General access to a novel class of silyl heterocycles

Article abstract of DOI:10.1007/s10593-006-0280-2

2-Trimethylsilyl-substituted five-membered heterocycles can be accessed through the reaction of bromo(methoxy)methyltrimethylsilane with 1,2-dithiols, 1,2-mercapto alcohols, 1,2-mercapto amines, and 1,2-hydroxy amines, leading to the formation of several

Full text of DOI:10.1007/s10593-006-0280-2

Chemistry of Heterocyclic Compounds, Vol. 42, No. 12, 2006
GENERAL ACCESS TO A NOVEL CLASS
OF SILYL HETEROCYCLES
A. Degl'Innocenti, A. Capperucci, I. Malesci, M. Acciai, and G. Castagnoli
2-Trimethylsilyl-substituted five-membered heterocycles can be accessed through the reaction of
bromo(methoxy)methyltrimethylsilane with 1,2-dithiols, 1,2-mercapto alcohols, 1,2-mercapto amines,
and 1,2-hydroxy amines, leading to the formation of several 2-silylated 1,3-dithiolanes, -oxathiolanes, -
thiazolidines, and -oxazolidines.
Keywords: dithiolanes, organosilanes, oxathiolanes, oxazolidines, silyl heterocycles, thiazolidines.
Five-membered ring heterocyclic derivatives are valuable intermediates in organic synthesis.
Compounds containing either one heteroatom, as furans and pyrrolidines, or two or more heteroatoms have
played an important role in different synthetic strategies. Among these the last structures 1,3-dioxolanes, [1, 2]
1,3-oxathiolanes, [3] 1,3-thiazolidines, 1,3-oxazolidines, imidazolines, and benzotriazole derivatives [4–8] have
found wide application as building blocks for the construction of more complex molecules [9].
Furthermore, the umpolung reactivity is a valuable synthetic strategy in organic synthesis providing
unconventional access to molecules through the formation of bonds via the inversion of normal reactivity [10].
In this context the development of synthetic equivalents of acyl anions has recently actracted a great deal of
interest for the synthetic potential that such reaction may disclose. In this connection suitably protected carbonyl
derivatives, in particular heterocyclic derivatives, represent a very interesting class of compounds able to act as
equivalents of acyl anions. For example, lithiated chiral oxazolidinones, including compounds derived from
camphor [11], have been reported to react with aldehydes, ketones, and imines leading to enantioselective
formylation of the carbonyl compounds [12, 13]. Oxazolidines also found application as chiral formyl anion
equivalents, either for direct metalation in the presence of (–)-sparteine [14] or through transmetalation
of tributylstannyl derivative with BuLi and condensation with benzaldehyde [15, 16]. Also thiazolidines upon
treatment with BuLi and aldehydes or ketones in the presence of (–)-sparteine afforded products with high ee but
moderate diastereoselectivity [17, 18]. Isopropyl N-Boc-thiazolidines have been used as chiral organolithium
compounds in the addition to aldehydes, leading to products with good stereoselectivity [19]. Thiazolidines are also
metalated with BuLi via their respective formamidines, but 40-50% fragmentation of the heterocyclic ring has been
observed [20]. Chiral dioxolanes [15] and dioxolanones [2] also found application as acyl anion equivalents.
1,3-Dithianes have been also widely used in umpolung reactivity. They can be easily metalated with
BuLi and reacted with a wide range of electrophiles, thus evidencing their behavior as useful synthons and
umpoled reagents [21-27]; though several methods for their unmasking have been reported [28, 29],
_______
* Dedicated to Prof. Edmunds Lukevics on the occasion of his 70th birthday
__________________________________________________________________________________________
Department of Organic Chemistry and HBL, University of Florence, 50019 Sesto Fiorentino, Italy; e-mail:
alessandro.deglinnocenti@unifi.it. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1838-1844,
December, 2006. Original article submitted September 20, 2006.
1568
0009-3122/06/4212-1568©2006 Springer Science+Business Media, Inc.

they still suffer from the generally harsh conditions of their unblocking. On the other hand, dioxolanes,
oxathiolanes, dithiolanes, and thiazolidines have been reported to be unmasked under milder conditions, thus
possibly giving a broader spectrum of application to such molecules in the generation of formyl and acyl anion
equivalents. Unfortunately such heterocycles suffer generally from the difficulties in their functionalization
under strong basic conditions. For instance, 1,3-dithiolanes, upon treatment with bases, have been reported to
undergo either deprotonation at C-2 with subsequent cycloelimination to dithiocarboxylate anions and ethylene
derivatives [30] or at C-4 to afford products derived from thiocarbonyl derivatives and vinyl thiolate anion [31,
32]. A similar behavior was evidenced in unsubstituted dioxolanes, whose anions undergo fragmentation.
Thiazolidines are also reported to be metalated but only in the presence of specific nitrogen-protecting groups.
Only two examples of functionalization under the basic conditions of dithiolanes bearing an electron
withdrawing group are reported in the literature, thus evidencing the still present need for a general protocol for
their functionalization [33-36]. Thus, taking advantage of the reactivity of organosilanes, which seems to occur
via a pentacoordinated silicon species, and not a free carbanion, we envisaged that the functionalization of the
C–Si bond in silyl heterocycles could possibly lead to the solution of this problem, and to the development of
a novel and general functionalization methodology for such labile heterocycles [37].
As direct access to silyl heterocycles is difficult or even prevented, an alternative access to such
molecules had to be devised.
A general access to silyl dithiolanes, and, more generally, to five-membered ring silylated heterocycles
could be envisaged through the simple reaction of bifunctional molecules such as dithiols, amino alcohols,
amino thiols, and mercapto alcohols with formyl trimethylsilane. The difficulties in the generation and handling
of such a labile molecule [38] led to the search for a possible synthetic equivalent, and we envisaged the reagent
of choice in bromo(methoxy)methyltrimethylsilane (2). Such molecule, in fact, can be obtained in quantitative
yield by the treatment of the commercially available methoxymethyltrimethylsilane (1) with bromine [39-41]
and subsequent one-pot treatment with the required thiol.
Thus, we reacted 1,2-ethanedithiol with bromomethoxy derivative 2, and we were able to isolate in 72%
yield the corresponding 2-trimethylsilyl-1,3-dithiolane (3a), which could be easily purified on TLC. This
reaction evidenced the ability of compound 2 to act as a real synthetic equivalent of formylsilane and opened the
way to a possible general route to access a wide variety of silyl heterocycles.
R
R
S
R
R
MeO
SiMe3
Br2
SH
HS
MeO
SiMe3
S
Br
SiMe3
1
2
3a,b
a R = H, b R = Me
In a similar way, by reacting meso-butane-2,3-dithiol both cis and trans diastereoisomers of meso-
4,5-dimethyl-2-trimethylsilyl-1,3-dithiolane (3b) were isolated.
To study the generality of this reaction, the freshly prepared compound 2 was reacted in situ with
β-mercaptoethanol (4a) at room temperature in the presence of diisopropylethylamine (DIPEA) and stirred
overnight. In these conditions the intermediate compound 5a was obtained. To achieve cyclization after stirring
the reaction mixture overnight in the presence of DIPEA, conc. HCl was added in the same flask and the mixture
was stirred for 90 min to obtain the desired 2-trimethylsilyl-1,3-oxathiolane (6a).
Due to the instability of these heterocycles under acidic conditions the treatment with HCl must not be longer
than 90 min. If the intermediate product is treated under acidic conditions for a longer time, oxathiolane ring
fragmentation was observed.
5-(Methoxymethyl)-2-trimethylsilyl--1,3-oxathiolane (6b) as a mixture of diastereoisomers (6:1) was
obtained similarly.
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R
R
O
R
DIPEA
CH2Cl2
OH
conc. HCl
90 min
+
2
S
OH
S
OMe
HS
SiMe3
4a,b
SiMe3
5a,b
6a,b
a R = H, b R = CH2OMe
The reaction of silane 2 in situ with N-protected aminoethanethiols 7a,c afforded smoothly 2-silyl-
N-protected 1,3-thiazolidines 8a and 8c.
Moreover, in the reaction with N-Boc-protected 2-amino-4-methylpentane-1-thiol (7b) the substituted
4-isobutyl-N-Boc-2-trimethylsilylthiazolidine (8b) was obtained as an equimolar mixture of cis and trans isomers.
R
N
S
BOC
R
1. DIPEA
SiMe3
8a,b
+
2
NHPg
HS
+
2. H
7a–c
N
S
Cbz
8c
SiMe3
8 a R = H, b R = i-Bu
7 a R = H, Pg = BOC, b R = i-Bu, Pg = BOC, c R = H, Pg = Cbz;
The treatment of compound 2 with a suitably N-protected 2-aminoethanol 9 led to the isolation of the
N-Cbz-2-silyl-1,3-oxazolidine 10.
1. DIPEA
N
O
Cbz
+
Cbz
2
NH
HO
+
2. H
9
SiMe3
10
In conclusion we have reported a novel and general protocol for the synthesis of a variety of silyl
heterocycles through simple and high yielding procedures, thus opening the way to a thorough investigation of
their chemistry.
EXPERIMENTAL
13
1H and C NMR spectra were recorded on a Varian Gemini 200 (200 and 50 MHz, respectively) and a
Varian Mercury 400 (400 and 100 MHz, respectively) in CDCl3 solutions. Chemical shifts are given relative to
the residue signals of the solvent (1H: 7.26 ppm; 13C: 77.0 ppm). Mass spectra were obtained on a
MFC500/QMD 1000 Carlo Erba instrument and a Shimadzu GCMS-QP5050/GC 17A apparatus with electron
impact ionization (70 or 30 eV).
2-Trimethylsilyl-1,3-dithiolanes 3a,b (General procedure). A solution of bromine (2.57 mmol) in dry
CCl4 (4 ml) was added dropwise with stirring under N2 to a solution of methoxymethyltrimethylsilane
(2.57 mmol) in dry CCl4 (5 ml). The mixture was stirred until the bromine disappeared. A solution of the suitable
dithiol (2.57 mmol) in CH2Cl2 (5 ml) was then added and the mixture was stirred overnight. After
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washing with water the organic layer was dried (Na2SO4) and evaporation of the solvent gave the crude product,
which was purified by chromatography (column or TLC).
2-Trimethylsilyl-1,3-dithiolane (3a). TLC petroleum ether–diethyl ether, 20:1 (72%). 1H NMR
spectrum, δ, ppm (J, Hz): 0.18 (9H, s, SiMe3); 3.06-3.29 (4H, m); 3.53 (1H, s, HCSiMe3). 13C NMR spectrum, δ,
ppm: -2.3 (Si(CH3)3); 37.6; 39.4. Mass spectrum, m/z (Irel, %): 178 (41) [M]+, 135 (97), 105 (72), 91 (25), 75
(63), 73 (100), 59 (81). Found, %: C 40.16; H 8.12. C6H14S2Si. Calculated, %: C 40.40; H 7.91.
meso-4,5-Dimethyl-2-trimethylsilyl-1,3-dithiolane (3b). TLC hexanes–ethyl acetate, 200:1 (82%). A
1
mixture of the two isomers, 1:3. cis Isomer: H NMR spectrum, δ, ppm (J, Hz): 0.14 (9H, s, SiMe3);
1.25-1.28 (6H, m); 3.50-3.64 (2H, m); 3.69 (1H, s, HCSiMe3). 13C NMR spectrum, δ, ppm: -2.5 (Si(CH3)3);
16.5; 36.9; 53.2. Mass spectrum, m/z (Irel, %): 206 (5) [M]+, 150 (11), 135 (32), 73 (100), 59 (16). Found, %:
1
C 46.20; H 8.90. C8H18S2Si. Calculated, %: C 46.54; H 8.79. trans Isomer: H NMR spectrum, δ, ppm (J, Hz):
0.16 (9H, s, SiMe3); 1.31 (6H, d, J = 6.6, CH3); 3.62–3.75 (2H, m); 3.81 (1H, s, HCSiMe3). 13C NMR spectrum,
δ, ppm: -2.6 (Si(CH3)3); 15.5; 33.7; 53.4. Mass spectrum, m/z (Irel, %): 206 (30) [M]+, 163 (25), 150 (71),
135 (100), 73 (89), 59 (56). Found, %: C 46.32; H 9.06. C8H18S2Si. Calculated, %: C 46.54; H 8.79.
2-Trimethylsilyl-1,3-oxathiolanes 6a,b (General procedure). A solution of freshly prepared
bromo(methoxy)methyltrimethylsilane (2) (4.24 mmol) (obtained from methoxymethyl trimethylsilane and
bromine in CCl4) was slowly added at room temperature with a solution of the appropiate β-mercapto alcohol
(4.24 mmol) and DIPEA (5.08 mmol) in 23 ml of anhydrous CH2Cl2. The mixture was stirred overnight and then
treated with HCl 12M (5 ml) for 90 min. After dilution with CH2Cl2 and addition with water, the mixture was
stirred for 5-10 min and then transferred into a separatory funnel. Solid NaHCO3 was slowly added to neutralize
the solution. The organic phase was washed with water (2 × 15 ml) and dried over Na2SO4. Evaporation of the
solvent afforded crude compounds, which were chromatographically purified.
1
2-Trimethylsilyl-1,3-oxathiolane (6a). TLC petroleum ether–ethyl acetate, 4:1 (70%). H NMR spectrum,
δ, ppm (J, Hz): 0.13 (9H, s, SiMe3); 2.79-2.93 (1H, m, HCHS); 3.03-3.12 (1H, m, HCHS); 3.49-3.62 (1H, m,
HCHO); 4.37-4.47 (1H, m, HCHO); 4.50 (1H, s, HCSiMe3). 13C NMR spectrum, δ, ppm: -3.7 (Si(CH3)3); 32.8
(CH2S); 73.7 (CH2O); 78.7 (HCSiMe3). Mass spectrum, m/z (Irel, %): 163 (2) [M+1]+, 162 (1) [M]+, 134 (25), 119
(100), 89 (15), 73 (74). Found, %: C 44.22; H 8.74. C6H14OSSi. Calculated, %: C 44.39; H 8.69.
5-(Methoxymethyl)-2-trimethylsilyl-1,3-oxathiolane (6b). A mixture of diastereoisomers (6:1) was
purified on TLC (petroleum ether–ethyl acetate, 100:1, 52%). Major isomer: 1H NMR spectrum, δ,
ppm (J, Hz): 0.12 (9H, s, SiMe3); 2.65 (1H, dd, J = 8.2, J = 10, HCHS); 3.09 (1H, dd, J = 6.0, J = 10, HCHS);
3.40 (3H, s, CH3O); 3.44-3.61 (2H, m, CH2O); 3.96-4.08 (1H, m, CHO); 4.65 (1H, s, HCSiMe3). 13C NMR
spectrum, δ, ppm: -3.6 (Si(CH3)3); 35.1 (CH2S); 59.4 (CH3O); 73.5 (CH2O); 77.2 (HCSiMe3); 84.5 (CHO). Mass
spectrum, m/z (Irel, %): 206 (2) [M]+, 146 (55), 135 (60), 119 (100), 91 (49), 73 (57), 59 (38). Found, %: C 46.38;
1
H 8.84. C8H18O2SSi. Calculated, %: C 46.56; H 8.79. Minor isomer: H NMR spectrum, δ, ppm (J, Hz): 0.12
(9H, s, SiMe3); 2.82–2.88 (2H, m, CH2S); 3.33–3.40 (2H, m, CH2O, partially overlapped with signal at 3.38);
3.38 (3H, s, CH3O); 4.38–4.51 (1H, m, CHO); 4.68 (1H, s, HCSiMe3). 13C NMR spectrum, δ, ppm: –3.5
(Si(CH3)3); 39.9 (CH2S); 59.3 (CH3O); 72.7 (CH2O); 77.2 (HCSiMe3); 82.1 (CHO). Mass spectrum, m/z (Irel, %):
206 (1) [M]+, 146 (21), 135 (32), 119 (100), 91 (23), 73 (63), 59 (24).
2-Trimethylsilyl-1,3-thiazolidines 8a–c (General procedure). To a freshly prepared solution of
compound 2 (5 mmol) (obtained from methoxymethyltrimethylsilane and bromine in CCl4) were added at room
temperature anhydrous CH2Cl2 (10 ml), the appropriate N-protected 2-mercapto amine (5 mmol) dissolved in
5 ml of CH2Cl2, and DIPEA (1.2 eq). The mixture was stirred overnight, then treated with p-toluenesulfonic acid
(0.1 eq) for 90 min. The resulting mixture was washed with water and brine, and the organic phase dried over
Na2SO4. Evaporation of the solvent afforded the crude N-protected thiazolidines, which were purified by flash
chromatography.
tert-Butyl 2-(trimethylsilyl)thiazolidine-3-carboxylate (8a). Petroleum ether–ethyl acetate, 8:1 (81%).
Rf 0.8. 1H NMR spectrum, δ, ppm (J, Hz): 0.10 (9H, s, SiMe3); 1.46 (9H, s, (CH3)3); 2.81-2.92 (2H, m, CH2S);
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3.10-3.30 (1H, m, HCHN); 4.10-4.35 (1H, m, HCHN); 4.60 (1H, s, HCSiMe3). 13C NMR spectrum, δ, ppm: -2.6
(Si(CH3)3); 28.3; 30.5; 49.4; 50.9; 80.1; 154.0. Mass spectrum, m/z (Irel, %): 263 (0.5) [M+2]+, 204 (89), 188
(10), 160 (100), 132 (56), 100 (63), 88 (94), 73 (98). Found, %: C 50.30; H 9.05; N 5.08. C11H23NO2SSi.
Calculated, %: C 50.53; H 8.87; N 5.36.
tert-Butyl 4-isobutyl-2-(trimethylsilyl)thiazolidine-3-carboxylate (8b). A mixture of diastereo-
isomers (1:1) was purified on TLC petroleum ether–ethyl acetate, 10:1 (53%). Rf 0.9 and 0.7. trans Isomer:
1H NMR spectrum, δ, ppm (J, Hz): 0.12 (9H, s, SiMe3); 0.95 (6H, d, J = 5.8, CH(CH3)2); 1.38-1.61 (3H, m,
CH2CH(CH3)2); 1.47 (9H, s, (CH3)3); 2.63 (1H, dd, J = 3.8, J = 11.6, HCHS); 3.02 (1H, dd, J = 6.4, J = 11.6,
HCHS); 4.22-4.37 (1H, m, HCHN); 4.7 (1H, s, HCSiMe3). 13C NMR spectrum, δ, ppm: -1.9 (Si(CH3)3); 21.9;
23.8; 25.8; 28.6; 36.1; 43.8; 52.3; 60.0; 80.0; 157.1. Mass spectrum, m/z (Irel, %): 260 (07) [M–57]+, 188 (5), 144
(7), 73 (56), 57 (100). Found, %: C 56.48; H 10.08; N 4.60. C15H31NO2SSi. Calculated, %: C 56.73; H 9.84;
1
N 4.41. cis Isomer: H NMR spectrum, δ, ppm (J, Hz): 0.15 (9H, s, SiMe3); 0.94 (6H, d, J = 5.8, CH(CH3)2);
1.40-1.59 (3H, m, CH2CH(CH3)2); 1.46 (9H, s, (CH3)3); 2.65 (1H, dd, J = 2.0, J = 1.2, HCHS); 3.03 (1H, dd,
J = 6.8, J = 11.2, HCHS); 4.10 (1H, s, HCSiMe3); 4.26-4.36 (1H, m, HCHN). 13C NMR spectrum, δ, ppm: -
0.6 (Si(CH3)3); 22.1; 23.5; 25.7; 28.5; 36.0; 42.4; 52.1; 59.5; 80.1; 157.1. Mass spectrum, m/z (Irel, %): 260 [M–
57]+ (07), 188 (5), 144 (7), 73 (56), 57 (100). Found, %: C 56.60; H 9.98; N 4.39. C15H31NO2SSi.
Calculated, %: C 56.73; H 9.84; N 4.41.
Benzyl 2-(trimethylsilyl)thiazolidine-3-carboxylate (8c). Petroleum ether–ethyl acetate, 4:1 (56%).
1
Rf 0.8. H NMR spectrum, δ, ppm (J, Hz): 0.09 (9H, s, SiMe3); 2.85-2.92 (2H, m, CH2S); 3.23-3.42 (1H, m,
HCHN); 4.11-4.37 (1H, m, HCHN); 4.61 (1H, s, HCSiMe3), 5.09 (1H, d, J = 14, HCHPh); 5.18 (1H, d, J = 14,
13
HCHPh); 7.33-7.37 (5H, m, H–CPh). C NMR spectrum, δ, ppm: -2.5 (Si(CH3)3); 30.6; 50.0; 51.8; 67.5; 128.1;
128.4; 128.6; 139.8; 154.6. Mass spectrum, m/z (Irel, %): 204 (8) [M–91]+, 160 (22), 91 (100), 73 (44). Found, %:
C 56.66; H 7.44; N 4.45. C14H21NO2SSi. Calculated, %: C 56.91; H 7.16; N 4.74.
2-Trimethylsilyl-1,3-oxazolidines (General procedure). To a freshly prepared solution of compound 2
(8.7 mmol) (obtained from methoxymethyltrimethylsilane and bromine in CCl4) was added at room temperature
anhydrous CH2Cl2 (10 ml), the appropriate N-protected β-amino alcohol (8.7 mmol) dissolved in 5 ml of CH2Cl2,
and DIPEA (1.2 eq). The mixture was stirred overnight, then was treated with p-toluenesulfonic acid (0.1 eq) for
90 min. The resulting mixture was washed with water and brine, and the organic phase was dried over Na2SO4.
Evaporation of the solvent afforded the crude N-protected oxazolidines, which were purified by flash
chromatography.
Benzyl 2-(trimethylsilyl)oxazolidine-3-carboxylate (10). Petroleum ether/ethyl acetate, 5:1 (52%).
1H NMR spectrum, δ, ppm (J, Hz): 0.07 (9H, s, SiMe3); 3.19-3.34 (1H, m, HCHN); 3.63-3.81 (2H, m,
HCHN+HCHO); 4.01-4.12 (1H, m, HCHO); 4.77 (1H, s, HCSiMe3); 5.10-5.16 (2H, m, CH2Ph); 7.35-7.37 (5H,
m, H–CPh). 13C NMR spectrum, δ, ppm: -3.1 (Si(CH3)3); 45.7; 67.0; 67.9; 85.7; 127.9; 128.3; 138.6; 156.5. Mass
spectrum, m/z (Irel, %): 264 (0.5) [M–15]+, 188 (7), 144 (26); 91 (100), 73 (43). Found, %: C 59.96; H 7.72;
N 5.17. C14H21NO3Si. Calculated, %: C 60.18; H 7.58; N 5.01.
Financial support by the National Project ''Stereoselezione in Sintesi Organica. Metodologie ed
Applicazioni'' (MUR, Roma) is gratefully acknowledged. M. A. thanks Ente Cassa di Risparmio di Firenze for
financial support. Ente Cassa di Risparmio di Firenze is also acknowledged for granting a 400 MHz NMR
spectrometer. Mrs. B. Innocenti is acknowledged for carrying out elemental analyses.
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