Regioselective cross-coupling of allylindium reagents with activated benzylic bromides-a simple and efficient procedure for the synthesis of terminal alkenes

Article abstract of DOI:10.1016/j.tetlet.2007.08.025

Allylindium reagents undergo facile and highly regioselective cross-coupling with benzylic and cinnamyl bromides in THF at room temperature without any catalyst producing terminal alkenes in high yields. The addition is highly regioselective and is found to provide γ-adducts in all reactions.

Full text of DOI:10.1016/j.tetlet.2007.08.025

Tetrahedron Letters 48 (2007) 7374–7379
Regioselective cross-coupling of allylindium reagents
with activated benzylic bromides—a simple and efficient
procedure for the synthesis of terminal alkenes
Brindaban C. Ranu,* Subhash Banerjee and Laksmikanta Adak
Department of Organic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India
Received 3 May 2007; revised 30 July 2007; accepted 6 August 2007
Available online 10 August 2007
Abstract—Allylindium reagents undergo facile and highly regioselective cross-coupling with benzylic and cinnamyl bromides in
THF at room temperature without any catalyst producing terminal alkenes in high yields. The addition is highly regioselective
and is found to provide c-adducts in all reactions.
ꢀ 2007 Elsevier Ltd. All rights reserved.
The allylation of an organic substrate is a very useful
process as it provides a double bond, which can be fur-
ther functionalized.1 The most frequently used methods
for allylation include addition of allylsilanes2 or allylst-
annanes3 to organic electrophiles catalyzed by Lewis
acids. However, the difficulties associated with the prep-
aration of regiochemically defined allylstannanes, their
tendency to undergo allylic isomerization, removal of
tin from the product, and finally the toxicity of tin, re-
strict the use of allylstannanes as nucleophilic coupling
partners.3b Allylsilanes are also not very user-friendly
being volatile. Allyl-magnesium and -lithium reagents
are also used for allyl transfer to alkyl chains by halo-
gen–metal exchange, however, these reagents show low
functional group tolerance.4
selective cross-coupling of allylindium reagents with
benzyl bromides leading to terminal alkenes (Scheme 1).
The experimental procedure is very simple.7 A mixture
of the benzyl bromide and allylindium reagent, prepared
in situ from the reaction of indium metal and allyl bro-
mide in THF, was stirred at room temperature until
completion of the reaction (TLC). Workup and purifica-
tion then provided the pure product. The products were
1
characterized from their spectroscopic (IR, H, and 13C
NMR) data.
Several substituted benzylic bromides underwent cross-
coupling reactions with a variety of allylindium reagents
at room temperature to produce the corresponding ter-
minal alkenes, Table 1. As is evident from the results
in Table 1, c-addition products were obtained from
the reactions of all the substituted allyl bromides (entries
3, 5, 6, 9, 10, 19, 20, and 22) although in principle, there
was the possibility of formation of two products arising
from a- and c-attack. The coupling reaction was found
Organoindium reagents are of considerable interest in
organic synthesis because of their environmentally
benign characteristics and their synthetic utility for
carbon–carbon bond formation.5 As part of our
continued activities in the area of indium-mediated reac-
tions6 we are investigating novel applications of organo-
indium reagents in organic synthesis. Previously, we
have described the indium mediated homocoupling of
alkyl and aryl halides.6a Here, we demonstrate the regio-
1
R
In, THF
1
RBr
R = benzyl, R1= H, Me, CO2Me
Scheme 1.
+
Br
R
R
rt
Keywords: Indium metal; Allylation; Cross-coupling; Regioselectivity;
Terminal alkenes.
*
Corresponding author. Tel.: +91 33 24734971; fax: +91 33
24732805; e-mail: ocbcr@iacs.res.in
0040-4039/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.08.025

B. C. Ranu et al. / Tetrahedron Letters 48 (2007) 7374–7379
Table 1. Cross-coupling of organoindium reagents with benzyl bromides
7375
Entry
RBr
R1
Product
Time (min)
Yielda (%)
75
Ref.
Br
1
H
90
10
Me
Me
Me
Me
2
3
H
60
60
80
70
10
Br
Br
Me
11
Me
Ph
Ph
Ph
Ph
4
5
H
20
30
90
80
10
12
Br
Br
Me
Me
Ph
Ph
6
CO2Me
60
85
13
Br
CO2Me
Br
7
8
H
H
120
30
70
90
14
15
Cl
Cl
Br
MeO
MeO
Br
Br
9
10
11
Me
45
30
40
80
90
85
16
—
19
Me
MeO
MeO
MeO
MeO
CO2Me
CO2Me
Br
OMe
Br
H
OMe
12
H
85
78
20
OMe
OMe
Br
13
14
H
H
45
85
30
19
21
Me
Me
Br
240
O2N
O2N
Br
15
H
240
37
—
NO2
NO2
(continued on next page)

7376
B. C. Ranu et al. / Tetrahedron Letters 48 (2007) 7374–7379
Table 1 (continued)
Entry
RBr
R1
H
Product
Time (min)
15
Yielda (%)
90
Ref.
—
Br
16
BnO
O
BnO
O
Br
Br
17
18
H
H
15
20
92
90
17
—
O
O
O
O
Br
Br
Me
19
20
Me
35
60
80
85
—
—
O
O
O
O
CO2Me
CO2Me
Br
Br
21
22
H
30
40
86
80
—
—
O
O
O
O
Me
Me
Br
23
24
H
H
25
20
85
90
18
22
Br
OMe
OMe
a Yields refer to those of pure isolated products fully characterized by spectroscopic data.
to be more facile when an electron donating group was
present at the para-position of the aromatic ring of the
benzylic bromides (entries 8–10, 13, and 16–22) com-
pared to unsubstituted (entry 1) and more particularly,
electron withdrawing group-substituted benzyl bro-
mides (entries 7, 14, and 15). The ortho-electron donat-
ing substituted benzyl bromide (entry 11) also reacted
in the same way as p-substituted examples. The reac-
tions of m-methoxy-substituted benzyl bromide (entry
12) and cinnamyl bromide (entry 24) were comparable
with their unsubstituted counterparts. However, ali-
phatic bromides remained virtually inert as was found
by reaction with n-pentyl bromide; even after heating
for 10 h at 70 ꢁC, the reaction only proceeded margin-
ally (10%). Both primary and secondary benzyl bro-
mides participated in this coupling reaction. Several
functional groups present on the aromatic ring of the
benzylic bromides, such as Cl, OMe, OBn, dioxymethyl-
ene, allyloxy, and propargyloxy remained unaffected
under the reaction conditions.
In general, the reactions were very clean, fast, and high
yielding. No Lewis acid catalysts, as usually employed
for allylation reactions, were required in this procedure.
The facile addition of c-bromocrotonate to benzyl bro-
mides provides an easy access to important substituted
b,c-unsaturated carboxylic esters (entries 6, 10, and
20). The highly selective c-addition products provided
by this reaction offer advantages over other existing pro-
cedures, which give mixtures of products.2–4

B. C. Ranu et al. / Tetrahedron Letters 48 (2007) 7374–7379
7377
As is evident from the results, this cross-coupling reac-
tion is very much influenced by the electronic effects of
the substitutents on the benzyl bromides. Thus, for a
better understanding of the transition state, a Hammett
correlation diagram has been drawn plotting r (substit-
utent constant) values of different substituents versus
logI (I = ratio of signal intensity of product with that
of starting material after a certain period of time). The
plot shows a negative slope (q) of À1.956 (Fig. 1), which
indicates the involvement of a partial benzylic carbocat-
ion type intermediate. As the q value of a typical SN1
reaction is approx. À4.5,8 here the reaction is neither a
typical SN1 nor a classical SN2 process. Thus, a full po-
sitive charge at the benzylic carbon is unlikely for this
reaction and a non-classical and non-concerted cyclic
transition state (A) is thus postulated (Scheme 2), which
favors c-addition. This mechanism explains the facile
reaction with an electron donating substituent, which
stabilizes the benzylic carbocation favorably compared
to an electron withdrawing group or with no substitu-
tion. This also gains support by the inertness of the alkyl
bromide, which fails to provide sufficient stabilization of
the carbocation to allow the reaction to progress.
regioselective addition of benzyl bromides to allylindi-
ums. We are not aware of any report addressing the cat-
alyst-free cross-coupling of benzyl bromides and
allylindiums, although similar Pd-catalyzed couplings
of allylindiums with aryl halides have been reported.9
Moreover, this procedure offers marked improvements
with regard to operational simplicity, short reaction
times, general applicability to substituted allylindiums,
excellent regioselectivity, no requirement for a catalyst,
and typically high isolated yields (75–95%). We believe,
this reaction will find suitable applications in organic
synthesis.
Acknowledgments
We are pleased to acknowledge the financial support
from CSIR, New Delhi, [Grant No. 01(1936)/04/
EMR-1], for this investigation. S.B. and L.A are thank-
ful to CSIR for their fellowships.
References and notes
1. (a) Farina, V.; Krishnan, B. J. Am. Chem. Soc. 1991, 113,
9585–9595; (b) Yamamoto, Y.; Hatsuya, S.; Yamada, J.-i.
J. Org. Chem. 1990, 55, 3118–3128; (c) Labadie, J. W.;
Tueting, D.; Stille, J. K. J. Org. Chem. 1983, 48, 4634–
4642.
In conclusion, the present procedure provides a novel
protocol for the synthesis of terminal alkenes via highly
2. (a) De, S. K.; Gibbs, R. A. Tetrahedron Lett. 2005, 46,
8345–8350; (b) Kim, S. H.; Shin, C.; Pae, A. N.; Koh, H.
Y.; Chang, M. H.; Chung, B. Y.; Cho, Y. S. Synthesis
2004, 1581–1584.
3. (a) Farina, V.; Krishnamurthy, V.; Scott, W. J. Org.
React. 1997, 50, 1; (b) Stille, J. K. Angew Chem., Int. Ed.
Engl. 1986, 25, 508–524.
p
-OCH
3
1.0
0.5
p
-CH
3
p
-H
4. Fleming, F. F.; Zhang, Z.; Liu, W.; Knochel, P. J. Org.
Chem. 2005, 70, 2200–2205, and references cited therein.
5. (a) Cintas, P. Synlett 1995, 1087–1096; (b) Li, C.-J.
Tetrahedron 1996, 52, 5643–5668; (c) Li, C.-J.; Chan, T. H.
Tetrahedron 1999, 55, 11149–11176; (d) Chauhan, K. K.;
Frost, C. G. J. Chem. Soc., Perkin Trans. 1 2000, 3015–
3019; (e) Ranu, B. C. Eur. J. Org. Chem. 2000, 2347–2356;
(f) Ghosh, R. Indian J. Chem. 2001, 40B, 550–557; (g)
Podelech, J.; Maier, T. C. Synthesis 2003, 633–655; (h)
Nair, V.; Ros, S.; Jayan, C. N.; Pillai, B. S. Tetrahedron
2004, 68, 1959–1982.
0.0
p
-Cl
-0.5
-1.0
gradient,
ρ = -1.956
p
-NO
2
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
σ
6. (a) Ranu, B. C.; Dutta, P.; Sarkar, A. Tetrahedron Lett.
1998, 39, 9557–9558; (b) Ranu, B. C.; Hajra, A.; Jana, U.
Figure 1. Hammett plot.
Br
S
In
THF
1
1
Br
In2Br3
R
R
THF
3
δ +
δ+
In
δ −
Br
CH2
1
In
+
R
1
Br
δ −
R
S
S
A
Scheme 2. A possible mechanism for the regioselective cross-coupling reaction of the organoindium reagent and benzyl bromides.

7378
B. C. Ranu et al. / Tetrahedron Letters 48 (2007) 7374–7379
Tetrahedron Lett. 2000, 41, 531–533; (c) Ranu, B. C.;
5.09 (s, 2H), 5.88–5.97 (m, 1H), 7.04 (d, J = 8.74 Hz, 2H),
7.24 (d, J = 8.73 Hz, 2H), 7.35–7.50 (m, 5H); 13C NMR
(CDCl3, 75 MHz) d 34.6, 35.8, 70.1, 114.8 (2C), 114.9,
127.6 (2C), 127.9, 128.6 (2C), 129.4 (2C), 134.3, 137.3,
138.3, 157.1. Anal. Calcd for C17H18O: C, 85.67; H, 7.61.
Found: C, 85.49; H, 7.51.
Samanta, S.; Hajra, A. Synlett 2002, 987–989; (d) Ranu, B.
C.; Das, A.; Samanta, S. Synlett 2002, 727–730; (e) Ranu,
B. C.; Dey, S. S.; Hajra, A. Tetrahedron 2002, 58, 2529–
2532; (f) Ranu, B. C.; Hajra, A.; Dey, S. S.; Jana, U.
Tetrahedron 2003, 59, 813–819; (g) Ranu, B. C.; Samanta,
S. J. Org. Chem. 2003, 68, 7130–7132; (h) Ranu, B. C.;
Das, A.; Hajra, A. Synthesis 2003, 1012–1014; (i) Ranu, B.
C.; Samanta, S. Tetrahedron 2003, 59, 7901–7906; (j)
Ranu, B. C.; Mandal, T.; Samanta, S. Org. Lett. 2003, 5,
1439–1441; (k) Ranu, B. C.; Mandal, T. J. Org. Chem.
2004, 69, 5793–5795; (l) Ranu, B. C.; Mandal, T. Synlett
2004, 1239–1242; (m) Ranu, B. C.; Das, A. Tetrahedron
Lett. 2004, 45, 6875–6877; (n) Ranu, B. C.; Jana, R.;
Samanta, S. Adv. Synth. Catal. 2004, 346, 446–450; (o)
Ranu, B. C.; Das, A. Adv. Synth. Catal. 2005, 347, 712–
714; (p) Ranu, B. C.; Chattopadhyay, K.; Banerjee, S. J.
Org. Chem. 2006, 71, 423–425; (q) Ranu, B. C.; Mandal,
T. Tetrahedron Lett. 2006, 47, 2859–2861; (r) Ranu, B. C.;
Mandal, T. Tetrahedron Lett. 2006, 47, 5677–5680; (s)
Ranu, B. C.; Mandal, T. Tetrahedron Lett. 2006, 47, 6911–
6914; (t) Ranu, B. C.; Chattopadhyay, K.; Jana, R.
Tetrahedron Lett. 2007, 48, 3847–3850.
1-Allyloxy-4-but-3-enyl-benzene (Table 1, entry 18): Col-
orless liquid; IR (neat) 2923, 2855, 1510, 1240, 997, 916,
; d 2.35 (q,
825 cmÀ1 1H NMR (CDCl3, 300 MHz)
J = 7.59 Hz, 2H), 2.67 (t, J = 7.77 Hz, 2H), 4.53 (d,
J = 5.23 Hz, 2H), 4.97–5.08 (m, 2H), 5.26–5.45 (m, 2H),
5.82–5.91 (m, 1H), 6.02–6.10 (m, 1H), 6.85 (d,
J = 8.55 Hz, 2H), 7.11 (d, J = 8.49 Hz, 2H); 13C NMR
(CDCl3, 75 MHz) d 34.6, 35.9, 68.9, 114.7 (2C), 114.9,
117.6, 129.4 (2C), 133.6, 134.3, 138.3, 156.9. Anal. Calcd
for C13H16O: C, 82.94; H, 8.57. Found: C, 82.84; H, 8.43.
1-Allyloxy-4-(2-methyl-but-3-enyl)-benzene (Table 1,
entry 19): Colorless liquid; IR (neat) 3080, 2960, 2918,
2864, 1510, 1242, 1222, 1028, 997, 914, 833 cmÀ1 1H
;
NMR (CDCl3, 300 MHz) d 0.78 (d, J = 6.42 Hz, 3H),
2.17–2.27 (m, 1H), 2.39–2.45 (m, 2H), 4.31 (d,
J = 5.19 Hz, 2H), 4.70–4.77 (m, 2H), 5.05–5.24 (m, 2H),
5.53–5.62 (m, 1H), 5.81–5.88 (m, 1H), 6.63 (d,
J = 8.48 Hz, 2H), 6.85 (d, J = 8.50 Hz, 2H); 13C NMR
(CDCl3, 75 MHz) d 19.7, 39.8, 42.7, 69.3, 113.2, 114.8
(2C), 117.9, 130.5 (2C), 133.4, 133.9, 144.4, 157.2. Anal.
Calcd for C14H18O: C, 83.12; H, 8.97. Found: C, 83.01; H,
8.85.
7. Representative procedure for coupling of allylindium with 4-
methoxybenzyl bromide (Table 1, entry 8). 4-Methoxy-
benzyl bromide (201 mg, 1 mmol) in THF (2 mL) was
added to a well stirred mixture of allylindium reagent,
prepared in situ by the reaction of indium metal (115 mg,
1 mmol) and allyl bromide (182 mg, 1.5 mmol) in THF
(2 mL). The reaction mixture was stirred at room temper-
ature for 30 minutes until completion of the reaction
(TLC). The reaction mixture was quenched with brine,
extracted with diethyl ether (3 · 10 mL), and dried over
Na2SO4. Evaporation of the solvent gave a crude residue,
which was purified by short column chromatography over
silica gel to provide a colorless liquid (147 mg, 90%),
which was identified as 1-but-3-enyl-4-methoxybenzene by
2-(4-Allyloxy-benzyl)-but-3-enoic acid methyl ester (Table
1, entry 20): Colorless liquid; IR (neat) 2951, 2924, 2856,
1736, 1510, 1242, 1176, 1026, 997, 924 cmÀ1 1H NMR
;
(CDCl3, 300 MHz) d 2.75–3.06 (m, 2H), 3.25–3.36 (m,
1H), 3.63 (s, 3H), 4.50 (d, J = 5.26 Hz, 2H), 5.05–5.13 (m,
2H), 5.25–5.43 (m, 2H), 5.79–5.87 (m, 1H), 6.00–6.06 (m,
1H), 6.82 (d, J = 8.51 Hz, 2H), 7.06 (d, J = 8.50 Hz, 2H);
13C NMR (CDCl3, 75 MHz) d 37.7, 51.9, 52.4, 68.9, 114.7
(2C), 117.6, 117.7, 130.1 (2C), 130.9, 133.5, 135.5, 157.3,
173.9. Anal. Calcd for C15H18O3: C, 73.15; H, 7.37%.
Found: C, 73.01; H, 7.25.
1
comparison of its spectroscopic data (IR, H NMR, and
13C NMR) with those reported.15
This procedure was followed for the reactions listed in
Table 1. The products (entries 1–9, 11–14, 17, 23, and 24)
are known compounds and were identified by comparison
of their spectroscopic data (IR, 1H NMR, and 13C NMR)
with those reported (see references in Table 1). New
compounds (entries 10, 15, 16, and 18–22) were charac-
terized from their spectroscopic data and elemental
analysis. These data are presented below:
1-But-3-enyl-4-prop-2-ynyloxybenzene (Table 1, entry 21):
Colorless liquid; IR (neat) 3294, 3074, 2924, 2856, 1639,
1610, 1510, 1298, 1263, 1219, 1031, 914, 825 cmÀ1 1H
;
NMR (CDCl3, 300 MHz) d 2.39 (q, J = 7.37 Hz, 2H), 2.54
(t, J = 2.23 Hz, 1H), 2.70 (t, J = 6.40 Hz, 2H), 4.69 (d,
J = 2.23 Hz, 2H), 5.01–5.23 (m, 2H), 5.85–5.95 (m, 1H),
6.95 (d, J = 8.52 Hz, 2H), 7.16 (d, J = 8.48 Hz, 2H); 13C
NMR (CDCl3, 75 MHz) d 34.6, 35.8, 55.9, 75.4, 78.9, 114.9
(2C), 115.0, 129.3 (2C), 134.9, 138.2, 155.8. Anal. Calcd for
C13H14O: C, 83.83; H, 7.58. Found: C, 83.70; H, 7.41.
1-(2-Methyl-but-3 enyl)-4 prop-2-ynyloxybenzene (Table 1,
entry 22): Colorless liquid; IR (neat) 3296, 2963, 2920,
2864, 1610, 1510, 1217, 1032, 916, 835, 673 cmÀ1; 1H NMR
(CDCl3, 300 MHz) d 1.00 (d, J = 6.31 Hz, 3H), 2.38–2.46
(m, 1H), 2.51 (d, J = 7.00 Hz, 2H), 2.64 (t, J = 6.28 Hz,
1H), 4.68 (d, J = 2.15 Hz, 2H), 4.93–4.99 (m, 2H), 5.76–
5.87 (m, 1H), 6.91 (d, J = 8.45 Hz, 2H), 7.10 (d,
J = 8.48 Hz, 2H); 13C NMR (CDCl3, 75 MHz) d 19.7,
39.8, 42.7, 56.6, 75.7, 79.2, 113.2, 114.9 (2C), 130.5 (2C),
134.2, 144.4, 156.2. Anal. Calcd for C14H16O: C, 83.96; H,
8.05. Found: C, 83.79; H, 7.92.
2-(4-Methoxybenzyl)-but-3-enoic acid methyl ester (Table
1, entry 10): Colorless liquid; IR (neat) 2958, 2934, 1736,
1514, 1248, 1167, 1034, 822 cmÀ1 1H NMR (CDCl3,
;
300 MHz) d 2.38–2.67 (m, 2H), 3.11–3.18 (m, 1H), 3.62 (s,
3H), 3.65 (s, 3H), 5.11–5.16 (m, 2H), 5.70–5.85 (m, 1H),
6.76–6.86 (m, 4H); 13C NMR (CDCl3, 75 MHz) d 34.1,
48.6, 51.3, 51.9, 118.1, 122.9 (2C), 134.4 (2C), 134.5, 145.0,
166.4, 172.9. Anal. Calcd for C13H16O3: C, 70.89; H, 7.32.
Found: C, 70.77; H, 7.25.
1-But-3-enyl-3-nitrobenzene (Table 1, entry 15): Colorless
liquid; IR (neat) 3026, 2976, 1639, 1599, 1493, 1450, 1032,
1
912, 698 cmÀ1; H NMR (CDCl3, 300 MHz) d 2.26–2.34
(m, 2H), 2.60 (t, J = 7.5 Hz, 2H), 4.93–5.04 (m, 2H), 5.77–
5.85 (m, 1H), 7.49–7.60 (m, 1H), 7.71 (d, J = 7.8 Hz, 1H),
8.12 (d, J = 8.0 Hz, 1H), 8.24 (s, 1H); 13C NMR (CDCl3,
75 MHz) d 5.5, 36.2, 115.4, 123.6, 124.3, 130.3, 135.4,
138.3, 140.1, 147.8 cmÀ1; Anal. Calcd for C10H11NO2: C,
67.78; H, 6.26; N, 7.90. Found: C, 67.62; H, 6.15; N, 7.78.
1-But-3-enyl-4-benzyloxybenzene (Table 1, entry 16): Col-
orless liquid; IR (neat) 3030, 2962, 2916, 1510, 1454, 1242,
1026, 912, 734 cmÀ1; 1H NMR (CDCl3, 300 MHz) d 2.37–
2.45 (m, 2H), 2.72 (t, J = 7.78 Hz, 2H), 5.01–5.16 (m, 2H),
8. Clayden, J.; Greeves, N.; Warren, S.; Wothers, P. Organic
Chemistry; Oxford University Press, 2005, pp 1090–1100.
9. (a) Lee, P. H.; Seomoon, D.; Lee, K.; Kim, S.; Kim, H.;
Kim, H.; Shim, E.; Lee, M.; Lee, S.; Kim, M. Adv. Synth.
Catal. 2004, 346, 1641–1645; (b) Lee, K.; Lee, J.; Lee, P.
H. J. Org. Chem. 2002, 67, 8265–8268; (c) Lee, P. H.;
Sung, S.-Y.; Lee, K. Org. Lett. 2001, 3, 3201–3204.
10. Saito, T.; Nishimoto, Y.; Yashida, M.; Baba, A. J. Org.
Chem. 2006, 71, 8516–8522.

B. C. Ranu et al. / Tetrahedron Letters 48 (2007) 7374–7379
7379
11. Lehmkuhl, H.; Nehl, H. J. Organomet. Chem. 1973, 60, 1–
10.
17. Datta, S.; Chang, C.-L.; Yeh, K.-L.; Liu, R.-S. J. Am.
Chem. Soc. 2003, 125, 9294–9295.
12. Baron, W. J.; Henrick, M. E.; Jones, M., Jr. J. Am. Chem.
Soc. 1973, 95, 6286–6294.
18. Ikeda, Y.; Nakamura, T.; Yorimitsu, H.; Oshima, K. J.
Am. Chem. Soc. 2002, 124, 6514–6515.
13. Carruthers, W.; Coggins, P.; Weston, J. B. J. Chem. Soc.,
Perkin Trans. 1 1991, 611–616.
19. Hirao, T.; Fujii, T.; Ohshiro, Y. Tetrahedron Lett. 1994,
35, 8005–8008.
14. Johnson, D. K.; Ciavarri, J. P.; Ishmael, F. T.; Schillinger,
K. J.; Van Geel, T. A. P.; Stratton, S. M. Tetrahedron
Lett. 1995, 36, 8565–8568.
15. Uemura, M.; Yorimitsu, H.; Oshima, K. Chem. Commun.
2006, 4726–4728.
20. Elings, J. A.; Downing, R. S.; Sheldon, R. A. J. Org.
Chem. 1999, 64, 837–846.
21. Selvakumar, N.; Reddy, B. Y.; Kumar, S. G.; Iqbal, J.
Tetrahedron Lett. 2001, 42, 8395–8398.
22. Taber, D. F.; Wang, Y.; Pahutski, T. F., Jr. J. Org. Chem.
2000, 65, 3861–3863.
16. Hirao, T.; Fujii, T.; Ohshiro, Y. Tetrahedron Lett. 1994,
35, 8005–8008.