Formal [4 + 1]-Cycloaddition of homopropargyl alcohols to diazo dicarbonyl compounds giving substituted tetrahydrofurans

Article abstract of DOI:10.1021/ol403746r

A novel formal [4 + 1]-cycloaddition of readily available homopropargyl alcohols with diazo dicarbonyl compounds is described, which involves tandem O-H insertion/Conia-ene cyclization under cooperative Rh(II)/Zn(II) catalysis. This reaction provides easy access to various substituted tetrahydrofurans and exhibits complete E-selectivity in the case of nonterminal alkynes.

Full text of DOI:10.1021/ol403746r

Letter
pubs.acs.org/OrgLett
Formal [4 + 1]-Cycloaddition of Homopropargyl Alcohols to Diazo
Dicarbonyl Compounds Giving Substituted Tetrahydrofurans
Fumiya Urabe, Shohei Miyamoto, Keisuke Takahashi, Jun Ishihara, and Susumi Hatakeyama*
Graduate School of Biomedical Sciences, Nagasaki University, Nagasaki 852-8521, Japan
S
* Supporting Information
ABSTRACT: A novel formal [4 + 1]-cycloaddition of readily
available homopropargyl alcohols with diazo dicarbonyl
compounds is described, which involves tandem O−H
insertion/Conia-ene cyclization under cooperative Rh(II)/
Zn(II) catalysis. This reaction provides easy access to various
substituted tetrahydrofurans and exhibits complete E-selectiv-
ity in the case of nonterminal alkynes.
etrahydrofurans are ubiquitous structural elements found
in numerous biologically active natural products.1 Among
Scheme 2. Formal [4 + 1]-Cycloaddition Giving
Tetrahydrofurans
T
the many approaches toward their synthesis,2,3 cycloaddition
reactions4 provide us with powerful and efficient strategies.
Recently, in the course of our campaign directed toward the
synthesis of natural products utilizing an In(OTf)3-catalyzed
Conia-ene reaction,5 we discovered a new methodology for the
synthesis of tetrahydrofurans which consists of formal [4 + 1]-
cycloaddition of homopropargyl alcohols to diazo dicarbonyl
compounds.
During the examination of Rh(II)-catalyzed O−H insertion
of 1 onto 2, we unexpectedly found that the cyclized product 4
was directly formed possibly via Rh(II)-catalyzed Conia-ene
reaction of 3 although the yield was less than 10% (Scheme 1).
Scheme 1. Formation of 4 via a O−H Insertion/Conia-Ene
Process
at room temperature (Table 1). It was observed that the
cyclization step was very slow in the absence of a Lewis acid
although tetrahydrofuran 8aa was produced directly in very
poor yield (Table 1, entry 1). As a result, among the additives
5,8c
9c
examined, In(OTf)3
and ZnCl2 were found to effectively
promote the cyclization to provide 8aa in acceptable yields
(Table 1, entries 2 and 3). Since the In(OTf)3-catalyzed
conditions were found to exhibit a limited substrate scope,16
the conditions using ZnCl2 were selected for further
optimization.
After the experiments shown in Table 2 were conducted, we
identified the reaction conditions listed in entry 3 to be
optimum. Thus, when 6a was reacted with 1 equiv of 5a using 1
mol % of Rh2(esp)2 and 10 mol % of ZnCl2 in CH2Cl2 at room
temperature, the cycloaddition completed within 4 h to give
8aa in 83% yield. When ZnCl2 was reduced to 5 mol % or less,
the cyclization became very sluggish (Table 2, entries 1−4).
Toluene, THF, and MeCN were found to be inferior to
CH2Cl2 as solvent (Table 2, entries 5−7).
This observation allowed us to envisage that, provided the
second cyclization step is facilitated by the addition of an
appropriate Lewis acid,6−12 the O−H insertion reaction of 5
with 6 and the following cyclization reaction of 7 could be
achieved in a cascade manner13 to directly produce
tetrahydrofuran 8 (Scheme 2). To our knowledge, this type
of transformation, formally regarded as a [4 + 1]-cyclo-
addition14 as depicted in 9, has not been reported to date.
To assess the feasibility of our envisaged tetrahydrofuran
synthesis, we first surveyed Lewis acids as an additive in the
reaction of 5a and 6a using the Rh2(esp)2 catalyst15 in CH2Cl2
Received: December 25, 2013
Published: January 13, 2014
© 2014 American Chemical Society
1004
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Letter
a
a
Table 1. Reaction of 5a and 6a
Table 3. Substrate Scope of Diazo Dicarbonyl Compounds
b
yield (%)
7aa
entry
Lewis acid
time (h)
8aa
1
2
3
4
5
6
7
8
9
none
72
3
80
0
4
93
68
1
In(OTf)3
ZnCl2
6
16
78
81
64
23
21
73
AuCl(PPh3), AgOTf
Pd(OAc)2
AgOTf
6
3
13
20
22
43
15
6
Ni(acac)2
4
CuOTf·1/2PhH
FeCl3
6
6
a
Reaction conditions: 5a (10 equiv), 6a (1 equiv), Rh2(esp)2 (1 mol
%), Lewis acid (5 mol %), 4 Å MS (30 mg/mL), CH2Cl2 (0.5 M), rt.
b
7aa and 8aa were inseparable, and the yields were calculated by their
1
ratio determined by H NMR.
a
Table 2. Optimization of the Synthesis of 8aa
b
yield (%)
entry 5a (equiv) ZnCl2 (mol %)
solvent
time (h) 7aa
8aa
1
2
3
4
5
6
7
10
10
1
5
20
10
1
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
toluene
MeCN
THF
6
7
16
0
68
86
83
47
22
4
4
0
1
96
18
19
23
47
37
57
0
1
10
10
10
1
a
1
20
The reactions were conducted using 4 Å MS (30 mg/mL) in CH2Cl2
b
c
d
a
(0.5 M). Isolated yield. ZnCl2 was not employed. Addition of
Reaction conditions: 5a (indicated amount), 6a (1 equiv), Rh2(esp)2
ZnCl2 (10 mol %) led to the complex mixture.
(1 mol %), ZnCl2 (indicated amount), 4 Å MS (30 mg/mL), solvent
b
(0.5 M), rt. 7aa and 8aa were inseparable, and the yields were
1
calculated by their ratio determined by H NMR.
the yield was very low. However, the addition of ZnCl2 caused
the formation of a complex mixture possibly due to the
reactivity of the cyclic malonic ester functionality under Lewis
acidic conditions (Table 3, entry 6).
Having established optimized reaction conditions, we next
evaluated the substrate scope with the diazo dicarbonyl
compounds (Table 3). It was found that compounds 6b−d
similarly underwent a cycloaddition reaction with 5a to afford
the corresponding cyclized products 8ab−8ad in moderate to
good yields. However, in the cases of 6b and 6c, side products
8ab′ and 8ac′ were also formed via an ester exchange reaction
while the corresponding ester exchange product was not
observed for 6a (Table 3, entries 1−3). Hence, 1.5 equiv of 5a
was necessarily employed to obtain 8ab and 8ac in acceptable
yields (entries 2 and 3). Reaction of O−H insertion product
7ab17 with 5a under the same conditions using Rh2(esp)2,
ZnCl2, and 4 Å molecular sieves in CH2Cl2 did not afford 8ab′,
indicating that the ester exchange reaction occurred prior to
O−H insertion. In the case of more reactive 6d, ZnCl2 could be
reduced to 1 mol %, and 8ad was still obtained in good yield
(Table 3, entry 4). Furthermore, the cyclization proceeded even
in the absence of ZnCl2 to give 8ad in moderate yield (Table 3,
entry 5). Reaction of 5a with diazo compound 2 also afforded
the cyclized product 4a directly under the conditions using
Rh2(esp)2 alone as seen in the reaction of Scheme 1 although
Scheme 3 illustrates the substrate scope with alkynols in the
reaction with 6a. This cycloaddition method is applicable to
various substituted homopropargyl alcohols. The presence of a
substitutent at the C1 or C2 position did not negatively affect
the cyclization, and tetrahydrofurans 8ba−8ea were obtained in
good yields, albeit moderate diastereoselectivities.18 cis-/trans-
Octahydrobenzofuran derivatives 8fa and 8ga were also
obtained in good yields as a diastereoisomeric mixture.18
Furthermore, 8ha could be synthesized in good yield from 5h
having gem-methyl groups next to the alkynic triple bond. It is
noteworthy that, in the cases of nonterminal alkynes 5i−5l, the
cyclization proceeded with complete E-selectivity to produce
8ia−8la in good to excellent yields. Scheme 4 shows the
limitation of this method. In the case of tertiary alcohol 5m, the
O−H insertion step was so slow that either 7ma or 8ma could
not be produced effectively. Instead, 5m underwent competitive
cycloaddition of 6a to the triple bond to give cyclopropene 10.
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a
Scheme 3. Substrate Scope of Alkynols
Scheme 5. Control Experiments
cyclization step effectively while it does not promote the O−H
insertion.
These observations allowed us to propose the reaction
mechanism depicted in Scheme 6 where Rh(II) and Zn(II)
Scheme 6. Proposed Reaction Mechanism
a
Condition A: 5b−h (1 equiv), 6a (1 equiv), Rh2(esp)2 (1 mol %),
ZnCl2 (10 mol %), 4 Å MS (30 mg/mL), CH2Cl2 (0.5 M), rt;
Condition B: 5i−l (1 equiv), 6a (1 equiv), Rh2(esp)2 (1 mol %),
ZnCl2 (10 mol %), 4 Å MS (30 mg/mL), (CH2Cl)2 (0.5 M), rt 30
min, then reflux.
Scheme 4. Limitation of the Cycloaddition Method
catalysts worked independently without exerting a deleterious
influence on each other. Namely, Rh(II) reacts with diazo
dicarbonyl compound 6 to form Rh carbenoid 16 which in turn
undergoes the O−H insertion reaction with alkynol 5 via 17 to
generate 7.19 Then intermediate 7 is involved in the Zn(II)-
mediated Conia-ene reaction giving tetrahydrofuran 8 with an
E-configuration via carbometalation of Zn enolate 18 to 19.9a
This mechanism explains the highly selective production of the
E-isomers 8ia−8la.
In conclusion, we have developed a novel formal [4 + 1]-
cycloaddition of readily available homopropargyl alcohols with
diazo dicarbonyl compounds, which features broad applicability
as well as atom economical efficiency and operational
simplicity. The present work provides new entry to
tetrahydrofurans.
ASSOCIATED CONTENT
■
In addition, this method cannot be applied to the synthesis of
tetrahydropyrans and oxetanes as observed in the reactions of
alkynols 11 and 14 with 6a.
S
* Supporting Information
Experimental procedures, characterization data, H and 13C
1
In order to probe the reaction mechanism, control
experiments were carried out as shown in Scheme 5. When
the reaction of 5a with 6a was conducted using 10 mol % of
ZnCl2 alone, no reaction occurred and 6a was recovered
quantitatively. On the other hand, 7aa smoothly underwent the
cyclization reaction under the conditions using 10 mol % of
ZnCl2 to produce 8aa in high yield. These results and the result
listed in entry 1 of Table 1 suggest that (i) Rh2(esp)2
participates in the O−H insertion step while it does not
promote the cyclization effectively; (ii) ZnCl2 catalyzes the
NMR spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: susumi@nagasaki-u.ac.jp.
Notes
The authors declare no competing financial interest.
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Letter
8414. (e) Liu, L.; Wei, L.; Lu, Y.; Zhang, J. Chem.Eur. J. 2010, 16,
11813.
ACKNOWLEDGMENTS
■
This work was supported by the Grant-in-Aid for Scientific
Research (A) (22249001), Grant-in-Aid for Scientific Research
(C) (25460017) and Grant-in-Aid for Young Scientists (B)
(21790019) from JSPS, Grant-in-Aid for Scientific Research on
Innovative Areas “Reaction Integration” (No. 2105) (22106538
and 24106736) from MEXT, and The Naito Foundation.
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