Synthesis of end-blocked thienyl oligomers incorporating benzo[c]thiophene

Article abstract of DOI:10.1016/j.tetlet.2006.11.178

A straightforward synthesis of end-capped bithienyl, quaterthienyl and sexithienyl systems incorporating benzo[c]thiophene units is presented.

Full text of DOI:10.1016/j.tetlet.2006.11.178

Tetrahedron Letters 48 (2007) 779–784
Synthesis of end-blocked thienyl oligomers
incorporating benzo[c]thiophene
Arasambattu K. Mohanakrishnan,* P. Amaladass and J. Arul Clement
Department of Organic Chemistry, University of Madras, Guindy Campus, Chennai 600 025, India
Received 21 September 2006; revised 23 November 2006; accepted 30 November 2006
Available online 19 December 2006
Abstract—A straightforward synthesis of end-capped bithienyl, quaterthienyl and sexithienyl systems incorporating benzo[c]thio-
phene units is presented.
ꢀ 2006 Published by Elsevier Ltd.
Electronic properties of linear conjugated oligomers
R
have acquired growing importance in many areas of
modern chemistry. In particular, thiophene oligomers
are frequently used as semiconducting materials in
molecular electronic and optical devices.1 The thiophene
oligomers can advantageously replace polythiophene in
organic-based electronic devices such as Schotty diodes
and field effect transistors.2
R
S
S
S
S
S
S
1a R = H
1b R =
1c R = CH2O-
n
-C6H13
n
-C6H13
Scheme 1.
During the last 15 years, the synthesis and characteriza-
tion of 1,3-dithienylbenzo[c]thiophene 2 and several of
its derivatives have been reported,8 Scheme 2.
Among the higher oligomers of thiophene, the hexamer,
a-sexithiophene 1a has been incorporated successfully
into electronic and optical data processing devices.3
a-Sexithiophenes and higher oligomers, however, are
practically insoluble in organic solvents and thus are
difficult to purify. Moreover, sophisticated high tempera-
ture vacuum deposition techniques are needed to
incorporate them as thin films in devices. In particular,
unsubstituted oligothiophenes suffer from instability as
a result of the relatively higher chemical reactivity
at the a-carbons of the terminal thiophene ring.4 On
the other hand, b-substitution of thiophene lowers the
degree of conjugation and carrier mobility due to the
out of plane twisting of the thiophene rings in the b-
substituted system.5 In contrast, substitution at the
reactive a-positions of the terminal thiophenes (1b and
1c) results in chemically stable oligomers,6 Scheme 1.
Several workers have reported the electrochemical and
chemical synthesis of a,x-dialkyl sexithiophenes and
their utilization in field effect transistors (FETs).7
The above-mentioned reports of oligothiophenes used
for field effect transistors (FETs) and other electronic
applications encouraged us to initiate our studies on
the synthesis of more soluble and easily processable
derivatives of oligothiophenes/sexithiophenes incorpo-
rating one or two benzo[c]thiophene units.
In order to realize this objective, our initial attempts
to synthesize one-end-blocked 1,3-dithienylbenzo[c]thio-
phene 4a starting from the known bromo-benzo[c]thio-
phene 38e and n-hexylmagnesium bromide using
Ni(dppp)Cl2 under Kumada conditions were unsuccess-
ful, Scheme 3. Use of other Ni complexes such as
Ni(PPh3)2Cl2 or Ni(dppe)Cl2 was also ineffective.
S
S
Keywords: Benzo[c]thiophene; End-capped thienyl oligomers; Dimeri-
S
zation; Electronic properties.
2
*
Corresponding author. Tel.: +91 44 24451108; fax: +91 44
22300488; e-mail: mohan_67@hotmail.com
Scheme 2.
0040-4039/$ - see front matter ꢀ 2006 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2006.11.178

780
A. K. Mohanakrishnan et al. / Tetrahedron Letters 48 (2007) 779–784
i)
n-C6H13Br/Mg, ether
X
Me
Br
ii) Ni(dppp)Cl2
S
S
S
S
S
S
3
4a
Scheme 3.
MgBr
Yield
Entry
i)
R
R
S
6
O
60%
55%
50%
n
-hexyl
2-ethylhexyl
-butyl
4a
4b
4c
O
5
S
4
ii) aq.NH4Cl
iii) Lawesson's reagent/DCM
room temperature
S
S
S
R
n
Scheme 4.
However, one-end-blocked terthienyl analogues 4a–c
were prepared smoothly via conventional ring opening
of lactone 5 using freshly prepared 5-alkyl thienylmag-
nesium bromides 69 in dry THF, followed by thionation
and intramolecular cyclization, Scheme 4.
Kita and co-workers reported a simple synthesis of
2,20-bithiophene derivatives involving oxidative cou-
pling of the corresponding alkylthiophenes using a
combination of phenyliodide bis(trifluoroacetate) and
BF3ÆOEt2.13 A mild FeCl3 mediated oligomerization of
b-substituted 1,3-dithienylbenzo[c]thiophenes led to the
isolation of annelated sexithiophene and nonathiophene
analogues.8e
Similarly, one-end-blocked quaterthienyl system 8 was
prepared from the known lactone 78e via lactone ring
opening with 5-n-hexylthienylmagnesium bromide 6a9
followed by thionation and intramolecular cyclization
using Lawesson’s reagent, Scheme 5.
In order to identify suitable conditions to dimerize
benzo[c]thiophenes, various conditions were explored.
Dimerization of 4a using CAN in acetonitrile led to
the isolation of 9a in low yield (ꢀ10%). Similarly, dimeri-
zation using the lithio derivative of 4a (generated using
n-BuLi) and CuCl2 was also found to proceed in low
yield. The expected dimerization of one-end-blocked
terthienyl system 4 was carried out smoothly using two
different sets of conditions, Scheme 6. The conditions
employed for the dimerization of benzo[c]thiophenes 4
and the results obtained are outlined in Table 1.
Having prepared one-end-blocked benzo[c]thiophenes 4
and 8, the next task was to dimerize them in a controlled
manner to afford the respective products in reasonable
yields. In general, the controlled oligomerization/dimer-
ization of several thienyl monomers is reported via
oxidation of the corresponding thienyl a-carbanion
using CuCl2.10 A regioselective oligomerization of
3-(alkylsulfanyl)thiophenes using FeCl3 has been
reported.11 Mustafa and Shepherd reported a simple
dimerization of b-trimethylsilyl substituted terthio-
phenes using ceric ammonium nitrate12 (CAN). Recently
The dimerization of hexylated terthiophene 4a using
anhydrous FeCl3 at room temperature for 12 h followed
i)
n
-C6H13
MgBr
S
6a
O
O
S
ii) aq. NH4Cl
S
S
S
n
-C6H13
iii) Lawesson's reagent/DCM
room temperature
55%
8
7
S
S
Scheme 5.
i) FeCl3/DCM
ii) NH2NH2.H2O
(or)
4
S
S
i) PIFA/BF3.OEt2
-78 oC, 6 h
S
S
9
S
S
R
R
ii) aq. NaHCO3
Scheme 6.

A. K. Mohanakrishnan et al. / Tetrahedron Letters 48 (2007) 779–784
Table 1. Dimerization of benzo[c]thiophenes
781
Entry
Benzo[c]thiophene14
Dimer15
Yield (%)/mp
55,a 60b (liquid)
60,a 65b (liquid)
50,a 54b (liquid)
R
R
R
S
1
S
S
S
S
S
S
S
S
9a R = n-hexyl
4a R =
n
-hexyl
9b R = 2-ethylhexyl
4b R = 2-ethylhexyl
9c R = n-butyl
4c R = n-butyl
R
R
R
65,a 35b (220 ꢁC)
60,a 40b (160 ꢁC)
25,a 35b (163 ꢁC)
S
S
S
S
S
n-C6H13
S
2
9d R = O-
9e R = OMe
9f R = Me
4d R = O-
4e R = OMe
4f R = Me
n
-C6H13
R
R
R
3
4
5
50,a 65b (liquid)
S
S
S
S
S
S
S
S
S
4g R =
n
-hexyl
9g R = n-hexyl
27,a 22b (liquid)
S
S
S
S
S
S
4h
9h
No reaction
n
-C6H13
S
S
S
S
8
a Isolated yield using FeCl3–DCM.
b Isolated yield using PIFA–BF3ÆOEt2.
by quenching with hydrazine hydrate afforded the
corresponding dimer 9a in 55% yield. Under similar con-
ditions, dimerization of one-end-blocked benzo[c]thio-
phenes 4b and 4c afforded 9b and 9c in 60% and 50%
yields, respectively (Table 1, entry 1).
yields (Table 1, entry 4). In the case of one-end-blocked
benzo[c]thiophene 8, the dimerization was found to be
unsuccessful using both sets of conditions.
Finally, the end-capped benzo[c]thiophene based tetra-
thienyl systems 11a–c were prepared via the lactone ring
opening of commercially available diphthalide 10 using
excess 5-substituted-2-thienylmagnesium bromide9 in
dry THF, Scheme 7. Work-up followed by thionation
led to expected products 11a–c16 in 65%, 56% and 60%
yields, respectively. Using identical conditions, ring
opening of 10 with freshly prepared p-tolylmagnesium
bromide led to the formation of p-tolyl-capped
benzo[c]thiophene 1316 in 51% yield.
It should be noted that during the above-mentioned
dimerization using anhydrous FeCl3, a small amount
(ꢀ5%) of the starting materials 4a–c was recovered even
on extended reaction (more than 24 h for entry 1).
Dimerization of 4a–c using PIFA–BF3ÆOEt2 at ꢁ78 ꢁC
in dry THF gave the respective products 9a–c in
somewhat better yields. Additionally, under the PIFA–
BF3ÆOEt2 conditions, the monomeric benzo[c]thio-
phenes 4a–c were fully consumed. The arylated bithienyl
systems 4d–f were converted into the respective dimers
9d–f using anhydrous FeCl3 as well as PIFA–BF3ÆOEt2
(Table 1, entry 2). The b-substituted benzo[c]thiophene
4g was converted into the known product 9g in higher
yield using PIFA–BF3ÆOEt2 compared to anhydrous
FeCl3 (Table 1, entry 3). The naphthylated benzo[c]thio-
phene 4h afforded the corresponding dimer 9h in poor
The UV–vis spectra of the benzo[c]thiophenes exhibited
strong absorption between 438 and 510 nm, the exact
values are presented in Table 2. The introduction of
alkyl a-substituents (n-hexyl, 2-ethylhexyl, n-butyl) into
2 (433 nm)8e increased the absorption (Table 2, 4a–c). A
similar effect was observed for 8 compared to the parent
benzo[c]thiophene (458 nm).8e The long-wavelength

782
A. K. Mohanakrishnan et al. / Tetrahedron Letters 48 (2007) 779–784
MgBr
i)
R
S
6
R
R
O
O
S
S
S
S
O
O
ii) aq.NH4Cl
iii) Lawesson's reagent/DCM
room temperature
11
10
Yield
65%
Entry
R
n
-hexyl
11a
11b
11c
2-ethylhexyl
56%
60%
n
-butyl
Br
i) Mg/THF, reflux
ii) 10/THF
H3C
CH3
S
S
iii) aq.NH4Cl
Me
iv) Lawesson's reagent/DCM
room temperature
51%
13
12
Scheme 7.
Table 2. UV–vis spectral data of benzo[c]thiophenes
Benzo[c]thiophenes
4a
4b
4c
8
9a
9b
9c
9d
9e
9f
9g
9h
11a
11b
11c
13
kmax (nm) (DCM)
440
442
438
468
504
510
508
495
480
490
484
476
468
472
494
449
Table 3. Fluorescence spectral data of benzo[c]thiophenes
Benzo[c]thiophenes
4a
4c
8
9a
9e
9d
11a
13
kexcitation (nm) (CHCl3)
kemission (nm) (CHCl3)
440
561
438
543
468
584
504
600
480
582
495
602
468
531
449
600
absorption bands for dimers 9a–c were shifted by 64 nm,
68 nm and 70 nm from the respective monomers 4a–c.
Acknowledgements
The authors thank DST, New Delhi (SR/S1/OC-37/
2005) and UGC-Potential for excellence for financial
support. P.A. thanks CSIR for a CSIR-SRF fellowship.
J.A. thanks UGC-Potential for excellence for the fellow-
ship. Financial support from the Department by DST-
FIST is also acknowledged. Authors thank SAIF, IIT
Madras, for high-resolution NMR spectral data.
Qualitative fluorescence spectral data for some of the
benzo[c]thiophenes are presented in Table 3. Monomeric
benzo[c]thiophenes 4a, 4c and 8, emit light at 563 nm,
543 nm and 584 nm, respectively. Bis-benzannelated
tetra-thiophene 11a, and bis-benzannelated symmetrical
bi-thiophene 13, emit light at 531 nm and 600 nm,
respectively. Dimeric benzo[c]thiophene analogs 9a, 9d,
and 9e emit light in the 582–602 region.
References and notes
In summary, one-end-blocked benzannelated terthienyl
and quaterthienyl systems are prepared in reasonable
yields. Dimerization of the one-end-blocked terthienyl
system led to the formation of benzo[c]thiophene
incorporated sexithiophenes in reasonable yields using
anhydrous FeCl3 or PIFA–BF3ÆOEt2. Except for
benzo[c]thiophenes possessing alkoxy units, the dimeri-
zation yield was good when PIFA–BF3ÆOEt2 was used.
Apart from benzo[c]thiophene 13, all the other
benzo[c]thiophenes were found to be soluble in common
organic solvents. The highly soluble nature of these end-
blocked benzo[c]thiophenes may allow them to find use
in LED and field effect transistor (FET) applications.
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Then it was treated with Lawesson’s reagent (1.67 g,
4.14 mmol) at room temperature for 6 h. The solvent was
removed, and the residue was gently heated (fume hood)
on a steam bath with ethanol (20 mL). Purification of the
crude product by column chromatography (neutral alu-
mina; eluent:hexane) afforded 4b as a thick yellow liquid
(1.86 g, 55%).
Spectral data: For 4b: 1H NMR (CDCl3, 400 MHz): d
0.99–0.98 (m, 7H), 1.40–1.57 (m, 8H), 2.81 (d,
J = 6.80 Hz, 2H), 6.63 (d, J = 3.2 Hz, 1H), 6.89–6.91 (m,
1H), 6.96 (d, J = 3.6 Hz, 1H), 7.03 (t, J = 3.7 Hz, 1H),
7.22–7.29 (m, 2H), 7.45–7.47 (m, 2H), 8.06 (m, 1H). 13C
NMR (CDCl3, 100.6 MHz): d 10.78, 14.09, 22.97, 25.54,
28.77, 32.22, 33.78, 34.63, 121.41, 121.68, 122.85, 125.13,
125.55, 126.48, 126.80, 127.77, 129.27, 133.19, 134.78,
135.21, 135.77, 144.27, 145.10, 146.12. MS m/z (%): 410
(22, M+), 311 (73), 215 (54), 114 (08). C24H26S3: C, 70.19;
H, 6.38; S, 23.43. Found: C, 70.10; H, 6.31; S, 23.59.
For 8: 1H NMR (CDCl3, 500 MHz): d 0.92 (t, J =
6.87 Hz, 3H), 1.27–1.36 (m, 4H), 1.43 (t, J = 7.20 Hz, 2H),
1.71–1.77 (m, 2H), 2.85 (t, J = 7.65 Hz, 2H), 6.81 (d,
J = 3.8 Hz, 1H), 7.03–7.05 (m, 1H), 7.11–7.16 (m, 3H),
7.18–7.21 (m, 1H), 7.22–7.25 (m, 3H), 7.94–7.97 (m, 2H).
13C NMR (CDCl3, 125.6 MHz): d 14.23, 22.72, 28.96,
30.37, 31.70, 123.76, 124.60, 124.65, 125.01, 125.47,
125.82, 128.05, 146.86. C26H24S4: C, 67.20; H, 5.21; S,
27.60. Found: C, 67.08; H, 5.25; S, 27.62.
15. Representative procedure for 9a–h:
Dimerization using FeCl3:
A solution of 4b (0.2 g,
0.48 mmol) in CH2Cl2 (30 mL) was treated with FeCl3
(0.07 g, 0.48 mmol) under a nitrogen atmosphere at room
temperature for 12 h. The reaction mixture was then
treated with a dilute solution of NH2NH2ÆH2O (3 · 5 mL).
9. The required 5-substituted thienylmagnesium bromides 6
were prepared as mentioned below:
Mg/THF
Cat. I2
R
o
n
-BuLi
RBr
/-78 C
NBS
R
BrMg
S
R
S
N2
Br
S
S
DMF, 40 h
62%
6
reflux
10. (a) Ba¨uerle, P.; Fischer, T.; Bidlingmeir, B.; Stable, A.;
Rabe, J. P. Angew. Chem., Int. Ed. Engl. 1995, 34, 303–
307; (b) Groenendaal, L. B.; Jonas, F.; Freitag, D.;
Pielartzik, H.; Reynolds, J. R. Adv. Mater. 2000, 12, 481–
494.
11. Barbarella, G.; Zambianchi, M.; Di Toro, R.; Colonna,
M., Jr.; Iarossi, D.; Goldoni, F.; Bongini, A. J. Org.
Chem. 1996, 61, 8285–8292.
12. Mustafa, A. H.; Shepherd, M. K. Chem. Commun. 1998,
2743–2744.
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Maruyama, A.; Kita, Y. Org. Biomol. Chem. 2003, 1,
1647–1649.
The organic layer was separated and dried (Na2SO4).
Removal of the solvent followed by column chromato-
graphic purification (silica gel; EtOAc–hexane; 1:1) gave
dimer 9b as a dark brown liquid (0.23 g, 60%).
Dimerization using PIFA–BF3ÆOEt2: BF3ÆEt2O (0.074 g,
0.532 mmol) and PIFA (0.226 g, 0.532 mmol) were added
sequentially to a stirred solution of 4a (0.2 g, 0.590 mmol)
in CH2Cl2 (20 mL) at ꢁ78 ꢁC under a nitrogen atmo-
sphere. The reaction mixture was stirred at the same
temperature for 6 h. Aqueous work-up with saturated
NaHCO3 (10 mL) at 0 ꢁC followed by column chromato-
graphic purification (silica gel; EtOAc–hexane; 1:1) gave
dimer 9a as a red sticky liquid (0.23 g, 60%).
14. For the preparation of 4d–h: see Ref. 8h.
Spectral data: For 9b: 1H NMR (CDCl3, 400 MHz): d
0.91–1.21 (m, 12H), 1.41–1.72 (m, 18H), 2.85 (d,
J = 5.88 Hz, 4H), 6.83–6.91 (m, 2H), 7.16–7.23 (m, 4H),
7.37–7.40 (m, 6H), 7.83–8.06 (m, 4H). 13C NMR (CDCl3,
100.6 MHz): 10.68, 14.09, 22.96, 25.39, 28.73, 32.34, 34.16,
41.29, 121.53, 121.77, 123.23, 124.48, 125.58, 125.93,
126.33, 126.54, 128.94, 130.47, 135.65, 136.18, 139.08,
141.75, 145.18, 146.79. GC MS: (818 M+), C48H50S6: C,
70.37; H, 6.15; S, 23.48. Found: C, 70.24; H, 6.21; S, 23.55.
Representative procedure for the preparation of 4a–c and 8:
A solution of 2-bromo-5-(2-ethylhexyl)thiophene (2.28 g,
8.32 mmol) in dry THF (20 mL) was added dropwise
(ꢀ15 min) to a refluxing mixture of magnesium turnings
(0.239 g, 9.98 mmol) containing a catalytic amount of
iodine (20 mg) in dry THF (100 mL) under N2. The
reaction mixture was refluxed for 2 h to ensure the
completion of Grignard formation. The Grignard reagent
was slowly added via an addition funnel to a solution of
3-(2-thienyl)phthalide 5 (1.7 g, 8.32 mmol) in dry THF
(30 mL) at room temperature. The reaction mixture was
stirred for 6 h at room temperature and then poured over
ice-cold ammonium chloride solution. The crude product
was extracted into DCM (75 mL) and dried (Na2SO4).
For 9d: 1H NMR (400 MHz, CDCl3):
d 0.91 (t,
J = 6.84 Hz, 6H), 1.34–1.47 (m, 8H), 1.51–1.55 (m, 4H),
1.81 (quin, J = 7.3 Hz, 4H), 4.01 (t, J = 6.6 Hz, 4H), 7.04
(d, J = 8.8 Hz, 4H), 7.11–7.21 (m, 4H), 7.24 (d,
J = 3.88 Hz, 2H), 7.27 (d, J = 3.92 Hz, 2H), 7.59 (d,
J = 8.76 Hz, 4H), 7.77 (d, J = 8.8 Hz, 2H), 8.02 (d,

784
A. K. Mohanakrishnan et al. / Tetrahedron Letters 48 (2007) 779–784
J = 8.8Hz, 2H). MS (MALDI-TOF) calcd 782.2. Found:
7.51 mmol) in dry THF (50 mL) at 0 ꢁC. The reaction
mixture was stirred for 6 h at room temperature and then
poured over ice-cold ammonium chloride solution. The
intermediate was extracted into CH2Cl2 (2 · 50 mL) and
dried (Na2SO4). The dried extract was treated with
Lawesson’s reagent (3.05 g, 7.51 mmol) and then stirred
at room temperature for 6 h. Work-up and column
chromatographic purification (neutral alumina, hexane)
furnished 11c as a black solid (2.44 g, 60%); mp 80 ꢁC.
Spectral data: For 11c: 1H NMR (CDCl3, 500 MHz): d
1.02 (t, J = 3.85 Hz, 6H), 1.44–1.50 (m, 4H), 1.71–1.77 (m,
4H), 2.85 (t, J = 7.27 Hz, 4H), 6.82 (d, J = 3.05 Hz, 2H),
6.91–7.21 (m, 6H), 7.85 (d, J = 9.2 Hz, 2H), 8.01 (d,
J = 9.2 Hz, 2H). 13C NMR (CDCl3, 125.6 MHz): d 13.99,
22.38, 30.07, 33.86, 121.74, 122.02, 123.82, 124.79, 125.06,
125.43, 133.11, 134.84, 136.92, 146.78. C32H30S4: C, 70.80;
H, 5.57; S, 23.63. Found: C, 70.64; H, 5.53; S, 23.83.
For 13: 1H NMR (CDCl3, 400 MHz): d 2.43 (s, 6H), 7.09–
7.11 (m, 4H), 7.30 (d, J = 7.08 Hz, 4H), 7.60 (d,
J = 8.0 Hz, 4H), 7.83–7.86 (m, 4H). 13C NMR (CDCl3,
100.6 MHz): d 21.26, 121.21, 121.85, 124.35, 129.11,
129.79, 131.14, 134.87, 135.75, 136.89, 137.62. MS m/z
(%): 446 (22, M+), 298 (64), 208 (72), 165 (54). C30H22S2:
C 80.68; H, 4.96; S, 14.36. Found: C, 80.79; H, 4.91; S,
14.30.
782.6. C48H46S4O2: C, 73.62; H, 5.92; S, 16.38. Found: C,
73.56; H, 6.10; S, 16.45.
For 9e: H NMR (400 MHz, CDCl3): d 3.90 (s, 6H), 7.05
1
(d, J = 8.8 Hz, 4H), 7.22–7.27 (m, 6H), 7.29 (d,
J = 3.92 Hz, 2H), 7.59 (d, J = 8.8 Hz, 4H), 7.75 (d,
J = 8.8Hz, 2H), 8.01 (d, J = 8.8 Hz, 2H). MS (MALDI-
TOF) C38H26O2S4: calcd 642.0. Found: 642.4.
C38H26S4O2: C, 70.99; H, 4.08; S, 19.95. Found: C,
70.85; H, 4.19; S, 20.05.
1
For 9h: H NMR (400 MHz, CDCl3): 6.79–6.81 (m, 4H),
6.96–7.18 (m, 4H), 7.27–7.37 (m, 4H), 7.22 (d, J =
4.41 Hz, 2H), 7.42 (d, J = 7.84 Hz, 2H), 7.46 (d,
J = 0.96 Hz, 2H), 7.48 (d, J = 1.44 Hz, 2H) 7.75 (d, J =
8.28 Hz, 2H), 7.79 (d, J = 8.28 Hz, 2H), 7.86 (d,
J = 8.81 Hz, 2H), MS (MALDI-TOF) C44H26S4: calcd
682.0. Found: 681.83. C44H26S4: C, 77.38; H, 3.84; S,
18.78. Found: C, 77.51; H, 3.80; S, 18.69.
16. Representative procedure for 11a–c and 13: 2-Bromo-5-
butylthiophene (4.93 g, 22.5 mmol) was added slowly to a
refluxing mixture of magnesium turnings (0.65 g,
27.13 mmol) and iodine (20 mg) in dry THF and then
refluxed for 6 h to ensure completion of Grignard forma-
tion. The cooled Grignard reagent was added slowly via
an addition funnel to a solution of diphthalide 10 (2 g,