Stereoselective synthesis of conjugated bisallenols as precursors of novel bis(2,5-dihydrofuran) derivatives

Article abstract of DOI:10.1002/adsc.200800469

A stereoselective synthesis of conjugated bis(α-hydroxyallenes) by copper-mediated S_N2′-substitution is described. Their silver- or gold-catalyzed cycloisomerization affords highly functionalized 2-allenyl-substituted 2,5-dihydrofurans and bis(

Full text of DOI:10.1002/adsc.200800469

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DOI: 10.1002/adsc.200800469
Stereoselective Synthesis of Conjugated Bisallenols as Precursors
of Novel Bis(2,5-dihydrofuran) Derivatives
Manojkumar Poonotha and Norbert Krausea,*
a
Dortmund University of Technology, Organic Chemistry II, Otto-Hahn-Strasse 6, 44227 Dortmund, Germany
Fax : (+49)-231-755-3884; e-mail: norbert.krause@tu-dortmund.de
Received: July 28, 2008; Revised: November 21, 2008; Published online: December 19, 2008
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200800469.
Abstract: A stereoselective synthesis of conjugated
bis(a-hydroxyallenes) by copper-mediated SN2’-sub-
stitution is described. Their silver- or gold-catalyzed
cycloisomerization affords highly functionalized 2-
allenyl-substituted 2,5-dihydrofurans and bis(2,5-di-
hydrofurans) under axis-to-center chirality transfer.
drofurans. We take advantage of the cycloisomeriza-
tion of a-hydroxyallenes[7] to 2,5-dihydrofurans which
is known to occur with axis-to-center chirality transfer
when catalyzed by (anhydrous) acid,[8] silver,[9] or gold
salts[10,11] – the latter method is often superior in terms
of efficiency and functional group tolerance
(Scheme 1).
Keywords: bisallenes; chirality transfer; cycloisome-
rization; gold catalysis; silver catalysis
Scheme 1. Cycloisomerization of a-hydroxyallenes to 2,5-di-
hydrofurans.
The rich and fascinating chemistry of conjugated bi-
sallenes,[1] a species containing one conjugated and
two cumulated diene systems, has inspired chemists in
the past decades. Ever since the isolation of the
We started our approach to bis(a-hydroxyallenes)
parent 1,2,4,5-hexatetraene (bisallenyl) by Hopf,[2] from (E)-3-methylpent-2-en-4-yn-1-ol which was con-
these have been used as 4p components in [4+2]cy- verted into the bisoxiranes 1a–c by protection,
cloaddition reactions furnishing a wide variety of sub- Glaser–Hay coupling (CuCl/TMEDA/O2)[12] and ep-
stituted carbo- and heterocyclic products.[3] These re- oxidation with mCPBA. Whereas the NMR spectra
actions turned out to be an attractive method of pre- show a single set of signals, a slight splitting and/or
paring [2.2]paracyclophanes as well.[4] The first broadening observed in the HPLC seems to indicate
iron(0)-catalyzed [4+1]cycloaddition[5] reaction was that the bisoxiranes 1 were formed as a mixture of
also reported using conjugated bisallenes and carbon the meso- and dl-diastereomers.[13] All attempts to
monoxide furnishing cyclopentenones under mild con- separate these isomers failed, so that the subsequent
ditions. Although the chemistry of (unfunctionalized) reactions were carried out with the mixture. For the
conjugated bisallenes is very rich, there are only few allene formation by SN2’-substitution of propargyloxir-
reports on functionalized derivatives,[3f,6] and no sys- anes, various transition metals can be used.[7] Whereas
tematic study on the synthesis and transformation of the iron-catalyzed reaction of bisoxiranes 1 with
functionalized bisallenes has been reported to date. Grignard reagents,[14] as well as the rhodium-catalyzed
This is quite intriguing since the development of new SN2’-substitution with arylboronic acids[15] did not
methods for the synthesis of functionalized conjugat- afford any of the desired bisallene, we were pleased
ed bisallene derivatives can provide precursors for to observe a smooth conversion of the bisoxiranes 1
highly complex carbo- and heterocycles, employing ef- into the bis(a-hydroxyallenes) 2 by reaction with
ficient and atom-economical routes.
magnesium cuprates in THF.[7d,16] Thus, treatment of
Herein, we disclose a novel, convenient and stereo- the O-methylated bis(propargyloxirane) 1a with the
selective approach to conjugated bis(a-hydroxyal- cuprate obtained from isopropyl or tert-butylmagnesi-
lenes), as well as their cyclization to bis(2,5-dihydro- um chloride and CuBr·SMe2 at ꢀ658C and warming
furan) derivatives and 2-allenyl-substituted 2,5-dihy- of the reaction mixture to room temperature gave the
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conjugated bisallenes 2a and 2b in 68% and 74% out of these diastereomers, only the meso1- and dl1-
yield, respectively (Scheme 2). When CuCN was used isomers were obtained in a 1:1 ratio. The copper-
as copper salt, the yields were improved to 85% (2a) mediated SN2’-substitution of propargyloxiranes is
known to proceed with high anti-selectivity in most
cases, and it is often accompanied by epimerization of
the allenic chirality axis (induced by single electron
transfer from the cuprate or other reactive copper
species present in the reaction mixture).[7d,20] In the
present case, both the meso- and dl-isomer of 1a react
in an anti,anti-manner. Moreover, epimerization pro-
cesses can be excluded since these would probably
lead to the formation of a statistical mixture of all
diastereomeric bis(a-hydroxyallenes). Rather, it
seems that meso-1a gives meso-2a/b, and dl-1a affords
dl-2a/b.[21]
Under the above reaction conditions, only a single
diastereomer of bisallenes 2c and 2d was obtained
with low yield when 1a was treated with the magnesi-
um cuprate derived from EtMgBr or PhMgCl and
CuBr·SMe2 (Table 1, entries 1 and 2). Moreover, bis-
AHCTUNGTREG(NNUN propargyloxiranes) 1b and 1c bearing benzyl or TBS
protecting groups also furnished a single diastereomer
of the corresponding bisallenol when treated with var-
ious magnesium cuprates (entries 4–7). This observa-
Scheme 2. Copper-mediated SN2’-substitution of bisoxirane
1a to bisallenols 2a/b.
and 82% (2b). Both bisallenes were obtained as a 1:1 tion along with the fact that the yield of some bisalle-
mixture of diastereomers which were separated by nols is higher than 50% (entries 4–6) indicates that
flash column chromatography; the relative configura- the bis(propargyloxiranes) 1b and 1c are not exactly
tion was determined with the X-ray structures of 1:1-meso/dl-mixtures, but rather enriched in one dia-
meso-2a,[17] meso-2b,[18] and dl-2b.[19]
stereomer.[13] Only the reaction of 1b with i-PrMgCl
In principle, each propargyloxirane unit of bisoxir- and CuBr·SMe2 gave two diastereomeric bis(a-hy-
anes 1 can undergo a syn- or anti-selective SN2’-substi- droxyallenes) 2f (entry 3). Since the products 2c–i are
tution, so that 8 diastereomeric bisallenes can be not crystalline, we were unable determine their rela-
formed (Scheme 3). It is very interesting to note that tive configuration. In contrast to 1a, reaction of bisox-
Scheme 3. Possible diastereomers of bis(a-hydroxyallenes).
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Stereoselective Synthesis of Conjugated Bisallenols
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Table 1. Copper- mediated SN2’-substitution of bisoxiranes 1
to bis(a-hydroxyallenes) 2.
Table 2. Synthesis of monosubstitution products 3 from bis-
AHCTUNGTRENNGoUN xiranes 1.
Entry
1
R1
R2
3
Yield [%]
Entry
1
R1
R2
2
Yield [%]
1
2
3
4
5
6
1a
1a
1b
1c
1c
1c
Me
Me
Bn
TBS
TBS
TBS
i-Pr
t-Bu
i-Pr
n-Bu
i-Pr
t-Bu
3a
3b
3c
3d
3e
3f
49
53
49
57
54
59
1
2
3
4
5
6
7
1a
1a
1b
1b
1c
1c
1c
Me
Me
Bn
Bn
TBS
TBS
TBS
Et
Ph
2c
2d
2e
2f
2g
2h
2i
12
33
i-Pr
t-Bu
i-Pr
t-Bu
Bn
55[a]
54
57
58
21
[a]
Mixture of 2 diastereomers in a ratio of 54:46 (55% com-
bined yield).
place with axis-to-center chirality transfer. As ob-
served previously for simple a-hydroxyallenes,[9c] the
presence of water in the reaction mixture should be
irane 1c with magnesium cuprates derived from avoided since it strongly decelerates the cyclization.
CuCN gave only complex product mixtures. A second cyclization of the 2-allenyl-substituted
Further experimentation revealed that the reaction 2,5-dihydrofurans 4 to the corresponding bis(2,5-dihy-
of the propargyloxirane units of 1 with magnesium drofurans) could not be achieved with gold or silver
cuprates occurs stepwise with quite different rates. salts alone. This is probably due to the steric hin-
Whereas the first SN2’-substitution was found to be drance at the allene caused by two adjacent isopropyl
extremely fast even at ꢀ908C, the second substitution groups. In accordance with this assumption, treatment
requires higher temperatures (ꢀ208C to 08C). An ex- of the ethyl- or benzyl-substituted bisallenes 2c and 2i
ception is the phenylmagnesium cuprate which re- with 0.3 equiv. of AgNO3 afforded the bis(2,5-dihy-
quires rather higher temperatures (ca. ꢀ108C) al- drofurans) 5c and 5i with good yield after prolonged
ready for the first substitution step. For R2 ¼Ph, the reaction times (16–24 h; Scheme 4). In the case of
monosubstitution products 3 can be isolated with 49– R2 =i-Pr, the second cyclization step was realized by
59% yield by quenching the reaction mixture at taking advantage of the accelerating effect of N-iodo-
ꢀ908C (Table 2).
With various bis(a-hydroxyallenes) 2 at hand, we tions.[23] Thus, treatment of 4a (obtained from meso-
examined their cycloisomerization to bis(2,5-dihydro- 2a) with 2 mol% of gold(III) bromide in the presence
furans). Unfortunately, the use of various gold(I) or of 1.2 equiv. of NIS in CH2Cl2 afforded the richly
gold(III) salts in non-polar (dichloromethane, tolu- functionalized bis(2,5-dihydrofuran) 6a with 61%
succinimide (NIS) on gold-catalyzed transforma-
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
ene) or polar solvents (THF, acetonitrile) did not yield after just 10 min at room temperature
afford any dihydrofuran product, but resulted in no (Scheme 4).
conversion (Ph3PAuCl+AgOTf or AgSbF6; AuCl3 +
Unfortunately, all attempts to extend the silver-
2,2’-bipyridine[10c]) or decomposition of the starting mediated cycloisomerization to bis(a-hydroxyallenes)
material (AuCl; AuCl3; AuBr3; AuCl3 +CuCl2[22]). In 2 bearing very bulky substituents R2 (t-Bu, Ph) failed
the presence of p-toluenesulfonic acid in dichlorome- even at higher temperatures or under microwave con-
thane,[8] an extremely slow formation of monocyclized ditions. In the presence of NIS, however, the previ-
products was observed. In contrast to this, treatment ously unsuccessful gold-catalyzed cyclization of the
of the bisallenols 2a/e/g bearing two isopropyl groups bis(a-hydroxyallenes) 2 could be achieved (Table 3).
with substoichiometric amounts (0.25 equiv.) of silver Thus, treatment of meso- or dl-2a with 2 mol% of
nitrate in acetone at room temperature[9] induced a AuBr3 and 1.2 equiv. of NIS in CH2Cl2 provided the
rapid monocyclization to afford the corresponding 2- iodinated monocyclization products 7a and 7a’ with
allenyl-substituted 2,5-dihydrofurans 4 with excellent 60% and 78% yield, respectively, after just 10 min re-
yield (Scheme 4). In each case, a single diastereomer action time (entries 1 and 2). Even the particularly
was obtained, indicating that the cyclization takes bulky tert-butyl-substituted bisallenes 2b/f were con-
Adv. Synth. Catal. 2009, 351, 117 – 122
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Scheme 4. Silver- and gold-catalyzed cycloisomerization of bis(a-hydroxyallenes) 2.
verted into the corresponding 2,5-dihydrofurans 7 were obtained as a single diastereomer, indicating
with high yield under these conditions (entries 3 and that also these cyclizations take place with complete
5). In contrast to these substrates, the diphenyl-substi- axis-to-center chirality transfer. In contrast to the bis-
tuted bisallene 2d afforded not only the monocycliza-
ACHTUNGTRENaNGNU llenes 2, the monosubstitution products 3 failed to
tion product 7d (38%), but also traces of the sterically deliver a cycloisomerization product when subjected
crowded bis(2,5-dihydrofuran) 8d (3% yield, entry 4). to silver or gold catalysis (with or without NIS), but
In this case, use of AgNO3 in acetone instead of furnished complex product mixtures.
AuBr3 caused a slight shift of the product ratio from
In conclusion, we have developed an efficient and
7d (25%) to 8d (9% yield). All heterocyclic products stereoselective access to previously unknown bis(a-
hydroxyallenes) 2 by anti-selective SN2’-substitution of
bis(propargyloxiranes) 1 with magnesium cuprates. It
was found that the bisallenol formation takes place in
Table 3. Gold-catalyzed cycloisomerization of bis(a-hy-
ACHTUNGTRENNUNGdroxyallenes) 2 in the presence of N-iodosuccinimide (NIS).
two steps allowing the isolation of the monosubstitu-
tion products 3 at very low temperatures. In some
cases only one diastereomer of the bis(a-hydroxyal-
lene) was isolated. The isopropyl-substituted bisalle-
nols 2a/e/g undergo a facile silver-mediated monocyc-
lization to 2-allenyl-substituted 2,5-dihydrofurans 4,
whereas the less bulky substrates 2c/i afford the bis-
AHCTUNGTREG(NNNU dihydrofurans) 5. A gold-catalyzed cyclization of the
bisallenes 2 or the monocyclization products 4 is pos-
sible in the presence of N-iodosuccinimide, giving rise
to the formation of the iodinated heterocycles 6–8.
All cyclizations proceed with complete chirality trans-
fer from the allenic chirality axis to the new stereo-
genic center(s). At this point, it should be noted that
the bis-tetrahydrofurandiol structure of the type 5 is
found in Annonaceous acetogenins[24] (ACGs) pos-
sessing high anticancer activities. Thus, by taking ad-
vantage of combined coinage-metal catalysis, our
method may provide an alternative access to these
highly interesting natural products. Moreover, the in-
troduction of iodine into the dihydrofurans broadens
the opportunities for subsequent transformations. Fur-
ther studies on the synthesis and application of func-
tionalized bisallenes are currently underway.
Entry
2
R1 R2
7 (Yield [%]) 8 (Yield [%])
1
2
meso-2a Me i-Pr 7a (60)
dl-2a Me i-Pr 7a’ (78)
meso-2b Me t-Bu 7b (81)
2d
2f
–
–
–
3
4[a]
5
Me Ph
Bn t-Bu 7f (69)
7d (38)
8d (3)
–
[a]
Treatment of 2c with AgNO3 (10 mol%) and NIS
(1.2 equiv.) in acetone for 15 min at room temperature
afforded 25% of 7d and 9% of 8d.
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Stereoselective Synthesis of Conjugated Bisallenols
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Experimental Section
Synthesis of 2-Allenyl-3-iodo-2,5-dihydrofuran 7b;
Representative Procedure
Synthesis of Conjugated Bisallenol meso-2b and dl-
2b; Representative Procedure
The bisallene meso-2b (50 mg, 0.14 mmol) was dissolved in
dry dichloromethane (3 mL) and N-iodosuccinimide (37 mg,
0.17 mmol) as well as AuBr3 (1.1 mg, 3 mmol) were added.
After 10 min stirring at room temperature, TLC control in-
dicated complete consumption of the starting material. The
reaction mixture was passed through a short pad of Celite
and the solvent was removed. The residue was subjected to
flash chromatography on silica gel using cyclohexane-ethyl
acetate (4:1) as eluent to afford the 7b as a brownish-red
liquid; yield: 54 mg (81%).
Anhydrous LiBr (416 mg, 4.8 mmol) was quickly weighed
and transferred into a three-neck round-bottom flask. The
flask was evacuated, heated with a heat gun and then cooled
to room temperature under an argon atmosphere.
CuBr·SMe2 (985 mg, 4.8 mmol) and dry THF (18 mL) were
added and the suspension was vigorously stirred until it
became homogeneous and was then cooled to ꢀ258C. tert-
Butylmagnesium chloride (2.0M solution in THF, 2.4 mL,
4.8 mmol) was added dropwise. After stirring the mixture
for 30 min at ꢀ258C, it was cooled to ꢀ658C, and a THF so- Supporting Information
lution of the bis(propargyloxirane) 1a (200 mg, 0.79 mmol in
General remarks, characterization data and NMR spectra
2 mL THF) was added dropwise. The mixture was stirred for
10 min at ꢀ658C and slowly warmed up to room tempera-
ture. The completion of the reaction was proved by the
TLC. The reaction was quenched by the addition of aqueous
saturated NH4Cl (3 mL) and stirred for 30 min at room tem-
perature. It was then filtered through Celite and dried with
sodium sulfate. After removal of the solvent, the residue
was subjected to flash chromatography on silica gel using cy-
clohexane-ethyl acetate (2:1) as eluent to afford the conju-
gated bisallenols meso-2b (yield: 107 mg, 36.5%) and dl-2b
(yield: 110 mg, 37.5%) as colorless solids.
are given in the Supporting Information.
Acknowledgements
Financial support by the European Commission (Marie
Curie Early Stage Training, MEST-CT-2005–019780) is grate-
fully acknowledged.
The same procedure was used for the synthesis of the
monoallenol adduct 3b with the only change that 1a was
added at ꢀ908C and the mixture was stirred for 10 min at
ꢀ908C before being quenched with methanol at the same
temperature.
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[18] CCDC 691406 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained
122
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Adv. Synth. Catal. 2009, 351, 117 – 122