Asymmetric syntheses of (-)-methyl shikimate and (-)-5a-carba-β-d-gulopyranose from d-arabinose via Mukaiyama-type intramolecular aldolization

Article abstract of DOI:10.1016/j.tetasy.2008.12.028

New asymmetric syntheses of (-)-methyl shikimate 1 and (-)-5a-carba-β-d-gulopyranose 11 from d-arabinose through a common route which employed Mukaiyama-type intramolecular aldolization as a key step were described.

Full text of DOI:10.1016/j.tetasy.2008.12.028

Tetrahedron: Asymmetry 20 (2009) 78–83
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Tetrahedron: Asymmetry
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Asymmetric syntheses of (ꢀ)-methyl shikimate and (ꢀ)-5a-carba-b-D-
gulopyranose from D-arabinose via Mukaiyama-type intramolecular aldolization
*
Shi-Ling Liu, Xiao-Xin Shi , Yu-Lan Xu, Wei Xu, Jing Dong
Department of Pharmaceutical Engineering, School of Pharmacy, East China University of Science and Technology, PO Box 363, 130 Mei-Long Road, Shanghai 200237, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
New asymmetric syntheses of (ꢀ)-methyl shikimate 1 and (ꢀ)-5a-carba-b-
D-gulopyranose 11 from D-
arabinose through a common route which employed Mukaiyama-type intramolecular aldolization as a
key step were described.
Received 4 September 2008
Accepted 8 December 2008
Available online 23 February 2009
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
2. Results and discussion
(ꢀ)-Shikimic acid has currently been used as a chiral starting
material for the manufacture of a potent antiinfluenza drug, osel-
tamivir phosphate (Tamiflu),1 which would protect humans from
a possible influenza pandemic. (ꢀ)-Shikimic acid can be isolated
from plant sources, for example, it was first isolated from the fruit
of Illicium religiosum in 1885 by Eykman.2 However, the isolation of
(ꢀ)-shikimic acid from natural materials is limited in quantity, so it
may retard the use of (ꢀ)-shikimic acid in a large-scale synthesis of
oseltamivir phosphate. Accordingly, endeavor should be made by
synthetic chemists to develop a practical approach to (ꢀ)-shikimic
acid.
The two original syntheses of racemic shikimic acid were re-
ported almost simultaneously by the groups of Raphael3 and
Smissman.4 A series of methods over the last decades have been
found to synthesize racemic or enantiomerically pure shikimic
acid, and an excellent review has summarized these methods.5
They can be briefly classified into four categories: (a) syntheses
based on the Diels–Alder reaction;3,4,6 (b) syntheses from benzene
and its derivatives;7 (c) syntheses from (ꢀ)-quinic acid;8 and (d)
syntheses from carbohydrates.9
As outlined in Scheme 1, the Wittig olefination of D-arabinose
with (ethoxycarbonylmethylene)triphenylphosphorane in dioxane
affords an unsaturated ester 2 in 74% yield according to the mod-
ified version of a known procedure.12 Then, Pd/C-catalyzed hydro-
genation of 2 in methanol at room temperature produced saturated
ester 3 in an almost quantitative yield. Protection of the primary
hydroxyl group of compound 3 by reacting with 1.2 equiv of tri-
phenylmethyl chloride in the presence of 2 equiv of pyridine and
0.2 equiv of 4-dimethylaminopyridine (DMAP) in DMF afforded
compound 4 in 90% yield. The reaction of compound 4 with
12 equiv of allyl bromide in the presence of 3.5 equiv of sodium hy-
dride in dry DMF produced the lactone 5 in 76% yield; meanwhile,
both secondary hydroxyl groups were masked by two allyl groups.
In order to efficiently reduce the formation of the by-product in
which all three secondary hydroxyl groups were allylated, we al-
lowed the reaction to continue for 2 h after the addition of the first
batch of sodium hydride (1.5 equiv) to allow the lactonization to be
matured, and then added the second batch of sodium hydride
(2.0 equiv) and allyl bromide to protect the remaining two hydro-
xyl groups. Removal of the triphenylmethyl group of compound 5
with a catalytic amount of concentrated HCl in a mixed solvent of
acetonitrile–water (40:1) at 0 °C produced compound 6 in 98%
yield. Swern oxidation of the alcohol 6 affords an aldehyde 7 in
91% yield. Other attempts to prepare aldehyde 7 from alcohol 6
by using pyridinium chlorochromate (PCC)13 or Dess–Martin re-
agent14 as an oxidant failed. We found that aldehyde 7 is not stable
and should be used for the next step as soon as possible.
Carbohydrates that are naturally abundant and inherently rich
in chiral centers have played an important role in the syntheses
of natural products.10 The resemblance of configuration of the
three adjacent hydroxyl groups in
mic acid prompted us to investigate a novel synthesis of (ꢀ)-shiki-
mic acid from -arabinose. Though Bestmann and Heid have
reported a synthesis of (ꢀ)-shikimic acid from
-arabinose,9a here-
in, we would like to disclose a different concise synthesis of (ꢀ)-
D-arabinose to that of (ꢀ)-shiki-
D
D
The key step for constructing the skeleton of the title com-
pounds is the annulation of the aldehyde 7. We first tried the intra-
molecular aldolization of 7 under strongly basic conditions using
LDA or LHDMS in THF at low temperature. The reaction was com-
plicated, and none of the desired product was isolated, although
Rassu and his colleagues obtained moderate yields in a similar
intramolecular aldolization.15 Fortunately, when we employed a
methyl shikimate 1 also from D-arabinose, in which the key step
was a Mukaiyama-type intramolecular aldolization.11
* Corresponding author.
E-mail address: xxshi@ecust.edu.cn (X.-X. Shi).
0957-4166/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2008.12.028

S.-L. Liu et al. / Tetrahedron: Asymmetry 20 (2009) 78–83
COOEt
79
TBSO
O
O
1
OH
O
O
a
b
D-arabinose
5
2
TBSO
74%
99%
OTBS
O
4
3
HO
OH
B
Si
Si
RO
RO
H
OR
OH
2
OR
8
re/si face attack (favored)
COOEt
COOEt
H
OHC
O
TBSO
OH
OTr
c
d
O
O
OR
RO
A (R=Allyl)
1
5
90%
76%
HO
OH
3
OH
HO
OH
H
2
4
3
OR
2-epi-8
OH
4
C
OR
RO
O
TBSO
OR
OTr
OH
re/re face attack (disfavored)
Figure 1. Pathway for the intramolecular aldolization.
O
O
e
O
O
O
O
Allyl
98%
O
O
Allyl
Allyl
Allyl
6
5
and would form a thermodynamically less stable product 2-epi-8
where the tert-butyldimethylsilyloxy group at the C-2 position
was axial. Coordination of the silyl reagent with the aldehyde
group may play an important role, it could accelerate the reaction
rate, and the bulkiness of the coordinated silyl group should also
promote the diastereoselectivity.
CHO
O
g
f
O
O
91%
81%
O
Allyl
Allyl
7
O
O
CO2Me
TBSO
The compound 8 was then treated with 0.25 equiv of PdCl2 and
with 1.5 equiv of HF in methanol at reflux. The silyl group and both
allyl groups of compound 8 were removed simultaneously, along
with the alcoholysis of the lactone. Exposure of the crude product
to 10 equiv of Ac2O and 20 equiv of pyridine in the presence of
0.2 equiv of DMAP in dichloromethane at room temperature fur-
nished compound 9 in 86% yield. The compound 9 was treated
with 5.0 equiv of DBU at room temperature in dichloromethane
to afford (ꢀ)-methyl 3,4,5-triacetylshikimate 10 in 83% yield.
Compound 10 was finally transformed into the title compound
(ꢀ)-methyl shikimate 1 in 92% yield after methanolysis of 10 in
absolute methanol at reflux in the presence of catalytic ammonia.
Pseudo-sugars, also referred to as carbasugars, are a subclass of
the largely represented family of cyclitols.17 Carbasugars display a
wide range of biological properties owing to their close resem-
blance to carbohydrates.17,18 Some of carbasugars have been pro-
ven to be potent inhibitors of a variety of glycosidase enzymes.19
In this work, we found that bicyclic compound 8 could be a very
AcO
AcO
h
O
86%
OAc
Allyl
O
OAc
9
CO2Me
Allyl
8
CO2Me
j
i
OAc
83%
OH
92%
AcO
HO
OAc
OH
1
10
Scheme 1. Synthesis of (ꢀ)-methyl shikimate 1. Reaction conditions: (a) 1.3 equiv
of Ph3PCH@CHCOOEt, rt for 8 h, then 60 °C for 4 h in dioxane under N2; (b) 10% of
Pd/C, rt for 50 h in methanol under H2; (c) 1.2 equiv of triphenylmethyl chloride,
0.2 equiv of DMAP and 2 equiv of pyridine, rt overnight in DMF; (d) 1.5 equiv of
NaH, rt for 2 h in dry DMF; then 12 equiv of allyl bromide, 2.0 equiv of NaH, rt for
7 h; (e) cat. HCl, 0 °C for 4 h in a mixed solvent of acetonitrile and water (40:1); (f)
2.5 equiv of oxalyl chloride, 5.0 equiv of DMSO and 6 equiv of Et3N, in CH2Cl2, ꢀ78
to ꢀ60 °C for 3 h, then 0 °C for 2 h; (g) 3 equiv of DIPEA, 3 equiv of TBSOTf, in
CH2Cl2, 0 °C for 0.5 h, then rt for 2 h. (h) 1.5 equiv of HF, 0.25 equiv of PdCl2,
refluxing for 2 h in methanol; then 20 equiv of pyridine, 0.2 equiv of DMAP,
10 equiv of Ac2O, 0 °C to rt overnight in CH2Cl2; (i) 5 equiv of DBU, rt for 6 h in
CH2Cl2; (j) cat. NH4OH, refluxing in methanol for 5 h.
useful intermediate for the synthesis of carbasugar 5a-carba-b-D-
gulopyranose 11 (Scheme 2). The bicyclic lactone 8 was first trea-
ted with 10 equiv of diisobutylaluminum hydride (DIBAL-H) in
dichloromethane at ꢀ78 to ꢀ20 °C to produce a crude product,
which was then treated with sodium borohydride in methanol at
0 °C to give pure alcohol 12 in 94% yield. Here, it should be pointed
out that treatment of the crude product with sodium borohydride
was necessary, otherwise it would have been contaminated by a
lactol (15–20%). Alcohol 12 was allowed to react with 2.0 equiv
of tetrabutylammonium fluoride (TBAF) in THF at 0 °C to remove
the silyl group, and the crude product was subjected to acylation
with 10 equiv of Ac2O, 20 equiv of pyridine, and 0.2 equiv of DMAP
in dichloromethane at 0 °C to room temperature to furnish com-
pound 13 in 88% yield. The structure and stereochemistry of the
compound 13 were also confirmed by 1H NMR, 13C NMR, 1H–1H
COSY, and 1H–1H NOESY spectra. After the deprotection of both al-
lyl groups of compound 13 with 0.25 equiv of PdCl2 in methanol at
reflux, the crude product was further treated with 20 equiv of
Ac2O, 20 equiv of pyridine, and 0.2 equiv of DMAP in dichloro-
methane, and thus compound 14 could be obtained in 87% yield.
We have also tried the direct transformation of compound 12 into
compound 14 in one step, in which the silyl group and both allyl
groups were supposed to be removed simultaneously, but the yield
was quite low, so the two-step transformation of 12 into 14 should
be followed. Finally, the five acetyl groups of compound 14 were
removed in a one-pot reaction by methanolysis in the presence
Mukaiyama-type aldolization protocol, the desired cyclization via
an intramolecular aldolization occurred.16 The aldehyde 7 was
treated with 3.0 equiv of tert-butyldimethylsilyl trifluoromethane-
sulfonate (TBSOTf) and with 3.0 equiv of diisopropylethylamine
(DIPEA) in dichloromethane at 0 °C to room temperature to pro-
duce a bicyclic lactone 8 in 81% yield. The structure and stereo-
chemistry of compound 8 were unequivocally identified by 1H
NMR, 13C NMR, 1H–1H COSY, and 1H–1H NOESY spectra. It was
noteworthy that use of an excess of TBSOTf and DIPEA was neces-
sary, when 3 equiv of both the reagents were used, the diastereose-
lectivity of the intramolecular aldolization was very high, and
almost no other stereoisomer could be detected by TLC. When less
than 2 equiv of TBSOTf and DIPEA were used, the reaction rate was
decreased and the yield was lowered; moreover, the product 8 was
contaminated with around 2% of 2-epi-8. A plausible pathway for
the intramolecular aldolization is drawn in Figure 1, during the
nucleophilic cycloaddition of the silyl enol ether intermediate A,
a re/si face attack (conformation B) was favored and should be fol-
lowed to form a thermodynamically more stable product 8 where
the tert-butyldimethylsilyloxy group at the C-2 position was equa-
torial. While a re/re face attack (conformation C) was disfavored

80
S.-L. Liu et al. / Tetrahedron: Asymmetry 20 (2009) 78–83
OH
4. Experimental
TBSO
O
4.1. General methods
a
Compound 8
94%
OH
Melting points are uncorrected. 1H NMR and 13C NMR spectra
were acquired on Bruker AM-500. Chemical shifts were given on
the delta scale as parts per million (ppm) with tetramethylsilane
(TMS) as the internal standard. IR spectra were recorded on Nicolet
Magna IR-550. Mass spectra were recorded on HP5989A. Optical
rotations of chiral compounds were measured on WZZ-1S auto-
matic polarimeter at room temperature. Column chromatography
was performed on silica gel. All chemicals were analytically pure.
Allyl
O
Allyl
12
OAc
AcO
c
b
88%
87%
OAc
O
Allyl
O
Allyl
13
OAc
OH
4.2. (E)-(4R,5S,6R)-Ethyl 4,5,6,7-tetrahydroxy-hept-2-enoate 2
AcO
AcO
HO
HO
d
To a solution of (ethoxycarbonylmethylene)triphenylphospho-
rane (30.00 g, 86.11 mmol) in dry dioxane (150 mL), was added
98%
OAc
OH
OAc
14
OH
11
D-arabinose (10.00 g, 66.61 mmol). The mixture was stirred under
an atmosphere of N2 at room temperature for 8 h, then warmed
to 60 °C and stirred for 4 h. The resulting clear solution was con-
centrated in vacuum to give a semisolid residue, water (250 mL)
was added, and the suspension was vigorously stirred for 3 h and
then filtered. After that the filtrate was extracted with CH2Cl2
(3 ꢁ 50 mL). The aqueous solution was evaporated under reduced
pressure to dryness to give a residue, which was recrystallized
from ethanol to afford a white solid 2 (10.86 g, 49.29 mmol) in
Scheme 2. Synthesis of (ꢀ)-5a-carba-b-
D-gulopyranose 11. Reaction conditions: (a)
10 equiv of DIBAL-H, ꢀ78 to ꢀ20 °C for 2 h in CH2Cl2; then 3 equiv of NaBH4, 0 °C for
0.5 h in methanol; (b) 2 equiv of TBAFꢃ3H2O, 0 °C for 3 h in THF; then 20 equiv of
pyridine, 0.2 equiv of DMAP, 10 equiv of Ac2O, 0 °C to rt overnight in CH2Cl2; (c)
0.25 equiv of PdCl2, refluxing for 2 h in methanol; then 20 equiv of pyridine,
0.2 equiv of DMAP, 10 equiv of Ac2O, 0 °C to rt overnight in CH2Cl2; (d) cat. NH4OH,
rt for 12 h, and then refluxing for 16 h in methanol.
74% yield, mp 138–139 °C, ½a D25
ꢂ
¼ þ15:1 (c 0.4, MeOH). 1H NMR
of a catalytic amount of ammonia, and thus another title com-
pound 5a-carba-b-D-gulopyranose 11 was obtained in almost
(D2O) d 1.24 (t, J = 7.0 Hz, 3H), 3.55–3.67 (m, 2H), 3.68–3.82 (m,
2H), 4.18 (q, J = 7.0 Hz, 2H), 4.56–4.64 (m, 1H), 6.10 (d,
J = 15.8 Hz, 1H), 7.01 (dd, J1 = 15.7 Hz, J2 = 4.2 Hz, 1H). MS m/z (rel-
ative intensity) 221 (M++1, 0.3), 203 (0.1), 189 (1), 171 (3), 157 (2),
147 (3), 143 (6), 130 (22), 101 (30), 84 (53), 73 (100), 57 (71), 43
(39). IR (KBr) 3318, 2954, 1721, 1666, 1449, 1386, 1270, 1165,
quantitative yield.
It is worthy to point out that 2D 1H NMR analyses support the
stereochemistry of compound 8 and compound 13 as drawn in Fig-
ure 2. In 1H–1H NOESY spectra of 8, H-2 correlates with axial H -
a
1087, 730, 650 cmꢀ1
.
5a, which suggests that H-2 is at axial position in the chair confor-
mation of the hexanoid ring, thus the stereogenic center C-2 should
possess an (S)-configuration. H-1 correlates with vicinal H-2, but it
does not correlate with H-3; moreover, the correlation spot be-
4.3. (4R,5S,6R)-Ethyl 4,5,6,7-tetrahydroxy-heptanoate 3
tween H-1 and H -5a is obviously greater than the correlation spot
a
To a solution of 2 (5.00 g, 22.70 mmol) in methanol (150 mL),
was added Pd/C (500 mg). The suspension was purged with hydro-
gen gas several times, and then was well stirred under an atmo-
sphere of H2 at room temperature for 50 h. After the catalyst Pd/
C was filtered off, the clear solution was evaporated under a vac-
uum to give a neat white solid 3 (4.99 g, 22.45 mmol) in 99% yield,
between H-1 and Hb-5a, which suggests that H-1 is at an equatorial
position, and the stereogenic center C-1 should also possess an (S)-
configuration. In 1H–1H NOESY spectra of 13, H-5 correlates with
vicinal H-4, H -5a and axial H-1, which suggests that H-5 is at axial
a
position of the chair conformation, and C-5 has a (R)-configuration.
H-4 correlates with H-3 and H-5, but it does not correlate with H-2,
which suggests that H-4 is at an equatorial position, and C-4 has an
(S)-configuration.
½
a 2D5
ꢂ
¼ þ9:2 (c 0.5, H2O). 1H NMR (D2O) d 1.15 (t, J = 7.2 Hz, 3H),
1.65–1.76 (m, 2H), 2.35–2.47 (m, 2H), 3.32 (dd, J1 = 1.7 Hz,
J2 = 8.3 Hz, 1H), 3.53 (dd, J1 = 11.8 Hz, J2 = 6.5 Hz, 1H), 3.59–3.65
(m, 1H), 3.69–3.78 (m, 2H), 4.06 (q, J = 7.1 Hz, 2H). MS m/z (relative
intensity) 223 (M++1, 0.05), 177 (0.2), 159 (1), 145 (2), 127 (4), 115
(21), 98 (57), 85 (100), 73 (18), 57 (35), 43 (37). IR (KBr) 3548,
3301, 2981, 2909, 1712, 1459, 1443, 1375, 1229, 1087, 1012,
O
1
AcO
Hβ
5a
H
3
OAc
H
TBSO
RO
4
O
5
5
2
H
H
Hβ
872, 664 cmꢀ1
.
Hα
H
OAc
2
H
3
1
4
RO
5a
H
H
H
OR
H
Hα
RO
13 (R=Allyl)
Figure 2. Stereochemistry and NOE of compounds 8 and 13.
4.4. (4R,5S,6R)-Ethyl 4,5,6-trihydroxy-7-triphenylmethoxy-
heptanoate 4
8 (R=Allyl)
To a cold (ice bath) solution of 3 (2.00 g, 9.00 mmol) in DMF
(15 mL), were added DMAP (0.22 g, 1.80 mmol), pyridine
(1.45 mL, 18 mmol), and triphenylmethyl chloride (3.02 g,
10.83 mmol). After the addition was finished, the mixture was al-
lowed to warm to room temperature and stirred overnight. Dis-
tilled water (100 mL) and benzene (100 mL) were added and
vigorously stirred for 0.5 h. After the mixture was transferred into
a separatory funnel, the aqueous phase was separated, and the or-
ganic phase was successively washed with dilute aqueous solution
of acetic acid, water, and brine. The organic solution was dried over
3. Conclusion
In conclusion, the work described herein provides an efficient
common approach to (ꢀ)-methyl shikimate 1 and (ꢀ)-5a-carba-
b-D-gulopyranose 11. The compound 1 could be obtained via a
10-step synthesis in around 23.8% overall yield from
and compound 11 could be obtained via an 11-step synthesis in
around 25.5% overall yield from -arabinose.
D-arabinose,
D

S.-L. Liu et al. / Tetrahedron: Asymmetry 20 (2009) 78–83
81
anhydrous MgSO4, and then concentrated to give a syrup, which
4.7. (4R,5S,6R)-5,6-Diallyloxy-7-oxy-heptano-1,4-lactone 7
was purified by chromatography to furnish compound 4 (3.77 g,
8.12 mmol) in 90% yield, ½a D25
ꢂ
¼ þ6:8 (c 2.6, ethyl acetate). 1H
A solution of oxalyl chloride (1.054 g, 8.30 mmol) in dichloro-
methane (20 mL) was cooled to ꢀ78 °C by a dry ice-acetone bath,
a solution of DMSO (1.30 g, 16.64 mmol) in dichloromethane
(40 mL) was then dropwise added within a period of 30 min. After
the addition was finished, the mixture was stirred at ꢀ78 °C for one
more hour, then a solution of 6 (0.85 g, 3.32 mmol) in dichloro-
methane (10 mL) was added. After triethylamine (2.11 g,
20.85 mmol) was added, the stirring was continued at ꢀ78 to
ꢀ60 °C for 2 h. The mixture was then allowed to warm to 0 °C,
and stirring was continued at 0 °C for 2 h. The reaction was
quenched by adding water (50 mL) and the mixture was well stir-
red for 15 min. The aqueous phase was separated and extracted
once more with dichloromethane (20 mL). Extracts were combined
and dried over anhydrous Na2SO4. The organic solution was con-
centrated under a vacuum to produce a crude oil, which was puri-
NMR (CDCl3) d 1.15 (t, J = 7.1 Hz, 3H), 1.63–1.72 (m, 1H), 1.76–
1.88 (m, 1H), 2.29–2.42 (m, 2H), 2.69 (d, J = 7.0 Hz, 1H), 2.79 (s,
1H), 2.97 (d, J = 3.6 Hz, 1H), 3.21–3.28 (m, 2H), 3.37 (dd,
J1 = 5.6 Hz, J2 = 5.7 Hz, 1H), 3.63–3.3.69 (m, 1H), 3.77 (dd,
J1 = 5.3 Hz, J2 = 5.4 Hz, 1H), 4.03 (q, J = 7.2 Hz, 2H), 7.14–7.28 (m,
9H), 7.32–7.39 (m, 6H). MS m/z (relative intensity) 465 (M++1,
0.02), 418 (0.7), 341 (2), 259 (6), 243 (100), 228 (5), 183 (10),
165 (36), 105 (11), 85 (4), 77 (4), 44 (5). HRMS (FAB) calcd for
(C28H32O6 + Na+): 487.2097, found: 487.2097. IR (KBr) 3442,
2931, 1731, 1490, 1447, 1070, 706 cmꢀ1
.
4.5. (4R,5S,6R)-5,6-Diallyloxy-7-triphenylmethoxy-heptano-1,4-
lactone 5
Compound 4 (2.79 g, 6.00 mmol) was dissolved in dry DMF
(20 ml), which was freshly distilled over calcium hydride. The
first batch of NaH (393 mg, 55%, 9.01 mmol) was added, and
the suspension was stirred at room temperature for 2 h. Allyl
fied by flash chromatography to afford aldehyde 7 (0.77 g,
3.03 mmol) in 91% yield. 1H NMR (CDCl3) d 2.00–2.10 (m, 1H),
2.21–2.32 (m, 1H), 2.37–2.49 (m, 1H), 2.50–2.61 (m, 1H), 3.60–
3.69 (m, 1H), 3.95–4.07 (m, 3H), 4.07–4.17 (m, 2H), 4.67–4.74
(m, 1H), 5.08–5.29 (m, 4H), 5.72–5.88 (m, 2H), 9.71 (d, J = 1.6 Hz,
1H). HRMS (EI) calcd for C13H18O5: 254.1311, found: 254.1315.
bromide (8.60 g, 71.08 mmol) and
a second batch of NaH
(524 mg, 55%, 12.01 mmol) were added, and then stirring was
continued at room temperature for 7 h. The mixture was filtered
through a thin layer of Celite to remove the suspended solid, the
filtrate was then partitioned between benzene (100 mL) and
water (100 mL). Aqueous layer was separated, organic layer
was dried over anhydrous MgSO4, and concentrated under a re-
duced pressure to give a crude oil, which was purified by flash
chromatography to produce 5 (2.28 g, 4.57 mmol) in 76% yield,
4.8. (1S,2S,3S,4S,5R)-2-(tert-Butyldimethylsilyloxy)-3,4-diallyloxy-6-
oxa-bicyclo[3,2,1]octan-7-one 8
To
a solution of diisopropylethyl amine (DIPEA, 0.504 g,
3.90 mmol) in anhydrous dichloromethane (10 mL) at 0 °C (ice
bath) under argon, was added TBSOTf (1.03 g, 3.90 mmol), and
the resulting mixture was stirred at 0 °C for 15 min. A solution of
aldehyde 7 (0.33 g, 1.30 mmol) in dichloromethane (5 mL) was
then added. The mixture was stirred at 0 °C for 30 min, then was
allowed to warm to room temperature and stirred at room temper-
ature for 2 h. The reaction was quenched with water (20 mL) and
saturated NH4Cl aqueous solution until pH 5, organic layer was
separated and the aqueous layer was extracted twice with dichlo-
romethane (20 mL ꢁ 2). The combined extracts were dried over
anhydrous MgSO4 and concentrated under a vacuum. The crude
oil was purified by flash chromatography to give 8 (0.388 g,
mp 137–138 °C. ½a D25
ꢂ
¼ ꢀ5:7 (c 3, ethyl acetate). 1H NMR (CDCl3)
d 2.10–2.21 (m, 1H), 2.28–2.38 (m, 1H), 2.41–2.49 (m, 1H), 2.55–
2.65 (m, 1H), 3.01 (dd, J1 = 10.1 Hz, J2 = 2.6 Hz, 1H), 3.53–3.61 (m,
2H), 3.72 (dd, J1 = 8.6 Hz, J2 = 2.0 Hz, 1H), 3.86–3.98 (m, 2H), 4.04
(dd, J1 = 12.2 Hz, J2 = 5.6 Hz, 1H), 4.18 (dd, J1 = 12.2 Hz, J2 = 5.7 Hz,
1H), 4.95–5.08 (m, 3H), 5.23 (dd, J1 = 10.3 Hz, J2 = 1.3 Hz, 1H),
5.35 (dd, J = 17.2 Hz, J = 1.6 Hz, 1H), 5.53–5.65 (m, 1H), 5.95–
6.06 (m, 1H), 7.18–7.35 (m, 9H), 7.45–7.58 (m, 6H). MS m/z (rel-
ative intensity) 498 (M+, 0.03), 421 (0.2), 283 (0.3), 259 (1), 243
(100), 228 (3), 199 (1), 165 (24), 105 (3), 85 (3), 77 (2), 41 (7).
HRMS (FAB) calcd for (C32H34O5 + Na+): 521.2304, found:
521.2313. IR (KBr) 3385, 2983, 1764, 1449, 1184, 1100, 1039,
1.05 mmol) in 81% yield, ½a D25
ꢂ
¼ þ56:0 (c 1.05, EtOH). 1H NMR
(CDCl3) d 0.09 (s, 3H), 0.12 (s, 3H), 0.92 (s, 9H), 2.15–2.21 (m,
1H), 2.31 (d, J = 12.3 Hz, 1H), 2.55 (dd, J1 = 3.5 Hz, J2 = 4.8 Hz, 1H),
3.41 (dd, J1 = 4.8 Hz, J2 = 8.6 Hz, 1H), 3.91 (dd, J1 = 8.6 Hz,
J2 = 3.2 Hz, 1H), 4.01 (t, J = 4.5 Hz, 1H), 4.05–4.11 (m, 2H), 4.16
(dd, J1 = 12.9 Hz, J2 = 5.3 Hz, 1H), 4.27 (dd, J1 = 12.9 Hz, J2 = 5.5 Hz,
1H), 4.65 (t, J = 5.0 Hz, 1H), 5.14–5.22 (m, 2H), 5.23–5.30 (m, 2H),
5.83–5.95 (m, 2H). 13C NMR (CDCl3) d 179.46, 139.29, 139.20,
122.19, 121.64, 85.87, 77.58, 77.43, 77.02, 76.52, 48.92, 34.16,
30.21, 22.50, ꢀ0.09, ꢀ0.30. HRMS (EI) calcd for C19H32O5Si:
368.2019, found: 368.2031. IR (KBr) 3081, 2930, 2857, 1795,
702 cmꢀ1
.
4.6. (4R,5S,6R)-5,6-Diallyloxy-7-hydroxy-heptano-1,4-lactone 6
Compound 5 (2.00 g, 4.01 mmol) was dissolved in a mixed
solvent of acetonitrile (20 mL) and water (0.5 mL). After the solu-
tion was cooled to 0 °C by an ice bath, concentrated hydrochloric
acid (0.1 mL) was added, the mixture was stirred at 0 °C for 4 h,
and the reaction was traced by TLC. After the reaction was com-
plete, sodium carbonate (2.00 g, 18.86 mmol) powder was added,
and stirring was continued for 0.5 h. The solid was filtered and
rinsed with acetonitrile, the filtrate was evaporated in a vacuum
to yield a crude product, which was purified by flash chromatog-
1645, 1462, 1362, 1255, 1117, 987, 836, 778 cmꢀ1
.
4.9. (1S,2S,3R,4S,5R)-Methyl 2,3,4,5-tetraacetoxy-cyclohexane-
carboxylate 9
raphy to give 6 (1.00 g, 3.92 mmol) in 98% yield, ½a D25
¼ ꢀ37:5 (c
ꢂ
1.4, ethyl acetate). 1H NMR (CDCl3) d 2.04–2.13 (m, 1H), 2.21–
2.31 (m, 1H), 2.36–2.44 (m, 1H), 2.53–2.62 (m, 1H), 3.44 (dd,
J1 = 8.5 Hz, J2 = 2.0 Hz, 1H), 3.48–3.54 (m, 1H), 3.61 (dd,
J1 = 12.2 Hz, J2 = 2.2 Hz, 1H), 3.84 (dd, J1 = 12.2 Hz, J2 = 3.0 Hz,
1H), 3.96–4.10 (m, 3H), 4.24 (dd, J1 = 12.6 Hz, J2 = 5.1 Hz, 1H),
4.72–4.79 (m 1H,), 5.12 (dd, J1 = 10.6 Hz, J2 = 10.9 Hz, 2H),
5.17–5.28 (m, 2H), 5.77–5.91 (m, 2H). HRMS (EI) calcd for
C13H20O5: 256.1311, found: 256.1343. IR (KBr) 3474, 2922,
1774, 1645, 1460, 1423, 1361, 1186, 1097, 1038, 926, 655,
To a solution of 8 (0.299 g, 0.81 mmol) in methanol (15 mL),
was added hydrofluoric acid (48% in water, 51 mg, 1.22 mmol)
and palladium chloride (36 mg, 0.20 mmol), and the reaction mix-
ture was heated at reflux and stirred for 2 h. The solvent was re-
moved off by evaporation under a reduced pressure to give a
residue. Dichloromethane (30 mL), pyridine (1.28 g, 16.2 mmol),
and DMAP (20 mg, 0.164 mmol) were added. The resulting mixture
was stirred and cooled to 0 °C, and acetic anhydrous (0.835 g,
8.2 mmol) was then added. After addition, stirring was continued
at room temperature overnight. The reaction was quenched with
530 cmꢀ1
.

82
S.-L. Liu et al. / Tetrahedron: Asymmetry 20 (2009) 78–83
aqueous solution of sodium carbonate (10%, 20 mL). The organic
layer was separated, and aqueous layer was extracted twice with
dichloromethane (30 mL ꢁ 2), combined extracts were wished
with dilute aqueous HCl solution (2 M, 30 mL), and then dried over
anhydrous MgSO4. Evaporation of the solvent gave crude product,
which was purified by flash chromatography to furnish 9 (0.262 g,
and NaBH4 (125 mg, 3.30 mmol) was added. After stirring at 0 °C
for 30 min, water (20 mL) was added, and the aqueous solution
was extracted twice with dichloromethane (2 ꢁ 20 mL). The ex-
tracts were combined and dried over anhydrous MgSO4. Evapora-
tion of the solvent gave a crude product, which was purified by
chromatography to furnish 12 (0.38 g, 1.02 mmol) in 94% yield,
0.70 mmol) in 86% yield, ½a D20
ꢂ
¼ ꢀ3:3 (c 1.5, CHCl3). 1H NMR
½
a 2D5
ꢂ
¼ ꢀ38:3 (c 1.2, EtOH). 1H NMR (CDCl3) d 0.06 (s, 3H), 0.07
(CDCl3) d 1.94–2.05 (m, 1H), 1.99 (s, 3H), 2.05 (s, 3H), 2.09 (s,
3H), 2.13 (s, 3H), 2.32 (ddd, J1 = 13.5 Hz, J2 = 3.2 Hz, J3 = 4.0 Hz,
1H), 3.03 (ddd, J1 = 13.4 Hz, J2 =3.4 Hz, J3 = 4.1 Hz, 1H), 3.67 (s,
3H), 5.08–5.19 (m, 2H), 5.34–5.42 (m, 2H). 13C NMR (CDCl3) d
170.77, 170.25, 169.98, 168.93, 168.91, 69.96, 68.84, 68.60, 68.34,
52.28, 39.89, 26.51, 20.93, 20.73, 20.67, 20.64. HRMS (EI) calcd
for C16H22O10: 374.1213, found: 374.1207.
(s, 3H), 0.87 (s, 9H), 1.22 (dd, J1 = 2.3 Hz, J2 = 12.0 Hz, 1H), 1.42
(dd, J1 = 12.1 Hz, J2 = 24.2 Hz, 1H), 1.72 (ddd, J1 = 12.2 Hz,
J2 = 5.6 Hz, J3 = 5.5 Hz, 1H), 3.45 (dd, J1 = 9.5 Hz, J2 = 2.8 Hz, 1H),
3.49–3.58 (m, 2H), 3.67 (dd, J1 = 3.5 Hz, J2 = 3.4 Hz, 1H), 3.84–3.93
(m, 1H), 3.97 (dd, J1 = 12.6 Hz, J2 = 5.9 Hz, 1H), 4.01–4.18 (m, 4H),
5.17 (d, J = 10.3 Hz, 2H), 5.21–5.32 (m, 2H), 5.81–5.97 (m, 2H).
13C NMR (CDCl3) d 135.23, 134.78, 117.41, 117.25, 81.65, 75.93,
72.01, 70.65, 69.08, 68.49, 63.79, 37.94, 26.61, 25.73, 17.90,
ꢀ4.44, ꢀ5.19. HRMS (EI) calcd for C19H36O5Si: 372.2332, found:
372.2340.
4.10. (ꢀ)-Methyl 3,4,5-O-triacetyl shikimate 10
To a solution of 9 (0.260 g, 0.69 mmol) in dichloromethane
(10 mL), was added 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU,
0.530 g, 3.48 mmol). The reaction mixture was then stirred at room
temperature for 6 h. After the reaction was complete, dichloro-
methane (20 mL) and dilute aqueous HCl solution (2 N, 25 mL)
were added, and stirring was continued for 15 min. The organic
layer was separated, and the aqueous layer was extracted once
more with dichloromethane (20 mL). The extracts were combined
and dried over anhydrous MgSO4. Evaporation of the solvent pro-
duced a crude product, which was purified by flash chromatogra-
4.13. (1R,2S,3R,4S,5R)-5-Acetoxymethyl-1,4-diacetoxy-2,3-
diallyloxy-cyclo-hexane 13
A solution of 12 (0.12 g, 0.32 mmol) in THF (6 mL) was cooled to
0 °C, TBAFꢃ3H2O (205 mg, 0.65 mmol) was added, and stirring was
then continued for 3 h. After the solvent was removed, the residue
was allowed to dissolve in dichloromethane (6 mL). Pyridine
(0.515 g, 6.51 mmol), DMAP (7.9 mg, 0.065 mmol), and Ac2O
(0.34 g, 3.33 mmol) were added at 0 °C, the mixture was then stir-
red at room temperature overnight. Dichloromethane (25 mL) and
aqueous solution of dilute HCl (1 M, 15 mL) were added, and after
that the mixture was well stirred for 15 min; the organic phase
was separated and washed with saturated aqueous NaHCO3 solu-
tion (10 mL). The organic phase was dried and concentrated to give
a crude product, which was purified by chromatography to pro-
phy to give 10 (0.180 g, 0.57 mmol) in 83% yield, ½a D25
¼ ꢀ172:4
ꢂ
(c 0.47, CHCl3) [lit.9c
½
a 2D6
ꢂ
¼ ꢀ173 (c 0.47, CHCl3)]. 1H NMR (CDCl3)
d 2.04 (s, 3H), 2.06 (s, 3H), 2.07 (s, 3H), 2.43 (dd, J1 = 18.5 Hz,
J2 = 5.2 Hz, 1H), 2.92 (dd, J1 = 18.4 Hz, J2 = 4.8 Hz, 1H), 3.77 (s,
3H), 5.22–5.32 (m, 2H), 5.71–5.79 (m, 1H), 6.75 (d, J = 4.0 Hz,
1H). 13C NMR (CDCl3) d 169.92, 169.81, 169.82, 165.86, 132.69,
131.18, 67.70, 66.75, 66.00, 52.18, 28.50, 20.94, 20.71, 20.68. HRMS
(EI) calcd for C14H18O8: 314.1002, found: 314.1003.
duce 13 (109 mg, 0.28 mmol) in 88% yield, ½a D25
¼ ꢀ43:7 (c 1.1,
ꢂ
MeOH). 1H NMR (CDCl3) d 1.38 (dd, J1 = 12.4 Hz, J2 = 24.3 Hz, 1H),
1.95–2.05 (m, 1H), 2.03 (s, 3H), 2.05 (s, 3H), 2.06 (s, 3H), 2.42–
2.53 (m, 1H), 3.49 (dd, J1 = 2.9 Hz, J2 = 10.0 Hz, 1H), 3.80–3.87 (m,
2H), 4.01 (dd, J1 = 10.9 Hz, J2 = 8.2 Hz, 1H), 4.08 (d, J = 5.5 Hz, 2H),
4.13 (dd, J1 = 12.9 Hz, J2 = 5.7 Hz, 1H), 4.22 (dd, J1 = 12.9 Hz,
J2 = 5.6 Hz, 1H), 5.06–5.20 (m, 4H), 5.21–5.32 (m, 2H), 5.82–5.93
(m, 2H). 13C NMR (CDCl3) d 170.85, 170.28, 169.85, 134.90,
134.67, 117.33, 116.83, 77.92, 74.52, 72.54, 71.70, 71.34, 69.65,
63.76, 32.88, 27.91, 21.21, 20.86, 20.78. HRMS (EI) calcd for
C19H28O8: 384.1784, found: 384.1784.
4.11. (ꢀ)-Methyl shikimate 1
To a solution of 10 (0.302 g, 0.96 mmol) in absolute methanol
(6 mL), was added two drops of concentrated ammonium hydrox-
ide. The solution was heated at reflux and stirred at this tempera-
ture for 5 h. The solution was then concentrated to dryness to give
crude pale yellow solid, which was crystallized in ethyl acetate to
afford compound 1 (0.166 g, 0.88 mmol) in 92% yield, mp 115–
116 °C (lit.9c 115–116.5 °C), ½a 2D5
ꢂ
¼ ꢀ131:5 (c 0.75, EtOH) [lit.9c
½
a 2D5
ꢂ
¼ ꢀ132 (c 0.75, EtOH)]. 1H NMR (D2O)
d
2.16 (dd,
4.14. (1R,2S,3R,4S,5R)-1,2,3,4-O-Tetraacetyl-5-acetoxymethyl-
cyclohexane-1,2,3,4-tetrol 14
J1 = 18.4 Hz, J2 = 4.8 Hz, 1H), 2.68 (dd, J1 = 18.4 Hz, J2 = 5.2 Hz, 1H),
3.69 (s, 3H), 3.64–3.73 (m, 1H), 3.93–3.99 (m, 1H), 4.37 (t,
J = 4.0 Hz, 1H), 6.74 (d, J = 4.0 Hz, 1H). 13C NMR (D2O) d 168.92,
137.03, 129.50, 70.99, 66.52, 65.71, 52.49, 30.25. HRMS (EI) calcd
for C8H12O5: 188.0685; found: 188.0688.
Compound 13 (160 g, 0.416 mmol) was dissolved in methanol
(8 mL), PdCl2 (18.5 mg, 0.104 mmol) was added, and the mixture
was heated at reflux. After stirring was continued at reflux for
2 h, methanol was removed under a reduced pressure. Dichloro-
methane (15 mL), pyridine (0.66 g, 8.32 mmol), and DMAP
(10.2 mg, 0.084 mmol) were added, the mixture was cooled to
0 °C, and Ac2O (0.85 g, 8.32 mmol) was added. The mixture was
then allowed to warm to room temperature and stirred overnight.
Dichloromethane (25 mL) and aqueous solution of dilute HCl (1 M,
25 mL) were added, and after that the mixture was well stirred for
15 min, the organic phase was separated and washed with satu-
rated aqueous NaHCO3 solution (20 mL). The organic phase was
dried and concentrated to give a crude product, which was purified
by chromatography to produce compound 14 (141 mg,
4.12. (1R,2S,3S,4S,5R)-4-(tert-Butyldimethylsilyloxy)-2,3-diallyloxy-
5-hydroxy-methyl cyclohexanol 12
A solution of compound 8 (0.40 g, 1.08 mmol) in dichlorometh-
ane (20 mL) was stirred and cooled to ꢀ78 °C by a dry-ice bath. A
solution of DIBAL-H (1 M, 10.8 mL, 10.80 mmol) in toluene was
added within 5 min. After addition, the mixture was stirred at
ꢀ78 to ꢀ20 °C for 2 h. The reaction was quenched with aqueous
HCl solution (1 M, 13 mL). The mixture was allowed to warm to
room temperature, then aqueous layer was separated and ex-
tracted twice with dichloromethane (2 ꢁ 20 mL). The extracts were
combined and dried over anhydrous MgSO4. Removal of the sol-
vent under reduced pressure produced a residue that was allowed
to dissolve in methanol (10 mL). The solution was cooled to 0 °C,
0.362 mmol) in 87% yield, ½a D25
ꢂ
¼ ꢀ22:3 (c 1, CHCl3). 1H NMR
(CDCl3) d 1.46–1.56 (m, 1H), 1.93 (s, 3H), 1.95–2.05 (m, 1H), 1.98
(s, 6H), 2.06 (s, 6H), 2.33–2.43 (m, 1H), 3.80 (dd, J1 = 6.5 Hz,
J2 = 11.0 Hz, 1H), 3.98 (dd, J1 = 8.3 Hz, J2 = 11.1 Hz, 1H), 5.02–5.05

S.-L. Liu et al. / Tetrahedron: Asymmetry 20 (2009) 78–83
83
(m, 1H), 5.05–5.16 (m, 2H), 5.26–5.35 (m, 1H). 13C NMR (CDCl3) d
170.70, 170.31, 170.03, 169.26, 169.10, 70.40, 68.96, 68.36, 68.17,
63.20, 33.60, 27.56, 20.97, 20.78, 20.74, 20.71, 20.68. HRMS (EI)
calcd for C17H24O10: 388.1369, found: 388.1341.
4. Smissman, E. E.; Suh, J. T.; Oxman, M.; Daniels, R. J. Am. Chem. Soc. 1959, 81,
2909.
5. Jiang, S.; Singh, G. Tetrahedron 1998, 54, 4697.
6. (a) Koreeda, M.; Ciufolini, M. A. J. Am. Chem. Soc. 1982, 104, 2308; (b) Koreeda,
M.; Teng, K.; Murata, T. Tetrahedron Lett. 1990, 31, 5997; (c) Koreeda, M.; Jung,
K.-Y.; Ichita, J.-J. J. Chem. Soc., Perkin Trans. 1 1989, 2129; (d) Ogawa, S.; Aoki, Y.;
Takagaki, T. Carbohydr. Res. 1987, 164, 499; (e) Barlett, P. A.; McQuaid, L. A. J.
Am. Chem. Soc. 1984, 106, 7854; (f) Choy, W.; Reed, L. A. J. Org. Chem. 1983, 48,
1137; (g) Masamune, S.; Reed, L. A. J. Org. Chem. 1983, 48, 4441; (h) Takahashi,
T.; Namiki, T. Chem. Pharm. Bull. 1988, 36, 3213; (i) Pawlak, J. L.; Berchtold, G. A.
J. Org. Chem. 1987, 52, 1765; (j) Kamikubo, T.; Ogasawara, K. Chem. Lett. 1996,
987–988; (k) Hiroya, K.; Ogasawara, K. Chem. Commun. 1998, 2033; (l)
Ogasawara, K.; Yoshida, N. Org. Lett. 2000, 2, 1461.
7. (a) Coblens, K. E.; Muralidharan, V. B.; Ganem, B. J. Org. Chem. 1982, 47, 5041;
(b) Ikota, N.; Ganem, B. J. Am. Chem. Soc. 1978, 100, 351; (c) Birch, A. J.; Kelly, L.
F.; Weerasuria, D. V. J. Org. Chem. 1988, 53, 278; (d) Johnson, C. R.; Adams, J. P.;
Collins, M. A. J. Chem. Soc., Perkin Trans. 1 1993, 1; (e) Johnson, C. R.; Ple, P. A.;
Adams, J. P. J. Chem. Soc., Chem. Commun. 1991, 1006; (f) Takeuchi, M.;
Taniguchi, L.; Ogasawara, K. Synthesis 2000, 1375.
8. (a) Dangschat, T.; Fischer, M. Biochim. Biophys. Acta 1950, 4, 199; (b) Grewe, R.;
Buttner, H.; Burmeister, G. Angew. Chem. 1957, 69, 61; (c) Shinada, T.; Yoshida,
Y.; Ohfune, Y. Tetrahedron Lett. 1998, 39, 6027; (d) Alves, C.; Barros, M. T.;
Maycock, C. D., et al Tetrahedron 1999, 55, 8443.
9. (a) Bestmann, H. J.; Heid, H. A. Angew. Chem., Int. Ed. Engl. 1971, 10, 336; (b)
Suami, T.; Tadano, K.; Ueno, Y.; Iimura, Y. Chem Lett. 1985, 37; (c) Tadano, K.;
Ueno, Y.; Iimura, Y.; Suami, T. J. Carbohydr. Chem. 1987, 6, 245; (d) Fleet, G. W.
J.; Shing, T. K. M.; Warr, S. M. J. Chem. Soc., Perkin Trans. 1 1984, 905; (e) Mirza,
S.; Vasella, A. Helv. Chim. Acta 1984, 67, 1562; (f) Mirza, S.; Harvey, J.
Tetrahedron Lett. 1991, 32, 4111; (g) Jiang, S.; Mekki, B.; Singh, G. Tetrahedron
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4.15. 5a-Carba-b-D-gulopyranose [(1R,2S,3R,4S,5R)-5-hydroxy-
methyl-cyclo-hexane-1,2,3,4-tetrol] 11
To a solution of compound 14 (92 mg, 0.237 mmol) in methanol
(10 mL), was added concentrated ammonia (0.20 mL). The mixture
was stirred at room temperature for 12 h, and then was heated at
reflux. After stirring was continued at reflux for 16 h, methanol was
removed under a reduced pressure to produce a glassy solid that
was dried in vacuum for 5 h to afford the title compound 11
(41.4 mg, 0.232 mmol) in 98% yield,15,20
½
a 2D5
ꢂ
¼ ꢀ58:3 (c 0.4,
MeOH). 1H NMR (D2O) d 1.23–1.33 (m, 1H), 1.71–1.79 (m, 1H),
1.95–2.03 (m, 1H), 3.45–3.53 (m, 1H), 3.56–3.63 (m, 2H), 3.68–
3.77 (m, 1H), 3.91–3.96 (m, 2H). 13C NMR (D2O) d 72.95, 72.61,
69.71, 69.05, 62.39, 36.47, 29.20. HRMS (EI) calcd for C7H14O5:
178.0841, found: 178.0853.
Acknowledgments
10. Inch, T. D. Tetrahedron 1984, 40, 3161.
We thank the Chinese National Science Foundation (No. A-
20172015) and Shanghai Educational Development Foundation
(The Dawn Program: No. 03SG27) for the financial support of this
work.
11. (a) Mukaiyama, T.; Narasaka, K.; Banno, K. Chem. Lett. 1973, 1011; (b)
Mukaiyama, T.; Banno, K.; Narasaka, K. J. Am. Chem. Soc. 1974, 96, 7503; (c)
Mukaiyama, T. Org. React. 1982, 28, 203.
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13. Corey, E. J.; Suggs, J. W. Tetrahedron Lett. 1975, 16, 2647.
14. Dess, D. B.; Martin, J. C. J. Am. Chem. Soc. 1991, 113, 7277.
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