Total Synthesis, Configuration Assignment, and Cytotoxic Activity Evaluation of Protulactone A

Article abstract of DOI:10.1021/acs.jnatprod.7b00212

The first total synthesis and absolute configuration assignment of protulactone A (1) has been achieved. Four stereoisomers, 1a, ent-1a, 1b, and ent-1b, of this natural polyketide were prepared by chiral pool synthesis starting from l- and d-arabinose, re

Full text of DOI:10.1021/acs.jnatprod.7b00212

Article
pubs.acs.org/jnp
Total Synthesis, Configuration Assignment, and Cytotoxic Activity
Evaluation of Protulactone A
†
‡
†
†
†
§
̌
Martin Markovic, Peter Koos, Tomas Carny, Saskia Sokoliova, Nikola Bohacikova, Jan Moncol′,
̌
́
̌
́
̌
́
́
́
̌
́
́
and Tibor Gracza*,†
§
†Department of Organic Chemistry, Institute of Organic Chemistry, Catalysis and Petrochemistry, and Department of Inorganic
Chemistry, Institute of Inorganic Chemistry, Technology and Materials, Slovak University of Technology, Radlinskeh
Bratislava, Slovakia
‡Georganics Ltd., Korenico
̌
va 1, SK-811 03 Bratislava, Slovakia
́
o 9, SK-812 37
S
* Supporting Information
ABSTRACT: The first total synthesis and absolute config-
uration assignment of protulactone A (1) has been achieved.
Four stereoisomers, 1a, ent-1a, 1b, and ent-1b, of this natural
polyketide were prepared by chiral pool synthesis starting from
L- and D-arabinose, respectively. The absolute and relative
configurations of all isomers were assigned by single-crystal X-
ray analysis. Target compounds were screened for their in vitro
cytotoxicity toward certain human tumor cells (NCI60 cancer
cell line panel).
rotulactones A (1) and B ((−)-2) were isolated in 2010
P
from the marine-derived fungus Aspergillus sp. SF-5044.1
The substructure unit of these two polyketide-derived fungal
metabolites, a bicyclic lactone, is not a distinctive structure for
the natural compounds isolated from fungi of the Aspergillus
genus. However, similar compounds occur in plants of various
families (e.g., Goniothalamus giganteus Hook f., Thomas
(Annonaceae) and Polyalthia crassa).2 The structures and
relative configurations of protulactones A and B were
determined by extensive NMR analysis. The absolute
configuration of the rigid bicyclic skeleton of protulactone B
((−)-2) was assigned by Mosher’s MTPA ester method.
However, the relative configuration of the C-7 center and the
absolute configuration of 1 have not been described.1 The
biological activities of these two natural compounds have not
been evaluated presumably due to the limited supply from
natural sources (only 3.7 mg of 1 was isolated from 2.0 g of the
EtOAc extract). Naturally occurring and structurally similar
lactones, goniofufurone ((+)-3), 7-epi-goniofufurone ((+)-4),
goniopypyrone ((+)-6),2a,b and crassalactone C ((+)-5),2c have
exhibited significant cytotoxic activity in tests with several
human tumor cell lines.2,3 The incomplete configuration
assignment and promising biological activity of 1 led us to
develop a total synthesis of protulactone A with the aim to
establish its absolute structure and evaluate its antiproliferative
activities against the NCI60 cancer cell line panel.
skeleton with a relative gluco configuration and different
configurations at the C-7 center. The proposed retrosynthetic
strategy of protulactone A (1) is shown in Scheme 1. In both
selected routes, the methyl moiety is introduced at the end of
Scheme 1. Retrosynthetic Analysis of the Protulactone A
RESULTS AND DISCUSSION
■
Because the absolute and C-7 configuration of the isolated
compound 1 are unknown, we have targeted four different
stereoisomers. We herein report the synthesis of all of these
isomers of 1 (1a, 1b, ent-1a, and ent-1b) containing the bicyclic
Received: March 10, 2017
© XXXX American Chemical Society and
American Society of Pharmacognosy
A
DOI: 10.1021/acs.jnatprod.7b00212
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Scheme 2. Synthesis of Lactones 8 and ent-10
Scheme 3. Synthesis of Isomers 13, 14, ent-13, and ent-14
the synthesis via the Grignard addition on aldehyde A,
providing both C-7 stereoisomers. The optically pure
heptonolactone A having the required D- or L-gluco
configuration can be obtained by a condensation reaction of
arabinose B with Meldrum’s acid4 or using Pd-catalyzed
carbonylative bicyclization of unsaturated tetraol5a C.
excellent yields (91%/94%). Selective deprotection of the
primary hydroxy group using cerium(IV) ammonium nitrate8
in aqueous isopropyl alcohol provided primary alcohols 12/ent-
12 (67%/67%). These alcohols were then subjected to the
Dess−Martin oxidation followed by an addition of methyl-
magnesium bromide to the formed aldehyde. However, the
addition reaction provided a poorly separable diastereomeric
mixture of alcohols 13/14 and ent-13/ent-14 in moderate yield.
After several MPLC purifications, the analytically pure
intermediates 13, ent-13, 14, and ent-14 were isolated as
white solids. Recrystallization of the individual materials from
CH2Cl2/hexanes (1:10) furnished colorless needles suitable for
single-crystal X-ray analysis. Accordingly, the absolute L-gluco
configuration of the bicyclic skeleton and S-configuration of the
newly formed stereocenter at C-7 in compound 13 were
established (Figure 1). The alcohol 14 represents the C-7
epimer (7R). An X-ray study of the enantiomeric intermediates
ent-13 and ent-14 confirmed the D-gluco configuration of the
bicyclic moiety with the 7S/7R-configuration (Figure 1).9
Finally, crystalline lactones 13, 14, ent-13, and ent-14 were
then transformed in two steps into the final compounds. Thus,
the acetylation of the free hydroxy group and buffered
tetrabutylammonium fluoride (TBAF) deprotection of the
silyl moiety provided 1a/1b and ent-1a/ent-1b in good yields
(Scheme 4).
First, we applied a known oxycarbonylation strategy6 for the
stereoselective construction of the [3.3.0] bicyclic core of
intermediary heptonolactone A. The key intermediates C, the
known alkenol5a 9, and partially protected tetraol5b−e 7 were
obtained by standard carbohydrate chemistry procedures from
D-mannitol and L-arabinose, respectively (Scheme 2).5
The crucial steps in this approach, homogeneous Pd-
catalyzed carbonylation reactions of alkenes 7 and 9 using
Fe(CO)5 as an in situ donor of a stoichiometric amount of
carbon monoxide, afforded corresponding optically pure
lactones 8 (L-gluco, 76%) and ent-10 (D-gluco, 47%).7 In the
second approach, the required [3.3.0] bicyclic structural unit
was assembled by a single reaction of the corresponding aldose
with Meldrum’s acid in the presence of triethylamine.4
Consequently, we prepared the known 3,6-anhydro-2-deoxy-
L-gluco-heptono-1,4-lactone (10) and its enantiomer ent-10 in
one step in acceptable yields (30%/28%) using L- or D-
arabinose.4b While both synthetic approaches provided lactone
ent-10 in similar (combined) yields, the second route has been
chosen to prepare the key aldehyde A (Scheme 3).
Isolated stereoisomer 1a was identified as the natural
protulactone A, despite the fact that the value of specific
rotation, [α]2D5 +35.6 (c 0.34, MeOH), of the synthetic sample
1a differs from the literature value, [α]2D5 +6 (c 0.34, MeOH)
The syntheses of both enantiomers were run in parallel and
continued with the silyl protection of free hydroxy groups.
Thus, bis-silylated lactones 11 and ent-11 were isolated in
B
DOI: 10.1021/acs.jnatprod.7b00212
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Figure 1. Ball-and-stick view of crystal structures 13, 14, ent-13, and ent-149
Scheme 4. Synthesis of Protulactone A (1a) and Its Isomers
1b, ent-1a, and ent-1b
Scheme 5. Structures of Natural Protulactone A (1a), 7-
Epimer 1b, and Unnatural Enantiomers ent-1a and ent-1b
1
(Scheme 5). The H and 13C NMR spectroscopic data of our
synthetic product 1a matched well the reported values for the
isolated natural compound.1,10 The proton NMR spectrum of
the second isomer 1b does exhibit some differences in the
chemical shifts. While the chemical shift of the H-5 proton (br
d, 1H, J = 4.8 Hz) of the natural substance is 4.32 ppm,1 the
corresponding H-5 proton of the diastereomer 1b is shifted to
4.15 ppm (d, 1H, J = 4.5 Hz).
Despite the inconsistency in the specific rotation data of
isolated material 1 and our isomer 1a (designated as natural
protulactone A), the stereogenic centers of 1a are consistent
with the configuration of natural protulactone B (−)-2.
According to the literature,11 we have also outlined a possible
biosynthetic pathway involving intermediates 16 and 17, which
can be transformed to the individual natural products by an
intramolecular Michael addition (Scheme 6). As follows,
lactones 16 and 17 can be derived from the same biosynthetic
precursor 15, pointing to the configuration uniformity of
proposed protulactone A (1a).
Moreover, all prepared stereoisomers of 1 were then
evaluated for their in vitro cytotoxicity against the NCI60
cancer cell line panel at the developmental therapeutics
C
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Nagel). The compounds were visualized by UV fluorescence and by
dipping the plates in an aqueous H2SO4 solution of cerium sulfate/
ammonium molybdate followed by charring with a heat gun.
Scheme 6. Proposed Biosynthetic Pathway of Natural
Protulactones A (1a) and B ((−)-2)
Data collection and cell refinement of 13, 14, ent-13, and ent-14
were made on a Stoe StadiVari diffractometer using a Pilatus 300 K
HPAD detector and the microfocus source Xenocs FOX3D with Cu
Kα. The structures were solved using SHELXT or SIR-2011 and
refined by the full-matrix least-squares procedure with SHELXL. The
structures were drawn using the OLEX2 package.12 The absolute
configurations of all enantiomers were determined. The Flack
parameters were calculated by the classical Flack method (for 13
and ent-14) or Parsons method (for 14 and ent-13).13
Synthesis of 3-O-Benzyl-1-O-tert-butyldiphenylsilyl-L-xylo-
hex-5-enitol (7). 5-O-tert-Butyldiphenylsilyl-L-arabinose. L-Arabi-
nose (8.00 g, 53.29 mmol, 1 equiv) was dissolved in DMF (120 mL) at
100 °C. After cooling the solution to 55 °C, imidazole (7.26 g, 106.57
mmol, 2 equiv) and tert-butyl(chloro)diphenylsilane (13.9 mL, 53.29
mmol, 1 equiv) were added, and the reaction mixture was stirred at the
same temperature for 2.5 h. The reaction mixture was then
concentrated to half of its volume, and EtOAc (800 mL) was added.
The organic phase was then washed with deionized H2O (4 × 200
mL), dried over anhydrous Na2SO4, filtered, and concentrated under
reduced pressure. The residue was purified by MPLC (gradient
EtOAc/hexanes, 0/100 to 70/30, then isocratic EtOAc/hexanes, 70/
30), providing the desired product as a light yellow oil (75%, 15.51 g):
TLC Rf 0.30 (EtOAc/hexanes, 70/30); [α]2D5 −17.7 (c 0.97, MeOH);
IR (ATR) νmax 502, 699, 1043, 1105, 1427, 2929, 3373 cm−1;
HRESIMS m/z 411.15996 [M + Na]+ (calcd for C21H28NaO5Si,
411.15982); NMR data were in good agreement with published data.5b
1,2-O-Isopropylidene-5-O-tert-butyldiphenylsilyl-β-L-arabinose.
To a solution of 5-O-tert-butyldiphenylsilyl-L-arabinose (3.36 g, 8.64
mmol, 1 equiv) in dry acetone (34 mL) were added CuSO4 (6.35 g,
39.8 mmol, 4.6 equiv) and camphorsulfonic acid (0.20 g, 0.87 mmol,
0.1 equiv), and the reaction mixture was stirred at room temperature
(rt) overnight. Subsequently, the reaction mixture was neutralized
using NaHCO3 (1.45 g, 17.31 mmol, 2 equiv). The resulting mixture
was then left to stir for 20 min and filtered through fiberglass filter
paper. The filtrate was then concentrated under reduced pressure, and
the residue was purified using MPLC (gradient EtOAc/hexanes, 0/100
to 25/75, then isocratic EtOAc/hexanes, 25/75), providing the desired
product as a colorless oil (3.20 g, 86%): TLC Rf 0.25 (EtOAc/hexanes,
25/75); [α]2D5 −13.1 (c 1.05, MeOH); IR (ATR) νmax 701, 1014, 1063,
2931, 3444 cm−1; HRESIMS m/z 451.19121 [M + Na]+ (calcd for
C24H32NaO5Si, 451.19112); NMR data were in good agreement with
published data.5b
1,2-O-Isopropylidene-3-O-benzyl-5-O-tert-butyldiphenylsilyl-β-L-
arabinose. To a solution of 1,2-O-isopropylidene-5-O-tert-butyldiphe-
nylsilyl-β-L-arabinose (3.20 g 7.47 mmol, 1 equiv) in dry
tetrahydrofuran (THF) (72 mL) under an argon atmosphere was
added portionwise NaH (0.85 g, 22.41 mmol, 3 equiv, 63% dispersion
in mineral oil). Subsequently, benzyl bromide (1.77 mL, 14.94 mmol,
2 equiv) was added, and the reaction mixture was stirred at rt for 12 h.
The reaction was quenched with saturated NH4Cl solution. The layers
were then separated, and the aqueous layer was extracted using EtOAc
(3 × 160 mL). Combined organic extracts were dried over anhydrous
Na2SO4, filtered, and concentrated under reduced pressure. The
residue was purified using MPLC (gradient EtOAc/hexanes, 0/100 to
5/95, then isocratic EtOAc/hexanes, 5/95), providing 1,2-O-
isopropylidene-3-O-benzyl-5-O-tert-butyldiphenylsilyl-β-L-arabinose as
a colorless oil (3.41 g, 88%): TLC Rf 0.10 (EtOAc/hexanes, 5/95);
[α]2D5 −13.4 (c 0.96, MeOH); IR (ATR) νmax 504, 699, 1017, 1065,
1102, 2856, 2931 cm−1; HRESIMS m/z 541.23814 [M + Na]+ (calcd
for C31H38NaO5Si, 541.23807); NMR data were in good agreement
with published data.5c
program, Division of Cancer Treatment and Diagnosis,
National Cancer Institute. Interestingly, the screening of all
prepared isomers of protulactone 1 at a single dose of 10 μM
has shown low toxicity against the tested tumor cells (see one-
dose mean graphs in the SI) in contrast to highly active
structurally related lactones (goniofufurone (3), 7-epi-goniofu-
furone (4), and crassalactone C (5)).
In summary, we have developed a divergent synthesis of four
stereoisomers of naturally occurring protulactone A from L- and
D-arabinose. The absolute configurations of all prepared
stereoisomers were established on the basis of the single-crystal
X-ray analysis of the intermediates 13, 14, ent-13, and ent-14.
The 3,6-anhydro-2,8-dideoxy-7-O-acetyl-L-glycero-L-gluco-octa-
no-1,4-lactone ((+)-1a) (3R,4R,5S,6R,7S) was identified as
the naturally occurring lactone 1 based on the comparison of
the specific rotation and NMR spectroscopic data. The
screening of the prepared compounds against the NCI60 cancer
cell line panel has not shown significant cytotoxicity. However,
these results bring new insights into the relationship between
structure and biological activity of such bicyclic lactones.
Finally, the presented approach is applicable in the synthesis of
substantial amounts of new epimers and C-7 analogues of
protulactone A (1), making it suitable for structure−activity
relationship studies.
EXPERIMENTAL SECTION
■
General Experimental Procedures. Melting points were
obtained using a Boecius apparatus and are uncorrected. Optical
rotations were measured with a JASCO P-2000 polarimeter and are
given in units of 10−1 deg·cm2·g−1. FTIR spectra were obtained on a
Nicolet 5700 spectrometer (Thermo Electron) equipped with a Smart
Orbit (diamond crystal ATR) accessory, using the reflectance
technique (4000−400 cm−1). 1H and 13C NMR spectra were recorded
on either a 300 (75) MHz Unity Inova or a 600 (151) MHz VNMRS
spectrometer from Varian. Standard chemical shifts are referenced to
the corresponding solvent residual peak (CDCl3: δH 7.26 ppm, δC
77.16 ppm; CD3OD: δH 3.31 ppm, δC 49.00 ppm; DMSO-d6: δH 2.50
ppm, δC 39.52 ppm) or tetramethylsilane as internal standard. High-
resolution mass spectra (HRMS) were recorded on an OrbitrapVelos
mass spectrometer (Thermo Scientific) with a heated electrospray
ionization source. The mass spectrometer was operated with full scan
(50−2000 amu) in positive or negative FT mode (at a resolution of
100 000). The analyte was dissolved in MeOH and infused via syringe
pump at a rate of 5 mL/min. The heated capillary was maintained at
275 °C with a source heater temperature of 50 °C, and the sheath,
auxiliary, and sweep gases were at 10, 5, and 0 units, respectively.
Source voltage was set to 3.5 kV. Flash column liquid chromatography
was performed on Kieselgel 60 silica gel (40−63 μm, 230−400 mesh),
and analytical thin-layer chromatography (TLC) was performed on
aluminum plates precoated with either 0.2 mm (DC-Alufolien, Merck)
or 0.25 mm silica gel 60 F254 (ALUGRAM SIL G/UV254, Macherey-
3-O-Benzyl-5-O-tert-butyldiphenylsilyl-L-arabinose. To a solution
of 1,2-O-isopropylidene-3-O-benzyl-5-O-tert-butyldiphenylsilyl-β-L-ara-
binose (2.12 g, 4.09 mmol, 1 equiv) in CH2Cl2 (33 mL) at 0 °C was
added trifluoroacetic acid (TFA) (4 mL, 44.96 mmol, 11 equiv, 90%
w/w). The solution was stirred at 0 °C for 3 h followed by the
addition of saturated NaHCO3 solution until no evolution of CO2 was
D
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observed. The resulting layers were separated, and the aqueous phase
was extracted using CH2Cl2 (3 × 180 mL). Combined organic extracts
were dried over anhydrous Na2SO4, filtered, and concentrated under
reduced pressure. The residue was purified using MPLC (gradient
EtOAc/hexanes, 0/100 to 35/65, then isocratic EtOAc/hexanes, 35/
65), providing 3-O-benzyl-5-O-tert-butyldiphenylsilyl-L-arabinose as a
colorless oil (1.68 g, 86%): TLC Rf 0.22 (EtOAc/hexanes, 35/65);
[α]2D5 −18.5 (c 1.07, MeOH); IR (ATR) νmax 504, 699, 1047, 1105,
1427, 2856, 2929, 3400 cm−1; HRESIMS m/z 501.20696 [M + Na]+
(calcd for C28H34NaO5Si, 501.20677); NMR data were in good
agreement with published data.5d
3-O-Benzyl-1-O-tert-butyldiphenylsilyl-L-xylo-hex-5-enitol
(7). To a suspension of MePPh3Br (2.99 g, 8.36 mmol, 4 equiv) in dry
THF (50 mL) at 0 °C under an argon atmosphere was added
dropwise nBuLi (3.43 mL, 8.57 mmol, 4.1 equiv of 2.5 M in hexane).
The reaction mixture was left to stir at 0 °C for 5 min. Subsequently,
the solution of 3-O-benzyl-5-O-tert-butyldiphenylsilyl-L-arabinose
(1.00 g, 2.09 mmol, 1 equiv) in dry THF (10 mL) was added, and
the reaction mixture was left to stir for another 2.5 h. The reaction was
quenched with a saturated solution of NH4Cl (15 mL). The resulting
layers were separated, and the aqueous phase was extracted using
EtOAc (3 × 100 mL). Combined organic extracts were dried over
anhydrous Na2SO4, filtered, and concentrated under reduced pressure.
The residue was purified by MPLC (gradient EtOAc/hexanes, 0/100
to 20/80, then isocratic EtOAc/hexanes, 20/80), providing the desired
product 7 as a light yellow oil (0.75 g, 76%): TLC Rf 0.18 (EtOAc/
hexanes, 20/80); [α]2D5 −13.4 (c 0.48, CHCl3), −17.6 (c, 0.76 MeOH);
IR (ATR) νmax 504, 699, 738, 1070, 1105, 2929, 3417 cm−1; 1H NMR
(300 MHz, CDCl3) δH 7.60−7.56 (m, 4H), 7.37−7.24 (m, 6H), 7.20−
7.08 (m, 5H), 5.96 (ddd, J = 17.3, 10.6, 5.0 Hz, 1H), 5.30 (dt, J = 17.3,
1.7 Hz, 1H), 5.14 (dt, J = 10.6, 1.7 Hz, 1H), 4.53 (d, J = 11.2 Hz, 1H),
4.44 (d, J = 11.2 Hz, 1H), 4.36 (ddt, J = 5.0, 3.2, 1.7 Hz, 1H), 3.84
(ddd, J = 7.2, 5.7, 3.8 Hz, 1H), 3.76 (dd, J = 10.4, 3.8 Hz, 1H), 3.69
(dd, J = 10.4, 5.7 Hz, 1H), 3.51 (dd, J = 7.2, 3.2 Hz, 1H), 3.16 (br s,
2H), 0.99 (s, 9H) ppm; 13C NMR (75 MHz, CDCl3) δC 138.1, 137.8,
135.51, 135.47, 133.1, 132.9, 129.7, 128.2, 127.9, 127.7, 115.5, 80.0,
73.5, 71.7, 71.6, 64.9, 26.8, 19.2 ppm; HRESIMS m/z 499.22766 [M +
Na]+ (calcd for C29H36NaO4Si, 499.22751).
3,6-Anhydro-2-deoxy-7-O-tert-butyldiphenylsilyl-L-gluco-
heptono-1,4-lactone (8). To alkenol 7 (0.40 g, 0.84 mmol, 1 equiv),
anhydrous LiOAc (0.22 g, 3.36 mmol, 4 equiv), and anhydrous CuCl2
(0.45 g, 3.36 mmol, 4 equiv) in a 7 mL screw cap vial was added dry
AcOH (3.36 mL, 0.25 M according to 7). The reaction mixture was
stirred for 15 min at rt followed by the addition of PdCl2(CH3CN)2
(0.02 g, 0,08 mmol, 0,1 equiv) and Fe(CO)5 (28.3 μL, 0.04 mmol,
0.25 equiv). Caution: The vial must be immediately closed after the
addition of Fe(CO)5 to the reaction mixture! The mixture was then
stirred for 35 min at 60 °C and concentrated under reduced pressure.
The crude product was absorbed onto silica gel and purified using
MPLC (gradient EtOAc/Hex, 0/100 to 25/75, then isocratic EtOAc/
hexanes, 25/75). The desired product 8 was obtained as a colorless oil
(0.32 g) in 76% yield: TLC Rf 0.2 (EtOAc/hexanes, 25/75); [α]D25
+9.02 (c 1.00, CHCl3), +10.81 (c 1.02, MeOH); IR (ATR) νmax 503,
699, 1047, 1105, 1143, 1786, 2929 cm−1; 1H NMR (300 MHz,
CDCl3) δH 7.68−7.62 (m, 4H), 7.47−7.26 (m, 11H), 4.90 (dd, J = 4.4,
1.1 Hz, 1H), 4.79 (td, J = 4.4, 2.2 Hz, 1H), 4.65 (d, J = 11.7 Hz, 1H),
4.55 (d, J = 11.7 Hz, 1H), 4.25 (dd, J = 5.7, 1.1 Hz, 1H), 4.00 (dt, J =
5.7, 4.0 Hz, 1H), 3.79 (dd, J = 14.5, 4.0 Hz, 1H), 3.76 (dd, J = 14.5, 4.0
Hz, 1H), 2.77−2.64 (m, 2H), 1.04 (s, 9H) ppm; 13C NMR (75 MHz,
CDCl3) δC 174.9, 137.1, 135.8, 135.7, 133.4, 133.2, 129.9, 128.7, 128.2,
128.0, 127.9, 88.0, 85.4, 83.3, 77.5, 72.7, 63.1, 36.1, 26.9, 19.4 ppm;
HRESIMS m/z 525.20649 [M + Na]+ (calcd for C30H34NaO5Si,
525.20677).
purified using MPLC (first separation: isocratic MeOH/CH2Cl2, 10/
90, the second separation, gradient EtOAc/hexanes, 0/100, to EtOAc/
hexanes, 100/0, then isocratic EtOAc/hexanes, 100/0), providing the
desired compound 10 as a white crystalline solid (4.22 g, 30%): TLC
Rf 0.18 (MeOH/CH2Cl2, 10/90); mp 50−52 °C; [α]2D5 +44.3 (c 1.03,
MeOH); IR (ATR) νmax 566, 655, 993, 1026, 1754, 2935, 3311 cm−1;
1H NMR (300 MHz, DMSO-d6) δH 5.60 (d, J = 5.0 Hz, 1H), 4.81 (t, J
= 5.7 Hz, 1H), 4.75 (dd, J = 4.4, 1.0 Hz, 1H), 4.69 (t, J = 4.8 Hz,1H),
4.02 (t, J = 4.9 Hz, 1H), 3.68 (dd, J = 10.5, 4.8 Hz, 1H), 3.46 (ddd, J =
11.7, 5.5, 4.4 Hz, 1H), 3.38 (dd, J = 11.7, 5.8 Hz, 1H), 2.86 (dd, J =
18.2, 5.6 Hz, 1H), 2.44 (d, J = 18.1 Hz, 1H) ppm; 13C NMR (75 MHz,
CD3OD) δC 175.6, 90.1, 87.2, 75.4, 61.2, 39.5, 36.0 ppm; HRESIMS
m/z 175.06014 [M + H]+ (calcd for C7H11O5, 175.06010).
3,6-Anhydro-2-deoxy-5,7-di-O-tert-butyldimethylsilyl-L-
gluco-heptono-1,4-lactone (11). To a solution of 10 (4.22 g, 24.25
mmol, 1 equiv) in anhydrous DMF (70 mL) were added imidazole
(8.25 g, 121.27 mmol, 5 equiv) and tert-butyldimethylsilyl chloride
(TBSCl) (12.79 g, 84.89 mmol, 3.5 equiv). The mixture was left to stir
at 50 °C for 3 h and then diluted with EtOAc (450 mL). The organic
phase was washed with deionized H2O (3 × 150 mL), dried over
Na2SO4, filtered, and concentrated under reduced pressure.
Purification of the residue using MPLC (gradient EtOAc/hexanes,
0/100, to EtOAc/hexanes, 10/90, then isocratic EtOAc/hexanes, 10/
90) provided the desired product 11 as a colorless oil (8.84 g, 91%):
TLC Rf 0.24 (EtOAc/hexanes, 10/90); [α]2D5 +17.2 (c 1.11, MeOH);
IR (ATR) νmax 776, 832, 1046, 1108, 1253, 1790, 2929 cm−1; 1H NMR
(300 MHz, CDCl3) δH 4.79 (td, J = 4.5, 2.2 Hz, 1H), 4.70 (dd, J = 4.5,
1.3 Hz), 4.34 (dd, J = 5.1, 1.3 Hz, 1H), 3.80 (dt, J = 5.1, 4.3 Hz, 1H),
3.70 (dd, J = 9.8, 4.3 Hz, 1H), 3.66 (dd, J = 9.8, 4.3 Hz, 1H), 2.74−
2.62 (m, 2H), 0.90 (s, 9H), 0.89 (s, 9H), 0.13 (s, 3H), 0.11 (s, 3H),
0.054 (s, 3H), 0.05 (s, 3H) ppm; 13C NMR (75 MHz, CDCl3) δC
175.2, 90.6, 87.6, 77.3, 76.9, 62.4, 36.2, 26.0, 25.8, 18.5, 18.1, −4.7,
−4.9, −5.2, −5.3 ppm; HRESIMS m/z 403.23302 [M + H]+ (calcd for
C19H39O5Si2, 403.23305).
3,6-Anhydro-2-deoxy-5-O-tert-butyldimethylsilyl-L-gluco-
heptono-1,4-lactone (12). To a solution of 11 (8.50 g, 21.11 mmol,
1 equiv) in isopropyl alcohol (210 mL) were added ceric ammonium
nitrate (CAN) (11.57 g, 21.11 mmol, 1 equiv) and deionized H2O (6
mL). The mixture was left to stir at rt for 6 h. Subsequently, the
reaction mixture was diluted with EtOAc (1500 mL) and washed with
saturated NaHCO3 solution (400 mL) and deionized H2O (3 × 250
mL). The separated organic phase was then dried over Na2SO4,
filtered, and concentrated under reduced pressure, affording a crude
solid product. The resulting white solid was then suspended in hot
hexanes (75 mL), cooled to rt, and filtered, providing pure product 12
(4.08 g, 67%): TLC Rf 0.16 (EtOAc/hexanes, 35/65); mp 105.6−
106.4 °C; [α]2D5 +30.3 (c 1.05, MeOH); IR (ATR) νmax 779, 833, 1035,
1
1052, 1762, 2954, 3458 cm−1; H NMR (300 MHz, CDCl3) δH 4.81
(td, J = 4.5, 2.1 Hz, 1H), 4.74 (dd, J = 4.5, 1.4 Hz, 1H), 4.26 (dd, J =
5.8, 1.4 Hz, 1H), 3.87−3.80 (m, 2H), 3.66 (dd, J = 12.6, 5.9 Hz, 1H),
2.78−2.65 (m, 2H), 1.69 (t, J = 5.8 Hz, 1H), 0.90 (s, 9H), 0.14 (s, J =
7.6 Hz, 3H), 0.11 (s, J = 7.6 Hz, 3H) ppm; 13C NMR (75 MHz,
CDCl3) δC 175.0, 90.8, 87.0, 77.3, 77.0, 61.9, 36.0, 25.8, 18.0, −4.7,
−4.9 ppm; HRESIMS m/z 289.14659 [M + H]+ (calcd for
C13H25O5Si, 289.14658).
3,6-Anhydro-2,8-dideoxy-5-O-tert-butyldimethylsilyl-L-glyc-
ero-L-gluco-octano-1,4-lactone (13) and 3,6-Anhydro-2,8-di-
deoxy-5-O-tert-butyldimethylsilyl-D-glycero-L-gluco-octano-
1,4-lactone (14). To a solution of 12 (1.00 g, 3.47 mmol, 1 equiv) in
CH2Cl2 (30 mL) were added Dess−Martin reagent (2.21 g, 5.20
mmol, 1.5 equiv) and solid NaHCO3 (1.46 g, 17.34 mmol, 5 equiv).
The resulting suspension was left to stir at rt for 3 h and then filtered
through a column filled with 20 g of silica gel. The crude product was
eluted with an EtOAc/hexanes mixture (30/70, 200 mL), and the
filtrate concentrated to dryness under reduced pressure, providing a
crude aldehyde.
3,6-Anhydro-2-deoxy-L-gluco-heptono-1,4-lactone (10). To a
solution of L-arabinose (12.00 g, 79.97 mmol, 1 equiv) and Meldrum’s
acid (11.53 g, 79.97 mmol, 1 equiv) in anhydrous dimethylformamide
(DMF) (42 mL) was added triethylamine (11.15 mL, 79.97 mmol, 1
equiv). The solution was left to stir at 40−50 °C under an argon
atmosphere for 10 days. The mixture was then concentrated under
reduced pressure and absorbed onto silica gel. The crude product was
The crude aldehyde and anhydrous LiCl (0.29 g, 6.93 mmol, 2
equiv) were then dissolved in anhydrous THF (14 mL), and a
MeMgBr solution (1.27 mL, 3.81 mmol, 1.1 equiv, 3 M in Et2O) was
added dropwise (0.05 mL/min) at −78 °C under an argon
E
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Journal of Natural Products
Article
atmosphere. The solution was left to stir for 2 h at −78 °C and then
left to warm to rt over 10 min. The reaction was quenched by the
addition of a saturated NH4Cl solution (5 mL). The layers were
separated, and the aqueous phase was extracted with EtOAc (2 × 25
mL). The combined organic extracts were dried over Na2SO4, filtered,
and concentrated under reduced pressure. Purification of the residue
using MPLC (isocratic MeOH/EtOAc/hexanes, 1/20/80) provided a
mixture of diastereomers 13 and 14 as a white solid (0.41g, 39%, 13/
14 = 3/2). A second MPLC purification (150 equiv of SiO2, isocratic
EtOAc/hexanes, 25/75) of this mixture provided pure products 13
and 14.
anhydrous CH2Cl2 (1 mL) was added, and the reaction mixture was
left to stir for 1.5 h. The mixture was then diluted with CH2Cl2 (25
mL), washed with deionized H2O (2 × 10 mL), dried over anhydrous
Na2SO4, filtered, and concentrated under reduced pressure.
Subsequently, the crude product was dissolved in THF (1.5 mL),
and a solution of TBAF·3H2O (125 mg, 0.40 mmol, 1.5 equiv) and
AcOH (26 μL, 0.40 mmol, 1.5 equiv) in THF (1.5 mL) was added.
The resulting mixture was left to stir for 2 h and then concentrated
under reduced pressure to one-fifth of its volume. The crude product
was purified using MPLC (gradient EtOAc/hexanes, 0/100, to
EtOAc/hexanes, 100/0, then isocratic EtOAc/hexanes, 100/0),
providing 1b as a colorless oil (47 mg, 77%): TLC Rf 0.19 (EtOAc/
hexane, 40/60); [α]2D5 +53.6 (c 1.15, MeOH), +50.4 (c 0.34, MeOH),
+65.3 (c 1.10, CHCl3); IR (ATR) νmax 553, 608, 945, 1041, 1731,
1782, 3451 cm−1; 1H NMR (300 MHz, CD3OD) δH 5.04 (dq, J = 6.5,
5.0 Hz, 1H), 4.83−4.79 (m, 2H, 3-H), 4.15 (d, J = 4.5 Hz, 1H), 3.81
(t, J = 5.0 Hz, 1H), 2.85 (dd, J = 18.2, 4.8 Hz, 1H), 2.57 (dd, J = 18.2,
0.6 Hz, 1H), 2.02 (s, 3H), 1.26 (d, J = 6.5 Hz, 3H) ppm; 13C NMR
(75 MHz, CD3OD) δC 177.6, 172.3, 92.0, 90.1, 79.2, 77.6, 71.0, 37.0,
21.0, 16.8 ppm; HRESIMS m/z 231.08636 [M + H]+ (calcd for
C10H15O6, 231.08631).
3,6-Anhydro-2-deoxy-D-gluco-heptono-1,4-lactone (ent-10).
Procedure A. To alkenol 9 (0.12 g, 0.81 mmol, 1 equiv), anhydrous
LiOAc (0.27 g, 4.05 mmol, 5 equiv), and anhydrous CuCl2 (0.54 g,
4.05 mmol, 5 equiv) in a 7 mL screw cap vial was added dry AcOH
(3.24 mL, 0.25 M according to 7). The reaction mixture was stirred for
15 min at rt followed by the addition of PdCl2(CH3CN)2 (0.02 g, 0.08
mmol, 0.1 equiv) and Fe(CO)5 (53 μL, 0.4 mmol, 0.5 equiv). Caution:
The vial must be immediately closed after the addition of Fe(CO)5 to the
reaction mixture! The mixture was then stirred for 1 h at 60 °C and
concentrated under reduced pressure. The crude product was
absorbed onto silica gel and purified using MPLC (gradient EtOAc/
hexanes, 0/100 to 100/0). The desired product ent-10 was obtained as
a white crystalline solid (0.66 g) in 47% yield.
1
Aldehyde: TLC Rf 0.25 (EtOAc/hexanes, 25/75); H NMR (300
MHz, CDCl3) δH 9.53 (d, J = 1.5 Hz, 1H), 5.03 (t, J = 4.1 Hz, 1H),
4.67 (dd, J = 4.1, 0.9 Hz, 1H), 4.58 (dt, J = 1.5, 0.9 Hz, 1H), 4.30 (t, J
= 1.5 Hz, 1H), 2.87 (d, J = 18.3 Hz, 1H), 2.74 (dd, J = 18.3, 4.1 Hz,
1H), 0.90 (s, 9H), 0.14 (s, 6H) ppm; 13C NMR (75 MHz, CDCl3) δC
199.5, 174.3, 92.0, 87.8, 79.4, 78.2, 36.3, 25.7, 18.1, −4.84 ppm.
Compound 13: TLC Rf 0.32 (EtOAc/hexanes, 40/60); mp 61.7−
63.5 °C; [α]2D5 +35.3 (c 1.00, CHCl3); IR (ATR) νmax 776, 835, 1114,
1
1772, 2860, 2929, 3483 cm−1; H NMR (300 MHz, CDCl3) δH 4.78
(td, J = 4.2, 2.2 Hz, 1H), 4.69 (dd, J = 4.2, 0.8 Hz, 1H), 4.40 (dd, J =
5.3, 0.8 Hz, 1H), 3.93 (qd, J = 6.5, 4.3 Hz, 1H), 3.68 (dd, J = 5.3, 4.3
Hz, 1H), 2.76−2.62 (m, 2H), 1.20 (d, J = 6.5 Hz, 3H), 0.90 (s, 9H),
0.15 (s, 3H), 0.13 (s, 3H) ppm; 13C NMR (75 MHz, CDCl3) δC
175.1, 91.0, 90.6, 77.3, 76.3, 66.8, 36.1, 25.8, 18.7, 17.9, −4.7, −4.9
ppm; HRESIMS m/z 325.14413 [M + Na]+ (calcd for C14H26NaO5Si,
325.14417).
Compound 14: TLC Rf 0.26 (EtOAc/hexanes, 40/60); mp 105.0−
106.5 °C; [α]2D5 +23.2 (c 1.00, CHCl3); IR (ATR) νmax 783, 835, 1060,
1
1763, 2856, 2956, 3459 cm−1; H NMR (300 MHz, CDCl3) δH 4.80
(td, J = 4.4, 2.2 Hz, 1H), 4.72 (dd, J = 4.4, 1.3 Hz, 1H), 4.24 (dd, J =
5.6, 1.3 Hz, 1H), 3.79 (qd, J = 6.5, 5.6 Hz, 1H), 3.60 (t, J = 5.6 Hz,
1H), 2.78−2.65 (m, 2H), 1.25 (d, J = 6.5 Hz, 3H), 0.90 (s, 9H), 0.15
(s, 3H), 0.12 (s, 3H) ppm; 13C NMR (75 MHz, CDCl3) δC 175.0,
90.9, 90.7, 77.7, 77.2, 67.4, 36.0, 25.8, 19.7, 18.0, −4.5, −4.9 ppm;
HRESIMS m/z 325.14404 [M + Na]+ (calcd for C14H26NaO5Si,
325.14417).
3,6-Anhydro-2,8-dideoxy-L-glycero-L-gluco-octano-1,4-lac-
tone (Protulactone A, 1a). To a solution of 4-dimethylamino-
pyridine (DMAP) (100 mg, 0.82 mmol, 2.2 equiv) in anhydrous
CH2Cl2 (1 mL) was added AcCl (53 μL, 0.74 mmol, 2 equiv), and the
mixture was left to stir for 15 min. Subsequently, the solution of 13
(112 mg, 0.37 mmol, 1 equiv) in anhydrous CH2Cl2 (2 mL) was
added, and the reaction mixture was left to stir for 1.5 h followed by
the addition of CH2Cl2 (25 mL). The resulting mixture was then
washed with deionized H2O (2 × 10 mL), dried over anhydrous
Na2SO4, filtered, and concentrated under reduced pressure, providing
a crude product.
Procedure B. To a solution of D-arabinose (15.00 g, 99.96 mmol, 1
equiv) and Meldrum’s acid (14.41 g, 99.96 mmol, 1 equiv) in
anhydrous DMF (53 mL) was added triethylamine (13.94 mL, 99.96
mmol, 1 equiv). The solution was left to stir at 40−50 °C under an
argon atmosphere for 10 days. The mixture was then concentrated
under reduced pressure and absorbed onto silica gel. The crude
product was purified using MPLC (first separation: isocratic MeOH/
CH2Cl2, 10/90, second separation, gradient EtOAc/hexanes, 0/100, to
EtOAc/hexanes, 100/0, then isocratic EtOAc/hexanes, 100/0),
providing the desired lactone ent-10 as a white, crystalline solid
(4.86 g, 28%): TLC Rf 0.27 (MeOH/CH2Cl2, 15/85); mp 50−52 °C;
[α]2D5 −43.9 (c 1.03, MeOH); IR (ATR) νmax 499, 1006, 1021, 1743,
2939, 3439, 3515 cm−1; 1H NMR (300 MHz, CD3OD) δH 4.83−4.78
(m, 2H), 4.17 (d, J = 5.4 Hz, 1H), 3.81 (td, J = 5.6, 3.8 Hz, 1H), 3.70
(dd, J = 11.9, 3.8 Hz, 1H), 3.58 (dd, J = 11.9, 5.8 Hz, 1H), 2.85 (dd, J
= 18.2, 5.1 Hz, 1H), 2.59 (d, J = 18.2 Hz, 1H) ppm; 13C NMR (75
MHz, CD3OD) δC 177.8, 92.2, 88.4, 78.9, 77.2, 62.8, 36.9 ppm;
HRESIMS m/z 175.06022 [M + H]+ (calcd for C7H11O5, 175.06010);
all spectroscopic data were in good agreement with published data.5a
3,6-Anhydro-2-deoxy-5,7-di-O-tert-butyldimethylsilyl-D-
gluco-heptono-1,4-lactone (ent-11). To a solution of ent-10 (4.19
g, 22.06 mmol, 1 equiv) in anhydrous DMF (72 mL) were added
imidazole (8.19 g, 120.3 mmol, 5 equiv) and TBSCl (12.69 g, 84.21
mmol, 3.5 equiv). The mixture was left to stir at 50 °C for 3 h and
then diluted with EtOAc (450 mL). The organic phase was then
washed with deionized H2O (3 × 150 mL), dried over Na2SO4,
filtered, and concentrated under reduced pressure. Purification of the
residue using MPLC (gradient EtOAc/hexanes, 0/100, to EtOAc/
hexanes, 10/90, then isocratic EtOAc/hexanes, 10/90) provided the
desired product ent-11 as a colorless oil (9.11 g, 94%): TLC Rf 0.5
(EtOAc/hexanes, 30/70); [α]2D5 −21.6 (c 3.30, MeOH); IR (ATR)
Subsequently, the crude product was dissolved in THF (2.5 mL),
and a solution of TBAF·3H2O (175 mg, 0.56 mmol, 1.5 equiv) and
AcOH (32 μL, 0.56 mmol, 1.5 equiv) in THF (2.5 mL) was added.
The reaction mixture was left to stir for 2 h and then concentrated
under reduced pressure to one-fifth of its volume. The crude product
was purified using MPLC (gradient EtOAc/hexanes, 0/100, to
EtOAc/hexanes, 100/0, then isocratic EtOAc/hexanes, 100/0),
providing protulactone A (1a) as a colorless oil (67 mg, 79%): TLC
Rf 0.17 (EtOAc/hexanes, 40/60); [α]2D5 +38.2 (c 1.1, MeOH), +35.6 (c
0.34, MeOH), +35.2 (c 1.05, CHCl3); IR (ATR) νmax 505, 552, 945,
1
960, 1728, 1780, 3446 cm−1; H NMR (300 MHz, CD3OD) δH 5.07
(dq, J = 6.6, 4.0 Hz, 1H), 4.82−4.77 (m, 2H), 4.33 (d, J = 4.7 Hz, 1H),
3.80 (dd, J = 4.7, 4.0 Hz, 1H), 2.90−2.80 (m, 1H), 2.53 (dd, J = 18.1,
0.8 Hz, 1H), 2.00 (s, 3H), 1.25 (d, J = 6.6 Hz, 3H) ppm; 13C NMR
(75 MHz, CD3OD) δC 177.5, 172.1, 92.2, 90.2, 79.2, 76.6, 70.6, 37.0,
21.0, 16.3 ppm; HRESIMS m/z 231.08639 [M + H]+ (calcd for
C10H15O6, 231.08631).
3,6-Anhydro-2,8-dideoxy-D-glycero-L-gluco-octano-1,4-lac-
tone (7-epi-protulactone A, 1b). To a solution of DMAP (71 mg,
0.58 mmol, 2.2 equiv) in anhydrous CH2Cl2 (1 mL) was added AcCl
(38 μL, 0.53 mmol, 2 equiv), and the mixture was left to stir for 15
min. Subsequently, the solution of 14 (80 mg, 0.26 mmol, 1 equiv) in
1
νmax 671, 833, 1046, 1108, 1790, 2858, 2929 cm−1; H NMR (300
MHz, CDCl3) δH 4.77 (td, J = 4.5, 2.2 Hz, 1H), 4.69 (dd, J = 4.5, 1.3
Hz, 1H), 4.33 (dd, J = 5.1, 1.3 Hz, 1H), 3.80 (dt, J = 5.1, 4.3 Hz, 1H),
3.69 (dd, J = 12.0, 4.3 Hz, 1H), 3.65 (dd, J = 12.0, 5.1 Hz, 1H), 2.73−
2.60 (m, 2H), 0.88 (s, 9H), 0.87 (s, 9H), 0.12 (s, 3H), 0.09 (s, 3H),
F
DOI: 10.1021/acs.jnatprod.7b00212
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Journal of Natural Products
Article
0.04 (s, 3H), 0.036 (s, 3H) ppm; 13C NMR (75 MHz, CDCl3) δC
175.1, 90.6, 87.6, 77.2, 76.8, 62.4, 36.2, 26.0, 25.8, 18.5, 18.0, −4.7,
−4.9, −5.2, −5.3 ppm; HRESIMS m/z 403.23354 [M + H]+ (calcd for
C19H39O5Si2, 403.23305).
CDCl3) δH 4.79 (td, J = 4.4, 2.2 Hz, 1H), 4.71 (dd, J = 4.4, 1.3 Hz,
1H), 4.24 (dd, J = 5.6, 1.3 Hz, 1H), 3.78 (qd, J = 6.5, 5.6 Hz, 1H), 3.59
(t, J = 5.6 Hz, 1H), 2.77−2.65 (m, 2H), 1.24 (d, J = 6.5 Hz, 3H), 0.90
(s, 9H), 0.15 (s, 3H), 0.12 (s, 3H) ppm; 13C NMR (75 MHz, CDCl3)
δC 175.0, 90.9, 90.7, 77.7, 77.2, 67.4, 36.0, 25.8, 19.8, 18.0, −4.5, −4.9
ppm; HRESIMS m/z 325.14400 [M + Na]+ (calcd for C14H26NaO5Si,
325.14417).
3,6-Anhydro-2,8-dideoxy-D-glycero-D-gluco-octano-1,4-lac-
tone (ent-1a). To a solution of DMAP (135 mg, 1.11 mmol, 2.2
equiv) in anhydrous CH2Cl2 (2 mL) was added AcCl (72 μL, 1.01
mmol, 2 equiv), and the mixture was left to stir for 15 min.
Subsequently, the solution of ent-13 (152 mg, 0.50 mmol, 1 equiv) in
anhydrous CH2Cl2 (2 mL) was added, and the resulting solution was
left to stir for 1.5 h. The reaction mixture was then diluted with
CH2Cl2 (25 mL), washed with deionized H2O (2 × 15 mL), dried
over anhydrous Na2SO4, filtered, and concentrated under reduced
pressure.
Subsequently, the crude product was dissolved in THF (3.4 mL),
and a solution of TBAF·3H2O (238 mg, 0.75 mmol, 1.5 equiv) and
AcOH (43 μL, 0.75 mmol, 1.5 equiv) in THF (3.4 mL) was added.
The resulting mixture was left to stir for 2 h and then concentrated
under reduced pressure to one-fifth of its volume. The crude product
was purified using MPLC (gradient EtOAc/hexanes, 0/100, to
EtOAc/hexanes, 100/0, then isocratic EtOAc/hexanes, 100/0),
providing ent-1a as a colorless oil (91 mg, 79% over 2 steps): TLC
Rf 0.17 (EtOAc/hexanes, 40/60); [α]2D5 −37.5 (c 1.16, MeOH), −33.8
(c 0.34, MeOH), −35.2 (c 1.10, CHCl3); IR (ATR) νmax 552, 1040,
1240, 1729, 1780, 2937, 3437 cm−1; 1H NMR (300 MHz, CD3OD) δH
5.07 (dq, J = 6.6, 4.0 Hz, 1H), 4.82−4.77 (m, 2H), 4.33 (d, J = 4.7 Hz,
1H), 3.80 (dd, J = 4.7, 4.0 Hz, 1H), 2.89−2.80 (m, 1H), 2.52 (dd, J =
18.1, 0.8 Hz, 1H), 2.00 (s, 3H), 1.25 (d, J = 6.6 Hz, 3H) ppm; 13C
NMR (75 MHz, CD3OD) δC 177.5, 172.1, 92.2, 90.1, 79.1, 76.6, 70.6,
37.0, 21.0, 16.3 ppm; HRESIMS m/z 231.08639 [M + H]+ (calcd for
C10H15O6, 231.08631).
3,6-Anhydro-2-deoxy-5-O-tert-butyldimethylsilyl-D-gluco-
heptono-1,4-lactone (ent-12). To a solution of ent-11 (9.11 g,
22.62 mmol, 1 equiv) in isopropyl alcohol (237 mL) was added CAN
(12.40 g, 22.62 mmol, 1 equiv) and deionized H2O (7 mL). The
mixture was left to stir at rt for 6 h. Subsequently, the mixture was
diluted with EtOAc (1500 mL) and washed with saturated NaHCO3
solution (400 mL) and deionized H2O (3 × 250 mL). The separated
organic phase was then dried over Na2SO4, filtered, and concentrated
under reduced pressure, affording a crude solid product. The resulting
white solid was then suspended in hot hexanes (75 mL), cooled to rt,
and filtered, providing pure product ent-12 (4.40 g, 67%): TLC Rf 0.27
(EtOAc/hexanes, 35/65); mp 106.5−107.8 °C; [α]2D5 −29.3 (c 0.90,
MeOH); IR (ATR) νmax 434, 833, 1052, 1762, 2856, 2952, 3460 cm−1;
1H NMR (300 MHz, CDCl3) δH 4.81 (td, J = 4.5, 2.1 Hz, 1H), 4.74
(dd, J = 4.5, 1.4 Hz, 1H), 4.26 (dd, J = 5.8, 1.4 Hz, 1H), 3.86−3.81 (m,
2H), 3.65 (dd, J = 12.6, 5.9 Hz, 1H), 2.78−2.65 (m, 2H), 0.90 (s, 9H),
0.14 (s, 3H), 0.11 (s, 3H) ppm; 13C NMR (75 MHz, CDCl3) δC
175.0, 90.8, 87.0, 77.4, 77.1, 62.0, 36.0, 25.8, 18.0, −4.7, −4.9 ppm;
HRESIMS m/z 289.14663 [M + H]+ (calcd for C13H25O5Si,
289.14658).
3,6-Anhydro-2,8-dideoxy-5-O-tert-butyldimethylsilyl-D-glyc-
ero-D-gluco-octano-1,4-lactone (ent-13) and 3,6-Anhydro-2,8-
dideoxy-5-O-tert-butyldimethylsilyl-L-glycero-D-gluco-octano-
1,4-lactone (ent-14). To a solution of ent-12 (1.00 g, 3.47 mmol, 1
equiv) in CH2Cl2 (30 mL) were added Dess−Martin reagent (2.21 g,
5.20 mmol, 1.5 equiv) and solid NaHCO3 (1.46 g, 17.34 mmol, 5
equiv). The resulting suspension was left to stir at rt for 3 h and then
filtered through a column filled with 20 g of silica gel. The crude
product was eluted with an EtOAc/hexanes mixture (30/70, 200 mL)
and the filtrate concentrated under reduced pressure, providing a
crude aldehyde.
3,6-Anhydro-2,8-dideoxy-L-glycero-D-gluco-octano-1,4-lac-
tone (ent-1b). To a solution of DMAP (42 mg, 0.34 mmol, 2.2
equiv) in anhydrous CH2Cl2 (0.5 mL) was added AcCl (22 μL, 0.31
mmol, 2 equiv), and the mixture was left to stir for 15 min at rt.
Subsequently, a solution of ent-14 (47 mg, 0.16 mmol, 1 equiv) in
anhydrous CH2Cl2 (1 mL) was added, and the resulting solution was
left to stir for 1.5 h. The reaction mixture was then diluted with
CH2Cl2 (15 mL), washed with deionized H2O (2 × 5 mL), dried over
anhydrous Na2SO4, filtered, and concentrated under reduced pressure.
Subsequently, the crude product was dissolved in THF (1 mL), and
a solution of TBAF·3H2O (73 mg, 0.23 mmol, 1.5 equiv) and AcOH
(13 μL, 0.23 mmol, 1.5 equiv) in THF (1 mL) was added. The
resulting mixture was left to stir for 2 h and then concentrated under
reduced pressure to one-fifth of its volume. The crude product was
purified using MPLC (gradient EtOAc/hexanes, 0/100, to EtOAc/
hexanes, 100/0, then isocratic EtOAc/hexanes, 100/0), providing ent-
1b as a colorless oil (27 mg, 75% over 2 steps): TLC Rf 0.19 (EtOAc/
hexanes, 40/60); [α]2D5 −56.8 (c 1.15, MeOH), −55.8 (c 0.34, MeOH)
−65.5 (c 1.30, CHCl3); IR (ATR) νmax 463, 1039, 1239, 1728, 1779,
2939, 3437 cm−1; 1H NMR (300 MHz, CD3OD) δH 5.04 (dq, J = 6.5,
5.0 Hz, 1H), 4.83−4.79 (m, 2H), 4.15 (d, J = 4.5 Hz, 1H), 3.81 (t, J =
5.0 Hz, 1H), 2.89−2.80 (m, 1H), 2.58 (dd, J = 18.2, 0.6 Hz, 1H), 2.02
(s, 3H), 1.26 (d, J = 6.5 Hz, 3H) ppm; 13C NMR (75 MHz, CD3OD)
δC 177.6, 172.3, 92.0, 90.1, 79.2, 77.6, 71.0, 37.0, 21.0, 16.8 ppm;
HRESIMS m/z 231.08635 [M + H]+ (calcd for C10H15O6,
231.08631).
The crude aldehyde and andydrous LiCl (0.29 g, 6.93 mmol, 2
equiv) were then subsequently dissolved in anhydrous THF (14 mL),
and MeMgBr solution (1.27 mL, 3.81 mmol, 1.1 equiv, 3 M in Et2O)
was added dropwise (0.05 mL/min) at −78 °C under an argon
atmosphere. The solution was left to stir for 2 h at −78 °C and then
left to warm to rt over 10 min. The reaction was quenched with
saturated NH4Cl solution (5 mL). The layers were separated, and the
aqueous phase was extracted with EtOAc (2 × 25 mL). The combined
organic extracts were dried over Na2SO4, filtered, and concentrated
under reduced pressure. Purification of the residue using MPLC
(isocratic MeOH/EtOAc/hexanes, 1/20/80) provided a mixture of
diastereomers ent-13 and ent-14 as white solids (0.42g, 40%, ent-13/
ent-14 = 3/2). A second MPLC purification (150 equiv of SiO2,
isocratic EtOAc/hexanes, 25/75) of this mixture provided pure
products ent-13 and ent-14.
1
ent-Aldehyde: TLC Rf 0.25 (EtOAc/hexanes, 40/60); H NMR
(300 MHz, CDCl3) δH 9.55 (d, J = 1.5 Hz, 1H), 5.04 (t, J = 4.1 Hz,
1H), 4.68 (dd, J = 4.1, 0.9 Hz, 1H), 4.59 (ddd, J = 2.3, 1.5, 0.9 Hz,
1H), 4.30 (t, J = 1.5 Hz, 1H), 2.88 (d, J = 18.3 Hz, 1H), 2.75 (dd, J =
18.3, 4.1 Hz, 1H), 0.91 (s, 9H), 0.15 (s, 6H) ppm; 13C NMR (75
MHz, CDCl3) δC 199.6, 174.2, 92.1, 87.9, 79.5, 78.3, 36.3, 25.7, 18.1,
−4.8, −4.84 ppm.
Compound ent-13: TLC Rf 0.18 (EtOAc/hexanes, 35/65); mp
61.2−63.0 °C; [α]2D5 −35.8 (c 1.00, CHCl3); IR (ATR) νmax 547, 834,
1048, 1774, 2858, 2929, 3467 cm−1; 1H NMR (300 MHz, CDCl3) δH
4.78 (td, J = 4.2, 2.2 Hz, 1H, H-5), 4.69 (dd, J = 4.2, 0.8 Hz, 1H, H-1),
4.41 (dd, J = 5.3, 0.8 Hz, 1H), 3.94 (qd, J = 6.5, 4.3 Hz, 1H), 3.68 (dd,
J = 5.3, 4.3 Hz, 1H), 2.76−2.62 (m, 2H), 1.20 (d, J = 6.5 Hz, 3H), 0.90
(s, 9H), 0.15 (s, 3H), 0.13 (s, 3H) ppm; 13C NMR (75 MHz, CDCl3)
δC 175.0, 91.0, 90.6, 77.3, 76.3, 66.8, 36.1, 25.8, 18.7, 17.9, −4.5, −4.9
ppm; HRESIMS m/z 325.14409 [M + Na]+ (calcd for C14H26NaO5Si,
325.14417).
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jnat-
prod.7b00212.
Compound ent-14: TLC Rf 0.13 (EtOAc/hexanes, 35/65); mp
104.2−106.3 °C; [α]D25 −22.3 (c 1.00, CHCl3); IR (ATR) νmax 561,
835, 1041, 1765, 2856, 2927, 3462 cm−1; 1H NMR (300 MHz,
1H and 13C NMR spectra for all new compounds; one-
dose mean graphs (PDF)
G
DOI: 10.1021/acs.jnatprod.7b00212
J. Nat. Prod. XXXX, XXX, XXX−XXX

Journal of Natural Products
Article
(12) (a) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Adv. 2015,
A71, 3−8. (b) Burla, M. C.; Caliandro, R.; Camalli, M.; Carrozzini, B.;
Cascarano, G. L.; Giacovazzo, C.; Mallamo, M.; Mazzone, A.; Polidori,
G.; Spagna, R. J. Appl. Crystallogr. 2012, 45, 351−356. (c) Sheldrick,
G. M. Acta Crystallogr. 2015, C71, 3−8. Dolomanov, O.; Bourhis, L. J.;
Gildea, R. I.; Howard, J. A. K.; Puschmann, H. J. Appl. Crystallogr.
2009, 42, 339−341.
(13) (a) Flack, H. D. Acta Crystallogr., Sect. A: Found. Crystallogr.
1983, A39, 876−881. (b) Parsons, S.; Flack, H. D.; Wagner, T. Acta
Crystallogr., Sect. B: Struct. Sci., Cryst. Eng. Mater. 2013, B69, 249−259.
crystallographic data (CIF) of compounds 13, 14, ent-13,
and ent-14 (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: tibor.gracza@stuba.sk. Tel: +421 2 59325160.
ORCID
■
Tibor Gracza: 0000-0002-7667-0663
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Slovak Grant Agencies (VEGA
No. 1/0488/14, APVV-14-0147, and ASFU, Bratislava, ITMS
project no. 26240120025). This article was created also with
̌
̌
the support of the MSVVaS of the Slovak Republic within the
Research and Development Operational Programme for the
project “University Science Park of STU Bratislava” (ITMS
project no. 26240220084) cofunded by the European Regional
Development Fund.
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DOI: 10.1021/acs.jnatprod.7b00212
J. Nat. Prod. XXXX, XXX, XXX−XXX