Total synthesis of (29S,37S)-isomer of malevamide E, a potent ion-channel inhibitor

Article abstract of DOI:10.1039/c2ob26533h

The first total synthesis of (29S,37S)-malevamide E (1), a potent ion channel inhibitor, has been achieved in a convergent fashion involving Julia-Kocienski olefination, Urpi acetal aldol and Shiina macrolactonization reactions as the key steps. The strategy developed herein is amenable for the synthesis of the other possible isomers in search for the correct stereoisomer of the naturally occurring molecule.

Full text of DOI:10.1039/c2ob26533h

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Tushar Kanti Chakraborty et al.
Total synthesis of (29S,37S)-isomer of malevamide E, a potent ion-channel inhibitor

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Total synthesis of (29S,37S)-isomer of malevamide E, a
potent ion-channel inhibitor†‡
Cite this: Org. Biomol. Chem., 2013, 11,
257
Praveen Kumar Gajula,a Shrikant Sharma,b Ravi Sankar Ampapathib and
Tushar Kanti Chakraborty*a
Received 3rd August 2012,
Accepted 11th September 2012
DOI: 10.1039/c2ob26533h
www.rsc.org/obc
The first total synthesis of (29S,37S)-malevamide E (1), a potent taking up a simple approach to make all possible diastereo-
ion channel inhibitor, has been achieved in a convergent fashion mers of the polyketide subunit. Herein, we report the first total
involving Julia–Kocienski olefination, Urpi acetal aldol and Shiina synthesis of (29S,37S)-isomer of malevamide E (1).
macrolactonization reactions as the key steps. The strategy devel-
Retrosynthetically (Scheme 1), (29S,37S)-malevamide E (1)
oped herein is amenable for the synthesis of the other possible can be obtained by macrolactonization of acyclic hydroxy acid
isomers in search for the correct stereoisomer of the naturally 3, which, in turn, could be prepared by coupling the two key
occurring molecule.
intermediates 4 and 5, the former with the long chain fatty
Introduction
Extracts of cyanobacteria belonging to the genus Symploca
(Oscillatoriaceae) have given rise to scores of biologically active
natural products with interesting structures.1 One such
example is a new depsipeptide, malevamide E, an N-methyl-
ated analogue of dolastatin 14, isolated from field-collected
colonies of the filamentous cyanobacterium Symploca laete-
Viridis.2 Dolastatin 14 was previously reported by Pettit and
co-workers3 in exceptionally low yield from D.auricularia.
Malevamide E demonstrated a dose-dependent (2–45 μM) inhi-
bition of store-operated Ca2+ entry in thapsigargin-treated
human embryonic kidney (HEK) cells indicating an inhibitory
effect on Ca2+ release-activated Ca2+ (CRAC) channels.
Although, there has been considerable interest in the total
synthesis of dolastatin 14, to date, neither the total synthesis
nor the stereochemical configuration of dolastatin 14 or malev-
amide E has been reported. All four diastereomers of its poly-
ketide-derived subunit, dolatrienoic acid were earlier
synthesized as part of the effort toward the full synthesis of
dolastatin 14 by two independent groups.4 Encouraged by the
properties and coupled with our interest in total synthesis of
marine natural products,5 we embarked on the challenge of
aCSIR-Central Drug Research Institute, Lucknow-226 001, India.
E-mail: charkraborty@cdri.res.in
bNMR Centre, SAIF, CSIR-Central Drug Research Institute, Lucknow-226 001, India
†CDRI communication no. 8314.
‡Electronic supplementary information (ESI) available: Experimental procedure
and characterisation data for all the new compounds. See DOI: 10.1039/
c2ob26533h
Scheme 1 Retrosynthetic analysis.
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acid moiety and the latter possessing the heptapeptide com-
ponent. The salient feature of our synthesis is the stereo-
selective construction of the aliphatic fragment involving two
key steps: (i) enantioselective addition of a titanium enolate
derived from a chiral thiazolidinethione which was expected to
fix the C29 stereocenter; and (ii) construction of C32–C33 bond
using Julia–Kocienski olefination of the subunits 6 and 7.
Scheme 3 Synthesis of sulphone 6. (i) (a) BH3 SMe2, THF, 0 °C, rt, 2 h, (b) H2O2,
NaOH, 80%; (ii) (a) PTSH, DIAD, PPh3, THF, rt, 30 min, (b) (NH4)6Mo7O24H2O,
H2O2, EtOH, rt, 15 min, 85%.
Results and discussion
Our synthetic endeavour began with the synthesis of aldehyde
7 (Scheme 2) which started with the preparation of 4-(tert-
butyldiphenylsiloxy)butanal dimethyl acetal 8 from butanediol
according to the reported procedure.6 An acetal aldol reaction
between the resultant dimethyl acetal 8 and (S)-N-acetyl-4-iso-
propyl-1,3-thiozolidine-2-thione (9), according to the con-
ditions described by Urpi,7 provided the aldol product 10 in
72% yield in 3 : 1 dr in favour of the required isomer. The dia-
stereomerically pure products could be easily separated using
simple column chromatography.8 Reductive cleavage of the
chiral auxiliary of the major isomer (the minor isomer can be
used for the synthesis of another diastereomer of the mole-
cule) afforded aldehyde, which was subjected to a Wittig–
Horner reaction with N-methyldiethylphosphoacetamide (11)
at −40 °C in presence of NaH to obtain the α,β-unsaturated
amide 12 in 78% yield exclusively as an E-isomer.9 The amide
12 was reduced with DIBAL-H to the corresponding aldehyde
followed by Wittig homologation with Ph3PvCH(CH3)CO2Et
in benzene under reflux conditions to provide the α,β-unsatu-
rated ester 13 in 80% yield. Desilylation of 13 with TBAF in
THF gave a primary alcohol 14 in 75% yield, which on oxi-
dation under Parikh–Doering conditions gave the aldehyde 7.
On the other hand, sulfone 6 was synthesized (Scheme 3)
starting from a known compound 15,4 which was prepared by
silyl protection of the corresponding alcohol. Hydroboration
and oxidation of the terminal olefin moiety of 15 afforded
primary alcohol 16 in 80% yield, which was further converted
into the desired sulfone 6 in 85% yield via Mitsunobu reaction
Scheme 4 Synthesis of compound 4. (i) LiHMDS, THF : HMPA, −78 °C, alde-
hyde 7, −78 °C, 3 h, 72%; (ii) TBAF : AcOH (1 : 1), H2O, DMF, 50 °C, 12 h, 85%;
(iii) LiOH, THF : MeOH : H2O (3 : 1 : 1), 0 °C, rt, 1 h, 74%.
using 1-phenyl-1H-tetrazol-5-thiol (PT-SH) and diisopropyl azo-
dicarboxylate (DIAD) and oxidation in one pot using catalytic
amounts of ammonium molybdate and H2O2.
Synthesis of compound 17, as depicted in Scheme 4, was
performed through crucial Julia–Kocienski olefination.10
Accordingly, sulfone 6 was treated with LiHMDS in dry THF :
HMPA (4 : 1) at −78 °C followed by slow addition of aldehyde
7, over a period of 10 min and the same temperature was
maintained for 3 h to furnish the desired E-olefinic compound
17 in 72% yield along with 7% of the Z-isomer. An attempt to
accomplish the deprotection of silyl group with TBAF in THF
was ineffective and no desired product was obtained even at
higher temperatures. Alternatively, desilylation of 17 using
TBAF and AcOH (1 : 1) in presence of H2O in DMF for 12 h
gave deprotected 18 in 85% yield.11 Saponification of 18 with
LiOH in THF : MeOH : H2O (3 : 1 : 1) at 0 °C afforded corre-
sponding hydroxy acid 4, which was used directly in the next
step without further characterization.
The synthesis of the peptide segment, depicted in
Scheme 5, started from the tetrapeptide 19, which was syn-
thesized from commercially available protected amino acids by
standard solution phase peptide synthesis conditions using
1-ethyl-3-(3-(dimethylamino)propyl)carbodiimide hydrochlo-
ride (EDCI) and 1-hydroxybenzotriazole hydrate (HOBt) as
coupling agents and dry CH2Cl2 as solvent. For synthesis of 24,
N-methylation of dipeptide 22 and tetrapeptide 19 was carried
out using silver oxide and MeI in DMF to provide the corre-
sponding N-methylated dipeptide 23 and tetrapeptide 20,
respectively. From the 2D-TOCSY and ROESY spectrum of 20,
at least 4 conformations were present in the NMR time scale
out of which conformations 1A, 1B and 2A, 2B showed
exchange peaks with each other (see ESI, Fig. 1‡). For 1A,1B
Scheme 2 Synthesis of compound 7. (i) 9, TiCl4, DIPEA, SnCl4, CH2Cl2, −78 °C,
1 h, 72%; (ii) (a) DIBAL, CH2Cl2, −78 °C, 15 min, 96%; (b) 11, NaH, THF,
−40 °C,1 h, 78%; (iii) (a) DIBAL, CH2Cl2, −78 °C, 15 min, 97%; (b) Ph3PvC(CH3)
COOEt, benzene, reflux, 3 h, 80%; (iv) TBAF, THF, 0 °C, rt, 1 h, 85%; (v) SO3·Py,
Et3N, DCM : DMSO (2 : 1), 0 °C, rt, 30 min, 96%.
4
3
strong nOes between Pro Hδ → Val Hα confirmed trans con-
figuration and absence of such nOes for 2A, 2B suggests cis
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In conclusion, we have developed a method for the total
syntheses of (29S,37S)-isomer of malevamide E (1) in a conver-
gent fashion involving Julia–Kocienski olefination, Urpi acetal
aldol and Shiina macrolactonization as the key steps. Our strat-
egy is amenable for the synthesis of other possible diastereo-
mers whose synthesis is under progress and will be reported
in due course.
Experimental section
Compound 25
To a solution of 18 (50 mg, 0.15 mmol) in THF : MeOH : H2O
(3 : 1 : 1, 3 mL) at 0 °C, LiOH·H2O (18.5 mg, 0.44 mmol) was
added and stirred from 0 °C to room temperature for 1 h. The
reaction mixture was then acidified to pH ∼ 2 with 1 N HCl. It was
diluted with EtOAc, washed with brine, dried (Na2SO4), filtered
and concentrated in vacuo to obtain the crude acid 4, which was
used directly in the next step without further characterization.
Scheme 5 Synthesis of peptide fragment and completion of the total syn-
thesis. (i) Ag2O, MeI, DMF, 0 °C, rt, 12 h, 72%; (ii) (a) LiOH, THF : MeOH : H2O
(3 : 1 : 1) 0 °C, rt, 1 h; (b) TFA salt of Boc-deprotected 23, BOP-Cl, DIPEA, CH2Cl2,
0 °C, rt, 12 h, 76%; (iii) Ag2O, MeI, DMF, 0 °C, rt, 12 h, 68%; (iv) (a) TFA, CH2Cl2,
0 °C, rt, 1 h; (b) Boc-D-Val-OH, BOP-Cl, DIPEA, CH2Cl2, 0 °C, rt, 12 h, 79%; (v) (a)
TFA, CH2Cl2, 0 °C, rt, 1 h; (b) 4, EDCI, HOBt, CH2Cl2, 0 °C, rt, 15 min, then added
amine from step (a), DIPEA, CH2Cl2, rt, 12 h, 75%; (vi) LiOH, THF : MeOH : H2O
(3 : 1 : 1), 0 °C, rt, 1 h, 96%; (vii) MNBA, DMAP, CH2Cl2, rt, 12 h, 62%.
In round bottom flask,
a solution of 24 (99.8 mg,
0.12 mmol) in CH2Cl2 (2 mL) was taken. To this solution, tri-
fluoroacetic acid (1 mL) was added at 0 °C and slowly warmed
to room temperature and stirred for 1 h. The reaction mixture
was then concentrated in vacuo and azeotroped with CH2Cl2 3
times (3 × 10 mL) to give the Boc-deprotected compound 5.
The above prepared acid 4 was dissolved in CH2Cl2 (3 mL)
and cooled to 0 °C. It was then sequentially treated with HOBt
(29.7 mg, 0.22 mmol) and EDCI (42.0 mg, 0.22 mmol). After
10 min, compound 5, prepared above and dissolved in CH2Cl2
(2 mL), was added to the reaction mixture followed by the
addition of DIPEA (0.08 mL, 0.44 mmol). After stirring for 12 h
at room temperature, the reaction mixture was diluted with
CHCl3, washed with saturated NH4Cl solution, 1 N HCl, satu-
rated NaHCO3 solution, water, brine, dried (Na2SO4), filtered
and concentrated in vacuo. Purification by column chromato-
graphy (silica gel, 2% to 3% MeOH in CHCl3 as eluent)
afforded 25 (100 mg, 75%) as a white solid. Rf = 0.3 (SiO2, 10%
MeOH in CHCl3); specific rotation [α]2D5 = −66.6 (c 0.15,
CHCl3); IR (neat): νmax 3404, 2361, 2111, 1637, 1445, 1218,
configuration around proline peptide bond. Other two confor-
mations may be attributed to the cis/trans isomerism of the
one of the tertiary amides of 20 as these classes of peptides are
prone to exhibit cis/trans conformations.12 Coupling of the
N-methylated tetrapeptide 20 with N-methylated dipeptide 23
using BOP-Cl gave the hexapeptide 21 in 76% yield.
Finally, Boc deprotection of 21 using TFA in CH2Cl2 fol-
lowed by coupling with Boc-D-Val-OH using BOP-Cl as a coup-
ling reagent gave the heptapeptide component 24. NMR
studies on hexapeptide 21 and heptapeptide 24 suggested
presence of at least 8 conformations and may be attributed
again to the cis/trans isomerisation of the tertiary amides (see
ESI, Fig. 2 and 3‡).
With both the halves of the molecule now ready to be
coupled, the heptapeptide 24 was treated with 30% TFA in
CH2Cl2 at 0 °C to effect Boc deprotection and coupled with the
hydroxy acid 4 following the EDCI/HOBt protocol to obtain the
coupled product 25 in 75% yield. The hydroxy ester 25 on
saponification with LiOH in THF : MeOH : H2O (3 : 1 : 1) at 0 °C
afforded corresponding hydroxy acid 3, which was used
directly in the next step without further characterization. The
stage was now set to carry out the crucial macrolactonization
reaction. Lactonization was successfully carried out using
Shiina anhydride protocol13 using 2-methyl-6-nitrobenzoic
anhydride (MNBA) in presence of DMAP in CH2Cl2 at room
temperature to afford the target molecule 1 in 62% yield.
1020, 769, 669 cm−1 1H NMR (400 MHz, CDCl3): δ see
;
Table 6;‡ 13C NMR (400 MHz, CDCl3): δ 173.09, 171.82, 170.66,
170.21, 170.04, 169.55, 169.14, 168.32, 137.62, 137.16, 133.89,
130.68, 130.24, 130.14, 129.98, 129.54, 129.04, 128.59, 127.89,
79.97, 68.08, 60.04, 58.73, 56.92, 56.73, 55.47, 54.97, 53.72,
52.44, 52.27, 49.85, 47.36, 39.04, 38.89, 37.32, 35.73, 33.73,
32.60, 32.08, 31.88, 30.97, 30.06, 29.81, 29.46, 28.60, 28.30,
27.06, 26.47, 25.94, 25.07, 23.63, 22.79, 19.95, 19.81, 19.52,
19.44, 19.23, 18.09, 16.98, 14.26, 12.98; MS (ESIMS): m/z (%):
1157 (100) [M + Na]+; HRMS (ESIMS): calcd for C61H99N8O12
[M + H]+: 1135.7282, found: 1135.7255.
1
NMR spectral data, both H and 13C, and specific rotation
of the synthetic 1 were entirely not in agreement with the data
reported for the isolated natural compounds. However, the
detailed 2D-NMR analysis of 1 showed similar chemical shifts
for the aliphatic region and deviated for certain residues of the
peptoid portion.14 From the assigned major conformation, it
is worth noting that chemical shifts of the Ala, Asn and Phe
match well with the natural product.
Compound 1
To a solution of 25 (55 mg, 0.05 mmol) in THF : MeOH : H2O
(3 : 1 : 1, 3 mL) at 0 °C, LiOH·H2O (6.11 mg, 0.14 mmol) was
added and stirred from 0 °C to room temperature for 1 h. The
reaction mixture was then acidified to pH ∼ 2 with 1 N HCl. It
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was diluted with EtOAc, washed with brine, dried (Na2SO4), fil-
tered and concentrated in vacuo. The hydroxy acid 3, thus
obtained, after flash chromatography (52 mg, 96%), was
directly used in the next step without further characterization.
To a stirred solution of 2-methyl-6-nitrobenzoic anhydride
(MNBA, 18.98 mg, 0.06 mmol) and DMAP (13.48 mg,
0.11 mmol) in CH2Cl2 (16 mL) was added a solution of
hydroxy acid 3 (52 mg, 0.05) in CH2Cl2 (10 mL) over a period of
12 h at room temperature using a syringe pump. Thereafter,
saturated NaHCO3 solution (20 mL) was added and the
aqueous layer was extracted with CHCl3 (3 × 50 mL). The com-
bined organic layers were dried with Na2SO4, filtered and con-
centrated in vacuo. Purification by column chromatography
(silica gel, 2% to 3% MeOH in CHCl3 as eluent) afforded 1
(31 mg, 62%) as a colorless oil. Rf = 0.3 (SiO2, 10% MeOH in
CHCl3); specific rotation [α]2D5 = −22.6 (c 0.09, CH2Cl2); IR (neat):
νmax 3703, 3572, 3332, 2926, 2567, 2361, 1635, 1445, 1219, 1096,
770 cm−1; 1H NMR (400 MHz, CDCl3): δ see Table 8 in ESI;‡ 13C
NMR (150 MHz, CDCl3): δ see Table 8 in the ESI;‡ MS (ESIMS):
m/z (%): 1103 (100) [M + H]+; HRMS (ESIMS): calcd for
C60H95N8O11 [M + H]+: 1103.7120, found: 1103.7085.
V. J. Paul, J. Nat. Prod., 2003, 66, 620–624; (i) P. G. Williams,
W. Y. Yoshida, R. E. Moore and V. J. Paul, J. Nat. Prod.,
2004, 67, 49–53; ( j) T. L. Simmons, K. L. McPhail,
E. Ortega-Barrá, S. L. Mooberry and W. H. Gerwick, Tetrahe-
dron Lett., 2006, 47, 3387–3390.
2 B. Adams, P. Pörzgen, E. Pittman, W. Y. Yoshida,
H. E. Westenburg and F. D. Horgen, J. Nat. Prod., 2008, 71,
750–754.
3 G. R. Pettit, Y. Kamano, C. L. Herald, C. Dufresne,
R. B. Bates, J. M. Schmidt, R. L. Cerny and H. Kizu, J. Org.
Chem., 1990, 55, 2989–2990.
4 (a) S. Mouné, G. Niel, M. Busquet, I. Eggleston and P. Jouin,
J. Org. Chem., 1997, 62, 3332–3339; (b) J. J. Duffield and
G. R. Pettit, J. Nat. Prod., 2001, 64, 472–479.
5 (a) T. K. Chakraborty, V. R. Reddy and P. K. Gajula, Tetra-
hedron, 2008, 64, 5162–5167; (b) T. K. Chakraborty,
V. R. Reddy and A. K. Chattopadhyay, Tetrahedron Lett.,
2006, 47, 7435–7438; (c) T. K. Chakraborty and V. R.
Reddy, Tetrahedron Lett., 2006, 47, 2099–2102;
(d) T. K. Chakraborty and S. Das, Tetrahedron Lett., 2001,
42, 3387–3390; (e) T. K. Chakraborty and S. Das, Chem.
Lett., 2000, 80–81; (f) T. K. Chakraborty, S. Ghosh and
S. Dutta, Tetrahedron Lett., 2001, 42, 5085–5088;
(g) T. K. Chakraborty, S. Ghosh, P. Laxman, S. Dutta and
R. Samanta, Tetrahedron Lett., 2005, 46, 5447–5550;
(h) T. K. Chakraborty and V. R. Suresh, Tetrahedron Lett.,
1998, 39, 9109–9112; (i) T. K. Chakraborty and V. R.
Suresh, Tetrahedron Lett., 1998, 39, 7775–7778;
( j) T. K. Chakraborty and D. Thippeswamy, Synlett, 1999,
150–152; (k) T. K. Chakraborty, D. Thippe swamy,
V. R. Suresh and S. Jayaprakash, Chem. Lett., 1997, 563.
6 H. Okada, T. Mori, Y. Saikawa and M. Nakata, Tetrahedron
Lett., 2009, 50, 1276–1278.
Acknowledgements
The authors wish to thank the UGC (P.K.G.) and CSIR, New
Delhi (S.S.) for research fellowships. P.K.G thanks Dr Dipankar
Koley for his support. The authors are thankful to SAIF,
CSIR-CDRI for providing the spectroscopic and analytical data
and also to Prof Wesley Yoshida, University of Hawaii, Manoa
for providing the proton spectrum of malevamide E.
7 (a) A. Cosp, E. Llacer, P. Romea and F. Urpí, Tetrahedron
Lett., 2006, 47, 5819–5823; (b) A. Cosp, P. Romea, F. Urpí
and J. Vilarrasa, Tetrahedron Lett., 2001, 42, 4629–4631.
8 To prove the stereochemistry of adduct, compound 10 was
converted to a known compound (+)-(3S)-1,6-Dihydroxy-3-
methoxy-hexane, the spectral data of which was matched with
the reported data. For reference see: P. Ackermann, H. Tobler
and C. Ganter, Helv. Chim. Acta, 1972, 55, 2731–2743.
9 T. J. Heckrodt and J. Mulzer, J. Am. Chem. Soc., 2003, 125,
4680–4681.
Notes and references
1 (a) G. G. Harrigan, H. Luesch, W. Y. Yoshida, R. E. Moore,
D. G. Nagle, V. J. Paul, S. L. Mooberry, T. H. Corbett and
F. A. Valeriote, J. Nat. Prod., 1998, 61, 1075–1077;
(b) G. G. Harrigan, H. Luesch, W. Y. Yoshida, R. E. Moore,
D. G. Nagle and V. J. Paul, J. Nat. Prod., 1999, 62, 655–658;
(c) H. Luesch, W. Y. Yoshida, R. E. Moore, V. J. Paul,
S. L. Mooberry and T. Corbett, J. Nat. Prod., 2002, 65, 16–20;
(d) F. D. Horgen, W. Y. Yoshida and P. J. Scheuer, J. Nat.
Prod., 2000, 63, 461–467; (e) F. D. Horgen, E. B. Kazmierski, 10 J. Liu, K. Xu, J. He, L. Zhang, X. Pan and X. She, J. Org.
H. E. Westenburg, W. Y. Yoshida and P. J. Scheuer, J. Nat. Chem., 2009, 74, 5063–5066.
Prod., 2002, 65, 487–491 The malevamides are named after 11 H. Higashibayashi, K. Shinko, T. Ishizu, K. Hashimoto,
the Hawaiian word “malewa” (ma lé va), which means “to H. Shirahama and M. Nakata, Synlett., 2000, 1306–1308.
sway”. M. K. Pukui and S. H. Elbert, Hawaiian Dictionary, 12 Q. Sui, D. Borchardt and D. L. Rabenstein, J. Am. Chem.
Revised and Enlarged Edition, University of Hawaii Press, Soc., 2007, 129, 12042–12048.
Honolulu, 1986, p. 232, referring to the graceful movement 13 (a) I. Shiina, M. Kubota, H. Oshiumi and M. Hashizume,
of the filamentous Symploca colonies caused by the ever-
constant ocean swells; (f) P. G. Williams, W. Y. Yoshida, R.
E. Moore and V. J. Paul, J. Nat. Prod., 2002, 65, 1336–1339;
(g) P. G. Williams, W. Y. Yoshida, R. E. Moore and
V. J. Paul, J. Nat. Prod., 2003, 66, 1006–1009;
J. Org. Chem., 2004, 69, 1822–1830; (b) I. Shiina,
M. Hashizume, Y. S. Yamai, H. Oshiumi, T. Shimazaki,
Y. J. Takasuna and R. Ibuka, Chem.–Eur. J., 2005, 11, 6601–
6608; (c) For a review, see: .I. Shiina,Chem. Rev., 2007, 107,
239–273.
(h) P. G. Williams, W. Y. Yoshida, R. E. Moore and 14 For δ values see ESI Table 8.‡
260 | Org. Biomol. Chem., 2013, 11, 257–260
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