Directing the regioselectivity of rhodium(I) catalysed cyclisation of 2-alkynyl benzoic acids

Article abstract of DOI:10.1016/j.poly.2013.06.010

Rhodium(I) dicarbonyl complexes 1-4 containing chelating N-donor ligands bis(pyrazolyl)methane (bpm), bis(imidazolyl)methane (bim), tris(pyrazolyl) toluidine (tpt) or tris(imidazolyl)methanol (tim) were investigated as catalysts for the hydroalkoxylation of alkynyl benzoic acids (5a-g). The regioselectivity of the reaction was shown to be highly dependent on the nature of the terminal alkyne substituent (R) of the alkynol substrate. It was also determined that the presence of a third uncoordinated N-donor group in complexes 3 and 4 suppressed the catalytic efficiency of these complexes, and that the selectivity of the reaction for forming either endocyclic (6) or exocyclic (7) hydroalkoxylation products was influenced by the pendant hydroxyl group present in complex 4. We used ¹³C NMR spectroscopy to quantify the polarity of the alkynyl benzoic acid C≡C bond and our efforts to correlate this measure of bond polarity to the observed regioselectivity of the reaction are discussed.

Full text of DOI:10.1016/j.poly.2013.06.010

Polyhedron 61 (2013) 248–252
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Directing the regioselectivity of rhodium(I) catalysed cyclisation
of 2-alkynyl benzoic acids
⇑
Bradley Y.-W. Man, Astrid Knuhtsen, Michael J. Page, Barbara A. Messerle
School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
Rhodium(I) dicarbonyl complexes 1–4 containing chelating N-donor ligands bis(pyrazolyl)methane
(bpm), bis(imidazolyl)methane (bim), tris(pyrazolyl)toluidine (tpt) or tris(imidazolyl)methanol (tim)
were investigated as catalysts for the hydroalkoxylation of alkynyl benzoic acids (5a–g). The regioselec-
tivity of the reaction was shown to be highly dependent on the nature of the terminal alkyne substituent
(R) of the alkynol substrate. It was also determined that the presence of a third uncoordinated N-donor
group in complexes 3 and 4 suppressed the catalytic efficiency of these complexes, and that the selectiv-
ity of the reaction for forming either endocyclic (6) or exocyclic (7) hydroalkoxylation products was influ-
enced by the pendant hydroxyl group present in complex 4. We used 13C NMR spectroscopy to quantify
the polarity of the alkynyl benzoic acid C„C bond and our efforts to correlate this measure of bond polar-
ity to the observed regioselectivity of the reaction are discussed.
Received 16 May 2013
Accepted 9 June 2013
Available online 20 June 2013
Keywords:
Rhodium
Hydroalkoxylation
Catalysis
Tris(imidazolyl)methanol
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
and 4 containing the ligands tris(pyrazolyl)toluidine (tpt) and
tris(imidazolyl)methanol (tim), respectively, where the ligand
Phthalide [1] and isocoumarin [2] compounds that contain a
lactone functional group fused with a phenyl ring (Scheme 1) are
an important class of natural products prevalent in a wide range
of biologically active pharmaceutical, antifungal and pesticidal
agents. The cyclisation of 2-alkynyl benzoic acids represents an
important and atom efficient approach to the synthesis of both
groups of compounds depending on the regioselectivity of the
reaction. A number of different catalysts are known to facilitate
this transformation, including late transition metal complexes of
Pd [3], Cu [4], Ag [5], Au [6] and Ir [7], weak bases such as DBU,
Et3N and KOAc, and strong acids such as H2SO4 and CF3SO3H [8].
Unfortunately, for a majority of these systems a mixture of regioi-
somers is obtained decreasing the overall yield of desired product
and leading to expensive separation procedures. Therefore the
development of catalysts that can selectively yield one regioisomer
in preference to the other is a highly sought after goal.
We have previously demonstrated that rhodium(I) complexes
containing bidentate nitrogen donor ligands are highly effective
catalysts for the intramolecular cyclisation of aliphatic alkynoic
acids [9] and other related C–O bond forming reactions [10]. Some
of the most active catalysts for these transformations are dicar-
bonyl Rh(I) complexes 1 and 2 containing the ligands bis(pyrazol-
yl)methane (bpm) and bis(imidazolyl)methane (bim), respectively.
More recently we reported the analogous rhodium(I) complexes 3
coordinates in a bidentate j
2-fashion through only two of the
N-donor groups [11]. The presence of a pendant coordinating
group, as is found in 3 and 4, has been shown in many examples
to strongly influence the catalytic properties of a complex. For
example dicarbonyl rhodium(I) complexes containing j
2-tris(pyr-
azolyl) donor ligands are far more active catalysts for the hydrofor-
mylation of alkenes compared to related complexes containing
bis(pyrazolyl) chelates [12]. Cavell and co-workers also found that
donor functionalised bis(imidazolyl) chelates gave palladium(II)
Heck reaction catalysts [13] and chromium(III) ethylene oligomer-
ization catalysts [14] of various activities depending on the nature
of the pendant coordinating group. The pendant hydroxyl group in
complex 4 also has the potential to participate in metal binding
[15] or intermolecular hydrogen bonding [16] interactions which
may similarly influence the reactivity of this complex.
We report here the activity of complexes 1–4 as catalysts for the
intramolecular cyclisation of 2-alkynyl benzoic acids to yield
phthalide and isocoumarin products. We were particularly inter-
ested in whether the presence of the pendant N-donor or hydroxyl
groups in complexes 3 and 4 would have an impact on the effi-
ciency and regioselectivity of this reaction. The identity of the ter-
minal alkyne substituent (R) also has a significant impact on the
reaction selectivity [3c,4,6]. To understand this influence we have
used 13C NMR spectroscopy to determine the C„C bond polarity
for a variety of alkyne substrates. Our efforts to correlate this
measure of bond polarity with the observed regioselectivity of
the reaction are also reported.
⇑
Corresponding author. Tel.: +61 293854653.
E-mail address: b.messerle@unsw.edu.au (B.A. Messerle).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.06.010

B.Y.-W. Man et al. / Polyhedron 61 (2013) 248–252
249
X
X
N
N
N
N
X
X
H2N
N
Rh
N
N
HO
N
OC
CO
N
N
1
N
N
N
N
N
N
N
N
Rh
Rh
N
N
OC
CO
OC
CO
Rh
3
4
OC
CO
X = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BArF)
2
phthalide as the sole products. Fig. 1 shows an NMR stack plot fol-
lowing the cyclisation of 5a catalysed by the Rh(I) complex with a
bidentate ligand, 2. The reaction was complete in less than 1.3 h for
all substrates and catalysts (Table 1), with the exception of the cyc-
lisation of the phenyl substituted alkyne (5b) catalysed by 4 which
took 6 h. Such reaction efficiency is comparable to the most active
catalysts reported for this transformation [3–7].
Several clear trends can be seen by comparing the reactivity of
catalysts 1–4 for the cyclisation of substrates 5a–c (Table 1).
Firstly, for all catalysts the regioselectivity of the reaction is
strongly dependent on the alkyne substituent R. For terminal al-
kynes the exocyclic phthalide product 7a is favoured (entries 1
and 2), however for phenyl and pentyl substituted alkynes the
2. Results and discussion
2.1. Cyclisation of terminal, aryl and alkyl substituted alkynyl benzoic
acids (5a–c)
We investigated the catalysed cyclisation of 2-(ethynyl)benzoic
acid (5a, R = H), 2-(phenylethynyl)benzoic acid (5b, R = Ph) and 2-
(1-heptynyl)benzoic acid (5c, R = C5H11) to form the corresponding
isocoumarin (6a–c) and phthalide (7a–c) products using com-
plexes 1–4 as catalysts (Table 1). The reactions were carried out
using 1 mol% of catalyst at 60 °C in C6D6 and the reaction progress
was monitored by 1H NMR spectroscopy. The reaction was ob-
served to proceed very cleanly to yield the isocoumarin and
R
R
R
cat.
O
cat.
H
O
O
O
O
O
isocoumarin
phthalide
Scheme 1. Isocoumarin and phthalide regioisomers formed upon the catalysed cyclisation of 2-alkynyl benzoic acids.
Table 1
Reaction time and product ratio for the hydroalkoxylation of 5a–c using 1 mol% of catalyst 1–4 at 60 °C in C6D6.
R
R
R
1 mol% cat.
O
+
OH
O
C6D6, 60 oC
O
O
)
O
)
5a
6a
7a
)
R= H (
Ph(
R= H (
Ph(
R= H (
7b
Ph( )
5b
C5H11
6b
)
(
)
5c
6c
7c
C5H11 ( )
)
C5H11
(
)
Entry
Catalyst
R
Reaction time/ha
Product ratio (6:7)
1
2
3
4
5
6
7
8
2
4
2
4
2
4
1
3
H
H
Ph
Ph
C5H11
C5H11
Ph
0.42
0.67
0.20
6.00
0.27
1.28
0.15
1.25
0.31:1
0.01:1
1:0.11
1:0.50
1:0.00
1:0.00
1:0.04
1:0.05
Ph
a
Time at >99% conversion.

250
B.Y.-W. Man et al. / Polyhedron 61 (2013) 248–252
Fig. 1. 1H NMR spectra for the cyclisation of 5a catalysed by 2 in C6D6 showing clean formation of the isocoumarin (6a) and phthalide (7a) reaction products.
O
O
O
δ
O
δ
O
R
R
R
TS-endoa
Rh
[
]
6
O
or
O
O
R
[Rh]
O
O
δ
R
A
δ
[
]
Rh
7
TS-exo
Scheme 2. Proposed mechanism for the Rh(I) catalysed hydroalkoxylation of 2-alkynyl benzoic acids.
higher proportion of the exo-cyclic product 7b is also obtained
(6:7 = 1:0.5, entry 4). These results suggest that the structure of
the tris(imidazolyl)methanol (tim) ligand in catalyst 4 has a direct-
ing influence on the reaction which favours formation of the exo-
cyclic products. This trend does not extend to substrate 5c
(R = C5H11) where both catalysts yield exclusively the endocyclic
product 6c. To confirm that the difference in product distribution
was not a result of interconversion of 6–7 over the longer time
frames required with catalyst 4 we monitored the reactions for
several hours after completion. No change in the product ratios
was observed.
Table 2
13C NMR chemical shifts (d) of the methyl esters 8a–c.
R
13C chemical shift (d)/ppm
R
C2
C1
C2
D
d (C1–C2)
.
C1
O
H
Ph
C5H11
82.4
88.3
79.2
82.1
94.4
96.1
0.3
OMe
R=
8a
)
H (
À6.1
Ph(8b)
À16.9
C5H11 (8c)
endocyclic isocoumarin products 6b and 6c are favoured (entries
3–8). Comparing the results for catalyst 2 containing the bidentate
bis(imidazolyl)methane ligand, and catalyst 4 containing the tri-
dentate tris(imidazolyl)methanol ligand, we see a large difference
in the proportion of minor product formed. For example, with cat-
alyst 2 when R = H both exo- and endo-cyclic products are formed
in significant yield, in a ratio of approx. 1:3 (6:7, entry 1), and for
R = Ph the endo-cyclic product is clearly favoured, at a ratio of
6:7 ꢀ 10:1 (entry 3). However when catalyst 4 is used cyclisation
of the terminal alkyne (R = H) yields almost exclusively the exo-
cyclic product 7a (6:7 = 0.01:1, entry 2), and for R = Ph a much
In contrast to this, catalyst 1 containing the bidentate bis(pyraz-
olyl)methane ligand and catalyst 3 containing the tridentate
tris(pyrazolyl)toluidine ligand both give similar product ratios for
the cyclisation of 5b (R = Ph, entries 7 and 8). These results are also
similar to that obtained with catalyst 2 (entry 3) containing the
bidentate bis(imidazolyl)methane ligand and all three species can
be assumed to react in a similar manner. Catalyst 4 is unique in
that the ligand contains a protic OH group located proximal to
the reactive metal centre. For catalyst 4 the directing influence of
the tris(imidazolyl)methanol (tim) ligand is most likely due to its
pendant hydroxyl group, which has the potential to form hydro-

B.Y.-W. Man et al. / Polyhedron 61 (2013) 248–252
251
Table 3
Correlation of the Hammet constant (
using catalyst 2.
r
) for para-substituents (X), alkyne 13C NMR chemical shifts (d) for 8d–g and the reaction time and product ratios for the cyclisation of 5d–g
X
X
X
X
C2
C1
2
1 mol%
+
O
OH
O
OMe
O
C6D6, 60 oC
O
O
O
X= OMe (5d)
X= OMe (6d)
X= OMe (7d)
Me (7e)
X= OMe (8d)
Me (5e)
Me (6e)
Me (8e)
Cl (5f)
Cl (6f)
Cl (7f)
Cl (8f)
5g
6g
7g
C(O)Me ( )
C(O)Me (
)
C(O)Me (
)
8g
C(O)Me (
)
Entry
X
r
13C chemical shift (d)/ppm
Reaction time/ha
Product ratio (6:7)
C1
C2
D
d (C1–C2)
1
2
3
4
5
OMe
Me
H
Cl
C(O)Me
À0.268
À0.170
0
0.227
0.500
86.9
87.7
88.3
89.3
91.5
94.5
94.7
94.4
92.9
93.3
À7.6
À7.0
À6.1
À3.6
À1.8
3.67b
5.68b
0.20
2.13
0.25
1:0.12
1:0.00
1:0.11
1:0.09
1:0.18
a
Time at >99% conversion.
Maximum conversion of 90% achieved.
b
gen-bonding interactions with the carboxylic acid moiety of the
substrate. Further work is ongoing to understand this effect.
At all times the reactions catalysed by complexes 3 and 4
containing tridentate ligands were found to be slower than the
reactions catalysed by the bidentate ligand complexes 1 and 2. This
is perhaps unsurprising as the third N-donor in 3 and 4 is likely to
inhibit coordination of the substrate via steric shielding of the me-
tal centre and/or competition for binding to vacant metal coordina-
tion sites.
the nucleophilic oxygen leading to formation of the endocyclic
isocoumarin product 6c, which is in fact the exclusive product ob-
served experimentally (Table 1, entry 5). This preference is reduced
for 5b (
D
d(8b) = À6.1 ppm), with formation of a small proportion
of the exocyclic product 7b (ca. 10%, Table 1, entry 3). Finally, for
the terminal alkyne 5a (R = H) the bond polarity is reversed
(Dd(8a) = 0.25 ppm) and the reaction now favours formation of
the exocyclic product 7a, although as the magnitude of the C„C
polarisation is less pronounced a greater proportion of the minor
regioisomer 6a (ca. 25%) is also formed (Table 1, entry 1).
2.2. Mechanistic considerations
The first step in the transition metal catalysed cyclisation of
alkynyl benzoic acids is the coordination of the alkyne moiety to
an electrophilic metal centre (A), which activates the alkyne to-
wards nucleophilic attack by the carboxylate group (Scheme 2)
[6,10a,17]. The regioselectivity of the reaction then depends on
which transition state (TS-exo or TS-endo, Scheme 2) is most sta-
ble, which is strongly influenced by the initial polarity of the
C„C bond. We were interested in understanding how the nature
of the alkyne substituent R affects the C„C bond polarity and
thereby which regioisomer is preferred from this reaction. A
well-known measure of the relative electron density at a particular
carbon atom is the 13C NMR chemical shift [18], where for an al-
kyne bonded carbon pair the higher the observed chemical shift,
the less shielded the atom nucleus and the less electron density
about the carbon atom. In order to minimise the impact of hydro-
gen bonding on determining the electronic structure of the alkyne
we acquired the 13C NMR spectra of the methyl esters 8a–c rather
than their parent acids. The 13C NMR chemical shift (d) of the al-
kyne carbons C1 and C2 of 8a–c are reported in Table 2, along with
2.3. Tuning of C„C bond polarity
We were interested in establishing whether we could fine tune
the regioselectivity of the cyclisation reaction by varying the elec-
tronic properties of the R substituent of the substrate and hence
the polarity of the C„C bond. We therefore investigated the hydro-
alkoxylation of the 2-(phenylethynyl)benzoic acids 5d–g, contain-
ing
a series of electron donating or electron withdrawing
substituents (X) in the para-position of the terminal phenyl ring
(Table 3). These results are associated with the alkyne 13C NMR
chemical shifts of the corresponding methyl esters 8d–g and a
Hammet constant (
we can see that as the electron withdrawing nature of the phenyl
substituent increases (i.e. increasing ) the polarity of the alkyne
bond decreases (smaller d). We would therefore expect a corre-
r) for the para-substituent [19]. Satisfyingly,
r
D
sponding increase in the proportion of exocyclic product 7d–g
formed from the cyclisation of the alkynoic acids 5d–g. Unfortu-
nately we see no correlation between alkyne bond polarity and
product ratio (6:7). This suggests that the relatively small change
in bond polarity observed here for the group of substrates 5d–g
(which is much less than that seen for the group of substrates
5a–c) is insufficient to direct the regioselectivity of the reaction.
Surprisingly, variation of the para-substituent (X) had a large im-
pact on the reaction rate. Particularly slow rates were observed
for the cyclisation of 5d and 5e containing the electron donating
OMe and Me substituents, respectively, however a clear correlation
between alkyne bond polarity and reaction rate was not observed.
the difference in chemical shift (
the bond polarisation.
D
d) which gives an indication of
A good correlation between the sign and magnitude of
D
d and
the observed regioselectivity of the cyclisation reaction catalysed
by 2 was observed. For example, the polarisation is greatest for
8c (R = C5H11
,
D
d = À16.9 ppm) with the d+ charge residing on the
C2 carbon which has the higher chemical shift. For substrate 5c
(R = C5H11) the large d+ charge on C2 would favour its attack by

252
B.Y.-W. Man et al. / Polyhedron 61 (2013) 248–252
3. Conclusions
Appendix A. Supplementary data
We have shown that the rhodium(I) compounds 1–4 are effec-
tive catalysts for the hydroalkoxylation of a variety of alkynyl ben-
zoic acids. The presence of a pendant N-donor group in complexes
3 and 4 led to a decrease in catalyst activity relative to complexes 1
and 2. Interestingly, the pendant hydroxyl group in complex 4
appeared to have a significant impact on the regioselectivity of
the catalyst leading to an increased preference for forming the
exocyclic phthalide products (7a,b). Overall, the regioselectivity
of these reactions was dominated by the nature of the terminal
alkyne substituent (R). The use of 13C NMR spectroscopy provided
an accurate description of alkyne bond polarity, however, no corre-
lation was observed between this measurement and the observed
regioselectivity or rate of the reaction.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2013.06.010.
References
[1] (a) T. Tsuchida, M. Kobayashi, K. Kaneko, H. Mitsuhashi, Chem. Pharm. Bull. 35
(1987) 4460;
(b) P.M. Hon, C.M. Lee, T.F. Choang, K.Y. Chui, H.N.C. Wong, Phytochemistry 29
(1990) 1189;
(c) T. Naito, T. Katsuhara, K. Niitsh, Y. Ikeya, M. Okada, H. Mitsuhashi,
Phytochemistry 31 (1992) 639.
[2] (a) R.D. Barry, Chem. Rev. 64 (1964) 229;
(b) S. Ohta, Y. Kamata, T. Inagaki, Y. Masuda, S. Yamamoto, M. Yamashita, I.
Kawasaki, Chem. Pharm. Bull. 41 (1993) 1188.
[3] (a) D. Villemin, D. Goussu, Heterocycles 29 (1989) 1255;
(b) H.Y. Liao, C.-H. Cheng, J. Org. Chem. 60 (1995) 3711;
(c) N.G. Kundu, M. Pal, B. Nandi, J. Chem. Soc., Perkin Trans. 1 (1998) 561.
[4] S. Inack-Ngi, R. Rahmani, L. Commeiras, G. Chouraqui, J. Thibonnet, A. Duchene,
M. Abarbri, J.-L. Parrain, Adv. Synth. Catal. 351 (2009) 779.
[5] F. Bellina, D. Ciucci, P. Vergamini, R. Rossi, Tetrahedron 56 (2000) 2533.
[6] E. Marchal, P. Uriac, B. Legouin, L. Toupet, P. van de Weghe, Tetrahedron 63
(2007) 9979.
[7] X. Li, A.R. Chianese, T. Vogel, R.H. Crabtree, Org. Lett. 7 (2005) 5437.
[8] (a) M. Uchiyama, H. Ozawa, K. Takuma, Y. Matsumoto, M. Yonehara, K. Hiroya,
T. Sakamoto, Org. Lett. 8 (2006) 5517;
(b) C. Kanazawa, M. Terada, Tetrahedron Lett. 48 (2007) 933.
[9] (a) S. Elgafi, L.D. Field, B.A. Messerle, J. Organomet. Chem. 607 (2000) 97;
(b) J.H.H. Ho, D.S. Black, B.A. Messerle, J.K. Clegg, P. Turner, Organometallics 25
(2006) 5800.
[10] (a) B.A. Messerle, K.Q. Vuong, Organometallics 26 (2007) 3031;
(b) J.H.H. Ho, R. Hodgson, J. Wagler, B.A. Messerle, Dalton Trans. (2010) 4062;
(c) J.H.H. Ho, S. Choy, S.A. Macgregor, B.A. Messerle, Organometallics 30 (2011)
5978.
[11] B.Y.-W. Man, M. Bhadbhade, B.A. Messerle, New J. Chem. 35 (2011) 1730.
[12] W. Klaui, D. Schramm, G. Schramm, Inorg. Chim. Acta 357 (2004) 1642.
[13] T. Rüther, M.C. Done, K.J. Cavell, E.J. Peacock, B.W. Skelton, A.H. White,
Organometallics 20 (2001) 5522.
[14] T. Rüther, N. Braussaud, K.J. Cavell, Organometallics 20 (2001) 1247.
[15] A.J. Canty, J.M. Patrick, A.H. White, J. Chem. Soc., Dalton Trans. (1983) 1873.
[16] L. Benisvy, S. Halut, B. Donnadieu, J.-P. Tuchagues, J.-C. Chottard, Y. Li, Inorg.
Chem. 45 (2006) 2403.
[17] T. Fjermestad, J.H.H. Ho, S.A. Macgregor, B.A. Messerle, D. Tuna,
Organometallics 30 (2011) 618.
4. Experimental
We have previously reported the preparation of the rhodium
complexes 1–4. The alkynoic acids (5a–g) and esters (8a–g) were
prepared following modified literature procedures [6,20]. The
experimental method and spectroscopic data for compounds 5c,
5g and 8g that have not been reported previously are included in
the Supporting information. Characterisation of the cyclised
isocoumarin (6a–f) [6,21] and phthalide (7a–f) [6,22] products
were identified by comparison of their 1H NMR spectroscopic data
with reported literature values where available. The 1H and 13C
NMR spectral assignments for the new compounds 6g and 7g are
included in the Supporting information.
4.1. Generalised procedure for metal catalysed hydroalkoxylation
reactions
Catalysed hydroalkoxylation reactions were conducted on a
small scale in NMR tubes fitted with a concentric Teflon valve.
Deuterated benzene was dried over Na/benzophenone, distilled
into the NMR tube containing the catalyst precursor (complexes
1–4, 1 mol%) and substrate (ca. 25 mg), and the sample heated to
60 °C inside the NMR spectrometer. The temperature in the NMR
spectrometer was calibrated using neat ethylene glycol and a
K-type thermocouple. The progress of the reaction was monitored
by 1H NMR spectroscopy by comparing the integration of starting
material and product proton resonances. The identity of the prod-
uct was confirmed by comparison with the literature and/or char-
acterisation using standard 2D NMR spectroscopic techniques.
[18] (a) H. Spiesecke, W.G. Schneider, Tetrahedron Lett. 14 (1961) 468;
(b) K. Izawa, T. Okuyama, T. Fueno, Bull. Chem. Soc. Jpn. 46 (1973) 2881;
(c) A.J. Amass, P.E. Brough, M.E. Colclough, I.M. Philbin, M.C. Perry, Des.
Monomers Polym. 7 (2004) 413.
[19] The Hammet constant provides a measure of the electronic nature of the
substituent; a negative
r value implies the substituent is electron donating,
while a positive value implies the substituent is electron withdrawing C.
Hansch, A. Leo, Substituent Constants for Correlation Analysis in Chemistry
and Biology, Wiley-Interscience, NY, 1979.
[20] C.J. Moody, P. Shah, P. Knowles, J. Chem. Soc., Perkin Trans. 1 (1988) 3249.
[21] (a) R. Mancuso, S. Mehta, B. Gabriele, G. Salerno, W.S. Jenks, R.C. Larock, J. Org.
Chem. 75 (2010) 897;
(b) M. Hellal, J.-J. Bourguignon, F.J.-J. Bihel, Tetrahedron Lett. 49 (2008) 62;
(c) V. Subramanian, V.R. Batchu, D. Barange, M. Pal, J. Org. Chem. 70 (2005)
4778.
Acknowledgements
[22] (a) R. Rossi, A. Carpita, F. Bellina, P. Stabile, L. Mannina, Tetrahedron 59 (2003)
2067;
We gratefully acknowledge financial support from the Univer-
sity of New South Wales, the Australian Research Council, and
the Australian government for an Australian Postgraduate Award
(B.Y.-W.M.).
(b) H. Yang, G.-Y. Hu, J. Chen, Y. Wang, Z.-H. Wang, Bioorg. Med. Chem. Lett. 17
(2007) 5210.