A general approach to terminal allenols

Article abstract of DOI:10.1055/s-0032-1317949

We have demonstrated a very general method for the preparation of essentially any terminal 2,3-allenol from the corresponding alkynols, which may be easily available from propargylic alcohols by alkylation, or from terminal alkynes by deprotonation and 1,

Full text of DOI:10.1055/s-0032-1317949

592▌
PRACTICAL SYNTHETIC PROCEDURES
pAracGticael snynethreaticlprAocepduprersoach to Terminal Allenols
General Approach to Terminal Allenols
Jinqiang Kuang,a Xi Xie,a Shengming Ma*a,b
a
Shanghai Key Laboratory of Green Chemistry and Chemical Process, Department of Chemistry, East China Normal University, 3663 North
Zhongshan Lu, Shanghai 200062, P. R. of China
b
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry,
Chinese Academy of Sciences, 345 Linglin Lu, Shanghai 200032, P. R. of China
Fax +86(21)62609305; E-mail: masm@sioc.ac.cn
PSP
Received: 11.10.2012; Accepted after revision: 03.12.2012
No244
Abstract: We have demonstrated a very general method for the preparation of essentially any terminal 2,3-allenol from the corresponding
alkynols, which may be easily available from propargylic alcohols by alkylation, or from terminal alkynes by deprotonation and 1,2-addi-
tion with aldehydes or ketones, and subsequent base-catalyzed triple-bond migration.
Key words: copper(I) iodide, dicyclohexylamine, allenols, long carbon chain, chirality
CuI (0.5 equiv)
Cy2NH (1.8 equiv)
OH
R2
R3
OH
R2
n
R1
OH
R3 R2
n
R3
(CH2O)n (2.5 equiv)
dioxane, reflux
60–88%
R1 = H, n-alkyl
R2, R3 = alkyl, aryl
n = 0, 1, 4, 7, 8
Scheme 1 CuI-mediated synthesis of terminal allenols with dicyclohexylamine
Introduction
bromide with aldehydes11 or ketones,12 and the Crabbé re-
action of terminal propargylic alcohols.5a–c As noted
above, compared with the traditional conditions of CuBr
and diisopropylamine, allenes can be prepared in higher
yields from alkynes and paraformaldehyde with CuI and
dicyclohexylamine.5e,f Many functional groups, such as
mesylate, hydroxy, ether, and amide, are tolerated under
these conditions. In this paper, we wish to report our ex-
ploration of a general synthesis of basically all allenols,
including optically active examples, with different carbon
chains between the allene moiety and the hydroxy group
(Scheme 1).5c,13
During the last 10–15 years, allenes have been shown to
be versatile intermediates in organic synthesis.1,2 In addi-
tion to this, many natural products and pharmaceuticals
with an allene moiety have been identified.3 Thus, syn-
thetic methods for allenes from readily available organic
compounds4 are of high interest for chemists who use al-
lenes as the starting materials for their target syntheses.
Crabbé and co-workers reported the first CuBr-mediated
reaction to form terminal allenes from 1-alkynes and
formaldehyde in the presence of diisopropylamine.5a–d
Based on this, recently we have developed a modified
one-step procedure for converting terminal alkynes into Scope and Limitations
terminal allenes in higher yields by applying CuI, para-
Firstly, we investigated the synthesis of 2,3-allenols using
formaldehyde, and dicyclohexylamine.5f,g We have also
developed a ZnI2-mediated protocol for the synthesis of
1,3-disubstituted allenes, including chiral examples.5h–k
our modified method (Conditions B) and, at the same
time, we conducted reactions using the traditional Crabbé
reaction conditions as a comparison (Conditions A); the
As a type of synthetically attractive functionalized allene, results are summarized in Table 1. For secondary propar-
allenols are important synthetic moieties in organic syn- gylic alcohols with an alkyl substituent at the α-position,
thesis due to the combination of reactive allene moiety the reaction afforded the corresponding allenols 2a and 2b
and the hydroxy functionality. Syntheses of 2,5-dihydro- in 68% and 77% isolated yield, respectively (Table 1, en-
furans,6 5,6-dihydro-2H-pyrans,7 epoxides,8 3-halo-3-al- tries 1 and 2). For propargylic alcohols bearing an aryl
kenals,9 and other derivatives10 from allenols have been group at the α-position, allenols 2c–e were obtained in
reported. Thus, an easy and efficient procedure for obtain- 75–88% isolated yield (Table 1, entries 3–5). The reaction
ing allenols is of high value. Usually, 2,3-allenols can be of tertiary propargylic alcohols substituted with an alkyl
prepared by the metal-mediated reaction of propargylic and an aryl group at the α-position afforded allenols 2f and
2g in 84% and 85% isolated yield, respectively (Table 1,
entries 6 and 7). Cyclic alcohols also worked smoothly
(Table 1, entries 8–10).
SYNTHESIS 2013, 45, 0592–0595
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DOI: 10.1055/s-0032-1317949; Art ID: SS-2012-H0798-PSP
© Georg Thieme Verlag Stuttgart · New York

PRACTICAL SYNTHETIC PROCEDURES
General Approach to Terminal Allenols
593
Table 2 Copper(I)-Mediated Synthesis of 3,4-Allenols from Homo-
Table 1 Copper(I)-Mediated Synthesis of 2,3-Allenols from Propar-
propargylic Alcohols and Paraformaldehydea
gylic Alcohols and Paraformaldehydea
CuBr (0.5 equiv), i-Pr2NH (1.8 equiv)
CuBr (0.5 equiv), i-Pr2NH (1.8 equiv)
(CH2O)n (2.5 equiv), dioxane, reflux
(CH2O)n (2.5 equiv), dioxane, reflux
conditions A
conditions A
OH
R2
R1
OH
R1
R2
R1
R2
CuI (0.5 equiv), Cy2NH (1.8 equiv)
(CH2O)n (2.5 equiv), dioxane, reflux
OH
R2 R1
1
CuI (0.5 equiv), Cy2NH (1.8 equiv)
(CH2O)n (2.5 equiv), dioxane, reflux
HO
2
1
2
conditions B
conditions B
Entry
1
Conditions A
Conditions B
Time Yield
Entry 1
Conditions A
Time Yield
Conditions B
R1; R2
Time
(h)
Yield
(%) of 2 (h)
R1; R2
Time
Yield
(%) of 2
(h)
(%) of 2 (h)
(%) of 2
1
2
H; n-C7H15 (1a)
H; n-C9H19 (1b)
H; Ph (1c)
2.3
2.3
2.2
2.6
2.6
3.3
2.2
3.5
2.3
3.0
51
65
64
73
65
70
65
54
53
51
2.3
2.3
3.0
2.6
2.6
2.3
2.8
2.5
2.5
3.0
68 (2a)
77 (2b)
75 (2c)
88 (2d)
87 (2e)
84 (2f)
85 (2g)
69 (2h)
69 (2i)
65 (2j)
1
2
3
H; 4-BrC6H4 (1k)
H; 4-MeOC6H4 (1l)
Me; 4-Tol (1m)
2.0
2.1
3.0
39
37
49
2.5
2.5
2.8
64 (2k)
77 (2l)
3
64 (2m)
a Reactions were carried out on a 2.0-mmol scale in 3.0 mL of 1,4-di-
oxane.
4
H; 4-ClC6H4 (1d)
H; 4-MeOC6H4 (1e)
Me; 4-Tol (1f)
Me; 3-Tol (1g)
-(CH2)6- (1h)
5
6
bearing an electron-rich 4-methoxyphenyl substituent af-
forded allenol 2l in 77% isolated yield (Table 2, entries 1
and 2). Tertiary homopropargylic alcohol 1m was also
converted into the corresponding allenol 2m in 64% iso-
lated yield (Table 2, entry 3).
7
8
9
-(CH2)5- (1i)
10
-(CH2)3- (1j)
Since ω-alkynols with a long carbon chain can be pre-
pared conveniently from propargylic alcohols by
alkylation14 or from 1-alkynes by deprotonation and 1,2-
addition with aldehydes or ketones,15 and subsequent
base-catalyzed triple-bond migration,16 a series of ω-alle-
nols 2n–p with different carbon chains between the allene
moiety and the hydroxy group could be efficiently synthe-
sized using the current protocol (Scheme 2).
a Reactions were carried out on a 2.0-mmol scale in 3.0 mL of 1,4-di-
oxane.
Secondly, we expanded the scope of substrates to second-
ary and tertiary homopropargylic alcohols. When the sec-
ondary homopropargylic alcohol bears a 4-bromophenyl
substituent at the α-position, allenol 2k was obtained in
64% isolated yield, while homopropargylic alcohol 1l
Finally, it was observed that optically active ω-allenols
such as (R)-2p and (S)-2p could be prepared without
racemization17 (Scheme 3).
It can be seen that the replacement of diisopropylamine by
dicyclohexylamine and of CuBr by CuI improved the re-
action yields, and almost all types of terminal allenols
have been synthesized. Of course, it should be noted that
this procedure only works for terminal allenols.13h–j
R1
R2X, LiNH2
R1
n
OH
base
R2
OH
R1
OH
EtMgBr
RCHO
H2N
NH2
R2
In conclusion, we have demonstrated a very general meth-
od for the preparation of essentially any terminal allenols
from the corresponding alkynols, which may be easily
available from propargylic alcohols by alkylation, or from
terminal alkynes by deprotonation and 1,2-addition with
aldehydes or ketones, and subsequent base-catalyzed tri-
ple-bond migration, by applying 0.5 equivalents of CuI,
1.8 equivalents of dicyclohexylamine, and 2.5 equivalents
of paraformaldehyde. Due to the diverse utility of allenols
in organic chemistry and the broad generality, this method
will be of interest since it is easy to undertake and of high
efficiency. Further studies in this area are being pursued
in our laboratory.
CuBr (0.5 equiv), i-Pr2NH (1.8 equiv)
(CH2O)n (2.5 equiv), dioxane, reflux
conditions A
OH
R
n
OH
n
CuI (0.5 equiv), Cy2NH (1.8 equiv)
(CH2O)n (2.5 equiv), dioxane, reflux
R
1
2
conditions B
(2 mmol)
conditions A conditions B
products
2n
2o
R = H, n = 7
R = H, n = 8
40%
37%
49%
68%
66% (72%)a
71%
R = Et, n = 4 2p
Scheme 2 Synthesis of allenols 2n–p with different carbon chains be-
tween the hydroxy group and the allene moiety. Yield of a 29.6-
mmol-scale reaction.
a
© Georg Thieme Verlag Stuttgart · New York
Synthesis 2013, 45, 592–595

594
J. Kuang et al.
PRACTICAL SYNTHETIC PROCEDURES
OH
Et
n-Bu
88% ee
KOH
96%
MeOH
OH
OAc
OH
OAc
n-BuLi
CH3CH2CHO
85%
Et
43%, 99% ee
Et
n-Bu
+
Novozym 435
60 °C
Et
n-Bu
n-Bu
n-Bu
ref. 17
44%
CuI (0.5 equiv)
Cy2NH (1.8 equiv)
HO
HO
HO
OH
Li, t-BuOK
Et
Et
Et
Et
Et
H2N(CH2)2NH2
(CH2O)n (2.5 equiv)
dioxane, reflux
(S)-1p, 71%
(R)-1p, 67%
(S)-2p, 60%, 99% ee
n-Bu
99% ee
CuI (0.5 equiv)
Cy2NH (1.8 equiv)
HO
OH
Li, t-BuOK
Et
H2N(CH2)2NH2
(CH2O)n (2.5 equiv)
dioxane, reflux
(R)-2p, 63%, 89% ee
n-Bu
88% ee
Scheme 3 Copper(I)-mediated synthesis of optically active allenols from optically active alkynols and paraformaldehyde
1H and 13C NMR spectra were recorded in CDCl3 with a Bruker in-
phy on silica gel (petroleum ether–EtOAc, 10:1) afforded allenol
2o5e as a liquid; yield: 243.2 mg (66%).
1
strument operated at 300 MHz for H NMR and 75 MHz for 13C
IR (neat): 3330, 2925, 2854, 1956, 1463, 1057 cm–1.
NMR. Chemical shifts (δ) are given in parts per million (ppm). In-
frared spectra were recorded on a Bruker FT-IR spectrophotometer.
Mass spectra were recorded on an Agilent spectrometer in EI mode.
HRMS spectra were recorded on a Waters spectrometer in EI mode.
Cy2NH, i-Pr2NH, CuI, CuBr, and (CH2O)n were purchased from
Sinopharm Chemical Reagent Co., Ltd. and were used without fur-
ther treatment. 1,4-Dioxane was dried over sodium wire with ben-
zophenone as indicator and freshly distilled before use.
1H NMR (300 MHz, CDCl3): δ = 5.08 (quintet, J = 6.8 Hz, 1 H,
C=CH), 4.64 (dt, J = 6.6, 3.3 Hz, 2 H, C=CH2), 3.69–3.57 (m, 2 H,
OCH2), 2.05–1.91 (m, 2 H, C=CCH2), 1.63–1.48 (m, 2 H, CH2),
1.47–1.21 (m, 13 H, OH and 6 × CH2).
13C NMR (75.4 MHz, CDCl3): δ = 208.4, 90.0, 74.5, 63.0, 32.8,
29.5, 29.4, 29.3, 29.1, 29.0, 28.2, 25.7.
MS (EI, 70 eV): m/z (%) = 165 (19.37) [M+ – OH], 41 (100).
Dodeca-10,11-dien-1-ol (2o); Typical Procedure for Conditions
A
Compounds 2a–p were prepared according to the typical proce-
dures for conditions A and B, and the experimental data are given
in the supporting information.
(CH2O)n (149.9 mg, 5.0 mmol), CuBr (143.8 mg, 1.0 mmol), al-
kynol 1o (336.6 mg, 2.0 mmol), i-Pr2NH (363.6 mg, 3.6 mmol), and
1,4-dioxane (3 mL) were added sequentially into a dried reaction
tube equipped with a reflux condenser under a nitrogen atmosphere.
The resulting mixture was stirred under reflux. When the reaction
was complete (after 2.4 h, as monitored by TLC), the mixture was
cooled to r.t. and filtered. Et2O (10 mL) and H2O (5 mL) were add-
ed. Then, several drops of concd aq HCl were added to separate the
organic layer from the aqueous layer clearly. The aqueous solution
was separated and then extracted with Et2O (3 × 10 mL). The com-
bined organic layer was then dried (anhyd MgSO4). Filtration, con-
centration, and column chromatography on silica gel (petroleum
ether–EtOAc, 10:1) afforded allenol 2o as a liquid; yield: 134.2 mg
(37%).
Dodeca-10,11-dien-1-ol (2o); Procedure for Large-Scale Reac-
tion
To a 250-mL three-neck flask equipped with a reflux condenser
were added (CH2O)n (2.2253 g, 74.0 mmol), CuI (2.8193 g, 14.8
mmol), alkynol 1o (4.9951 g, 29.6 mmol), 1,4-dioxane (20 mL, to
rinse the alcohol), Cy2NH (9.6467 g, 53.5 mmol), and 1,4-dioxane
(22 mL, to rinse the amine). The resulting mixture was refluxed
with stirring. When the reaction was complete (after 3 h, as moni-
tored by TLC), the resulting mixture was cooled to r.t. and filtered.
The filtrate was acidified with concd HCl to pH 1–2. Then, Et2O (20
mL) was added to allow the separation of the organic layer from the
aqueous layer. The aqueous layer was extracted with Et2O (3 × 10
mL). The combined organic layer was washed with brine until
pH 6–7 and dried (anhyd MgSO4). Filtration, concentration, and
column chromatography on silica gel (petroleum ether–EtOAc,
10:1) afforded the terminal allenol 2o; yield: 3.8875 g (72%).
Dodeca-10,11-dien-1-ol (2o); Typical Procedure for Conditions
B
(CH2O)n (150.1 mg, 5.0 mmol), CuI (190.8 mg, 1.0 mmol), alkynol
1o (336.6 mg, 2.0 mmol), Cy2NH (651.8 mg, 3.6 mmol), and 1,4-
dioxane (3 mL) were added sequentially into a dried reaction tube
equipped with a reflux condenser under a nitrogen atmosphere. The
resulting mixture was stirred under reflux. When the reaction was
complete (after 2.4 h, as monitored by TLC), the mixture was
cooled to r.t. and filtered. Et2O (10 mL) and H2O (5 mL) were add-
ed, and then the aqueous solution was separated and extracted with
Et2O (3 × 10 mL). The combined organic layer was then dried (an-
hyd MgSO4). Filtration, concentration, and column chromatogra-
Acknowledgment
Financial support from the National Basic Research Program of
China (No. 2009CB825300) and the National Natural Science
Foundation of China (No. 21232006) is greatly appreciated. We
thank Minyan Wang in our group for reproducing the results pre-
sented in the text.
Synthesis 2013, 45, 592–595
© Georg Thieme Verlag Stuttgart · New York

PRACTICAL SYNTHETIC PROCEDURES
General Approach to Terminal Allenols
595
B.; Wan, B.; Wang, Y.; Xie, X.; Yu, Q.; Yuan, W.; Ma, S.
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Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synthesis.SnuIomprifgtooatrinSugpIoi
n
nfomartit
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