Unexpected thermal transformation of aryl 3-arylprop-2-ynoates: Formation of 3-(diarylmethylidene)-2,3-dihydrofuran-2-ones

Article abstract of DOI:10.1002/1522-2675(20011219)84:12<3548::AID-HLCA3548>3.0.CO;2-2

A number of aryl 3-arylprop-2-ynoates 3 has been prepared (cf. Table 1 and Schemes 3-5). In contrast to aryl prop-2-ynoates and but-2-ynoates, 3-arylprop-2-ynoates 3 (with the exception of 3b) do not undergo, by flash vacuum pyrolysis (FVP), rearrangement to corresponding cyclohepta[b]furan-2(2H)-ones 2 (cf. Schemes 1 and 2). On melting, however, or in solution at temperatures >150% the compounds 3 are converted stereospecifically to the dimers 3-[(Z)-diarylmethylidene]-2,3-dihydrofuran-2-ones (Z)-11 and the cyclic anhydrides 12 of 1,4-diarylnaphthalene-2,3-dicarboxylic acids, which also represent dimers of 3, formed by loss of one molecule of the corresponding phenol from the aryloxy part (cf. Scheme 6). Small amounts of diaryl naphthalene-2,3-dicarboxylates 13 accompanied the product types (Z)-11 and 12, when the thermal transformation of 3 was performed in the molten state or at high concentration of 3 in solution (cf. Tables 2 and 4). The structure of the dihydrofuranone (Z)-11c was established by an X-ray crystal-structure analysis (Fig. 1). The structures of the dihydrofuranones 11 and the cyclic anhydrides 12 indicate that the 3-arylprop-2-ynoates 3, on heating, must undergo an aryl O → C(3) migration leading to a reactive intermediate, which attacks a second molecule of 3, finally under formation of (Z)-11 or 12. Formation of the diaryl dicarboxylates 13, on the other hand, are the result of the well-known thermal Diels-Alder-type dimerization of 3 without rearrangement (cf. Scheme 7). At low concentration of 3 in decalin, the decrease of 3 follows up to ca. 20% conversion first-order kinetics (cf. Table 5), which is in agreement with a monomolecular rearrangement of 3. Moreover, heating the highly reactive 2,4,6-trimethylphenyl 3-(4-nitrophenyl)prop-2-ynonate (3f) in the presence of a twofold molar amount of the much less reactive phenyl 3-(4-nitrophenyl)prop-2-ynonate (3g) led, beside (Z)-11f, to the cross products (Z)-11fg, and, due to subsequent thermal isomerization, (E)-11fg (cf. Scheme 10), the structures of which indicated that they were composed, as expected, of rearranged 3f and structurally unaltered 3g. Finally, thermal transposition of [¹⁷O]-3i with the ¹⁷O-label at the aryloxy group gave (Z)- and (E)-[¹⁷O₂]-11i with the ¹⁷O-label of rearranged [¹⁷O]-3i specifically at the oxo group of the two isomeric dihydrofuranones (cf. Scheme 8), indicating a highly ordered cyclic transition state of the aryl O → C(3) migration (cf. Scheme 9).

Full text of DOI:10.1002/1522-2675(20011219)84:12<3548::AID-HLCA3548>3.0.CO;2-2

3548
Helvetica Chimica Acta Vol. 84 (2001)
Unexpected Thermal Transformation of Aryl 3-Arylprop-2-ynoates:
Formation of 3-(Diarylmethylidene)-2,3-dihydrofuran-2-ones
by Vit Lellek and Hans-J¸rgen Hansen*
Organisch-chemisches Institut der Universit‰t Z¸rich, Winterthurerstrasse 190, CH-8057 Z¸rich
Dedicated to Conrad Hans Eugster on the occasion of his 80th birthday
A number of aryl 3-arylprop-2-ynoates 3 has been prepared (cf. Table 1 and Schemes 3 5). In contrast to
aryl prop-2-ynoates and but-2-ynoates, 3-arylprop-2-ynoates 3 (with the exception of 3b) do not undergo, by
flash vacuum pyrolysis (FVP), rearrangement to corresponding cyclohepta[b]furan-2(2H)-ones 2 (cf. Schemes 1
and 2). On melting, however, or in solution at temperatures > 1508, the compounds 3 are converted
stereospecifically to the dimers 3-[(Z)-diarylmethylidene]-2,3-dihydrofuran-2-ones (Z)-11 and the cyclic
anhydrides 12 of 1,4-diarylnaphthalene-2,3-dicarboxylic acids, which also represent dimers of 3, formed by loss
of one molecule of the corresponding phenol from the aryloxy part (cf. Scheme 6). Small amounts of diaryl
naphthalene-2,3-dicarboxylates 13 accompanied the product types (Z)-11 and 12, when the thermal trans-
formation of 3 was performed in the molten state or at high concentration of 3 in solution (cf. Tables 2 and 4).
The structure of the dihydrofuranone (Z)-11c was established by an X-ray crystal-structure analysis (Fig. 1).
The structures of the dihydrofuranones 11 and the cyclic anhydrides 12 indicate that the 3-arylprop-2-ynoates 3,
on heating, must undergo an aryl O ! C(3) migration leading to a reactive intermediate, which attacks a second
molecule of 3, finally under formation of (Z)-11 or 12. Formation of the diaryl dicarboxylates 13, on the other
hand, are the result of the well-known thermal Diels-Alder-type dimerization of 3 without rearrangement (cf.
Scheme 7). At low concentration of 3 in decalin, the decrease of 3 follows up to ca. 20% conversion first-order
kinetics (cf. Table 5), which is in agreement with a monomolecular rearrangement of 3. Moreover, heating the
highly reactive 2,4,6-trimethylphenyl 3-(4-nitrophenyl)prop-2-ynonate (3f) in the presence of a twofold molar
amount of the much less reactive phenyl 3-(4-nitrophenyl)prop-2-ynonate (3g) led, beside (Z)-11f, to the cross
products (Z)-11fg, and, due to subsequent thermal isomerization, (E)-11fg (cf. Scheme 10), the structures of
which indicated that they were composed, as expected, of rearranged 3f and structurally unaltered 3g. Finally,
thermal transposition of [17O]-3i with the 17O-label at the aryloxy group gave (Z)- and (E)-[17O2]-11i with the
17O-label of rearranged [17O]-3i specifically at the oxo group of the two isomeric dihydrofuranones (cf.
Scheme 8), indicating a highly ordered cyclic transition state of the aryl O ! C(3) migration (cf. Scheme 9).
1. Introduction. Recently, we have demonstrated that flash vacuum pyrolysis
(FVP) of highly substituted phenyl prop-2-ynoates 1, according to the original
procedure of Trahanovsky et al. [1], represents an excellent method for the synthesis of
polyalkylated cyclohepta[b]furan-2(2H)-ones 2 [2] (Scheme 1).
Brown and Eastwood [3] have found that FVP of phenyl but-2-ynoate (3a) at 65 08
leads in a maximum yield of only 5% to the corresponding cyclohepta[b]furan-2(2H)-
one 4a (Scheme 2). Nonetheless, this experiment is in agreement with the hypothesis
that the high-temperature rearrangements 1 ! 2 is connected with the alkyne > alke-
nylidene equilibrium (ÀCꢀCÀH > ÀCHˆC:), which favors, below 6008, completely
the alkyne structures, but is shifted towards the alkenylidene structure above 6008.
Intramolecular addition of the unsaturated carbene part to the adjacent CˆC bond of
the PhO residue induces then the formation of 2. In the case of 3a, the discussed
equilibrium is less pronounced at 6508 due to the bad migratory aptitude of the Me

Helvetica Chimica Acta Vol. 84 (2001)
3549
Scheme 1
O
O
O
FVP
H
O
R
R
1
2
group in the alkyneÀalkenylidene rearrangement1). In our experiments, we were
interested in the high-temperature behavior of phenyl 3-phenylprop-2-ynoate (3b)
with the hope that the migratory aptitude of a Ph substituent might be better than that
of a Me group. However, the yield of the expected 3-phenylcyclohepta[b]furan-2(2H)-
one (4b)2) did not exceed 2%, even under optimal FVP conditions (Scheme 2).
Moreover, when we performed FVP experiments with 2,4,6-trimethylphenyl 3-
phenylprop-2-ynoate (3c), the formation of the corresponding cyclohepta[b]furan-
2(2H)-one 4c was not observed at all. However, in the preheating phase of 3c for its
evaporation for FVP we observed a thermal transformation of 3c already in the molten
state at temperatures > 1508. Below, we will report on this unprecedented thermal
transformation of aryl 3-arylprop-2-ynoates.
2. Synthesis of Aryl 3-Arylprop-2-ynoates. We synthesized a number of highly
substituted phenyl prop-2-ynoates 3 with the sodium salts 5' of the corresponding prop-
2-ynoic acids 5 and highly substituted phenyl carbonochloridates 6, according to a
procedure described in [2] (Table 1). The procedure is not applicable to phenyl
carbonochloridates that carry no o-substituents (cf. [2]) as shown once more by the
Scheme 2
O
R2
O
O
R2
R2
FVP
R1
R2
O
R1
R2
R2
R1 = Me, R2 = H:
R1 = Ph, R2 = H:
R1 = Ph, R2 = Me:
3a
3b
3c
4a (4 - 5 %)
4b (2 %)
4c (none)
1
2
)
Calculations on isodesmic reactions show that the transition-state energies of the ethyneÀethenylidene and
propyneÀpropenylidene transformation differ by 90 120 kJ/mol. We thank Dr. R. W. Kunz for these
calculations.
)
For an efficient synthesis of 4b, see [4].

3550
Helvetica Chimica Acta Vol. 84 (2001)
Table 1. Synthesis of Highly Substituted Aryl Pro-2-ynoates 3
O
O
O
Cl
O
Ð CO
R1
2
R1
COONa
+
THF,
reflux
R2
R2
(Ð NaCl)
5'
6
3
R1
No.
R2
No.
Prop-2-ynoates
No.
Yield [%]a)
Ph
Ph
5'a
5'a
5'b
5'b
5'b
5'c
2,4,6-Me3
2,3,4,5,6-Me5
2,3,4,5,6-Br5
2,4,6-Me3
H
6a
6b
6c
6a
6d
6a
3c
3d
3e
3f
3g
3h
73
69
92
80
4-NO2C6H4
4-NO2C6H4
4-NO2C6H4
Me
41 b)
50
2,4,6-Me3
a) Yield have not been optimized. b) Side products are formed.
reaction of 5'a with phenyl carbonochloridate (6d; Scheme 3). The expected
propynoate 3b is formed in a yield of only 41% due to the fact that the mixed
anhydride of 5a and 6b is attacked by the nucleophiles PhOÀ and PhÀCꢀCÀCOOÀ at
both CˆO groups. The Diels-Alder-like cyclization of 3-arylpropynoic acid anhydrides
with concomitant H-migration to the corresponding cyclic anhydride of naphthalene-
2,3-dicarboxylic acids is a well-known reaction (cf. [5] and refs. cit. therein). The
esterification of 3-(4-nitrophenyl)prop-2-ynoic acid could be realized with the
carbonochloridates 6a and 6c in excellent yields, whereas the established procedure
with the acid chloride 9b, which is easily available from the acid and oxalyl chloride in
the presence of a catalytic amount of DMF at ambient temperature in CH2Cl2, and the
corresponding sodium phenolates in THF resulted in the formation of a number of
products (cf. Scheme 4). Also 3b could not be synthesized in pure form from 9c and
PhONa. Fortunately, Okajima had already synthesized 3b by reacting 9c with phenol in
Scheme 3
O
O
Ph
O
Cl
O
O
PhO
PhO
Ð CO
2
Ph
+
+
+
5'a
O
O
THF,
reflux
(Ð NaCl)
O
6d
3b (41 %)
7b (73 %)a)
8 (93 %)a)
a) Yield with respect to the expected amount, based on 41% yield of 3b.

Helvetica Chimica Acta Vol. 84 (2001)
3551
Scheme 4
(COCl)2/cat. DMF
COCl
R
COOH
R
20¡/CH2Cl2
R = NO2 : 5b
R = MeO: 5d
9a
9b
O
O
Cl
ONa
O
O
Ð15¡
THF
NO2
+
+
9a
NO2
3f (49 %)a)
10 (26 %)a)
O
ONa
O
R
R
R
R
R
Ð15¡
THF
+
9b
OMe
R
R = Me : 3i (79 %)b)
R = Br : 3k (74 %)b)
a) Yield over both steps aqueous workup. b) Yield over both steps after chromatography on silica gel.
boiling benzene in the presence of Mg turnings [6]. Indeed, by following this protocol,
we obtained 3b in a yield of 81% (Scheme 5). The method was also applicable to the
synthesis of (2-methylphenyl) 3-phenylprop-2-ynoate (3l), which was isolated in 63%
Scheme 5
O
O
OH
O
Mg
R
R
Ph
+
Ph
C
benzene,
reflux
Cl
9ca)
R = H: 3b (81 %)
3l (63 %)
R = Me:
a) See Scheme 4.

3552
Helvetica Chimica Acta Vol. 84 (2001)
yield. On the other hand, the acid chloride/PhONa procedure is well-suited for the
synthesis of the highly substituted phenyl 3-(4-methoxyphenyl)prop-2-ynoates 3i and
3k (Scheme 4).
3. Thermal Transformation of the Aryl 3-Arylprop-2-ynoates 3. When the more or
less colorless esters 3 are heated in the molten state or in concentrated solutions in
solvents such as decalin at temperatures ꢁ 1508, an intense yellow color develops
slowly, which can be attributed to the formation of the 3-(diarylmethylidene)-2,3-
dihydrofuran-4-carboxylates (Z)-11 (Scheme 6). In all cases, the lactones (Z)-11 are
accompanied by smaller amounts of the cyclic anhydrides 12, the diaryl naphthalene-
2,3-dicarboxylates 13, and some (E)-11, which is formed by thermal isomerization of
(Z)-11 during the reaction. The product balance of the reactions in the molten state
does not account for 100%, since a certain amount of polymeric material is also formed,
which was not further investigated. The results of the realized thermal transformations
are collected in Table 2. The furan-carboxylates 11 and the diaryl naphthalene-
dicarboxylates 13 represent dimers of 3, whereas the cyclic anhydrides 12 indicate the
loss of one molecule of the corresponding phenols in the course of the dimerization of
3. A first inspection of Scheme 6 and Table 2 reveals that the formation of (Z)-11 and 12
is accompanied by an O ! C migration of the aryl group of the aryloxy part in one
molecule of 3. The COO group of the second molecule of 3 enters the dimers (Z)-11
unchanged and is found as (aryloxy)carbonyl group at C(4)3) of the furan ring. The loss
of one molecule of the corresponding phenol must occur after O ! C migration from
the second molecule of 3 that enters into the reaction leading to 12. On the other hand,
the formation of 13 takes place without change of the ester part of 3, since it is found
unaltered in the naphthalene-2,3-dicarboxylate part of 13. The formation of 1-
phenylnaphthalene-2,3-dicarboxylates on heating of 3-phenylprop-2-ynoates at 2008
has already been described by Pfeiffer and Mˆller in 1907 [7]4) and, later, it was found
that the dimerization of 3-phenylprop-2-ynoates slowly takes place already in solution
at ambient temperature in the presence of an acid (cf. [8] and earlier refs. cit. therein).
Moreover, Michael and Bucher reported even ten years earlier that 3-phenylprop-2-
ynoic acid, on treatment with Ac2O, forms the cyclic anhydride of 1-phenylnaphtha-
lene-2,3-dicarboxylic acid [9] (cf. also [10]).
The formation of (Z)-11 and 12 is dependent on the number of o-substituents in the
aryloxy part of the esters 3, but seems not to be influenced by the electronic nature of
the substituents, since the reaction occurs both with the 2,4,6-trimethylphenoxy and
with the 2,4,6-tribromophenoxy residue in 3. However, when the 2,4,6-trimethylphe-
noxy group is replaced with a 2-methylphenoxy moiety, as realized in the ester 3l, the
product formation declines distinctly, and higher temperatures for the conversion of 3l
are necessary; and ester 3b, which bears no substituent at its phenoxy part, provides, up
to 2008, neither furanone (Z)-11b nor anhydride 12b.
3
)
)
The C-atom numberings for 11 and 12 correspond to the systematic nomenclature (see Table 3 and Exper.
Part).
As a Zurich reminiscence, it is worthwhile to mention that Paul Pfeiffer and his co-worker performed the
investigation −Zur Polymerisation des Phenylpropiols‰ureesters× in the old −Universit‰tslaboratorium× in
Zurich shortly before the new chemistry building at the R‰mistrasse, founded by Alfred Werner, was ready
for use (summer term 1909).
4

Helvetica Chimica Acta Vol. 84 (2001)
3553
Scheme 6
R'
O
O
Rn
150 Ð 210¡
(E)-11a)
+
COO
Rn
R'
Rn
O
O
R'
3
(Z)-11
Rn
O
Rn
COO
COO
+
O
R'
R'
Rn
O
R'
R'
12
13
a) Formed thermally from (Z)-11.
Table 2. Product Composition of the Thermal Transformation of Aryl 3-Arylprop-2-ynoates 3a)
Furanones 11b)
(Z) [%] (E) [%] [%]
Propynoate 3
Temp. Time
Anhydrides 12 Diester 13 Recovered 3
No.c) R'
Rn
[8]
[h]
[%]
[%]
c
d
e
f
g
i
H
H
2,4,6-Me3
2,3,4,5,6-Me5 150
150
18
24
1
38
30
< 0.5d)
14
n.d.e)f)
n.d.
n.d.f)
n.d.
37
9
g
NO2 2,3,4,5,6-Br5 220
NO2 2,4,6-Me3
NO2
MeO 2,4,6-Me3
MeO 2,4,6-Br3
H
À )
150
18517
200
210
200
16
68
6
n.d.
n.d.
n.o.
0
0
H
1.8
0.8
n.d.
24
n.d.
0
12
548
154
254
19
< 0.5d)
8
0
k
l
n.o.h)
2-Me
2
7
n.d.
a) All reactions were performed in molten phase. b) Yields of purified compounds with respect to reacted 3. c)
Identification letter for 3 and 11 13. d) Limit of detection by HPLC. e) n.d.: Compound observed by TLC and/
or HPLC, but amount not determined; see also Table 5. f) After 100 h at 1508, relative ratio 11c/12c/13c
6.5: 2.5: 1. g) Mostly decomposition was observed. h) n.o. ˆ not observed.

3554
Helvetica Chimica Acta Vol. 84 (2001)
On the other side, the electronic effect of p-substituents at the 3-arylprop-2-ynoic
part of the esters 3 on the rate and also on the cleanness of the transformations is most
markedly. A p-acceptor substituent favors the reaction. We obtained the best results
with the 3-(4-nitrophenyl)prop-2-ynoate 3f, which underwent complete transformation
already after 16 h heating at 1508 with an isolated yield of 74% of (Z)-11f and (E)-11f.
The unsubstituted 3-phenylprop-2-ynoate 3c led, under similar conditions, to starting
material, and only 38% of (Z)-11c in the presence of 12c and some 13c were formed.
Furthermore, whereas the propynoate 3b exhibits no thermal reactivity at all up to
2008, its analog 3g, derived from 3-(4-nitrophenyl)prop-2-ynoic acid (5g), gave, at 1858,
2.6% of (E)-11g and (Z)-11g, but mainly the naphthalene-2,3-dicarboxylate 13g (24%;
Table 2). A p-donor substituent such as a MeO group retards the reaction of the esters
3. The thermal transformation of the p-MeO-substituted prop-2-ynoate 3i did not
proceed to a visible extent at 1508. The temperature had to be raised to 2008 so that
completion of product formation was realized within 12 h, leading to 25% of (Z)-11i
and 4% of (E)-11i besides 19% of the cyclic anhydride 12i. Diester 13i was also present,
but its amount was not determined. The thermal reaction of the corresponding Br-
substituted ester 3k was complete after 5-h heating at 2108 and led to the formation of
48% of the furanone (Z)-11k.
The described thermal transformation of the esters 3 into the lactones 11 seems to
be bound to 3-aryl substituents at the propynoic-acid part, since we were not able to
thermally react 2,4,6-trimethylphenyl but-2-ynoate (3h). We also recognized that the
transformations observed can only be realized thermally, since irradiation of our model
ester 3c in toluene gave no reaction at all.
3. Characterization of the New Products.
3.1. 3-(Diarylmethylidene)-2,3-
dihydrofuran-2-ones 11. Since the furan ring of 11 is only surrounded by aryl groups,
a purely spectroscopic structure assignment, especially with respect to the presence of
(Z)- and (E)-isomers, was impossible. The structure of the yellow main product from
the thermal transformation of the ester 3c was, therefore, solved by an X-ray
crystal-structure determination (Fig. 1). It established the presence of an unsaturated
five-membered lactone ring and also revealed the (Z)-configuration at the methylidene
group. In Table 3, some of the torsion angles V are listed. The five-membered lactone
ring is almost planar (mean deviation from a least-squares plane: 2.0 pm). However,
C(3') of the methylidene substituent as well as C(4') of the carbonyl group at C(4)
deviate markedly from this plane (above (19.6 pm) and below (43.0 pm), resp.). As a
consequence, the corresponding torsion angles V(O(1)ÀC(2)ÀC(3)ÀC(3')) and
V(C(5)ÀC(4)ÀC(3)ÀC(3')) are deflected, with 170.0(5)8 and À 171.3(6)8, respective-
ly, markedly from 1808. Similarly, V(C(2)ÀC(3)ÀC(4)ÀC(4')) and V(O(1)ÀC(5)À
C(4)ÀC(4')) amount to À 157.6(5)8 and 161.1(5)8, respectively, leading to an average
deviation of 208 from an ideal antiperiplanar arrangement. In turn, the torsion angle
between C(3') and C(4'), i.e., V(C(3')ÀC(3)ÀC(4)ÀC(4')), amounts to 28.0(9)8.
C(1a)-Atom of the Ph substituent at C(5) of the endocyclic CˆC bond is also turned
out of the plane of the lactone ring by 7.8 pm, which is also indicated by
V(C(2)ÀO(1)ÀC(5)ÀC(1a)) ˆ 174.9(4)8 as well as by V(C(4')ÀC(4)ÀC(5)ÀC(1a))
ˆ À17(1)8. The 2,4,6-trimethylphenyl substituent in (Z)-position at C(3') is
as
expected heavily turned out of conjugation with the methylidene group (cf.

Helvetica Chimica Acta Vol. 84 (2001)
3555
Fig. 1. Stereoscopic view of the X-ray crystal structure of (2,4,6-trimethylphenyl) 2,3-dihydro-2-oxo-5-phenyl-3-
[(Z)-(phenyl)(2,4,6-trimethylphenyl)methylidene]furan-4-carboxylate ((Z)-11c)
V(C(3)ÀC(3')ÀC(1b)ÀC(2b)), whereas the Ph substituent at the methylidene
group is tilted with respect to the p-plane to a much smaller extent (cf.
V(C(3)ÀC(3')ÀC(1c)ÀC(2c)).
Table 3. Selected Torsions Angles V from the X-Ray Crystal Structure of 2,3-Dihydrofuran-2-one (Z)-11c a)
Atoms
V [8]
Atoms
V [8]
À 5.0(6)
OˆC(2)ÀC(3)ÀC(3')
À 5(1)
O(1)ÀC(2)ÀC(3)ÀC(4)
O(1)ÀC(5)ÀC(4)ÀC(3)
OˆC(2)ÀC(3)ÀC(4)
C(2)ÀC(3)ÀC(3')ÀC(1b)
C(4)ÀC(3)ÀC(3')ÀC(1c)
C(3)ÀC(3')ÀC(1b)ÀC(2b)
C(3)ÀC(3')ÀC(1c)ÀC(2c)
C(4')ÀC(4)ÀC(5)ÀC(1a)
C(4)ÀC(5)ÀC(1a)ÀC(2a)
OˆC(4')ÀC(4)ÀC(5)
21.1(9)
16.2(9)
70.5(7)
36.0(9)
0.0(6)
179.8(7)
178.5(5)
170.0(5)
À 171.3(6)
À 157.6(5)
161.1(5)
28.0(9)
OˆC(2)ÀO(1)ÀC(5)
O(1)ÀC(2)ÀC(3)ÀC(3')
C(5)ÀC(4)ÀC(3)ÀC(3')
C(2)ÀC(3)ÀC(4)ÀC(4')
O(1)ÀC(5)ÀC(4)ÀC(4')
C(3')ÀC(3)ÀC(4)ÀC(4')
À 17(1)
À 17(1)
52.6(9)
3.2(6)
C(2)ÀC(3)ÀC(4)ÀC(5)
a) Numbering, which is also valid for the NMR data (including the Exper. Part), is shown below:
4
5
5
4
3
6
c
6
d
3
2
3
5
O
2
1
2
1
1
4
3'
b
O
4'
6
3 4
6
5
2
5
O
1
O
a
4
2
3

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Helvetica Chimica Acta Vol. 84 (2001)
The (E)-configuration can be attributed to the second yellow compound that
accompanied (Z)-11c. When a solution of pure (Z)-11c in decalin was heated at 160
1658 during 24 h, (E)-11c was obtained in 30% yield together with 70% of (Z)-11c. The
(E)-isomer was also formed, when pure (Z)-11c was irradiated in CDCl3 solution in a
NMR tube with (366 Æ 25)-nm light, to give, after 33 h, without any decomposition a
photostationary state with 84% of (E)-11c and 16% of (Z)-11c. These observations and
comparison of the UV/VIS (cf. Fig. 2), and the 1H- and 13C-NMR spectra, and
especially 1H-NOE measurements (cf. Fig. 3) of (Z)-11c and (E)-11c indicated
unequivocally that both forms are indeed stereoisomers with opposite configurations at
the 3-methylidene group. The pure (E)-isomers, beside the (Z)-isomers, were also
isolated and fully characterized in the case of 11d, 11f, 11g5) and 11i.
The UV/VIS spectra (hexane) of the (Z)- and (E)-configured furanones 11 are
characterized by a broad absorption band around 400 nm (log e 4.20) with an average
value of (401 Æ 10) nm for the (Z)-forms and (398 Æ 10) nm for the (E)-forms (cf.
Fig. 2. UV/VIS Spectra (hexane) of a) (Z)-11c, b) (E)-11c, c) (Z)-11f and d) (E)-11f
5
)
The stereochemical descriptors are reversed in this case due to the priority rules, i.e., (E)-11g and (Z)-11g
correspond to the (Z)- and (E)-configuration, respectively, of the other furanones 11.

Helvetica Chimica Acta Vol. 84 (2001)
3557
Exper. Part). As examples, the spectra of (Z)-11c, (E)-11c, (Z)-11f, and (E)-11f are
displayed in Fig. 2. The absorption band of the (Z)-isomers (Fig. 2,a and c) exhibits at
its short-wavelength flank a shoulder at ca. 350 360 nm, which is not visible in the
more or less symmetric band of the (E)-isomers (Fig. 2,b and d). This small difference
is observable in all spectra of the (Z)- and (E)-isomers (cf. Exper. Part) and may be
used for an assignment of the configuration at the exocyclic CˆC bond.
The IR spectra (CHCl3) of (Z)-11 and (E)-11 show two strong CˆO absorption
bands at (1780 Æ 9) and (1734 Æ 7) cmÀ1, which can be attributed to the CˆO vibrations
of the unsaturated lactone function and of the ester group at C(4), respectively. The
lactone band for the (Z)-isomers appears at higher wave-numbers than that for the
(E)-isomers (average values: (1782 Æ 9) cmÀ1 for (Z)-forms and (1777 Æ 9) cmÀ1 for
(E)-forms). However, the difference is not significant enough to allow, in all cases, an
unambiguous assignment of the configuration at the methylidene group (cf. Exper.
Part).
1
Compounds (Z)-11 and (E)-11 exhibit similar chemical shifts so that the H- and
13C-NMR spectra do not allow an unequivocal configurational assignment. However,
1
based on the established furanone ring of (Z)-11c (Fig. 1), H-NOE measurements
allow the correct assignment of the configuration of (Z)-11 and (E)-11. As an example,
the signal-enhancement effects for (Z)-11c and (E)-11c are depicted in Fig. 3. The
observed effects between the two 2,4,6-trimethylphenyl moieties are only compatible
with the (E)-configured diastereoisomer of 11c. This observation allows also the
assignment of all other signals to the H-atoms and Me groups at the phenyl rings (cf.
Exper. Part).
m
s
O
O
O
O
m
s
O
O m
m
O
m
O
w
s
m
1
Fig. 3. Observed H-NOE effects of (Z)-11c and (E)-11c (CDCl3)
3.2. Cyclic Anhydrides 12. Since compounds of this type are principally well-known
(see, e.g., [5]) and exhibit characteristic UV spectra, we isolated only anhydrides 12i
and 12l in 19% and 8% yield, respectively, from the corresponding reaction mixtures
(Table 2), in order to determine the position of the substituents at the naphthalene ring
1
by H-NOE measurements. For example, the signal of HÀC(8)3) appears in the
1H-NMR spectrum (CDCl3) of 12i at 7.59 ppm as d with 3J(8,7) ˆ 9.0 Hz. It interferes in

3558
Helvetica Chimica Acta Vol. 84 (2001)
1H-NOE experiments strongly with the s of MeÀC(2'',6'') of the 2,4,6-trimethylphenyl
group at C(9). In agreement with this finding, the d of HÀC(5) with 4J(5,7) ˆ 2.5Hz at
7.29 ppm shows a strong reciprocal enhancement effect with the signal of HÀC(2',6') of
the MeO-substituted Ph ring at C(4), which appear as d (Jo ˆ 6.5Hz) with fine
structure (f.s.) at 7.44 ppm.
The cyclic anhydride 12l showed two d×s with f.s. and J ꢂ 8 Hz at 7.97 and 7.77 ppm,
followed by two t×s with f.s. and J ꢂ 8 Hz at 7.70 and 7.68 ppm, which can be attributed to
HÀC(5 8) at the naphthalene ring. Because the d with f.s. at 7.77 ppm induced, on
irradiation, a strong enhancement effect on the s at 2.05ppm of Me ÀC(2'') of the o-
toluyl group at C(9), it had to be attributed to HÀC(8). The identification of the other
1
1
signals follows from further H-NOE and H-COSY measurements. They are in full
agreement with the anticipated structure of 12l.
The cyclic anhydride structure of 12i and 12l was further verified by their IR spectra
(CHCl3), which exhibited nÄas at 1842 and 1835cm À1, and nÄs at 1773 and 1777 cmÀ1,
respectively, due to the vibrationally coupled CˆO groups.
All the other cyclic anhydrides 12 were solely identified by their UV spectra,
recorded during HPLC analyses, as well as by their Rf values and fluorescence
quenching properties in TLC analyses.
3.3. Diaryl Naphthalene-2,3-dicarboxylates 13. Again, we isolated and characterized
only two compounds of this type, namely, those from the thermal reaction of ester 3g
1
and 3l. They were obtained in 24 and 7% yield, respectively. The H- and 13C-NMR
spectra as well as 1H-NOE measurements allowed complete elucidation of the
structures. Whereas the UV spectra of the cyclic anhydrides 12 and the diaryl
naphthalene-2,3-dicarboxylates 13 are, with the exception of one absorption band in
the 250-nm region, quite similar (cf. Exper. Part), they can be nicely differentiated by
their IR spectra (CHCl3), since they exhibit only one CˆO vibrational band at 1745
(13g) and 1740 cmÀ1 (13l).
All other diaryl naphthalene-2,3-dicarboxylates 13 were mostly identified by their
UV spectra, recorded during HPLC analyses, as well as by their Rf values and
fluorescence quenching properties in TLC analyses.
4. Mechanistical Studies. That the main products (Z)-11, 12, and 13 of the thermal
reaction of the esters 3 represent dimers of 3, whereby 12 is formed by loss of the
corresponding phenol of the aryloxy part of one molecule of 3, indicates that
bimolecular reactions play the decisive role. Since some further uncontrolled side
reactions such as polymerizations also took place, clean kinetic experiments could not
be performed. Nonetheless, the product composition of the thermal reaction of 3c in
dependence of the initial concentration of 3c after 13.5-h heating at 185 1908, as
summarized in Table 4, is in agreement with a bimolecular reaction pathway of 3c. As
we have already mentioned, (Z)-11c is thermodynamically favored, but its formation is
kinetically controlled. Indeed, AM1 calculation of the structure of (Z)-11c as well as of
(E)-11c, with the crystal conformation of (Z)-11c as start geometry (cf. Fig. 1), led to
DDHf 8 ˆ 0.92 kcal ¥ molÀ1. The equilibrium mixture of (Z)-11c/(E)-11c in hexane in the
presence of I2 at ambient temperature consists of 84% of (Z)-11c and 16% of (E)-11c.
By the assumption that DDHf 8 ꢂ DDGf 8, i.e., DDSf 8 ꢂ 0, the calculated composition of
83% of (Z)-11e and 17% of (E)-11c is in excellent agreement with the observed

Helvetica Chimica Acta Vol. 84 (2001)
3559
composition6). Also in cases where (Z)-11 is accompanied by small amounts of (E)-11,
the latter isomer is formed thermally from (Z)-11. As an example, Fig. 4 represents the
course of the reaction of the most reactive 1-(4-nitrophenyl)prop-2-ynoate 3f at 1758 in
PhNO2. At the beginning of the transformation, only (Z)-11f is recognizable. The
amount of (E)-11f generated depends upon (Z)-11f provided by the reaction of 3f.
This fact leads to a typical convexity of the curve of the formation of (Z)-11f and, in
turn, a concavity of the curve of the appearance of (E)-11f.
Fig. 4. Progress of the thermal transformation of 3f (nitrobenzene, 1758)
Table 4. Thermal Transformation of Propynoate 3c in Various Media a)
Medium
Conc.
Time
[h]
Temp.
Amount [%] of
(E)-11c
[mol/l]
4.518 0
[8]
3c
(Z)-11c
12c
n.d.
13c
Neat b)
15 37
13.1591085
160
13.5
13.5
75.3
24
160
13.5
38
< 0.5n.d.
Neat c)
3.8 .9 25
30
185 190
185 190
185 190
160
60.2
185 190
721.70
21.8
41.1
11.2
< 0.5n.d.
22.1
Decalin b)
Decalin c)
0.524
0.25
0.125
0.063
0.5
0.524
0.125
0.524
43
n.d.
35.8
31.9
26.6
49.9
5
52.3
57.0
75.3
56.8
47.9
55.1
5
39.1
41.9
45.6
41.8
3.0
0.8
0.1
5.1
25.3
27.8
3.2
Anisole c)
Nitrobenzene c)
3.9
39.3
42.75
22.6
42.1
34.6
0.8
o-Xylene c)
160
1.8
7.1
a) See also Table 2. b) Yields of isolated material. c) Product composition according to HPLC analysis; (Z)-
11c ‡ (E)-11c ‡ 12c ‡ 13c ˆ 100%.
A closer inspection of Table 4 reveals that the relative amount of 13c drops signifi-
cantly with the decrease of the initial concentration of 3c. In contrast to (Z)-11c/(E)-11c
The AM1 calculations gave DHf 8 ˆ À22.11 kcal ¥ molÀ1 for (Z)-11c and À 20.19 kcal ¥ molÀ1 for (E)-11c.
6
)

3560
Helvetica Chimica Acta Vol. 84 (2001)
and 12c, which are also formed in decreasing amounts, but with a more or less constant
ratio, when the appearance of 13c is no longer detectable at the given reaction time.
One also recognizes that the transformation of 3c exhibits almost no solvent effect.
This is true for the overall rate as well as for the product composition after a given time.
As already mentioned in the preceding chapter, the furanones (Z)-11 and the cyclic
anhydrides 12 are formed by an O ! C migration of the aryl group of the aryloxy part
of the esters 3, whereas the aryloxy group of 3 is found unchanged in the dimers 13.
Hence, we conclude that there exist two independent modes of thermal reactivity. One
mode is quite obvious. The naphthalene-2,3-dicarboxylates 13 are formed in a Diels-
Alder-type cycloaddition of two molecules of 3 with concomitant prototropic
rearrangement. Such dimerization reactions have first been reported by Pfeiffer and
Mˆller [7] (Scheme 7). The structure of the fully characterized 13g (R' ˆ NO2, Rn ˆ H)
is in agreement with this assumption. Finally, it is a well-known reactivity of 3-arylprop-
2-ynoic anhydrides that undergo such [4 ‡ 2]-type reactions intramolecularly already at
temperatures < 1008 [9][10] (see also the formation of 7b in Scheme 3). In contrast to
the formation of 13, the formation of the furanones (Z)-11 and the cyclic anhydrides 12
must take place via a common precursor U(1), formed thermally in a rate-determining
monomolecular process of 3, which leads to bonding between C(3) of the propynoic
part and C(1) of the aryloxy group. Intermediate U(1) must then react rapidly in at least
two or more independent steps with a second unchanged molecule of 3 to give (Z)-11
and a second product, which, by further loss of the corresponding phenol, yields the
cyclic anhydrides 12 (Scheme 7).
To gain more insight into the fate of the two O-atoms of the COO group of 3 after
its rearrangement into the reactive intermediate U(1), we have synthesized 2,4,6-
trimethyl[17O]phenol with a [17O] enrichment of 8 atom-% from 2,4,6-trimethylphe-
nyldiazonium tetrafluoroborate [11] and H2[17O] in the presence of CF3COOH in
analogy to the procedure described in [12]. Reaction of the Na salt of the 2,4,6-
trimethyl[17O]phenol with the acid chloride 9b (cf. Scheme 4) gave [17O]-3i with d(17O)
in the expected range of 198 ppm for an aryloxy 17O-atom (CDCl3; external standard
H2[17O]; cf. [13]) (Scheme 8). Heating of [17O]-3i without solvent at 2108 during 5h
gave (Z)-[17O2]-11i and (E)-[17O2]-11i7), in 25and 5% yield, respectively. The 17O-
NMR spectra (CDCl3) of both isomers are depicted in Fig. 5. The observed 17O-shifts
for (Z)-[17O2]-11i and (E)-[17O2]-11i reveal that one of the 17O-atoms is now found
exclusively in the oxo group at C(2) of (Z)-[17O2]-11i and (E)-[17O2]-11i, since d(17O)
values of 340 and 314 ppm, respectively, are quite typical for 17O-shifts of the oxo group
of lactones (cf. [14]), whereas the other shifts observed (190 and 198 ppm, resp.)
correspond to the aryloxy group of the starting ester, i.e., they represent the unchanged
2,4,6-trimethylphenyl[17O]oxycarbonyl group at C(4) in (Z)-[17O2]-11i and (E)-[17O2]-
11i8).
7
)
The cyclic anhydride 12i was also formed. Unfortunately, it decomposed when we tried to purify it by
column chromatography.
Our experiment also excludes O ! O aryl migration in analogy to the Chapman rearrangement (cf. [15] and
refs. cit. therein) prior to the monomolecular rearrangement of 3i. Similarly, O ! O aryl migration in the
exocyclic ester part of (Z)-11i and (E)-11i can be excluded.
8
)

Helvetica Chimica Acta Vol. 84 (2001)
3561
Scheme 7
(Z)-11
+ 3
U(1)
3
U(2)
Rn
Ð
OH
12
Rn
O
O
Rn
O
R'
O
R'
Rn
COO
COO
R'
Rn
R'
13
These findings indicate that there exists, for the aryl propynoates 3, a defined
thermal bond-reorganization process whereby the O-atom of their aryloxy groups is
finally converted, by O ! C(3) aryl group migration, to the oxo group of the (Z)-
configured furanones 11. In other words, the observed cis-relation of the migrating aryl
residue with respect to the formed oxo group provides an evidence for the formation of
a highly reactive intermediate (ˆ U(1)) with a fixed geometry, which is immediately
trapped by a second unchanged molecule of 3 under preservation of the cis-relation.
According to the −principle of least motion×, the postulated intermediate U(1) might
have the structure of a s,p-diradical 14 or any other related structure in which the
identity of the two O-atoms of the thermally transformed 3 is preserved (Scheme 9).
The intermediate s,p-diradicals could be trapped at their reactive s-radical site by a
second molecule of 3 under formation of the p2-diradicals 15a and 15b. Cyclization of
15a under concomitant cleavage of the OÀaryl bond would yield stereospecifically
(Z)-11, whereas cyclization of 15b leads to the p2-diradicals 16, which, after cleavage of

3562
Helvetica Chimica Acta Vol. 84 (2001)
Scheme 8
17OH
+
N2
MeO
H2[17O]
+ 9ba)
CF3COOH
83 %
17O
Ð
BF4
(8-atom-% 17O)
O
[17O]-3i
210¡/5 h
neat
OMe
17O
17O
MeO
+
O
O
17O
17O
O
O
OMe
OMe
(Z)-[17O2]-11i (25%)
(E)-[17O2]-11i (5%)
a) See Scheme 4.
the OÀaryl bond, form the cyclic anhydrides 12 under loss of the corresponding
phenols.
The proposed mechanism should allow cross-reactions between two esters 3,
varying in reactivity. Indeed, when the most reactive nitro ester 3f, which mainly forms
(Z)-11f and, by thermal isomerization, some (E)-11f at 1508 (cf. Table 2), was heated
in decalin (c ˆ 0.65m) in the presence of 2 mol-equiv. of the ester 3g, which mostly
reacts to 13g (cf. Table 2), (Z)-11f and the mixed forms (Z)-11fg and (E)-11fg were
obtained in the orange-colored furanone fraction in 23, 10, and 4% yield, respectively
(Scheme 10). The structure of (Z)-11fg and (E)-11fg were established by their 1H- and
13C-NMR, and mass spectra. The mixed furanones (E)- and (Z)-11gf with the reverse
combination of both esters were not present in detectable amounts in the reaction
mixture of the cross experiment, in agreement with the finding that ester 3g forms only
reluctantly (E)-11g and (Z)-11g (cf. Table 2). On the other hand, that (Z)-11fg and
(E)-11fg are formed in the cross-experiment demonstrates that it is indeed the first step
to U(1) (ˆ14) that determines the course of the further reaction. This fact can also be
deduced from a kinetic experiment with ester 3f in the absence or presence of a twofold
molar amount of ester 3g in PhNO2 at (175 Æ 1)8. In the case where U(1) is formed in
the rate-determining step, the decrease of 3f should follow first order kinetics at low
conversions. Fig. 6 demonstrates that this is indeed the case. Moreover, kÀ1(3f) for 3f ‡
2 3g is approximately three times as large as kÀ1(3f) for 3f alone, as one would expect

Helvetica Chimica Acta Vol. 84 (2001)
3563
Fig. 5. 17O-NMR Spectra (CDCl3; external ref. H2[17O]): a) [17O]-3i; b) (Z)-[17O2]-11i and c) (E)-[17O2]-11i
from the thermal transformation of [17O]-3i (molten phase, 2108/5h)
for the formation of a highly reactive intermediate U(1) from 3f, which should exhibit
similar second-order rate constants with 3f and 3g, i.e., k2[U(1)][3f] ꢂ k2[U(1)][3g].
We measured kobs(150 Æ 18) in 0.1m decalin solution in the range of 0 20%
conversion for a number of differently substituted propynoates 3, which all followed
first-order kinetics (see Table 5 and Fig. 7). The fastest reaction is observed for the 4-
NO2-substituted ester 3f, whereas the 4-MeO-substituted ester 3i exhibits only a
moderate rate enhancement with respect to the unsubstituted ester 3c. These obser-
vations may indicate a certain charge transfer from the 2,4,6-trimethylphenoxy moiety
to the aryl group at C(3) of the propynoates in the transition state. This assumption is
supported by the slight rate enhancement observed, when the 2,4,6-trimethylphenoxy
group of 3c is replaced with a 2,4,6-tribromophenoxy group as in 3k, whereas an

3564
Helvetica Chimica Acta Vol. 84 (2001)
Scheme 9

Helvetica Chimica Acta Vol. 84 (2001)
3565
Scheme 10
O2N
O2N
+
O
O
O
O
3f
3g
(1 : 2 mol-equiv.)
170¡/6 h decalin
NO2
O
O
O2N
O
O
+
(Z)-11f (23 %)
+
O
O
O
O
NO2
NO2
(Z)-11fg (10 %)
(E)-11fg (4 %)
unsubstituted PhO group as in 3b does not lead to any rearrangement at 1508 and
above. Therefore, we assume that the transition state for the formation of U(1) from
the propynoates 3 is characterized by bond formation between C(3) of the ethynyl part
and C(1) of the aryloxy group of 3, favored by an electron density flux from the aryloxy
to the aryl substituent and ending with the aryl O ! C migration observed. Its also of
interest to note that the bimolecular step between U(1) and 3, which leads finally to
(Z)-11 and 12, is strongly influenced by the nature of the 4-substituent of the aryl group
at C(3) (cf. Table 5). This would mean according to our proposed mechanism
(Scheme 9), that the p2-diradicals 15a and 15b are long-lived enough to be converted to
each other, whereby the NO2-substitued diradicals favor the formation of (Z)-11 and
the MeO-substituted diradicals the formation of 12.
The starting conformation for the described rearrangement of the propynoates 3
must be s-cis. The X-ray crystal-structure analysis of the model ester 3c shows the 2,4,6-
trimethylphenyl residue is in a perfect s-trans and parallel arrangement with respect to
the CꢀC bond of the acid part (see Exper. Part). In addition, the Ph ring at C(3) of the
acid part is in an periplanar orientation to the CˆO group. Just the same arrangement
is found by AM1 calculations as the energetically most favorable one of 3c (Fig. 8).
However, only 0.7 kcal ¥ molÀ1 higher in DHf 8 lies the calculated s-cis conformation of
3c, again with a perfect parallel arrangement of the mesityl residue with respect to the

3566
Helvetica Chimica Acta Vol. 84 (2001)
*
*
Fig. 6. Kinetics of the thermal transformation of 3f (– –) and of a 1:2 molar mixture of 3f and 3g (– –;
PhNO2, 175 Æ 18; cinit. of 3f ˆ 0.1m). kÀ1 (3f) ˆ (8.52 Æ 0.48) ¥ 10À4 minÀ1 (r2 ˆ 0.995) for 3f, and kÀ1 (3f) ˆ
(2.31 Æ 0.09) ¥ 10À3 minÀ1 (r2 ˆ 0.998) for 3f ‡ 2 3g).
triple bond, and the Ph group at C(3) of the propynoic part exhibits once more a
periplanar orientation with the CˆO group, causing an edge-to-face interaction with
the 2,4,6-trimethylphenyl residue. This latter conformation with a d(C(3) ¥¥¥ Caryloxy(1))
value of 355 pm must be responsible for the formation of U(1). At 1508, it should be
present in the s-trans/s-cis equilibrium mixture of 3c to an extent of ca. 30%, if we
assume that DDSf 8 ꢂ 09). It is of interest to note that the IR spectrum of 3c in CHCl3
solution shows in the stretching region of the CꢀC bond two frequencies at 2235and
2207 cmÀ1 of nearly equal intensities (see Fig. 9). At first sight, we thought that these
bands might represent nÄ(CꢀC) of s-trans and s-cis 3c at thermal equilibrium10).
However, since we observed this band splitting also in the IR spectrum of crystalline 3c
in a fixed s-trans conformation, it must have another origin. Indeed, two bands in the
2240 2210-cmÀ1 region have already been found for a number of 4-substituted ethyl 3-
9
)
Microwave and IR spectra of methyl prop-2-ynoate are in accordance with an almost exclusive s-trans
conformation of this ester in the temperature range of À 70 to 208 [16]. Irradiation of s-trans methyl prop-2-
ynoate in an Ar matrix at 20.4 K generates a new species to which the s-cis conformation has been assigned
[17].
Indeed, the calculated IR frequencies of s-trans- and s-cis phenyl prop-2-ynoate give Dn ˆ 12 cmÀ1 with the
lower wave-number for the s-cis conformation. We thank Prof. J. Hutter from our Institute for these
calculations.
10
)

Helvetica Chimica Acta Vol. 84 (2001)
3567
Fig. 7. Kinetics of the thermal tranformation of propynoate 3f (decalin, 150 Æ 18; cinit. ˆ 0.1m). kobs ˆ kÀ1 [3f] ˆ
(8.84 Æ 0.89) ¥ 10À4 minÀ1 (r2 ˆ 0.996; up to 19% conversion of 3f).
Table 5. Initial Rate Constants of the Transformation of Some Aryl 3-Arylpropynoates 3 in Decalin a)
Propynoates 3
R1
Decrease of 3
Product ratio b)
Furanones 11/
Anhydrides 12
R2
No.
Range [%]
kobs [10À5 minÀ1]c)
krel (1508)
Ph
2,4,6-Me3
2,4,6-Me3
2,4,6-Me3
2,4,6-Br3
3c
3f
3i
100 93
100 81
100 92
100 86
1.72 Æ 0.18
88.4 Æ 8.9
2.88 Æ 0.26
12.0 Æ 0.7
1
50
1.7
7
1.1 : 1
4-NO2ÀC6H4
4-MeOÀC6H4
Ph
> 200 : 1 d)
1 : 18
3k
1.4 : 1
a) Initial concentration: 0.1m; temperature: (150 Æ 1)8; diaryl naphthalene-2,3-dicarboxylates 13 were formed in
undetectable amounts under these conditions. b) Average value of the indicated range of transformation of 3
c
(for details, see Exper. Part). ) Correlation coefficients (r2) for linear regression analysis: 0.999 0.996.
d) Anhydride 12f was not detectable within the indicated range of transformation.
phenylprop-2-ynoates, and they have been attributed to a Fermi interaction with a
combination or overtone band in this region [18] (see also [19]).
The calculated lowest-energy conformation of 3-phenylprop-2-ynoic acid displays
an s-cis arrangement of the OH group with all atoms in one plane. The calculated IR
frequencies of this conformation show, in the low-frequency region (202 cmÀ1), a
transoid in-plane vibration that brings C(3) closer to the O-atom of the OH group10)11)
11
)
One may consider mesityl cinnamate (ˆ2,4,6-trimethylphenyl 3-phenylprop-2-enoate) in its s-cis
conformation as a model for this mode of vibration. The AM1 calculation of this conformation provides
an interatomic distance between C(3) and CMes(1) of 265pm, i.e., both C-atoms are close enough for bond
formation.

3568
Helvetica Chimica Acta Vol. 84 (2001)
Fig. 8. AM1-Calculated s-cis and s-trans conformation of propynoate 3c
(cf. also [20]). We assume that the unimolecular thermal rearrangement of the aryl 3-
arylprop-2-ynoates 3 to U(1) follow a vibrational mode as discussed (Scheme 11).
Scheme 11
R
≠
R
R
X
R
O
R
O
R
O
R
R
R
¥
¥
X
X
O
O
O
3
14
Fig. 9. CꢀC Stretching region of the IR spectra of 3c: a) in CDCl3; b) as crystals in KBr

Helvetica Chimica Acta Vol. 84 (2001)
3569
We thank Prof. M. Hesse and his co-workers for mass spectra, our NMR Laboratory and, especially, Nadja
Walch for specific NMR measurements, Dr. A. Linden for the two X-ray crystal-structure analyses, and our
Analytical Laboratory for elemental analyses. The financial support of this work by the Swiss National Science
Foundation is gratefully acknowledged.
Experimental Part
General. High-performance liquid chromatography (HPLC): Bischoff HPLC pump model 2200 with an
anal. column Spherisorb Nitrile (250 Â 3 mm), with a photodiode array detector (Waters 991). Prep. HPLC: Du
Pont Instruments 830 liquid chromatograph, with a prep. column (250 Â 20 mm) and a stationary phase
Spherisorb S5 CN. Flash vacuum pyrolysis (FVP): was performed on an apparatus equipped with a Tube
Furnace Type F 21100 (Barnstead/Thermolyne Corp.), a 30-cm long Pyrex tube filled with Pyrex O-rings. The
material was slowly evaporated from an air-bath-heated flask and collected, after pyrolysis at 6508/8 Pa, on a
cold finger, cooled with liq. N2. 2,4,6-Trimethylphenyl and 2,3,4,5,6-pentamethylphenyl carbonochloridates
were synthesized according to [2]. The 4-NO2 and 4-MeO derivatives of 3-phenylprop-2-ynoic acid were
prepared according to [21]. UV Spectra: Perkin-Elmer Lambda 19 spectrometer. IR Spectra: Perkin-Elmer
Spectrum One spectrometer. NMR Spectra: Bruker instruments ARX 300 (300/75MHz), Avance DRX 500
(500/125 MHz), and Avance DRX 600 (600/150 MHz); chemical shifts (d [ppm] relative to CD(H)Cl3 (7.27/
77.00 ppm). 17O-NMR Spectra: Bruker spectrometer Avance DRX 500 (67.8 MHz) and Avance DRX 600
1
(81.37 MHz); chemical shifts relative to external H2O at 300 K. For complete assignments of H-NMR signals,
COSY, TOCSY, NOESY, ROESY 2D- or 1D-NMR methods were applied. For complete assignments of
13C-NMR signals, HSQC and HMBC 2D-NMR methods were employed. The degree of 17O-labeling in the 17O-
enriched 2,4,6-trimethylphenol was determined by mass spectrometry, with the calculated natural-abundance
correction of [M ‡ 1] and [M ‡ 2] peaks. Electron-impact mass spectra (EI-MS): Finnigan SSQ 700 and Varian
MAT 90 mass spectrometer. Chemical-ionization mass spectra (CI-MS): Varian MAT 90 mass spectrometer;
ionization via CH4. Electrospray-ionization mass spectra (ESI-MS): Finnigan TSQ 700 instrument.
1. General Procedures. 1.1. 3-Arylprop-2-ynoyl Chlorides 9. Oxalyl chloride (5equiv.) and several drops
of dry DMF were added dropwise to a 0.5m soln. of the acids 5 in CH2Cl2. After 5h stirring, the excess of oxalyl
chloride and CH2Cl2 were removed in vacuo, and the residue was dried at 358/250 kPa for 1 h. The crude acid
chlorides 9 were immediately used in the next step.
1.2. Aryl 3-Arylprop-2-ynoates and Aryl But-2-ynoates 3. Procedure A (with the acid chlorides): To the
stirred suspension of NaH (1 equiv.) in dry THF at 08 was added dropwise a 0.5m THF soln. of the phenol (1
equiv.) so that a 0.25m soln. of PhONa was formed. It was cooled to À 158, and 1 equiv. of a 0.5m soln. of 9 in dry
THF was added dropwise under stirring. After 5h, the mixture was allowed to warm to r.t. Stirring was
continued for 15h. Then, the solvent was removed in vacuo, and the residue was further purified by CC (silica
gel; hexane/Et2O 9 :1).
Procedure B (with the Na salts 5' of the acids): The Na salts of the 3-arylprop-2-ynoic or but-2-ynoic acids 5
was prepared from the acid, dissolved in MeOH, by titration with 5% aq. NaOH soln. Before use, the salt was
dried several h in high vacuum at 508. A suspension of 5' (1.05equiv.) and the carbonochloridates 6 (1 equiv.) in
dry THF was stirred and heated at 408 for 1 h. Then, the temp. of the stirred suspension was slowly increased
within 5h to reflux temp. After 10 h, the mixture was cooled to r.t. and filtered. The solvent was removed in
vacuo, and the residue was further purified by CC (silica gel; hexane/Et2O 1 :1).
1.2.1. Phenyl 3-Phenylprop-2-ynoate (3b). According to [6], acid 5a was converted, via its chloride 9c, to 3b;
yield 81%.
Data of 3b: M.p. 40.1 40.88 (hexane/Et2O). IR (CHCl3): 2238s/2213s (CꢀC), 1720s (CˆO), 1285s
(CÀO). 1H-NMR (CDCl3): 7.69 (m, 2 arom. H); 7.55 (m, 1 arom. H); 7.43 7.53 (m, 4 arom. H); 7.34 (m, 1 arom.
H); 7.25( m, 2 arom. H). 13C-NMR (CDCl3): 152.25 (s); 150.14 (s); 133.11 (d); 130.95( d); 129.51 (d); 128.61 (d);
.
‡
‡
126.31 (d); 121.40 (d); 119.22 (s); 88.63 (s); 80.21 (s). EI-MS: 222 (5, M ), 129 (100, [PhÀCꢀCÀCO] ). Anal.
calc. for C15H10O2 (222.24): C 81.07, H 4.54; found: C 80.77, H 4.66.
1.2.2. 2,4,6-Trimethylphenyl 3-Phenylprop-2-ynoate (3c). According to Procedure B, 5'a and 6a (2.25g,
11.33 mmol) yielded colorless crystals of 3c (2.20 g, 73%).
Data of 3c: M.p. 62.8 64.88 (hexane/Et2O). UV (hexane): lmax 264 (4.29); lmin 228 (3.96). IR (CHCl3):
2234s/2206s (CꢀC), 1720vs (CˆO), 1606w (arom. CÀH), 1285s/1182vs (CÀO). 1H-NMR (CDCl3): 7.67 (d-
like, J ˆ 7.7, 2 arom. H); 7.50 (t-like, J ˆ 7.4, 1 arom. H); 7.43 (t-like, J ˆ 7.7, 2 arom. H); 6.93 (s, 2 arom. H); 2.31 (s,
Me); 2.23 (s, 2 Me). 13C-NMR (CDCl3): 152.06 (s); 145.36 (s); 135.88 (s); 133.20 (d); 131.88 (s); 130.96 (d);

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Helvetica Chimica Acta Vol. 84 (2001)
.
‡
129.70 (d); 129.33 (d); 128.63 (d); 119.29 (s); 88.18 (s); 80.16 (s); 20.16 (q); 16.21 (q). EI-MS: 264 (5, M ), 129
‡
(100, [PhÀCꢀCÀCO] ). Anal. calc. for C18H16O2 (264.32): C 81.79, H 6.10; found: C 81.84, H 6.20.
1.2.3. 2,3,4,5,6-Pentamethylphenyl 3-Phenylprop-2-ynoate (3d). According to Procedure B, 5a' (0.445g,
2.64 mmol) and 6b (0.588 g, 2.59 mmol) yielded colorless crystals of 3d (0.522 g, 69%) after recrystallization
from hexane/Et2O.
Data of 3d: M.p. 146.0 147.08 (hexane/Et2O). IR (CHCl3): 2233s/2213m (CꢀC), 1717s (CˆO), 1287s/
1182s (CÀO). 1H-NMR (CDCl3): 7.73 (dd, Jm ˆ 1.3, Jo ˆ 8.4, 2 arom. H); 7.56 (m, 1 arom. H); 7.49 (m, 2 arom.
H); 2.31 (s, MeÀC(3,4,5)); 2.24 (s, MeÀC(2,6)). 13C-NMR (CDCl3): 152.50 (s); 145.19 (s); 133.54 (s); 133.18
(d); 130.87 (d); 128.58 (d); 125.22 (s); 119.39 (s); 87.98 (s); 80.31 (s); 16.57 (q); 16.38 (q); 13.38 (q). EI-MS:
.
‡
‡
‡
‡
292.0 (80, M ), 277.0 (75, [M À Me] ), 162.9 (50, [M À PhÀCꢀCÀCO] ), 128.8 (100, [PhÀCꢀCÀCO] ).
Anal. calc. for C20H20O2 (292.37): C 82.16, H 6.89; found: C 81.93, H 6.92.
1.2.4. 2,3,4,5,6-Pentabromophenyl 3-(4-Nitrophenyl)prop-2-ynoate (3e). 1.2.4.1. 2,3,4,5,6-Pentabromophen-
yl Carbonochloridate (6c). A 500-ml flask was charged with toluene (30 ml), CH2Cl2 (40 ml), and 2,3,4,5,6-
pentabromophenol (36.88 g, 75.5 mmol). A soln. of phosgene in toluene was added to the stirred suspension,
which was then cooled to 08. N,N-Dimethylaniline (10 ml, 79 mmol) was cautiously added through a dropping
funnel within 15min, while maintaining the temp. at 0 8. Within a few min, the mixture became a clear soln. After
2 h, the mixture was allowed to warm to r.t. and stirred for additional 15h. Toluene (300 ml) and, after cooling to
08 again, cold H2O was cautiously added, and the org. layer was separated. The org. layer was washed
successively with dil. HCl, NaOH, and H2O, and then dried (MgSO4). The solvent was removed, and 6c
crystallized from toluene (39.4 g, 95%).
Data of 6c :M.p. 122.0 123.78 (toluene). IR (CHCl3): 1801s/1790s/1775s (CˆO), 1088vs (CÀO).
13C-NMR (CDCl3): 147.42 (s); 146.62 (s); 128.87 (s); 128.70 (s); 120.11 (s). EI-MS: 545.5/547.5/549.4/551.4/553.4/
.
‡
555.6 (0.2/2.1/4.6/5.2/3.3/0.5, M ). Anal. calc. for C7Br5ClO2 (551.05): C 15.26, Cl 6.43; found: C 15.31, Cl 6.45.
1.2.4.2. Formation of 3e. According to Procedure B, 5'b (0.123 g, 0.575 mmol) and 6c (0.302 g, 0.548 mmol)
gave beige crystals of 3e (0.333 g, 92%).
Data of 3e: M.p. 197.3 200.08 (THF). IR (CHCl3): 2240m (CꢀC), 1749s (CˆO), 1527s/1349vs (NO2),
1279s/1134vs (CÀO). 1H-NMR (CDCl3): 8.31 (dt, Jo ˆ 8.9, Jm ˆ 2.0, 2 arom. H); 7.87 (dt, Jo ˆ 8.9, Jm ˆ 2.0, 2
arom. H). 13C-NMR (CDCl3): 149.07 (s); 148.57 (s); 145.64 (s); 134.24 (d); 128.79 (s); 128.00 (s); 125.14 (s);
123.87 (d); 120.78 (s); 86.98 (s); 82.31 (s). Anal. calc. for C15H4Br5NO4 ¥ 0.25THF (679.74): C 28.27, H 0.89, N
2.06; found: C 28.20, H 0.87, N 2.07 (The elemental analysis was repeated two times with identical results, after
drying a few days at 608/0.02 Torr).
1.2.5. 2,4,6-Trimethylphenyl 3-(4-Nitrophenyl)prop-2-ynoate (3f). According to Procedure B, 5'b (0.223 g,
1.05mmol) and 6a (0.249 g, 1.25mmol) gave, after CC (hexane/Et 2O 9 :1) and crystallization from hexane/Et2O
pale beige crystals of 3f (0.248 g, 80%). Procedure A gave, after repetitive CC and crystallization, a maximum
yield of 49% of 3f.
Data of 3f: M.p. 146.0 147.08 (hexane/Et2O). UV (hexane): lmax 293 (sh, 4.26), 284 (4.35), 280 (sh, 4.34),
213 (sh, 4.38); lmin 241 (3.74). IR (CHCl3): 2240m (CꢀC), 1722s (CˆO), 1526s/1349vs (NO2), 1287s/1169vs
(CÀO). 1H-NMR (CDCl3): 8.29 (dt, Jo ˆ 8.9, Jm ˆ 2.0, 2 arom. H); 7.82 (dt, Jo ˆ 8.9, Jm ˆ 2.0, 2 arom. H); 6.93 (s,
2 arom. H); 2.30 (s, MeÀC(4)); 2.21 (s, MeÀC(2,6)). 13C-NMR (CDCl3): 151.26 (s); 148.66 (s); 145.09 (s);
136.15( s); 133.85( d); 129.48 (s); 129.37 (d); 125.87 (s); 123.71 (d); 84.58 (s); 83.41 (s); 20.67 (q); 16.14 (q). EI-
.
‡
‡
‡
MS (C18H15NO4, 309.32): 309.0 (40, M ), 174.0 (100, [NO2C6H4ÀCꢀCÀCO] ), 127.9 (100, [C9H4O] ). Anal.
calc. for C18H15NO4 (309.32): C 69.89, H 4.89, N 4.53; found: C 69.69, H 4.87, N 4.42.
1.2.6. Phenyl 3-(4-Nitrophenyl)prop-2-ynoate (3g). According to Procedure B, 5'b (2.50 g, 11.73 mmol) and
6d (1.62 ml, 12.9 mmol) gave yellowish crystals of 3g (1.285g, 41%).
Data of 3g: M.p. 150.2 150.88 (hexane/CH2Cl2). UV (hexane): lmax 300 (sh, 4.19), 284 (4.35), 281 (sh,
4.34), 215(sh, 4.19); lmin 241 (3.83). IR (CHCl3): 2242m/2219m (CꢀC), 1728s (CˆO), 1526s/1349vs (NO2),
1286s/1169vs (CÀO). 1H-NMR (CDCl3): 8.27 (dt, Jo ˆ 8.9, Jm ˆ 2.1, 2 arom. H); 7.80 (dt, Jo ˆ 8.9, Jm ˆ 2.1, 2
arom. H); 7.44 (tt, Jo ˆ 7.7, Jm ˆ 2.1, 2 arom. H); 7.31 (tt, Jo ˆ 8.0, Jm ˆ 2.3, 1 arom. H); 7.20 (dq, Jo ˆ 7.5, Jm ˆ 2.5, 2
arom. H). 13C-NMR (CDCl3): 151.46 (s); 149.86 (s); 133.77 (d); 129.59 (d); 126.56 (d); 125.77 (s); 123.71 (d);
.
‡
‡¥
121.16 (s); 84.96 (s); 83.51 (s). EI-MS (C15H9NO4; 267.24): 267.0 (25, M ), 239.0 (25, [M À CO] ), 173.9 (100,
‡
‡
[NO2C6H4ÀCꢀCÀCO] ), 127.9 (95, [C9H4O] ). Anal. calc. for C15H9NO4 ¥ 0.1 H2O (269.04): C 66.97, H 3.45, N
5.21; found: C 66.97, H 3.36, N 5.03.
1.2.7. 2,4,6-Trimethylphenyl But-2-ynoate (3h). From 5'c (0.058 g, 0.545 mmol) and 6a (0.110 g,
0.545 mmol), 3h (0.055 g, 50%) was obtained as a glassy mass, according to Procedure B.
1
Data of 3h: IR (CHCl3): 2291s/2237s (CꢀC), 1720s (CˆO), 1251s (CÀO). H-NMR (CDCl3): 6.80 (s, 2
arom. H); 2.19 (s, MeÀC(4')); 2.08 (s, MeÀC(2',6')); 1.99 (s, MeÀC(3)). 13C-NMR (CDCl3): 151.60 (s); 145.19

Helvetica Chimica Acta Vol. 84 (2001)
3571
(s); 135.73 (s); 129.57 (s); 129.21 (d); 87.45( s); 71.88 (s); 20.65( q); 16.08 (q); 3.83 (q). EI-MS (C13H14O2; 202.25)
.
‡
‡
‡
‡
202.1 (20, M ), 187.1 (10, [M À CH3] ), 136.1 (50, [M À CH3ÀCꢀCÀCO] ), 67.1 (100, [CH3ÀCꢀCÀCO] ).
1.2.8. 2,4,6-Trimethylphenyl 3-(4-Methoxyphenyl)prop-2-ynoate (3i). Following 1.1 and then procedure A,
acid 5b (0.190 g, 1.08 mmol) and mesitol (0.147 g, 1.08 mmol) gave light beige crystals of 3i (0.251 g, 79%).
Data of 3i: M.p. 54.7 59.08 (hexane/Et2O). UV (hexane): lmax 296 (4.29), 282 (4.32), 277 (sh, 4.28), 211
(sh, 4.36); lmin 292 (4.26), 232 (3.68). IR (CHCl3): 2210s/2203s (CꢀC), 1713s (CˆO), 1605s (arom.), 1256s
(CÀO). 1H-NMR (CDCl3): 7.47 (d, Jo ˆ 8.9, 2 arom. H); 6.80 (d, Jo ˆ 8.9, 2 arom. H); 6.79 (s, 2 arom. H); 3.73 (s,
MeO); 2.18 (s, MeÀC(4)); 2.09 (s, MeÀC(2,6)). 13C-NMR (CDCl3): 161.78 (s); 152.30 (s); 145.41 (s); 135.74 (s);
135.18 (d); 129.75( s); 129.26 (d); 114.33 (d); 111.03 (d); 89.16 (s); 79.61 (s); 55.33 (q); 20.70 (q); 16.19 (q). EI-
.
‡
‡
MS (C19H18O3; 294,34): 294.1 (1, M ), 159.0 (100, [CH3OC6H4ÀCꢀCÀCO] ). Anal. calc. for C19H18O3 ¥
0.125H 2O (296.59): C 76.94, H 6.20; found: C 76.88, H 6.49.
1.2.9. 2,4,6-Tribromophenyl 3-(4-Methoxyphenyl)prop-2-ynoate (3k). According to 1.1 and then Procedure
A, acid 5b (0.200 g, 1.14 mmol) and 2,4,6-tribromophenol (0.394 g, 1.14 mmol) gave light beige crystals of 3k
(0.408 g, 74%) after crystallization from MeOH.
Data of 3k: M.p. 106.7 109.28 (MeOH). UV (hexane): lmax 303 (4.43), 292 (sh, 3.78), 287 (4.48), 277 (sh,
3.21); lmin 298 (4.32), 241 (3.91). IR (CHCl3): 2221s/2200s (CꢀC), 1734s (CˆO), 1604s (arom.), 1256s (CÀO).
1H-NMR (CDCl3): 7.75( s, 2 arom. H); 7.63 (d, Jo ˆ 8.8, 2 arom. H); 6.94 (d, Jo ˆ 8.8, 2 arom. H); 3.87 (s,
MeOÀC(4)). 13C-NMR (CDCl3): 162.11(s); 149.67(s); 144.95( s); 135.46 (d); 134.83 (d); 120.24 (s); 118.40 (s);
‡
114.39 (d); 110.44 (s); 91.40 (s); 78.92 (s); 55.37 (q). CI-MS: 503.7/505.7/507.7/509.7 (30/97/100/33, [M ‡ NH4] ).
Anal. calc. for C16H9Br3O3 (488.95): C 39.30, H 1.86; found: C 39.37, H 1.99.
1.2.10. 2-Methylphenyl 3-Phenylprop-2-ynoate (3l). According to [6], 5a and 2-methylphenol were
converted via chloride 9c to 3l; yield 63%.
Data of 3l: M.p. 60.3 62.28 (hexane). IR (CHCl3): 2236s/2206s (CꢀC), 1722vs (CˆO), 1285s/1153vs
(CÀO). 1H-NMR (CDCl3): 7.87 (dt, Jo ˆ 6.8, Jm ˆ 1.5, arom. H); 7.73 (tt, Jo ˆ 7.4, Jm ˆ 1.4, arom. H); 7.64 (tt, Jo ˆ
7.3, Jm ˆ 1.7, 2 arom. H); 7.52 7.39 (m, 3 arom. H); 7.34 (dd, Jo ˆ 7.5, Jm ˆ 1.6, arom H); 2.51 (s, MeÀC(2)).
13C-NMR (CDCl3): 152.19 (s); 148.80 (s); 133.21 (d); 131.30 (d); 131.00 (s); 130.22 (d); 128.65( d); 121.79 (d);
.
.
‡
‡
119.31 (s); 88.57 (s); 80.14 (s); 16.17 (q). EI-MS (C16H12O2, 236.27): 236.1 (5, M ), 158.1 (100, [M À C6H6] ),
‡
129.0 (100, [PhÀCꢀCÀCO] ). Anal. calc. for C16H12O2 ¥ 0.125H O (238.52): C 80.57, H 5.18; found: C 80.83,
2
H 5.17.
2. Thermolysis of the Aryl 3-Arylprop-2-ynoates 3. 2.1. 2,4,6-Trimethylphenyl 3-Phenylprop-2-ynoate
(3c). A stirred 0.5m soln. of 3c (0.060 g, 0.227 mmol) in decalin was heated at 1608 during 24 h. Decalin was
distilled off, and the residual orange-colored solid was purified by CC (silica gel; toluene). Crystallization from
hexane gave pure (Z)-11c (0.020 g, 33%) and recovered 3c (0.008 g, 13%). Heating molten 3c gave, after 18 h
and chromatography, 24% of (Z)-11c and 22% of recovered 3c. Heating a 2m soln. of 3c in o-xylene during 260 h
led to 15% of (Z)-11c and 1% of (E)-11c, which were again isolated by chromatography.
Data of 2,4,6-Trimethylphenyl 2,3-Dihydro-2-oxo-5-phenyl-3-[(Z)-(phenyl)(2,4,6-trimethylphenyl)methy-
lidene]furan-4-carboxylate ((Z)-11c). Yellow crystals. M.p. 152.1 153.48. UV (hexane; see Fig. 2,a): lmax 391
(4.20), 356 (sh, 4.11), 261 (sh, 4.13), 249 (sh, 4.11); lmin 302 (3.88). IR (CHCl3): 1774m (CˆO, five-ring lactone),
1731m (CˆO, aryl ester), 1010s (CÀO). 1H-NMR (600 MHz, CDCl3): 8.01 (dd, Jo ˆ 7.9, Jm ˆ 1.4, HÀC(2a,6a));
7.48 (td, Jo ˆ 7.3, Jm ˆ 2.0, HÀC(4a)); 7.41 7.44 (m, HÀC(3a,5a)), HÀC(2c,6c)); 7.27 7.36 (m, HÀC(3c,4c,5c));
6.97 (s, HÀC(3b,5b)); 6.68 (s, HÀC(3d,5d)); 2.36 (s, MeÀC(4b)); 2.23 (s, MeÀC(4d)); 2.10 (s, MeÀC(2b,6b)); 1.52
(s, MeÀC(2d,6d)). 13C-NMR (150 MHz, CDCl3): 163.83 (s, C(2)); 160.59 (s, COÀC(4)); 159.81 (s, C(5)); 158.11
(s, C(3')); 145.61 (s, C(1d)); 139.98 (s, C(1c)); 138.07 (s, C(4b)); 136.57 (s, C(1b)); 135.30 (s, C(4d)); 135.12 (s,
C(2b,6b)); 131.36 (d, C(4a)); 130.23 (d, C(4c)); 130.19 (d, C(2c,6c)); 129.92 (d, C(2a,6a)); 129.86 (s, C(2d,6d));
129.00 (d, C(3d,5d)); 128.87 (d, C(3b,5b) and C(3c,5c)); 128.24 (s, C(1a)); 128.07 (d, C(3a,5a)); 121.45( s, C(3));
111.40 (s, C(4)); 21.29 (q, MeÀC(4b)); 20.58 (q, MeÀC(4d)); 20.00 (q, MeÀC(2b,6b)); 15.95 (q, MeÀC(2d,6d)).
‡
‡
‡
CI-MS (C36H32O4; 528.64): 546.1 (100, [M ‡ NH4] ); 529.1 (67, [M ‡ H] ); 393.0 (15, [M À Me3C6H2O] ). EI-
‡
MS: 393.1 (100, [M À Me3C6H2O] ). Anal. calc. for C36H32O4 ¥ 0.125H 2O (530.89): C 81.43, H 6.12; found:
C 81.40, H 6.31.
Data of (E)-11c. Yellow solid. M.p. 120.1 125.48. UV (hexane; see Fig. 2,b): lmax 383 (4.20), 281 (sh, 3.79),
266 (sh, 3.87), 243 (sh, 4.07); lmin 316 (3.56). IR (CHCl3): 1769vs (CˆO; five-ring lactone), 1729vs (CˆO, aryl
ester), 1010s (CÀO). 1H-NMR (600 MHz, CDCl3): 7.81 (dt, Jo ˆ 7.2, Jm ˆ 1.6, HÀC(2a,6a)); 7.44 (tt, Jo ˆ 7.4, Jm
ˆ
2.0, HÀC(4a)); 7.42 (tt, Jo ˆ 7.2, Jm ˆ 1.4, HÀC(4b)); 7.36 7.39 (m, HÀC(3a,5a), and HÀC(2b,6b)); 7.33 (t, Jo ˆ 7.8,
HÀC(3b,5b)); 6.83 (s, HÀC(3c,5c)); 6.65( s, HÀC(3d,5d)); 2.33 (s, MeÀC(4c)); 2.15( s, MeÀC(4d)); 2.10 (s,
MeÀC(2c,6c)); 1.41 (s, MeÀC(2d,6d)). 13C-NMR (150 MHz, CDCl3): 163.96 (s, C(2)); 161.10 (s, C(5)); 160.45 (s,

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Helvetica Chimica Acta Vol. 84 (2001)
C(3')); 158.48 (s, COÀC(4)); 144.89 (s, C(1d)); 138.48 (s, C(4c)); 137.67 (s, C(1c)); 137.34 (s, C(1b)); 136.29 (s,
C(2c,6c)); 135.24 (s, C(4d)); 131.20 (d, C(4a)); 131.11 (d, C(2b,6b)); 130.65( d, C(4b)); 130.34 (d, C(2a,6a)); 129.91
(s, C(2d,6d)); 129.17 (d, C(3c,5c)); 129.04 (d, C(3d,5d)); 128.86 (s, C(1a)); 127.90 (d, C(3b,5b)); 127.65( d, C(3a,5a));
121.99 (s, C(3)); 111.52 (s, C(4)); 21.03 (q, MeÀC(4c)); 20.61 (q, MeÀC(4d)); 20.51 (q, MeÀC(2c,6c)); 14.10 (q,
‡
‡
MeÀC(2d,6d)). CI-MS (C36H32O4; 528.64): 546.4 (100, [M ‡ NH4] ); 529.4 (97, [M ‡ H] ); 393.2 (19, [M À
‡
Me3C6H2O] ).
2.2. 2,3,4,5,6-Pentamethylphenyl 3-Phenylprop-2-ynoate (3d). Molten 3d (0.220 g, 0.751 mmol) was stirred
at 1508 during 24 h. At first, the formed orange solid was purified by CC (silica gel; hexane/CH2Cl2 1 :1). Subse-
quent prep. HPLC (hexane/i-PrOH 95:5) gave pure ( Z)-11d (0.060 g, 27%) and (E)-11d (0.028 g, 13%). The
same reaction, carried out at 1808, yielded, after 18 h, 27% of (Z)-11d and 3% of (E)-11d.
Data of 2,3,4,5,6-Pentamethylphenyl 2,3-Dihydro-2-oxo-5-phenyl-3-[(Z)-(phenyl)(2,3,4,5,6-pentamethyl-
phenyl)methylidene]furan-4-carboxylate ((Z)-11d): Yellow crystals. M.p. 251.2 253.98 (hexane/i-PrOH 95:5).
UV (hexane): lmax 384 (4.08), 346 (sh, 3.98), 268 (sh, 3.96), 241 (sh, 4.27); lmin 308 (3.79). IR (CHCl3): 1778s
1
(CˆO; five-ring lactone), 1735s (br.; CˆO, aryl ester), 1149s (CÀO). H-NMR (600 MHz, CDCl3): 8.02 (d,
Jo ˆ 7.2, HÀC(2a,6a)); 7.47 (t, Jo ˆ 7.3, HÀC(4a)); 7.41 7.44 (m, HÀC(3a,5a), HÀC(2c,6c)); 7.27 7.36 (m,
HÀC(3c,4c,5c)); 2.31 (s, MeÀC(4b)); 2.24 (s, MeÀC(3b,5b)); 2.13 (s, MeÀC(4d), MeÀC(2b,6b)); 2.05( s,
MeÀC(3d,5d)); 1.49 (s, MeÀC(2d,6d)). 13C-NMR (150 MHz, CDCl3): 163.88 (s, C(2)); 161.11 (s, COÀC(4));
160.59 (s, C(5)); 159.61 (s, C(3')); 145.61 (s, C(1d)); 140.50 (s, C(1c)); 137.10 (s, C(1b)); 135.12 (s, C(4b)); 133.16 (s,
C(3d,5d)); 132.78 (s, C(3b,5b)); 132.71 (s, C(4d)); 131.26 (d, C(4a)); 130.33 (d, C(2c,6c)); 130.07 (d, C(4c)); 130.02
(d, C(2a,6a)); 129.99 (s, C(2b,6b)); 128.86 (d, C(3c,5c)); 128.37 (s, C(1a)); 128.05( d, C(3a,5a)); 125.52 (s, C(2d,6d));
121.43 (s, C(3)); 110.53 (s, C(4)); 17.66 (q, MeÀC(2b,6b)); 17.01 (q, MeÀC(4b)); 16.47 (q, MeÀC(3b,5b),
.
‡
MeÀC(4d)); 16.28 (q, MeÀC(3d,5d)); 13.07 (q, MeÀC(2d,6d)). EI-MS (C40H40O4; 584.74): 584.0 (0.2, M ); 421.0
‡
(100, [M À Me5C6O] ). Anal. calc. for C40H40O4 ¥ 0.25H2O (589.2): C 81.53, H 6.93; found: C 81.50, H 6.87.
Data of (E)-11d. Yellow crystals. M.p. 247.2 249.98 (hexane/i-PrOH 95:5). UV (hexane): lmax 383 (4.18),
273 (sh, 3.95), 243 (sh, 4.16); lmin 318 (3.73). IR (CHCl3): 1777s (CˆO; five-ring lactone), 1724s (br.; CˆO, aryl
1
ester). H-NMR (600 MHz, CDCl3): 7.84 (d, Jo ˆ 7.8, HÀC(2a,6a)); 7.44 (t, Jo ˆ 7.2, HÀC(4a)); 7.41 (t, Jo ˆ 7.2,
HÀC(4b)); 7.39 (d, Jo ˆ 7.2, HÀC(2b,6b)); 7.38 (t, Jo ˆ 7.8, HÀC(3a,5a)); 7.34 (t, Jo ˆ 7.8, HÀC(3b,5b)); 2.25( s,
MeÀC(4c)); 2.16 (s, MeÀC(3,5)); 2.11 (s, MeÀC(4d)); 2.09 (s, MeÀC(2c,6c)); 2.04 (s, MeÀC(3d,5d)); 1.29 (s,
MeÀC(2d,6d)). 13C-NMR (150 MHz, CDCl3): 164.13 (s, C(2)); 162.54 (s, C(3')); 160.76 (s, C(5)); 159.13 (s,
COÀC(4)); 144.84 (s, C(1d)); 138.41 (s, C(1c)); 137.98 (s, C(1b)); 135.63 (s, C(4c)); 133.09 (s, C(3d,5d)); 132.94 (s,
C(3c,5c)); 132.61 (s, C(4d)); 131.40 (s, C(2c,6c)); 131.23 (d, C(2b,6b)); 131.09 (d, C(4a)); 130.36 (d, C(2a,6a)); 130.32
(d, C(4b)); 128.90 (s, C(1a)); 127.88 (d, C(3a,5a)); 127.46 (d, C(3b,5b)); 125.48 (s, C(2d,6d)); 122.47 (s, C(3)); 111.62
(s, C(4)); 18.37 (q, MeÀC(2c,6c)); 16.87 (q, MeÀC(4c)); 16.56 (q, MeÀC(3c,5c)); 16.53 (q, MeÀC(4d)); 16.29 (q,
.
‡
MeÀC(3d,5d)); 12.39 (q, MeÀC(2d,6d)). EI-MS (C40H40O4 ; 584.74): 584.0 (0.4, M ); 421.0 (100, [M À
‡
Me5C6O] ). Anal. calc. for C40H40O4 ¥ H2O (602.75): C 79.71, H 7.02; found: C 79.50, H 6.75.
2.3. 2,4,6-Trimethylphenyl 3-(4-Nitrophenyl)prop-2-ynoate (3f). Molten 3f (0.185g, 0.598 mmol) was
heated for 16 h at 1508. CC (silica gel; hexane/ Et2O 1:1) gave a pure mixture (Z)-11f/(E)-11f (0.137 g, 74%) in
a ratio of 92 :8. For the analyses, the isomers were separated by HPLC (hexane/CH2Cl2/MeOH 80 :20 :0.5).
Data of 2,4,6-Trimethylphenyl 2,3-Dihydro-5-(4-nitrophenyl)-3-[(Z)-(4-nitrophenyl)(2,4,6-trimethylphe-
nyl)methylidene]-2-oxofuran-4-carboxylate ((Z)-11f): Orange solid. M.p. 134.9 138.58 (hexane/(t-Bu)2O). UV
(hexane; see Fig. 2,c): lmax 403 (4.18), 378 (sh, 4.16), 292 (sh, 4.11), 258 (4.24); lmin 322 (4.00), 252 (4.24). IR
(CHCl3): 1794m (CˆO, five-ring lactone), 1729m (CˆO, aryl ester), 1525s/1347vs (NO2). 1H- NMR (600 MHz,
CDCl3): 8.29 (d, Jo ˆ 8.4, HÀC(3a,5a)); 8.19 (d, Jo ˆ 8.4, HÀC(2a,6a) and HÀC(3c,5c)); 7.54 (d, Jo ˆ 8.4,
HÀC(2c,6c)); 7.00 (s, HÀC(3b,5b)); 6.71 (s, HÀC(3d,5d)); 2.36 (s, MeÀC(4b)); 2.16 (s, MeÀC(4d)); 2.11 (s,
MeÀC(2b,6b)); 1.62 (s, MeÀC(2d,6d)). 13C-NMR (150 MHz, CDCl3): 162.39 (s, C(2)); 159.52 (s, C(5)); 159.16 (s,
COÀC(4)); 157.53 (s, C(3')); 149.28 (s, C(4a)); 148.19 (s, C(4c)); 146.06 (s, C(1c)); 144.82 (s, C(1d)); 139.19 (s,
C(4b)); 136.05( s, C(4d)); 135.24 (s, C(1b)); 134.85( d, C(2b,6b)); 133.94 (s, C(1a)); 131.30 (d, C(2a,6a)); 130.55 (d,
C(2c,6c)); 129.39 (d, C(3d,5d)); 129.33 (d, C(3b,5b)); 129.05( s, C(2d,6d)); 123.86 (d, C(3c,5c)); 123.20 (s, C(3));
123.13 (d, C(3a,5a)); 111.40 (s, C(4)); 21.20 (q, MeÀC(4b)); 20.50 (q, MeÀC(4d)); 20.03 (q, MeÀC(2b,6b)); 16.00
.
‡
‡
(q, MeÀC(2d,6d)). EI-MS (C36H30N2O8; 618.63): 618.2 (1, M ), 483.1 (100, [M À Me3C6H2O] ). Anal. calc. for
C36H30N2O8 (618.62): C 69.89, H 4.89, N 4.53; found: C 69.94, H 5.01, N 4.33.
Data of (E)-11f: Yellow crystals. M.p. 253.5 255.38 (hexane/CH2Cl2). UV (hexane; see Fig. 2,d): lmax 396
(4.11), 283 (sh, 3.95), 262 (4.07), 216 (sh, 4.32); lmin 323 (3.71), 242 (4.00). IR (CHCl3): 1783s (CˆO; five-ring
1
lactone), 1735s (CˆO; aryl ester), 1525vs/1347vs (NO2). H-NMR (600 MHz, CDCl3): 8.26 (dt, Jo ˆ 8.7, Jm
ˆ
1.6, HÀC(3a,5a)); 8.20 (dt, Jo ˆ 8.9, Jm ˆ 1.8, HÀC(3b,5b)); 8.05( dt, Jo ˆ 9.0, Jm ˆ 2.0, HÀC(2a,6a)); 7.53 (dt, Jo ˆ
8.9, Jm ˆ 1.9, HÀC(2b,6b)); 6.87 (s, HÀC(3c,5c)); 6.69 (s, HÀC(3d,5d)); 2.28 (s, MeÀC(4c)); 2.17 (s, MeÀC(4d));

Helvetica Chimica Acta Vol. 84 (2001)
3573
2.11 (s, MeÀC(2c,6c)); 1.45( s, MeÀC(2d,6d)). 13C-NMR (150 MHz, CDCl3): 162.94 (s, C(2)); 159.16 (s, C(5));
158.92 (s, C(3')); 157.58 (s, COÀC(4)); 149.23 (s, C(4a)); 148.70 (s, C(4b)); 144.50 (s, C(1d)); 143.25( s, C(1b));
139.65( s, C(4c)); 136.48 (s, C(1c)); 135.96 (s, C(2c,6c)); 135.88 (s, C(4d)); 134.34 (s, C(1a)); 131.80 (d, C(2b,6b));
131.42 (d, C(2b,6b)); 129.60 (d, C(3c,5c)); 129.41 (s, C(2d,6d)); 129.34 (d, C(3d,5d)); 123.61 (s, C(3)); 123.19 (d,
C(3a,5a)); 122.97 (d, C(3b,5b)); 113.27 (s, C(4)); 21.04 (q, MeÀC(4c)); 20.59 (q, MeÀC(4d), MeÀC(2c,6c)); 15.69
‡
‡
(q, MeÀC(2b,6b)). CI-MS (C36H30N2O8; 618.63): 636.3 (100, [M ‡ NH4] ), 619.3 (7, [M ‡ H] ), 483.2 (21, [M À
‡
2,4,6-Me3C6H2O] ). Anal. calc. for C36H30N2O8 (618.62): C 69.89, H 4.89, N 4.53; found: C 69.67, H 4.84, N 4.46.
2.4. Phenyl 3-(4-Nitrophenyl)prop-2-ynoate (3g). Molten ester 3g (0.800 g, 2.99 mmol) was heated under
Ar during 17 h at 1808. Prep. TLC on silica gel (hexane/CH2Cl2 7:3) gave an orange colored fraction (0.240 g),
which contained (Z)-11g and (E)-11g as well as diester 13g. Dimer 13g (0.190 g, 24%) was fractionally
crystallized from cold CH2Cl2. The mother liquor was evaporated. The residue was submitted to prep. HPLC.
Repetitive runs (hexane/CH2Cl2/MeOH 80 :20 :0.5) yielded finally pure (E)-11g (0.014 g, 1.8%) and (Z)-11g
(0.006 g, 0.8%).
Data of Phenyl 2,3-Dihydro-5-(4-nitrophenyl)-3-[(E)-(4-nitrophenyl)(phenyl)methylidene]-2-oxofuran-4-
carboxylate ((E)-11g 5)). Orange crystals. M.p. 193.3 197.48 (hexane/CH2Cl2). UV (hexane/i-PrOH 9 :1): lmax
408 (4.11), 265(4.16); lmin 348 (3.77), 245(4.12). IR (CHCl 3): 1785m (CˆO: five-ring lactone), 1741m (CˆO;
aryl ester), 1525s/1349vs (NO2). 1H-NMR (600 MHz, CDCl3): 8.31 (dt, Jo ˆ 9.0, Jm ˆ 1.9, HÀC(3a,5a)); 8.18 (dt,
Jo ˆ 8.7, Jm ˆ 2.0, HÀC(3c,5c)); 8.10 (dt, Jo ˆ 9.0, Jm ˆ 2.1, HÀC(2a,6a)); 7.56 (t, Jo ˆ 7.5, H ÀC(4b)); 7.44 7.48 (m,
HÀC(2c,6c) and HÀC(3b,5b)); 7.35( dd, Jo ˆ 7.9, Jm ˆ 0.8, HÀC(2b,6b)); 7.20 (tt, Jo ˆ 7.3, Jm ˆ 2.0, HÀC(3d,5d));
7.14 (tt, Jo ˆ 7.3, Jm ˆ 1.7, HÀC(4d)); 6.50 (dd, Jo ˆ 7.6, Jm ˆ 1.1, HÀC(2d,6d)). 13C-NMR (150 MHz, CDCl3):
163.05( s, C(2)); 160.63 (s, COÀC(4)); 157.43 (s, C(3')); 155.30 (s, C(5)); 149.52 (s, C(1d)); 149.18 (s, C(4a));
148.65( s, C(4c)); 147.14 (s, C(1c)); 137.29 (s, C(1b)); 133.03 (s, C(1a)); 131.74 (d, C(4b)); 131.48 (d, C(2c,6c));
131.44 (d, C(2b,6b)); 129.69 (d, C(2a,6a)); 129.39 (d, C(3d,5d)); 128.55 (d, C(3b,5b)); 126.49 (d, C(4d)); 123.91 (d,
C(3c,5c)); 123.78 (d, C(3a,5a)); 122.19 (s, C(3)); 119.83 (d, C(2d,6d)); 113.32 (s, C(4)). CI-MS (C30H18N2O8;
‡
‡
‡
534.47): 552.2 (100, [M ‡ NH4] ), 505.2 (37, [M À CHO] ), 414.2 (68, [M À (C6H5O ‡ CHO)] ).
Data of (Z)-11g 5): Orange crystals. M.p. 205.4 212.88 (hexane/CH2Cl2). UV (CH2Cl2): lmax 412 (4.32), 296
(sh, 4.18), 264 (4.32); lmin 343 (3.93), 244 (4.25). IR (CHCl3): 1786s (CˆO, five-ring lactone), 1741s (CˆO, aryl
ester), 1524s/1348vs (NO2). 1H-NMR (600 MHz, CDCl3): 8.31 (dt, Jo ˆ 8.9, Jm ˆ 1.8, HÀC(3a,5a)); 8.28 (dt, Jo ˆ
9.0, Jm ˆ 1.8, HÀC(3b,5b)); 8.15( dt, Jo ˆ 8.9, Jm ˆ 1.9, HÀC(2a,6a)); 7.52 7.54 (m, HÀC(2b,6b), HÀC(4c)); 7.45( t,
Jo ˆ 7.7, H ÀC(3c,5c)); 7.31 (d, Jo ˆ 7.4, HÀC(2c,6c)); 7.20 (t, Jo ˆ 7.7, H ÀC(3d,5d)); 7.14 (t, Jo ˆ 7.3, HÀC(4d)); 6.34
(d, Jo ˆ 7.4, HÀC(2d,6d)). 13C-NMR (150 MHz, CDCl3): 163.37 (s, C(2)); 161.09 (s, COÀC(4)); 157.24 (s, C(3'));
155.00 (s, C(5)); 149.49 (s, C(1d)); 149.13 (s, C(4a)); 148.94 (s, C(4b)); 144.67 (s, C(1b)); 140.06 (s, C(1c)); 132.99
(s, C(1a)); 132.13 (d, C(2b,6b)); 131.41 (d, C(4c)); 130.85( d, C(2c,6c)); 129.64 (d, C(2a,6a)); 129.34 (d, C(3c,5c));
129.23 (d, C(3d,5d)); 126.30 (d, C(4d)); 123.77 (d, C(3a,5a)); 123.44 (d, C(3b,5b)); 122.19 (s, C(3)); 120.56 (d,
C(2d,6d)); 113.74 (s, C(4)). CI-MS (C30H18N2O8, 534.47): 552.2 (100, [M ‡ NH4] ), 505.2 (8, [M À CHO] ), 441.1
‡
‡
‡
(35, [M À C6H5O] ).
Data of Diphenyl 7-Nitro-1-(4-Nitrophenyl)naphthalene-2,3-dicarboxylate (13g). Yellowish crystals. M.p.
256.4 257.88 (CH2Cl2). UV (CH2Cl2): lmax 359 (3.49), 285 (sh, 4.02), 257 (4.48); lmin 333 (3.32), 240 (4.32). IR
1
(CHCl3): 1745s (CˆO, aryl ester), 1528s (NO2), 1351vs (NO2), 1190s (CÀO). H-NMR (500 MHz, CDCl3):
9.06 (s, HÀC(4)); 8.49 (d, Jo ˆ 8.5, HÀC(3',5')); 8.47 (dd, J(6,5) ˆ 9.0, J(6,8) ˆ 2.1, HÀC(6)); 8.42 (d, J(8,6) ˆ
1.7, HÀC(6)); 8.32 (d, J(5,6) ˆ 9.0, HÀC(5)); 7.74 (d, Jo ˆ 8.6, HÀC(2',6')); 7.48 (t, Jo ˆ 7.9, HÀC(3''',5''')); 7.34
(t, Jo ˆ 7.4, HÀC(4''')); 7.29 (d, Jo ˆ 7.4, HÀC(2''',6''')); 7.25( t, Jo ˆ 7.8, HÀC(3'',5'')); 7.17 (t, Jo ˆ 7.4, HÀC(4''));
6.71 (d, Jo ˆ 7.6, HÀC(2'',6'')). 13C-NMR (CDCl3): 165.55, 163.28 (2s, 2 CˆO); 150.47 (s); 150.05 (s); 148.49 (s);
148.14 (s); 141.64 (s); 138.68 (s); 134.94 (s); 132.75( s); 132.62 (d); 131.73 (d); 131.65( d); 129.75( d); 129.53 (d);
127.71 (s); 126.60 (d); 126.37 (d); 123.95( d); 122.52 (d); 121.81 (d); 121.46 (d); 120.93 (d). CI-MS: 552.0 (100,
‡
‡
‡
[M ‡ NH4] ), 505.2 (8, [M À CHO] ), 441.1 (35, [M À C6H5O] ). Anal. calc. for C30H18N2O8 (534.47): C 67.42,
H 3.39, N 5.24; found: C 67.64, H 3.53, N 5.18.
2.5. 2,4,6-Trimethylphenyl 3-(4-Methoxyphenyl)prop-2-ynoate (3i). Molten ester 3i (0.140 g, 0.476 mmol)
was stirred during 12 h at 2008. CC (silica gel, hexane/CH2Cl2 1:1) of the orange colored solid gave as a first
fraction (Z)-11i (0.035g, 25%). Subsequent prep. HPLC (hexane/i-PrOH 95:5) of the second orange colored
fraction led to pure (E)-11i (0.006 g, 4%) and 12i (0.020 g, 19%).
Data of 2,4,6-Trimethylphenyl 2,3-Dihydro-5-(4-methoxyphenyl-3-[(Z)-(4-methoxyphenyl)(2,4,6-trime-
thylphenyl)methylidene]-2-oxofuran-4-carboxylate ((Z)-11i). Yellow-orange crystals. M.p. 209.2 214.58
(EtOH). UV (hexane): lmax 406 (4.40), 361 (sh, 4.10), 313 (4.07), 291 (4.08), 239 (sh, 4.25); lmin 334 (3.99),
304 (4.06), 269 (4.03). IR (CHCl3): 1772m (CˆO, five-ring lactone), 1735s (CˆO, aryl ester), 1605s (arom.). 1H
NMR (600 MHz, CDCl3): 8.01 (d, Jo ˆ 9.0, HÀC(2a,6a)); 7.36 (d, Jo ˆ 9.0, HÀC(2c,6c)); 6.95( s, HÀC(3b,5b)); 6.92

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Helvetica Chimica Acta Vol. 84 (2001)
(d, Jo ˆ 9.0, HÀC(3a,5a)); 6.87 (d, Jo ˆ 9.0, HÀC(3c,5c)); 6.70 (s, HÀC(3d,5d)); 3.85( s, MeOÀC(4a)); 3.82 (s,
MeOÀC(4c)); 2.35( s, MeÀC(4b)); 2.18 (s, MeÀC(4d)); 2.08 (s, MeÀC(2b,6b)); 1.58 (s, MeÀC(2d,6d)). 13C-NMR
(150 MHz, CDCl3): 164.16 (s, C(2)); 161.99 (s, C(4a)); 161.39 (s, C(3')); 161.37 (s, C(4c)); 159.04 (s, C(4)); 156.63
(s, COÀC(4)); 145.87 (s, C(1d)); 137.86 (s, C(4b)); 136.75( s, C(1b)); 135.32 (s, C(2b,6b)); 135.26 (s, C(4d)); 132.73
(s, C(1c)); 132.00 (d, C(2c,6c)); 131.70 (d, C(2a,6a)); 129.97 (s, C(2d,6d)); 129.04 (d, C(3d,5d)); 128.78 (d, C(3b,5b));
120.59 (s, C(1a)); 119.92 (s, C(2)); 114.43 (d, C(3c,5c)); 113.58 (d, C(3a,5a)); 109.04 (s, C(3)); 55.35 (q,
MeOÀC(4a)); 55.35 (q, MeOÀC(4c)); 21.27 (q, MeÀC(4b)); 20.57 (q, MeÀC(4d)); 19.89 (q, MeÀC(2b,6b)); 15.94
‡
(q, MeÀC(2d,6d)). CI-MS (C38H36O6; 588.69): 589.0 (100, [M ‡ H] ). Anal. calc. for C38H36O6 ¥ 0.125H 2O
(590.94): C 76.94, H 6.20; found: C 77.17, H 6.31.
Data of (E)-11i. Yellow crystals. M.p. 206.6 208.48 (hexane/CH2Cl2). UV (hexane): lmax 408 (4.39), 300
(sh, 4.05), 286 (4.07), 241 (4.22); lmin 344 (3.86), 279 (4.06). IR (CHCl3): 1763s (CˆO, five-ring lactone), 1725s
(CˆO, aryl ester), 1603s (arom.). 1H-NMR (500 MHz, CDCl3): 7.82 (dt, Jo ˆ 9.0, Jm ˆ 2.4, HÀC(2a,6a)); 7.34 (dt,
Jo ˆ 8.9, Jm ˆ 2.4, HÀC(2b,6b)); 6.89 (dt, Jo ˆ 9.0, Jm ˆ 2.4, HÀC(3a,5a)); 6.84 (d, Jo ˆ 8.9, Jm ˆ 2.5, HÀC(3b,5b));
6.84 (s, HÀC(3c,5c)); 6.68 (s, HÀC(3d,5d)); 3.85( s, MeOÀC(4b)); 3.82 (s, MeOÀC(4a)); 2.28 (s, MeÀC(4c)); 2.18
(s, MeÀC(4d)); 2.08 (s, MeÀC(2c,6c)); 1.47 (s, MeÀC(2d,6d)). 13C-NMR (150 MHz, CDCl3): 164.42 (s, C(2));
161.85( s, C(4a), C(4b)); 160.33 (s, C(4)); 159.53 (s, C(3')); 158.81 (s, COÀC(4)); 144.98 (s, C(1d)); 138.32 (s,
C(4d)); 137.83 (s, C(1c)); 136.60 (s, C(2c,6c)); 135.13 (s, C(4c)); 133.17 (d, C(2b,6b)); 132.12 (d, C(2a,6a)); 130.01 (s,
C(2d,6d)); 129.96 (s, C(1b)); 129.07 (d, C(3c,5c)); 129.00 (d, C(3d,5d)); 121.10 (s, C(1a)); 120.63 (s, C(2)); 113.34 (d,
C(3a,5a)); 113.13 (d, C(3b,5b)); 110.50 (s, C(3)); 55.35 (q, MeOÀC(4a)); 55.35 (q, MeOÀC(4b)); 21.03 (q,
.
‡
MeÀC(4c)); 20.63 (q, MeÀC(4d)); 20.45( q, MeÀC(2c,6c)); 15.61 (q, MeÀC(2d,6d)). EI-MS: 588.1 (6, M ); 452.9
‡
(100, [M À Me3C6H2O] ), 168.9 (80). Anal. calc. for C38H36O6 (588.69): C 77.53, H 6.16; found: C 77.54,
H 6.14.
Data of 6-Methoxy-4-(4-methoxyphenyl)-9-(2,4,6-trimethylphenyl)-1H,3H-naphtho[2,3-c]furan-1,3-dione
(12i). Light beige crystals. M.p. 224.0 226.18 (hexane/CH2Cl2). UV (hexane): lmax 369 (3.35), 330 (3.66), 323
(3.66), 268 (sh, 4.35), 265 (4.36); lmin 365(3.33), 329 (3.66), 311 (3.63), 239 (3.79). IR (CHCl 3): 1842m
(nÄasym(CˆO), cyclic anhydride), 1773s (nÄsym(CˆO), cyclic anhydride), 1614m (arom.). 1H-NMR (500 MHz,
CDCl3): 7.59 (d, J(8,7) ˆ 9.0, HÀC(8)); 7.44 (dt, Jo ˆ 6.5, Jm ˆ 1.5, HÀC(2',6')); 7.29 (d, J(5,7) ˆ 2.5, HÀC(5));
7.26 (dt, J(7,8) ˆ 9.0, J(7,5) ˆ 2.5, HÀC(7)); 7.14 (dt, Jo ˆ 6.5, Jm ˆ 1.5, HÀC(3',5')); 7.07 (s, HÀC(3)); 3.96 (s,
MeOÀC(4')); 3.79 (s, MeOÀC(6)); 2.43 (s, MeÀC(4'')); 1.89 (s, MeÀC(2'',6'')). 13C-NMR (125MHz, CDCl 3):
162.53, 161.98 (2s, C(2,3)); 160.87 (s, C(6)); 160.09 (s, C(4')); 141.62 (s, C(1)); 140.53 (s, C(4)); 138.58 (s, C(4a));
138.29 (s, C(4'')); 135.76 (s, C(2'',6'')); 131.21 (d, C(2',6')); 131.04 (s, C(8a)); 130.30 (s, C(1'')); 129.49 (d, C(8));
128.61 (d, C(3'',5'')); 125.67 (s, C(1')); 123.23 (s, C(2) or C(3)); 122.26 (d, C(5)); 120.75 (s, C(3) or C(2)); 114.02
(d, C(3',5')); 107.67 (d, C(7)); 55.49 (q, MeOÀC(6)); 55.33 (q, MeOÀC(4')); 21.29 (q, MeÀC(4'')); 20.11 (q,
.
‡
‡
MeÀC(2'',6'')). EI-MS (C29H24O5; 452.50): 453,1 (30, [M ‡ 1] ), 452.1 (100, M ). Anal. calc. for C29H24O5 ¥
0.25H 2O (457.00): C 76.22, H 5.40; found: C 76.24, H 5.68.
2.6. 2,4,6-Tribromophenyl 3-(4-Methoxyphenyl)prop-2-ynoate (3k). Molten ester 3k (0.075g, 0.153 mmol)
was stirred during 5h at 210 8. CC (hexane/CH2Cl2 1:1) gave pure (Z)-11k (0.036 g, 48%). The product was
recrystallized from hexane/CH2Cl2 (0.033 g).
Data of 2,4,6-Tribromophenyl 2,3-Dihydro-5-(4-methoxyphenyl)-3-[(Z)-(4-methoxyphenyl)(2,4,6-tribro-
mophenyl)methylidene]-2-oxofuran-4-carboxylate ((Z)-11k). Orange crystals. M.p. 135.6 137.98 (hexane/
CH2Cl2). UV (hexane): lmax 407 (4.42), 325(3.98), 276 (4.04), 242 (sh, 4.28), 223 (sh, 4.71); lmin 349 (3.92), 306
(3.94), 275(4.04). IR (CHCl 3): 1782s (CˆO, five-ring lactone); 1740s (CˆO, aryl ester); 1604s (arom.).
1H-NMR (600 MHz, CDCl3): 7.98 (d, Jo ˆ 9.0, HÀC(2a,6a)); 7.82 (s, HÀC(3b,5b)); 7.56 (s, HÀC(3d,5d)); 7.36 (d,
Jo ˆ 9.0, HÀC(2c,6c)); 6.94 (d, Jo ˆ 9.0, HÀC(3a,5a)); 6.87 (d, Jo ˆ 9.0, HÀC(3c,5c)); 3.88 (s, MeOÀC(4a)); 3.81 (s,
MeOÀC(4c)). 13C-NMR (150 MHz, CDCl3): 164.45( s, C(2)); 163.71 (s); 162.91 (s); 161.61 (s); 157.46 (s); 152.49
(s); 145.41 (s); 140.84 (s); 135.00 (d); 134.98 (d); 132.74 (d); 132.68 (d); 129.90 (s); 123.99 (s); 122.99 (s); 120.64
(s); 120.58 (s); 119.96 (s); 118.67 (s); 114.44 (d); 113.76 (d); 106.58 (s); 55.66 (q); 55.57 (q). CI-MS: 982.2 (28,
‡
‡
‡
‡
[M ‡ H ‡ 10] ), 980.6 (78, [M ‡ H ‡ 8] ), 978.7 (100, [M ‡ H ‡ 6] ), 976.5(60, [ M ‡ H ‡ 4] ), 974.6 (18, [M ‡
‡
‡ 2] ). Anal. calc. for C32H18Br6O6 ¥ 0.67 H2O (989.90): C 38.83, H 1.97; found: C 38.60, H 2.01.
2.7. 2-Methylphenyl 3-Phenylprop-2-ynoate (3l). Molten ester 3l (2.80 g, 11.8 mmol) was heated in a sealed
glass tube during 15h at 200 8. Repeated CC (silica gel, hexane/Et2O 9 :1 ) gave a 2 :1 mixture (Z)-11l/(E)-11l
(0.155 g, 6%), 12l (0.180 g, 8%), and 13l (0.188 g, 7%).
Data of 2-Methylphenyl 2,3-Dihydro-3-[(Z)- and (E)-(2-methylphenyl)(phenyl)methylidene]-2-oxo-5-
phenylfuran-4-carboxylate ((E)-11f/(Z)-11l): Orange solid (2 :1 mixture). M.p. 76.5 78.0 8 (hexane/Et2O). UV
(hexane): lmax 385(4.09), 300 (sh, 3.87), 241 (sh, 4.26); lmin 327 (3.82). IR (CHCl3): 1778s (CˆO, five-ring
lactone), 1737s (CˆO, aryl ester), 1168vs (CÀO). 1H-NMR of the (Z)-isomer (600 MHz, CDCl3): 8.04 (dd,

Helvetica Chimica Acta Vol. 84 (2001)
3575
Jo ˆ 7.8, Jm ˆ 1.2, HÀC(2a,6a)); 7.48 (tt, Jo ˆ 7.3, Jm ˆ 1.2, HÀC(4a)); 7.47 7.43 (m, HÀC(3a,5a), HÀC(2c,6c));
7.40 7.37 (m, HÀC(5b), HÀC(3c,4c,5c)); 7.30 (d, Jo ˆ 7.3, HÀC(6b)); 7.29 (t, Jo ˆ 7.3, HÀC(4b)); 7.18 (d, Jo ˆ 7.5,
HÀC(3b)); 7.07 (d, Jo ˆ 7.5, H ÀC(3d)); 7.01 (td, Jo ˆ 7.5, Jm ˆ 1.0, HÀC(4d)); 6.93 (td, Jo ˆ 7.7, Jm ˆ 1.3, HÀC(5d));
5.68 (d, Jo ˆ 7.8, HÀC(6d)); 2.16 (s, MeÀC(2b)); 2.19 (s, MeÀC(2d)). 1H-NMR of the (E)-isomer (600 MHz,
CDCl3): 7.89 (d-like, Jo ˆ 7.8, H ÀC(2a,6a)); 7.45( t, Jo ˆ 7.2, H ÀC(4a)); 7.43 7.38 (m, HÀC(3a,5a),
HÀC(2b,3b,4b,5b,6b), HÀC(6c)); 7.36 (td, Jo ˆ 7.5, Jm ˆ 1.6, HÀC(4c)); 7.32 (td, Jo ˆ 7.5, Jm ˆ 1.3, HÀC(5c)); 7.16
(d, Jo ˆ 7.5, H ÀC(3c)); 7.07 (d, Jo ˆ 7.5, H ÀC(d3)); 7.03 (td, Jo ˆ 7.4, Jm ˆ 1.0, HÀC(4d)); 6.97 (td, Jo ˆ 7.9, Jm ˆ 1.4,
.
HÀCd(5d)); 5.94 (d, Jo ˆ 7.8, HÀC(6d)); 1.92 (s, MeÀC(2c)); 1.84 (s, MeÀC(2d)). EI-MS: 472.0 (20, M ), 365.0
‡
‡
‡
‡
(95, [M À CH3C6H4O] ), 104.8 (100, [CH3C6H4O] ). ESI-MS: 373.2 (100, [M ‡ H] ). Anal. calc. for C32H24O4
(472.53): C 81.34, H 5.12; found: C 81.59, H 5.39.
Data of 4-(2-Methylphenyl)-9-phenyl-1H,3H-naphtho[2,3-c]furan-1,3-dione (12l). Yellowish crystals. M.p.
224.8 228.58 (hexane/Et2O). UV (hexane): lmax 357 (3.79), 340 (3.77), 317 (4.02), 260 (4.71), 252 (4.71), 205
(sh, 4.66); lmin 351 (3.65), 337 (3.76), 289 (3.82), 257 (4.70), 232 (4.26). IR (CHCl3): 1835m (nÄasym(CˆO), cyclic
anhydride), 1777s (nÄsym(CˆO), cyclic anhydride), 1611w (arom.). 1H-NMR (600 MHz, CDCl3): 7.97 (m,
HÀC(5)); 7.77 (m, HÀC(8)); 7.70 (m, HÀC(6)); 7.68 (m, HÀC(7)); 7.63 7.61 (m, HÀC(3'',4'',5'')); 7.53 (m,
HÀC(2''))12); 7.51 (td, Jo ˆ 7.6, Jm ˆ 1.4, HÀC(4')); 7.49 (m, HÀC(6''))12); 7.46 (d, Jo ˆ 7.6, HÀC(3')); 7.41 (t, Jo ˆ
7.5, H ÀC(5')); 7.24 (dd, Jo ˆ 7.4, Jm ˆ 1.0, HÀC(6')); 2.05( s, MeÀC(2')). 13C-NMR (CDCl3): 161.91, 161.73 (2s,
CˆO); 142.55 (s); 142.08 (s); 136.26 (s); 135.99 (s); 133.46 (s); 133.37 (s); 130.27 (d); 129.98 (d); 129.93 (d);
129.76 (d); 129.30 (d); 129.07 (d); 128.98 (d); 128.93 (d); 128.48 (d); 128.39 (d); 125.93 (d); 122.54 (s); 122.11
.
‡
‡
‡
(s); 19.77 (q). EI-MS: 365.0 (60, [M ‡ 1] ), 364.0 (95, M ), 289.0 (100, [M À 75] ). Anal. calc. for C25H16O3
(364.39): C 82.40, H 4.43; found: C 82.33, H 4.54.
Data of 2-Methylphenyl 1-Phenylnaphthalene-2,3-dicarboxylate (13l). Yellowish crystals. M.p. 224.8
228.58 (hexane/Et2O). UV (hexane): lmax 340 (3.45), 326 (3.39), 285 (3.95), 275 (3.93), 244 (4.85), 243 (sh,
4.84), 204 (sh); lmin 331 (3.34), 315(3.25), 279 (3.92), 272 (3.92). IR (CHCl 3): 1740vs (CˆO), 1172s/1116s
(CÀO). 1H-NMR (600 MHz, CDCl3): 8.95( s, HÀC(4)); 8.12 (d, J(5,4) ˆ 7.8, HÀC(5)); 7.66 7.69 (m,
HÀC(6)); 7.59 7.51 (m, HÀC( 7,8 )) ; 7.53 ( s, HÀC(2',3',4',5',6')); 7.31 (d, J ˆ 7.8, HÀC(3''')); 7.28 (td, Jo ˆ 6.7,
Jm ˆ 1.6, HÀC(5''')); 7.24 7.20 (m, HÀC(4''',6''')); 7.07 (dd, J ˆ 5.7, 3.6, HÀC(3'')); 7.01 7.03 (m, HÀC(4'', 5''));
6.60 (dd, J ˆ 5.7, J ˆ 3.6, HÀC(6'')); 2.32 (s, MeÀC(2''')); 1.81 (s, MeÀC(2'')). 13C-NMR (150 MHz, CDCl3):
166.60 (s, COÀC(2)); 164.04 (s, COÀC(3)); 149.50 (s, C(1''')); 149.24 (s, C(1'')); 139.27 (s, C(1)); 136.40
(s, C(1')); 134.64 (s, C(8a)); 132.44 (s, C(4a)); 132.23 (d, C(4)); 131.22 (d, C(3''')); 130.93 (d, C(3')); 130.79
(d, C(3'')); 130.58 (s, C(2)); 130.47 (s, C(2''')); 130.30 (s, C(2'')); 129.44 (d, C(5,7)); 128.33 (d, C(2',4')); 127.87
(d, C(6)); 127.19 (d, C(8)); 127.03 (d, C(5''')); 126.47 (d, C(4'')); 126.27 (d, C(6''')); 125.63 (d, C(5'')); 124.11
(d, C(3)); 122.05( d, C(4''')); 121.81 (d, C(6'')); 16.34 (q, MeÀC(2''')); 15.66 (q, MeÀC(2'')). EI-MS: 472.1
.
‡
‡
(3, M ), 365.0 (100, [M À CH3C6H4O] ). Anal. calc. for C32H24O4 (472.53): C 81.34, H 5.12; found: C 81.39,
H 5.23.
2.8 Cross Experiment with 3f and 3g. A molten mixture of 3f (0.100 g, 0.323 mmol) and 3g (0.173 g,
0.646 mmol) was stirred under Ar during 6 h at 1708. CC (silica gel; hexane/CH2Cl2 1:1) gave an orange colored
fraction, which was submitted to prep. HPLC. A first run (hexane/i-PrOH 95:5) delivered the pure dimer ( Z)-
11f (0.023 g, 23%) and a mixture (Z)-11fg/(E)-11fg. Subsequent prep. HPLC (hexane/ CH2Cl2/MeOH
80 :20 :0.5) of the mixed fraction led to pure (Z)-11fg (0.019 g, 10%) and (E)-11fg (0.007 g, 4%).
Data of Phenyl 2,3-Dihydro-5-(4-nitrophenyl)-3-[(Z)-(4-nitrophenyl)(2,4,6-trimethylphenyl)methylidene]-
2-oxofuran-4-carboxylate ((Z)-11fg). Orange solid. M.p. 131.2 136.58 (hexane/CH2Cl2). UV (hexane): lmax 406
(4.21), 382 (sh, 4.14), 263 (4.15); lmin 318 (3.80), 251 (4.14). IR (CHCl3): 1787m (CˆO, five-ring lactone),
1740m (CˆO, aryl ester), 1525s (NO2), 1349vs (NO2). 1H-NMR (600 MHz, CDCl3): 8.32 (dt, Jo ˆ 8.9, Jm ˆ 1.9,
HÀC(3a,5a)); 8.21 (dt, Jo ˆ 9.0, Jm ˆ 1.9, HÀC(2a,6a)); 8.16 (dt, Jo ˆ 8.8, Jm ˆ 1.8, HÀC(3c,5c)); 7.52 (dt, Jo ˆ 8.8,
Jm ˆ 1.8, HÀC(2c,6c)); 7.20 (tt, Jo ˆ 8.1, Jm ˆ 1.9, HÀC(3d,5d)); 7.14 (t, Jo ˆ 7.4, HÀC(4d)); 7.01 (s, HÀC(3b,5b));
6.52 (dt, Jo ˆ 8.7, Jm ˆ 1.9, HÀC(2d,6d)); 2.36 (s, MeÀC(4b)); 2.14 (s, MeÀC(2b,6b)). 13C-NMR (150 MHz,
CDCl3): 162.24 (s, C(2)); 160.80 (s, COÀC(4)); 156.14 (s, C(5)); 155.80 (s, C(3')); 149.61 (s, C(1d)); 149.27 (s,
C(4a)); 148.33 (s, C(4c)); 145.60 (s, C(1c)); 139.37 (s, C(4b)); 134.89 (s, C(2,6b)); 134.75( s, C(1b)); 132.95( s,
C(1a)); 130.31 (d, C(2c,6c)); 129.99 (d, C(2a,6a)); 129.43 (d, C(3d,5d)); 129.35( d, C(3b,5b)); 126.43 (d, C(4d));
12
)
The two Ph groups exhibit hindered rotation, which leads to a differentiation of HÀC(2'') and HÀC(6'') of
PhÀC(9), because one H-atom is in a cis relation and the other one in a trans relation to the Me group of o-
MeC6H4ÀC(4).

3576
Helvetica Chimica Acta Vol. 84 (2001)
124.31 (s, C(3)); 124.02 (d, C(3c,5c)); 123.71 (d, C(3a,5a)); 119.71 (d, C(2d,6d)); 112.11 (s, C(4)); 21.27 (q,
‡
MeÀC(4b)); 20.12 (q, MeÀC(2b,6b)). CI-MS (C33H24N2O8, 576.56): 594.1 (100, [M ‡ NH4] ).
Data of (E)-11fg. Yellow crystals. M.p. 258.8 260.68 (hexane/CH2Cl2). UV (hexane): lmax 404 (4.05), 265
(3.95); lmin 322 (3.50), 248 (3.90). IR (CHCl3): 1784m (CˆO, five-ring lactone), 1745m (CˆO, aryl ester), 1524s
(NO2), 1347vs (NO2). 1H-NMR (600 MHz, CDCl3): 8.30 (d, Jo ˆ 8.8, HÀC(3a,5a)); 8.20 (d, Jo ˆ 8.6,
HÀC(3b,5b)); 7.97 (d, Jo ˆ 8.7, HÀC(2a,6a)); 7.57 (d, Jo ˆ 8.5, HÀC(2b,6b)); 7.27 (t, Jo ˆ 7.9, HÀC(3d,5d)); 7.21
(t, Jo ˆ 7.6, HÀC(4d)); 6.88 (s, HÀC(3c,5c)); 6.51 (d, Jo ˆ 7.9, HÀC(2d,6d)); 2.36 (s, MeÀC(4c)); 2.11 (s,
MeÀC(2c,6c)). 13C-NMR (150 MHz, CDCl3): 162.76 (s, C(2)); 160.54 (s, COÀC(4)); 156.11 (s, C(3')); 152.98 (s,
C(5)); 149.52 (s, C(1d)); 149.06 (s, C(4a)); 148.63 (s, C(4b)); 142.49 (s, C(1b)); 140.09 (s, C(4c)); 136.50 (s, C(2c));
135.10 (s, C(1c)); 132.78 (s, C(1a)); 131.56 (d, C(2b,6b)); 129.57 (d, C(3c,5c)); 129.09 (d, C(3d,5d)); 128.77 (d,
C(2a,6a)); 126.26 (d, C(4d)); 124.28 (s, C(3)); 123.96 (d, C(3a,5a)); 123.07 (d, C(3b,5b)); 120.49 (s, C(2d,6d)); 114.33
‡
(s, C(4)); 21.03 (q, MeÀC(4c)); 20.33 (q, MeÀC(2c,6c)). CI-MS (C33H24N2O8, 576.56): 594.0 (100, [M ‡ NH4] ),
‡
‡
‡
577.0 (12, [M ‡ H] ), 547.1 (26, [M À CHO] ), 483.0 (66, [M À C6H5O] ).
3. Synthesis of 17O-Labeled Compounds. 3.1. 2,4,6-Trimethyl-[17O]phenol. (2,4,6-Trimethylphenyl)dia-
zonium tetrafluoroborate was prepared in quant. yield from 2,4,6-trimethylaniline (cf. [11]). The dry diazonium
salt (0.590 g, 2.52 mmol) and dry AcONa (0.248 g, 2.52 mmol) were dissolved in H2[17O] (0.454 ml, 25.2 mmol;
17O-enrichment: 19%; source: Isotec Inc.). After the addition of CF3COOH (3.86 ml, 50.4 mmol), the soln. was
heated under Ar at reflux during 3 d (for a similar procedure, see [12]). The solvent mixture was distilled off,
and the residue was submitted to CC (hexane/Et2O 1:1). The isolated 2,4,6-trimethylphenyl [17O]phenol was
.
.
‡
‡
recrystallized from hexane (0.285g, 83%). MS Analysis of the signals at m/e 136 (M ), 137 ([M ‡ 1] ), and
.
138 ([M ‡ 2] ) indicated 8 9% 17O-incorporation. 17O-NMR (81.4 MHz, CDCl3): 65( Dn1/2 ˆ 240).
3.2. 17O-(2,4,6-Trimethylphenyl) 3-(4-Methoxyphenyl)-[17O1]prop-2-ynoate ([17O]-3i). According to the
procedure described in 1.2.8, [17O]-3i was prepared from 5d (0.194 g, 1.10 mmol) and 2,4,6-trimethylphenyl-
[17O]phenol (0.150 g, 1.10 mmol). Yield of [17O]-3i: 0.249 g (77%). 17O-NMR (81.4 MHz, CDCl3; cf. Fig. 5): 198
(Dn1/2 ˆ 1660).
‡
3.3. Thermolysis of [17O]-3i. The molten, labeled ester (0.210 g, 0.713 mmol) was heated during 5h at 210 8.
CC (silica gel, hexane/CH2Cl2 1:1) of the orange colored solid gave as a first fraction (Z)-11i (0.052 g, 25%).
Subsequent prep. HPLC (hexane/i-PrOH 95:5) of the second orange colored fraction led to pure ( E)-11i
(0.0113 g, 5%). Unfortunately, labeled 12i had been destroyed in the course of the purification procedures.
Data of 17O-(2,4,6-Trimethylphenyl) 2,3-Dihydro-5-(4-methoxyphenyl-3-[(Z)-(4-methoxyphenyl)(2,4,6-tri-
methylphenyl)methylidene]-2-[17O1]oxofuran-4-[17O1]carboxylate ((Z)-[17O2]-11i). 17O-NMR (81.4 MHz,
CDCl3; cf. Fig. 5): 340 (Dn1/2 ˆ 2290); 190 (Dn1/2 ˆ 2290).
Data of ((E)-[17O2]-11i). 17O-NMR (67.8 MHz, CDCl3; cf. Fig. 5): 314 (Dn1/2 ˆ 7400); 198 (Dn1/2 ˆ 8600).
4. Kinetic Investigations. 4.1. Thermal Rearrangement of 3c. 4.1.1. Influence of Solvent on Reactivity of 3c.
A 0.5-ml glass tube was filled with a soln. of 3c (0.2 ml; for solvent and concentration, see Table 4), the solvent
was flushed with Ar, and the glass tube was sealed. At a fixed time (for time and temp., see Table 4) of the
reaction the tube was opened, the solvent was evaporated (608/0.04 mm), the residue redissolved in hexane/
CH2Cl2 (80 :20), and then subjected to HPLC (Spherisorb CN; hexane/CH2Cl2/MeOH 80 :20 :0.5). UV
detection was performed at the isosbestic point of (Z)-11c, 12c, and 13c at (280 nm). The ratio between the e
values of 3c, (Z)-11c, (E)-11c, 12c, and 13c at 280 nm is 1.7 :1.0 :0.8 :1.0 :1.0 (data for 12c and 13c were taken
from the almost identical UV spectra of 12l and 13l). Therefore, the per cent ratio, which was obtained from
HPLC analyses, was recalculated for the real molar ratio. The results are listed in Table 4.
4.1.2. Kinetics of the Thermal Rearrangement of 3c in Decalin. A 0.1m soln. of 3c in decalin was heated
under Ar at 150 Æ 18. At regular time intervals, samples (10 ml) were taken, dissolved in hexane/CH2Cl2 (80 :20),
then subjected to HPLC, and analyzed as described in 4.1.1. The results are listed below (cf. Table 5):Scheme 4
Entry
Time [min]
3c
(Z)-11c
12c
1
2
3
4
0
870
1380
2610
97.0
95.5
94.6
92.8
1.8
2.4
2.8
3.7
1.3
2.1
2.6
3.4
The decrease of 3c followed first order kinetics with kÀ1([3c]) ˆ (1.72 Æ 0.18) ¥ 10À5 minÀ1 (r2 ˆ 0.999).
4.2. Thermal Rearrangement of 3f. 4.2.1. In PhNO2. A 0.1m soln. of 3f in PhNO2 was heated under Ar at
175 Æ 18. At regular time intervals, samples (10 ml) were taken, the solvent was evaporated (608/0.04 mm), the

Helvetica Chimica Acta Vol. 84 (2001)
3577
residue redissolved in hexane/CH2Cl2 (80 :20), and then subjected to HPLC (Spherisorb CN; hexane/CH2Cl2/
MeOH 80 :20 :0.5). UV detection was performed at the isosbestic point of 3f, (Z)-11f, and (E)-11f (305nm).
The results are listed below (cf. Fig. 6):
Entry
Time [min]
3f
(Z)-11f
(E)-11f
1
2
3
4
5
6
7
8
9
0
30
60
100
0
0
95.90
93.21
91.34
89.2510.06
86.37
85.09
15.50
80.19
76.70
3.96
6.48
8.14
0.14
0.31
0.52
90
120
150
180
210
240
300
0.69
12.29
1.53
1.94
17.41
19.32
1.34
2.1
2.6
2.40
3.98
10
The decrease of 3f followed first order kinetics with kÀ1([3f]) ˆ (8.52 Æ 0.48) ¥ 10À4 minÀ1 (r2 ˆ 0.995).
4.2.2. In Decalin. A 0.1m soln. of 3f in decalin was heated under Ar at 150 Æ 18. At regular time intervals
samples (10 ml) were taken, dissolved in hexane/CH2Cl2 (80 :20), and then subjected to HPLC (Spherisorb CN;
hexane/CH2Cl2/MeOH 80 :20 :0.5). UV Detection at the isosbestic point of 3f, (Z)-11f, and (E)-11f (305nm).
The results are listed below (cf. Table 5 and Fig. 7):
Entry
Time [min]
3f
(Z)-11f
(E)-11f
1
2
3
4
5
6
7
8
0
900
2700
100
98.8
0
0
1.2
2.9
9.0
0.0
0.0
0.0
0.0
0.0
1.0
1.5
97.1
91.0
81.1
69.6
59.0
33.0
6300
14400
43200
81000
167400
18.9
30.4
40.0
48.9
The decrease of 3f (Entry 1 5) followed first order kinetics with kÀ1([3f]) ˆ (8.84 Æ 0.89) ¥ 10À4 minÀ1
(r2 ˆ 0.996).
4.3. Thermal Rearrangement of Mixed Prop-2-ynoate 3f and 3g in PhNO2. The 0.1m soln. of 3f with 2 mol-
equiv. amount of 3g was heated under Ar at 175 Æ 18. At regular time interval samples (5 ml) were taken, solvent
was evaporated (608/0.04 Torr), diluted with hexane/CH2Cl2 (80 :20), and subjected to HPLC (analyses were
performed on the two in-line Spherisorb CN columns: 3 mm 125 Â 40 mm and 5 mm 25 0Â 40; hexane/CH2Cl2/
MeOH 80 :20 :0.5). UV Detection at the isosbestic point 305 nm. The results are listed below (cf. Fig. 6):
Entry
Time [min]
3f
11f
11fg
1
2
0
30
100
92.22
0
3.54
0
4.24
3
4
5
6
7
8
9
10
60
90
85.85
81.50
77.59
70.77
65.47
61.53
57.21
49.69
5.96
7.36
8.71
10.69
11.68
12.03
12.86
14.28
8.19
11.14
13.70
18.55
22.84
26.44
29.93
36.03
120
150
180
210
240
300
The decrease of 3f followed first-order kinetics with kÀ1([3f]) ˆ (2.31 Æ 0.09) ¥ 10À3 minÀ1 (r2 ˆ 0.998).
4.4. Thermal Rearrangement of 3i in Decalin. A 0.1m soln. of 3i in decalin was heated under Ar at 150 Æ 18.
Analysis as described in 4.2.2. UV detection was performed at the isosbestic point of 3i, (Z)-11i, and 12i
(242 nm). The results are listed below (cf. Table 5):