Synthesis of floridoside

Article abstract of DOI:10.1080/07328300802408843

Floridoside (2-O-glycerol-α-D-galactopyranoside) is a natural glycerol glycoside found in red algae and is believed to play important roles in carbon storage, transport, and assimilation and in the regulation of osmotic balance. We describe here a rapid,

Full text of DOI:10.1080/07328300802408843

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Synthesis of Floridoside
Michel Weïwer a b c d , Trevor Sherwood a b c d & Robert J. Linhardt a
b c d
a Department of Chemical and Biological Engineering, Rensselaer
Polytechnic Institute, 110, 8th Street, Troy, NY, 12180, USA
b Department of Chemistry and Chemical Biology, Rensselaer
Polytechnic Institute, 110, 8th Street, Troy, NY, 12180, USA
c Department of Biology, Rensselaer Polytechnic Institute, 110, 8th
Street, Troy, NY, 12180, USA
d Pharmaceutical Research Institute, Albany College of Pharmacy,
Albany, New York, 12208, USA
Version of record first published: 29 Oct 2008
To cite this article: Michel Weïwer, Trevor Sherwood & Robert J. Linhardt (2008): Synthesis of
Floridoside, Journal of Carbohydrate Chemistry, 27:7, 420-427
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Journal of Carbohydrate Chemistry, 27:420–427, 2008
ꢀC
Copyright Taylor & Francis Group, LLC
ISSN: 0732-8303 print / 1532-2327 online
DOI: 10.1080/07328300802408843
Synthesis of Floridoside
Michel We¨ıwer, Trevor Sherwood, and Robert J. Linhardt
Department of Chemical and Biological Engineering, Department of Chemistry and
Chemical Biology, and Department of Biology, Rensselaer Polytechnic Institute, 110,
8th Street, Troy, NY 12180 USA; Pharmaceutical Research Institute, Albany College of
Pharmacy, Albany, New York 12208 USA
Floridoside (2-O-glycerol-α-D-galactopyranoside) is a natural glycerol glycoside found
in red algae and is believed to play important roles in carbon storage, transport,
and assimilation and in the regulation of osmotic balance. We describe here a rapid,
high-yield, and high-stereoselectivity synthesis of floridoside in which the key step
involves the 1,2-cis O-glycosylation of 1,3-dibenzylglycerol with ethyl 2,3,4,6-tetra-O-
benzyl-1-thio-β-D-galactopyranoside using iodonium dicollidine perchlorate (IDCP) or
N-iodosuccinimide/trimethylsilyl triflate (NIS/TMSOTf) as promoters.
Keywords Floridoside; Glycerol glycoside; 1,2-cis O-glycosylation; Thioglycoside
INTRODUCTION
Floridoside (2-O-glycerol-α-D-galactopyranoside) (Figure 1), first isolated from
Rhodophyceae Rhodymenia palmata in 1930 by Colin and Gue´guen,[1] is a nat-
ural glycerol galactoside found in red algae. Floridoside is believed to play a
crucial role as the main photosynthetic reserve product[2−4] and in the control
of intracellular osmotic regulation.[5−7] Moreover, it is the carbon precursor
for the synthesis of cell-wall polysaccharides in the red microalga Porphyrid-
ium sp. (Rhodophyta).[8] The overall metabolism of floridoside in red algae
involves its synthesis by floridoside phosphate synthase,[9] its degradation by
α-galactosidase, and the interconversion of UDP-galactose and UDP-glucose
catalyzed by UDP-D-galactose-4-epimerase.[10] Floridoside is a potent inhibitor
of cyprid settlement at nontoxic concentrations to nauplii (0.01 mg/mL−1 [11]
)
and possess feeding-deterrent activity against the sea urchin Strongylocen-
1
trotus intermedius.[12] Its crystal structure, chirality,[13] and complete H and
13C spectral assignment[14] have been recently described after extraction and
purification from algae. However, this process of isolation is complex and usu-
ally furnishes a low amount of floridoside. Thus, studies of the physiological
Received January 5, 2007; accepted April 30, 2007.
Address correspondence to Robert J. Linhardt, Biotechnology 4005, Rensselaer
Polytechnic Institute, Troy, NY 12180 USA. E-mail: linhar@rpi.edu
420

Synthesis of Floridoside
421
Figure 1: Structure of floridoside.
roles of floridoside remain difficult. While the synthesis of some glycosyl glyc-
erols has been described with good yields,[15,16] the synthesis of floridoside,
reported in 1965 by Austin et al., afforded only a 27% isolated yield using
a chloride donor and silver perchlorate and silver carbonate promoter.[17] We
describe here a rapid, high-yield, and high-stereoselectivity synthesis of flori-
doside involving the 1,2-cis O-glycosylation of 1,3-dibenzylglycerol with ethyl
2,3,4,6-tetra-O-benzyl-1-thio-β-D-galactopyranoside using iodonium dicollidine
perchlorate (IDCP) or N-iodosuccinimide/trimethylsilyl triflate (NIS/TMSOTf)
as promoters.
RESULTS AND DISCUSSION
The synthesis of floridoside (2-O-glycerol-α-D-galactopyranoside) 1 requires a
stereoselective 1,2-cis O-glycosylation. Although the stereoselectivity of the
glycosylation reaction is influenced by a number of other parameters, such as
temperature, solvent, concentration, and the nature of the promoter, the pres-
ence of a nonparticipating group at the C-2 position (most commonly a benzyl
group) is the principle method used for the stereoselective synthesis of 1,2-cis
O-glycosides.[18] The nature of the leaving group in the glycosyl donor is also of
great importance, and thioglycosides have been successfully applied to 1,2-cis
O-glycosylation reactions.[19] Thioglycoside 5 (Sch. 1) was thus selected as a
potential donor for this glycosylation reaction. Commercially available per-O-
acetylgalactopyranose 2 was treated with ethyl mercaptan in the presence of
catalytic amounts of indium (III) chloride and titanium (IV) chloride to afford
the corresponding thioglycoside in 85% yield.[20] Removal of the acetyl groups
using sodium methoxide in methanol afforded 4, which was benzylated using
sodium hydride and benzyl bromide. The desired thioglycoside donor 5 was
obtained in 88% yield (75% overall yield over three steps).
Thioglycoside donor 5 was then engaged in glycosylation reactions using
commercially available acceptor 1,3-dibenzyloxy-2-propanol (Sch. 2). Different

M. We¨ıwer et al.
422
Scheme 1: Synthesis of the thioglycoside donor 5. a) EtSH, InCl3/TiCl4, CH3CN, 85%;
b) MeONa/MeOH, quant. c) NaH, BnBr, DMF, 88%.
promoters were tested to optimize the yield and stereoselectivity of the glyco-
sylation reaction. As previously demonstrated by Demchenko et al.,[21] IDCP
and NIS/TMSOTf are effective promoters in 1,2-cis O-glycosylation reactions
involving thioglycosides. Both of these promoters were tested for the glycosy-
lation of 1,3-dibenzyloxy-2-propanol with thioglycoside donor 5.
OBn
OBn
O
OH
OH
O
OBn
OBn
O
OBn
OH
a)
BnO
b)
HO
BnO
SEt
+
OBn
OH
OBn
O
OBn
O
OBn
OH
6
5
α-floridoside1
OBn
OH
˚
Scheme 2: Synthesis of α-floridoside. a) IDCP, MS 4 A, toluene/dioxane 1/3, rt, 1 hr, 67% (α/β
◦
˚
5/1) or NIS/TMSOTf, MS 4 A, toluene/dioxane 1/3, 0 C, 1.5 hr, 88% (α/β 8/1). b) H2 (1 atm)
Pd-C, MeOH, rt, 24 hr, quant.
IDCP is not commercially available and was synthesized in two steps fol-
lowing literature procedures.[22] When IDCP was used as promoter, the max-
imum yield of perbenzylated galactosyl glycerol 6 was 67% with a moderate
α/β selectivity of 5/1. This modest yield could be explained by the presence
of a side product, 2,3,4,6-tetrabenzyl-β-D-galactopyranose, resulting from the
presence of traces of water in IDCP. The use of a combination of NIS and
TMSOTf afforded an improved yield and higher stereoselectivity, and no
2,3,4,6-tetrabenzyl-β-D-galactopyranose was observed on TLC after completion
of the reaction. When the reaction was performed at 0◦C, perbenzylated galac-
tosyl glycerol 6 was formed in 88% yield and α/β ratio of 8/1. Column chro-
matography on silica gel, using petroleum ether/ethyl acetate 9/1 as the eluent,
afforded the pure α-anomer as a viscous and colorless oil (71% isolated yield).

Synthesis of Floridoside
423
Debenzylation using 10% Pd/C under hydrogen atmosphere afforded florido-
side 1 as a white solid in quantitative yield. Floridoside 1 was thus obtained
in five steps from commercially available per-O-acetylgalactopyranose with an
overall yield of 53%.
CONCLUSION
We have described a rapid and high-yield synthesis of floridoside in-
volving a highly stereoselective 1,2-cis O-glycosylation reaction between
ethyl 2,3,4,6-tetra-O-benzyl-1-thio-β-D-galactopyranoside and 1,3-dibenzyl-
glycerol in the presence of iodonium dicollidine perchlorate (IDCP) or N-
iodosuccinimide/trimethylsilyl triflate (NIS/TMSOTf). This new pathway will
allow easy access to large quantities of floridoside and facilitate studies of its
physiological roles.
EXPERIMENTAL
General
All chemicals were purchased from Aldrich or Acros and used without fur-
ther purification. Solvents were dried and distilled using classical procedures
1
˚
and stored on molecular sieves 4 A. Nuclear magnetic resonance ( H NMR and
13C NMR) spectra were recorded at rt, in CDCl3 or D2O (Varian, 500 MHz).
Chemical shifts (δ) are indicated in ppm and coupling constants (J) in Hz.
HRMS and LRMS were recorded on Micromass Autospec high-resolution mass
spectrometer and Agilent 1100 series LC/MSD trap, respectively. Thin-layer
chromatography (TLC) was carried out using Merck plates of silica gel 60 with
fluorescent indicator and revealed with UV light (254 nm) when possible and
5% H2SO4 in EtOH. Optical rotations were measured at rt using a Perkin
Elmer 241 polarimeter. Elemental analyses were performed by Galbraith Lab-
oratories Inc. using a Perkin Elmer 240 CHN Analyzer. Flash chromatog-
raphy was performed using silica gel 230–400 mesh (Natland International
Corporation).
Ethyl 2,3,4,6-tetra-O-acetyl-1-thio-β-D-galactopyranoside (3)
To a stirred solution of per-O-acetyl-D-galactopyranose (4.683 g, 12 mmol)
in freshly distilled acetonitrile (50 mL), ethyl mercaptan (2.67 mL, 36 mmol)
and InCl3 (536 mg, 2.4 mmol) were added at rt, followed by the addition of
TiCl4 (264 µL, 2.4 mmol), and the reaction was monitored by TLC. At the end
point the reaction mixture was diluted with ethyl acetate and washed succes-
sively with saturated aq. Na2CO3 and brine, dried (Na2SO4), and concentrated.
Filtration of the crude product through a short column of silica gel afforded 3
in 85% yield. 1H NMR (500 MHz, CDCl3) δ (ppm): 5.43 (1H, d, J = 3.3 Hz, H-4),

M. We¨ıwer et al.
424
5.23 (1H, t, J = 9.7 Hz, H-2), 5.04 (1H, dd, J = 10.0 Hz and J = 3.3 Hz, H-3),
4.49 (1H, d, J = 9.7 Hz, H-1), 4.18–4.09 (2H, m, 2H-6), 3.94 (1H, td, J = 7.0 Hz,
t
Jd = 1.0 Hz, H-5), 2.78–2.65 (2H, m, CH2 SEt), 2.15, 2.06, 2.03, 1.98 (12H, 4 s,
4 Ac), 1.28 (3H, t, J = 7.5 Hz, CH3 SEt). ESIMS m/z, calcd for C16H24O9SNa
[M+Na]+ 415.1, found 415.1.
Ethyl 2,3,4,6-tetra-O-benzyl-1-thio-β-D-galactopyranoside (5)
Ethyl 2,3,4,6-tetra-O-acetyl-1-thio-β-D-galactopyranoside
3
(5.2 g,
13.3 mmol) was dissolved in anhydrous methanol (40 mL) and
NaOMe/methanol solution 1 M (70 mL, 70 mmol) was added slowly at
rt. The course of the reaction was monitored by TLC. After 1 hr, the re-
action was complete and was neutralized by addition of Amberlite IR 120
(H+ form). After filtration, the solvent was evaporated and a white solid of
ethyl-1-thio-β-D-galactopyranoside 4 (3.0 g, quantitative) was obtained and
used without further purification. Ethyl-1-thio-β-D-galactopyranoside 4 (2.67
g, 11.9 mmol) was then dissolved in anhydrous dimethylformamide (DMF, 20
mL) and slowly added to a suspension of sodium hydride (NaH, 60 mmol) in
anhydrous DMF (35 mL) at 0◦C. After 10 min, a solution of benzyl bromide
(5.8 mL, 48.8 mmol) in anhydrous DMF (20 mL) was added slowly and the
mixture was allowed to reach rt. After completion of the reaction, ethyl acetate
(200 mL) was added and the organic phase was washed with water and 0.1
M HCl. Purification of the residue on silica gel using petroleum ether/ethyl
1
acetate 9/1 to 8/2 as the eluent afforded 5 as a white solid, in 88% yield. H
NMR (500 MHz, CDCl3) δ (ppm): 7.20–7.10 (20H, m, H Ar), 4.96 (1H, d, J =
11.7 Hz, 1H CH2 Bn), 4.89 (1H, d, J = 10.1 Hz, 1H CH2 Bn), 4.81 (1H, d,
J = 10.1 Hz, 1H CH2 Bn), 4.74 (2H, s, CH2 Bn), 4.63 (1H, d, J = 11.7 Hz,
1H CH2 Bn), 4.50–4.40 (2H, m, CH2 Bn), 4.43 (1H, d, J = 9.5 Hz, H-1), 3.97
(1H, d, J = 2.2 Hz, H-4), 3.84 (1H, t, J = 9.5 Hz, H-2), 3.60–3.50 (4H, m,
H-3, H-5, 2 H-6), 2.79-2.63 (2H, m, CH2 SEt), 1.31 (t, J = 7.4 Hz, CH3 SEt).
13C NMR (125 MHz, CDCl3) δ (ppm): 138.64, 138.27, 138.19, 137.76 (four
quaternary aromatic carbons), 128.35, 128.33, 128.27, 128.22, 128.18, 128.15,
128.13, 128.08, 127.94, 127.84, 127.81, 127.79, 127.77, 127.71, 127.64, 127.56,
127.54, 127.48, 127.46, 127.44 (20 CH Ar), 85.21 (C-1), 83.98 (C-3), 78.33
(C-2), 77.09 (C-4), 75.68 (C-5), 74.33, 73.45, 72.56 (4 CH2 OBn), 68.70 (C-6),
24.69 (CH2 SEt), 15.01 (CH3 SEt). ESIMS m/z, calcd for C36H40O5SNa
[M+Na]+ 607.2, found 607.2 and for C36H40O5SK [M+K]+ 623.2,
found 623.2.
2-(Phenylmethoxy)-1-[(phenylmethoxy)methyl]ethyl-2,3,4,6-
tetrakis-O-(phenylmethyl)-α-D -galactopyranoside (6)
A mixture of ethyl 2,3,4,6-tetra-O-benzyl-1-thio-β-D-galactopyranoside 5
(100 mg, 0.171 mmol) and 1,3-dibenzyloxy-2-propanol (46 mg, 0.163 mmol) was

Synthesis of Floridoside
425
dissolved in toluene (5 mL) and evaporated under reduced pressure. This oper-
ation was repeated twice. Freshly activated (300◦C for 2 h in vacuo) molecular
˚
sieves 4 A (244 mg), freshly distilled toluene (2 mL), and dioxane (6 mL) were
added and the mixture was stirred for 1 hr at rt. The mixture was cooled to
0◦C and IDCP (115 mg, 0.245 mmol) or NIS (46 mg, 0.204 mmol) and TM-
SOTf (19 µL, 0.098 mmol) were added and the course of reaction was moni-
tored by TLC. When the reaction was finished (1.5 hr), the molecular sieves
were filtered off and washed with dichloromethane (70 mL). The filtrate was
then washed with aqueous Na2S2O3 (2 × 20 mL) and water (3 × 20 mL),
dried (Na2SO4), and evaporated under reduced pressure. Purification on silica
gel, using petroleum ether/ethyl acetate 9/1 as the eluent, afforded the pure
α-anomer as a viscous and colorless liquid, in 71% isolated yield. 1H NMR
(500 MHz, CDCl3) δ (ppm): 7.41–7.22 (30H, m, HAr), 5.27 (1H, t, J = 3.3 Hz,
H-1), 4.96 (1H, dd, J = 11.4 Hz, J = 2.9 Hz, 1H CH2 Bn), 4.87 (1H, dd, J =
11.7 Hz and J = 2.6 Hz, 1H CH2 Bn), 4.75 (1H, dd, J = 11.7 Hz and J = 2.6
Hz, 1H CH2 Bn), 4.72 (2H, d, J = 2.8 Hz, CH2 Bn), 4.59 (1H, dd, J = 11.4
Hz and J = 2.9 Hz, 1H CH2 Bn), 4.54 (2H, d, J = 2.4 Hz, CH2 Bn), 4.52 (1H,
dd, J = 11.8 Hz and J = 2.6 Hz, 1H CH2 Bn), 4.48 (1H, dd, J = 11.8 Hz and
J = 2.6 Hz, 1H CH2 Bn), 4.40 (1H, dd, J = 11.8 Hz and J = 2.6 Hz, 1H CH2
Bn), 4.36 (1H, dd, J = 11.8 Hz and J = 2.6 Hz, 1H CH2 Bn), 4.23–4.19 (1H,
m, H-4), 4.17–4.11 (1H, m, H-1ꢁ), 4.06 (1H, dt, Jd = 9.6 Hz and J = 3.4 Hz,
t
H-2), 3.98 (1H, dt, Jd = 9.6 Hz and J = 2.9 Hz, H-3), 4.00 (1H, d, J = 2.9
t
Hz, H-5), 3.68–3.48 (6H, m, 4H-2’, 2H-6). 13C NMR (125 MHz, CDCl3)δ (ppm):
139.26, 139.05, 138.93, 138.58, 138.48, 138.40 (six quaternary aromatic car-
bons), 128.61, 128.59, 128.58, 128.47, 128.43, 128.08, 128.02, 127.87, 127.84,
127.81, 127.79, 127.77, 127.74, 127.69, 127.66 (30 CH Ar), 97.24 (C-1), 79.18
(C-3), 76.55 (C-2), 75.40 (C-4), 75.03 (CH2 Bn), 75.00 (CH2 Bn), 73.62 (C-1ꢁ),
73.60 (CH2 Bn), 73.38 (CH2 Bn), 73.36 (CH2 Bn), 72.91 (C-5), 70.81 (CH2 Bn),
70.65 (1C-2’), 69.45 (1C-2’), 69.17 (C-6). ESIMS m/z, calcd for C51H54O8Na
[M+Na]+ 817.4, found 817.4. HRMS calcd for C51H54O8Na 817.3716, found
817.3703. Elem. anal. calcd for C51H54O8: C, 77.05; H, 6.85. Found: C, 76.72;
H, 7.00.
Floridoside (2-O-α-D-galactopyranosylglycerol) (1)
2-Benzyl-1-(benzylmethyl)ethyl-2,3,4,6-tetra-O-benzyl-α-D-galactopyrano-
side 6 (58 mg, 0.073 mmol) in methanol (2 mL) was stirred under hydrogen
atmosphere in the presence of palladium on activated carbon (10%, 7 mg) for
2 d at rt. The resulting mixture was filtered through celite and washed with
methanol. After evaporation of the solvent, floridoside (18 mg) was recovered
as a white solid in quantitative yield. [α]D+168◦ (c 0.02, water), lit. [α]D+164◦
1
(c 0.97, water), H NMR (500 MHz, D2O) δ (ppm): 5.14 (1H, d, J = 3.9 Hz,
H-1), 4.09 (1H, t, J = 6.2 Hz, H-5), 3.99 (1H, d, J = 3.3 Hz, H-4), 3.89 (1H,

M. We¨ıwer et al.
426
dd, J = 10.0 Hz and J = 3.3 Hz, H-3), 3.82 (1H, dd, J = 10.0 Hz and J = 3.9
Hz, H-2), 3.73 (2H, d, J = 6.2 Hz, 2H-6), 3.82–3.68 (5H, m, H-1ꢁ, 4H-2ꢁ). 13C
NMR (125 MHz, D2O) δ (ppm): 100.98 (C-1), 81.60 (1 C-2ꢁ), 73.92 (C-5), 72.18
(C-3), 72.12 (C-4), 71.30 (C-2), 64.24 (C-1ꢁ), 63.97 (C-6), 63.18 (1 C-2ꢁ). ESIMS
m/z, calcd for C9H18O8Na [M+Na]+ 277.1, found 277.1. HRMS calculated for
C9H18O8Na 277.0899, found 277.0915.
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