Synthesis of 7-O-galloyl-d-sedoheptulose

Article abstract of DOI:10.1016/j.carres.2007.04.015

A facile synthetic approach to 7-O-galloyl-d-sedoheptulose (1), a natural product with notable immunosuppressant activity, was developed. The starting material, 2,7-anhydro-d-sedoheptulose (2), was converted in three steps into 1,3,4,5-tetra-O-benzyl-d-se

Full text of DOI:10.1016/j.carres.2007.04.015

Carbohydrate Research 342 (2007) 1510–1513
Note
Synthesis of 7-O-galloyl-D-sedoheptulose
Yupeng Xie and Yimin Zhao*
Beijing Institute of Pharmacology and Toxicology, 27 Taiping Road, 100850 Beijing, China
Received 28 November 2006; received in revised form 18 April 2007; accepted 22 April 2007
Available online 29 April 2007
Abstract—A facile synthetic approach to 7-O-galloyl-D-sedoheptulose (1), a natural product with notable immunosuppressant activ-
ity, was developed. The starting material, 2,7-anhydro-D-sedoheptulose (2), was converted in three steps into 1,3,4,5-tetra-O-benzyl-
D-sedoheptulose (5), a key intermediate that allows specific functionalization at C-7 of the sedoheptulpyranose. After regioselective
esterification of 5 with 3,4,5-tri-O-benzylgalloyl acid, followed by catalytic debenzylation (Pd–C), 1 was obtained in an overall yield
of 60%. The spectroscopic data and TLC behavior of 1 were found to be identical to that of the natural product.
ꢀ 2007 Elsevier Ltd. All rights reserved.
Keywords: Sedoheptulose; D-Altro-hept-2-ulose; 7-O-Galloyl-D-sedoheptulose; 2,7-Anhydro-D-sedoheptulose; Acetolysis; Galloylation
7-O-Galloyl-D-sedoheptulose (1) is a natural product
that was first isolated by Zhang et al. from the water
extract of the fruit of Cornus officinalis Sieb et Zucc.1
In traditional Chinese medicine, the fruit of C. officinalis
is considered to have beneficial effects on liver and kid-
ney functions. It has been reported that oral administra-
tion of water extracts of C. officinalis fruits could
suppress type II collagen-induced arthritis and the pro-
duction of collagen II-specific antibodies in rats,2 as well
as attenuate dextrane induced nephropathy in mice.3
Compound 1 was isolated in our laboratory by bioas-
say-guided fractionation. It showed notable suppressant
effects on the antibody production response in the
plaque-forming cell assay. The antibody production
response was reduced by 96% at 40 lg/mL concentration
when the compound was tested in vitro, and by 47%
when it was orally administered to mice at a dose of
40 mg/kg d for 7 days. These results inspired us to syn-
thesize this compound and related derivatives for further
pharmacological investigations. Sedoheptulose (sys-
temic name D-altro-hept-2-ulose) normally occurs in
nature in the form of its phosphate esters. These per-
form vital functions during the photosynthesis of plants
and in the sugar metabolism of animal tissues. Other
derivatives of sedoheptulose have rarely been reported
except for 11 and 1,7-di-O-galloyl-D-sedoheptulose4
from C. officinalis, and 7-O-caffeoyl-D-sedoheptulose
from Nyssa sylvatica.5 Synthetic studies on these com-
pounds have not been reported. The structure of 1 is
simple. The major problem to be overcome in the syn-
thesis of 1 is to selectively protect the multiple hydroxyl
groups to ensure the specific galloylation at the 7-OH of
the sugar. This problem was solved by using 2,7-anhy-
dro-D-sedoheptulose (2), a commercially available form
of sedoheptulose, as the starting material. The bicyclic
skeleton of 2 simplified the procedure of hydroxyl pro-
tection. After fully protecting the free hydroxyls of 2
and opening the 2,7-anhydro ring, functional groups
can be specifically introduced at C-7.
The synthesis of 1 (Scheme 1) started with perbenzyl-
ation of 2. Treatment of 2 with sodium hydride
(1.5 equiv per hydroxyl group), followed by benzylation
with benzyl bromide (1.1 equiv per hydroxyl group) in
the presence of a catalytic amount of tetra-butylammo-
nium iodide (TBAI) (0.1 equiv),6 gave the desired
1,3,4,5-tetra-O-benzyl-2,7-anhydro-D-sedoheptulose (3)
in excellent yield (96%).
In order to open the anhydro ring of 3, Lewis acids
such as Me3SiI7 and anhydrous ZnCl2,8 as well as the
ion-exchange resin Dowex 50-W (H+),9 were tested as
catalysts. The reaction conditions and results are
*
Corresponding author. Tel.: +86 10 66931648; fax: +86 10
68211656; e-mail: zhaoym@nic.bmi.ac.cn
0008-6215/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carres.2007.04.015

Y. Xie, Y. Zhao / Carbohydrate Research 342 (2007) 1510–1513
1511
Reaction of 5 with 3,4,5-tri-O-benzyl-galloyl acid (6)
in the presence of DCC and DMAP in anhydrous
pyridine afforded 1,3,4,5-tetra-O-benzyl-7-O-(tri-O-benz-
yl-galloyl)-D-sedoheptulose (7) in 80% yield. Since the
primary hydroxyl 7-OH is more susceptible than the
semi-acetal hydroxyl at C-2 toward esterification, and
the bulky benzyl groups adjacent to C-2 possibly hinder
the acylation of 2-OH, the galloylation of 5 proceeds
regioselectively and gives 7 as the main product. The
O
O
OAc
OBn
a
b
O
OBn
O
O
BnO
OBn
OH
OBn
OH
BnO
OBn
HO
OH
OAc
OBn
2
4
3
OH
OH
OBn
OBn
O
O
O
O
OH
OBn
c
OH
d
OBn
e
HO
O
OH
O
OBn
OBn
BnO
O
1
structure of 7 was confirmed by H NMR, 13C NMR,
BnO
OBn
OH
OH
OBn
1H–1HCOSY, and HMBC spectra. The C-2 signal could
be observed at 98.0 ppm in the 13C NMR spectrum. The
1H NMR spectrum shows a singlet at 5.68 ppm that
integrates to one H and is exchangeable in D2O. This
signal is correlated with C-2 in the HMBC spectrum,
suggesting that there is a free hydroxyl at C-2 and that
the galloyl is linked to C-7 of the protected sedoheptu-
lose. The low-field shift of the H-7 signal from 3.80 to
4.24 ppm also indicates the galloylation of 7-OH.
OH
OBn
OH
OH
5
7
1
Scheme 1. Reagents and conditions: (a) DMF, NaH, TBAI, BnBr,
0 ꢁC ! rt, overnight, 96%; (b) Ac2O, BF3ÆEt2O, ꢀ30 ꢁC, 75 min, 85%;
(c) K2CO3, MeOH, pH = 10, rt, 1 h, 100%; (d) 3,4,5-tri-O-benzyl-
galloyl acid (6), DMAP, DCC, Py, 0 ꢁC, 48 h; rt, 48 h, 80%; (e) 10%
Pd–C, EtOAc–EtOH, rt, 7 h, 98%.
summarized in Table 1. It was found that the internal
acetal could not be opened by Me3SiI, and the reactions
catalyzed by ZnCl2 and Dowex 50-W led to decomposi-
tion of 3 and a very low yield (<10%) of the expected
product. It was known that the internal acetal in pro-
tected anhydro sugars undergoes effective acetolysis in
the presence of TFA/Ac2O10 or BF3ÆEt2O/Ac2O.11 We
found that the TFA/Ac2O-mediated reaction gave a
very low yield of the ring-opened product (<10%),
whereas the BF3ÆEt2O/Ac2O-catalyzed reaction pro-
ceeded quite quickly at 0 ꢁC; however, the benzyl pro-
tecting groups were partially exchanged by acetyl
groups. The exchange was found to occur at first in
position 1, and then in the other positions successively.
By lowering the reaction temperature to ꢀ30 ꢁC and
controlling the amount of BF3ÆEt2O to 0.25–0.35% of
the quantity of 3, the acetylation occurred only on C-7
and C-2 and a good yield (>85%) of 1,3,4,5-tetra-O-benz-
yl-2,7-di-O-acetyl-D-sedoheptulose (4) was achieved.
The acetyl groups of 4 were removed by stirring a
methanol solution of 4 containing a small amount of
water. The solution was adjusted to pH 10–11 with
potassium carbonate at rt, and a quantitative yield of
the deacetylated product 1,3,4,5-tetra-O-benzyl-D-sedo-
heptulose (5) was obtained. The pH must be strictly con-
trolled, since side products are obtained when the pH is
over 11.
The final removal of the benzyl protecting groups on
both sugar moieties and the galloyl group by hydrogen-
olysis in the presence of a catalytic amount of 10% Pd/C
afforded the desired compound 1. In aqueous solution,
7-O-galloyl-D-sedoheptulose was found to exist in three
tautomeric forms (see Fig. 1), and the proportion of b-F,
a-F, and a-P was estimated to be 57:24:19 according to
the intensities of their C-2 signals at 102.4 ppm,
105.3 ppm, and 98.5 ppm in the 13C NMR spectrum.
1
The 13C and H NMR data of 1 are identical to those
of the natural 7-O-galloyl-D-sedoheptulose reported in
the literature.1
1. Experimental
1.1. General methods
Melting points were determined with an X4 melting
point apparatus (uncorrected, Beijing science and tech-
nology company). Optical rotations were measured with
a PE-243 polarimeter. IR spectra were measured with a
Nicolet Manga spectrometer (Micronicolet, America).
1H and 13C NMR spectra were recorded with a Mercury
400 spectrometer for solutions in CDCl3 or DMSO-d6.
Table 1. Reaction conditions and results of the ring-opening reaction of 3
Entry
Acidic media
T (ꢁC)
Time (h)
Yielda (%)
1
2
3
4
5
6
7
Me3SiI (1.1 equiv) in CHCl3
CF3COOH in Ac2O
CF3COOH in Ac2O
Anhydrous ZnCl2 in Ac2O/HOAc (2:1)
Dowex 50-W(H+) in Ac2O
BF3ÆEt2O in Ac2O
0
ꢀ15
0
0
80
0
ꢀ30
0.4
0.6
1.0
3.0
4.0
0.4
1.25
Nb
10
9
5
8
52
87
BF3ÆEt2O in Ac2O
a Isolated yield of 4.
b No ring-opening product was found.

1512
Y. Xie, Y. Zhao / Carbohydrate Research 342 (2007) 1510–1513
1
CHCl3); Rf 0.45 (3.5:1 cyclohexane–EtOAc); H NMR
(400 MHz, CDCl3) d: 7.25–7.41 (m, 20H, 4PhH), 4.52–
4.87 (m, 8H, 4PhCH2), 4.64 (m, 1H, H-6), 4.06 (d, 1H,
J = 8.7 Hz, H-3), 3.92 (d, 1H, J = 10.6 Hz, H-1), 3.73–
3.79 (m, 2H, H-4, H-7), 3.65 (m, 1H, H-5 ), 3.57 (d,
1H, J = 7.6 Hz, H-7), 3.52 (d, 1H, H-1); 13C NMR
(100 MHz, CDCl3) d: 138.4 (4Ph, C-1), 138.0, 137.9,
137.8, 127.4–128.4 (m), 108.0 (C-2), 79.6 (C-3), 78.7
(C-4), 75.2 (2PhCH2), 74.2 (C-5), 73.6 (PhCH2), 72.3
(PhCH2), 71.6 (C-6), 68.9 (C-1), 65.9 (C-7); ESI-MS
(m/z ): 570 ðMþNH4þÞ, 575 (M+Na+), 591 (M+K+);
HRESI-MS: Calcd for C35H40NO6: 570.6952; found:
570.6946 ðMþNH4þÞ.
OH
OH
OH
O
O
OH
OH
OH
OH
CH2O
OH
CH2OH
CH2O
HO
HO
O
O
OH
OH
CH2OH
OH
α-F
OH
β-F
OH
O
OH
OH
O
OH
O
OH
HO
1.3. 1,3,4,5-Tetra-O-benzyl-2,7-di-O-acetyl-D-
sedoheptulose (4)
OH
OH
α-P
Figure 1. Tautomeric forms of 7-O-galloyl-D-sedoheptulose in aque-
ous solution.
A suspension of 3 (5 g, 9.06 mmol) in cold Ac2O (30 mL,
ꢀ30 ꢁC) was stirred for 10 min under nitrogen. BF3Æ
Et2O (98 lL) was added into the mixture and stirred
at ꢀ30 ꢁC for a further 75 min, then quenched with
Et3N (1 mL), diluted with toluene (5 mL), and concen-
trated in vacuo. Column chromatography (silica gel,
4:1 cyclohexanes–EtOAc) yielded pure 4 (5.04 g, 85%)
as a colorless syrup. [a]D +27 (c 0.6, CHCl3); Rf 0.45
Chemical shifts were given in ppm downfield from inter-
nal Me4Si. ESI-MS spectra were obtained with an
API3000 spectrometer (ABI, America), HRESI-MS
was obtained with a Micromass LCT. The progress of
reactions were monitored by TLC on GF254 plates
(Haiyang Chemical Factory, Qingdao, China). Column
chromatography was performed on silica gel H (100–
200 or 200–300 mesh, Haiyang Chemical Factory, Qing-
dao, China) and Sephadex LH-20 (Amersham Pharma-
cia Biotech, Sweden). The solvent systems indicated are
volume–volume ratios. Components were detected by
spraying the plates with 20% concd H2SO4 in EtOH
and heating. Solutions were concentrated below 40 ꢁC
under reduced pressure.
1
(4:1 cyclohexane–EtOAc); H NMR (400 MHz, CDCl3)
d: 7.13–7.33 (m, 20H, 4PhH), 4.30–4.63 (m, 11H, H-6,
H-7 and 8PhCH2), 4.29 (m, 1H, H-3), 3.99 (d, 1H,
J = 10.6 Hz, H-1), 3.92 (d, 1H, H-1), 3.73–3.78 (m,
2H, H-4, H-5), 2.00 (s, 3H, CH3), 1.93 (s, 3H, CH3);
ESI-MS (m/z ): 677 (M+Na+), 693 (M+K+); HRESI-
MS: Calcd for C39H42NaO9: 677.7352; found:
677.7349 (M+Na+).
1.4. 1,3,4,5-Tetra-O-benzyl-D-sedoheptulose (5)
1.2. 1,3,4,5-Tetra-O-benzyl-2,7-anhydro-D-
sedoheptulose (3)
A suspension of 4 (9 g, 13.76 mmol) in MeOH–H2O
(95:4, 99 mL) was adjusted to pH 10–11 with anhydrous
K2CO3 under stirring, and then stirred for 1 h at rt and
filtered thereafter. The filtrate was evaporated under
reduced pressure to dryness at 30 ꢁC. The residue was
purified by column chromatography (silica gel, 2:1
cyclohexanes–EtOAc) to afford 5 (7.84 g, 100%) as a
colorless syrup. [a]D +14 (c 0.5, CHCl3); Rf 0.45 (2:1
To a suspension of NaH (6.2 g, 258.33 mmol, washed
three times with cyclohexane before use) in DMF
(60 mL) was added 2,7-anhydro-D-sedoheptulose 2
(5 g, 26.04 mmol) portionwise over 30 min at 0 ꢁC under
nitrogen. The mixture was stirred at rt for 2 h. TBAI
(1 g, 2.71 mmol) was added to the thick mixture, fol-
lowed by the dropwise addition of BnBr (14 mL,
118.11 mmol) over 45 min. The reaction mixture was
stirred overnight until it turned clear. The solvent was
removed under reduced pressure and the residue was
evaporated in vacuo to dryness. The remaining solid
was triturated with Et2O (3 · 50 mL). The combined
Et2O extract was washed with water (200 mL) and brine
(200 mL) and dried over Na2SO4. After removal of
Et2O, the product was purified by column chromatogra-
phy (silica gel, 3.5:1 cyclohexane–EtOAc) to give 3
(13.8 g, 96%) as a colorless syrup. [a]D +5.7 (c 0.7,
1
cyclohexane–EtOAc); H NMR (400 MHz, CDCl3) d:
7.24–7.36 (m, 20H, 4PhH), 5.78 (s, 1H, C2–OH), 4.41–
4.83 (m, 8H, 4PhCH2), 4.44 (s, 1H, C7–OH), 4.22 (m,
1H, H-4), 4.13 (dt, 1H, J = 2.8, 9.8 Hz, H-6), 3.89 (dd,
1H, J = 12.1, 3.6 Hz, H-7), 3.80 (dd, 1H, H-7), 3.75
(d, 1H, J = 10.1 Hz, H-1), 3.60 (d, 1H, J = 2.2 Hz, H-
3), 3.50 (dd, 1H, J = 2.2, 9.8 Hz, H-5), 3.36 (d, 1H, H-
1); 13C NMR (100 MHz, CDCl3) d: 137.8 (4Ph C-1),
137.6 (2C), 136.6, 127.4–128.4 (m), 97.9 (C-2), 73.6–
73.8 (4Ph CH2), 73.1 (C-5), 71.8 (C-3), 71.7 (C-4), 71.5
(C-6), 67.5 (C-1), 62.1 (C-7); ESI-MS (m/z): 588
ðMþNH4þÞ, 593 (M+Na+), 609 (M+K+); HRESI-

Y. Xie, Y. Zhao / Carbohydrate Research 342 (2007) 1510–1513
1513
MS: Calcd for C35H42NO7: 588.7105; found: 588.7112
ðMþNH4þÞ.
1.7. 7-O-Galloyl-D-sedoheptulose (1)
A suspension of 7 (2 g, 2.02 mmol) and 10% Pd–C
(800 mg) in EtOAc–EtOH (1:1, 20 mL) was stirred at
rt while H2 was bubbled through (20 mL/min) for 7 h.
After removing the Pd–C by filtration, the filtrate was
concentrated under reduced pressure at 30 ꢁC and the
residue was purified by column chromatography
(Sephadex LH-20, MeOH) to give pure 1 (0.72 g, 98%)
as a colorless syrup. [a]D +7.2 (c 0.2, MeOH); Rf 0.7
1.5. 3,4,5-Tri-O-benzyl-galloyl acid (6)12
A mixture of gallic acid (20 g, 11.76 mmol) and anhy-
drous K2CO3 (134 g, 97.10 mmol) in DMF (100 mL)
was stirred at rt for 1 h until the solids dissolved. BnBr
(88 mL) was added dropwise into the solution over
30 min at 40 ꢁC under nitrogen. The reaction mixture
was stirred for 3 h at 40 ꢁC and then filtered. The filtrate
was evaporated to dryness in vacuo. The residue was
dissolved in aqueous ethanol (50%, 400 mL) containing
5 M NaOH and refluxed for 3 h. The solution was di-
luted with H2O (100 mL), adjusted to pH 2 with concd
HCl and stirred for 0.5 h at rt. The precipitate was col-
1
(4:1:5 n-butanol–HOAc–H2O, upper layer); H NMR
(400 MHz, 1:1 Me2CO-d6-D2O) d: 7.02 (s, 2H, galloyl-
H), 4.33 (m, b-F-H-4), 4.28 (m, a-F-H-7), 4.25 (m, b-
F-H-7), 4.12–4.16 (m, a-F-H-4, a-P-H-6, b-F-H-3),
3.99–4.00 (m, b-F-H-6, a-F-H-6, a-P-H-4), 3.85 (m, a-
P-H-3), 3.83 (m, a-P-H-5), 3.77 (m, b-F-H-5), 3.52–
3.57 (m, a-F-H-1, a-P-H-7), 3.42 (s, b-F-H-1), 3.41 (s,
a-P-H-1); 13C NMR (100 MHz, 1:1 Me2CO-d6-D2O) d:
167.3 (C@O), 145.1 (2C, galloyl C-3, C-5), 138.5 (galloyl
C-4), 120.3 (galloyl C-1), 109.4 (2C, galloyl C-2, C-6),
105.3 (a-F-C-2), 102.4 (b-F-C-2), 98.2 (a-P-C-2), 82.0,
81.1 (b-F-C-5), 76.9, 76.4 (b-F-C-3), 76.0 (b-F-C-4),
71.5, 70.5 (b-F-C-6), 69.5, 68.1, 67.2, 65.7 (b-F-C-7),
64.8, 64.2, 63.9 (a-P-C-1), 63.2 (a-F-C-1), 62.9 (b-F-C-
1); ESI-MS (m/z): 385 (M+Na+); HRESI-MS: Calcd
for C14H18NaO11: 385.2759; found: 385.2751 (M+Na+).
lected and recrystallized from methanol to afford needles
cmꢀ1: 3398,
KBr
max
of 6 (42.9 g, 83%): mp 119–120 ꢁC; IR c
1747 (C@O), 1675, 1573, 1499, 1427, 1330, 1127, 763,
1
729, 690; H NMR (400 MHz, DMSO-d6) d: 7.23–7.49
(m, 17H, Ar-H), 5.18 (s, 4H, 2 PhCH2), 5.04 (s, 2H,
PhCH2); ESI-MS (m/z): 441 (M+H+), 458 ðMþNH þÞ.
4
1.6. 1,3,4,5-Tetra-O-benzyl-7-O-(tri-O-benzyl-galloyl)-
D-sedoheptulose (7)
A mixture of 5 (2.4 g, 4.21 mmol), 6 (2.2 g, 5 mmol),
DCC (1.3 g, 6.31 mmol) and DMAP (5.1 mg,
0.04 mmol) in anhydrous pyridine (22 mL) was stirred
at 0 ꢁC for 48 h and then at rt for 48 h. The soln was fil-
tered and the filtrate was evaporated under reduced
pressure to dryness. The residue was purified by column
chromatography (silica gel, 5:1 cyclohexane–EtOAc) to
give pure 5 (3.34 g, 80%) as a colorless syrup. [a]D
+25.7 (c 0.7, CHCl3); Rf 0.45 (5:1 cyclohexane–EtOAc);
1H NMR (400 MHz, CDCl3) d: 7.13–7.40 (m, 37H,
Ar-H), 5.68 (s, 1H, C2–OH), 5.01–5.12 (m, 6H, 3PhCH2),
4.36–4.70 (m, 8H, 4PhCH2), 4.55 (m, 1H, H-6), 4.53 (m,
1H, H-5), 4.38 (m, 1H, H-4), 4.24 (d, 1H, J = 11.8 Hz,
H-7), 3.85 (m, 1H, H-3), 3.80 (d, 1H, H-7), 3.73 (d,
1H, J = 9.8 Hz, H-1), 3.40 (d, 1H, H-1); 13C NMR
(100 MHz, CDCl3) d: 165.9 (C@O), 152.3 (2C galloyl
C-3 and C-5), 142.1 (galloyl C-4), 136.6–137.9 (m),
127.6–128.6 (m), 108.9 (2C galloyl C-2 and C-6), 98.0
(C-2), 75.0 (PhCH2), 73.9 (C-5), 73.9 (PhCH2), 73.8
(PhCH2), 73.7 (PhCH2), 73.2 (C-3), 72.0 (C-4), 71.4
(C-6), 71.3 (PhCH2), 70.8 (2C, 2PhCH2), 65.8 (C-1),
64.3 (C-7); ESI-MS (m/z): 1015 (M+Na+); HRESI-
MS: Calcd for C63H60NaO11: 1016.1337; found:
1016.1330 (M+Na+).
References
1. Zhang, Y. W.; Chen, Y. W.; Zhao, S. P. Acta Pharm.
Sinica 1999, 34, 153–155.
2. Guo, L. L.; Zhou, Y.; Wang, X. D.; Zhang, L.; Zhao, S.
P.; Ge, D. Y. Chin. J. Immunol. 2002, 18, 197–199.
3. Huang, L. M.; Liang, H.; Xing, J. Y.; Shi, P.; Jiang, Y. D.
Chin. Pharmacol. Bull. 2004, 20, 1150–1152.
4. Lee, S. H.; Takashi, G. I. Phytochemistry 1989, 28, 3469–
3472.
5. Li, X. C.; Hala, N. E.; Alice, M. C. Phytochemistry 2000,
53, 1033–1037.
6. Christian, G.; Marie, L. M.; Tanguy, S. Carbohydr. Res.
2002, 337, 1769–1774.
7. Jung, M. E.; Lyster, M. A. J. Org. Chem. 1977, 42, 3761–
3763.
8. Yang, G.; Ding, X.; Kong, F. Tetrahedron Lett. 1997, 38,
6725–6729.
9. Martin, P. E.; Steven, V.; Ley, S. G. J. Chem. Soc., Chem.
Commun. 1983, 72, 630–633.
10. Anjali, R. S.; Ganguli, J. K. Tetrahedron: Asymmetry
2005, 16, 411–424.
11. Umino, A.; Hinou, H.; Matsuoka, K. J. Carbohydr. Chem.
1998, 17, 231–239.
12. Clinton, R. O.; Geissman, T. A. J. Am. Chem. Soc. 1943,
65, 85–87.