One-pot synthesis of 2-(dicyanomethylene)-1,2-dihydropyridine derivatives

Article abstract of DOI:10.1016/j.tetlet.2014.03.056

The synthesis of 2-(dicyanomethylene)-1,2-dihydropyridine derivatives from the reactions of arylmethylidene derivatives of malononitrile dimers with 1,3-dicarbonyl compounds is described.

Full text of DOI:10.1016/j.tetlet.2014.03.056

Tetrahedron Letters xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
One-pot synthesis of 2-(dicyanomethylene)-1,2-dihydropyridine
derivatives
a
a
a
a
Ivan N. Bardasov a, , Anastasiya U. Alekseeva , Denis L. Mihailov , Oleg V. Ershov , Oleg E. Nasakin ,
⇑
Viktor A. Tafeenko b
a Chuvash State University, Moskovsky pr., 15, Cheboksary, Russia
b Lomonosov Moscow State University, Leninskie gory 1, Moscow, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of 2-(dicyanomethylene)-1,2-dihydropyridine derivatives from the reactions of arylmeth-
ylidene derivatives of malononitrile dimers with 1,3-dicarbonyl compounds is described.
Ó 2014 Elsevier Ltd. All rights reserved.
Received 14 January 2014
Revised 27 February 2014
Accepted 12 March 2014
Available online xxxx
Keywords:
Heterocyclic compounds
Cyano compounds
Domino synthesis
Pyridine
1,3-Dicarbonyl compounds
Michael addition
1,3-Dicarbonyl compounds are important chemical substrates
for the synthesis of various medicines that contain an azaheterocy-
clic fragment. Examples include vitamin B6, antipyrine, aminopy-
rine, aminoglutethimide, and various analgesics, antibacterial,
and antimalarial medications. In addition, 1,3-dicarbonyl com-
pounds are important for the synthesis of new biologically active
compounds, such as functionally substituted pyridin-2-ones, for
example, 1 and their hydrogenated analogs,1a–e for example, inhib-
itors of Rho-associated kinase,1a and the glycine site on the NMDA
receptor, and for AMPA antagonist activity,1b inhibition of PARP
activity,1c cytotoxic activity,1d and as a1a adrenergic receptor
antagonists.1e
The chemical properties of the oxo group are similar to those of
the ylidenemalononitrile fragment (Fig. 1).2 Therefore, as a poten-
tial bioactive compound, 2-(dicyanomethylene)-1,2-dihydropyri-
dine (2) might be interesting as a structural analog of compound
1. Metal complexes of compounds with the general formulas 1
and 2 are important in optical recording media.3
The reaction is thought to involve the Michael addition of 1,3-
dicarbonyl compounds 3 to the arylmethylidene derivatives 4 of
malononitrile dimer (Scheme 2). Subsequent cyclization of the
amino group to the carbonyl group leads to the formation of
intermediate A. Elimination of water then leads to the formation
of
4-aryl-3-cyano-2-(dicyanomethyl)-6-methyl-1,2,3,4-tetrahy-
dropyridin-2-ide salt 5, which in some cases can be isolated in
yields of 57–75%.5
The salts
5 probably exist in equilibrium with 4-aryl-5-
cyano-6-(dicyanomethylene)-2-methyl-1,4,5,6-tetrahydropyridine
derivatives 6, which can be easily dehydrogenated into the final
4-aryl-5-cyano-6-(dicyanomethylene)-2-methyl-1,6-dihydropyr-
idine derivatives 7. This may explain the low yields of compounds
5 and 6.
The synthesis of compounds 6 and 7 in one step would be useful
in order to avoid the isolation of compound 5. Mono-, di-, and
tripotassium or sodium phosphate was used as the base for the
In this Letter, we describe a new approach to 2-(dicyanometh-
ylene)-1,2-dihydropyridine derivatives 5–8 using the base-initi-
ated reaction between 1,3-dicarbonyl compounds
3
and
R2
arylmethylidene derivatives 4 of malononitrile dimer4 (Scheme 1
CN
N
R1
O
N
H
and Table 1).
CN
1
2
⇑
Corresponding author. Tel.: +7 9083030163; fax: +7 8352450279.
E-mail address: bardasov.chem@mail.ru (I.N. Bardasov).
Figure 1. Pyridin-2-ones 1 and 2-(dicyanomethylene)-1,2-dihydropyridine (2).
http://dx.doi.org/10.1016/j.tetlet.2014.03.056
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Bardasov, I. N.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.03.056

2
I. N. Bardasov et al. / Tetrahedron Letters xxx (2014) xxx–xxx
O
Ar
NH2
O
O
NaNO2
CN
CN
CN
R
+
Ar
R
CN
CN
B
CN
N
H
3
4
7
K
2 H
P
NaOH or
NaOEt
O
[O]
4
Ar
O
Ar
O
Ar
H
N
O
CN
BH+
CN
CN
H+
O
R
R
CN
CN
HN
CN
CN
R
N
H
5
N
H
6
8
CN
Scheme 1. Michael addition 1,3-dicarbonyl compound 3 to the arylmethylidene derivatives 4 of malononitrile dimer.
The reaction of compounds 3 and 4 under catalysis by a strong
base proceeds via an alternative intramolecular process, that leads
to the formation of methyl and ethyl 8-aryl-3-(dicyanomethylene)-
Table 1
Synthesis of compounds 5-8
Reagent
R
Substrate Ar
Product
Yielda (%)
1-methyl-5-oxo-2,6-diazabicyclo[2.2.2]octane
derivatives
8
(Scheme 3) in yields of 26–45%.10
5
6
7
8
3a
3a
3a
3a
3a
3a
3b
3b
3b
3b
3c
3c
3c
3d
3e
3f
OC2H5
OC2H5
OC2H5
OC2H5
OC2H5
OC2H5
OCH3
OCH3
OCH3
OCH3
NH2
NH2
NH2
CH3
N(CH3)2 4a
NHPh
Ph
Ph
4a
4b
4c
4d
4e
4f
4a
4d
4c
4f
C6H5
a
b
c
d
e
f
g
h
i
j
k
l
m
n
o
p
q
r
62 67 64 38
It is assumed that under the action of a strong base, there is the
possibility of formation of intermediate B. In intermediate B,
depending on the relative positions of the hydroxy and cyano
2-ClC6H4
4-FC6H4
4-H3CC6H4
4-(CH3)2NC6H4
3-BrC6H4
C6H5
4-H3CC6H4
4-FC6H4
3-BrC6H4
C6H5
2-ClC6H4
4-H3CC6H4
C6H5
C6H5
C6H5
70
65
57
—
—
—
—
—
—
67 29
71 26
—
—
60 35
74
—
—
66 63 69 40
—
—
—
—
—
—
75
—
—
—
—
—
90 63
93 71
—
—
—
—
80
4a
4b
4d
4a
83 76 45
—
—
—
—
81
85
68
—
—
—
65 42
4a
4a
4d
4f
89 78
94 77
—
—
—
—
3g
3g
3g
C6H5
4-H3CC6H4
3-BrC6H4
—
—
62
74
Ph
s
a
Yield of isolated product.
formation of compound 6.6 The formation of compound 7 in one
step is possible using sodium nitrite as the base.7 Apparently, so-
dium nitrite functions as both the basic catalyst and the oxidant.8
The structures of compounds 5–7 were confirmed by IR, 1H and
13C NMR spectroscopy, mass spectrometry, and by X-ray diffrac-
tion analysis of compound 7n (Fig. 2).9
Figure 2. ORTEP diagram of 2-[5-acetyl-3-cyano-6-methyl-4-phenylpyridin-2(1H)-
ylidene]malononitrile (7n) as a solvate with 1,4-dioxane.
O
Ar
O
Ar
NH2
CN
CN
O
O
CN
R
R
+B
CN
+
-BH+
CN
CN
Ar
O
CN
R
N
H
A
CN
CN
HO
H N
O
2
4
3
O
Ar
O
Ar
Ar
CN
CN
BH+
CN
H+
BH+
R
+BH+
-H2O
[O]
R
R
CN
CN
CN
N
H
N
H
5
N
H
6
CN
CN
CN
7
Scheme 2. Proposed mechanism for the synthesis of compounds 5–7.
Please cite this article in press as: Bardasov, I. N.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.03.056

I. N. Bardasov et al. / Tetrahedron Letters xxx (2014) xxx–xxx
3
O
Ar
Ar
O
Ar
Ar
H
N
NH
O
CN
CN
O
CN
R
+BH+
-B
O
R
O
CN
CN
CN
CN
HN
HN
N
H
R
R
N
H
B
HO
O
CN
CN
CN
8
A
Scheme 3. Proposed mechanism for the synthesis of 2,6-diazabicyclo[2.2.2]octanes 8.
Lee, D. J. Med. Chem. 2007, 50, 6–9; (b) Carling, R. W.; Leeson, P. D.; Moore, K.
W.; Smith, J. D.; Moyes, Ch. R.; Mawer, I. M.; Thomas, S.; Chan, T.; Baker, R.;
Foster, A. C.; Grimwood, S.; Kemp, J. A.; Marshall, G. R.; Tricklebank, M. D.;
Saywell, K. L. J. Med. Chem. 1993, 36, 3397–3408; (c) Shinkwin, A. E.; Whish, W.
J. D.; Threadgill, M. D. Bioorg. Med. Chem. 1999, 7, 297–308; (d) Deady, L. W.;
Rogers, M. L.; Zhuang, L.; Baguley, B. C.; Denny, W. A. Bioorg. Med. Chem. 2005,
13, 1341–1355; (e) Nantermet, Ph. G.; Barrow, J. C.; Selnick, H. G.; Homnick, C.
F.; Freidinger, R. M.; Chang, R. S. L.; O’Malley, S. S.; Reiss, D. R.; Broten, Th. P.;
Ransom, R. W.; Pettibone, D. J.; Olah, T.; Forray, C. Bioorg. Med. Chem. Lett. 2000,
10, 1625–1628.
2. (a) Wallenfels, K.; Friedrich, K.; Rieser, J.; Ertel, W.; Thieme, H. K. Angew. Chem.,
Int. Ed. Engl. 1976, 15, 261–270; (b) Karlsen, H.; Kolsaker, P.; Romming, C.;
Uggerud, E. Acta Chem. Scand. 1998, 52, 391–398; (c) Karlsen, H.; Kolsaker, P.;
Romming, Ch.; Uggerud, E. J. Chem. Soc., Perkin Trans. 2 2002, 404–409.
3. Clariant International Ltd. WO 2006/10773; 2006; Chem. Abstr. 2006, 144,
172705
4. (a) Junek, H.; Wolny, B. Monatsh. Chem. 1976, 107, 999–1006; (b) Gazit, A.;
Yaish, P.; Gilon, Ch.; Levitzki, A. J. Med. Chem. 1989, 32, 2344–2352. Typical
procedure for the preparation of arylmethylidene derivatives of malononitrile
dimer 4a–f.
A mixture of the appropriate aromatic aldehyde (10 mmol),
malononitrile dimer (10 mmol), and piperidine acetate (0.1 mmol) in EtOH
(50 mL) was stirred at 70 °C for 15–20 min. After cooling, the resulting
precipitate was filtered and washed with i-PrOH.
5. Typical procedure for the preparation of 4-aryl-3-cyano-2-(dicyanomethyl)-6-
methyl-1,2,3,4-tetrahydropyridin-2-ide salts 5.
A mixture of 1,3-dicarbonyl
compound (10 mmol), 2-amino-4-arylbuta-1,3-diene-1,1,3-tricarbonitrile
3
(4) (10 mmol), and diethylamine (12 mmol) in EtOH (10 mL) was heated at
reflux temperature with vigorous stirring for 8–10 min. After cooling, the
resulting precipitate was filtered, washed with i-PrOH and recrystallized from
refluxing EtOAc. Piperidine (12 mmol) was used as the base for the synthesis of
compound 5g instead of diethylamine. Compound 5a. Mp 164–165 °C; 1H NMR
(500.13 MHz, DMSO-d6): d 1.11 (3H, t, J = 7.0 Hz, CH3), 1.16 (6H, t, J = 7.2 Hz,
2CH3), 2.28 (3H, s, CH3), 2.93 (4H, q, J = 7.2 Hz, 2CH2), 3.97 (2H, q, J = 7.1 Hz,
CH2), 4.29 (1H, s, CH), 7.08 (2H, d, J = 7.3 Hz, C6H5), 7.14 (1H, t, J = 7.3 Hz, C6H5),
7.24 (2H, t, J = 7.6 Hz, C6H5), 8.07 (1H, s, NH), 8.14 (1H, br s, NH+2). 13C NMR
(125.76 MHz, DMSO-d6): d 10.93, 14.04, 18.58, 39.73, 41.03, 41.31, 58.91,
67.56, 100.92, 122.06, 122.12, 126.00, 126.38, 128.08, 145.99, 147.10, 147.34,
166.57. IR (mineral oil, cmÀ1): 3279–3169 (NH), 2220, 2167 (CN), 1685 (C@O),
1580 (C@C). MS (EI, 70 eV): m/z (%) 332 [M]+ (21). Anal. Calcd for C23H27N5O2:
C, 68.13; H, 6.71; N, 17.27. Found: C, 67.96; H, 6.94; N, 17.23.
Figure 3. ORTEP diagram of ethyl (7R⁄,8S⁄)-3-(dicyanomethylene)-1-methyl-5-
oxo-8-phenyl-2,6-diazabicyclo[2.2.2]octane-7-carboxylate (8a).
groups, there may be a flagpole interaction and formation of a pyr-
an ring. Iminolactone-lactam rearrangement then leads to com-
pounds 8,11 which contain four asymmetric centers. In our case,
according to 1H NMR and 13C NMR spectroscopy (8a), only one dia-
stereomer was observed. An unambiguous determination of the
position of the substituents was achieved by X-ray diffraction anal-
ysis using a single crystal of compound 8a (Fig. 3).12
6. Typical procedure for the preparation of 4-aryl-5-cyano-6-(dicyanomethylene)-2-
methyl-1,4,5,6-tetrahydropyridine derivatives 6a,g. A mixture of 1,3-dicarbonyl
compound 3 (10 mmol), 2-amino-4-arylbuta-1,3-diene-1,1,3-tricarbonitrile (4)
(10 mmol), and K2HPO4 (12 mmol) in EtOH (70 mL) was refluxed for 3–4 h.
After completion of the reaction (TLC), the solution was filtered and neutralized
with 25% HCl (to pH = 4). The resulting precipitate was filtered and washed
with i-PrOH and Et2O. Compound 6a. Mp 141–142 °C (dec.); 1H NMR
(500.13 MHz, DMSO-d6): d 1.13 (3H, t, J = 7.1 Hz, CH3), 2.57 (3H, s, CH3), 4.10
(2H, dq, J = 7.1, 3.0 Hz, CH2), 4.55 (1H, d, J = 2.1 Hz, CH), 4.62 (1H, br s, CH), 7.24
(2H, d, J = 7.3 Hz, C6H5), 7.31 (1H, t, J = 7.3 Hz, C6H5), 7.37 (2H, t, J = 7.4 Hz,
C6H5), 11.26 (1H, s, NH). 13C NMR (125.76 MHz, DMSO-d6): d 13.87, 17.53,
34.77, 39.11, 59.67, 60.45, 106.99, 111.55, 113.07, 115.32, 126.80, 128.13,
128.94, 136.01, 146.10, 156.13, 164.99. IR (mineral oil, cmÀ1): 3240–3123
(NH), 2227, 2210 (CN), 1704 (C@O), 1589 (C@C). MS (EI, 70 eV): m/z (%) 332
[M]+ (16), 259 [MÀ73]+ (53). Anal. Calcd for C19H16N4O2: C, 68.66; H, 4.85; N,
16.86. Found: C, 68.78; H, 4.80; N, 16.87. Typical procedure for the preparation of
4-aryl-5-cyano-6-(dicyanomethylene)-2-methyl-1,4,5,6-tetrahydropyridine
derivatives 6h,i,k,p,q. A solution of compound 5 (10 mmol) in a mixture of 1,4-
dioxane (15 mL) and H2O (30 mL) was neutralized with 5% H2SO4. The resulting
precipitate was filtered and washed with H2O, and then recrystallized from a
mixture of 1,4-dioxane:H2O.
In
conclusion,
2-(dicyanomethylene)-1,2-dihydropyridine
derivatives have been obtained for the first time, as a result of a
domino process in one synthetic operation between 1,3-dicarbonyl
compounds 3 and the arylmethylidene derivatives of malononitrile
dimer 4. Our goal is further modification of the substituents and a
study of the biological activity of these products.
Acknowledgements
This study was carried out in the framework of the basic part of
the State assignment of the Ministry of Education and Science of
the Russian Federation
Supplementary data
7. Typical procedure for the preparation of 4-aryl-5-cyano-6-(dicyanomethylene)-2-
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.
03.056.
methyl-1,6-dihydropyridine derivatives 7.
A
mixture of 1,3-dicarbonyl
compound (10 mmol), 2-amino-4-arylbuta-1,3-diene-1,1,3-tricarbonitrile
3
(4) (10 mmol) ,and NaNO2 (12 mmol) in EtOH (70 mL) was refluxed for 5–
6 h. After completion of the reaction (TLC), the solution was filtered and
neutralized with 1% HCl (to pH = 4). The precipitate was triturated with H2O
(20 mL), filtered, and washed with H2O (40 mL), and then recrystallized from a
mixture of 1,4-dioxane:i-PrOH. Compound 7a. Mp 224–225 °C (dec.); 1H NMR
(500.13 MHz, DMSO-d6): d 0.72 (3H, t, J = 7.1 Hz, CH3), 2.39 (3H, s, CH3), 3.82
(2H, q, J = 7.1 Hz, CH2), 6.66 (1H, br s, NH), 7.24-7.28 (2H, m, C6H5), 7.46 (3H, t,
J = 3.2 Hz, C6H5). 13C NMR (125.76 MHz, DMSO-d6): d 13.23, 21.92, 42.39, 60.80,
94.27, 115.05, 118.20, 119.86, 127.92, 128.34, 129.18, 136.22, 155.55, 156.51,
References and notes
1. (a) Goodman, K. B.; Cui, H.; Dowdell, S. E.; Gaitanopoulos, D. E.; Ivy, R. L.; Sehon,
C. A.; Stavenger, R. A.; Wang, G. Z.; Viet, A. Q.; Xu, W.; Ye, G.; Semus, S. F.; Evans,
Ch.; Fries, H. E.; Jolivette, L. J.; Kirkpatrick, R. B.; Dul, E.; Khandekar, S. S.; Yi, T.;
Jung, D. K.; Wright, L. L.; Smith, G. K.; Behm, D. J.; Bentley, R.; Doe, Ch. P.; Hu, E.;
Please cite this article in press as: Bardasov, I. N.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.03.056

4
I. N. Bardasov et al. / Tetrahedron Letters xxx (2014) xxx–xxx
158.35, 165.91. IR (mineral oil, cmÀ1): 3232–3080 (NH), 2220, 2204 (CN), 1720
10. Typical procedure for the preparation of 8-aryl-3-(dicyanomethylene)-1-methyl-5-
oxo-2,6-diazabicyclo[2.2.2]octane derivatives 8. mixture of 1,3-dicarbonyl
compound (10 mmol), 2-amino-4-arylbuta-1,3-diene-1,1,3-tricarbonitrile
(C@O), 1607 (C@C). MS (EI, 70 eV): m/z (%) 330 [M]+ (47), 285 [MÀ45]+ (86).
Anal. Calcd for C19H14N4O2: C, 69.08; H, 4.27; N, 16.96. Found: C, 69.06; H, 4.45;
N, 16.72.
A
3
(4) (10 mmol) and NaOEt (12 mmol) in EtOH (10 mL) was refluxed for 1–2 h.
After completion of the reaction (TLC), the solution was filtered and
neutralized with 1% HCl (to pH = 4). The precipitate was triturated with H2O
(20 mL), filtered, and washed with i-PrOH and H2O (40 mL). NaOMe (12 mmol)
in MeOH (10 mL) was used as the base for the synthesis of compound 8g.
Compound 8a. Mp 264–265 °C (dec.); 1H NMR (500.13 MHz, DMSO-d6): d 1.20
(3H, t, J = 7.1 Hz, CH3), 1.67 (3H, s, CH3), 3.34 (1H, d, J = 5.6 Hz, CH), 3.51–3.52
(1H, m, CH), 3.65 (1H, dd, J = 5.6, 2.4 Hz, CH), 4.10-4.18 (2H, m, CH2), 7.29–7.40
(5H, m, C6H5), 9.76 (1H, d, J = 1.6 Hz, NH), 10.89 (1H, s, NH). 13C NMR
(125.76 MHz, DMSO-d6): d 13.82, 18.85, 43.66, 46.55, 54.15, 57.32, 61.16,
69.59, 114.01, 115.59, 127.44, 127.80, 128.85, 138.51, 166.40, 168.73, 170.01.
IR (mineral oil, cmÀ1): 3321–3156 (NH), 2211 (CN), 1712 (C@O), 1601 (C@C).
MS (EI, 70 eV): m/z (%) 174 [MÀ176]+ (82), 91 [MÀ259]+ (8). Anal. Calcd for
8. (a) Xia, J.-J.; Wang, G.-W. Synthesis 2005, 2379–2383; (b) Zolfigol, M. A.; Kiany-
Borazjani, M.; Sadeghi, M. M.; Mohammadpoor-Baltork, I.; Memarian, H. R.
Synth. Commun. 2000, 30, 551–558; (c) Zolfigol, M. A.; Shirini, F.;
Choghamarani, A. Gh.; Mohammadpoor-Baltork, I. Phosphorus, Sulfur Silicon
Relat. Elem. 2003, 178, 1709–1716; (d) Niknam, Kh.; Zolfigol, M. A.; Razavian, S.
M.; Mohammadpoor-Baltork, I. Heterocycles 2005, 65, 657–660; (e) Niknam,
Kh.; Zolfigol, M. A.; Rabani, F. Heterocycl. Commun. 2006, 12, 183–186; (f)
Niknam, Kh.; Zolfigol, M. A.; Razavian, S. M.; Mohammahpoor-Baltork, I. J.
Heterocycl. Chem. 2006, 43, 199–202; (g) Hashemi, M. M.; Ghafuri, H.; Karimi-
Jaberi, Z. Monatsh. Chem. 2006, 137, 197–200; (h) Kadutskii, A. P.; Kozlov, N. G.;
Pashkovskii, F. S. Russ. J. Org. Chem. 2009, 45, 399–403; (i) Plotniece, A.; Pajuste,
K.; Kaldre, D.; Cekavicus, B.; Vigante, B.; Turovska, B.; Belyakov, S.; Sobolev, A.;
Duburs, G. Tetrahedron 2009, 65, 8344–8349; (j) Schade, D.; Lanier, M.;
Okolotowicz, K.; Gilley, C.; Bushway, P.; Wahlquist, Ch.; Mercola, M.; Willems,
E.; Cashman, J. R. J. Med. Chem. 2012, 55, 9946–9957.
C
19H18N4O3: C, 65.13; H, 5.18; N, 15.99. Found: C, 65.16; H, 5.22; N, 15.96.
11. Fedoseev, S. V.; Ershov, O. V.; Belikov, M. Yu.; Lipin, K. V.; Bardasov, I. N.;
Nasakin, O. E.; Tafeenko, V. A. Tetrahedron Lett. 2013, 54, 2143–2145.
9. Crystallographic data (excluding structure factors) for the structure 7n in this
Letter have been deposited with the Cambridge Crystallographic Data Centre as
supplementary publication number CCDC 973315. Copies of the data can be
obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge,
CB2 1EZ, UK [fax: +44 (0) 1223 336033 or e-mail: deposit@ccdc.cam.ac.uk].
12. Crystallographic data (excluding structure factors) for the structure 8a in this
Letter have been deposited with the Cambridge Crystallographic Data Centre
as supplementary publication number CCDC 973314. Copies of the data can be
obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge,
CB2 1EZ, UK [fax: +44 (0) 1223 336033 or e-mail: deposit@ccdc.cam.ac.uk].
Please cite this article in press as: Bardasov, I. N.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.03.056