methyl 6-Methylheptalene-5-carboxylates (isomers b or B)
a
and Methyl 10-Methylheptalene-1-carboxylates (isomers a and A) )
Substituents
p(1),p(2)-Heptalene precursors
C(1) Acid D(1) E(1)
No. No. [%] No. [%] No. [%]
35b 81 36b 72
p(1),p(2)-Heptalenes
No. [%]b)
a/bb)
1
2
i
R ¼H, R ¼Pr
16b 34b 88
19b 37b 62
20b 65b 83
23b 41b 52
22b 45b 72
21b 49b 71
26b 53b 92
24b 57b 82
1
43
1 : 3.5 (r.t.)
p(1) ¼PhCH¼CHꢀCH¼CH
p(2) ¼PhCH¼CHꢀCH¼CH
1
2
i
c
R ¼H, R ¼Pr
38b > 70 39b 47 ) 40 16
66b 64 67b 85 68 25
42b 90 43b 75 44 32
n.d.
p(1) ¼4-O NꢀC H CH¼CHꢀCH¼CH
2
6
4
p(2) ¼4-MeOꢀC H CH¼CHꢀCH¼CH
6
4
1
2
i
R ¼H, R ¼Pr
p(1) ¼4-MeOꢀC H CH¼CHꢀCH¼CH
1 : 5.7 (r.t.)
1 : 5.7 (r.t.)
6
4
p(2) ¼4-O NꢀC H CH¼CHꢀCH¼CH
2
6
4
1
2
i
R ¼H, R ¼Pr
p(1) ¼4-Me NꢀC H CH¼CH
2
6
4
p(2) ¼4-O NꢀC H CH¼CHꢀCH¼CH
2
6
4
1
2
i
d
R ¼H, R ¼Pr
p(1) ¼4-MeOꢀC H CH¼CH
46b 86 47b 79 48 ꢁ3 (54) ) 1 : 4.6 (558)
6
4
p(2) ¼4-O NꢀC H CH¼CH
2
6
4
1
2
i
R ¼H, R ¼Pr
p(1) ¼4-O NꢀC H CH¼CH
50b 80 51b 77 52 74
54b 77 55b 86 56 28
58b 77 59b 72 60 30
1 : 2.6 (r.t.)
1 : 4.5 (r.t.)
1 : 3.5 (808)
n.d.
2
6
4
p(2) ¼4-MeOꢀC H CH¼CH
6
4
1
2
R ¼Me, R ¼H
p(1) ¼4-MeOꢀC H CH¼CH
6
4
p(2) ¼4-O NꢀC H CH¼CHꢀCH¼CH
2
2
6
4
1
R ¼Me, R ¼H
p(1) ¼PhCH¼CHꢀCH¼CH
p(2) ¼PhCH¼CHꢀCH¼CH
1
2
e
R ¼Me, R ¼H
25b 61b < 85 ) 62b 55 63b 49 64 60
p(1) ¼4-MeOꢀC H CH¼CHꢀCH¼CH
6
4
p(2) ¼4-O NꢀC H CH¼CHꢀCH¼CH
2
6
4
a
)
See Schemes 3, 5, 6, and 11; all compounds with (1E,3E)- and (all-E)-configuration, respectively.
b) Isopropyl series (R ¼H, R ¼Pr): yields are given for the DBS mixtures a/b. Methyl series (R ¼Me,
1
2
i
1
2
R ¼H): yields are given for the easily crystallizing b forms. In parentheses, temperature of DBS
c
equilibration (n.d. ¼not determined). ) (1E,3E)-39b was obtained as a 2 : 1 mixture with the
d
corresponding lactone (yield 25%; cf. [1]). ) The yield in parentheses resulted from the Tebbe reaction
II
of (E)-47b (ꢁ40%), followed by Pd catalyzed Heck coupling with 1-iodo-4-nitrobenzene (cf. Path b in
e
Scheme 10). ) (1E,3E)-61b contained still some nonreacted diester (1E,3E)-24b.
discussed substitution reaction, in contrast to 13b, which reacts smoothly with NaI in
9
acetone at room temperature (Scheme 6) due to the unoccupied C(10) position ) [8].
It was of interest for us to compare the X-ray crystal structures of (1E,3E)-16b,
derived from 13b, and of (1E,3E)-24b, derived from 14b, both with a 4-phenylbuta-1,3-
dien-1-yl group at C(1). A stereoscopic view of the (M)-configured structures is
9)
The thermal equilibrium 13b > 13a takes place already at room temperature with ca. 0.5% of 13a
in equilibrium [8].

Helvetica Chimica Acta – Vol. 96 (2013)
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Helvetica Chimica Acta – Vol. 96 (2013)
Fig. 4. a) Stereoscopic view of the X-ray crystal structure of dimethyl 1-(chloromethyl)-6,8,10-
trimethylheptalene-4,5-dicarboxylate (14b) (atoms with 50% probability ellipsoids). b) Stereoscopic
view of the AM1 calculated structure of dimethyl 5-(chloromethyl)-6,8,10-trimethylheptalene-1,2-
dicarboxylate (14a)
displayed in Figs. 5 and 6. The buta-1,3-dien-1-yl chain adopts in both cases an almost
perfect s-trans-conformation at their formal s-bonds (cf. Table 2). As expected from
our former investigations, the average value of the two torsion angles at the central s-
bond (C(5a)ꢀC(10a)) of (1E,3E)-16b with three peri-substituents is slightly smaller
than that of (1E,3E)-24b with four peri-substituents. This leads to quite similar
interatomic distances between C(10)ꢀH and C(2’) of (1E,3E)-16b and C(10)ꢀCH and
3
C(2’) of (1E,3E)-24b, respectively (cf. last three lines of Table 2). This fact means that,
in both cases, the 4-phenylbuta-1,3-dien-1-yl chain at C(1) can adopt a sterically
undisturbed s-trans orientation with respect to C(1)¼C(2) of the heptalene core
providing thus optimal conjugation, which is further transmitted via
C(1)¼C(2)ꢀC(3)¼C(4) to the ester C¼O group at C(4). This effect of conjugation is
documented by V(C(3)¼C(4)ꢀC¼O) of 308 for the (M)-configured (1E,3E)-16b and
ꢀ
1688 for the (M)-configured (1E,3E)-24b (see Footnote c of Table 2).
The X-ray structures of the p(1),p(2)-substituted heptalene-5-carboxylates (all-E)-
52b and (all-E)-64b demonstrate that there is also enough free space around C(4) for

Helvetica Chimica Acta – Vol. 96 (2013)
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Fig. 5. Stereoscopic view of the X-ray crystal structure of dimethyl 9-isopropyl-6-methyl-1-[(1E,3E)-4-
phenylbuta-1,3-dien-1-yl]heptalene-4,5-dicarboxylate ((1E,3E)-16b) (atoms with 50% probability ellip-
soids)
Fig. 6. Stereoscopic view of the X-ray crystal structure of dimethyl 6,8,10-trimethyl-1-[(1E,3E)-4-
phenylbuta-1,3-dien-1-yl]heptalene-4,5-dicarboxylate ((1E,3E)-24b) (atoms with 50% probability ellip-
soids)
an extended p(2) substituent, so that an (E)-configured 2-phenylethenyl as well as an
(1E,3E)-configured 4-phenylbuta-1,3-dien-1-yl group can take the s-trans-conforma-
tion with respect to the C(3)¼C(4) bond of the heptalene core – optimal for an effective
conjugation between the two p-systems (Figs. 7 and 8, and Table 2). Moreover, as
expected (cf. Figs. 5 and 6), the s-trans-conformation is also maintained between p(1)
and C(1)¼C(2), in a way that p(1)ꢀC(1)¼C(2) and p(2)ꢀC(4)¼C(3) form an almost
perfect (all-s-trans)-conformation in the crystals, which is only interrupted by the given
s-cis-arrangement of C(1)¼C(2)ꢀC(3)¼C(4) with V ꢀ308 to ꢀ338, i.e., in a range
where almost full through-conjugation is still possible. Heptalene (all-E)-64b is found
in the crystals in two conformations A and B (Fig. 8,a and b), which differ only in their
relative position of MeOOCꢀC(5) and of 4-MeO of p(1). The conformation at

1
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Helvetica Chimica Acta – Vol. 96 (2013)
s(C(5)ꢀC(¼O)) of (all-E)-52b and the form B of (all-E)-64b are similar with
V(C(5a)¼C(5)ꢀC¼O) ¼598 and 428, respectively (cf. Table 2). The position of the
C¼O group seems to be stabilized by weak H-bonding with one of the H-atoms of
MeꢀC(6) (d ¼273 pm for (all-E)-52b and 253 pm for (all-E)-64b) as well as with
HꢀC(2’’) of p(2) (d ¼267 pm for (all-E)-52b and 281 pm for (all-E)-64b). In form A,
MeOOCꢀC(5) is turned at s(C(5)ꢀC(¼O) by ca. 1808 (V(C(5a)¼C(5)ꢀC¼O) ¼
ꢀ
1348) as compared with form B. Weak H-bonding of the MeO O-atom with one H-
atom of MeꢀC(6) (d ¼259 pm) and with HꢀC(2’’) (d ¼305 pm) seem to stabilize this
conformation. It is also of interest to note that the remote 4-MeO group of p(1) at C(1)
occupies opposite positions in forms A and B of (all-E)-64b. The torsion angles V of
MeꢀOꢀC ꢀC amount (by clockwise counting of the fragment OꢀC ꢀC ) to ꢀ18 (A)
ip
o
ip
o
and ꢀ1668 (B), respectively.
Fig. 7. Stereoscopic view of the X-ray crystal structure of methyl 9-isopropyl-4-[(E)-2-(4-methoxyphe-
nyl)ethenyl]-6-methyl-1-[(E)-2-(4-nitrophenyl)ethenyl]heptalene-5-carboxylate ((all-E)-52b) (atoms
with 50% probability ellipsoids)
NMR Analyses. The assignments of almost all H- and C-atoms of the p(1)- and
p(2)-substituted heptalene-4,5-dicarboxylates and heptalene-5-carboxylates were real-
ized with the usual NMR techniques. The corresponding DBS forms (i.e., a) of the
mentioned heptalenes were mostly characterized only in the thermodynamically
controlled mixtures of the a and b forms (cf. Table 1). Of special interest for us,
however, was the question of how the p-substituents, which in the crystals occupied
always the s-trans-conformation, would behave in solution, especially after the DBS
1
process of the heptalene core. For answering these questions, we performed H-NOE
measurements on heptalenes (1E,3E)-16b, (1E,3E)-19b, (all-E)-60b, and (all-E)-60a in
1
0
CDCl and for the DBS isomers of (all-E)-60 also in C D ). The results of the
3
6
6
1
measurements are visualized in Figs. 9 – 12. As expected, strong (s) to medium (m) H-
10)
(all-E)-60a was obtained on irradiation of pure (all-E)-60b with filtered light of 400 – 500 nm in 60%
yield. It was freed from residual (all-E)-60b by column chromatography (for details, see Exper.
Part).

Helvetica Chimica Acta – Vol. 96 (2013)
1505
Fig. 8. Stereoscopic views of the X-ray crystal structures of methyl 1-[(1E,3E)-4-(4-methoxyphenyl)buta-
,3-dien-1-yl]-6,8,10-trimethyl-4-[(1E,3E)-4-(4-nitrophenyl)buta-1,3-dien-1-yl]heptalene-5-carboxylate
(all-E)-64b): a) Conformation A and b) conformation B (atoms with 50% probability ellipsoids)
1
(
NOEs were observed in all cases between the ortho-H-atoms of the terminal phenyl
groups and HꢀC(3’) and HꢀC(4’) as well as HꢀC(3’’) and HꢀC(4’’), respectively. The
1
detection of a strong H-NOE between HꢀC(1’) of the buta-1,3-dien-1-yl chain and
HꢀC(2) of the heptalene core of (1E,3E)-16b (Fig. 9) is in perfect agreement with an s-
trans conformation at C(1)ꢀC(1’) also in solution (CDCl ). This fact is fully supported
3
1
by the presence of a weak (w) H-NOE between HꢀC(2’) and HꢀC(10) in accordance
with the interatomic distance of these two H-atoms in the crystals structure of (1E,3E)-
1
1
6b (cf. Fig. 5 and Table 2). Similar H-NOEs were found for (1E,3E)-19b, the 4-NO
2
1
analog of (1E,3E)-16b (Fig. 10). Despite the fact that the H-NOE between HꢀC(1’)
and HꢀC(2) was not identifiable (n.i.) in CDCl due to the nearness of the signals of the
3
1
two H-atoms, the realized strong H-NOE of HꢀC(2’) and HꢀC(10) indicates the
prevailing s-trans conformation of the p-substituent and the heptalene core. Never-
1
theless, the observed weak H-NOE of HꢀC(1’) and HꢀC(10) speaks for the presence

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Helvetica Chimica Acta – Vol. 96 (2013)
Fig. 9. 1H-NOE Effects (600 MHz, CDCl ) of dimethyl 9-isopropyl-6-methyl-1-[(1E,3E)-4-phenylbuta-
3
1
,3-dien-1-yl]heptalene-4,5-dicarboxylate ((1E,3E)-16b). s ¼strong, m ¼medium, w ¼weak, n ¼no
effect.
Fig. 10. 1H-NOE Efects (600 MHz, CDCl ) of dimethyl 9-isopropyl-6-methyl-1-[(1E,3E)-4-(4-nitro-
3
phenyl)buta-1,3-dien-1-yl]heptalene-4,5-dicarboxylate ((1E,3E)-19b). n.i. ¼not identifiable due to over-
lapping shifts.
of a minor amount of the s-cis-conformer with respect to C(1’)ꢀC(1) in the case of
(
1E,3E)-19b in CDCl solution.
3
1
Most interesting were the H-NOE investigations of (all-E)-60b and (all-E)-60a in
CDCl as well as in C D (Figs. 11 and 12), since both DBS isomers were available in
3
6
6
pure form as already mentioned. Several enhancement effects were not identifiable in
1
1
CDCl due to the nearness of the H-signals. However, the realized weak H-NOE
3
between HꢀC(2’) and MeꢀC(10) of (all-E)-60b indicates an s-trans arrangement of
11
C(2)¼C(1)ꢀC(1’)¼C(2’) ). The s-trans-conformation of the p(2)-substituent with
11)
For the comparable X-ray structures A and B of (all-E)-64b, one finds an average interatomic
distance between HꢀC(2’) and the closest HꢀCH ꢀC(10) of 290 pm, which is in accordance with the
2
1
above discussed H-NOE.

Helvetica Chimica Acta – Vol. 96 (2013)
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1
respect to the heptalene core is secured by a strong H-NOE of HꢀC(3) and HꢀC(1’’)
1
in C D as solvent. Support for the s-trans-conformation is given by a medium H-NOE
6
6
between HꢀC(1’’) and MeOOCꢀC(5), an effect, which is not found in CDCl . It seems
3
that MeOOCꢀC(5) of (all-E)-60b can attain different conformations, comparable with
those (A and B) found in the crystal structure of (all-E)-64b.
1
The H-NOE measurement of (all-E)-60a in CDCl and in C D were of particular
3
6
6
interest in view of the relaxed conformations of the two 4-phenylbuta-1,3-dien-1-yl
substituents with respect to the heptalene core. The observed effects (Fig. 12) clearly
showed that the DBS process (all-E)-60b !(all-E)-60a is followed also by a 1808 turn
of both 4-phenylbuta-1,3-dien-1-yl substituents, so that the new conformations
C(1)¼C(2)ꢀC(1’’)¼C(2’’) and C(5a)¼C(5)ꢀC(1’)¼C(2’) of (all-E)-60a are s-trans
configured as it is found also for the other s-bonds of the two unsaturated substituents.
In other words, (all-E)-60a exists in solution mainly in all-s-trans conformation.
MeOOCꢀC(1) seems to occupy also in (all-E)-60a two favored conformations as
discussed for (all-E)-60b.
Fig. 11. 1H-NOE Effects (600 MHz) of methyl 6,8,10-trimethyl-1,4-bis[(1E,3E)-4-phenylbuta-1,3-dien-1-
yl]heptalene-5-carboxylate ((all-E)-60b): a) in CDCl and b) in C D
3
6
6
3
Of interest is also to note that the size of J(HꢀC(2’),HꢀC(3’)) and of
3
J(HꢀC(2’’),HꢀC(3’’)) of the 4-phenylbuta-1,3-dien-1-yl substituents indicate with
3
1
0 – 8 Hz (cf. s-trans-buta-1,3-diene, J ¼10.4 Hz [18]) also a preferred s-trans-
3
3
conformation at the central s-bond, whereby we mostly found that J p(1) > J p(2).
This finding would be in agreement with a higher internal s-trans mobility of the p(2)
chains as compared to those of p(1). Steric interference with the neighboring
MeOOCꢀC(5) group seems to be responsible for this effect.

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Helvetica Chimica Acta – Vol. 96 (2013)
Fig. 12. 1H-NOE Effects (600 MHz) of methyl 6,8,10-trimethyl-2,5-bis[(1E,3E)-4-phenylbuta-1,3-dien-1-
yl]heptalene-1-carboxylate ((all-E)-60a): a) in CDCl and b) in C D
3
6
6
UV/VIS Spectra of the DBS Isomers. We measured most of the spectra in the course
of anal. HPLC separations with CH Cl /hexane mixtures as mobile phase (see also
2
2
12
[
1][2][9]) ) to avoid thermal DBS isomerizations, especially in the case of the
thermally p-mobile heptalenes derived from 5b (Scheme 3). In some cases, we used for
i
optimal HPLC separation of the DBS isomers, 4% PrOH/hexane as eluant. For
crystalline B isomers, configurationally stable at room temperature, at least for the time
of measurement, MeCN was used as standard solvent.
We reported already on the solvatochromism of the much stronger polarized
merocyanine system of methyl 1-[(E)-2-(dialkylamino)ethenyl]heptalene-5-carboxy-
lates [19]. The solvent dependence of the position of band I and III of dimethyl 6,8,10-
trimethyl-1-[(E)-2-(pyrrolidin-1-yl)ethenyl]heptalene-4,5-dicarboxylate
((E)-69b)
and its DBS isomer (E)-69a is listed in Table 3. One recognizes that the heptalene
band I of the through-conjugated form (E)-69b exhibits a strong positive solvatochro-
mic effect of þ37 nm in going from hexane to MeCN. The effect is much less
pronounced for the heptalene band III (þ13 nm). Almost the same effects are found
for (E)-69a what demonstrates that the heptalene p-skeleton in total takes part in the
charge-transfer (CT) transitions. However, what changes in going by DBS from (E)-
69a to (E)-69b is a stronger localization of the CT transition in the through-conjugated
p-chain of (E)-69b, determined by length and electronic character of the p(1) and p(2)
substituents at C(1) and C(4), respectively. Band II in p-substituted heptalenes is
mostly difficult to determine and at best to make out as a shoulder sitting on the longer-
12)
The UV/VIS spectra were measured directly with the photodiode-array detector of the HPLC
system (see Exper. Part). The eluant was in all cases 15 – 30% CH Cl /hexane, CH Cl containing as
2
2
2
2
additive 0.5% MeOH.

Helvetica Chimica Acta – Vol. 96 (2013)
1509
Table 3. Solvatochromism of Heptalene Band I and III of 69a and 69ba)
Solvent
Hexane
(E)-69a [nm]
(E)-69b [nm]
I
III
I
III
427
445
465
331
334
338
431
451
468
330
339
343
Hexane/CH Cl2b)
2
MeCN
a) For details, see [19]. b) Ratio 1.86 : 1 (v/v).
wavelength flanc of band III in A-type heptalenes or it may cause a certain asymmetry
of band I at the lower-wavelength side of B-type heptalenes. The UV/VIS spectra of
(all-E)-1a and (all-E)-1b (Fig. 1) as well as those of its 6,8,10-trimethyl variant (all-E)-
60a and (all-E)-60b (Fig. 13) illustrate these facts. The introduction of a MeO and a
NO group as established electron-donor and electron-acceptor substituents at the 4-
2
position of the phenyl groups at the p-chain termini, to enhance the CT character of the
electronic transitions, has a marked influence, in that the position of the heptalene band
I is strongly shifted to longer wavelengths (Table 4).
An inspection of the UV/VIS data and the corresponding Figs. 13 – 20 show that the
heptalene band I is bathochromically shifted by the additional donor/acceptor
1
2
i
substituents by 28 – 37 nm taking the isopropyl series (R ¼H, R ¼Pr) with (all-E)-
b as reference, or by at least 30 nm taking (all-E)-60b as reference for (all-E)-64b. And
it seems that a 4-NO group at p(1) and a 4-MeO group at p(2) shift band I stronger
1
2
(
þ37 nm) than in the reverse situation (þ28 nm), a fact which is also evident by
comparison of the position of band I of (all-E)-52b and (all-E)-48b with the shorter p-
chains (Table 4). Nevertheless, Dl ((all-E)-52b ꢀ(all-E)-48b) is with 11 nm about
equal to Dl ((all-E)-68b ꢀ(all-E)-40b).
The influence of the donor/acceptor substituents on the position of heptalene band
III is smaller than on that of band I and amounts to ca. þ14 nm (Table 4). In three
cases, we found only a shoulder on the lower-wavelength flanc of band I at ca. 400 nm,
i.e., ca. 20 nm higher than in the other cases (Table 4). Therefore, we assume that this
shoulder in the spectra of (all-E)-56b, (all-E)-64b, and (all-E)-68b represents heptalene
band II of these heptalenes, and heptalene band III is weak and hidden under the
further falling flanc of the dominating band I.
The UV/VIS spectra of the type-A heptalenes are easier to analyze since the
dominating absorption is caused here by heptalene band III (Table 5 and Figs. 1 and
13 – 20). Heptalene band II is sitting in most cases as clearly recognizable shoulder on

1
510
Helvetica Chimica Acta – Vol. 96 (2013)
Table 4. Absorption Maximum of the Heptalene Bands I and III of the p(1),p(2)-Substituted Type-B
a
Methyl Heptalene-5-carboxylates )
p(1),p(2)b)
Others
No.
I [nm]
440
III [nm]
359
Remarks
i
PhCH¼CHꢀCH¼CH
PhCH¼CHꢀCH¼CH
9- Pr, 6-Me
(all-E)-1b
Fig. 1
i
4
4
4
4
4
4
4
4
4
4
4
4
-O NꢀC H CH¼CHꢀCH¼CH
9- Pr, 6-Me
(all-E)-40b
(all-E)-68b
(all-E)-44b
(all-E)-48b
(all-E)-52b
(all-E)-56b
(all-E)-60b
(all-E)-64b
477
468
486
439
450
451
439
469
378
Fig. 19
Fig. 18
Fig. 17
Fig. 14
Fig. 15
Fig. 16
Fig. 13
Fig. 20
2
6
4
-MeOꢀC
6
H
4
CH¼CHꢀCH¼CH
i
401 (sh)c)
378
-MeOꢀC H CH¼CHꢀCH¼CH
9- Pr, 6-Me
6
4
-O NꢀC H CH¼CHꢀCH¼CH
2
6
4
i
-Me NꢀC H CH¼CH
9- Pr, 6-Me
2
6
4
-O NꢀC H CH¼CHꢀCH¼CH
2
6
4
i
-MeOꢀC H CH¼CH
9- Pr, 6-Me
370
6
4
-O NꢀC H CH¼CH
2
6
4
i
-O NꢀC H CH¼CH
9- Pr, 6-Me
356
2
6
4
-MeOꢀC H CH¼CH
6
4
-MeOꢀC H CH¼CH
6,8,10-Me3
6,8,10-Me3
6,8,10-Me3
400 (sh)c)
365
6
4
-O NꢀC H CH¼CHꢀCH¼CH
2
6
4
PhCH¼CHꢀCH¼CH
PhCH¼CHꢀCH¼CH
4
4
-MeOꢀC H CH¼CHꢀCH¼CH
400 (sh)c)
6
4
-O NꢀC H CH¼CHꢀCH¼CH
2
6
4
a) See Figs. and Exper. Part for further details. b) All p(1),p(2) substituents have (E)-configuration.
c
)
Most probably heptalene band II; see text.
Fig. 13. UV/VIS Spectra (CH Cl /hexane) of (all-E)-60a and (all-E)-60b
2
2
the longer-wavelength flanc of band III, whereas band I appears only as tailing of band
III, or at best as a weak broad shoulder in the range where the type-B heptalenes show
the maximum of band I. As a rule, one finds band III of the type-A heptalenes at
shorter wavelength than band III of the B forms.
Since we got a number of p(1) substituted heptalenes of type C(1), D(1), and E(1)
in pure crystalline form, we measured their UV/VIS spectra in MeCN (Table 6). It was
interesting to notice that band I of these type-B heptalenes appeared as broad maxima
or shoulders at comparably long wavelength with respect to the corresponding
p(1),p(2)-disubstituted type-B heptalenes derived from them – also if one takes into

Helvetica Chimica Acta – Vol. 96 (2013)
1511
i
Fig. 14. UV/VIS Spectra (4% PrOH/hexane) of (all-E)-48a and (all-E)-48b [1]
Fig. 15. UV/VIS Spectra (CH Cl /hexane) of (all-E)-52a and (all-E)-52b
2
2
Fig. 16. UV/VIS Spectra (CH Cl /hexane) of (all-E)-56a and (all-E)-56b
2
2
account a polar solvent shift of ca. þ17 nm in going from CH Cl /hexane to MeCN (cf.
2
2
Table 3). Most obvious is this bathochromic shift for the corresponding D(1)
heptaleno[3,4-c]furan-3-ones (E)-42b (Table 6). The correction of the solvent shift
of þ17 nm of the band I position of (E)-42b leads in comparison to its p(1),p(2)-

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512
Helvetica Chimica Acta – Vol. 96 (2013)
Fig. 17. UV/VIS Spectra (CH Cl /hexane) of (all-E)-44a and (all-E)-44b
2
2
Fig. 18. UV/VIS Spectra (CH Cl /hexane) of (all-E)-68a and (all-E)-68b
2
2
Fig. 19. UV/VIS Spectra (CH Cl /hexane) of (all-E)-40a and (all-E)-40b
2
2
disubstituted analog (all-E)-44b to almost the same wavelength of absorption. There
must be reasons for the fact that the structure of (E)-42b compensates structurally and
electronically the effect of the p(2) substituent 4-(4-nitrophenylbuta-1,3-dien-1-yl) of

Helvetica Chimica Acta – Vol. 96 (2013)
1513
Fig. 20. UV/VIS Spectra (CH Cl /hexane) of (all-E)-64a and (all-E)-64b
2
2
Table 5. Dominating Absorption Maximum of the p(1),p(2)-Substituted Type-A Methyl Heptalene-1-
a
carboxylates )
p(1),p(2)b)
Others
No.
l [nm]
Remarks
i
PhCH¼CHꢀCH¼CH
PhCH¼CHꢀCH¼CH
9- Pr, 6-Me
(all-E)-1a
343
Fig. 1
i
4
4
4
4
4
4
4
4
4
4
4
4
-O NꢀC H CH¼CHꢀCH¼CH
9- Pr, 6-Me
(all-E)-40a
(all-E)-68a
(all-E)-44a
(all-E)-48a
(all-E)-52a
(all-E)-56a
(all-E)-60a
(all-E)-64a
377
351
369
337
352
378
342
359
Fig. 19
Fig. 18
Fig. 17
Fig. 14
Fig. 15
Fig. 16
Fig. 13
Fig. 20
2
6
4
-MeOꢀC
6
H
4
CH¼CHꢀCH¼CH
i
-MeOꢀC H CH¼CHꢀCH¼CH
9- Pr, 6-Me
6
4
-O NꢀC H CH¼CHꢀCH¼CH
2
6
4
i
-Me NꢀC H CH¼CH
9- Pr, 6-Me
2
6
4
-O NꢀC H CH¼CHꢀCH¼CH
2
6
4
i
-MeOꢀC H CH¼CH
9- Pr, 6-Me
6
4
-O NꢀC H CH¼CH
2
6
4
i
-O NꢀC H CH¼CH
9- Pr, 6-Me
2
6
4
-MeOꢀC H CH¼CH
6
4
-MeOꢀC H CH¼CH
6,8,10-Me3
6,8,10-Me3
6,8,10-Me3
6
4
-O NꢀC H CH¼CHꢀCH¼CH
2
6
4
PhCH¼CHꢀCH¼CH
PhCH¼CHꢀCH¼CH
4
-MeOꢀC H CH¼CHꢀCH¼CH
6
4
4
-O NꢀC H CH¼CHꢀCH¼CH
2
6
4
a) See Figs. and Exper. Part for further details. b) All p(1),p(2) substituents have (E)-configuration.
(all-E)-44b. There are two structure arguments: The fusion of the five-membered ring
in D(1) structures leads to a certain flattening of the heptalene skeleton, accompanied
by a reduction of the torsion angles of the p-skeleton and a rigid almost perfect s-cis
13
orientation of the C¼O group with respect to the adjacent p-bonds ). Moreover, the
13)
In connection with the synthesis of new heptalenes [9b], we performed an X-ray analysis of the
structure of an analog of furanone (1E,3E)-35b, wherein MeꢀC(11) is substituted by an (E)-styryl
group. In comparison with the X-ray structure of heptalenedicarboxylate (1E,3E)-16b (cf. Table 2),
we found for the analog of (1E-3E)-35b torsion angles reduced by ca. 38, and V (O¼CꢀC(3a)¼C(4))
amounted to 12.58. AM1 Calculations of (E)-25b and (E)-42b with a Me N group at p(1) showed
2
just the same tendencies in V of the heptalene skeleton.

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Helvetica Chimica Acta – Vol. 96 (2013)
C¼O group of the furanone ring gains in electron-acceptor quality due to the three
neighboring O-atoms of the cyclic semi-orthoanhydride structure [11], a fact, which
enhances the polar character of the merocyanine part of the D(1) forms. An aldehyde
group at the heptalene skeleton seems to adopt always an almost perfect s-trans
arrangement with respect to the corresponding heptalene C¼C bond (cf. [20]), thus
allowing optimal conjugation within the merocyanine system of the E(1) forms (cf.
Table 6).
A last point should be regarded. The B forms of the heptalenes contain a
merocyanine chain, therefore, it is of interest to consider corresponding open-chain
phenyl capped polyene systems and their donor/acceptor substituted variants as linear
merocyanines (Fig. 21). Their longest-wavelength absorptions are close to those of the
corresponding heptalene structures. Therefore, it can be concluded that the incorpo-
rated heptalene p-system with exception of the directly involved two p-bonds does not
contribute markedly to the longest-wavelength absorption. A comparison of the
position of heptalene band I of (all-E)-48b with the one of (all-E)-52b and of (all-E)-
40b with the one of (all-E)-68b with an exchange of donor and acceptor group of p(1)
and p(2) (cf. Table 4) indicates that a 4-NO group at p(1) and a 4-MeO group at p(2)
2
lead to a bathochromic shift of 11 and 18 nm, respectively, measured against the reverse
situation. In other words, the merocyanine system of the B-type heptalenes are similar
Fig. 21. Position of the longest-wavelength band of a,w-arylpolyenes in comparison with that of
corresponding heptalene based polyenes

Helvetica Chimica Acta – Vol. 96 (2013)
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Helvetica Chimica Acta – Vol. 96 (2013)
to those of open-chain structures, however, they carry their own character, best
documented by the unique thermal and photochemical convertibility of the A and B
forms.
We studied the pure photochromism of heptalenes (all-E)-56a and (all-E)-56b at
room temperature in hexane/CH Cl 9 :1 in detail. Irradiation of (all-E)-56b with a Hg
2
2
high-pressure lamp (150 W) through an interference filter with transmittance at 439 ꢂ
20 nm led after 2 h to an almost complete conversion into (all-E)-56a (Fig. 22). On the
other hand, when the thus obtained solution of (all-E)-56a was then irradiated through
an interference filter with transmittance at 308 ꢂ20 nm for 2 h, a photo-equilibrated
1
:3 mixture of (all-E)-56a and (all-E)-56b was generated (Fig. 22).
Fig. 22. Testing the photochromic behavior of (all-E)-56a and (all-E)-56b
We are thankful to our NMR laboratory for specific NMR measurements and to our MS laboratory
for mass spectra. Financial support of this work by the Swiss National Science Foundation is gratefully
acknowledged.
Experimental Part
General. Anal. TLC: glass plates covered with silica gel 60 (SiO ) with fluorescence indicator 254 nm
2
(
Fluka; layer thickness 0.25 mm, grain size 5 – 17 mm); detection with UV light (254 or 366 nm) or with
appropriate spray reagents. Prep. column chromatography (CC): SiO 60 (Merck; grain size 40 – 63 mm).
2
Anal. HPLC: Waters-991 instrument, equipped with a photodiode-array detector and a Bischoff pump
and Jasco MD-910 instrument, equipped with a photodiode-array detector and a Milton-Roy-CM-4000
pump, Spherisorb-S5-CN column (Phase Separations; 5 mm, 20 ꢃ250 mm). M.p.: Mettler-FP-5 and -FP-
5
2 instruments; uncorrected. UV/VIS Spectra: recorded with the HPLC photodiode-array detectors
(
wavelength accuracy ꢂ1.5 nm) in relative intensities, or with a PerkinꢀElmer-Lambda-9 spectropho-
3
tometer; minima (min.) and maxima (max.) in nm; molar extinction coefficients e [dm /mol · cm] as log
e; sh ¼shoulder. IR Spectra: PerkinꢀElmer-FT-1600 IR spectrophotometer; solid compounds in KBr
ꢀ1 1
pills (ca. 0.1 g KBr/1 mg substance); in cm . H-NMR Spectra: Bruker-AC-300, -ARX-300, and -AMX
6
00 instruments; chemical shifts d in ppm with respect to Me Si (¼0 ppm), coupling constants J in Hz; if
4
13
not otherwise stated, at 300 MHz; f.s. ¼fine structure. C-NMR Spectra: Bruker-ARX-300 and -AMX-

Helvetica Chimica Acta – Vol. 96 (2013)
1517
1
13
6
3
00 instruments; at 75 and 150 MHz; internal reference signal CDCl at 77.00 ppm with J( C,D) ¼
3
1.5 Hz; multiplicities determined with DEPT technique; if not otherwise stated, at 75 MHz. Mass
spectra (MS): Varian MAT-112S and -CH-7A instruments for chemical ionization (CI) with NH3;
Finnigan-MAT-SSQ-700 instrument, for electron-impact ionization (EI) at 70 eV; source temp. 2508,
direct injection; in m/z, (rel. %).
1. Syntheses of Dimethyl Heptalene-4,5-dicarboxylates. 1.1. Dimethyl 9-Isopropyl-1,6-dimethylhepta-
lene-4,5-dicarboxylate (5b). The diester was prepared in average yields of 68% by thermal reaction of
guaiazulene with a three-fold molar amount of dimethyl but-2-ynedioate in toluene at 1308 during 24 h.
Recrystallization from Et O gave 5b. Yellow crystals. M.p. 133.3 – 137.38 ([6]: 1478 (Et O)).
2
2
1.2. Dimethyl 1-Formyl-9-isopropyl-6-methyl- and 6-Formyl-9-isopropyl-1-methylheptalene-4,5-di-
carboxylate (9b and 10b, resp.). Diester 5b (3.10 g, 9.10 mmol) was dissolved in xylene (70 ml), and then
SeO (1.01 g, 9.10 mmol) was added. The mixture was heated under reflux, and additional amounts of
2
2
SeO (each time 1.01 g) were added after 30 and 60 min. Xylene was distilled off and the residue
subjected to CC (SiO , hexane/Et O 4 :1): 9b (0.089 g, 3%) and 10b (0.128 g, 4%), in this order. Other
2
2
products which were visible on TLC were not isolated.
Data of 9b: Red crystals from hexane/Et O. M.p. 161.5 – 162.88. R (hexane/Et O 1:1) 0.24. UV/VIS
2
f
2
(
MeCN): max. 431 (2.80), 276 (sh, 4.12), 254 (4.15); min. 380 (2.79). IR (KBr): 2965m, 2843w, 1733s,
1
1
711s, 1679s, 1644w, 1594w, 1565w, 1526w, 1435m, 1385w, 1356w, 1344w, 1277s, 1223s, 1184m, 1155m,
1
170m, 1047m, 983m, 957w, 867w, 838m, 805m, 790w, 758m, 703w, 631w. H-NMR (CDCl ): 9.55 (s,
3
3
3
3
CHOꢀC(1)); 7.66 (d, J(2,3) ¼6.1, HꢀC(3)); 7.12 (d, J(2,3) ¼6.1, HꢀC(2)); 6.33 (d, J(7,8) ¼6.6,
3
3
HꢀC(8)); 6.22 (d, J(7,8) ¼6.5, HꢀC(7)); 5.71 (s, HꢀC(10)); 3.75, 3.71 (2s, COOMe); 2.50 (sept., J ¼6.8,
3
13
Me CHꢀC(9)); 1.96 (s, MeꢀC(6)); 1.08, 1.06 (2d, J ¼6.9, 6.8, Me CHꢀC(9)). C-NMR (CDCl ;
2
2
3
tentative assignment): 189.90 (d, CHO); 169.78, 167.21 (2s, COOMe); 149.19 (s, C(5a)); 148.78 (s, C(9));
1
1
43.65 (d, C(3)); 140.81 (s, C(1)); 138.47 (s, C(10a)); 136.65 (d, C(2)); 130.99 (d, C(10)); 128.44 (d, C(7));
27.84 (s, C(6)); 126.22 (d, C(8)); 123.94 (s, C(5)); 120.62 (s, C(4)); 52.50, 52.14 (2q, COOMe); 35.48 (d,
þ.
Me CHꢀC(9)); 22.92, 22.55 (2q, Me CHꢀC(9)); 22.26 (q, MeꢀC(6)). EI-MS: 355 (18), 354 (100, M ),
2
2
þ
þ
þ
3
40 (21), 339 (16, [M ꢀMe] ), 325 (21, [M ꢀCHO] ), 311 (21, [M ꢀ(Me þCO)] ), 295 (24, [M ꢀ
þ
COOMe] ), 279 (21), 251 (8), 235 (8), 212 (17), 207 (7), 198 (8), 197 (6), 193 (7), 191 (8), 189 (7),
1
78 (7), 165 (12). Anal. calc. for C H O (354.40): C 71.17, H 6.26; found: C 70.89, H 6.36.
21 22 5
Data of 10b: Orange crystals from hexane/Et O. M.p. 165.7 – 166.68. R (hexane/Et O 1:1) 0.12. UV/
2
f
2
VIS (MeCN): max. 400 (3.11), 330 (sh, 3.54), 279 (4.28); min. 257 (4.21), 227 (4.30). IR (KBr): 2965m,
2
1
908w, 2873w, 2843w, 1722s, 1677s, 1644w, 1608w, 1578m, 1528w, 1438m, 1385w, 1347w, 1300w, 1259s,
1
226m, 1192m, 1166m, 1094m, 1070w, 1053m, 1043m, 989w, 957w, 876w, 862w, 833w, 734w. H-NMR
3
3
(
CDCl ): 9.46 (s, CHOꢀC(6)); 7.47 (d, J(2,3) ¼6.4, HꢀC(3)); 7.06 (d, J(7,8) ¼6.6, HꢀC(7)); 6.54 (d,
3
3
3
J(7,8) ¼6.6, HꢀC(8)); 6.24 (d, J(2,3) ¼6.5, HꢀC(2)); 5.96 (s, HꢀC(10)); 3.70, 3.58 (2s, COOMe); 2.58
3
3
13
(
(
sept., J ¼6.8, Me CHꢀC(9)); 2.04 (s, MeꢀC(1)); 1.14, 1.12 (2d, J ¼6.9, 6.8, Me CHꢀC(9)). C-NMR
2
2
CDCl ; tentative assignment): 190.26 (d, CHO); 167.16, 167.01 (2s, COOMe); 157.16 (s, C(9)); 146.13 (d,
3
C(3)); 142.94 (s, C(5a)); 140.24 (d, C(2)); 137.47 (s, C(1)); 133.17 (s, C(10a)); 132.15 (s, C(6)); 131.07 (s,
C(4)); 127.74 (d, C(7)); 126.76 (d, C(10)); 126.62 (s, C(5)); 124.14 (s, C(8)); 52.00, 51.95 (2q, COOMe));
þ.
3
(
7
6.45 (d, Me CHꢀC(9)); 25.41, 22.52 (2q, Me CHꢀC(9)); 22.06 (q, MeꢀC(1)). EI-MS: 354 (8, M ), 296
2
2
þ
16), 295 (100, [M ꢀCOOMe] ), 165 (8). Anal. calc. for C H O (354.40): C 71.17, H 6.26; found: C
21
22
5
0.89, H 6.17.
.3. Dimethyl 1-(Bromomethyl)-9-isopropyl-6-methyl- and 6-(Bromomethyl)-9-isopropyl-1-methyl-
heptalene-4,5-dicarboxylate (11b and 12b, resp.). Diester 5b (1.00 g, 2.94 mmol) and NBS (0.55 g,
.08 mmol) were dissolved in CCl . Then, dibenzoyl peroxide (0.038 g, 0.12 mmol) was added to start the
1
3
4
bromination reaction under reflux, at an oil bath temp. of 908. After 2 h at 908, the mixture was cooled
and the formed succinimide removed by filtration. The residue of the clear CCl soln. was subjected to
4
CC (SiO , hexane/Et O 4 :1): crystalline mixture 12b/5b/11b 3.1:1.7:1. Recrystallization from EtOH
2
2
gave 12b (0.32 g, 26%) in a purity of ca. 95%.
1
3
Data of 12b: R (hexane/AcOEt 3 :1) 0.31. H-NMR (CDCl ): 7.50 (d, J(2,3) ¼7.1, HꢀC(3)); 6.42 (d,
f
3
3
3
3
J(7,8) ¼6.5, HꢀC(8)); 6.24 (dd, J(2,3) ¼7.2, HꢀC(2)); 6.22 (d, J(7,8) ¼6.5, HꢀC(7)); 5.85 (s,
2
3
HꢀC(10)); 4.26, 4.04 (AB,
J
AB
¼10.0, BrCH ꢀC(6)); 3.72, 3.68 (2s, COOMe); 2.49 (sept., J ¼6.9,
2
3
Me CHꢀC(9)); 2.20 (s, MeꢀC(1)); 1.09, 1.06 (2d, J ¼6.9, Me CHꢀC(9)).
2
2