Direct syntheses of spiro- and fused-hydrofurans by a tunable tandem semipinacol rearrangement/oxa-michael addition protocol

Article abstract of DOI:10.1002/chem.201300205

A highly chemoselective one-pot reaction has been developed involving a tandem semipinacol rearrangement/oxa-Michael addition sequence in which the in situ generated ketol diene intermediate can be transformed specifically to either the spiro- or fused-dihydrofuran products (see scheme). This one-pot tandem reaction represents a general synthetic methodology for the syntheses of the two different kinds of furan derivatives. Copyright

Full text of DOI:10.1002/chem.201300205

DOI: 10.1002/chem.201300205
Direct Syntheses of Spiro- and Fused-Hydrofurans by a Tunable Tandem
Semipinacol Rearrangement/Oxa-Michael Addition Protocol
Bao-Sheng Li, Wen-Xing Liu, Qing-Wei Zhang, Shao-Hua Wang, Fu-Min Zhang,
Shu-Yu Zhang, Yong-Qiang Tu,* and Xiao-Ping Cao*[a]
A significant number of biologically important natural
products and potential medicinal molecules contain cycloke-
tone spiro- and fused-dihydrofuran units,[1,2] with examples
including sieboldine A[1f] and the trichodermaketones A and
C[2d] (Figure 1). Over the past few years, several different
methods have been developed for the construction of these
units,[3,4] including a variety of asymmetric syntheses.[5,6] Un-
fortunately, however, these methods generally require the
preparation of complex substrates and subsequent multistep
derivations of the resulting products. With this in mind, the
development of a divergent and concise synthesis for the
Figure 1. Representative natural products containing the spiro- or fused-
construction of these structural units is still highly desirable.
The semipinacol rearrange-
hydrofuran units.
ment is a well-known chemical
reaction[7] and represents a class
of versatile transformations for
the construction of complex
structural motifs, especially
those with crowded quaternary
centers.[8] As a result, this reac-
tion has been applied to the ef-
ficient syntheses of many natu-
ral products and bioactive com-
pounds.[9] Over the past decade,
our own group[7c] and several
other chemists[7a,b,d] have direct-
Scheme 1. A tunable tandem reaction towards the syntheses of dihydrofuran derivatives. SR=semipinacol
ed a great deal of effort to-
wards the development of semi-
rearrangement.
pinacol rearrangement based synthetic methodologies for
the formation of structurally diverse quaternary centers. In
connection with our ongoing investigations regarding this re-
arrangement process and interests in constructing cycloke-
tone spiro- and fused-dihydrofuran units, we hypothesized
that these units might be derived from a readily available
diene–ketone intermediate[10,11] through a tandem semipina-
col rearrangement/oxa-Michael addition protocol under ap-
propriate conditions (Scheme 1). This tandem process would
be unprecedented and efficient for the synthesis of the
aforementioned structural motifs. Herein, we present our
preliminary results for this tandem protocol.
Our preliminary evaluation of the strategy was carried
out by using phenylfuranyl cyclobutanol 1a (R1 =trimethyl-
silyl, TMS) and dimethyl diazomalonate as the model sub-
strates (Table 1). Initially, a variety of different metal cata-
lysts were investigated, including copper and silver salts. Un-
fortunately, these attempts failed to afford any of the ex-
pected intermediate in dichloromethane. Pleasingly, when
[a] B.-S. Li, W.-X. Liu, Dr. Q.-W. Zhang, Prof. Dr. S.-H. Wang,
Prof. Dr. F.-M. Zhang, Dr. S.-Y. Zhang, Prof. Dr. Y.-Q. Tu,
Prof. Dr. X.-P. Cao
State Key Laboratory of Applied Organic Chemistry &
College of Chemistry and Chemical Engineering
Lanzhou University, Lanzhou 730000 (P. R. China)
Fax : (+86)931-8915557
E-mail: tuyq@lzu.edu.cn
[Rh2ACHTNUTRGNEU(GN Oct)4] (1 mol%) was used as the catalyst, substrate 1a
caoxplzu@163.com
reacted with dimethyl diazomalonate to form the intermedi-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300205.
ate 1e in 92% yield.[10]
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Chem. Eur. J. 2013, 19, 5246 – 5249

COMMUNICATION
Table 1. Optimization of reaction conditions.[a]
the current reaction conditions (3 equiv of THF·BF3 were
used). The results revealed that the use of the more stable
protecting group tert-butyldimetylsilyl (TBS) also provided
the spiro-ring products 1c/1c’, albeit with a lower total yield
of 59% (Table 1, entry 4). Interestingly, the fused-ring prod-
ucts 1d/1d’ (entry 5) were obtained in good yield (71%)
from intermediate 1 f by the simple replacement of
THF·BF3 with Et2O·BF3. A further investigation of the sol-
vents and several other acids demonstrated that substrate
1b with a TBS protecting group always gave the fused hy-
drofuran products 1d/1d’ in 18–71% yields (entries 5–12).
The use of trifluoroacetic acid (TFA; entry 8) provided the
only exception, resulting only in the decomposition of the
substrate. In conclusion, the chemoselectivity of this one-pot
reaction was highly dependent on the acidity of the
THF·BF3 or Et2O·BF3 additive and the stability of the pro-
tecting group.
Entry Solvent
Acid
R
t
Yield [%]
Yield [%]
[h][b] of 1c/1c’[c] of 1d/1d’[d]
1
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
1,2-DCE
CCl4
Et2O·BF3
(1.1 equiv)
THF·BF3
(1.1 equiv)
THF·BF3
(3.0 equiv)
THF·BF3
(3.0 equiv)
Et2O·BF3
(3.0 equiv)
TMS
4
–
61
78
59
–
–
2
TMS 24
–
3
TMS
TBS
TBS
8
8
4
4
8
8
8
4
6
6
6
–
With the optimized reaction conditions (Table 1, entry 3)
in hand, we proceeded to investigate the general scope of
the reaction for the conversion of compounds of type a to
the corresponding spiro-products c by means of path A.
Subsequently, a series of TMS-protected substrates 2a–11a
containing a variety of different substituents on the cyclo-
4
–
5
71
66
53
–
6
2AcOH·BF3 TBS
(3.0 equiv)
–
AHCTUNGTREGbNNNU utanol moiety were synthesized and subjected to the reac-
7
TBSOTf
(3.0 equiv)
TFA
(3.0 equiv)
Et2O·BF3
(3.0 equiv)
Et2O·BF3
(3.0 equiv)
Et2O·BF3
(3.0 equiv)
TBS
TBS
TBS
TBS
TBS
TBS
TBS
–
tion conditions. As shown in Table 2, the substrates with cis-
p-Me-phenyl, cis-p-Br-phenyl, diphenyl, and spiro-cycloalkyl
at the C3’-position gave the products in good yields of 64
(2c/2c’), 63 (3c/3c’), 61 (4c/4c’), and 75% (5c/5c’). Further-
more, the reaction exhibited a trend in migratory aptitude
similar to that observed in the normal semipinacol rear-
rangement, in that the tertiary carbon atom migrated prefer-
entially to the secondary carbon atom.[7d,12] Accordingly,
substrates bearing different substituents (e.g., di-n-butyl, di-
i-butyl, spiro-cycloalkyl, spiro-pyranyl, and ketospiro-cyclo-
alkyl) at the C2’-position tended to undergo C2’ carbon
atom migration reactions to give the corresponding spiro-
furans 6c/6c’, 7c/7c’, 8c/8c’, 9c/9c’, and 10c/10c’ containing
two contiguous quaternary centers in reasonable yields. Im-
portantly, the 2’,3’-cyclohexane-fused cyclobutanol substrate
11a also proved to be effective for generating the corre-
sponding tricyclic products 11c/11c’ in the best yield (85%)
of all of the products. Furthermore, the products 11c/11c’
might serve as a building block in the synthesis of the bio-
logically important sieboldine A (Figure 1). The relative
configurations of all of the spiro-furan products c and c’
were deduced based on their 1H NMR spectra and X-ray
diffraction of the products 1c, 3c, 4c, 6c, 7c, and 3c’.[13]
We then applied this one-pot protocol to the syntheses of
the fused hydrofurans d from various TBS-protected sub-
strates 2b–6b (Table 3). Pleasingly, this series of substrates
also produced the desired products in moderate to good
yields. When the larger 3’-BOM-substituted substrate 2b
(BOM=benzyloxymethyl) was subjected to the standard
conditions, the reaction afforded the desired products 2d/
2d’ in a combined yield of 74%. In addition, the reaction of
substrates 3b and 4b containing 3’,3’-spiro-alkyls of different
sizes led to the formation of 3d and 4d in acceptable yields
8
–
9
–
45
60
18
39
55
10
11
12
13
–
C6H5F
–
mesitylene Et2O·BF3
–
(3.0 equiv)
Et2O·BF3
toluene
–
(3.0 equiv)
[a] All reactions were carried out with 0.1 mmol of the substrate in
1.0 mL of solvent under [Rh2A(Oct)4] catalysis (1.0 mol%). [b] The time
CTHUNGTRENNUNG
required for the transformation of intermediate to products c or d.
[c] Isolated total yield of the spiro products c/c’ through path A. [d] Iso-
lated total yield of the fused products d/d’ through path B. 1,2-DCE=1,2-
dichloroethane; DCM=dichloromethane.
With the intermediate 1e in hand, we proceeded to opti-
mize the reaction conditions for the conversion of 1e to
either 1c or 1d in a one-pot manner. As shown in Table 1,
when Et2O·BF3 (1.1 equiv) was used as the Lewis acid to
promote the tandem reaction, none of the expected prod-
ucts, 1c or 1d, was formed; however, decomposition of the
substrate was observed (Table 1, entry 1). Fortunately, when
THF·BF3 (1.1 equiv) was used as
a replacement for
Et2O·BF3, the intermediate successfully afforded the spiro-
ring products 1c/1c’ in a combined yield of 61% following a
24 h reaction time (entry 2). In addition, the reaction time
was further reduced to 8 h and the yield was increased to
78% when three equivalents of THF·BF3 were used
(entry 3).
To investigate the influence of protecting groups on this
tandem reaction, substrate 1b (R1 =TBS) was subjected to
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Y.-Q. Tu, X.-P. Cao et al.
Table 2. The spiro-dihydrofuran products c and c’ synthesized through
(62 and 56%, respectively) relative to the yield (53%) of
8c/8c’. Furthermore, the application of the conditions to
substrate 5b, which did not contain any substituents on
the cyclobutanol moiety, gave the desired product 5d in
the best yield of all of the examples (81%). When diethyl
diazomalonate and 5b were subjected to the reaction con-
ditions, the corresponding product 6d was also produced
in a good yield of 76%. Unfortunately, however, the 3’,3’-
diphenyl-substituted substrate 6b[13] did not provide any
of the desired product under the same reaction conditions,
with the reaction resulting only in decomposition.
path A.[a]
On the basis of the experimental results above, a plausi-
ble reaction mechanism was proposed (Scheme 2). The
[a] All reactions were carried out under standard conditions in DCM.
Isolated yields (R2 =CO2Me). The relative configurations of the products
c were shown. The diastereomeric ratio (d.r.) values were confirmed by
1H NMR spectroscopy.
Scheme 2. A plausible mechanism for the tandem reaction.
formation of the spiro and fused products might be attri-
[14]
G
relative to
Table 3. The fused-dihydrofuran products d and d’ synthesized through
that of THF·BF3, with the former capable of inducing the
second 1,2-migration of the intermediate h and the subse-
quent oxa-Michael addition of the intermediate, leading in
turn to the formation of the fused-dihydrofuran product d.
In contrast, however, the latter directly underwent an oxa-
Michael addition of intermediate diene–ketone h and result-
ed in the spiro-dihydrofuran product c.
path B.[a]
In summary, we have successfully developed a novel one-
pot tandem process for the construction of spiro- or fused-
dihydrofuran products from simple furan derivatives under
mild reaction conditions. The method represents a useful
and highly efficient strategy for the syntheses of differential-
ly substituted spiro and fused compounds. Further studies
towards the application of this one-pot tandem reaction are
currently underway in our group.
Experimental Section
[a] All reactions were carried out under standard conditions in DCM.
Isolated yields (the single products d or d’ could not be isolated by
column chromatography). The d.r. values were confirmed by 1H NMR
spectroscopy; R2 =CO2Me. [b] R2 =CO2Et.
General procedure: Under an Ar atmosphere and ice bath, [Rh
2ACHTUNGTRENNUNG(Oct)4]
(1.0 mol%) was added to a solution of substrate a or b (0.1 mmolmLÀ1
)
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Direct Syntheses of Spiro- and Fused-Hydrofurans
COMMUNICATION
in DCM; then diazomalonate (1.2 equiv) was added dropwise to the reac-
tion system and stirred vigorously at RT. After the substrate a or b had
been fully transformed to intermediate e or f as monitored by TLC anal-
ysis, THF·BF3 or Et2O·BF3 (3.0 equiv) was injected slowly into the reac-
tion solution under an ice bath. The reaction was warmed to RT until full
conversion of the intermediate e or f to product as monitored by TLC
analysis. The reaction was quenched by careful addition of saturated
NaHCO3 aqueous solution under an ice bath. The resulting solution was
extracted with DCM, washed with brine, dried over MgSO4, and chroma-
tographed (ethyl acetate/petroleum ether) to give the product.
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Acknowledgements
We greatly acknowledge financial support from the “973” Program
(grant no. 2010CB833203), the NSFC (grant nos. 21072085, 20921120404,
21102061, 20972059, 21202073, 21290180, and 21272097), the Key Nation-
al S&T Program “Major New Drug Development” of Ministry of Health
of China (2012ZX09201101-003), and the “111” Program of the MOE,
the fundamental research funds for the central universities (lzujbky-2012-
60, 86).
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Keywords: chemoselectivity
addition · semipinacol rearrangements · tandem reactions
·
dihydrofurans
·
Michael
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Received: January 19, 2013
Published online: March 13, 2013
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