Diastereoselective construction of cyclopent-2-enone-4-ols from aldehydes and 1,2-allenones catalyzed by N-heterocyclic carbene

Article abstract of DOI:10.1039/c6cc07974a

Cyclopent-2-enone-4-ols have been prepared highly diastereoselectively from readily available aromatic aldehydes and 1,2-allenones catalyzed by N-heterocyclic carbene. Mechanistic studies showed that there are four possible pathways.

Full text of DOI:10.1039/c6cc07974a

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Reactivity differences of Sc
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Diastereoselective Construction of Cyclopent-2-enone-4-ols from
Aldehydes and 1,2-Allenones Catalyzed by N-Heterocyclic Carbene
Received 00th January 20xx,
Accepted 00th January 20xx
Dengke Ma, Chunling Fu and Shengming Ma*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Cyclopent-2-enone-4-ols
have
been
prepared
highly diastereoselectively (Scheme 1, eq 2).
diastereoselectively from readily available aromatic aldehydes
and 1,2-allenones catalyzed by N-heterocyclic carbene.
Mechanistic studies showed that there are four possible pathways.
Cyclopent-2-enone-4-ol unit exists in some natural products
1
and biological active compounds, thus, efficient synthetic
2
routes to these compounds are of high interest. Known
methods for their preparations from acyclic precursors
includes: cyclizations of 1,2-diketones with 3-oxocarboxylic
3
4
acid esters or amides, ketones, 1,3-bis(trimethylsiloxy)-1,3-
3
a,5
6
2-(trimethylsiloxy)allyltriemthylsilane, or 2,3-
dienes,
7
allenoates; transition metal-catalyzed intramolecular coupling Scheme 1 Stetter reaction and this reaction.
of
enthioates followed by desilylation; ring closing metathesis of In the beginning, we attempted the reaction of 4-
-OTBS or DPS-hepta-1,6-dien-3-ones followed by possible bromobenzaldehyde 1a with 1,2-allenyl ketone 2a using
ethyl
3-(triethylsilyloxy)-5-(tributylstannyl)pent-4(Z)-
8
5
9
deprotection;
Tandem Aldol–Brook reaction of β- Cs CO as the base and toluene as the solvent in the presence
2 3
(
phenylthio)acryloyl silanes with lithium enolates followed by of different commercially available N-hetereocyclic carbene
1
0
simultaneous desilylation and dephenylthiolation; tandem precursors
ozonolysis–Knoevenagel cyclization of tert-butyl 5-OTBS-3- 4aa, (E)-3aa, and (Z)-3aa, a new cyclic product 5aa (dr = 95:5)
oxohept-6-enoates; Nazarov reaction and desiylation of (E)- was observed in 40% yield when changing the solvent from
A to E. In addition to the C=C bond regio-isomer
1
1
1
2
4
-methoxymethyl-1-silyloxyhexa-1,4,5-trien-3-ols;
and the toluene to dioxane (Table S1, entry 10, see ESI†).
13
cyclization reactions involving cis-2-en-1,4-diketones or (Z)-4- Inspired by the observation above, reaction parameters
oxopent-2-enal.
1
4
including the loading of the NHC precursor A1 and Cs CO ,
2 3
On the other hand, traditionally, the treatment of aldehydes solvents, together with the reaction temperature were
with N-heterocyclic carbenes would afford the Breslow screened extensively. As a result, increasing the loading of
intermediate, subsequent Michael addition with α,β- Cs CO led to a complete transformation to product 5aa
.
2
3
unsaturated carbonyl compounds gave 1,4-diketones (the However, the diastereoselectivity was not satisfactory (Table 1,
1
5,16
o
Stetter reaction) (Scheme 1, eq 1).
when 1,2-allenone is treated with the Breslow intermediate, a diastereoselectivity with a lower yield (Table 1, entry 3). By
mixture of C-C double bonds regio-isomers 1,4-diketones S3
reducing the loading of base, slightly higher
S4 and S5, could be produced. Rather unexpectedly, the cyclic diastereoselectivity was observed with a lower yield (Table 1,
We hypothesized that entry 1). Running the reaction at 10 C gave a better
,
a
o
4
-hydroxycyclopentenone product was formed highly entries 4 and 5). As expected, the reaction at 50 C afforded
the product 5aa with a lower diastereoselectivity (81:19),
although the yield is 68% (Table 1, entry 6). After further
Laboratory of Molecular Recognition and Synthesis, Department of Chemistry,
Zhejiang University, Hangzhou 310027, Zhejiang, People’s Republic of China.
E-mail: masm@sioc.ac.cn
† Footnotes relaꢀng to the ꢀtle and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
optimization, the best diastereoselectivity was obtained with
10 mol % A1 and 25 mol % Cs CO using dioxane as solvent at
2
3
o
5 C (Table 1, entry 8). Other solvents, such as DMF, THF and
1
supplementary information available should be included here]. See CH
DOI: 10.1039/x0xx00000x
CN didn’t give a better result (Table 1, entries 11-13).
3
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a
o
Table 1 Optimization of the reaction conditions
temperature of 13 C, disappointingly, the diastereoselectivity
dropped to 80:20 (Table 3, entry 1). In order to run the
reaction at a lower temperature for better dr, we considered
o
to add CH
catalyst loading of 20 mol % for A1 and Cs
a mixed solvent of dioxane and CH Cl (3/1) afforded 72% of
2
Cl
2
as the co-solvent: reaction at -10 C under a
2
3
CO respectively in
2
2
the product and dr value was as high as 97:3 (Table 3, entry 6).
Thus, this has been defined as the optimized conditions.
a
Table 3 Further optimization using benzaldehyde
T
t
NMR Yield%
(E)-3aa (Z)-3aa 4aa
Entry
X
Y
o
b
(
C)
(h)
5aa (dr)
1
2
3
4
5
6
7
8
9
10 30
10 30
10 30
10 20
10 15
10 20
10 20
10 25
10 25
12 25
10 20
10 20
10 20
21
15
10
20
20
50
15
15
15
15
20
20
20
17
11.75
21.75
22
2
—
4
—
—
<1
1
—
—
61 (90:10)
57 (89:11)
49 (94:6)
63 (91:9)
45 (93:7)
68 (81:19)
55 (94:6)
51 (95:5)
49 (93:7)
55 (93:7)
—
<1
<0.5
<1
—
2
22
4
1
22
—
8
—
1
T
t
NMR Yield%
23
1
Entry
X
Y
o
(
b
C)
(h)
(E)-3ba (Z)-3ba 4ba
5ba (dr)
23
11
3
1
2
c,d
47.5
22.3
21.75
22
<1
—
—
—
—
<0.5
<1
—
1
10 25
10 25
15 25
20 25
13
0
20.5
22
—
48
36
—
2
—
11
9
—
11
9
83 (80:20)
11 (100:0)
27 (100:0)
82 (88:12)
76 (96:4)
72 (97:3)
1
0
4
2
3
c
1
1
—
3
0
22.3
12.25
8.5
d
c
1
2
3
<0.5
—
38 (94:6)
—
4
0
—
—
1
—
—
1
e
1
22
—
5
6
20 25 -10
20 20 -10
a
8.5
7
Reaction conditions: A solution of NHC precursor and Cs
2
CO
3
in dioxane (1 mL)
a
was stirred at rt for 10 min, then 1a, 0.4 mmol of 2a, and dioxane (1 mL) were
Reaction conditions: A solution of NHC precursor and Cs
2
CO
3
in mixed solvent (2
o
added sequentially. The resulting mixture was then stirred at T ( C) for the time
mL) was stirred at rt for 10 min, and then 1b, 0.4 mmol of 2a, and solvent (2 mL)
b
c
indicated in the Table. Determined by NMR of the crude product. DMF was
were added sequentially. The resulting mixture was then stirred at T for the time
d
e
b
c
3
used as solvent. THF was used as solvent. CH CN was used as solvent.
indicated in the Table. Determined by NMR of the crude product. The reaction
d
was conducted on 1 mmol of 2a. Only dioxane was used as the solvent.
To our delight, running the reaction at a concentration of 0.1
M for 1,2-allenone 2a proved to be the best condition for both Gladly, aromatic aldehydes substituted with electron-
the yield and the diastereoselectivity (Table 2, entry 3).
withdrawing p-Br (Table 4, entry 1) and p-Ph (Table 4, entry 6)
or electron-donating p-CH (Table 4, entry 7) were well
a
3
Table 2 Effect of Concentration
tolerated under the standard conditions. In addition, 2-
naphthaldehyde was also compatible (Table 4, entries 8 and 9).
2
1
When R group was hydrogen atom, R group could be linear
alkyl (Table 4, entries 1-8) or branched alkyl (Table 4, entry 9).
What’s more, the reaction of allenone containing removable
OBn group proceeded smoothly, which provided an
1
opportunity for further elaboration (Table 4, entry 5). When R
was benzyl group, the dr values are slightly lower (Table 4,
entries 4 and 8). Spirobicyclic compound 5bf could be formed
in 50% yield when propadienyl cyclohexylketone was used
(Table 4, entry 10). The structure of the product was further
c
t
NMR Yield%
Entry
b
(
M)
(h)
(E)- 3aa
(Z)-3aa
4aa
2
5aa (dr)
confirmed by X-ray single crystal diffraction study of 4R*,5R*-
17
1
2
3
4
0.2
0.13
0.1
23
11
—
3
1
—
<1
2
51 (95:5)
67 (88:12)
71 (94:6)
53 (94:6)
5bb (Figure 1).
21.3
22.5
21.75
—
<0.5
3
0.08
15
a
Reaction conditions: A solution of NHC precursor and Cs
CO
2 3
in dioxane (half)
was stirred at rt for 10 min, then 1a, 0.4 mmol of 2a, and dioxane (half) were
o
added sequentially. The resulting mixture was then stirred at 15 C for the time
b
indicated in the Table. Determined by NMR of the crude product.
4
R*,5R*-5bb
However, when benzaldehyde was applied under the optimal
conditions illustrated as entry 3 in Table 2 even at a lower
Figure 1 ORTEP representation of 4R*,5R*-5bb.
2
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a
Table 4 Substrates scope
Dr (4R*,5R*-5:
Yield of 5%
b
4
R*,5S*-5)
1
2
Entry
Ar/R /R
Before/After
separation
95:5/>99:1
96:4/99:1
95:5/99:1
92:8/92:8
96:4/99:1
94:6/96:4
94:6/99:1
92:8/91:9
95:5/98:2
—
NMR/isolated
1
2
3
4
5
6
7
8
4-BrC
Ph (1b)/n-C
Ph (1b)/n-C
Ph (1b)/Bn/H (2c)
Ph (1b)/-(CH OBn/H (2d)
4-PhC (1c)/n-C 11/H (2a)
4-MeC (1d)/n-C /H (2b)
6
H
4
(1a)/n-C
11/H (2a)
/H (2b)
5
H
11/H (2a)
54/53 (5aa)
80/76 (5ba)
75/63 (5bb)
62/57 (5bc)
74/65 (5bd)
5
H
3
H
7
Scheme 3 Mechanistic studies.
2
)
4
An isotopic labeling experiments using deuterated
benzaldehyde 1b-D was conducted. As a result, deuterated
product 5ba-D was obtained, with 19% D-incorporation in the
c
6
H
4
5
H
78/75 (5ca)
6
H
4
3
H
7
76/65 (5db)
60/57 (5ec)
2-naphthyl (1e)/Bn/H (2c)
2
1
-position, 34% D-incorporation in the 3-position methyl group,
2% D-incorporation in the 4-position hydroxyl group, and 16%
d
9
2-naphthyl (1e)/i-propyl/H (2e)
64/56 (5ee)
e
1
0
Ph (1b)/-(CH
2
5
) - (2f)
50/50 (5bf)
D-incorporation in the 5-position (eq 5).
a
Reaction conditions: A solution of NHC peecursor and Cs
2
CO
3
in mixed solvent
(
5 mL) was stirred at rt for 10 min, then 1, 1 mmol of 2, and solvent (5 mL) were
added sequentially. The resulting mixture was then stirred at T for the time
b
1
c
indicated in the Table. Determined by H NMR. 4R*,5R*-5ca (72%) and
d
4
4
R*,5S*-5ca (3%) could be separated by column chromatography on silica gel.
R*,5R*-5ee (55%) and 4R*,5S*-5ee (1%) could be separated by column
e
o
chromatography on silica gel. The reaction was conducted at -10 C for 32 h,
o
then warmed up to 50 C for 14.5 h. 4% of (E)-3bf, 1% of 4bf, and 4% of (Z)-3bf Based on the observation above, a possible mechanism was
were also formed.
then proposed. N-heterocyclic carbene generated in situ
reacted with aldehyde to give the Breslow intermediate Int-A
,
,
The reaction of aliphatic aldehyde 1f was also studied.
Nevertheless, the expected 4-alkyl substituted cyclopent-2-
enone-4-ol was not formed, only 6% of 6fa was observed,
together with 58% recovery of aldehyde 1f and 39% recovery
of 1,2-allenone 2a (Scheme 2). Obviously, the reactivity of
aliphatic aldehyde towards NHC precursor A1 is rather low in
such a reaction.
which would attack the central carbon of the allenone
2
18
forming a delocalized anion intermediate Int-B
formation of intermediate Int-B, four pathways are possible:
1) An intramolecular D-transfer occurred in intermediate Int-B
.
After the
(
led to intermediate Int-C and Int-C’. Then elimination of the
NHC produced the deuterated 1,4-enediketones Int-D. With
18,19
NHC acting as a Brønsted base,
the sequential Aldol
reaction proceeded to give the final product with deuteration
within the 2-position and the methyl group of 3-position
b
Scheme 4, route a); (2) H in Int-B may also transfer to the
(
delocalized anion, giving enolate-type intermediate Int-F
.
2
0
Scheme 2 The reaction of aliphatic aldehyde 1f.
Cyclic nucleophilic substitution occurred to give the cyclic
product with the deuteration in the hydroxyl group, finally
yielding the non-deuterated , accompanied with the
5
5
In order to gain insight into the reaction, possible intermediate
regeneration of NHC (Scheme 4, route b); (3) The deuterium in
the hydroxyl group of Int-F may also transfer to trap the
enolate, forming α-deuterated intermediate Int-G. Then a
(
E)-3ba and (Z)-3ba were isolated utilizing the condition of
entry 1 in Table S1 (see ESI†). As a result, cyclic product 5ba
couldn’t be formed when starting from (E)-3ba under sole 20
similar Aldol reaction like route a happened to give product
5
mol % of Cs
Cs CO , respectively (Scheme 3, eq 3). On the other hand, (Z)-
ba also couldn’t be transferred to 5ba catalyzed only by 20
mol % of Cs CO (Scheme 3, eq 2). However, 5ba was obtained
in only 34% NMR yield (dr = 97:3) together with 3% of 4ba
when 20 mol % of A1 and 20 mol % of Cs CO were used as
catalysts starting from (Z)-3ba, however, 41% recovery of (Z)-
ba was observed (Scheme 3, eq 4). These experiments
2 3
CO (Scheme 3, eq 1) or under 20 mol % of A1 and
with deuteration at the 5-position via Int-D’ (Scheme 4, route
a’); (4) Int-G could also form intermediate Int-H followed by a
2
3
3
pathway like route b to generate product
the 5-position (Scheme 4, route b’). Meanwhile, the trans-
diastereoselectivity of products may be explained by the
5 with deuteration at
2
3
5
2
3
stereo-electronic effect of oxygen atom in hydroxyl group
adjacent to the NHC acting as a leaving group.
3
indicated that there exist other pathways.
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5
6
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(
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Scheme 4 Possible reaction pathways.
(
In conclusion, we have developed an efficient
diastereoselective synthetic route to useful cyclopent-2-
enone-4-ols from readily available aromatic aldehydes and 1,2-
and R. C. Whitehead, J. Chem. Soc., Perkin Trans. 1, 2002,
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NHC in this transformation. Further investigation is being
carried out in our laboratory.
1
4
1509.
Financial support from the National Natural Science 15 (a) H. Stetter, Angew. Chem., Int. Ed. Engl., 1976, 15, 639; (b)
H. Stetter, H. Kuhlmann, In Organic Reactions; Vol. 40; L. A.
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Alaniz and T. Rovis, Synlett, 2009, 1189; (d) S. R. Yetra, A.
Research Program (2015CB856600) is greatly appreciated. S.
Ma is a Qiu Shi Adjunct professor at Zhejiang University. We
Patra and A. T. Biju, Synthesis, 2015, 47, 1357.
thank Mr. Yizhan Zhai of this group for reproducing the 16 For an intramolecular NHC-catalyzed hydroacylation reaction
preparation of 5bc and 5ee shown in Table 4.
of 2-(propa-1,2-dienyloxy)benzaldehyde to form chromones,
see: B. Alcaide, P. Almendros, I. Fern ndez, T. Martínez del
Campo and T. Naranjo, Chem. Eur. J., 2015, 21, 1533.
Crystal data for compound 4R*,5R*-5bb: C15
30.29, monoclinic, space group P 1 21/n 1, final R indices
á
1
7
18 2
H O , MW =
Notes and references
2
1
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25.844 (2) Å, α= 90.00°, β= 94.486 (8)°, γ= 90.00°, V = 1334.2
,
2
3
(2) Å , T = 293 (2) K, Z = 4, reflections collected/unique
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