The furan approach to oxacycles. Part 3: Stereoselective synthesis of 2,3-disubstituted tetrahydropyrans

Article abstract of DOI:10.1016/j.tet.2005.01.001

Commercially available furan 1 was converted to 2,3-trans and 2,3-cis-disubstituted tetrahydropyrans 2 and 3 using a highly efficient route to oxacycles, based on the oxidation of the furan ring with singlet oxygen. Tetrahydropyrans 2 and 3 could be easil

Full text of DOI:10.1016/j.tet.2005.01.001

Tetrahedron 61 (2005) 2021–2026
The furan approach to oxacycles. Part 3: Stereoselective synthesis
of 2,3-disubstituted tetrahydropyrans
a
David Alonso, Manuel Perez, Generosa Gomez, Berta Covelo and Yagamare Fall
a
a
b
a,
*
´
´
a
´
´
´
Departamento de Quımica Organica, Facultad de Quımica, Universidad de Vigo, 36200 Vigo, Spain
b
´
´
´
Departamento de Quımica Inorganica, Facultad de Quımica, Universidad de Vigo, 36200 Vigo, Spain
Received 3 November 2004; revised 23 December 2004; accepted 3 January 2005
Available online 20 January 2005
Abstract—Commercially available furan 1 was converted to 2,3-trans and 2,3-cis-disubstituted tetrahydropyrans 2 and 3 using a highly
efficient route to oxacycles, based on the oxidation of the furan ring with singlet oxygen. Tetrahydropyrans 2 and 3 could be easily separated
by column chromatography.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
oxacycles using either methoxyallene5 or furan6 as starting
material. We now describe the stereoselective synthesis of
2,3-disubstituted tetrahydropyrans using the furan approach.
Synthesis of 2 and 3 can be accomplished according to the
reaction sequence shown in Scheme 1.
The stereoselective preparation of 2,3-disubstituted tetra-
hydropyrans is an important synthetic goal since they
are structural units that are present in a wide variety of
bioactive natural products.1 Due to their biological
potency and structural complexity the fused cyclic
polyether class of natural products such as brevetoxins A
and B2 or ciguatoxin3 (Fig. 1) have received much attention.
Their unusual molecular architecture, a series of fused
cyclic ethers having regular trans–syn–trans stereo-
chemistry, has stimulated numerous iterative routes4 and
represent indeed challenging synthetic targets for organic
chemists.
Commercially available furan 1 (Avocado Research
Chemicals Ltd) reacted with LAH in ether to give alcohol
4 in 89% yield. Alcohol 4 was protected to afford silylether
5. Oxidation of 5 with singlet oxygen followed by treatment
with acetic anhydride in pyridine, afforded butenolide 6 in
89% yield (2 steps). Treatment of 6 with TBAF led to the
bicyclic compound 7 through an intramolecular Michael
addition (yield 81%). Finally after much experimentation
(see Table 1), the best conditions for opening bicyclic
lactone 7 were found: On reaction with LAH (10 equiv) in
the presence of BF3$OEt2, 7 afforded a 89% yield of diols
27,8 and 37 which could be easily separated by column
chromatography.
2. Discussion
We recently described the synthesis of seven-membered
Figure 1. Ciguatoxin.
Keywords: Furan; Singlet oxygen; Tetrahydropyran; Oxacycles; Marin toxins.
* Corresponding author. Tel.: C34 986 81 23 20; fax: C34 986 81 22 62; e-mail: yagamare@uvigo.es
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.01.001

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D. Alonso et al. / Tetrahedron 61 (2005) 2021–2026
Scheme 1.
At this point, we were not able to distinguish between the
two diols 2 and 3 so we decided to assign their
stereochemistry using chemical correlation. The stereo-
chemistry of trans and cis bicyclic lactones 8 and 11
(Table 1) was established using NOE experiments and 3D
NMR techniques (Fig. 2).
Opening of these lactones with LAH led to diols whose
spectroscopic data were identical to diols 2 and 3,
respectively (Scheme 2).
Recrystallization of diol 2 using a mixture of hexane
and ethyl acetate afforded crystals which were subjected to
Table 1

D. Alonso et al. / Tetrahedron 61 (2005) 2021–2026
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3. Experimental
3.1. General
Silica gel (230 mesh) was purchased from Merck. All other
chemicals used were of reagent grade and were obtained
1
from Aldrich Chemical Co. H NMR spectra (300 MHz)
and 13C NMR spectra (75.47 MHz) were recorded in a
Bruker AMX 300 spectrometer, using TMS as internal
standard (chemical shifts in d values, J in Hz). Mass spectra
were recorded on a Kratos MS-59 spectrometer. Flash
chromatography was performed on silica gel (Merck 60,
230–240 mesh) and analytical TCL on pre-coated silica gel
plates (Merck 60 F254, 0.25 mm).
Figure 2. NOE correlations for 8 and 11.
3.1.1. 3-(2-Furyl)propanol (4). To an ice cooled solution of
furan 1 (7.3 g, 43.4 mmol) in dry ether (70 mL) was added
LAH (3.3 g, 86.8 mmol) in small portions. At the end of the
addition the mixture was stirred for 12 h at room
temperature. Excess of hydride was destroyed by dropwise
addition of water at 0 8C. The resulting white precipitate
was filtered off and the organic phase was separated. The
aqueous phase was extracted with ether (3!30 mL) and
the combined organic phases were washed with brine
(3!100 mL), dried (Na2SO4), filtered and concentrated
affording 4.8 g of compound 4 [89%, yellowish oil, Rf: 0.50
(40% AcOEt/hexane)] which was used in the next reaction
without further purification.; 1H NMR (CDCl3, d): 7.28 (1H,
dd, JZ1.8, 0.7 Hz, CH), 6.26 (1H, dd, JZ3.1, 1.8 Hz, CH),
5.99 (1H, dd, JZ3.1, 0.8 Hz, CH), 3.64 (2H, t, JZ6.4 Hz,
Scheme 2.
X-ray crystallographic analysis, thus establishing the
structure to be that shown in Figure 3 and already
determined by chemical correlation.
In conclusion, a new and efficient method for the
stereoselective synthesis of trans and cis 2,3-disubstituted
tetrahydropyrans from a commercially available furan
derivative has been developed. The stereochemistry of
diols 2 and 3 has been unambiguously assigned using
chemical correlation and X-ray crystallographic analysis.8
With a view towards an iterative approach to polycyclic
ethers, diol 2 could be an excellent starting material. Both
2 and 3 are attractive synthons in diversity-oriented
synthesis9 or possible building blocks for new types of
ionophores with C2-symmetry.10 Work is now in progress
towards the enantioselective synthesis of highly substituted
tetrahydropyrans starting from furans possessing a chiral
side chain.
CH2), 2.71 (2H, t, JZ7.5 Hz, CH2), 1.88 (2H, m, CH2); 13
C
NMR (CDCl3, d): 155.49 (C), 140.78 (CH), 110.00 (CH),
104.86 (CH), 61.68 (CH2), 30.80 (CH2), 24.15 (CH2);
HRMS (EI) calcd for C7H10O2 126.0680, found 126.0677.
3.1.2. 2-(3-tert-Butyldipenylsilyloxypropyl)furan (5). To
a solution of alcohol 4 (4.84 g, 38.4 mmol) in DMF (48 mL)
was added imidazol (3.1 g, 46.1 mmol) and tert-butyl-
diphenylsilylchloride (TBDS)(12.67 g, 46.1 mmol). The
mixture was stirred at room temperature for 1 h, quenched
Figure 3. X-ray structure of 2.

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D. Alonso et al. / Tetrahedron 61 (2005) 2021–2026
with aqueous NH4Cl solution and extracted with diethyl
ether (3!50 mL). The combined organic phases were dried
over Na2SO4, filtered and the solvent evaporated to afford a
residue which was chromatographed on silica gel using 2%
EtOAc/hexane as eluent giving 12.9 g of furan 9 [92%,
yellow oil, Rf:0.82 (20% EtOAc/hexane)]. 1H NMR
(CDCl3, d): 7.69 (4H, dd, JZ5.8, 1.6 Hz, CHo–Ph), 7.42
(6H, m, CHp,m–Ph), 7.30 (1H, m, CH), 6.28 (1H, dd, JZ2.9,
1.7 Hz, CH), 5.97 (1H, dd, JZ2.6, 2.00 Hz, CH), 3.72 (2H,
m, CH2), 2.78 (2H, t, JZ7.7 Hz, CH2-1), 1.91 (2H, m, CH2),
1.08 (9H, s, CH3–tBu); 13C NMR (CDCl3, d): 155.91 (C),
140.72 (CH), 135.54 (CHo–Ph), 133.84 (C–Ph), 129.55
(CHp–Ph), 127.61 (CHm–Ph), 110.43 (CH), 104.77 (CH),
62.89 (CH2), 30.81 (CH2), 26.82 (CH3–tBu), 24.35 (CH2),
19.22 (C–tBu); HRMS (EI) calcd for C18H19O2Si [MK3!
CH3] 308.1187, found 308.1190.
17.2 Hz), 1.75 (1H, m), 1.66 (2H, m); 13C NMR (CDCl3, d):
176.00 (CO), 104.61 (C), 76.48 (CH), 65.39 (CH2), 49.35
(CH3O), 37.02 (CH2), 27.16 (CH2), 21.52 (CH2); HRMS
(EI) calcd for C8H12O4 172.0736, found 172.0743.
3.2. Obtention of compounds 8–12 by reduction of
bicyclic lactone 7 with Et3SiH–TMSOTf11a and Et3SiH–
BF3$Et2O11b
Reduction of 7 with Et3SiH–TMSOTf. To a solution of 7
(175 mg, 1.02 mmol) in dry dichloromethane (26 mL) was
added dropwise Et3SiH (0.98 mL, 6.12 mmol) and TMSOTf
(0.58 mL, 2.86 mmol) and the mixture stirred at room
temperature for 4 days. A few drops of pyridine and water
(25 mL) were added and the product extracted with ethyl
acetate (3!30 mL). The combined organic phases were
dried over Na2SO4, filtered and the solvent evaporated
giving a residue which was chromatographed on silica gel
using 25% EtOAc/hexane as eluent, affording 39 mg of 8
[27%, colourless oil, Rf: 0.69 (70% EtOAc/hexane)], 11 mg
of 11 [8%, colourless oil, Rf: 0.41 (70% EtOAc/hexane)]
and 48 mg of 12 [27%, colourless oil, Rf: 0.18 (70% EtOAc/
hexane)].
3.1.3. 5-[3-(tert-Butyldiphenylsilyloxi)propyl]-5-meth-
oxy-5H-furan-2-one (6). To a solution of furan 5
(12.89 g, 35.38 mmol) in dry methanol (200 mL) was
added Rose bengal (837 mg). The mixture was purged
several times with O2 (balloon), cooled to K78 8C and
irradiated with a 200 W lamp for 7 h, stirring under oxygen
atmosphere. The mixture was allowed to reach room
temperature and the solvent was evaporated. The residue
was rapidly passed through a column using 50% EtOAc/
hexane in order to get rid of the catalyst. After solvent
evaporation the residue was dissolved in pyridine (90 mL),
acetic acid (23 mL) and DMAP (catalytic) were added and
the mixture stirred at room temperature overnight. MeOH
(100 mL) was added and stirring continued for 30 min. The
methanol was rotatory evaporated and ether (200 mL)
added. The pyridine was removed using a 10% aqueous
solution of CuSO4 (4!100 mL). The ethereal phase was
dried over Na2SO4 and the solvent concentrated giving a
residue which was chromatographed on silica gel using 15%
EtOAc/hexane as eluent affording 12.6 g of butenolide 6
[89%, yellow oil, Rf: 0.33 (20% EtOAc/hexane)]. 1H NMR
Reduction of 7 with Et3SiH–BF3$Et2O. To a solution 7
(167 mg, 0.97 mmol) in dry dichloromethane (25 mL) was
added dropwise Et3SiH (0.47 mL, 2.92 mmol) and
BF3$Et2O (0.30 mL, 2.33 mmol) and the mixture stirred at
room temperature for 48 h. A few drops of pyridine and
water (25 mL) were added and the product extracted with
dichloromethane (2!25 mL). The aqueous phase was
saturated with NaCl and extracted with dichloromethane
(2!25 mL). The combined organic phases were dried over
Na2SO4, filtered and the solvent evaporated giving a residue
which was chromatographed on silica gel using 35%
EtOAc/hexane as eluent, affording 9 mg of 8 [6%, colour-
less oil, Rf: 0.69 (70% EtOAc/hexane)], 19 mg of 11 [14%,
colourless oil, Rf: 0.41 (70% EtOAc/hexane)] and 26 mg of
12 [16%, colourless oil, Rf: 0.18 (70% EtOAc/hexane)].
(CDCl3, d): 7.81 (4H, m, Ho–Ph), 7.56 (6H, m, Hm,p
–
Ph),7.24 (1H, d, JZ5.7 Hz, aCH), 6.36 (1H, d, JZ5.7 Hz,
aCH), 3.83 (2H, t, JZ6.1 Hz, CH2), 3.37 (3H, s, CH3O),
2.18 (2H, m, CH2), 1.80 (2H, m, CH2); 13C NMR (CDCl3,
d): 170.32 (CO), 153.89 (CH), 135.94 (CHo–Ph), 134.13
(C–Ph), 130.06 (CHp–Ph), 128.08 (CHm–Ph), 125.22 (CH),
111.58 (C), 63.64 (CH2), 51.54 (CH3O), 33.95 (CH2), 27.27
(CH3–tBu), 26.82 (CH2), 19.61 (C–tBu); HRMS (EI) calcd
for C19H21O4Si [MK3!CH3] 354.1243, found 354.1219.
1
3.2.1. trans-2,7-Dioxabicyclo[4.3.0]nonane-8-one (8). H
NMR (CDCl3, d): 4.02 (1H, dd, JZ11.8, 3.25 Hz), 3.82
(1H, m,), 3.55 (2H, m), 2.72 (1H, dd, JZ15.8, 6.9 Hz), 2.59
(1H, dd, JZ15.8, 12.2 Hz), 2.31 (1H, m), 1.81 (2H, m), 1.7
(1H, m).13C NMR (CDCl3, d): 172.55 (CO), 80.38 (CH),
79.07 (CH), 68.89 (CH2), 35.66 (CH2), 28.20 (CH2), 24.71
(CH2). HRMS (EI) calcd for C7H10O3 142.0630, found
142.0624.
3.1.4. 6-Methoxy-2,7-dioxabicyclo[4.3.0]nonan-8-one (7).
To a solution of butenolide 6 (5.62 g, 13.69 mmol) in dry
THF (207 mL) was added dropwise tetrabutylammonium
fluoride (13.69 mL of 1.0 M solution in THF, 13.69 mmol)
and the mixture was stirred at room temperature for 5 h.
Aqueous saturated solution of NaHCO3 (220 mL) was
added and the product extracted with ethyl acetate (3!
130 mL). The combined organic phases were dried over
Na2SO4, filtered and the solvent evaporated giving a residue
which was chromatographed on silica gel using 20%
EtOAc/hexane as eluent, affording 1.9 g of bicyclic lactone
7 [81%, yellow oil, Rf: 0.45 (30% EtOAc/hexane)]. 1H
NMR (CDCl3, d): 3.88 (1H, d, JZ4.4 Hz); 3.85 (1H, m),
3.37 (1H, dd, JZ11.7, 1.7 Hz), 3.33 (3H, s, OCH3), 2.88
(1H, dd, JZ17.2, 4.4 Hz), 2.52 (1H, m), 2.33 (1H, d, JZ
3.2.2. trans-(6-Methoxy-2-oxacyclohexyl)methylacetate
(9). 1H NMR (CDCl3, d): 3.85 (1H, m), 3.66 (3H, s,
CH3OCO), 3.53 (1H, td, JZ8.8, 3.7 Hz), 3.34 (1H, m), 3.31
(3H, s, CH3O), 2.90 (1H, td, JZ10, 4.3 Hz), 2.80 (aH, dd,
JZ15.1, 3.7 Hz), 2.40 (1H, dd, JZ15.1, 8.5 Hz), 2.25 (1H,
m), 1.66 (2H, m), 1.28 (1H, m).13C NMR (CDCl3, d):
172.07 (CO), 78.59 (CHOMe), 77.69 (CH), 67.79 (CH2),
56.16 (CH3O), 51.58 (CH3OCO), 37.99 (CH2), 28.33 (CH2),
25.09 (CH2). HRMS (EI) calcd for C8H13O3 [MKCH3O]
157.0865.1243, found 157.0873.
3.2.3. trans-[6-(Trimethylsilyloxy)-2-oxacyclohexyl]-
methylacetate (10). H NMR (CDCl3, d): 3.84 (1H, m),
1
3.67 (3H, s, CH3OCO), 3.50 (1H, td, JZ8.9, 3.4 Hz), 3.34

D. Alonso et al. / Tetrahedron 61 (2005) 2021–2026
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(1H, m), 3.30 (1H, m), 2.76 (1H, dd, JZ15.1, 3.42 Hz), 2.32
(1H, dd, JZ15.1, 9.0 Hz), 1.97 (1H, d, JZ9.3 Hz), 1.64
(2H, m), 1.45 (1H, m), 0.08 (9H, s, (CH3)3Si).13C NMR
(CDCl3, d): 172.19 (CO), 79.42 (CH), 70.85 (CH), 67.84
(CH2), 51.59 (CH3O), 8.02 (CH2), 33.30 (CH2), 25.5 (CH2),
0.3 (CH3)3Si); HRMS (EI) calcd for C10H19O4Si [MKCH3]
231.1053, found 231.1059.
studies. Manuel Marcos (CACTI, Vigo) is gratefully
acknowledged for the X-ray structure determination of 2.
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tet.2005.01.
001
3.2.4. cis-2,7-Dioxabicyclo[4.3.0]nonane-8-one (11). 1H
NMR (CDCl3, d): 4.37 (1H, t, JZ2.6 Hz), 4.20 (1H, dd, JZ
4.2, 2.4 Hz), 3.91 (1H, m), 3.42 (1H, m), 2.68 (1H, dd, JZ
17.2, 4.6 Hz), 2.53 (1H, d, JZ17.2 Hz), 2.34 (1H, d, JZ
13.6 Hz), 1.85 (2H, m), 1.48 (1H, m); 13C NMR (CDCl3, d):
176.2 (CO), 76.8 (CH), 72.8 (CH), 65.8 (CH2), 39.0 (CH2),
24.7 (CH2), 19.2 (CH2); HRMS (EI) calcd for C7H10O3
142.0630, found 142.0624.
References and notes
1. (a) Macias, F. A.; Molinillo, J. M. G.; Varela, R. M.; Torres,
A.; Fronczek, F. R. J. Org. Chem. 1994, 59, 8261–8266.
´
(b) Alvarez, E.; Candenas, M.-L.; Perez, R.; Ravelo, J. L.;
3.2.5. trans-[6-Hydroxy-2-oxacyclohexyl]methylacetate
(12). H NMR (CDCl3, d): 3.90 (1H, m), 3.53 (1H, td, JZ
1
8.8, 3.7 Hz), 3.36 (1H, m), 3.34 (3H, s, CH3O), 2.93 (1H,
m), 2.87 (1H, dd, JZ15.4, 3.7 Hz), 2.87 (1H, dd, JZ15.4,
8.4 Hz), 2.28 (1H, m), 1.67 (2H, m), 1.29 (1H, m); 13C NMR
(CDCl3, d): 176.3 (CO), 78.5 (CHO), 77.3 (CH), 67.9
(CH2), 56.3 (CH3O), 37.9 (CH2), 28.3 (CH2), 25.0 (CH2);
HRMS (EI) calcd for C8H12O3 [MKH2O] 156.0786, found
156.0791.
Martin, J. D. Chem. Rev. 1995, 95, 1953–1980. (c) Faulkner,
D. J. Nat. Prod. Rep. 1996, 13, 75–125. (d) Marata, K.; Satake,
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64, 37–48. (f) Liu, T.-Z.; Isobe, M. Tetrahedron 2000, 56,
5391–5404. (g) Takakura, H.; Noguchi, K.; Sasaki, M.;
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1090–1093. (h) Kadota, I.; Takamura, H.; Sato, K.;
Yamamoto, Y. J. Org. Chem. 2002, 67, 3494–3498.
3.3. Reduction of 7 using LAH, BF3$OEt2: obtention of
trans-2-(6-hydroxy-2-oxacyclohexyl)ethanol (2) and
cis-2-(6-hydroxy-2-oxacyclohexyl)ethanol (3)
2. Nicolaou, K. C.; Gunzner, J. L.; Shi, G.; Agrios, K. A.;
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Gartner, P.; Yang, Z. Chem. Eur. J. 1999, 5, 646–658.
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(b) Takai, S.; Isobe, M. Org. Lett. 2002, 4, 1183–1186.
(c) Inoue, M.; Hirama, M. Synlett 2004, 4, 577–595.
To a solution of 7 (179 mg, 1.04 mmol) in dry ether (15 mL)
was added BF3$OEt2 (0.32 mL, 2.5 mmol) and the mixture
stirred for 40 min at room temperature. LiAlH4 (396 mg,
10.44 mmol) was added in portions to the ice-cooled
mixture which was stirred at room temperature for 19 h.
The reaction was carefully quenched with cold water to
destroy excess of hydride. After filtration and solvent
evaporation the residue was chromatographed on silica gel
using 20–80% EtOAc/hexane as eluent to afford 89 mg of 2
(58%) and 47 mg of 3 (31%).
4. (a) Rainer, J. D.; Allwein, S. P. J. Org. Chem. 1998, 63,
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5310–5311. (b) Marmsater, F. P.; West, F. G. Chem. Eur. J.
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5. (a) Fall, Y.; Gomez, G.; Fernandez, C. Tetrahedron Lett. 1999,
´
´ ´
40, 8307–8308. (b) Perez, M.; Canoa, P.; Gomez, G.; Teijeira,
M.; Fall, Y. Synthesis 2004, in press.
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Lett. 2003, 44, 4467–4469. (b) Perez, M.; Canoa, P.; Gomez,
´
G.; Teran, C.; Fall, Y. Tetrahedron Lett. 2004, 45, 5207–5209.
´
1
3.3.1. Compound 2. H NMR (CDCl3, d): 3.89 (1H, m),
3.80 (2H, m), 3.58 (2H, m), 3.19 (1H, m), 2.11 (1H, m), 2.03
(1H, m), 1.80 (1H, m), 1.69 (2H, m), 1.40 (1H, m); 13C
NMR (CDCl3, d): 82.4 (CH), 70.1 (CH), 67.7 (CH2), 60.5
(CH2), 35.3 (CH2), 32.5 (CH2), 25.5 (CH2); HRMS (EI)
calcd for C7H14O3 146.0943, found 146.0936.
7. Two literature references can be found for an alternative
synthesis of compounds 2 and 3: (a) Clark, J. S.; Kettle, J. G.
Tetrahedron 1999, 55, 8231–8248. (b) Betancort, J. M.;
Martin, V. S.; Padron, J. M.; Palazon;, J. M.; Ramirez, M. A.;
Soler, M. A. J. Org. Chem. 1997, 62, 4570–4583.
8. X-ray crystal structure analysis of 2: Bruker Smart CCD
˚
1
3.3.2. Compound 3. H NMR (CDCl3, d): 4.02 (1H, m),
diffractometer, Mo Ka-radiation (lZ0.71073 A), T 293(2) K.
Crystal size: 0.36!0.22!0.20 mm3, colourless irregular
3.81 (2H, m), 3.58 (2H, m), 3.52 (1H, m), 2.00 (2H, m), 1.71
(2H, m), 1.44 (2H, m); 13C NMR (CDCl3, d): 79.6 (CH),
68.6 (CH2), 67.1 (CH), 60.6 (CH2), 34.3 (CH2), 30.5 (CH2),
20.1 (CH2); HRMS (EI) calcd for C7H12O2 [MKH2O]
128.0837, found 128.0834.
˚
prism, space group C2/c, monoclinic, aZ18.175(4) A, bZ
˚
˚
5.6732(13) A,
cZ15.521(4) A,
bZ102.577(4)8,
VZ
3
1562.0(6) A , ZZ8, rcalZ1.243 g/cm3,
q
rangeZ2.30–
˚
28.018, 4096 reflections collected, 1753 independent (Rint
Z
0.0544), empirical absorption correction (SADABS [1],
TminZ0.530, TmaxZ1.000) 93 parameters, final R indices
[IO2s], RZ0.0472, wRZ0.0832, GOFZ0.827. Structure
solution: direct methods (SHELXS97 [2]), refinement on F2
(SHELXL97 [2]). H atoms were calculated. Crystallographic
data (excluding structure factors) have been deposited with the
Cambridge Crystallographic Centre No. CCDC 238929.
Acknowledgements
This work was supported by a grant from the Xunta de
Galicia (PGIDIT04BTF301031PR). We also thank the
NMR service of the CACTI, University of Vigo, for NMR

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Copies of the data can be obtained free of charge on
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