O-monoacyltartaric acid catalyzed enantioselective conjugate addition of a boronic acid to dienones: Application to the synthesis of optically active cyclopentenones

Article abstract of DOI:10.1021/ol502526y

Enantioselective conjugate addition of styrylboronic acid to dienones was effectively catalyzed by an O-monoacyltartaric acid to afford monostyrylated products with good enantioselectivity. The RCM of the monostyrylated products using the Hoveyda-Grubbs II catalyst afforded optically active cyclopentenones, including a synthetic intermediate of the antitumor agent TEI-9826. The study shows that a diene additive such as 1,6-heptadiene or diallyl ether was essential for the RCM.

Full text of DOI:10.1021/ol502526y

Letter
pubs.acs.org/OrgLett
O‑Monoacyltartaric Acid Catalyzed Enantioselective Conjugate
Addition of a Boronic Acid to Dienones: Application to the Synthesis
of Optically Active Cyclopentenones
Masaharu Sugiura,* Ryo Kinoshita, and Makoto Nakajima
Graduate School of Pharmaceutical Sciences, Kumamoto University, 5-1 Oe-honmachi, Chuo-ku, Kumamoto 862-0973, Japan
*
S Supporting Information
ABSTRACT: Enantioselective conjugate addition of styryl-
boronic acid to dienones was effectively catalyzed by an O-
monoacyltartaric acid to afford monostyrylated products with
good enantioselectivity. The RCM of the monostyrylated
products using the Hoveyda−Grubbs II catalyst afforded
optically active cyclopentenones, including a synthetic
intermediate of the antitumor agent TEI-9826. The study
shows that a diene additive such as 1,6-heptadiene or diallyl ether was essential for the RCM.
ienones 1 (Scheme 1) are readily available via aldol
enantioselective addition of boronic acids or their esters to
17
1
D
condensations between acetone and an aldehyde (R =
enones or enals, respectively. However, the application of these
2
1
1
2
2
R ) or between an enone and an aldehyde (R ≠ R ). The
enantioselective methods to the reaction of multiconjugated
18
corresponding Horner−Wadsworth−Emmons reactions are also
carbonyl compounds 1 has not been explored.
3
,4
applicable to the preparation of 1. As dienones 1 possess two
We recently reported that O-(3,5-di-tert-butylbenzoyl)tartaric
acid 5 (Scheme 2) effectively catalyzed the enantioselective
5
electrophilic sites, the enantioselective conjugate addition of
nucleophiles to 1 is an interesting challenge in regio- and
chemoselectivities (Scheme 1). Several optically active products
Scheme 2. Asymmetric Synthesis of Cyclopentenones via
Enantioselective Conjugate Addition and RCM
2
−4 with stereogenic center(s) at the carbonyl β-position(s) can
be generated. Selective formation of a monoadduct 2 or 2′ may
allow further functionalization because of the remaining
unsaturated bond.
Scheme 1. Conjugate Addition to Dienones 1
Several research groups have focused on this issue and
developed highly enantioselective methods: Cu-catalyzed
1
9
conjugate addition of boronic acids to enones. We therefore
envisaged that the reaction of dienones 1 and styrylboronic acid
(6) might be catalyzed by 5 to selectively furnish monostyrylated
6
addition of dialkylzinc, Pd- or Ni-catalyzed addition of
7
8
allylboronate, Rh-catalyzed conjugate alkynylation, and orga-
20
9
product 2 and that the Ru-catalyzed RCM of diene products 2
might result in optically active cyclopentenones 7, which are an
important structural motif in natural products and pharmaceut-
nocatalyzed cyclization with malononitrile. However, such
examples are limited, despite the availability of dienones 1.
10
Meanwhile, non-metal-promoted conjugate addition of
21
icals, such as the antitumor agent TEI-9826 (Scheme 2).
Helmchen and co-workers described the asymmetric synthesis of
alkenylboronate to enones was first discovered by Suzuki and
1
1
co-workers. More than 15 years later, Chong and co-workers
7
via Ir-catalyzed enantioselective allylic alkylation, vinylation of
described the first organocatalyzed enantioselective method
1
2
utilizing 3,3′-disubstituted BINOL derivatives as catalysts.
13
14
Chiral secondary amines, thiourea derivatives, other BINOL
derivatives, or a resin-supported peptide catalyzed the
Received: August 27, 2014
Published: September 23, 2014
1
5
16
©
2014 American Chemical Society
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dx.doi.org/10.1021/ol502526y | Org. Lett. 2014, 16, 5172−5175

Organic Letters
Letter
the Weinreb amide products, and the RCM of the resulting
electron-rich substituents tighten the transition state for the
2
1b,c
terminal dienes.
Our strategy may be challenging not only
enantio-determining C−C bond formation, leading to high
22
because of the aforementioned selectivity issues in the conjugate
addition to dienones 1 but also because of the low reactivity
expected for conjugated internal dienes 2 in the RCM.
enantioselectivity; the carbonyl oxygen atoms of these enones
have higher Lewis basicity and, thus, coordinate more strongly to
23
the boron atom of the styrylboronic acid/catalyst complex. The
reaction of alkyl group substituted dienone 1f afforded the
desired monostyrylated product 2f with a somewhat higher
enantioselectivity, although a significant amount of bis-styrylated
product 3f was obtained (entry 9).
We initially investigated the reaction of dibenzylidene acetone
1a) and styrylboronic acid (6) with tartaric acid derived catalyst
(
5
under the conditions reported previously (1.2 equiv of 6 and
2.4 equiv of methanol in toluene at 50 °C; Table 1, entry 1). The
We next studied the reaction of unsymmetrical dienones 1g−i
(Scheme 3). The styryl group was chemoselectively added to the
Table 1. Conjugate Addition of Styrylboronic Acid (6) to
a
Symmetrical Dienones 1
Scheme 3. Reaction of Unsymmetrical Dienones
entry
R (1)
Ph (1a)
2
yield (%) ee (%)
3
yield (%)
b
c
1
2a
2a
2a
2a
2b
2c
2d
2e
2f
79
78
81
42
52
68
76
68
56
86
86
86
86
86
90
88
94
89
3a
3a
3a
3a
3b
3c
3d
3e
3f
16
d
2
Ph (1a)
16
9
3
Ph (1a)
e
f
4
Ph (1a)
58
5
6
7
8
9
p-BrC H (1b)
21
12
8
6
4
p-MeOC H (1c)
6
4
2-thienyl (1d)
1-naphthyl (1e)
n-C H (1f)
15
15
8
17
a
Unless otherwise noted, the reaction was conducted using dienone 1
(
0.3 mmol), styrylboronic acid (6) (0.3 mmol), methanol (0.6 mmol),
b
and catalyst 5 (10 mol %) in toluene (1 mL) at 50 °C. With 1 (0.3
mmol), 6 (0.36 mmol), methanol (0.72 mmol), and 5 (0.03 mmol).
c
d
e
dl/meso = 84/16, 99% ee (dl-isomer). For 14 h. With 1 (0.15
mmol), 6 (0.36 mmol), methanol (0.72 mmol), and 5 (0.03 mmol) for
f
24
4
8 h. dl/meso = 87/13, 99% ee (dl-isomer).
alkyl-substituted β-position of 1g with good enantioselectivity.
The reactions of 1h and 1i occurred at the less substituted β-
position to afford adducts 2h and 2i, respectively, with high
chemoselectivities and good enantioselectivities.
desired monostyrylated product (S)-2a was obtained in good
yield with good enantioselectivity and good chemoselectivity
(
selectivity between monoadduct 2 and bis-adduct 3). The bis-
Using the obtained monostyrylated products, the synthesis of
25
styrylated product 3a was obtained as a mixture of the dl- and
meso-isomers (84:16), and the enantiomeric excess of the dl-
isomer was 99% ee. These results indicate that the second
conjugate addition is independent of the first addition with nearly
the same enantioselectivity. To suppress the formation of 3a, the
reaction time was reduced to 14 h, but the chemoselectivity was
not improved (entry 2). Simply decreasing the amount of
boronic acid 6 to 1 equiv suppressed the formation of undesired
cyclopentenones via the RCM was investigated (Table 2). We
26
first examined the Schrodi−Grubbs catalyst in the presence of
1,6-heptadiene because Fukuyama and co-workers have recently
reported in a total synthesis that this combination was effective
27
for the RCM of a diene with low reactivity. Under their
conditions (addition of 5 mol % of catalyst every 24 h; total 15
mol %), the RCM of 2a afforded the desired cyclopentenone 7a
20
in a moderate yield (entry 1). The Grubbs II catalyst also
showed a similar activity with the diene additive (entry 2),
3
a (entry 3). The reaction using an excess amount (2.4 equiv) of
20
the boronic acid mainly afforded bis-adduct 3a after 48 h with
almost the same stereoselectivity as entry 1 (entry 4).
whereas the Grubbs I catalyst did not catalyze the cyclization
28
(entry 3). The use of the Hoveyda−Grubbs II catalyst further
improved the yield (entry 4). We found that portionwise
addition of the catalyst was not necessary, as the same yield was
obtained when the catalyst (15 mol %) was added in one portion
(entry 5). Decreasing the amount of the catalyst to 10 mol %
lowered the yield (entry 6); however, changing the solvent from
benzene to toluene and elevating the temperature to 80 °C
dramatically improved the yield, even with 10 mol % catalyst
(entry 7). The addition of twice the amount of 1,6-heptadiene
did not improve the yield (entry 8), but notably diene 2a did not
cyclize in the absence of the diene additive (entry 9). Diallyl ether
Under the conditions optimized for dienone 1a (entry 3), the
reactions of other symmetrical dienones 1b−f were performed
(
entries 5−9). Dienone 1b with electron-deficient p-bromo-
phenyl groups showed lower chemoselectivity than 1a (entry 5),
though the enantioselectivity of product 2b was the same as that
of 2a. Conversely, dienones 1c−e (entries 6−8) bearing
electron-rich aromatic rings tended to have higher enantiose-
lectivities than unsubstituted 1a. The highest enantioselectivity
(
94% ee) was observed for the reaction of 1e bearing bulky and
electron-rich 1-naphthyl groups (entry 8). We speculate that
5
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Organic Letters
Letter
Table 2. RCM of Monostyrylated Products
Scheme 5. RCM of Products 2f−h
entry
Ru catalyst
X
time (h)
yield (%)
a
1
Schrodi−Grubbs
Grubbs II
15
15
15
15
15
10
10
10
10
10
72
72
72
72
24
24
24
24
24
24
56
56
0
a
a
a
b
b
b
b
b
2
3
4
5
6
7
8
9
1
Grubbs I
Hoveyda−Grubbs II
Hoveyda−Grubbs II
Hoveyda−Grubbs II
Hoveyda−Grubbs II
Hoveyda−Grubbs II
Hoveyda−Grubbs II
Hoveyda−Grubbs II
84
84
73
98
85
0
styrylboronic acid to symmetrical and unsymmetrical dienones
to afford monostyrylated adducts with good enantioselectivity.
The RCM of the diene products using the Hoveyda−Grubbs II
catalyst proceeded smoothly in the presence of a diene additive
to afford optically active cyclopentenones, including a synthetic
intermediate of the antitumor agent TEI-9826. Further improve-
ments of the chemo- and enantioselectivities and the application
of tartaric acid derived catalysts to other reaction systems are now
in progress.
,
,
c
c
,
d
, ,
ce
b,c,f
0
81
b
a
The catalyst (5 mol %) was added every 24 h (total 15 mol %). The
catalyst was added in one portion. In toluene at 80 °C. With 1,6-
heptadiene (0.4 equiv). Without 1,6-heptadiene. With diallyl ether
0.2 equiv) instead of 1,6-heptadiene.
c
d
e
f
(
ASSOCIATED CONTENT
Supporting Information
■
*
S
was found to be as effective an additive as 1,6-heptadiene (entry
0).
The effect of the diene additives on this reaction system was
significant and may be attributed to the generation of an active
ruthenium methylidene complex in situ. This complex would
promote the RCM of less reactive conjugated internal alkenes
such as 2 (Scheme 4). Catalytic amounts of the diene additive can
be used because the methylidene species is regenerated along
with the formation of stable β-substituted styrenes 8.
Experimental details and characterization of all new products.
This material is available free of charge via the Internet at http://
pubs.acs.org.
1
29
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: msugiura@kumamoto-u.ac.jp.
Notes
The authors declare no competing financial interest.
Scheme 4. Probable Catalytic Cycle
ACKNOWLEDGMENTS
This work was partially supported by JSPS KAKENHI Grant No.
3590009.
■
2
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■
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