The first asymmetric total synthesis of (+)-coriandrone A and B

Article abstract of DOI:10.1039/c3ob41497c

The first enantioselective total synthesis of (+)-coriandrone A and B, two bioactive natural products, has been achieved in 10 steps and 11 steps starting from commercially available methyl 2-hydroxy-4-methoxybenzoate. Key reactions include a Claison rearrangement, a Shi-type epoxidation-cyclization sequence and ortho-metallation of t-butylbenzamides with (S)-(-)-propylene oxide reaction.

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Communications
The First Asymmetric Total Synthesis of (+)-Coriandrone A and B
Wenjing Wang, Jijun Xue,* Tian Tian, Yingdong Jiao and Ying Li*
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
DOI: 10.1039/b000000x
construction of it is a difficult task. Second, the assembly of the
chiral center in the dihydrobenzofuran/3-hydroxybenzopyan is
also a challenging task because of the bulky tertiary alcohol
moiety.6 To the best of our knowledge, there are no reports
40 concerning the synthesis of coriandrone A or B until now.
Shi’s epoxidation sequence7 and ortho-metallation of t-
butylbenzamide with (S)-(–)-propylene oxide reaction8 are
versatile tools for the introduction of chiral alcohol. In particular,
Shi’s dioxirane-catalysed tandem epoxidation/cyclisation
5
The first enantioselective total synthesis of (+)-coriandrone A
and B, two bioactive natural products, has been achieved in
10 steps and 11 steps starting from commercially available
methyl 2-hydroxy-4-methoxybenzoate. Key reactions include
Claison rearrangement, a Shi-type epoxidation-cyclization
10 sequence and ortho-metallation of t-butylbenzamides with (S)-
(–)-propylene oxide reaction.
Medicinal plants and natural products continue to play significant
roles in drug discovery and development.1 The medicinal plant
Coriandrum sativum has been used in traditional Chinese
15 medicine as a remedy for many ailments, such as indigestion and
abdominal distention. Coriander has also been used to treat acne
and anxiety.2 The essential oil and various extracts from
45 sequence is an efficient tool for heterocycle construction.7a,
7g
Herein we successfully employed this sequence in constructing
the two alcohol moieties of coriandrone A and B. Moreover, to
improve synthesis efficiency and potential analogue diversity, we
decided to adopt diversity-oriented synthesis, a technique for
50 transforming a collection of simple and similar starting materials
into a collection of more complex and diverse products.9 And the
divergent synthesis of both coriandrone A and B from the same
epoxide could be accomplished under either basic or acidic
conditions (Scheme1).
coriander
exhibit
antibacterial,3a
anticancerous
and
antimutagenic,3b antioxidant,3c, 3d, 3e and antidiabetic activities.3f
20
Figure 1. Structures of Coriandrin, Dihydrocoriandrin, and Coriandrone
A and B.
Coriandrone A and B were isolated from the aerial parts of
Coriandrum sativum along with other two known isocoumarins,
25 coriandrin and dihydrocoriandrin (Figure 1). Their structures
were determined by spectroscopic means and X-ray
crystallography.4 Emerging interest in the unique structural,
together with the considerable biological activity5 of coriandrone
A and B, have provided the motivation to complete an adaptable
30 and scalable total synthesis of these compounds. Despite the fact
that (+)-coriandrone A and B have a relatively simple structure,
the development of an efficient approach to this molecule and a
general route for the preparation of its analogues faces several
challenges. First, coriandrone A/B contains a regioselective and
35 highly functionalized benzene ring, the versatile and facile
55
Scheme 1. Retrosynthetic analysis of Coriandrone A and B
.
Our synthetic approach for the preparation of coriandrone A
and B is outlined in Scheme 1. coriandrone A and B share a same
8-hydroxy-3-methyl-3,4-dihydro-isocoumarin structural, so we
60 hypothesized that the isocoumarin motif of them could be
synthesized via ortho-metallation of t-butylbenzamides 1a and 1b
with (S)-(-)-propylene oxide reaction, then deprotection and
cyclization separately. It was also envisioned that the
constructions of 3-hydroxybenzopyan 1b and 2-Substituted
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dihydrobenzofuran 1a could be built by treatment of the common 45 followed by base treatment14 and formed the desired product in
chiral epoxide 2 under acidic and basic conditions respectively.
The common chiral epoxide 2 could be obtained by asymmetric
epoxidation of methyl 2-hydroxy-4-methoxy-3-(3-methylbut-2-
enyl)benzoate 3. Further analysis indicated that methyl 2-
hydroxy-4-methoxybenzoate 4 should be an ideal material.
49% yield for the two steps in this one-pot procedure.
5
Scheme 3. Synthesis of the epoxide 8 through the Sharpless asymmetric
dihydroxylation reaction. Ms=methanesulfonyl.
50
To obtain better enantioselectivity in the conversion of 6 to 2,
we explored different asymmetric epoxidations. Shi’s catalyst
system was attempted, which could be anticipated approach of
the dioxirane catalyst from the bottom face of osthenol and gave
the expoxide product with the desired configuration (as shown in
55 figure 2).
Scheme 2. Synthesis of the prenylated derivative 6. DBU=1,8-diaza-
bicyclo-[5.4.0]undec-7-ene,
10 dimethylformamide.
BnBr=Benzyl
bromide,
DMF=N,N-
The first key conversion in our synthesis was the introduction
of the prenyl fragment to compound 4 (Scheme 2). To prenylate
compound 4, several methods were explored. Direct prenylation
of 4 with prenyl bromide and potassium carbonate did not afford
15 the C-prenylation product 3. Reacting 4 with 1,1-dimethyl-2-
propenyl isobutyl carbonate and tetrakis(triphenylphosphine)-
palladium (Pd(PPh3)4) in THF and DMF did not yield the desired
product 5′.10a Furthermore, the reaction of 4 with 1,1-dimethyl
Figure 2. Anticipated selectivity for the Shi’s epoxidation.
However, Shi’s common epoxidation condition,15 using
OXONE, catalyst 17 and buffer A, offered the same
60 configuration of product but little improvement on ee value and
yield (entry 1 in table 1). Therefore, other ketone catalysts were
evaluated for this conversion. In 2011, Yasumasa Hamada and
co-workers reported a Shi’s ketone 18 for the enantioselective
epoxidation of O-silyl-protected ortho-allylphenols15c that was
65 more effective than ketone 17. Epoxidation of O-Bn-Protected
allylphenol 6 using Yasumasa Hamada´s conditions afforded
better results than Shi´s conditions, producing 8 in 76% ee and
55% yield (entry 2 in table 1). Notably, this reaction was
sensitive to temperature. When the temperature was increased
70 from -10 to 0 °C, the reaction time was shorter. Additionally, the
ee value improved slightly (79%), and the yield increased
dramatically (97%) (entry 3 in table 1). As we know, this kind of
dioxiranes is sensitive to pH;7h-7k therefore, buffer solutions were
screened. In buffer B, a slightly acidic solution, the yield
75 decreased from 97% to 54%, and the ee value decreased from
79% to 72%, demonstrating that buffer B was not suitable (entry
4 in table 1). After additional investigation, we found optimised
conditions: OXONE, catalyst 18, buffer A, 0 °C and 16 h.
10c]
propargyl chloride afforded only a small quantity of 5.[10b,
20 Alkylation of 4 with 1,1-dimethylprop-2-ynyl methyl carbonate
15 as the electrophile, DBU as the base, and CuCl2 as the catalyst
10e
produced the best results.10d,
Compared with similar
reactions,10d, 10e Compared to the reported similar reaction, the
conversion of 4 to 5 needs a wilder condition for the existing of
25 hydrogen bond between OH group and carbonyl group.
Successive round of alkylation of 4 proceeded with these reagents
o
o
at 0 C for 1 h and 38 C for 6 h, and propargyl ether 5 was
formed in a 68% yield. The Claison rearrangement is a versatile
method for C-allylation, so it was used to convert propargyl ether
30 5 to C-allyl product 3. The compound 5 was reduced using
Lindlar catalyst11 and subsequently heated in refluxing DMF to
produce Claisen adduct 3 successfully in an 84% yield for two
steps. The compound 3 was treated subsequently with benzyl
bromide in the presence of potassium carbonate and catalytic
35 amounts of potassium iodide in acetone and benzyl-protected
prenylated derivative 6 generated in a 95% yield.
These conditions were then applied in the epoxidation of 3.
80 When compound 3 was treated with ketone 18 and OXONE in
buffer A for 22 h, epoxide 2 was produced in 95% yield and 74%
ee. Surprisingly the chiral epoxide 2 converted to 9a upon
Based on our previous success in synthesising naturally
occurring benzopyrans,12 we attempted to produce the epoxide 8
via
40 However, the Sharpless asymmetric dihydroxylation of 6 using
commercially available AD-mix-
13 afforded diol (S)-7 in 78%
a Sharpless asymmetric dihydroxylation (Scheme 3).
extended reaction time. Hence,
a tandem procedure was
α
employed: compound 2 was treated with ketone 18 in buffer A
85 for 36 h to obtain compound 9a in 92% yield and 77% ee
(Scheme 4). This transformation depended primarily on the basic
yield and only 45% ee (determined by chiral HPLC). The
subsequent conversion of diol 7 to epoxide 8 proceeded by
selective mesylation using methanesulfonyl chloride and pyridine
2
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17
buffer solution, which converted the phenol to phenolate and 40 yield.16,
Deprotection of 13b with TBAF afforded the
therefore permitted nucleophilic attack on the epoxide.
corresponding diol 14b. 14b was treated with 50% aq. NaOH in
ethanol solution,18 and the base-mediated intramolecular
nucleophilic substitution led to coriandrone B in 88% yield over
the two steps.
Table 1. Reaction conditions of the Shi asymmetric epoxidations
Entry
Olefin
Catalyst
17
Buffer
T
Time
5.5 h
24 h
16 h
17 h
22 h
Yield %
8 (67)
8 (55)
8 (97)
8 (54)
2 (95)
ee %
47
1
2
3
4
5
6
6
6
6
3
A
A
A
B
A
-10 °C
-10 °C
0 °C
18
76
18
79
18
0 °C
72
18
0 °C
74
5
Buffer A: 0.05 M Na2B4O7·10H2O in 4 × 10-4 M aqueous Na2(EDTA); buffer B: 0.1
M aqueous KOH/0.1 M aqueous KH2PO4/H2O = 1/9/8, pH = 6.
45
Scheme 4. Total Synthesis of (+)-Coriandrone A TBTU = N,N,N',N'-
tetramethyl-O-(benzotriazol-1-yl)uronium tetrafluoroborate, DIPEA
Having the compound 9a in hand, the stage was set for
construction of the dihydro-isocoumarin moiety (Scheme 4). Two
primary challenges existed for this synthesis: the introduction of
=
N,N-diisopropylethyl-amine, TMEDA tetramethylethylenediamine,
=
50 TBSOTf = trimethylsilyl trifluoro-methanesulfonate, TBAF = tetrabutyl
ammonium fluoride.
10 the three-carbon chiral centre and the cyclisation to the
isocoumarin. For the former, (S)-(–)-propylene oxide was
considered as a reagent. For the latter, the base-mediated
nucleophilic displacement of t-butyl amide by the appropriate
alcohol was designed as the key strategy.
15
To introduce the tert-butyl amide, the intermediate 9a was
first converted into the corresponding acid 10a by treatment with
LiOH.2H2O in THF:MeOH:H2O (3:1:1). Condensation of acid
10a and tert-butylamine with TBTU and DIPEA afforded the N-
tert-butyl benzamide 11a. The desired intermediate 12a was
20 easily obtained from 11a by reacting with TBSOTf and 2,6-
lutidine in CH2Cl2 at room temperature (91% yield from 9a).
Treating 12a with n-BuLi (3 eq) and TMEDA formed the orange-
coloured dianion, which attacked (S)-(–)-propylene oxide and
17]
afforded the secondary alcohol 13a in 61% yield.[16,
25 Subsequently, a one-pot conversion of 13 to the target product
was attempted with acid. Unfortunately, the deprotected product
did not cyclise. The conversion was therefore performed
stepwise. First, deprotection with TBAF afforded the diol 14a.
We tried forming the
δ-lactone by acid-induced (p-TsOHꢀH2O)
30 intramolecular attack of the alcohol 13a at the amide group but
were unsuccessful.17 However, intramolecular cyclisation under
basic conditions (EtOH/50% aq. NaOH 1:1)18 led to coriandrone
A in 83% yield over both steps.
Scheme 5. Total synthesis of coriandrone B.
The spectroscopic data for the synthesised coriandrone B and
55 A matched the reported spectroscopic data for the natural
product. In addition, the synthesised materials’ optical rotation
values were close to the reported literature values.4 The structure
of Coriandrone B was determined by X-ray crystallography; its
cif data were deposited in the Cambridge Crystallographic Data
60 Centre: CCDC 938907.
Accordingly, the epoxide 2 was converted to compound 9b
35 when treated with formic acid (Scheme 5). Using the same
conditions as in the preparation of 12a from 9a, compound 12b
was synthesised in 93% yield from compound 9b. Treating 12b
with n-BuLi (3 eq) and TMEDA, followed by reaction with (S)-(–
)-propylene oxide, formed the secondary alcohol 13b in 54%
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Figure 3. X-ray crystal structure of Coriandrone B.
In summary, the first asymmetric total synthesis of (+)-
coriandrone B and coriandrone A was accomplished through a
divergent synthesis in 11 and 10 steps, respectively, from
commercially available 4. The synthesis afforded (+)-coriandrone
B and coriandrone A with good enantioselectivity in 21.6% and
26.3% overall yield, respectively. Key reactions included a
Claison rearrangement, a Shi epoxidation-cyclisation sequence,
5
8
10 and a reaction involving ortho-metallation of t-butylbenzamide
with (S)-(–)-propylene oxide. To improve efficiency, successive
reactions without purification and one-pot reactions were used.
This is the first asymmetric synthesis of these two targets, as well
as
a
new application of Shi’s asymmetric tandem
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