Synthesis of imidazoylthiocarbonyl intermediates for the radical deoxygenation of hindered secondary alcohols

Article abstract of DOI:10.1081/SCC-100104834

A practical and efficient synthesis of imidazoylthiocarbonyl derivatives of highly hindered alcohols was achieved using 1,1′-thiocarbonyldiimidazole in very concentrated mixtures of reagents. More diluted conditions give longer reaction times and the reco

Full text of DOI:10.1081/SCC-100104834

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SYNTHESIS OF IMIDAZOYLTHIOCARBONYL
INTERMEDIATES FOR THE RADICAL DEOXYGENATION
OF HINDERED SECONDARY ALCOHOLS
Daniel Oves a , Mónica Díaz a , Susana Fernández a , Miguel Ferrero a & Vicente Gotor b
a Departamento de Química Orgánica e Inorgánica, Facultad de Química , Universidad de
Oviedo , Oviedo, 33071, Spain
b Departamento de Química Orgánica e Inorgánica, Facultad de Química , Universidad de
Oviedo , Oviedo, 33071, Spain
Published online: 09 Nov 2006.
To cite this article: Daniel Oves , Mónica Díaz , Susana Fernández , Miguel Ferrero & Vicente Gotor (2001) SYNTHESIS
OF IMIDAZOYLTHIOCARBONYL INTERMEDIATES FOR THE RADICAL DEOXYGENATION OF HINDERED SECONDARY ALCOHOLS,
Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 31:15,
2335-2343, DOI: 10.1081/SCC-100104834
To link to this article: http://dx.doi.org/10.1081/SCC-100104834
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SYNTHETIC COMMUNICATIONS, 31(15), 2335–2343 (2001)
SYNTHESIS OF
IMIDAZOYLTHIOCARBONYL
INTERMEDIATES FOR THE RADICAL
DEOXYGENATION OF HINDERED
SECONDARY ALCOHOLS
Daniel Oves, Monica Dıaz, Susana Fernandez,
Miguel Ferrero, and Vicente Gotor*
Departamento de Quımica Organica e Inorganica, Facultad de
Quımica, Universidad de Oviedo, 33071 – Oviedo, Spain
ABSTRACT
A practical and efficient synthesis of imidazoylthiocarbonyl
derivatives of highly hindered alcohols was achieved using
1,10-thiocarbonyldiimidazole in very concentrated mixtures
of reagents. More diluted conditions give longer reaction
times and the recovery of unreacted starting alcohol.
Radical deoxygenation of alcohols is an important process in organic
synthesis.1 Barton and McCombie2 have shown that secondary alcohols can
be deoxygenated by a radical chain reduction of suitable thiocarbonyl deri-
vatives using tributyltin hydride and 2,20-azobis(2-methylpropionitrile)
(AIBN) as initiator. Under typical deoxygenation conditions S-methyldithio-
carbonyl,3 thiobenzoyl,4 imidazole-1-thiocarbonyl,5 phenoxythiocarbonyl6
or S-phenyldithiocarbonyl7 derivatives, among others, afford the deoxy
*Corresponding author.
2335
Copyright & 2001 by Marcel Dekker, Inc.
www.dekker.com

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OVES ET AL.
compound in high yield. The choice of the thiocarbonyl compound is deter-
mined more by structural features present in the alcohol than by the actual
reductive step.
Quinic acid, shikimic acid, and their derivatives are useful precursors in
the synthesis of important natural compounds. The chiral centres present in
their structures and the possible introduction of a variety of new functions
from the selective manipulation of the functional groups on the cyclohexane
ring makes these starting materials very attractive from the point of view of
organic synthesis.
In our ongoing research into the synthesis of A-ring precursors of
vitamin D3 analogues8 it was necessary to obtain the corresponding thio-
imidazolide derivative of various intermediates.9 Conventional procedures
reported in the literature5,10 involve the reaction of an alcohol with an excess
of 1,10-thiocarbonyldiimidazole (TCDI) in methylene chloride, 1,2-dichloro-
ethane or tetrahydrofuran at room temperature or under reflux. However,
low isolated yields of thioimidazolide compounds were obtained when these
conditions were employed in sterically hindered substrates, in which the
reactive hydroxyl function is surrounded by two O-tert-butyldimethylsilyl
groups. This bulky protecting group is very commonly used to regioselec-
tively protect one hydroxyl group among others and to ensure the stability
of the silyl ether, which is linked to the steric bulk of the substituents on the
silicon atom, for the successive transformations on the molecule.
In view of our unsuccessful experiments, we were encouraged to
investigate the appropriate conditions to perform the synthesis of thio-
midazolides efficiently.
DeLuca and co-workers have reported10a the synthesis of compound 2
with 91% yield by reaction of 3,5-disilylprotected quinate 1 with 1.6 equi-
valents of TCDI in methylene chloride at room temperature in 60h (Scheme 1).
Scheme 1.
When the same conditions were applied by us, GC analysis showed
thiomidazolide 2 as the minor compound after 96 h at room temperature,

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IMIDAZOYLTHIOCARBONYL INTERMEDIATES
2337
Table 1. Reaction Conditions to Prepare Thioimidazolide Derivative 2 from 1
TCDIa
(equiv.)
CH2Cl2
(ml)
conv (%)
[2, yieldb (%)]
Entry
1 (mmol)
t (h)
1
2
3
4
5
6
7
8
0.23
0.23
4.60
0.23
0.23
0.23
0.23
5.70
1.6 (A)
2.0 (C)
1.3
1.3
20.0
dropse
dropse
dropse
dropse
6.0
96
120
120
48
32
24
16c
> 98c [55]
> 99c [83]
52c
1.6 þ 1d (C)
1.2 (A)
1.2 (B)
1.2 (C)
1.2 (D)
1.2 (C)
78c
83f
100f [76]
100f [89]
24
8
aDifferent batches: A and B were approximately five months old, C had been open
approx, three weeks, and D opened just before starting the reactions. bThe numbers
in square brackets represents isolated yields of 2 for processes that are close to 100%
conversion. cCalculated by GC. dAfter 3 days, an extra equivalent was added.
eMinimum amount of solvent required to dissolve the reagents. fCalculated by NMR.
together with large amounts of starting material (entry 1, Table 1). With
the aim of increasing the degree of conversion, a larger excess of
TCDI (2 equiv.) was used (entry 2, Table 1). Thus, after five days, 98%
of conversion was achieved, although unfortunately a mixture of 4-TCI
derivative 2 and the corresponding 1,4-dithiocarbonyl compound, were
obtained in a ratio 4.5:1. To avoid the formation of the latter, we
added 1.6 equiv. at the beginning of the process, and an additional
equiv. of TCDI after 3 days (the point at which the reaction did
not evolve further), stirring being maintained for another two days (entry
3, Table 1). In these conditions, compound 2 was isolated with 83%
yield after purification by flash chromatography column, and only traces of
the 1,4-dithiocarbonyl derivative were obtained. The limitations, neverthe-
less, were the large excess of TCDI and the long reaction time needed.
After carrying out several experiments, we realized that both TCDI
batch and concentration could play a role in this process. We decided to
1
perform the experiments using different batches (mp and H-NMR spectra
of all of them are identical; see footnote a in Table 1) of TCDI, and the
minimum amount of solvent needed to dissolve the reagents. Moreover, we
carried out the reactions with only a slight excess of TCDI (1.2 equiv.) in
order not to waste this reagent. The importance of the concentration in the
course of the reaction was observed from entries 1 and 4 in Table 1. Even with
the smaller amount of TCDI, a 52% conversion was obtained in only two
days when just a few drops were used to dissolve the reagents, whilst in more

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2338
OVES ET AL.
dilute conditions four days were necessary to obtain a 16% of conversion.
If different batches of TCDI were used, shorter reaction times were needed to
achieve compound 2 with the newer reagent (entries 4–7, Table 1). Thus, old
batches of TCDI which were usually rejected because reactions did not evolve
more than 15% in standard conditions, in concentrated conditions, now gave
the desired product although in longer reaction times.
In order to scale-up the process, the reaction was carried out with
5.70 mmol of starting material and 1.2 equiv. of TCDI in 6 ml of methylene
chloride (entry 8, Table 1). Thus, derivative 2 was obtained in 8 h with 89%
isolated yield after purification by silica gel column.
Similar results were obtained when the above conditions were applied
to synthon 3 (Scheme 2).
Scheme 2.
Thus, entries 1 vs. 2 and 3 vs. 4 in Table 2 show the concentration
dependence in the formation of the corresponding imidazoylthiocarbonyl 4.
In highly concentrated conditions (entries 2 and 4, Table 2) total conver-
sions were obtained in short periods of time (11 and 6 h respectively) in
contrast to entries 1 and 3. The isolated yields were excellent (90–98%).
Table 2. Reaction Conditions to Prepare Thioimidazolide Derivative 4 from 3
TCDIa
(equiv.)
CH2Cl2
(ml)
convb (%)
[4, yieldc (%)]
Entry
3 (mmol)
t (h)
1
2
3
4
0.24
0.24
0.24
0.24
1.2 (C)
1.2 (C)
1.2 (D)
1.2 (D)
1.3
dropsd
1.3
dropsd
80
11
52
6
80
> 98 [90]
90
100 [98]
aDifferent batches: C had been open approx. three weeks, and D opened just before
b
c
starting the reactions. Conversion percentage of 3 calculated by NMR. The num-
bers in square brackets represents isolated yields of 4 for processes close to 100%
d
conversion. Minimum amount of solvent required to dissolve the reagents.

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IMIDAZOYLTHIOCARBONYL INTERMEDIATES
2339
These concentrated conditions were essential in the case of the strongly
sterically hindered substrate 5 (Scheme 3), which shows an inversion in the
5’-OTBDMSgroup with respect to compound 3. This characteristic means
that the substituents in positions 3, 4, and 5 are very close together in space,
and consequently the steric effects are increased substantially. Thus, the reac-
tion of 5 with TCDI in presence of N,N-dimethylaminopyridine as
catalyst and methylene chloride as solvent takes place with less than 10%
conversion after several days at room temperature followed by solvent reflux
(entry 1, Table 3). The change from methylene chloride to THF did not
improve the amount of 6 obtained.
Scheme 3.
On the other hand, when the smallest amount of solvent necessary
to dissolve the reagents and substrate 5 was used (entry 3, Table 3), the
thioimidazolide derivative 6 was obtained 86% yield in just 6 h at room
temperature.
Table 3. Reaction Conditions to Prepare Thioimidazolide Derivative 6 from 5
TCDIa
(equiv.)
Solvent
(ml)
convb (%)
[6, yieldc (%)]
Entry
5 (mmol)
t (h)
1
2
3
0.10
0.12
1.00
2.0
2.5
2.0
CH2Cl2 (3.0)
THF (5.0)
CH2Cl2 (1.0)
96d
96d
6
< 10
< 5
100 [86]
aBatch D (opened just before starting the reactions) was used with 0.5 equiv. DMAP
b
as catalyst. Conversion percentage of 5 calculated by GC. The numbers in square
c
d
brackets represent isolated yields of 6 for processes close to 100% conversion. The
reaction was carried out at rt for 3 d and 1 d under reflux.
In this paper, the concentration dependence of the preparation of
thiomidazolide derivatives from highly sterically hindered alcohols has
been demonstrated. The use of very concentrated solutions allows mild
reaction conditions and short reaction times, in addition to saving reagents

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2340
OVES ET AL.
and solvent, and thus making the reaction more environmentally friendly.
Moreover, old batches of TCDI can be used with these reaction conditions.
This is an improvement for the radical deoxygenation through thio-
midazolide intermediates of highly hindered alcohols that possess sensitive
functional groups, to the conditions used to introduce more reactive
thiocarbonyl compounds. These alcohols are present not only in derivatives
described in this paper, but in structures en route to other complex molecules
which possess very congested hydroxyl groups.
EXPERIMENTAL
General
Melting points were taken on samples in open capillary tubes using a
Gallenkamp apparatus and are uncorrected. IR spectra were recorded on a
Mattson 3000 Infrared Fourier Transform spectrophotometer using NaCl
plates. Flash chromatography was performed using Merck silica gel 60
1
(230–400 mesh). H-, 13C-NMR, and DEPT were obtained using a Bruker
AC-200 (1H, 200.13 MHz and 13C, 50.3 MHz) spectrometer for routine experi-
ments. Bruker AMX-400 spectrometer operating at 400.13 and 100.61 MHz
1
1
for H and 13C, respectively was used for the acquisition of H-1H homo-
nuclear and H-13C heteronuclear correlation experiments. The chemical
1
shifts are given in delta (d) values and the coupling constants (J) in Hertz
(Hz). Mass spectra (HRMSor MS) were recorded on a Finnigan MAT 95
spectrometer set at 70 eV EI (electron impact). The solvents were dried by
standard methods described in the literature. Gas chromatography was
carried out with flame ionization detection (FID) and a 25 m HP-1 capillary
column coated with methylsilicone gum using nitrogen as carrier gas. The
method used was: injector and detector temperatures set at 300ꢀC, column
initial temperature 250ꢀC (3 min), rate 18ꢀC/min, column final temperature
280ꢀC (10 min). With this method, compound 1 appeared at 4.8 min; 2 at
12.0 min; 5 at 5.7 min; and 6 at 12.6 min. Hydroxyquinate 1 was synthesised
as has been reported previously.10a Shikimate 3 was first reported by
Desmaele and Tanier.11 3,5-Disilyl protected compound 5 was obtained
from selective bis-protection of (À)-methyl 5-epi-shikimate.8c
General Procedure
1,10-Thiocarbonyldiimidazole was added to a solution of the alcohol in
methylene chloride or THF under nitrogen atmosphere (see details in Tables

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IMIDAZOYLTHIOCARBONYL INTERMEDIATES
2341
1, 2, and 3). The reaction mixture was stirred and monitored by GC. When
the reaction did not evolve further, the solvent was evaporated under
reduced pressure and the crude was purified by flash chromatography
using as eluent 40% EtOAc/hexane for compound 2, 15% EtOAc/hexane
for 4, and 10% EtOAc/hexane for 6. Yields are indicated in the tables.
Methyl (1sn,3R,4sn,5R)-3,5-di[(tert-Butyldimethylsilyl)oxy]-1-hydroxy-
4-[(imidazoylthioncarbonyl)oxy]cyclohexanecarboxylate (2). This compound
was previously reported.10a IR (NaCl): v 3250, 2953, 2923, 2887, 2857, 1742,
1
and 1532 cmÀ1; H NMR (CDCl3, 200 MHz): d À0.12 (s, 3H, MeSi), 0.00
(s, 3H, MeSi), 0.06 (s, 3H, MeSi), 0.08 (s, 3H, MeSi), 0.76 (s, 9H, Me3Csi),
2
3
0.89 (s, 9H, Me3CSi), 1.99 (dd, 1H, JHH 13.3, JHH 10.1 Hz, H-6a), 2.07
2
(ddd, 1H, JHH 14.5, JHH 4.3, JHH 2.7 Hz, H-2e), 2.27 (dd, 1H, JHH
3
4
2
3
2
3
4
14.6, JHH 2.7 Hz, H-2a), 3.31 (ddd, 1H, JHH 13.5, JHH 5.0, JHH
3
2.7 Hz, H-6e), 3.79 (s, 3H, OMe), 4.56 (ddd, 1H, JHH 10.1, 9.0, 4.9 Hz,
3
3
H-5), 4.64 (ddd, 1H, JHH 4.4, 2.8, 2.8 Hz, H-3), 5.50 (dd, 1H, JHH 9.0,
2.7 Hz, H-4), 7.05 (m, 1H, H-im), 7.62 (m, 1H, H-im), and 8.37 (m, 1H,
H-im); 13C NMR (CDCl3, 50.3 MHz): d À6.0 (MeSi), À5.3 (MeSi), À5.1
(MeSi), À4.5 (MeSi), 17.3 (Me3CSi), 17.4 (Me3CSi), 25.1 (Me3CSi), 25.3
(Me3CSi), 37.6, 42.6 (C-2 and C-6), 52.4 (OMe), 65.1, 68.0 (C-3 and C-5),
74.8 (C-1), 87.5 (C-4), 117.4 (im), 130.5 (im), 136.8 (im), 173.1 (C ¼ O), and
183.5 (C ¼ S); MS (70 eV, m/z): 487 (Mþ-tBu, 10%), 427 (9), 359 (21), 267
(47), 227 (40), 185 (54), and 73 (100); HRMS: Calcd for C20H35N2O6SSi2:
487.1754. Found: 487.1754.
Methyl (3R,4S,5R)-3,5-di[(tert-Butyldimethylsilyl)oxy]-4-[(imidazoyl-
thiocarbonyl)oxy]cyclohex-1-enecarboxylate (4). IR (NaCl): v 3135, 2940,
1
1722, 1656, 1531, and 1470 cmÀ1; H NMR (CDCl3, 200 MHz): d À0.01
(s, 3H, MeSi), 0.08 (s, 3H, MeSi), 0.10 (s, 6H, MeSi), 0.81 (s, 9H,
Me3CSi), 0.83 (s, 9H, Me3CSi), 2.29–2.68 (m, 2H, H-6), 3.76 (s, 3H, OMe),
4.44 (m, 1H, H-5), 4.30 (br s, 1H, H-3), 5.62 (m, 1H, H-4), 6.77 (m, 1H,
H-2), 6.99 (br s, 1H, H-im), 7.54 (br s, 1H, H-im), and 8.36 (br s, 1H,
H-im); 13C-NMR (CDCl3, 50.3 MHz): d À5.2 (MeSi), À5.0 (MeSi), À4.94
(MeSi), À4.89 (MeSi), 17.6 (Me3CSi), 17.7 (Me3CSi), 25.40 (Me3CSi), 25.43
(Me3CSi), 31.1 (C-6), 51.9 (OMe), 64.8, 65.0 (C-3 and C-5), 81.3 (C-4), 117.8
(im), 128.3 (C-1), 130.7 (im), 136.7 (im), 137.4 (C-2), 166.4 (C ¼ O), and 183.9
(C ¼ S); MS (70 eV, m/z): 526 (Mþ, 5%), 469 (100), 409 (41), 341 (51), 267
(42), 235 (31), and 185 (68); HRMS: Calcd for C24H42N2O5SSi2: 526.2353.
Found: 526.2342.
Methyl (3R,4S,5S)-3,5-di[(tert-Butyldimethylsilyl)oxy]-4-[(imidazoyl-
thiocarbonyl)oxy]cyclohex-1-enecarboxylate (6). IR (NaCl): v 3134, 2949,
1
2897, 1722, 1656, 1531, and 1464 cmÀ1; H NMR (CDCl3, 200 MHz): d
0.07 (s, 3H, MeSi), 0.09 (s, 3H, MeSi), 0.10 (s, 3H, MeSi), 0.11 (s, 3H,
MeSi), 0.77 (s, 9H, Me3CSi), 0.84 (s, 9H, Me3CSi), 2.39 (m, 1H, H-6),

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OVES ET AL.
2.70 (dd, 1H, 2JHH 17.5, 3JHH 6.4 Hz, H-6), 3.80 (s, 3H, OMe), 4.09 (m, 1H,
H-5), 4.62 (m, 1H, H-3), 6.13 (m, 1H, H-4), 6.61 (m, 1H, H-2), 6.99 (s, 1H,
H-im), 7.54 (s, 1H, H-im), and 8.22 (s, 1H, H-im); 13C NMR (CDCl3,
50.3 MHz): d À5.1 (MeSi), À5.05 (MeSi), À5.02 (MeSi), À4.9 (MeSi), 17.8
(Me3CSi), 17.9 (Me3CSi), 25.4 (Me3CSi), 25.5 (Me3CSi), 30.3 (C-6), 52.1
(OMe), 67.5 (C-5), 68.2 (C-3), 82.1 (C-4), 118.0 (im), 128.7 (C-1), 130.5 (im),
136.7 (im), 138.2 (C-2), 166.2 (C ¼ O), and 184.8 (C ¼ S); MS (70 eV, m/z):
526 (Mþ, < 1%), 469 (35), 341 (21), 267 (79), 227 (46), 185 (90), and 73
(100); HRMS: Calcd for C24H42N2O5SSi2: 526.2353. Found: 526.2351.
ACKNOWLEDGMENTS
Financial support from Comision Interministerial de Ciencia y
Tecnologıa (Spain; Project BIO98-0770) and Fundacion Prıncipe de
Asturias (‘‘Dr. Severo Ochoa’’ Award to M.F.) is gratefully acknowledged.
We also thank the Ministerio de Educacion y Cultura (Spain) for post-
doctoral fellowships (S.F. and M.F.).
REFERENCES
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8. (a) Gotor-Fernandez, V.; Ferrero, M.; Fernandez, S.; Gotor, V.
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10. (a) Periman, K.L.; Swenson, R.E.; Paaren, H.E.; Schnoes, H.K.;
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Received in the UK September 18, 2000

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