Total synthesis and glycosidase inhibition studies of (-)-gabosine J and its derivatives

Article abstract of DOI:10.1002/ejoc.201301782

The total syntheses of (-)-gabosine J and its two gabosinol derivatives, the reduced forms of (-)-gabosine J, were achieved by a chiral pool strategy that started from inexpensive and readily available D-mannitol. The desymmetrization of the C₂-symmetric tri-O-isopropylidene mannitol through a H₂SO₄/silica mediated hydrolysis of one of the two terminal ketals and a ring-closing methathesis (RCM) of the appropriately protected diene are the key steps in these syntheses. The preparation of (-)-gabosine J, which involved simple steps as well as inexpensive and readily available reagents and starting material, was completed in 14 steps with an overall yield of 13%. Similarly, gabosinol J-α and gabosinol J-β (two synthetic derivatives of gabosine J) were prepared in 13 steps in an overall yield of 11 and 6%, respectively. A preliminary investigation into the biological activity of these compounds revealed that (-)-gabosine J inhibits α-mannosidase with an IC₅₀ of 260 μM. Its reduced form gabosinol J-α inhibits β-galactosidase with an IC₅₀ of 600 μM, and gabosinol J-β inhibits β-glucosidase. This is the first synthesis of a gabosine that employs mannitol as the starting material and the first report of the biological activity of gabosine J.

Full text of DOI:10.1002/ejoc.201301782

FULL PAPER
DOI: 10.1002/ejoc.201301782
Total Synthesis and Glycosidase Inhibition Studies of (–)-Gabosine J and Its
Derivatives
Adiyala Vidyasagar[a] and Kana M. Sureshan*[a]
Keywords: Natural products / Total synthesis / Inhibitors / Cyclitols / Metathesis
The total syntheses of (–)-gabosine J and its two gabosinol
derivatives, the reduced forms of (–)-gabosine J, were
achieved by a chiral pool strategy that started from inexpen-
sive and readily available D-mannitol. The desymmetrization
of the C2-symmetric tri-O-isopropylidene mannitol through a
overall yield of 13%. Similarly, gabosinol J-α and gabosin-
ol J-β (two synthetic derivatives of gabosine J) were prepared
in 13 steps in an overall yield of 11 and 6%, respectively. A
preliminary investigation into the biological activity of these
compounds revealed that (–)-gabosine J inhibits α-mannosid-
H2SO4/silica mediated hydrolysis of one of the two terminal
ketals and a ring-closing methathesis (RCM) of the appropri- inhibits β-galactosidase with an IC50 of 600 μ
ately protected diene are the key steps in these syntheses.
The preparation of (–)-gabosine J, which involved simple
steps as well as inexpensive and readily available reagents
and starting material, was completed in 14 steps with an
ase with an IC50 of 260 μ
M
. Its reduced form gabosinol J-α
, and gabos-
inol J-β inhibits β-glucosidase. This is the first synthesis of a
M
gabosine that employs mannitol as the starting material and
the first report of the biological activity of gabosine J.
been proposed. No attempt has been made either to assign
its absolute configuration (e.g., specific rotation) or investi-
gate its biological activities.[13] In such a scenario, to assign
its absolute configuration, it is essential to prepare both
enantiomers in pure form and compare their chirooptical
properties with the natural product. Though many synthe-
ses of other gabosines have been reported, the first attempt
was made only recently to synthesize gabosine J.[14] On the
basis of the nonconcurrence of the spectroscopic data be-
tween a synthetically prepared enantiomer with the pro-
posed structure and the reported data of the natural prod-
uct, Figueredo et al.[14] revised the relative configuration of
gabosine J and synthesized its enantiomer, (+)-gabosine J.
No biological activity, however, has been reported. Herein,
we report the syntheses of the other enantiomer, (–)-gabos-
ine J, and two of its gabosinol derivatives by starting from
d-mannitol (1) along with the results of their glycosidase
inhibition studies.[15]
Introduction
Gabosines, natural products that belong to the family of
carbasugars, are secondary metabolites that are isolated
from Streptomyces sp. They have a polyhydroxylated cyclo-
hexenone or cyclohexanone core with a methyl or hy-
droxymethyl substituent as common structural features.
The variation of the number, position, and relative orienta-
tion of the hydroxy groups, the positions of the keto group
and methyl/hydroxymethyl substituents, the substitution
status of the hydroxy groups, and the absolute configura-
tion at each asymmetric carbon center allow gabosine to
have many theoretically possible isomers. Fifteen different
gabosines are known thus far to occur naturally (see Fig-
ure 1).[1] Gabosines exhibit a wide range of biological activi-
ties such as antibiotic,[2] antitumor,[3] and DNA binding
properties[4] as well as plant growth regulating effects,[5] gly-
cosidase inhibition,[6] and antiprotozoal activity.[1] Because
of the biological importance of these natural products,
many groups have developed methods to synthesize various
gabosines[7–11] and their derivatives[12] from carbo-
hydrates,[7] benzene derivatives,[8] (–)-quinic acid,[9] cyclo-
hexane-1,4-dione,[10] and l-tartaric acid.[11] Though gabos-
ine J was isolated in 1993, only its relative configuration has
Results and Discussion
Retrosynthetically, gabosine J can be obtained by a ring-
closing metathesis (RCM) of diene 13, which can be pre-
pared from the enal 11 (see Scheme 1). Several protection
and deprotection sequences can provide enal 11 from 6. Alk-
ene 6 can be derived from diol 3 through sequential selec-
tive protection, oxidation, and Wittig reactions, and diol 3
can be obtained from the selective hydrolysis of one of the
terminal ketals of tri-O-isopropylidene-mannitol, which can
be obtained from d-mannitol (1).
[a] School of Chemistry, Indian Institute of Science Education and
Research,
Thiruvananthapuram 695016, India
E-mail: kms@iisertvm.ac.in
http://www.iisertvm.ac.in/scientists/kana-m-sureshan/
personal-information.html
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201301782.
Eur. J. Org. Chem. 2014, 2349–2356
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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A. Vidyasagar, K. M. Sureshan
FULL PAPER
Figure 1. Gabosine family of secondary metabolites.
Scheme 1. Retrosynthetic analysis of (–)-gabosine J.
We started our synthesis with d-mannitol (1), an inex- single-crystal X-ray structure analysis of tetrol 7 proved its
pensive and readily available chiral pool compound (see structural identity (see Figure 2).
Scheme 2). Triketalization of 1 by using 2,2-dimethoxy-
Regioselective protection of the primary hydroxy group
propane in the presence of camphorsulfonic acid (CSA) of 7 with trityl chloride gave trityl ether 8, which upon
gave known triketal 2[16] in excellent yield. The selective de- exhaustive benzylation with benzyl bromide and sodium hy-
protection of one of the terminal isopropylidene groups by dride gave the fully protected compound 9. The chemoselec-
treatment with H2SO4/silica gave known diol 3[17] in 92% tive deprotection of the trityl ether by treatment with para-
yield. Diol 3 underwent a regioselective benzylation by toluenesulfonic acid (PTSA) in CHCl3/MeOH gave primary
using dibutyltinoxide[18] and benzyl bromide to give benzyl alcohol 10, which upon Dess–Martin periodinane oxi-
ether 4. A Swern oxidation[19] of alcohol 4 gave ketone 5 dation[20] gave aldehyde 11 in good (95%) yield. A Grignard
in excellent yield. Here, the prolonged reaction led to the reaction between aldehyde 11 and vinylmagnesium bromide
formation of an unwanted dimerized product, which was gave an inseparable mixture of allylic alcohols 12 (α/β, 65:35
formed in situ, as the result of a self-aldol reaction of by 1H NMR analysis) followed by a Dess–Martin oxidation
ketone 5. However, the required ketone 5 could be obtained to give enone 13. This strategy of chain elongation through
in good yield (94%) by the immediate workup of the reac- a Grignard reaction allows for the synthesis of higher
tion mixture. The Wittig reaction of ketone 5 with methyl- homologues [e.g., by Grignard reaction with allylmagne-
triphenylphosphonium bromide gave alkene 6 in good yield sium bromide (AllMgBr)] if required. The ring-closing me-
(92%). The deprotection of remaining isopropylidene tathesis of enone 13 using Grubbs catalyst (2nd generation,
groups by treatment with HCl in MeOH/CHCl3 gave tetrol G-II) gave cyclohexenone 14 in an unacceptable yield of
7 in excellent yield. Apart from the spectroscopic evidence, 27% along with unreacted starting material (50%) and sev-
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2014, 2349–2356

Total Synthesis and Inhibition Studies of (–)-Gabosines
Scheme 2. Total syntheses of (–)-gabosine J, gabosinol J-α and gabosinol J-β (DMF = N,N-dimethylformamide, DMSO = dimethyl sulfox-
ide, DCM = dichloromethane, THF = tetrahydrofuran, Tr = trityl, DIPEA = N,N-diisopropylethylamine, and DMP = Dess–Martin
periodinane, G-II = Grubbs’ 2nd generation catalyst).
eral byproducts (not isolated and characterized). The re-
To synthesize the two gabosinol derivatives, allylic
sults of several attempts to improve the yield were unsatis- alcohols 15 (27%) and 16 (51%) were separated chromato-
factory, and in all cases, approximately 50% of unreacted graphically. The stereochemistry of these diastereomers was
starting material 13 was recovered.
confirmed after acetylation to give 17 and 18, respectively.
The poor yield of cyclohexenone 14 in this key step The signal from the downshifted H-1 proton of 18 (as a
prompted us to take a detour for the synthesis of the target result of acetylation) had a coupling constant of 5.8 Hz,
compound. Thus, the RCM of the diastereomeric mixture which suggests an anti relationship between H-1 and H-2,
12 (α and β) with Grubbs II catalyst gave a mixture of all- whereas the signal from the downshifted H-1 proton of 17
ylic alcohols 15 and 16, which could be separated chro- (as a result of acetylation) had a smaller coupling constant,
matographically, in a combined yield of 78%. Gratifyingly, which suggests a syn relationship between H-1 and H-2.
the Dess–Martin oxidation of the mixture of diastereomers This stereochemical assignment was further confirmed by
15 and 16 gave enone 14 in 91% yield. Finally, the global NOESY and NOE experiments of compounds 17 and 18
debenzylation of 14 by treatment with BCl3 in DCM pro- as well as the spectral agreement of compound 15 with the
vided (–)-gabosine J in 71% yield. This corresponds to an sole product of a Luche reduction of ketone 14, which was
overall yield of 13% in 14 steps from d-mannitol.
expected to give an alcohol as a result of a hydride delivery
Eur. J. Org. Chem. 2014, 2349–2356
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A. Vidyasagar, K. M. Sureshan
FULL PAPER
Conclusions
In summary, we achieved the total synthesis of (–)-gabos-
ine J in 14 steps as well as the syntheses of its two reduced
forms by starting from inexpensive d-mannitol and utilizing
its C2 symmetry. Though many members of the gabosine
family have been synthesized from different chiral pools,
this is the first report of the preparation of a gabosine from
d-mannitol. Gabosine J and its derivatives were found to
have good to moderate glycosidase inhibition activities, and
this is the first report of their biological activity. The prelim-
inary data of this investigation reveals the possibility to de-
velop various analogues and study their structure–activity
relationships as well as to develop analogues that have im-
proved biological activities. In this context, it is pertinent
that our synthetic approach is amenable to the preparation
of various derivatives, which include ring-expanded ana-
logues.
Experimental Section
Figure 2. ORTEP diagram of compound 7.
General Methods: Chemicals and enzymes were purchased from
Sigma Aldrich. The solvents were purchased from Merck (LR
from the less hindered side of enone 14. Allylic alcohols 15
and 16 were individually subjected to debenzylation with grade) and were used after distillation. Thin layer chromatography
was carried out using precoated silica gel 60 F254 (Merck) plates.
Chromatograms were visualized under UV light and by dipping the
plates into a solution of either phosphomolybdic acid in MeOH or
anisaldehyde in ethanol followed by heating with a hot air gun. IR
spectra were recorded with an IR prestige-21 (Shimadzu) spectrom-
eter by using pellets with KBr (for solids) or a CHCl3 solution in a
NaCl cell. The 1H and 13C NMR, COSY, DEPT, and heteronuclear
multiple quantum coherence (HMQC) spectra were recorded with
an Avance III-500 (Bruker) NMR spectrometer. The chemical
shifts of the proton signals are reported in ppm (δ) relative to in-
BBr3 in DCM to give the two reduced forms of (–)-gabos-
ine J, gabosinol J-β and gabosinol J-α, respectively.
As no biological properties of gabosine J have been re-
ported to date, we carried out a preliminary investigation
of the biological activities of (–)-gabosine J and the two
hitherto unknown gabosinols that were synthesized. As ga-
bosines are a class of carbasugars, we investigated their abil-
ity to inhibit various glycosidase enzymes (see Table 1). To
our satisfaction, (–)-gabosine J inhibited α-mannosidase
(from jack beans) with an IC50 value of 260 μm. This ac- ternal tetramethylsilane (TMS, δ = 0.0 ppm) or with the solvent
1
referenced relative to TMS (CHCl3, δ = 7.26 ppm). The H NMR
spectroscopic data are reported as chemical shift {multiplicity [sin-
glet (s), doublet (d), triplet (t), quartet (q), and multiplet (m)], cou-
pling constants (Hz), and integration}. All NMR signals were as-
tivity is better than the standard mannosidase inhibitor, de-
oxymannojirimycin (DMJ), which is reported to have an
IC50 value of 840 μm.[21] This remarkable activity is not sur-
prising as both DMJ and (–)-gabosine J are isostructural to
d-mannose. The synthetic analogue gabosinol J-α inhibited
β-galactosidase with an IC50 value of 600 μm, and gabos-
inol J-β was a weak inhibitor of β-glucosidase.
signed on the basis of H and 13C NMR, COSY and HMQC ex-
1
periments. The 13C NMR spectra were recorded with complete pro-
ton decoupling. The chemical shifts of the carbon signals are re-
ported in ppm (δ) relative to TMS with the respective solvent reso-
nance as the internal standard. All NMR spectroscopic data were
collected at 25 °C. Melting points were measured with a Stuart
SMP30 melting point apparatus. Elemental analyses were carried
out with an Elementar Vario MICRO cube elemental analyzer.
Flash column chromatography was performed using silica gel (200–
400 mesh). The X-ray intensity data measurements of freshly grown
crystals of 7 were carried out at 296 K with a Bruker-KAPPA
APEX II CCD diffractometer with graphite-monochromatized
(λMo-K = 0.71073 Å) radiation. The X-ray generator was operated
at 50 kV and 30 mA. Data were collected with a scan width of 0.3°
at different settings of φ (0°, 90° and 180°) and by keeping the
sample to detector distance fixed at 40 mm and the detector posi-
tion (2θ) fixed at 24°. The X-ray data collection was monitored by
the SMART program (Bruker, 2003). All the data were corrected
for Lorentzian polarization and absorption effects using the
SAINT and SADABS programs (Bruker, 2003). SHELX-97 was
used for structure solution and full-matrix least-squares refinement
on F2. The molecular diagram was generated using ORTEP-3.
Table 1. Enzyme inhibition studies.
Enzyme
(–)-Gabosine J Gabosinol J-α Gabosinol J-β
α-Glucosidase
(Saccharomyces
cerevisae)
β-Glucosidase
(almonds)
n.i.[a]
n.i.
n.i.
n.i.
260 μm
n.i.
n.i.
n.i.
w.i.[a] (42%)
α-Mannosidase
(jack bean)
n.i.
n.i.
n.i.
n.i.
β-Mannosidase
(Helix pomatia)
α-Galactosidase
(green coffee beans)
β-Galactosidase
(bovine liver)
n.i.
n.i.
n.i.
n.i.
600 μm
[a] n.i.: no inhibition up to 2 mm concentration, w.i.: weak inhibi-
tion.
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Total Synthesis and Inhibition Studies of (–)-Gabosines
Geometrical calculations were performed using SHELXTL
(Bruker, 2003) and PLATON.
Na2SO4 and concentrated under reduced pressure. Purification of
the residue by column chromatography (AcOEt/petroleum ether,
1:5) yielded 6 (8.2 g, 92%) as an oily syrup; [α]2D5 = –2.32 (c = 0.8,
(R)-2-(Benzyloxy)-1-{(4R,4ЈR,5R)-2,2,2Ј,2Ј-tetramethyl-[4,4Ј-bi-
(1,3-dioxolan)]-5-yl}ethanol (4): To a solution of 3[15] (26 g,
99.2 mmol) in dry toluene was added Bu2SnO (29.5 g, 119 mmol),
and the resulting mixture was heated at reflux by using a Dean–
Stark apparatus for 16 h, at which time the reaction became clear.
To this was added BnBr (10.7 mL, 109 mmol), and the resulting
mixture was stirred for an additional 12 h. Upon completion, the
reaction was quenched by the addition of triethylamine. The mix-
ture was then heated at reflux for an additional 2 h and then passed
through filter paper. The solvents were evaporated under reduced
pressure, and the residue was diluted with ethyl acetate. This re-
sulting solution was washed with water, and the organic layer was
dried with Na2SO4 and concentrated under reduced pressure. Puri-
fication of the residue by column chromatography (AcOEt/petro-
leum ether, 1:3) yielded 4 (25.1 g, 72%) as a gum; [α]2D5 = +12.7 (c
CHCl ). IR: ν = 2930, 2856, 1640, 1410, 1390, 1245, 1215 cm–1. 1H
˜
3
NMR (500 MHz, CDCl3): δ = 7.37–7.28 (m, 5 H, Ar-H), 5.43 (s,
1 H, H7a), 5.37 (d, J = 1.1 Hz, 1 H, H7b), 4.55 (q, J = 12, 20 Hz,
2 H, CH2Ph), 4.45 (d, J = 7.6 Hz, 1 H, H3), 4.20 (d, J = 13 Hz, 1
H, H1a), 4.16 (q, J = 6.2, 12.4 Hz, 1 H, H5), 4.11–4.08 (m, 2 H,
H1b and H6a), 3.96 (q, J = 7.9, 14.2 Hz, 2 H, H6b and H4), 1.43 (s,
3 H, CH3), 1.42 (s, 3 H, CH3), 1.39 (s, 3 H, CH3), 1.34 (s, 3 H,
CH3) ppm. 13C NMR (125 MHz, CDCl3): δ = 142.7, 138.2, 128.3,
127.7, 127.6, 115.6, 109.6, 109.2, 80.5, 80.3, 76.5, 72.2, 69.8, 66.8,
27.0, 26.9, 26.4, 25.3 ppm. C20H28O5 (348.44): calcd. C 68.94, H
8.10; found C 69.16, H 8.08.
(2R,3S,4R)-5-[(Benzyloxy)methyl]hex-5-ene-1,2,3,4-tetraol (7): To a
solution of 6 (7.9 g, 22.7 mmol) in a mixture of MeOH/CHCl3 (1:1
v/v, 60 mL) was added HCl (2 m solution, 10 mL) at room temp.
The reaction mixture was stirred at room temp. for 6 h. Upon com-
pletion, the reaction was quenched by the addition of triethylamine.
= 0.7, CHCl ). IR: ν = 3420, 3145, 2943, 1432, 1120, 1045 cm–1.
˜
3
1H NMR (500 MHz, CDCl3): δ = 7.37–7.28 (m, 5 H, Ar-H), 4.63
(q, J = 12.2, 16.5 Hz, 2 H, CH2Ph), 4.18 (dd, J = 6.3, 8.4 Hz, 1 H, The solvents were evaporated under reduced pressure, and the resi-
H1a), 4.13–4.11 (m, 1 H, H2), 4.02 (dd, J = 6.3, 8.4 Hz, 1 H, H1b),
3.92 (d, J = 7.4 Hz, 1 H, H4), 3.86 (t, J = 7.7 Hz, 2 H, H3 and H5),
3.75 (dd, J = 2.5, 9.9 Hz, 1 H, H6a), 3.61 (dd, J = 6.2, 10 Hz, 1 H,
due was purified by column chromatography (AcOEt) to yield 7
(5.3 g, 88%) as a white solid; m.p. 80–82 °C. [α]2D5 = –1.7 (c = 0.6,
CH OH). IR: ν = 3377, 3290, 2939, 2868, 1429, 1406, 1311, 1255,
˜
3
H
6b), 3.44 (br. s, 1 H, -OH), 1.45 (s, 3 H, CH3), 1.38 (s, 6 H, CH3), 1207, 1074, 1037 cm–1. 1H NMR (500 MHz, CD3OD): δ = 7.37–
1.37 (s, 3 H, CH3) ppm. 13C NMR (125 MHz, CDCl3): δ = 138.2, 7.27 (m, 5 H, Ar-H), 5.35 (s, 1 H, H7a), 5.28 (s, 1 H, H7b), 4.58–
128.3, 127.7, 127.5, 110.0, 109.5, 80.6, 79.9, 76.4, 73.5, 71.8, 71.4,
67.5, 27.0, 26.9, 26.4, 25.1 ppm. C19H28O6 (352.43): calcd. C 64.75,
H 8.01; found C 64.46, H 7.76.
4.51 (m, 3 H, CH2Ph and H3), 4.18 (d, J = 12.6 Hz, 1 H, H1a),
4.08 (d, J = 12.6 Hz, 1 H, H1b), 3.80 (dd, J = 3.4, 11.1 Hz, 1 H,
H6a), 3.75–3.72 (m, 1 H, H4), 3.65–3.63 (m, 2 H, H6b and H5) ppm.
13C NMR (125 MHz, CD3OD): δ = 146.4, 138.1, 128.0, 127.6,
127.3, 113.1, 72.4, 71.8, 71.6, 70.8, 70.6, 63.7 ppm. C14H20O5
(268.31): calcd. C 62.67, H 7.51; found C 62.55, H 7.29.
2-(Benzyloxy)-1-{(4R,4ЈR,5S)-2,2,2Ј,2Ј-tetramethyl-[4,4Ј-bi(1,3-di-
oxolan)]-5-yl}ethanone (5): To a solution of oxalyl chloride (7.6 mL,
85.2 mmol) and DMSO (6.03 mL, 85.2 mmol) in dry DCM
(200 mL) at –78 °C was added a solution of 4 (20 g, 56.8 mmol) in
dry DCM. The reaction was stirred at the same temperature for
2.5 h. Then, Et3N (40 mL, 284 mmol) was added, and the mixture
was stirred for an additional 1 h. The solvents were evaporated un-
der reduced pressure, and the residue was diluted with ethyl acetate.
The solution was washed successively with saturated NaHCO3
solution, water, and then brine. The organic layer was dried with
Na2SO4 and concentrated under reduced pressure. Purification of
the residue by column chromatography (AcOEt/petroleum ether,
Crystal Data of Compound 7: C14H20O5, M = 268.3, colorless
block, 0.2ϫ0.15ϫ0.1 mm3, orthorhombic, space group P2(1)2(1)
2(1), a = 6.076(5) Å, b = 9.725(5) Å, c = 23.654(5) Å, V =
1397.7(14) Å3, Z = 4, T = 296(2) K, 2θmax = 56.3°, Dcalcd.
=
1.275 gcm–3, F(000) = 576.0, μ = 0.096 mm–1, 5984 reflections col-
lected, 2461 unique reflections (Rint = 0.0566), multiscan absorp-
tion correction, Tmin = 0.9810, Tmax = 0.9904, number of param-
eters = 218, number of restraints = 92, GoF = 1.039, R1 = 0.0504,
wR2 = 0.1342, R indices based on 2196 reflections with I Ͼ 2s(I)
(refinement on F2). Δρmax = 0.007, Δρmin = 0.001 eÅ–3. CCDC-
959760 (for 7) contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
1:4) yielded 5 (18.6 g, 94%) as a syrupy liquid; [α]2D5 = +1.06 (c =
1
1.1, CHCl ). IR: ν = 3001, 1709, 1410, 1322, 1208 cm–1. H NMR
˜
3
(500 MHz, CDCl3): δ = 7.29–7.19 (m, 5 H, Ar-H), 4.54 (s, 2 H,
CH2Ph), 4.41–4.32 (m, 3 H, H1a, H1b, and H3), 4.13 (dd, J = 6.6,
12.7 Hz, 1 H, H4), 4.10 (dd, J = 6.6, 11.7 Hz, 1 H, H5), 4.02 (dd, data_request/cif.
J = 6.2, 8.7 Hz, 1 H, H6a), 3.88 (dd, J = 4.8, 8.7 Hz, 1 H, H6b),
(2R,3S,4R)-5-[(Benzyloxy)methyl]-1-(trityloxy)hex-5-ene-2,3,4-
1.36 (s, 3 H, CH3), 1.32 (s, 3 H, CH3), 1.26 (s, 3 H, CH3), 1.25 (s,
3 H, CH3) ppm. 13C NMR (125 MHz, CDCl3): δ = 205.4, 137.1,
128.5, 128.0, 111.5, 109.9, 80.6, 78.2, 76.3, 73.3, 72.8, 66.6, 26.9,
26.4, 26.1, 25.1 ppm. C19H26O6 (350.41): calcd. C 65.13, H 7.48;
found C 65.44, H 7.19.
triol (8): To a suspension of 7 (1.5 g, 5.6 mmol) in dry DCM
(25 mL) under nitrogen were added DIPEA (1.7 mL, 10.0 mmol),
tetra-n-butylammonium iodide (TBAI, 2 mg), and trityl chloride
(1.71 g, 6.15 mmol) at 0 °C. The resulting mixture was stirred at
room temp. for 3 h. Upon completion of the reaction, the solvent
was evaporated under reduced pressure. The residue was purified
by column chromatography (AcOEt/petroleum ether, 1:1) to yield
8 (2.5 g, 90%) as an oily syrup; [α]2D5 = +4.6 (c = 0.2, CHCl3). IR:
(4S,4ЈR,5R)-5-[3-(Benzyloxy)prop-1-en-2-yl]-2,2,2Ј,2Ј-tetramethyl-
4,4Ј-bi(1,3-dioxolane) (6): To a suspension of methyltriphenylphos-
phonium bromide (11.8 g, 37.7 mmol) in dry THF (100 mL) at
–78 °C was added n-butyllithium (1.6 m solution, 25.7 mL,
37.7 mmol) under nitrogen. To the resulting orange colored solu-
tion was added a solution of 5 (9.0 g, 25.71 mmol) in dry THF
(60 mL). The mixture was stirred at the same temperature for 2 h.
Upon completion, the reaction was quenched by the addition of
saturated NH4Cl solution. The THF was then removed by evapora-
tion, and the residue was diluted with ethyl acetate. The solution
was washed with water and brine. The organic layer was dried with
ν = 3379, 3284, 2939, 2860, 1490, 1446, 1327, 1257, 1207, 1155,
˜
1076, 1033, 1014 cm–1. 1H NMR (500 MHz, CDCl3): δ = 7.23–7.16
(m, 20 H, Ar-H), 5.16 (d, J = 12 Hz, 2 H, H7a and H7b), 4.43 (ABq,
J = 11.7, 15.4 Hz, 2 H, CH2Ph), 4.34 (d, J = 5.4 Hz, 1 H, H3), 4.07
(d, J = 11.4 Hz, 1 H, H1a), 3.92 (d, J = 11.4 Hz, 1 H, H1b), 3.84–
3.82 (m, 1 H, H4 or H5), 3.63–3.60 (m, 1 H, H5 or H4), 3.32 (dd,
J = 4.3, 9.6 Hz, 1 H, H6a), 3.25 (dd, J = 6.1, 9.6 Hz, 1 H, H6b),
3.24 (d, J = 5.4 Hz, 1 H, -OH), 2.92 (d, J = 5.9 Hz, 1 H, -OH),
Eur. J. Org. Chem. 2014, 2349–2356
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
2353

A. Vidyasagar, K. M. Sureshan
FULL PAPER
2.61 (d, J = 5.2 Hz, 1 H, -OH) ppm. 13C NMR (125 MHz, CDCl3):
δ = 144.9, 143.7, 137.3, 128.6, 128.5, 128.0, 127.97, 128.92, 127.27,
127.21, 116.9, 87.0, 73.2, 72.7, 72.5, 71.3, 71.1, 64.9 ppm. C33H34O5
(510.63): calcd. C 77.62, H 6.71; found C 77.43, H 6.42.
(d, J = 11.6 Hz, 1 H), 4.54–4.51 (m, 3 H), 4.38 (d, J = 11.6 Hz, 1
H), 4.35 (d, J = 11.7 Hz, 1 H), 4.29–4.26 (m, 1 H), 4.04–4.02 (m,
4 H) ppm. 13C NMR (125 MHz, CDCl3): δ = 201.7, 142.2, 138.2,
138.0, 137.4, 128.4, 128.3, 128.2, 128.1, 127.9, 127.7, 117.8, 83.7,
81.9, 81.2, 74.6, 72.8, 72.6, 71.2, 70.3 ppm. C35H36O5 (536.67):
calcd. C 78.33, H 6.76; found C 78.54, H 6.75.
(2R,3S,4R)-2,3,4-Tribenzyloxy-5-benzyloxymethyl-1-(trityloxy)-
hex-5-ene (9): To a solution of 8 (2.2 g, 4.318 mmol) in dry DMF
at 0 °C were added NaH (60% suspension in mineral oil, 700 mg,
17.27 mmol) and benzyl bromide (2.04 mL, 17.27 mmol). The reac-
tion mixture was stirred at the same temperature for 1 h. Upon
completion of the reaction, the DMF was evaporated under re-
duced pressure, and the resulting residue was diluted with ethyl
acetate. This solution was washed with water, and the organic layer
was dried with Na2SO4 and concentrated under reduced pressure.
The crude product was purified by column chromatography (Ac-
OEt/petroleum ether, 1:19) to yield 9 (3.2 g, 96%) as an oily syrup;
(4S,5S,6R)-4,5,6-Tris(benzyloxy)-7-[(benzyloxy)methyl]octa-1,7-
dien-3-one (13): To a solution of 11 (1.9 g, 3.54 mmol) in dry THF
at –78 °C was added vinylmagnesium bromide (1 m solution in
THF, 9.5 mL, 9.34 mmol). Upon completion, the reaction was
quenched by the addition of saturated ammonium chloride solu-
tion. The THF was removed by evaporation, and the residue was
diluted with ethyl acetate. The resulting solution was washed with
water and brine. The organic layer was dried with sodium sulfate
and concentrated. The crude material was purified by column
chromatography (AcOEt/petroleum ether, 1:9) to yield 12 (1.64 g,
[α]2D5 = –12.0 (c = 0.2, CHCl ). IR: ν = 2926, 2858, 1726, 1672,
˜
3
1492, 1386, 1070 cm–1. 1H NMR (500 MHz, CDCl3): δ = 7.35–7.32 82 %) as a yellowish syrup that consisted of two diastereomers
(m, 6 H, Ar-H), 7.29–7.22 (m, 27 H, Ar-H), 7.06–7.05 (m, 2 H, Ar- (65:35). To a solution of this diastereomeric mixture (300 mg,
H), 5.42 (s, 1 H, H7a), 5.31 (s, 1 H, H7b), 4.78 (d, J = 11.4 Hz, 1
0.516 mmol) in dry DCM (8 mL) at room temp. was added Dess–
H), 4.59–4.52 (m, 5 H), 4.39 (d, J = 11.4 Hz, 1 H), 4.29–4.27 (m, Martin periodinane (331 mg, 0.774 mmol), and the mixture was
2 H), 4.10 (d, J = 12.8 Hz, 1 H), 4.04 (d, J = 12.8 Hz, 1 H), 3.95–
3.92 (m, 2 H), 3.54 (d, J = 10 Hz, 1 H), 3.39–3.36 (m, 1 H) ppm.
stirred for 3 h. Upon completion, the reaction was quenched by the
addition of saturated sodium thiosulfate solution. The mixture was
13C NMR (125 MHz, CDCl3): δ = 144.1, 142.9, 138.9, 138.4, then stirred for an additional 1 h. The organic layer was washed
138.39, 138.34, 138.33, 128.9, 128.4, 128.3, 128.1, 128.0, 127.9,
127.7, 127.6, 127.5, 127.35, 127.32, 126.9, 115.7, 86.6, 80.4, 80.3,
78.8, 74.6, 72.7, 72.4, 70.9, 70.6, 63.4 ppm. C54H52O5 (781.00):
calcd. C 83.05, H 6.71; found C 82.83, H 6.62.
successively with saturated NaHCO3 solution, water, and brine.
The organic layer was dried with Na2SO4 and concentrated under
reduced pressure. The crude material was purified by column
chromatography (AcOEt/petroleum ether, 1:9) to yield 13 (252 mg,
87%) as a yellowish oil; [α]2D5 = –9.0 (c = 0.2, CHCl ). IR: ν = 2972,
˜
3
(2R,3S,4R)-2,3,4-Tris(benzyloxy)-5-(benzyloxymethyl)hex-5-en-1-
ol (10): To a solution of 9 (3.1 g, 3.979 mmol) in a mixture of
MeOH/CHCl3 (1:1 v/v, 30 mL) was added PTSA (100 mg) at room
temp. The reaction mixture was stirred at room temp. for 3 h. Upon
completion of the reaction, the PTSA was neutralized by the ad-
dition of triethylamine. The solvents were evaporated under re-
duced pressure, and the crude material was purified by column
chromatography (AcOEt/petroleum ether, 1:5.6) to yield 10 (1.77 g,
2865, 1690, 1448, 1087 cm–1. 1H NMR (500 MHz, CDCl3): δ =
7.35–7.21 (m, 20 H, Ar-H), 6.81 (dd, J = 10.5, 17.4 Hz, 1 H, vinylic
CH2a), 6.33 (dd, J = 1.4, 17.4 Hz, 1 H, vinylic CH2b), 5.67 (dd, J
= 1.4, 10.5 Hz, 1 H, vinylic CH), 5.50 (s, 2 H, H7a and H7b), 4.66–
4.55 (m, 2 H), 4.53–4.52 (m, 3 H), 4.37–4.31 (m, 2 H), 4.29–4.23
(m, 3 H) ppm. 13C NMR (125 MHz, CDCl3): δ = 200.2, 142.3,
138.28, 138.27, 138.0, 137.4, 133.1, 128.5, 128.4, 128.33, 128.29,
128.26, 128.16, 128.1, 127.9, 127.8, 127.6, 127.58, 127.52, 117.2,
82.7, 81.6, 81.0, 74.9, 72.7, 72.2, 71.1, 70.4 ppm. C37H38O5
(562.70): calcd. C 78.98, H 6.81; found C 78.77, H 6.53.
83%) as a colorless oil; [α]2D5 = –8.2 (c = 0.6, CHCl ). IR: ν = 3338,
˜
3
2846, 1722, 1680, 1477, 1035 cm–1. 1H NMR (500 MHz, CDCl3):
δ = 7.31–7.22 (m, 20 H, Ar-H), 5.48 (s, 1 H, H7a), 5.42 (s, 1 H,
H7b), 4.72 (ABq, J = 11.1, 14.6 Hz, 2 H), 4.62 (d, J = 11.6 Hz, 1 (4R,5S,6S)-4,5,6-Tris(benzyloxy)-3-[(benzyloxy)methyl]cyclohex-
H), 4.54 (ABq, J = 12, 21.7 Hz, 2 H), 4.41 (d, J = 11.4 Hz, 1 H), 2-enone (14): To a solution of 13 (200 mg, 0.345 mmol) in DCM
4.30 (d, J = 11.6 Hz, 1 H), 4.29 (d, J = 11.4 Hz, 1 H), 4.20 (d, J = (10 mL) at room temp. was added Grubbs II catalyst (78 mg,
5.0 Hz, 1 H), 4.06 (ABq, J = 12.7, 27.6 Hz, 2 H), 3.90 (t, J =
5.3 Hz, 1 H), 3.81 (d, J = 4 Hz, 2 H), 3.67 (q, J = 5.2, 9.4 Hz, 1
H) ppm. 13C NMR (125 MHz, CDCl3): δ = 142.7, 138.2, 138.1,
0.086 mmol). The resulting solution was heated at reflux for 24 h.
The solvent was evaporated, and the residue was purified by col-
umn chromatography (AcOEt/petroleum ether, 1:6) to yield 14
128.4, 128.39, 128.34, 128.32, 128.28, 128.21, 127.6, 116.4, 81.1, (51 mg, 27 %) as a syrup as well as recovered starting material
80.4, 79.1, 75.2, 72.8, 71.4, 70.8, 70.4, 60.8 ppm. C35H38O5
(538.68): calcd. C 78.04, H 7.11; found C 78.29, H 7.03.
(98 mg). Data for 14: [α]2D5 = –47.8 (c = 0.3, CHCl ). IR: ν = 2965,
˜
3
2844, 1687, 1467, 1054 cm–1. 1H NMR (500 MHz, CDCl3): δ =
7.41–7.14 (m, 20 H, Ar-H), 6.15 (s, 1 H, H6), 4.94 (d, J = 11.8 Hz,
1 H, CHPh), 4.75 (d, J = 12.2 Hz, 1 H, CHPh), 4.61–4.56 (m, 3
H, CH2Ph), 4.52 (d, J = 11.8 Hz, 1 H, CHPh), 4.47–4.44 (m, 2 H,
CH2Ph), 4.39 (br. s, 1 H, H2), 4.22 (br. s, 1 H, H4), 4.11 (s, 2 H,
H7a and H7b), 4.07–4.06 (m, 1 H, H3) ppm. 13C NMR (125 MHz,
CDCl3): δ = 195.7, 138.0, 137.8, 137.5, 137.3, 128.5, 128.45, 128.41,
128.2, 128.0, 127.9, 127.8, 127.7, 124.9, 78.9, 74.8, 74.2, 72.9, 72.7,
72.6, 69.4 ppm. C35H34O5 (534.65): calcd. C 78.63, H 6.41; found
C 78.56, H 6.32.
(2S,3S,4R)-2,3,4-Tris(benzyloxy)-5-[(benzyloxy)methyl]hex-5-enal
(11): To a solution of 10 (1.0 g, 1.80 mmol) in dry DCM (35 mL)
at room temp. was added Dess–Martin periodinane (0.925 g,
2.163 mmol), and the mixture was stirred for 2 h. Upon comple-
tion, the reaction was quenched by the addition of saturated so-
dium thiosulfate solution. The mixture was then stirred for an ad-
ditional 1 h. The organic layer was washed successively with
NaHCO3, water, and then brine. The organic layer was dried with
Na2SO4 and concentrated under reduced pressure. The crude mate-
rial was purified by column chromatography (AcOEt/petroleum Ring-Closing Metathesis of Compound 12: To a solution of 12 (mix-
ether, 1:10) to yield 11 (0.916 g, 95%) as a syrup; [α]2D5 = –7.2 (c =
ture of two isomers, 1.7 g, 3.041 mmol) in DCM (70 mL) was
added Grubbs second generation catalyst (135 mg, 0.15 mmol) at
room temp., and the mixture was heated at reflux for 4 h. When
0.6, CHCl ). IR: ν = 3599, 2926, 2873, 1691, 1496, 1454, 1097,
˜
3
1074, 1018 cm–1. 1H NMR (500 MHz, CDCl3): δ = 9.65 (s, 1 H,
CHO), 7.34–7.25 (m, 20 H, Ar-H), 5.50 (s, 1 H, H7a), 5.45 (s, 1 H, the starting material was completely consumed, the solvent was
H7b), 4.72 (d, J = 11.4 Hz, 1 H), 4.67 (d, J = 11.4 Hz, 1 H), 4.59 evaporated under reduced pressure. The residue was purified by
2354
www.eurjoc.org
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2014, 2349–2356

Total Synthesis and Inhibition Studies of (–)-Gabosines
column chromatography (AcOEt/petroleum ether, 1:4) to yield 15
(0.441 g, 27%; Rf = 0.45) and 16 (0.815 g, 51%; Rf = 0.3).
ture for 1 h. The reaction was quenched by the addition of MeOH
(1 mL) and pyridine (2 mL). The solvents were evaporated under
reduced pressure, and the residue was diluted with water. The re-
sulting mixture was successively passed through a hydrogen carb-
onate resin (prepared by passing NaHCO3 through a Cl– resin)
and then a H+ ion resin. The water was evaporated under reduced
pressure, and the residue was purified by column chromatography
(AcOEt/2-propanol, 9:1) to yield gabosinol J-α (34 mg, 72%) as a
gum; [α]2D5 = +1.9 (c = 0.2, CH3OH). 1H NMR (500 MHz,
CD3OD): δ = 5.61 (t, J = 1.1 Hz, 1 H, H6), 4.09 (d, J = 4.9 Hz, 1
H, H1), 4.04 (s, 2 H, H7a and H7b), 4.01 (d, J = 4.2 Hz, 1 H, H4),
3.77 (dd, J = 2.4, 4.2 Hz, 1 H, H3), 3.67 (dd, J = 2.3, 6.5 Hz, 1 H,
H2) ppm. 13C NMR (125 MHz, CD3OD): δ = 140.1, 126.4, 75.0,
73.7, 70.5, 70.3, 63.7 ppm. C7H12O5 (176.17): calcd. C 47.72, H
6.87; found C 47.44, H 7.01.
Data for (1R,4R,5S,6R)-4,5,6-Tris(benzyloxy)-3-[(benzyloxy)meth-
yl]cyclohex-2-enol (15): M.p. 58–60 °C; [α]2D5 = –55.1 (c = 0.6,
CHCl ). IR: ν = 3342, 2962, 2883, 1492, 1434, 1309, 1029 cm–1. 1H
˜
3
NMR (500 MHz, CDCl3): δ = 7.23–7.17 (m, 18 H, Ar-H), 7.13–
7.11 (m, 2 H, Ar-H), 5.84 (d, J = 3.4 Hz, 1 H, H6), 4.68 (s, 2 H,
CH2Ph), 4.64 (d, J = 11.8 Hz, 1 H, CHPh), 4.55 (d, J = 11.8 Hz,
1 H, CHPh), 4.50 (d, J = 11.2 Hz, 1 H, CHPh), 4.42 (d, J = 12 Hz,
2 H, CH2Ph), 4.29 (d, J = 11.8 Hz, 1 H, CHPh), 4.15 (t, J = 5.5 Hz,
2 H, H1 and H4), 4.11 (d, J = 12 Hz, 1 H, H7a), 3.88–3.87 (m, 1
H, H2), 3.84–3.79 (m, 2 H, H3 and H7b) ppm. 13C NMR (125 MHz,
CDCl3): δ = 138.4, 138.2, 138.1, 137.8, 134.9, 128.8, 128.5, 128.44,
128.42, 128.3, 128.0, 127.9, 127.8, 127.7, 127.5, 79.2, 75.2, 73.8,
73.0, 71.9, 71.7, 70.3, 65.6 ppm. C35H36O5 (536.67): calcd. C 78.33,
H 6.76; found C 78.19, H 6.54.
Gabosinol J-β [(1R,2R,3S,4R)-5-(Hydroxymethyl)cyclohex-5-ene-
1,2,3,4-tetraol]: To a solution of 15 (0.150 g, 0.279 mmol) in dry
DCM (5 mL) at –40 °C was added BBr3 (1 m solution, 2 mL,
1.98 mmol) dropwise, and the solution was stirred at that tempera-
ture for 1 h. The reaction was quenched by the addition of MeOH
(1 mL) and pyridine (2 mL). The solvents were evaporated under
reduced pressure, and the residue was diluted with water. The re-
sulting mixture was successively passed through a hydrogen carb-
onate resin (prepared by passing NaHCO3 through a Cl– resin)
and then a H+ ion resin. The water was evaporated under reduced
pressure, and the residue was purified by column chromatography
(AcOEt/2-propanol, 9:1) to yield gabosinol J-β (32.8 mg, 69%) as
a hygroscopic liquid. An analytically pure sample, however, could
not be obtained even after several chromatography purifications;
Data for (1S,4R,5S,6R)-4,5,6-Tris(benzyloxy)-3-[(benzyloxy)methyl-
]cyclohex-2-enol (16): [α]2D5 = +11.3 (c = 0.2, CHCl ). IR: ν = 3340,
˜
3
2972, 2865, 1481, 1448, 1324, 1013 cm–1
.
1H NMR (500 MHz,
CDCl3): δ = 7.21–7.17 (m, 18 H, Ar-H), 7.12–7.10 (m, 2 H, Ar-H),
5.76 (s, 1 H, H6), 4.55 (d, J = 12.3 Hz, 1 H, CHPh), 4.49–4.46 (m,
4 H, H1 and CH2Ph), 4.38 (d, J = 7.2 Hz, 1 H, CHPh), 4.35 (s, 2
H, CH2Ph), 4.27 (d, J = 11.8 Hz, 1 H, CHPh), 4.06 (d, J = 12.3 Hz,
1 H, H7a), 3.95 (d, J = 3.0 Hz, 1 H, H4), 3.81–3.79 (m, 2 H, H3
and H7b), 3.61 (dd, J = 2.0, 7.9 Hz, 1 H, H2) ppm. 13C NMR
(125 MHz, CDCl3): δ = 138.4, 138.3, 138.29, 138.21, 134.3, 129.3,
128.5, 128.4, 128.38, 128.34, 128.0, 127.9, 127.86, 127.83, 127.8,
127.79, 127.7, 127.6, 80.4, 74.8, 74.0, 73.4, 72.5, 72.0, 71.7, 70.5,
68.1 ppm. C35H36O5 (536.67): calcd. C 78.33, H 6.76; found C
78.17, H 6.86.
[α]2D5 = –42 (c = 0.2, CH3OH). H NMR (500 MHz, CD3OD): δ =
1
5.54 (t, J = 1.3 Hz, 1 H, H6), 4.16 (d, J = 6.7 Hz, 1 H, H4), 4.14–
4.12 (m, 1 H, H1), 4.05 (br. s, 2 H, H7a and H7b), 3.91–3.88 (m, 1 H,
H2), 3.55 (dd, J = 2.0, 6.8 Hz, 1 H, H3) ppm. 13C NMR (125 MHz,
CD3OD): δ = 140.6, 125.2, 76.3, 73.0, 71.1, 69.0, 63.2 ppm. HRMS:
calcd. for C7H12O5 [M + Na] 199.0582; found 199.0192.
Alternate Procedure for the Synthesis of 14: To a solution of the
mixture of 15 and 16 (0.315 g, 0.587 mmol) in DCM (10 mL) was
added Dess–Martin periodinane (0.376 g, 0.881 mmol), and the
mixture was stirred at room temp. for 2 h. Upon completion, the
reaction was quenched by the addition of saturated sodium thio-
sulfate solution. The mixture was then stirred for an additional 1 h.
The organic layer was washed successively with NaHCO3, water,
and brine, dried with Na2SO4, and concentrated under reduced
pressure. The crude material was purified by column chromatog-
raphy (AcOEt/petroleum ether, 1:6) to give 14 (0.285 g, 91%) as a
syrup.
Acetylation of 15: To a solution of compound 15 (15 mg,
0.027 mmol) in pyridine at 0 °C was added Ac2O (30 μL). The reac-
tion mixture was stirred at the same temperature for 30 min. The
pyridine was then evaporated, and the residue was diluted with
ethyl acetate. The resulting solution was successively washed with
dilute HCl, saturated sodium hydrogen carbonate, water, and brine.
The organic layer was dried with sodium sulfate and concentrated
under reduced pressure. The residue was purified by column
chromatography (AcOEt/petroleum ether, 1:9) to yield the acetate
(–)-Gabosine J: To a solution of 14 (0.329 g, 0.6 mmol) in dry DCM
(15 mL) at –40 °C was added BCl3 (1 m solution, 3 mL, 2.99 mmol)
dropwise, and the solution was stirred at that temperature for 1 h.
The reaction was quenched by the addition of MeOH (1 mL) and
pyridine (2 mL). The solvents were evaporated under reduced pres-
sure, and the residue was diluted with water. The resulting mixture
was successively passed through a hydrogen carbonate resin (pre-
pared by passing NaHCO3 through a Cl– resin) and then a H+ ion
resin. The water was evaporated under reduced pressure, and the
residue was purified by column chromatography (AcOEt/2-prop-
anol/H2O, 8:2:1) to yield (–)-gabosine J (74 mg, 71%) as a colorless
1
17 (11 mg, 75%). H NMR (500 MHz, CDCl3): δ = 7.27–7.15 (m,
20 H, Ar-H), 5.59 (br. s, 1 H, H6), 5.39 (s, 1 H, H1), 4.70–4.64 (m,
4 H, CH2Ph), 4.55 (dd, J = 4.7, 10.9 Hz, 2 H, CH2Ph), 4.44 (d, J
= 11.7 Hz, 1 H, CHPh), 4.37–4.33 (m, 2 H, CHPh and H4), 4.16–
4.14 (m, 2 H, H2 and H7a), 3.88 (d, J = 12.8 Hz, 1 H, H7b), 3.74
(dd, J = 1.0, 5.8 Hz, 1 H, H3), 1.99 (s, 3 H, COCH3) ppm. 13C
NMR (125 MHz, CDCl3): δ = 170.6, 138.6, 138.4, 138.36, 138.31,
138.2, 128.4, 128.35, 128.31, 128.27, 128.0, 127.8, 127.6, 127.56,
127.52, 122.8, 80.8, 77.21, 76.9, 75.1, 74.7, 73.5, 72.1, 72.0, 69.9,
21.1 ppm.
oil. [α]2D5 = –58.2 (c = 0.2, CH3OH); ref.[14] (+)-gabosine J [α]2D5
=
+62 (c = 1.5, CH3OH). 1H NMR (500 MHz, CD3OD): δ = 6.00 (t,
J = 1.9 Hz, 1 H, H6), 4.44 (d, J = 2.7 Hz, 1 H, H2), 4.30 (dd, J =
1.9, 17.7 Hz, 1 H, H7a), 4.12 (dd, J = 1.9, 17.7 Hz, 1 H, H7b), 4.09–
4.06 (m, 2 H, H3 and H4) ppm. 13C NMR (125 MHz CD3OD): δ
= 197.9, 160.3, 120.2, 74.6, 71.9, 68.0, 61.3 ppm.
Acetylation of 16: The above procedure was followed by starting
1
from 16 to yield 18 (10 mg, 64%). H NMR (500 MHz, CDCl3): δ
= 7.23–7.10 (m, 20 H, Ar-H), 5.66 (br. s, 1 H, H6), 5.57 (d, J =
5.8 Hz, 1 H, H1), 4.60 (d, J = 12.2 Hz, 2 H, CH2Ph), 4.52–4.49 (m,
3 H, CH2Ph), 4.44–4.38 (m, 3 H, CH2Ph), 4.27 (d, J = 11.8 Hz, 1
Gabosinol J-α [(1S,2R,3S,4R)-5-(Hydroxymethyl)cyclohex-5-ene-
1,2,3,4-tetraol]: To a solution of 16 (0.150 g, 0.279 mmol) in dry H, CHPh), 4.07 (d, J = 12.7 Hz, 1 H, H3), 4.03–4.01 (m, 1 H, H4
DCM (5 mL) at –40 °C was added BBr3 (1 m solution, 2 mL,
2.0 mmol) dropwise, and the solution was stirred at that tempera-
or H7a), 3.81–3.78 (m, 3 H, H2, H7b, and H7a or H4), 1.94 (s, 3 H,
COCH3) ppm. 13C NMR (125 MHz, CDCl3): δ = 170.5, 138.4,
Eur. J. Org. Chem. 2014, 2349–2356
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
2355

A. Vidyasagar, K. M. Sureshan
FULL PAPER
kawa, K. Kato, K. Umezawa, Bioorg. Med. Chem. Lett. 2005,
15, 1111–114.
[4] A. Maier, C. Maul, M. Zerlin, S. Grabley, R. Thiericke, J. Anti-
biot. 1999, 52, 952–959.
138.3, 138.1, 128.4, 128.3, 128.0, 127.9, 127.8, 127.77, 127.66,
127.64, 127.56, 77.2, 76.5, 75.5, 74.9, 73.9, 72.6, 72.2, 71.7, 70.6.
70.2, 21.2 ppm.
[5] S. Grabley, J. Wink, P. Hammann, C. Giani, K. Hütter, A.
Zeeck, Hoechst AG, PCT WO 8912038 [Chem. Abstr. 1990,
113, 22211y].
Glycosidase Inhibition Assay: The enzyme inhibition studies were
carried out spectroscopically by using known procedures.[22] α-
Glucosidase (Saccharomyces cerevisae), β-glucosidase (from al-
monds), α-galactosidase (from green coffee beans), β-galactosidase
(from bovine liver), α-mannosidase (from jack bean), β-mannosid
ase (from Helix pomatia), and the respective p-nitrophenyl glycopyr-
anosides were purchased from Sigma Aldrich. Each enzyme solu-
tion was prepared in the appropriate buffer solution (0.1 U/mL).
The respective p-nitrophenyl glycoside solution (0.5 mm) was also
prepared in the same buffer solution. A stock solution (3 mm) of
the inhibitor (gabosine, gabosinol α, gabosinol β) was made in the
same buffer solution. The enzyme solution (25 μL) was added to
10–12 wells in a 96-well microplate. To this was added a measured
volume of the inhibitor stock solution such that upon dilution
(with respective buffer) to 150 μL, each well had a different concen-
tration of the inhibitor that ranged from 0.1 to 2.0 mm. The reac-
tion was initiated by the addition of the substrate (the respective
p-nitrophenyl glycoside, 0.5 mm, 10 μL) to each of the wells that
contained solution. The hydrolysis reaction was incubated at 37 °C
for 30 min, and then the reaction was stopped by the addition of
Na2CO3 (0.2 m solution, 30 μL). The absorbance (at 405 nm) of the
released p-nitrophenol was measured by using a microplate reader
(Spectra Max M5 spectrometer). The absorbance value for the
blank (solution containing all components except enzyme) was re-
duced from the total absorbance to get the corrected absorbance.
The percentage of inhibition was calculated using the equation,
percent inhibition = [(A–B)/A]100, where A is the corrected ab-
sorbance without inhibitor and B is the corrected absorbance of
the solution that contains the inhibitor. The IC50 value was calcu-
lated from a plot of concentration versus percentage inhibition.
Each experiment was done in triplicate in parallel.
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Supporting Information (see footnote on the first page of this arti-
1
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Acknowledgments
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Received: November 29, 2013
Published Online: February 6, 2014
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