One-pot asymmetric synthesis of 2-and 2,3-disubstituted tetrahydrofuran derivatives

Article abstract of DOI:10.1021/jo300414u

A novel and convenient one-pot asymmetric synthesis of 2- and 2,3-disubstituted tetrahydrofurans has been achieved in 56-81% yields and 86-99% ee from aliphatic and aromatic aldehydes via an allyl/crotyl/ alkoxyallylboration-hydroboration-iodination-cyclization reaction sequence.

Full text of DOI:10.1021/jo300414u

Note
pubs.acs.org/joc
One-Pot Asymmetric Synthesis of 2- and 2,3-Disubstituted
Tetrahydrofuran Derivatives
P. Veeraraghavan Ramachandran,* Hari N. G. Nair, and Pravin D. Gagare
Herbert C. Brown Center for Borane Research, Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette,
Indiana 47907-2084, United States
S
* Supporting Information
ABSTRACT: A novel and convenient one-pot asymmetric
synthesis of 2- and 2,3-disubstituted tetrahydrofurans has been
achieved in 56−81% yields and 86−99% ee from aliphatic and
aromatic aldehydes via an allyl/crotyl/alkoxyallylboration−
hydroboration−iodination−cyclization reaction sequence.
inane-based asymmetric allylboration and related reac-
substituted THF derivatives.9 Reported herein is a novel,
one-pot asymmetric synthesis of chiral THF derivatives in high
diastereo- and enantiomeric excesses from aldehydes via a
sequential allylboration−hydroboration−iodination−cyclization
reaction.
P
tions1 are significant C−C bond forming reactions that are
used for the stereoselective syntheses of simple and complex
molecules.2 Tandem reactions involving allyl-derived boranes
provide a one-pot procedure for the synthesis of either
intermediates or end-products. For example, Brown and co-
workers reported a tandem allylboration−protection−hydro-
boration−oxidation−deprotection−cyclization protocol for the
synthesis of chiral γ-lactones.3 Taking advantage of the
uniqueness of fluorinated alcohols, this protocol was applied
for the preparation of fluorinated γ-lactones, without the need
for the protection−deprotection sequence.4
We recently reported the allyboration of imines, followed by
hydroboration−oxidation in the same pot, for the preparation
of γ-aminobutyric acid derivatives.5 We also recently described
a one-pot synthesis of tetrahydropyrans via an allyl/
crotylboration−Prins cyclization.6 In continuation of our
projects on tandem reactions, a simple one-pot synthesis of
tetrahydrofurans (THF) involving an intramolecular Mitsuno-
bu reaction of the 1,4-diols obtained via allylboration−
oxidation was envisaged (Scheme 1). Substituted THF
derivatives are key components of a wide range of biologically
active natural products, such as lignans7 and annonceous
acetogenins.8 Because of this importance, the THF moiety
continues to attract the attention of organic chemists. Various
methodologies have been described for the synthesis of
Because of the ease of analysis of the products, we began the
project with the allylboration of 1-naphthaldehyde (2a) with
(−)-B-allyldiisopinocampheylborane [(−)-Ipc2BAll, 1a] in
Et2O at −100 °C.10,11 Analysis of the homoallylic alcohol
1
revealed an ee of 98% (as determined by the H NMR of its
MTPA ester). The hydroboration of the borinate intermediate
3a was carried out in the same pot with BH3−SMe2 and was
subjected to alkaline H2O2 oxidation.12 1H NMR analysis of the
crude reaction mixture revealed the formation of a 83:17
mixture of the 1,4- (5a) and 1,3-diols (5′a) (Scheme 2). HPLC
analysis of 5a on a Chiralcel OD-H column demonstrated that
there was a complete transfer of chirality from 1a to 5a.
Scheme 2. One-Pot Asymmetric Allylboration−
Hydroboration−Oxidation
Scheme 1. Retrosynthesis of THF Derivatives via
Cyclodehydration
Screening other hydroborating agents, such as BH3−THF, 9-
borabicyclo[3.3.1]nonane (9-BBN), Br2BH−SMe2, and dicy-
clohexylborane (Chx2BH)13 revealed the exclusive formation of
the 1,4-diol (5) with only Chx2BH and 9-BBN (Table 1).
Received: February 27, 2012
Published: May 18, 2012
© 2012 American Chemical Society
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The Journal of Organic Chemistry
Note
The allylboration of 1-naphthaldehyde was carried out in
Et2O at −100 °C for 5 min; the reaction mixture was then
transferred via cannula to a flask containing Chx2BH in Et2O at
rt. The solid Chx2BH was visibly consumed within 5 min, while
the 11B NMR spectrum of the intermediate borinate 4a
revealed two broad peaks at δ 52 and 84 ppm (corresponding
to a borinate and a trialkylborane, respectively). Iodination of a
1°-carbon attached to boron is reported to be much faster than
that of a 2°- or a 3°-carbon.17 Accordingly, the addition of 1
equiv of methanolic NaOMe to 4a revealed the disappearence
of the trialkylborane (11B NMR peak at δ 84 ppm) and the
formation of two different tetracoordinated boron species (11B
NMR peaks at δ 8 and 1 ppm).18 The peak due to the
intermediate borinate at δ 52 ppm was still present, though
there was a decrease in its size.19 Iodine was added at this stage
and stirred for 2 h, whereafter the 11B NMR spectrum revealed
the regeneration of the peak at δ 52 ppm and the disappearance
of the peaks due to the “ate” complexes. This could be
rationalized by the formation of Chx2BOMe upon iodination.
Workup provided only 10% yield of the corresponding THF
derivative 6a along with the corresponding iodinated product,
4-iodo-1-(naphthalen-1-yl)butan-1-ol, in 61% yield. The above
sequence of reactions provided 6a in 81% yield when 2 equiv of
methanolic NaOMe were used. Alternately, a high yield of 6a
can also be achieved by the treatment of 4a with 1 equiv of
methanolic NaOMe and I2, followed by the addition of 1 equiv
of aq NaOH (Scheme 4).
Table 1. Regioselectivity of Hydroboration of Homoallyl
Alcohol Borinate
diol
R′2BH
equiv
a
b
entry RCHO
R′2BH
5
yield, %
5:5′
1
2a
2a
2a
2a
2a
2a
2a
2b
2b
2b
2b
2b
2b
2c
2d
2e
2f
BH3−SMe2
BH3−THF
Br2BH−SMe2
9-BBN
1.2
1.2
1.2
1.2
1.2
1.5
2.0
1.2
1.2
1.2
1.2
1.5
2.0
1.5
1.5
1.5
1.5
5a
5a
5a
5a
5a
5a
5a
5b
5b
5b
5b
5b
5b
5c
5d
5e
5f
66
71
76
68
77
81
81
65
66
60
60
71
71
72
65
67
65
87:13
91:9
2
3
93:7
4
>98:<2
>98:<2
>98:<2
>98:<2
83:17
5
Chx2BH
Chx2BH
Chx2BH
BH3−SMe2
BH3-THF
9-BBN
6
7
8
9
88:12
10
11
12
13
14
15
16
17
96:4
Chx2BH
Chx2BH
Chx2BH
Chx2BH
Chx2BH
Chx2BH
Chx2BH
>98:<2
>98:<2
>98:<2
>98:<2
>98:<2
>98:<2
>98:<2
a
b
Isolated yield. The ratio of 5:5′ was determined by 1H NMR of the
crude reaction mixture.
Increasing the equivalents of Chx2BH from 1.2 to 1.5 improved
the yield of 5 to 81%. Further increasing the amounts of the
hydroborating agent did not enhance the yields. We also
performed the allylboration−hydroboration−oxidation se-
quence with p-trifluoromethylbenzaldehyde (2b); in this case,
Chx2BH gave the best regioselectivity. We therefore chose
Chx2BH for further reactions.
Scheme 4. One-Pot Allylboration−Hydroboration−
Iodination−Cyclization
The stage was now set to examine the intramolecular
Mitsunobu reaction.14 Evans and co-workers had reported that
the cyclodehydration of (S)-phenylethane-1,2-diol with various
phosphorus reagents resulted in significant racemization.15 The
application of this method for the synthesis of the THF
derivative from 1,4-diol 5a gave only 33% yield of the desired
product, (S)-2-(naphthalen-1-yl)tetrahydrofuran (6a). Unfortu-
nately, the enantiomeric excess dropped from 98 to 80% for 6a.
Low yields and a significant drop in ee were observed with the
chiral 1,4-diols derived from 2b and 2d as well.
Weissman et al. has described the use of Chx3P-DIAD as an
effective reagent system to achieve high stereoretention for the
Mitsunobu cyclodehydration during the preparation of chiral
styrene oxide derivatives.16 The high cost of Chx3P dissuaded
us from using the Weissman protocol, and an intramolecular
ether synthesis via the iodination of intermediate 4 appeared
attractive (Scheme 3).
Thus under optimized conditions, a 1 M solution of (−)-B-
allyldiisopinocampheylborane in pentane (3.1 mL) was added
at −100 °C to 2a (3 mmol) dissolved in dry ether (6 mL), and
the mixture was stirred for 5 min. The reaction mixture was
added to Chx2BH (0.8 g, 4.5 mmol) and allowed to warm to
room temperature, and then it was stirred for 5 min. Iodine
(1.52 g, 6 mmol) was added to the reaction mixture, followed
by a 1 M solution of sodium methoxide in methanol (6 mL, 6
mmol). After stirring for 2 h, the crude product was extracted
with ether, dried over anhydrous Na2SO4, concentrated and
purified by flash column chromatography to obtain 6a in 81%
yield and 98% ee. Once the conditions for the one-pot
allylboration−hydroboration−iodination−cyclization were op-
timized, the generality of the reaction was demonstrated with a
series of aldehydes possessing varying steric and electronic
properties (Table 2). The aldehyde with an electron-with-
drawing (p-CF3) group 2b provided product 6b in 70% yield
and 90% ee, whereas the aldehyde with an electron-donating
(p-MeO) group 2d provided the product 6d in 60% yield and
95% ee. Aliphatic aldehydes 2e and 2f formed 6e and 6f in 67
and 65% yields, respectively, and 96 and 92% ee, respectively.
The ee-values of the THF derivatives were determined by
HPLC analysis on a Chiralcel OD-H column and/or by GC
analysis on a CP-Chirasil-Dex CB column.
Scheme 3. Revised Retrosynthesis of THF Derivatives:
Iodination Approach
This methodology was then extended to include crotylbora-
tion- and methoxyallylboration−hydroboration−iodination−
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Note
Table 2. One-Pot Allylboration−Hydroboration−Iodination−Cyclization of Aldehydes
a
b
c
Isolated overall yield for 4 steps. Determined by HPLC analysis on a Chiralcel OD-H column. Determined by GC analysis on a CP-Chirasil-Dex
d
CB column. On the basis of the ee of the homoallylic alcohol.
reactions were monitored by thin-layer chromatography carried out on
0.25 mm silica plates (60F-254) using UV light as visualizing agent. In
general, reactions were carried out in dry solvents under nitrogen
cyclization to prepare the corresponding 2,3-disubstituted THF
derivative from 2a. E- and Z-crotylboration of 1-naphthalde-
hyde with (E)-B-crotyldiisopinocampheylborane (E-1b) and
(Z)-crotyldiisopinocampheylborane (Z-1b), followed by hydro-
boration−iodination−cylization provided the corresponding
diastereomerically pure 3-methyl-2-naphthyl-tetrahydrofurans,
trans-7a and cis-7a, in 90 and 86% ee, respectively.
Alkoxyallylboration of 1-naphthaldehyde using B-methoxyallyl-
diisopinocampheyl-borane (1c), followed by the hydrobora-
tion−iodination−cyclization reaction sequence afforded essen-
tially enantiomerically pure 2-methoxy-3-naphthyltetrahydro-
furan (8a) in 56% yield, >99% de and 99% ee (Table 2).
In conclusion, an efficient and convenient asymmetric
synthesis of 2-substituted and 2,3-disubstituted tetrahydrofur-
ans has been achieved via a novel one-pot allylboration−
hydroboration−iodination−cyclization protocol.
1
atmosphere, unless noted otherwise. H, 19F, and 13C NMR spectra
were recorded either on a 400 MHz or on a 300 MHz NMR
spectrometer. Chemical shifts, multiplicity (s, singlet; d, doublet; t,
triplet; q, quartet; m, multiplet), coupling constant in hertz (Hz), and
number of protons. Flash chromatography was performed using silica
gel 40−63 μm, 60 Å and hexane−ethyl acetate mixture as eluent.
Procedure of One-Pot Allylboration−Hydroboration−Oxi-
dation Reaction. Synthesis of 1,4-Diols. A pentane solution of
(−)-B-allyldiisopinocampheylborane (1 M, 3.1 mL, 3 mmol) was
added at −100 °C to aldehyde (3 mmol) dissolved in dry ether (6
mL) and maintained at −100 °C. After stirring for 5 min, this reaction
mixture was added to Chx2BH (0.8 g, 4.5 mmol) in ether (3 mL),
allowed to warm to room temperature, and stirred for 5 min. Reaction
was followed by TLC. After completion, the reaction mixture was
cooled to 0 °C and quenched by the addition of 3 N aq NaOH
solution (2 mL), followed by slow addition of 30% hydrogen peroxide
(2 mL), and stirred for 4 h at room temperature. Organic layer was
separated, and aqueous layer was extracted with ether (3 × 15 mL).
The combined organic layer was dried over anhydrous Na2SO4,
concentrated, and purified by flash column chromatography.
EXPERIMENTAL SECTION
■
General Information. All reactions were performed under inert
atmosphere. All solvents for routine isolation of products and
chromatography were reagent grade. Reaction flasks were dried in
an oven at 110 °C for 12 h. Air- and moisture-sensitive reactions were
performed under nitrogen atmosphere. Flash chromatography was
performed using silica gel (100−200 mesh) with indicated solvents. All
General Procedure for the Synthesis of THF-Derivative-
Cyclodehydration Method. Triisopropylphosphine (0.3 g, 1.87
mmol) was dissolved in anhydrous THF (3 mL) at rt, followed by
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The Journal of Organic Chemistry
Note
cooling to 5 °C and the addition of diisopropyl azodicarboxylate (0.35
mL, 1.8 mmol). The reaction was warmed to 15 °C. After 10 min, the
diol (1.20 mmol) dissolved in THF (3 mL) was added dropwise,
followed by warming to 25 °C, and then it was stirred for 6 h. Reaction
was followed by TLC. The reaction mixture was quenched by addition
of saturated NH4Cl solution, extracted with ether, dried over Na2SO4,
and purified using column chromatography.
General Procedure for the Preparation of THF-Derivative:
One-Pot Allylboration−Hydroboration−Iodination−Cycliza-
tion. Aldehyde (3 mmol) was dissolved in dry ether (6 mL). A 1
M solution of (−)-B-allyldiisopinocampheylborane (3.1 mL) in
pentane was added at −100 °C and stirred for 5 min. After stirring
for 5 min, this reaction mixture was added to Chx2BH (0.8 g, 4.5
mmol) in ether (3 mL), allowed to warm to room temperature, and
stirred for 5 min. Iodine (1.52 g, 6 mmol) was added to the reaction
mixture, followed by 1 M solution of sodium methoxide in methanol
(6 mL, 6 mmol), and the mixture was stirred for 2 h. Reaction was
followed by TLC. The crude product was extracted with ether, dried
over anhydrous Na2SO4, concentrated, and purified by flash column
chromatography.
added via cannula, and after 5 min, a 2.5 M solution of n-BuLi in
hexanes (76.4 mL 2.5 mmol) was added dropwise. The reaction
mixure was warmed to −55 °C, stirred there for 45 min, and again
cooled to −78 °C. A −78 °C cooled solution of (−)-Ipc2BOMe (3
mmol) in THF was added dropwise via cannula, and the reaction
mixture was stirred for 1 h. BF3·Et2O (3.6 mmol) was added dropwise,
followed by a dropwise addition of the −78 °C cooled solution of
aldehyde (2 mmol) in THF (1 mL) via cannula. The reaction mixture
was stirred at −78 °C for 3 h. Reaction mixture was warmed to 0 °C,
and dicyclohexylborane (7.5 mmol) was added. Reaction mixture was
warmed to room temperature and stirred for 5 min. Iodine (1.52 g, 6
mmol) was added to the reaction mixture, followed by a 1 M solution
of sodium methoxide in methanol (6 mL, 6 mmol), and the mixture
was stirred for 2 h. Reaction was followed by TLC. The crude product
was extracted with ether, dried over anhydrous Na2SO4, concentrated,
and purified by flash column chromatography.
(2S,3R)-3-Methyl-2-(naphthalen-1-yl)tetrahydrofuran (cis-
7a). cis-7a was obtained as pale-brown gummy oil in 63% yield
(0.267 mg).
1
Spectral data: H NMR (300 MHz, CDCl3) δ 7.94−7.81 (m, 2H),
7.77−7.66 (m, 2H), 7.53−7.41 (m, 3H), 5.64 (d, J = 6.0 Hz, 1H), 4.22
(dd, J = 15.3, 7.3 Hz, 1H), 3.99 (td, J = 8.3, 5.8 Hz, 1H), 2.94−2.75
(m, 1H), 2.48−2.22 (m, 1H), 1.85−1.73 (m, 1H), 0.45 (d, J = 7.1 Hz,
2H); 13C NMR (101 MHz, CDCl3) δ 136.2, 133.4, 130.5,128.9, 127.2,
125.7, 125.5, 125.3, 123.4, 122.9, 80.8, 66.5, 36.7, 34.6, 15.8; HRMS
(ESI) C15H16O calc. 212.1201, found 212.1221.
(S)-2-(Naphthalen-1-yl)tetrahydrofuran (6a). 6a was obtained
as pale-yellow liquid in 81% yield (0.481 mg).
1
Spectral data: H NMR (CDCl3, 300 MHz) δ 7.99−7.96 (m, 1H),
7.88−7.85 (m, 1H), 7.75 (d, J = 8.4 Hz, 1H), 7.64 (d, J = 7.2 Hz, 1H),
7.53−7.43 (m, 3H), 5.65 (t, J = 6.9 Hz, 1H), 4.28−4.20 (m, 1H),
4.07−4.00 (m, 1H), 2.62−2.51 (m, 1H), 2.12−1.86 (m, 3H); 13C
NMR (CDCl3, 75 MHz) δ 139.3, 130.4, 128.8, 127.4, 125.8, 125.5,
125.4, 123.4, 121.8, 77.9, 68.8, 33.8, 26.0; HRMS (ESI) C14H14O calc.
198.1045, found 198.1069.
(2S,3S)-3-Methyl-2-(naphthalen-1-yl)tetrahydrofuran (trans-
7a). trans-7a was obtained as yellow gummy oil in 66% yield (0.280
mg).
1
Spectral data: H NMR (300 MHz, CDCl3) δ 8.07 (d, J = 7.6 Hz,
(S)-2-(4-(Trifluoromethyl)phenyl)tetrahydrofuran (6b). 6b
1H), 7.83 (d, J = 2.2 Hz, 1H), 7.75 (d, J = 8.1 Hz, 1H), 7.56−7.40 (m,
4H), 5.14 (d, J = 6.4 Hz, 1H), 4.25 (dd, J = 14.1, 8.0 Hz, 1H), 4.12
(dd, J = 14.8, 7.8 Hz, 1H), 2.41 (dq, J = 13.5, 6.8 Hz, 1H), 2.28−2.14
(m, 1H), 1.73 (dt, J = 13.0, 6.7 Hz, 1H), 1.18 (d, J = 6.7 Hz, 3H); 13C
NMR (101 MHz, CDCl3) δ 137.9, 133.8, 131.1, 128.7, 127.8, 125.7,
125.3, 123.7, 123.3, 85.2, 85.2, 67.7, 41.1, 33.9, 17.7; HRMS (ESI)
C15H16O calc. 212.1201, found 212.1231
was obtained as pale-yellow liquid in 70% yield (0.454 mg).
Spectral data: H NMR (CDCl3, 300 MHz) δ 7.58 (d, J = 8.1 Hz,
1
2H), 7.44 (d, J = 8.1 Hz, 2H), 4.94 (t, J = 7.2 Hz, 1H), 4.14−3.92 (m,
1H), 3.99−3.92 (m, 1H), 2.42−2.32 (m, 1H), 2.05−1.96 (m, 2H),
1.82−1.70 (m, 1H). 19F NMR (CDCl3, 282 MHz) δ −62.33 (s, 3F);
13C NMR (CDCl3, 75 MHz) δ 147.8, 125.8, 125.3, 125.2, 80.0, 68.8,
34.7, 25.9; HRMS (ESI) C11H11F3O calc. 216.0762, found 216.0738.
(S)-2-Phenyltetrahydrofuran (6c). 6c was obtained as colorless
liquid in 74% yield (0.328 mg).
(2R,3R)-3-Methoxy-2-(naphthalen-1-yl)tetrahydrofuran (8a).
8a was obtained as pale-brown gummy solid in 56% yield (0.255 mg).
1
Spectral data: H NMR (CDCl3, 300 MHz) δ 7.98−7.97 (m, 1H),
1
Spectral data: H NMR (CDCl3, 300 MHz) δ 7.2−7.43 (m, 5H),
7.90−7.86 (m, 1H), 7.82−7.78 (m, 1H), 7.54−7.47 (m, 2H), 7.53−
7.43 (m, 3H), 5.54 (d, J = 3.6 Hz, 1H), 4.35−4.27 (m, 2H), 4.05 (dt, J
= 4.8 and 8.7 Hz, 1H), 2.77 (s, 3H), 2.42−2.20 (m, 2H); 13C NMR
(CDCl3, 75 MHz) δ 139.3, 130.4, 128.8, 127.4, 125.8, 125.5, 125.4,
123.4, 121.8, 77.9, 68.8, 33.8, 26.0; HRMS (ESI) C15H16O2 calc.
228.1150, found 228.1170
4.82 (t, J = 7.8 Hz, 1H), 4.08−4.15 (m, 1H), 3.96−3.85 (m, 1H),
2.25−2.36 (m, 1H), 2.06−1.96 (m, 2H), 1.88−1.72 (m, 1H); 13C
NMR (CDCl3, 75 MHz) δ 143.5, 128.4, 127.2, 125.7, 80.8, 68.8, 34.7,
26.1; HRMS (ESI) C10H12O calc. 148.0888, found 148.0899.
(S)-2-(4-Methoxyphenyl)tetrahydrofuran (6d). 6d was ob-
tained as gummy liquid in 60% yield (0.320 mg).
1
Spectral data: H NMR (CDCl3, 300 MHz) δ 7.28−7.25 (m, 2H),
ASSOCIATED CONTENT
* Supporting Information
6.90−6.86 (m, 2H), 4.83 (t, J = 9.0 Hz, 1H), 4.12−4.05 (m, 1H),
3.95−3.87 (m, 1H), 3.80 (s, 3H), 2.33−2.23 (m, 1H), 2.06−1.96 (m,
2H), 1.88−1.72 (m, 1H); 13C NMR (CDCl3, 75 MHz) δ 158.8, 135.4,
127.0, 113.7, 80.5, 68.5, 55.3, 34.5, 26.1; HRMS (ESI) C11H14O2 calc.
178.0994, found 178.0982.
■
S
Copies of the NMR spectra of the products. This material is
available free of charge via the Internet at http://pubs.acs.org.
(R)-2-Phenethyltetrahydrofuran (6e). 6e was obtained as clear
AUTHOR INFORMATION
Corresponding Author
*E-mail: chandran@purdue.edu.
Notes
■
liquid in 67% yield (353 mg).
1
Spectral data: H NMR (400 MHz, CDCl3) δ 7.37−7.05 (m, 5H),
3.94−3.54 (m, 3H), 2.82−2.70 (m, 1H), 2.69−2.61 (m, 1H), 2.04−
1.65 (m, 5H), 1.50−1.40 (m, 1H); 13C NMR (101 MHz, CDCl3) δ
141.9, 128.2, 128.1, 125.5, 78.3, 67.4, 37.2, 32.5, 31.1, 25.5; HRMS
(ESI) C12H16O calc. 176.1201 found 176.1235.
The authors declare no competing financial interest.
(R)-2-Heptyltetrahydrofuran (6f). 6f was obtained as light yellow
ACKNOWLEDGMENTS
We thank the Herbert C. Brown Center for Borane Research
for financial assistance of this project.
liquid in 65% yield (0.331 mg).
■
1
Spectral data: H NMR (400 MHz, CDCl3) δ 3.85−3.54 (m, 3H),
1.95−1.66 (m, 3H), 1.50 (m, 1H), 1.43−1.29 (m, 3H), 1.22 (m, 9H),
0.80 (t, J = 6.8 Hz, 3H); 13C NMR (101 MHz, CDCl3) δ 79.1, 67.2,
35.5, 31.6, 31.1, 29.5, 29.0, 26.1, 25.4, 22.4, 13.8; HRMS (ESI)
C11H22O calc. 170.1671, found 170.1641.
REFERENCES
■
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General Procedure for the Preparation of THF-Derivative:
One-Pot Crotylboration−Hydroboration−Iodination−Cycliza-
t
tion. A 1 M solution of BuOK in THF (2.5 mL, 2.5 mmol) is
added to THF (9 mL) and cooled to −78 °C. 2-Butene (5 mmol) was
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Note
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