C.M. Grison et al. / Bioorganic Chemistry 52 (2014) 50–55
51
(5:1 hexane/EtOAc) = 0.5) was used immediately in the synthesis
2. Experimental protocols
of compound 5.
All starting materials were commercially available research-
grade chemicals and used without further purification. Reactions
were monitored by TLC (Merck – 5535 – Kieselgel 60-F254),
detection being carried out by UV, by iodine vapor or by spraying
solution of H2SO4 15% in ethanol followed by heating. NMR spectra
were recorded on a Bruker DRX-300. Chemical shifts are expressed
as parts per million downfield from the internal standard
tetramethylsilane for 1H and 13C. Multiplicities are indicated by s
(singlet), d (doublet), dd (dedoubled doublet), t (triplet), q (quadru-
plet), m (multiplet), bs (broadened singlet); coupling constants are
reported in Hertz (Hz). IR spectra were recorded on a PerkinElmer
Spectrum 100 FT-IR spectrometer, in ATR mode, and are given in
cmꢀ1. High-resolution mass spectrometry (HRMS) data were re-
corded using the electrospray ionization technique in positive
mode (ESI+) with a tandem Q-TOF analyser (Bruker, 2009). Optical
2.1.3. Synthesis of isopropyl dichloroacetate 4
To 150 mL of anhydrous isopropanol (117.90 g, 1.961 mol,
13 equiv) were added dichloroacetic acid (19.35 g, 150 mmol,
1 equiv) and 2 mL of sulfuric acid (36 N). The solution was stirred
for 26 h at 100 °C. Isopropanol was then evaporated under reduced
pressure. The remaining yellow oil was dissolved in 60 mL of dieth-
ylether, washed with an aqueous solution of NaHCO3 8% until
pH = 8 and extracted three times with diethylether. The organic
layer was dried over MgSO4, filtered and concentrated under re-
duced pressure to give 24 g of compound 4 as yellow oil (96%
yield). The analytical data were consistent with literature [10].
2.1.4. Synthesis of 2,3-anhydro-2-C-chloro-6-O-[(1,1-
dimethylethyl)dimethylsilyl]-4,5-O-isopropylidene-D-erythro-hexonic
acid 1-methylethyl ester 5
A suspension of solid potassium (0.52 g, 13.2 mmol, 2 equiv) in
41 mL of isopropanol under nitrogen was stirred until complete
dissolution of potassium. After addition of 8 mL of diethylether,
the solution was cooled to 0 °C and a mixture of 3 (6.63 mmol,
rotations were measured using a 10 cm quartz cell (Jasco, P-1010).
T
Values for ½/ꢁD were obtained with the
D-line of sodium at the indi-
cated temperature T, using solutions of concentration (c) in units of
gꢂ100 mLꢀ1. Column chromatography was performed on silica gel
(MN Kieselgel 60, 0.063–0.2 mm/70–230 mesh, Macherey–Nagel).
All primers were purchased from Invitrogen (Saint-Aubin, France).
1 equiv) and isopropyl dichloroacetate
4 (2.26 g, 13.2 mmol,
2 equiv) in 32 mL of diethylether, was added dropwise for
30 min. The mixture was stirred for 2 h, neutralized by addition
of saturated HCl solution in diethylether and centrifuged
(1000 rpm, 15 min). The supernatant was taken off, concentrated
and purified by silica chromatography (5:1 hexane/EtOAc) to give
2,21 g of compound 5 as brownish oil (71% yield).
2.1. Chemical synthesis of 2-keto-3-deoxy-D-erythro-hexonic acid
isopropyl ester 8
5a (60%): Rf (5:1 hexane/EtOAc) = 0.5; 1H RMN (300 MHz,
CDCl3) d 0.09 (s, 6 H, Si–H3), 0.91 (s, 9 H, Si–tBu), 1.43–1.44 (s, 6
H, C–CH3), 1.46–1.48 (m, 6 H, CH–CH3), 3.50–3.52 (d, 1 H, CH–O),
3.80–3.90 (2dd, 2 H, CH2–OTBDMS), 4.05–4.30 (m, 2 H, 2 CH–O),
5.09–5.18 (h, 1 H, O–CH–); 13C NMR (75 MHz, CDCl3) d ꢀ5.3 (Si–
C(CH3)3), 16.7 (C(CH3)3), 21.5 (C(CH3)3), 25.7 (CH(CH3)2), 26.2
(C(CH3)2), 63.2 (CH–O(CCl)), 65.2 (CH2–O), 65.9 (Cl–C–O), 66.5
(CH–O), 67.8 (CH–O), 73.4 (CH(CH3)2), 111.5 (C(CH3)2), 163.7
(C@O).
2.1.1. Synthesis of 4-O-tert-butyldimethylsilyl-2,3-O-isopropylidene-
D-threitol 2
To a suspension of NaH (60% in oil, 0.18 g, 6.63 mmol, 1 equiv)
in THF (10 mL) was added, over 15 min under a stream of nitrogen
at 5 °C, solution of 2,3-O-isopropylidene- -threitol (1.30 g,
a
D
6.63 mmol, 1 equiv) in THF (12 mL). The suspension was stirred
for 45 min at 5 °C. Then tert-butyldimethylsilyl chloride (1 g,
6.63 mmol, 1 equiv) in 2 mL of THF was added. After stirring for
16 h, the reaction mixture was washed with 30 mL of saturated
NaHCO3 and extracted three times with diethylether (3 ꢃ 20 mL).
The organic layer was dried over MgSO4, filtered and concentrated
under reduced pressure. The residue was purified by silica chroma-
tography (3:1 hexane/EtOAc) to give 1.75 g of compound 2 as
yellow oil (98% yield).
5b (40%): Rf (5:1 hexane/EtOAc) = 0.5; 1H RMN (300 MHz,
CDCl3) d 0.09 (s, 6 H, Si–CH3), 0.91 (s, 9 H, Si–tBu), 1.31–1.32 (s, 6
H, C–CH3), 1.46–1.48 (m, 6 H, CH–CH3), 3.51 (d, 1 H, CH–O),
3.60–3.88 (2dd, 2 H, CH2–OTBDMS), 4.05–4.30 (m, 2 H, 2 CH–O),
5.09–5.18 (h, 1 H, O–CH–); 13C NMR (75 MHz, CDCl3) d ꢀ5.3 (Si–
C(CH3)3), 16.7 (C(CH3)3), 21.5 (C(CH3)3), 25.7 (CH(CH3)2), 26.2
(C(CH3)2), 62.4 (CH–O(CCl)), 64.6 (CH2–O), 65.9 (Cl–C–O), 66.5
(CH–O), 67.8 (CH–O), 73.4 (CH(CH3)2), 111.2 (C(CH3)2), 163.7
(C@O).
2: Rf (4:1 hexane/EtOAc) = 0.25; 1H RMN (300 MHz, CDCl3) d
0.10 (s, 6 H, Si–CH3), 0.92 (s, 9 H, Si–tBu), 1.44 (dd, 6 H, C–CH3),
2.35 (bs, 1 H, OH), 3.71–4.03 (m, 6 H, 2 CH2–O + 2 CH–O); 13C
NMR (75 MHz, CDCl3) d -5.3 (Si–(CH3)2), 18.4 (Si–C(CH3)3), 25.7
(C(CH3)3), 26.9 (C(CH3)2), 62.7 (CH2–OH), 73.2 (CH2–O), 79.3 (CH–
O), 110.9 (C(CH3)2); HRMS (ESI+) Cacld. for C13H29O4Si [M+H]+:
277.1835, found 277.1829.
HRMS (ESI+) Cacld. for C18H33ClNaO6Si [M+Na]+: 431.1633,
found 431.1635.
2.1.5. One-pot sequential synthesis of 3-deoxy 6-O-[(1,1-
2.1.2. Synthesis of 4-O-tert-butyldimethylsilyl-2,3-O-isopropylidene-
dimethylethyl)dimethylsilyl]-4,5-O-(1-methylethylylidene)-D-erythro-
D-threose 3
hexulosonic acid 1-methylethyl ester 7
To
a
cooled (ꢀ78 °C) solution of oxalyl chloride (1.09 g,
To a suspension of solid magnesium (0.08 g, 3.25 mmol,
8.62 mmol, 1.3 equiv) in CH2Cl2 (23 mL), under a stream of nitro-
gen, was added dropwise DMSO (1.35 g, 17.24 mmol, 2.6 equiv)
in CH2Cl2 (4 mL). Then compound 2 (1.80 g, 6.63 mmol, 1 equiv)
in 10 mL of CH2Cl2 was added dropwise. After stirring for 20 min,
triethylamine (3.35 g, 33.15 mmol, 5 equiv) was added and the
reaction was allowed to warm to 20 °C over a period of 1 h. The
heterogeneous mixture was extracted with water, diluted in (1:1
hexane/ether) and extracted three more times with water
(3 ꢃ 25 mL). Then the combined aqueous fractions were extracted
with (1:1 hexane/ether) and the combined organic layers were
dried over Na2SO4, filtered and concentrated under reduced pres-
sure. After coevaporation with toluene, the crude aldehyde 3 (Rf
1.3 equiv) in 22 mL of diethylether was added grinded iodine
(0.83 g, 3.25 mmol, 1.3 equiv). After stirring for 3 h at 35 °C in the
dark, compound 5 (1 g, 2.50 mmol, 1 equiv) in 14 mL of diethyl-
ether was added dropwise. The solution was refluxed for 2 h to
give 6a and 6b. A fraction of the reaction mixture was isolated to
characterize 6a and 6b. 6a and 6b were not enough stable to obtain
13C NMR data.
6a, 6b: Rf (5:1 hexane/EtOAc) = 0.57; 1H RMN (300 MHz, CDCl3)
d 0.08 (s, 12 H, Si–CH3), 0.91 (s, 18 H, Si–tBu), 1.34–1.40 (m, 24 H,
C–CH3, CH–CH3), 3.67–3.72 (2dd, 2 H, CH2–OTBDMS), 3.78–3.83
(m, 2 H, CH–O), 3.84–3.89 (2dd, 2 H, CH2–OTBDMS), 4.05–4.30
(d, 1 H, CH–O), 4.17–4.25 (d, 1 H, CH–O), 4.35–4.42 (d, 2 H,