Carbohydrate-templated asymmetric Diels-Alder reactions of masked ortho-benzoquinones for the synthesis of chiral bicyclo-[2.2.2]oct-5-en-2-ones

Article abstract of DOI:10.1002/anie.200802693

(Chemical Equation Presented) A sweet addition: Asymmetric [4 + 2] cycloaddition of masked ortho-benzoquinones with various alkenols by using sugars as chiral auxiliaries (see scheme; Bn: benzyl) affords the optically pure bicyclo[2.2.2]oct-5-en-2-ones wi

Full text of DOI:10.1002/anie.200802693

Communications
DOI: 10.1002/anie.200802693
Asymmetric Synthesis
Carbohydrate-Templated Asymmetric Diels–Alder Reactions of
Masked ortho-Benzoquinones for the Synthesis of Chiral Bicyclo-
[2.2.2]oct-5-en-2-ones**
Shun-Yuan Luo, Yeong-Jiunn Jang, Jing-Yuan Liu, Chrong-Shyua Chu, Chun-Chen Liao,* and
Shang-Cheng Hung*
Dedicated to Professor Chi-Huey Wong on the occasion of his 60th birthday
Bicyclo[2.2.2]oct-5-en-2-ones have been widely applied in
natural product synthesis for several decades.[1] Highly
reactive
6,6-dialkoxycyclohexa-2,4-dienones
(namely,
masked o-benzoquinones or MOBs)[2] and their orthoquinol
variants,[3] which can be conveniently generated by oxidation
of the corresponding 2-alkoxy- and 2-alkylphenols in an
alcoholic solvent, are often used for synthesizing the bicyclo-
[2.2.2]oct-5-en-2-ones in racemic form through in situ intra-[4]
or intermolecular[5] Diels–Alder reactions with various dien-
ophiles. However, two major hurdles are frequently encoun-
tered in these studies: avoiding the self-dimerization of the
MOBs[6] and preparing optically pure enantiomers.[7] For
example, oxidative addition of 2-methoxyphenol with meth-
anol led to a MOB intermediate, which immediately self-
dimerized to give the [4+2] cycloadducts. When an allyl or
homoallyl alcohol was used in the reaction, a racemic mixture
of the intramolecular cyclic products was obtained in very low
yield. For the synthesis of the chiral forms, (S)-1-phenyl-
ethanol (1) was initially studied. However, the reaction of 2-
allyloxyphenol (2) with 1 in the presence of PhI(OCOCF3)2,
via the MOB intermediates 3 and 4, furnished diastereomers 5
and 6 in only 15% and 9% yields, respectively (Scheme 1).
We report herein a new and straightforward asymmetric
methodology that involves carbohydrates as chiral auxiliaries
and that tackles these problems.[8]
Scheme 1. Low-yielding reaction of (S)-1-phenylethanol (1) and 2-
allyloxyphenol 2.
Our strategy, as illustrated in Scheme 2, entailed a three-
step protocol. Coupling of the 2,3,4,6-tetra-O-protected
hexopyranose 7[9] with a catechol 8 by Mitsunobu-type
glycosylation[10] could give the phenolic derivative 9. Oxida-
tive assembly of 9 with an alkenyl alcohol 10 would yield the
MOB intermediate 11, which could undergo intramolecular
[4+2] cycloaddition to furnish the adduct 12 in a one-pot
Scheme 2. Three-stepsugar-templated asymmetric synthesis of chiral
bicyclo[2.2.2]oct-5-en-2-ones from catechols. Pg: protecting group; Rm,
Ro: H or alkyl.
[*] Dr. S.-Y. Luo, Dr. Y.-J. Jang, Dr. J.-Y. Liu, Dr. C.-S. Chu,
Prof. Dr. C.-C. Liao, Prof. Dr. S.-C. Hung
Department of Chemistry, National Tsing Hua University
101, Section 2, Kuang-Fu Road, Hsinchu 300 (Taiwan)
Fax: (+886)3-5711082
manner. The glycone part is expected to control the diaste-
reoselective induction at the C6 position of cyclohexadienone
11 and increase the steric hindrance to avoid intermolecular
Diels–Alder dimerization of 11. Once the configuration of the
C6 position in 11 is fixed, the remaining new asymmetric
carbon atoms in compound 12 can be created and controlled
through intramolecular cyclization. Hydrolysis of 12 under
acidic conditions should provide the desired chiral bicyclo-
[2.2.2]oct-5-en-2-one 13 and recover the initial sugar 7 for
recycling and reuse.
E-mail: ccliao@mx.nthu.edu.tw
hung@mx.nthu.edu.tw
[**] This work was supported by the National Science Council of Taiwan
(grant nos.: NSC 95-2113M-007-027-MY3, NSC 95-2113M-007-028-
MY3, NSC 95-2627M-007-002, NSC 95-2323-B-007-005, NSC 96-
2627M-007-002, and NSC 96-2321-B-007-003).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200802693.
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The preparation of the sugar-derived phenols is depicted
in Scheme 3. Coupling of 2,3,4,6-tetra-O-benzyl-d-mannopyr-
anose (14) with catechol (15) by using a combination of
product 18 (77% in 3 steps). Application of this three-step
transformation to compound 22 afforded the a-d-mannosyl
phenol 17 in 70% overall yield.
triphenylphosphine
and
diisopropylazadicarboxylate
Table 1 outlines the conditions and results of the oxidative
coupling of compounds 17–19 and 24 with allyl alcohol
followed by an intramolecular Diels–Alder reaction in a one-
pot manner. In general, the reactions were carried out in two
stages, being initiated at low temperature and continued at
Table 1: One-pot oxidative coupling and intramolecular Diels–Alder
reactions of compounds 17–19 and 24 with allyl alcohol.
Entry SM[a] Oxidant[b]
T1 [8C] T2 [8C] Products (ratio) Yield [%]
1
2
3
4
5
6
7
8
17
17
19
19
24
18
18
18
PhI(OAc)2
PhI(OTFA)2 ꢀ30
PhI(OAc)2
ꢀ30
ꢀ30
RT
RT
RT
RT
0
25/26 (3.5:1)[c]
25/26 (3.6:1)[c]
29/30 (4.3:1)[c]
29/30 (3.1:1)[c]
31/32 (2.7:1)[c]
27/28 (2.8:1)[d]
27/28 (5.1:1)[d]
56
45
53
50
37
61
61
69
PhI(OTFA)2 ꢀ30
PhI(OAc)2
PhI(OAc)2
PhI(OAc)2
PhI(OAc)2
ꢀ30
ꢀ20
ꢀ30
ꢀ30
0
0
reflux 27/28 (7.6:1)[d]
[a] SM: starting material. [b] TFA: trifluoroacetyl. [c] The ratio was
determined by HPLC. [d] The ratio was determined by the yields of
isolated 27 and 28.
higher temperature. The former is expected to induce high
diastereoselectivity during the MOB formation, whereas the
latter is for the completion of the [4+2] cycloaddition. The
initial studies at room temperature or 08C did not give
satisfactory results and the oxidants were insoluble in solvents
below ꢀ408C, so the reaction was first conducted at ꢀ308C
for 6 h and then the temperature was raised to 08C or room
temperature or the mixture was heated to reflux for 16 h.
PhI(OAc)2-oxidized assembly of the per-O-benzylated a-d-
mannopyranosyl phenol 17 with allyl alcohol gave a mixture
of cycloadducts 25 and 26 in 56% yield (Table 1, entry 1).
When PhI(OTFA)2 was used (Table 1, entry 2), a similar ratio
of 25 and 26 was obtained in lower yield (45%). In the cases
with the per-O-benzylated a-d-glucopyranosyl phenol 19
(Table 1, entries 3 and 4), a mixture of the products 29 and
30 was generated in 53% and 50% yields, respectively. The
reaction of the per-O-acetylated b-d-glucopyranosyl phenol
24 with PhI(OAc)2 (Table 1, entry 5) furnished an isomeric
mixture of 31 and 32 in low yield (37%). When the per-O-
benzylated b-d-glucopyranosyl phenol 18 was oxidized at
ꢀ208C and then stirred at 08C (Table 1, entry 6), compounds
27 and 28 were isolated, after column chromatography on
silica gel, in a 2.8:1 ratio. By lowering of the temperature to
ꢀ308C (Table 1, entry 7), the ratio of 27 and 28 could be
improved to 5.1:1. When the latter part of the reaction was
carried out at reflux temperature (Table 1, entry 8), 27 and 28
were obtained in a 7.6:1 ratio and a slightly increased yield.
With this set of optimized conditions in hand, a variety of
alkenyl alcohols were investigated. With (E)-2-buten-1-ol
(33; Table 2, entry 1) the cycloadducts 37 and 38 were isolated
Scheme 3. Reagents and conditions: a) Ph3P, DIAD, THF, 08C!RT,
20 h, 17: 76%, 18: 60%, 19: 19%; b) 2, TMSOTf, CH2Cl2, 08C!RT,
2 d, 63%; c) 2, BF3·OEt2, CH2Cl2, 08C, 18 h, 70%; d) cat. [Pd(PPh3)4],
AcOH, 808C, 3 h, 77%; e) 1. NaOMe, MeOH; 2. NaH, BnBr, DMF;
3. cat. [Pd(PPh3)4], AcOH, 808C, 3 h, 17: 70%, 18: 77%. Bn: benzyl;
TMSOTf: trimethylsilyl trifluoromethanesulfonate; DMF: N,N-dimethyl-
formamide.
(DIAD) in THF afforded the single a-form phenol 17 in
76% yield. Treatment of the d-glucopyranose 16 under
similar conditions furnished the b-glycosylated phenol 18
and its a-anomeric isomer 19 in 60% and 19% yields,
respectively. An alternative approach with the per-O-acetyl-
ated sugars as starting materials was also investigated.
TMSOTf-activated coupling of compound 20 with 2-allylox-
yphenol (2) led to a derivative 22 (63%; recovered 20: 30%).
Similar conditions could not be applied to b-d-glucopyranosyl
pentaacetate (21). In this case, BF3·OEt2 was found to be a
better promoter than TMSOTf, and the desired product, 23,
was obtained in 70% yield. It should be noted that a-d-
glucopyranosyl pentaacetate did not react with 2 in the
presence of acid activators. [Pd(PPh3)4]-catalyzed cleavage of
the allyl group in 23 gave the corresponding phenol 24 in 77%
yield. Deacetylation of compound 23 (NaOMe, MeOH)
followed by per-O-benzylation (NaH, BnBr) provided the 2-
allyloxyphenyl 2,3,4,6-tetra-O-benzyl-b-d-glucopyranoside,
which underwent deallylation to yield the expected phenolic
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Table 2: PhI(OAc)2-oxidized intramolecular [4+2] cycloadditions of
Table 3: Hydrolysis of the per-O-benzylated b-d-glycopyranosyl bicyclo-
compound 18 with various allyl and homoallyl alcohols under opti-
[2.2.2]oct-5-en-2-ones under acidic conditions.
mized conditions.
Entry
Alcohol
Product (yield [%])[a]
Ratio
Entry
SM
Product
Yield [%]
[a]TD[a]
16 [%]
1
2
3
4
33
34
35
36
37 (72)+38 (11)
39 (67)
40 (74)
6.5:1
1
2
3
4
5
6
27
28
37
39
40
41
42
43
44
45
46
47
94
93
87
93
90
86
ꢀ287.5
+257.2
ꢀ300.4
ꢀ288.5
ꢀ311.1
ꢀ345.6
84
81
98
97
86
95
–
–
–
41 (47)
[a] The yields were obtained after purification by column chromatography
on silica gel.
[a] In 10ꢀ1 degcm3 gꢀ1 dmꢀ1 (42: T=248C, c 1.3, CHCl3; 43: T=258C,
c 0.5, CHCl3; 44: T=278C, c 1.4, CHCl3; 45: T=178C, c 1.0, CHCl3; 46:
198C, c 3.3, CHCl3; 47: T=208C, c 1.6, CHCl3).
in 72% and 11% yields, respectively. 2-Methyl-2-propen-1-ol
(34) led to compound 39 (67%) as a single product, and no
other diastereomers were found after purification (Table 2,
entry 2). Similar results were observed in the cases of 3-
methyl-2-buten-1-ol (35; Table 2, entry 3) and 3-methyl-3-
buten-1-ol (36; Table 2, entry 4), which furnished the products
40 and 41 in 74% and 47% yields, respectively.
The structures of compounds 27 and 28 were determined
through the chemical correlation method. First, the config-
urations of compounds 5 and 6 were individually confirmed
by X-ray single-crystal diffraction analyses.[11] Hydrolysis of 5
and 6 in a mixture of 3% aqueous HCl and acetic acid at
1008C gave the corresponding enantiomeric ketones 42
([a]2D4 = ꢀ287) and 43 ([a]2D5 = + 257), respectively (see
Table 3). The preferred configuration of compound 42, as
indicated by the peaks of its 13C NMR spectrum (see the
Supporting Information) at d = 203.4 (ketone) and 96.8 ppm
(hemiketal), is the hemiketalic ketone (not the hydroxy
diketone intermediate A). Similar phenomena have been
reported for other hemiketals.[12] Cleavage of compounds 27
and 28 under similar conditions did not workwell. Finally, a
solution of 0.04n H2SO4 in dioxane/water (2:1) was found to
hydrolyze 27 and 28, and the corresponding products 42 and
43 (Table 3, entries 1 and 2) were obtained in 94% and 93%
yields, respectively. Under these conditions, the initial sugar
16 was also recovered in excellent yield. The cleavages of
compounds 37 and 39–41 were individually carried out
(Table 3, entries 3–6), and the specific rotations of the
obtained products 44–47 exhibit high negative values, which
indicate that their skeletons are similar to that of 27.[13] The
13C NMR spectra of compounds 44–47 also show that their
configurations favor the hemiketal forms (see the Supporting
Information).
ꢀ
possesses a stronger dipole–dipole interaction with the C1’ O
bond in the pyranosyl ring. These effects presumably result in
a preference for conformer 48, which can be attacked by an
alkenyl alcohol from the b face.
In summary, we have developed a three-step synthesis of
optically pure bicyclo[2.2.2]oct-5-en-2-ones in good yields
through carbohydrate-templated asymmetric intramolecular
Diels–Alder reactions of MOBs. The per-O-benzylated sugar
moiety can inhibit self-dimerization of MOBs and induce high
diastereoselectivity. The strategy described herein should
provide access to substituted catechols for the preparation of
highly functionalized chiral bicyclo[2.2.2]oct-5-en-2-ones. A
variety of commercially available d and l sugars could be
applied for the synthesis of either enantiomer.
Experimental Section
General procedure for PhI(OAc)2-oxidized one-pot coupling and
Diels–Alder reaction of compound 18 with various alkenols: PhI-
A mechanism for the induction of high diastereoselectiv-
ity is proposed from examination of the possible reactive
conformations of the oxocarbenium intermediate (48 and 49)
that is presumably formed during the formation of the ketal
with the allylic or homoallylic alcohols. Through the p–p
interaction between the phenyl ring and the cationized
cyclohexadienone, the carbonyl group in 49 not only has a
higher steric hindrance with the anomeric proton but also
(OAc)2 (3.0 equiv) was added to
a solution of compound 18
(1.0 equiv) in anhydrous alkenyl alcohol (100 mL per 1 g of 18) and
CH2Cl2 (20 mL per 1 g of 18) at ꢀ308C under nitrogen. After the
mixture had been stirred for 6 h, the reaction flaskwas gradually
warmed up and the mixture was heated to reflux for 16 h. The solution
was cooled to room temperature, and the reaction was quenched by
saturated aqueous NaHCO3. The whole mixture was extracted three
times with ethyl acetate, and the combined organic layers were
washed with brine, dried over anhydrous MgSO4, filtered, and
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concentrated in vacuo. The residue was purified by flash column
chromatography on silica gel to afford the desired cycloadduct.
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Received: June 8, 2008
Revised: August 3, 2008
Published online: September 16, 2008
Keywords: asymmetric synthesis · benzoquinones ·
.
bicyclic compounds · chiral auxiliaries · cycloaddition
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[11] CCDC 689698 (5) and 689699 (6) contain the supplementary
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free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
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reference [7]) have revealed that chiral compounds with the
same skeletons and different substituent groups result in similar
curve patterns.
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