Two approaches to the enantioselective synthesis of (4R)-(-)-4-hydroxymethyl-4-thiobutyro-1,4-lactone

Article abstract of DOI:10.1016/S0957-4166(99)00568-6

Enantiomerically pure (4R)-4-hydroxymethyl-4-thiobutyro-1,4-lactone [(5R)-dihydro-5-(hydroxymethyl)-2(3H)-thiophenone (12)] and derivatives were synthesized by two enantiospecific sequences employing D-ribono-1,4-lactone (1) and L-glutamic acid (6) as chiral templates. The key step in the first approach was the SmI₂-promoted 2,3-deoxygenation of a 4-thio-L-lyxono-1,4-lactone derivative, prepared from 1. The other strategy, which starts from 6, involves the (5S)-dihydro-5-(p-tolylsulfonyloxymethyl)-2-(3H)-furanone (8) as chiral precursor. This was converted into a 4,5-thiirane derivative via the corresponding 4,5-epoxide. Regioselective opening of the thiirane ring by acetate followed by O-deacetylation gave 12 (40% overall yield from 8). Copyright (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0957-4166(99)00568-6

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 11 (2000) 765–771
Two approaches to the enantioselective synthesis of
(4R)-(−)-4-hydroxymethyl-4-thiobutyro-1,4-lactone
Patricia A. Zunszain and Oscar Varela ∗
CIHIDECAR-CONICET, Departamento de Química Orgánica, Facultad de Ciencias Exactas y Naturales,
Universidad de Buenos Aires, Ciudad Universitaria, Pabellón II, 1428 Buenos Aires, Argentina
Received 25 October 1999; accepted 24 November 1999
Abstract
Enantiomerically pure (4R)-4-hydroxymethyl-4-thiobutyro-1,4-lactone [(5R)-dihydro-5-(hydroxymethyl)-
2(3H)-thiophenone (12)] and derivatives were synthesized by two enantiospecific sequences employing
D-ribono-1,4-lactone (1) and L-glutamic acid (6) as chiral templates. The key step in the first approach was
the SmI2-promoted 2,3-deoxygenation of a 4-thio-L-lyxono-1,4-lactone derivative, prepared from 1. The other
strategy, which starts from 6, involves the (5S)-dihydro-5-(p-tolylsulfonyloxymethyl)-2-(3H)-furanone (8) as chiral
precursor. This was converted into a 4,5-thiirane derivative via the corresponding 4,5-epoxide. Regioselective
opening of the thiirane ring by acetate followed by O-deacetylation gave 12 (40% overall yield from 8). © 2000
Elsevier Science Ltd. All rights reserved.
1. Introduction
In the last two decades a number of natural products containing a thiolactone ring have been
isolated.1–4 Many of them such as thiolactomycin,1 thiotetromycin,2 thiocoumarins,3 and other
thiolactones4 displayed interesting biological activities. Small ring thiolactones have been prepared
by addition of thioacetic acid to unsaturated, straight chain acids,5 and from bismetallated derivatives
of thioacids and carbonyl compounds.6 Also, the reaction of S-(4-alkenyl)-dithiocarbonates with
tri-n-butyltin hydride afforded 1,4-thiolactones.7 More recently, saturated thiolactones have been
prepared from ω-halo acid chlorides, by a sulfur transfer reaction mediated by benzyltriethylammonium
tetrathiomolybdate,8 and ω-carboxyacylsilanes have been used as precursors of unsaturated silylated
thiolactones.9 Also, activated carboxythiolactones have been synthesized as small molecules useful for
the preparation of peptidil immunogen constructs.10 Despite the importance of thiolactones, relatively
little attention has been given to their synthesis in enantiomerically pure forms.11,12 For example,
∗
Corresponding author. E-mail: varela@qo.fcen.uba.ar
0957-4166/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(99)00568-6
tetasy 3202

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thioascorbic acid,13 thiolactomycin,14 and the anticonvulsant agent thiolosigamone,15 have been
obtained as racemates. We considered that thiolactones having a stereocenter at C-4 as controlling
element of asymmetry would be useful building blocks for the synthesis of natural products and their
sulfur containing analogs. Thus, we have already reported a convenient procedure for the synthesis
of optically pure aldopentono-1,4-thiolactones.16 In connection with these studies and others on the
synthesis and reactivity of 4-thiosugars,16–21 we wish to report here two sequences for the preparation
of (4R)-(−)-4-hydroxymethyl-4-thiobutyro-1,4-lactone (12) and derivatives, employing alternatively
D-ribono-1,4-lactone (1) or L-glutamic acid (6) as chiral templates.
2. Results and discussion
4-Thio-L-lyxono-1,4-lactone (2) constitutes a suitable precursor for the synthesis of (4R)-4-
hydroxymethyl-4-thiobutyro-1,4-lactone (12). Compound 2 was readily prepared,16 and in good
yield, starting from D-ribono-1,4-lactone (1). α,β-Deoxygenation of 2 would lead to the desired
dideoxythiolactone 5. Samarium iodide (SmI2) has been successfully employed for the deoxygenation
and unsaturation of common aldonolactone derivatives. Inanaga and co-workers22 described an easy
access to 2-deoxysugar lactones by SmI2-promoted deacetoxylation of per-O-acetyl aldonolactone
derivatives. Hanessian et al.23 employed SmI2 in THF for the 2,3-unsaturation of per-O-benzoyl
derivatives of aldonolactones. Similarly, we have observed that 5-O-silyl derivatives of 2,3-di-O-acyl-
D-ribonolactone reacted with SmI2–THF to give the corresponding butenolides.24 Such butenolides
could be readily hydrogenated to the corresponding 2,3-dideoxylactone derivatives, or they might even
undergo the SmI2-induced reduction of analogous α,β-unsaturated carbonyl compounds.25 This reaction
takes place in the presence of t-butanol–N,N-dimethylacetamide26 or just hexamethylphosphoramide.27
In order to apply these procedures to the synthesis of 5, 2,3-di-O-benzoyl-5-O-tert-butyldiphenylsilyl-
4-thio-L-lyxono-1,4-lactone (4) was prepared (Scheme 1). Treatment of 2 with 1.1 molar equivalent of
t-butylchlorodiphenylsilane in DMF, and in the presence of imidazole, selectively afforded the 5-O-silyl
derivative 3 in 77% yield. Conventional benzoylation of 3 gave the dibenzoate 4, which was treated with
a freshly prepared solution of SmI2 in THF,28 under a variety of conditions. The crude products were
1
monitored by H NMR, and it was observed that the 4-thiobutenolide was accompanied by variable
proportions of the dideoxythiolactone 5. When the reaction was performed in the presence of t-butanol as
proton source, no thiobutenolide was formed and compound 5 was isolated by column chromatography
in 53% yield.
Scheme 1. (a) ButPh2SiCl (1.1 molar equiv.), C3H4N2, DMF; (b) PhCOCl (BzCl), C5H5N; (c) 0.1 M SmI2 (3 molar equiv.),
THF, 0°C, 5 min; 0.1 M SmI2 (2 molar equiv.), THF, ButOH (2.5 molar equiv.), 0→25°C, 2 h

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767
A more direct, and higher yielding, sequence for the synthesis of the 4-thiolactone was attempted,
employing, in this case, L-glutamic acid (6) as chiral template (Scheme 2). Nitrous acid deamination of
6 afforded the (4S)-(+)-4-carboxy-1,4-butyrolactone, which was reduced with borane in THF to give 7.29
Tosylation of the free HO group of 7 with tosyl chloride in dry chloroform containing about 2 molar
equivalents of pyridine30 gave the tosylate 8 (83% yield from 7). On treatment with sodium methoxide in
methanol, compound 8 underwent opening of the lactone ring by methanolysis, followed by nucleophilic
attack of the resulting C-4 alkoxide to C-5, with displacement of the tosylate, affording the oxirane 9. The
formation of the oxirane ring was evidenced by the spectral data of 9. Its 13C NMR spectrum showed a
strong upfield shifting (> 20 ppm) for the signals of C-4 and C-5, which are now incorporated within the
three-membered oxirane ring, relative to the same signals of 8. Reaction of 9 with thiourea in methanol
produced the replacement of the ring oxygen atom by sulfur, to give 83% yield of the thiirane having
inverted configuration at C-4. The 13C NMR spectrum of 10 showed that the signals of the carbons
involved in the thiirane ring (C-4 and C-5) underwent a further upfield displacement (≈20 ppm).
Scheme 2. (a) TsCl (1.5 molar equiv.), C5H5N (2 molar equiv.), CHCl3, 0°C, 12 h (83%); (b) NaOMe, MeOH, 25°C, 50 min;
(c) (NH2)2CS (2 molar equiv.), MeOH, 25°C, 72 h; (d) KOAc (10 molar equiv.), 1:1 DMF:AcOH, reflux temp, 18 h; (e) 2:1
THF:6% aq. HCl, 35°C, 16 h (92%); (f) ButPh2SiCl (1.1 molar equiv.), C3H4N2, DMF (90%)
In order to promote the thiolactonization, compound 10 was dissolved in a mixture of KOAc in
AcOH−DMF, and heated at the reflux temperature. Under these conditions, opening of the thiirane
ring took place by regioselective nucleophilic attack of the acetate ion to C-5, with simultaneous
thiolactonization, to give 11 in 65% yield. Removal of the acetyl group of 11 by acid hydrolysis afforded
the free (4R)-4-hydroxymethyl-4-thiobutyrolactone (12) in 92% yield. The 13C NMR spectrum of 12
showed the signal characteristic of a thiolactone carbonyl at 208.7 ppm, which was strongly shifted
downfield with respect to that of the lactone carbonyl of 7. Conversely, the C-4 resonance of 12 showed
an intense upfield shifting relative to the same signal in 7. Silylation of 12 was performed as described
for 2, affording compound 5, which showed the same spectral and physical properties as the product
previously synthesized from 1.
In summary, we describe herein two alternative routes for the enantiospecific synthesis of (4R)-4-
hydroxymethyl-4-thiobutyro-1,4-lactone (12) and derivatives. The sequence that starts from L-glutamic
acid (6) was simple, direct and efficient, as 12 was obtained in about 40% yield from 8. Furthermore,
the enantiomer of 12, which possesses opposite configuration at C-4, could be prepared similarly starting
from 4-thio-D-ribono-1,4-lactone16 or from D-glutamic acid.

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P. A. Zunszain, O. Varela / Tetrahedron: Asymmetry 11 (2000) 765–771
3. Experimental
3.1. General methods
Solvents were dried and purified by appropriate standard procedures. Melting points were determined
with a Fisher–Johns apparatus and are uncorrected. Analytical thin layer chromatography (TLC) was
performed on 0.2 mm silica gel 60 F254 (Merck) aluminum supported plates. Detection was effected
by exposure to UV light or charring with 10% H2SO4 (v/v) in EtOH. Column chromatography was
performed with silica gel 60 (230–400 mesh, Merck). Optical rotations were measured with a Per-
kin–Elmer 343 polarimeter at 25°C. Nuclear magnetic resonance (NMR) were recorded on a Bruker
AC 200 spectrometer (1H at 200 MHz, 13C at 50 MHz) in CDCl3 with TMS as an internal standard.
3.2. 5-O-tert-Butyldiphenylsilyl-4-thio-L-lyxono-1,4-lactone (3)
To a solution of compound 216 (113 mg, 0.69 mmol) in anhydrous DMF (0.5 mL), imidazole (102
mg, 1.48 mmol) and tert-butylchlorodiphenylsilane (0.20 mL, 0.76 mmol) were added. The mixture was
stirred at room temperature for 5 h and then poured into water and extracted with CH2Cl2. The extract
was dried (MgSO4) and concentrated to a syrup, which was chromatographed with 4:1 hexane:EtOAc.
Fractions containing the product of Rf 0.90 (EtOAc) were concentrated to afford compound 3 (214 mg,
77%); [α]D=−54 (c 1.1, CHCl3); 1H NMR δ 7.70–7.32 (m, 10H), 4.53 (dd, 1H, J=4.0, 3.0 Hz), 4.32 (d,
1H, J=4.0 Hz), 4.2–3.9 (m, 3H), 1.03 (s, 9H); 13C NMR δ 204.8, 135.5, 132.7, 130.0, 127.8, 80.9, 78.3,
62.9, 49.7, 26.9, 19.4. Anal. calcd for C21H26O4SSi: C, 62.65; H, 6.51. Found: C, 62.52; H, 6.50.
3.3. 2,3-Di-O-benzoyl-5-O-tert-butyldiphenylsilyl-4-thio-L-lyxono-1,4-lactone (4)
A solution of 3 (78 mg, 0.194 mmol) in anhydrous pyridine (0.8 mL), cooled at 0°C, was stirred with
benzoyl chloride (0.4 mL, 3.4 mmol) for 1 h, when TLC indicated complete transformation of 3 into
a less polar product (Rf 0.20, 15:1 hexane:EtOAc). The mixture was poured into ice-water and stirred
for an additional 12 h. It was then extracted with CH2Cl2, and successively washed with 5% HCl, H2O,
saturated aqueous NaHCO3, and water. The organic extract was dried (MgSO4) and concentrated to a
syrup, which was chromatographed with 30:1 hexane:EtOAc, to afford compound 4 (92 mg, 78%); 1H
NMR δ 7.93–7.10 (m, 20H), 6.22 (dd, 1H, J=3.4, 4.0 Hz), 5.94 (d, 1H, J=4.0 Hz), 4.37 (dt, 1H, J=3.4,
7.5 Hz), 4.07 (dd, 1H, J=10.2, 7.5 Hz), 3.88 (dd, 1H, J=7.5, 10.2 Hz), 0.98 (s, 9H); 13C NMR δ 197.3,
165.1, 164.9, 135.6, 135.4, 130.1, 130.0, 128.7, 128.0, 127.8, 78.3, 69.5, 62.3, 47.8, 26.7, 19.1.
3.4. (4R)-4-(tert-Butyldiphenylsilyloxymethyl)-4-thiobutyro-1,4-lactone (5) ((5R)-dihydro-5-(tert-butyl-
diphenylsilyloxymethyl)-2(3H)-thiophenone)
To a solution of 4 (65 mg, 0.11 mmol) in recently distilled, dry THF (0.5 mL) cooled at 0°C was added
under Ar a freshly prepared28 0.1 M solution of SmI2 in THF (3.3 mL, 0.33 mmol). The solution changes
immediately from the deep blue color to yellow, and t-butanol (0.021 mL, 0.22 mmol) and an additional
amount of 0.1 M SmI2 in THF (2.8 mL, 0.28 mmol) were added. The mixture was allowed to reach room
temperature, and after 2 h of stirring it was diluted with CH2Cl2 and washed with 5% HCl, H2O, saturated
aqueous NaHCO3, and H2O. The organic layer was dried (MgSO4) and concentrated. The residue was
purified by column chromatography with 30:1 hexane:EtOAc, to give 5 (20 mg, 53%); [α]D=−49.6 (c
1
0.5; CHCl3); H NMR δ 7.67–7.36 (m, 10H), 4.03 (m, 1H, J≈6.3 Hz), 3.83 (d, 2H, J≈6.0 Hz), 2.53

P. A. Zunszain, O. Varela / Tetrahedron: Asymmetry 11 (2000) 765–771
769
(m, 2H), 2.29 (dq, 1H, J≈7.0, 13.0 Hz), 2.08 (dq, 1H, J≈6.5, 13.0 Hz), 1.06 (s, 9H); 13C NMR δ 207.0,
135.6, 130.0, 127.9, 127.8, 66.6, 51.9, 40.8, 27.5, 26.8, 19.3. Anal. calcd for C21H26O2SiS: C, 68.06; H,
7.07. Found: C, 68.25; H, 7.34.
3.5. (4S)-(+)-4-Hydroxymethyl-1,4-butyrolactone (7) ((5S)-dihydro-5-(hydroxymethyl)-2-(3H)-
furanone)
It was prepared from L-glutamic acid (6) via (4S)-(+)-4-carboxy-1,4-butyrolactone as already
described.29
3.6. (4S)-(+)-4-(p-Tolylsulfonyloxymethyl)-1,4-butyrolactone (8) ((5S)-(+)-dihydro-5-(p-tolyl-
sulfonyloxymethyl)-2(3H)-furanone)
To a solution of 7 (1.38 g, 11.9 mmol) in dry CHCl3 (12 mL), cooled at 0°C, pyridine (1.9 mL, 24.5
mmol) and tosyl chloride (3.43 g, 18 mmol) were added. After 12 h of stirring at 0°C, water (10 mL) was
added dropwise, and the stirring was maintained for 0.5 h. The mixture was diluted with CH2Cl2 and
it was successively washed with 0.5 N HCl, water, saturated aqueous NaHCO3, and water. The organic
extract was dried (MgSO4) and concentrated to give a crystalline residue. Compound 8 (Rf 0.44, 2:1
toluene:EtOAc) was recrystallized from EtOH (2.68 g, 83%); mp 85–86°C; [α]D=+46.2 (c 1.0, CHCl3);
lit.29 mp 85–87°C; [α]D=+47.
3.7. Methyl (4S)-4,5-epoxipentanoate (9)
Compound 8 (603 mg, 2.2 mmol) was added to a solution prepared by dissolving sodium (54 mg, 2.3
mmol) in anhydrous methanol (8.5 mL). The mixture was stirred at room temperature for 50 min, when
a single spot, faster migrating than 8 (Rf 0.67, 2:1 toluene:EtOAc), was detected by TLC. Evaporation
of the solvent afforded a residue which was extracted with CH2Cl2. The extract was concentrated and
the resulting syrup dissolved in hexane and filtered. Upon evaporation of the solvent, compound 9 was
obtained as a chromatographically homogeneous oil (228 mg, 79%); [α]D=−18 (c 1.1, CHCl3); lit.31
[α]D=−17.9; 13C NMR δ 173.2, 51.6, 51.1, 46.9, 30.1, 27.5.
3.8. Methyl (4R)-4,5-epithiopentanoate (10)
To a solution of 9 (0.37 g, 2.8 mmol) in dry methanol (44 mL) was added thiourea (0.47 g, 5.5 mmol).
The resulting solution was stirred at room temperature for 72 h, when TLC showed a single spot (Rf 0.73,
2:1 toluene:EtOAc) less polar than the starting 9. Evaporation of the solvent afforded a residue which was
extracted with ether. The extract was concentrated and the resulting syrup was dissolved in hexane and
filtered. Concentration of the solution gave chromatographically homogeneous, oily compound 10 (0.34
1
g, 83%); [α]D=+83 (c 1.1, CHCl3); H NMR δ 3.69 (s, 3H), 2.95 (dddd, 1H, J=5.1, 5.5, 6.1, 8.0 Hz),
2.53 (t, 2H, J≈7.2 Hz), 2.51 (dd, 1H, J=1.2, 6.1 Hz), 2.30 (dddd, 1H, J=5.1, 7.2, 8.0, 14.0 Hz), 2.19 (dd,
1H, J=1.2, 5.6 Hz), 1.65 (ddt, 1H, J=7.2, 8.0, 14.0 Hz); 13C NMR δ 173.1, 51.5, 34.7, 33.3, 31.5, 25.7.
Anal. calcd for C6H10O2S: C, 49.29; H, 6.89. Found: C, 48.92; H, 6.51.

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P. A. Zunszain, O. Varela / Tetrahedron: Asymmetry 11 (2000) 765–771
3.9. (4R)-Acetoxymethyl-4-thiobutyro-1,4-lactone (11) ((5R)-dihydro-5-(acetoxymethyl)-2(3H)-thio-
phenone)
Compound 10 (0.21 g, 1.39 mmol) was dissolved in a mixture of DMF (7 mL), AcOH (7 mL), and
KOAc (1.4 g, 14.4 mmol) and heated to the reflux temperature, under nitrogen. After 18 h the reaction
mixture showed by TLC a main spot (Rf 0.48, 2:1 toluene:EtOAc); it was diluted with CH2Cl2 (60 mL)
and successively washed with water, saturated aqueous NaHCO3, and water. The organic extract was
dried (MgSO4) and concentrated to a syrup, which was chromatographed using 20:1 hexane:EtOAc.
From the column unreacted starting 10 (50 mg) was recovered, and the fractions containing the product
of Rf 0.48 were pooled and concentrated to afford the thiolactone 11 (0.12 g, 65% yield based on reacted
1
10). It had [α]D=−87 (c 1.0, CHCl3); H NMR δ 4.37 (dd, 1H, J=6.3, 11.1 Hz), 4.22 (dd, 1H, J=6.6,
11.1 Hz), 4.11 (tdd, 1H, J=6.3, 6.6, ≈7.0 Hz), 2.71 (dq, 1H, J≈7.5, 17.0 Hz), 2.55 (dq, 1H, J≈7.5, 17.0
Hz), 2.38 (dq, 1H, J≈7.0, 13.0 Hz), 2.10 (s, 3H), 2.06 (dq, 1H, J≈7.0, 13.0 Hz); 13C NMR δ 207.0,
170.5, 66.3, 48.1, 40.6, 27.9, 20.7. Anal. calcd for C7H10O3S: C, 48.26; H, 5.78. Found: C, 48.26; H,
6.02.
3.10. (4R)-Hydroxymethyl-4-thiobutyro-1,4-lactone (12) ((5R)-dihydro-5-(hydroxymethyl)-2(3H)-thio-
phenone) and its tert-butyldiphenylsilyloxy derivative 5
Compound 11 (115 mg, 0.69 mmol) was dissolved in a mixture of THF (5 mL) and 6% aqueous HCl
(2.5 mL), and the resulting solution was stirred at 35°C for 16 h. The reaction mixture, which showed by
TLC a main spot having Rf 0.66 (EtOAc), was concentrated and the resulting syrup was purified through
a short column of silica gel with 2:1 hexane:EtOAc. Evaporation of the solvent led to the free thiolactone
12 (80 mg, 92%): [α]D=−106 (c 1.1, CHCl3); 1H NMR δ 4.04 (dq, 1H, J=5.3, ≈7.0 Hz), 3.78 (dd, 1H,
J=5.3, 12.5 Hz), 3.66 (dd, 1H, J=7.0, 12.5 Hz), 2.88 (bs, 1H, OH), 2.61 (dq, 1H, J≈7.0, 17.6 Hz), 2.57
(dq, 1H, J≈7.5, 17.6 Hz), 2.28 (dq, 1H, J≈7.5, 12.5 Hz), 1.97 (dq, 1H, J≈7.5, 12.5 Hz); 13C NMR δ
208.7, 65.3, 52.1, 41.1, 27.4. Anal. calcd for C5H8O4S: C, 36.58; H, 4.91; S, 19.53. Found: C, 36.71; H,
4.71; S, 19.49.
Silylation of 12 (127 mg, 0.96 mmol) with tert-butylchlorodiphenylsilane (293 mg, 106 mmol) in
anhydrous DMF (2 mL), and in the presence of imidazole (147 mg, 2.1 mmol), was performed as
described for 2. After column chromatography with 40:1 hexane:EtOAc, syrupy compound 5 (321 mg,
90%) was obtained; [α]D=−48.2 (c 0.9, CHCl3). It showed the same spectral properties as the product 5
obtained from 2.
Acknowledgements
Financial support from ANPCYT (National Agency for Promotion of Science and Technology, PICT
01698) and from the University of Buenos Aires (Project TW70) is gratefully acknowledged. O.V. is a
Research Member of the National Research Council of República Argentina (CONICET).
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