Amines Overman-JOC2005

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Asymmetric Allylboration of a,b-Enals as a Surrogate for the Enantioselective
Synthesis of Allylic Amines and
ŋ
-Amino Acids.
P. Veeraraghavan Ramachandran,* Thomas E. Burghardt, and M. Venkat Ram Reddy
Herbert C. Brown Center for Borane Research, Department of Chemistry, Purdue University, 560 Oval Dr., West
Lafayette, IN 47907-2084
chandran@purdue.edu
Abstract
O
R
1
O
1. (-)Ipc
2
B
'Allyl'
2. Overman
rearrangement
R
1
HN
CCl
3
R
1
NH
2
O
R
R
2
R
3
R
4
51-60% yield (2 steps)
88-94% ee
R
R
R
2
OH
Optically pure allylic amines have been synthesized from
ŋ
,
Ȳ
-unsaturated aldehydes via allylboration with
(–)-B-allyldiisopinocampheylborane, followed by Overman rearrangement. By incorporating crotyl and
alkoxyallylboration, functionalization at
Ō
-position was readily accomplished. Applying this methodology, the
synthesis of several chiral
ŋ
-amino acids has been achieved.
R=HO-CH
2
,
C
6
H
5
, C
6
F
5
R
1
=Me, H
Allylic amines are very useful synthons in organic
synthesis.
1
They are widely used for the preparation of
alkaloids, carbohydrates, and other synthetic
intermediates.
2
Dienyl amines have been recently
evaluated as linkers in the synthesis of taxoid analogues.
3
Conversion of analogous amines to
Ō
-amino ketones,
precursors for the synthesis of
ŋ
-methyl piperidines,
which are common structural features found in numerous
natural products has also been reported.
4
Allylic amines
are also attractive starting materials for the synthesis of
ŋ
-
and
Ȳ
-amino acids.
5
Owing to their importance, there
have been many reports in the literature for the
preparation of allylic amines.
6
,
7
Preparation of
homoallylic alcohols in high enantiomeric excess via
——————————————————
(1) Johannsen, M.; Jørgensen, K. A.
Chem. Rev.
1998
,
98
, 1689.
(2) (a) Trost, B. M.
Angew. Chem.
1989
,
101,
1899. (b) Trost, B.
M.; Van Vranken, D. L.
J. Am. Chem. Soc.
1993
,
115,
444. (c)
Ichikawa, Y.; Ito, T.; Nishiyama, T.; Isobe, M.
Synlett
2003
,
7
, 1034.
(3) Ojima, I.; Lin, S.; Inoue, T.; Miller, M. L.; Borella, C. P.; Geng,
X.; Walsh, J. J.
J. Am. Chem. Soc.
2000
,
122
, 5343.
(4) (a) Reginato, G.; Mordini, A.; Verrucci, M.; Degl’Innocenti, A.;
Capperucci, A.
Tetrahedron: Asymm.
2000
,
11
, 3759. (b) Comins,
D. L.; Weglarz, D. A.
J. Org. Chem.
1991
,
56,
2506.
(5) (a) Burgess, K.; Liu, L. T.; Pal, B.
J. Org. Chem
.
1993
,
58
, 4758.
(b) Bower, J. F.; Jumnah, R.; Williams, A. C.; Williams, J. M. J.
J.
Chem. Soc. Perkin Trans. 1
1997
, 1411. (c) Savage, I.; Thomas, E.
J.; Wilson, P. D.
J. Chem. Soc. Perkin Trans 1
1999
, 3291.
(6) (a) Rehn, S.; Ofial, R. A.; Mayr, H.
Synthesis
2003
,
12,
1790. (b)
Wipf, P.; Janjic, J.; Stephenson, C. R. J.
Org. Biomol. Chem.
2004
,
2
,
443. (c) Barluenga, J.; Rodríguez, F.; Álvarez-Rodrigo, L.; Zapico, J.
M.; Fañanás, F. J.
Chem. Eur. J.
2004
,
10
, 109.
(7) (a) Overman, L. E.
J. Am. Chem. Soc.
1974
,
96,
597. (b) Lurain,
A. E.; Walsh P. J.
J. Am. Chem. Soc.
2003
,
125,
10677. For reviews,
see: (c) Overman, L. E.
Acc. Chem. Res.
1980
,
13,
218. (d) Sato, H.;
Oishi, T.; Chida, N.
J. Synth. Org. Chem. Jpn.
2004
,
62
, 693.
asymmetric allylboration is a well-established protocol.
8
Our recent work
9
on the applications of allylboration using
ŋ
-pinene-based chiral auxiliaries prompted us to undertake
a project involving tandem asymmetric allylboration-
Overman rearrangement
7
for the synthesis of optically
active allylic amines and
ŋ
-amino acids. We chose three
ŋ
-pinene-based reagents for asymmetric allylboration (
1-
3
, Figure 1).
10
1
: R = R' = H
2
: R = H, R' = Me
3
: R = OMEM, R' = H
FIGURE 1.
ŋ
-Pinene-based 'allyl'borating reagents.
We selected several
ŋ
,
Ȳ
-unsaturated aldehydes,
containing a variety of functional groups and substitutions.
The required aldehydes
4b-g
were synthesized using
several literature procedures. Aldehyde
4b
was prepared
via LiAlH
4
reduction of commercially available ethyl
Ȳ
-
methyl cinnamate and subsequent Dess-Martin
periodinane (DMP) oxidation.
11
4c
was made via S
N
2'
substitution with ethylmagnesium bromide on ethyl 2-
——————————————————
(8) (a) Hoffmann, R. W.
Pure Appl. Chem.
1988
,
60,
123. (b)
Brown, H. C.; Ramachandran, P. V.
J. Organomet. Chem.
1995
,
500
,
1.
(9) Ramachandran, P. V.; Reddy, M. V. R.; Brown, H. C.
Pure Appl.
Chem.
2003
,
75
, 1263.
(10) (a) Brown, H. C.; Jadhav, P. K.
J. Am. Chem. Soc.
1983
,
105
,
2095. (b) Brown, H. C.; Bhat, K. S.
J. Am Chem. Soc.
1986
,
108
,
293. (c) Brown, H. C.; Jadhav, P. K.; Bhat, K. S.
J. Am. Chem. Soc.
1988
,
110
, 1535.
(11) Dess, D. B.; Martin, J. C.
J. Org. Chem.
1983
,
48
, 4155.
2
R'
R
1
[(acetyloxy)(phenyl)methyl]acrylate,
12
followed by
LiAlH
4
reduction-DMP oxidation sequence, and
4d
was
made from (2
Z
)-butene-1,4-diol.
13
Aldehydes
4e-g
were
prepared from methyl (2
S
)-3-hydroxy-2-methylpropanoate
via Wittig-Horner olefination with (triphenyl–
phosphoranylidene)acetaldehyde
5c
.
14
The aldehydes
4a-
g
were reacted with (–)-
B
-allyldiisopinocampheylborane
(
1
), followed by oxidative workup to afford the desired
homoallylic allylic alcohols
5a-g
in 68-83% yields and in
89-94% ee as determined by HPLC analysis (Scheme 1,
Table 1, Entries 1-7). On the basis of previous reports,
10a
the
S
isomer of homoallylic alcohols was obtained. These
alcohols were converted to the corresponding
trichloroacetimidates by treatment with 0.1 eq of sodium
bis(trimethylsilyl)amide in THF at –42 °C, followed by
the addition of trichloroacetonitrile and warming to room
temperature. THF was removed in vacuo and the crude
trichloroacetimidates were diluted with xylene and
refluxed for 6-14 h (TLC analysis) in the presence of 1.1
eq of K
2
CO
3
.
15
The desired amides
6a-g
were obtained in
good yields (74-89%) (Scheme 1, Table 1, Entries 1-7).
Overman rearrangement has been reported to proceed with
complete retention of stereochemistry.
7
Verification of
the enantioselectivity of amides
6a
,
6b
and
6e
using
HPLC revealed that they retained the high optical purity,
94%, 91%, and 93% ee, respectively, of the homoallylic
alcohols achieved during allylboration.
Scheme 1
a
4b
furnished the corresponding homoallylic alcohols
7a
,
8a
, and
8b
in >98% de and 89%, 94%, and 93% ee,
respectively, as determined by HPLC analysis (Scheme 2,
Table 1, Entries 8-10). These densely functionalized
alcohols were converted to the corresponding amides
9a
,
10a
, and
10b
via the trichloroacetimidates, followed by
rearrangement as described above, without loss of optical
activity (HPLC analysis of
10b
indicated 92% ee).
Scheme 2
a
O
R
1
O
a
R
1
OH
b, c
HN
CCl
3
R
1
R
R
R
2
R
3
R
R
2
R
3
4a
,
4b
7a
,
8a
,
8b
9a
,
10a
,
10b
4a
: R=Ph,R
1
=H
4b
: R=Ph,R
1
=Me
7a
,
9a
: R=Ph, R
1
=R
3
=H, R
2
=Me
8a
,
10a
: R=Ph, R
1
=R
2
=H, R
3
=OMEM
8b
,
10b
: R=Ph, R
1
=Me, R
2
=H, R
3
=OMEM
a
Reagents and conditions: a. (1) 2 or 3, then 4a or 4b; –78
°C, 5 h; (2) NaOH, H
2
O
2
; –78 °C
ŗ
RT, 14 h. b. (1)
NaN(SiMe
3
)
2
; THF, –42 °C, 0.5 h; (2) Cl
3
CCN; –42 °C
ŗ
RT, 1 h. c. K
2
CO
3
; xylene reflux, 10-14 h.
Unnatural amino acids are very important in
bioorganic chemistry.
16
Having achieved the preparation
of the allylic amines in high ee, we demonstrated an
application of this methodology by preparing
representative a-amino acids
11a
,
11b
,
11d
, and
11e
from
trichloroacetamides
6a
,
6b
,
6d
, and
6e
, respectively.
Accordingly, ozonolysis of
6a
in CH
2
Cl
2
at –78 °C,
followed by quenching with Me
2
S gave the intermediate
aldehyde, which upon oxidation with sodium chlorite in
tert
-butanol in the presence of 2-methylbut-2-ene and
aqueous NaH
2
PO
4
provided the corresponding acid in
good yield. We found this two-step procedure to be more
convenient and reliable than a one-step oxidation with
NaIO
4
-RuCl
3
, which gave varying results.
Removal of the trichloroacetamide group required
refluxing of the material with conc. HCl for 1 h. (2
R
)-
aminophenyl acetic acid hydrochloride (
11a
) and (
2R
)-2-
amino-2-phenylpropionic acid hydrochloride (
11b
) were
obtained by this sequence in 64% and 62% yield,
respectively. Alternatively, we first deprotected
6a
by
stirring with aqueous alcoholic NaOH for 24 h (the
reaction could be shortened to 2 h by refluxing the
mixture), followed by the protection of the amine using di-
tert
-butyl dicarbonate. This
N
-Boc-protected amine was
subjected to ozonolysis and oxidation with NaClO
2
to
afford
N
-Boc-protected amino acid. The Boc group was
then conveniently removed by treatment with ethereal
hydrogen chloride for 0.5 h at RT. The amino acid
11a
was obtained in 73% yield. We applied the latter method
for the synthesis of an aliphatic
ŋ
-amino-
Ȳ
-hydroxy acid
11d
and a fluorinated amino acid
11e
(Scheme 3).
O
R
1
O
R
1
OH
a
b, c
HN
CCl
3
R
1
R
R
R
2
R
2
R
R
2
4a-g
5a-g
6a-g
4a, 5a, 6a
: R = Ph, R
1
=R
2
=H
4b, 5b, 6b
: R = Ph, R
1
=Me, R
2
=H
4c, 5c, 6c
: R = Ph, R
1
=H, R
2
=
n
-Pr
4d, 5d, 6d
: R = TBSOCH
2
, R
1
=R
2
=H
4e, 5e, 6e
: R = C
6
F
5
, R
1
=R
2
=H
4f, 5f, 6f
: R = (
S
)-TBSOCH
2
CH(Me), R
1
=R
2
=H
4g, 5g, 6g
: R = (
S
)-PMBOCH
2
CH(Me), R
1
=R
2
=H
a
Reagents and conditions: a. (1) 1, then 4a-g; –100
ŗ
–78
°C, 3 h; (2) NaOH, H
2
O
2
; –78 °C
ŗ
RT, 14 h. b. (1)
NaN(SiMe
3
)
2
; THF, –42 °C, 0.5 h; (2) Cl
3
CCN; –42 °C
ŗ
RT, 1 h. c. K
2
CO
3
; xylene reflux, 6-14 h.
Additionally, to show the versatility of the current
protocol in the synthesis of more functionalized allylic
amines, we incorporated crotyl and alkoxyallylboration
using
2
and
3
. These reagents,
10
upon reaction with
4a
or
——————————————————
(12) Amri, H.; Rambaud, M.; Villiéras, J.
J. Organomet. Chem.
1990
,
384
, 1.
(13) Ramachandran, P. V.; Liu, H.; Reddy, M. V. R.; Brown, H. C.
Org. Lett.
2003
,
5
, 3755.
( 14 ) Kalesse, M.; Chary, K. P.; Quitschalle, M.; Burzlaff, A.;
Kasper, C.; Scheper, T.
Chem. Eur. J.
2003
,
9
, 1129.
(15) Nishikawa, T.; Asai, M.; Ohayabu, N.; Isobe, M.
J. Org. Chem.
1998
,
63
, 188.
——————————————————
(16) (a) Dougherty, D. A.
Curr. Op. Chem. Biol.
2000
,
4
, 645. (b)
Bittker, J. A.; Phillips, K. J.; Liu, D. R.
Curr. Op. Chem. Biol.
2002
,
6
, 367. (c) Yodar, N. C.; Kumar, K.
Chem. Soc. Rev.
2002
,
31
, 335.
2
Table 1 - Synthesis of
ŋ
,
Ȳ
-unsaturated homoallylic alcohols and allylic amides.
Enal
ʧʧʧʧʧʧʧʧ
Homoallylic alcohol
ʧʧʧʧʧʧʧʧʧ
ʧ
Amide
ʧ
Entry
#
#
R
R
1
R
2
R
3
R
4
% yld
a
% ee
b
(de)
c
#
% yld
a
1
4a
5a
Ph
H
H
H
H
73
94
6a
89
2
4b
5b
Ph
Me
H
H
H
73
91
6b
79
3
4c
5c
Ph
H
n
-Pr
H
H
71
91
6c
89
4
4d
5d
TBSO-CH
2
-
H
H
H
H
68
91
6d
86
5
4e
5e
C
6
F
5
H
H
H
H
74
93
6e
74
6
4f
5f
(
S
)-TBSO-CH
2
-
CH(CH
3
)-
H
H
H
H
79 89 (>98)
6f
75
7
4g
5g
(
S
)-PMBO-CH
2
-
CH(CH
3
)-
H
H
H
H
83
88 (30)
d
6g
84
8
4a 7a
Ph H H Me H 69 89 (>98)
9a
74
9
4a 8a
Ph H H H OMEM 76 94 (>98)
10a
80
10
4b 8b
Ph Me H H OMEM 78 93 (>98)
10b
77
a
All yields are of pure isolated products.
b
% enantiomeric excess was determined by HPLC using Chiracel OD-H column and
isopropyl alcohol-hexanes as the mobile phase.
c
Ratio of diastereomers was determined with
1
H NMR spectroscopy.
d
The low de
could be due to the partial racemization of the starting aldehyde during Wittig-Horner reaction.
Comparison of the optical rotation of the obtained
amino acids with those reported in the literature
17
revealed the configuration and assured that the
enantioselectivity induced during the preparation of
homoallylic alcohols has been transferred during the
Overman rearrangement.
Scheme 3
a
O
procedure for the synthesis of several chiral
ŋ
-amino
acids. The simplicity of the protocol, high
enantioselectivities in the allylboration step, complete
retention of stereochemistry during the rearrangement,
and the importance of allylic amines and unnatural
ŋ
-
amino acids make this methodology very attractive. We
believe that the presented procedure will find further
applications in organic synthesis.
Experimental Section
Representative experimental procedures
:
Preparation of ((2
E
)-3-phenylbut-2-enal (4b):
To
ethyl (2
E
)-3-phenylbut-2-enoate (1.0 mL, 5.4 mmol), diluted
with THF (20 mL) and cooled to 0 °C, was added LiAlH
4
(1
M in THF; 5.5 mL, 5.5 mmol) and the reaction was stirred at
RT for 1 h. Excess LiAlH
4
was quenched with water; the
product was extracted with Et
2
O (3×30 mL) and dried with
MgSO
4
. After removal of the solvent under reduced
pressure, the resulting alcohol was diluted with CH
2
Cl
2
and
Dess-Martin periodinane (2.8 g, 6.6 mmol) was added. After
stirring at RT for 0.5 h, the solvent was removed; the residue
was extracted with pentane (3×50 mL) and filtered through
Celite. After evaporation of the solvent, the obtained
product was purified on silica gel (flash; 96:4 hexanes:ethyl
acetate) to afford 0.7 g (4.8 mmol, 89% yield) of
4b
.
1
H
NMR (300 MHz, CDCl
3
,
Ō
): 2.57 (d,
J
= 0.72 Hz, 3H), 6.40
(dq,
J
= 1.24 Hz, 7.88 Hz, 1H), 7.40-7.55 (m, 5H), 10.19 (d,
J
= 7.83 Hz, 1H);
13
C NMR (75 MHz, CDCl
3
,
Ō
): 16.4, 126.3,
127.3, 128.8, 130.1, 140.6, 157.7, 191.3.
Preparation of (4
S
,5
E
)-6-phenylhepta-1,5-dien-4-ol
(5b):
To
1
(1 M in pentane; 6 mL, 6 mmol), diluted with
Et
2
O (3 mL) and cooled to –100 °C, was added
4b
(0.7 g, 4.8
mmol) diluted with Et
2
O (6 mL) pre-cooled to –78 °C. The
mixture was stirred for 3 h, while it was allowed to warm to
–78 °C. To the reaction mixture at –78 °C was added 3 M
aq. NaOH (1.6 mL) and (slowly!) 30% aq. H
2
O
2
(1.3 mL),
and the reaction was left stirring for 14 h under positive N
2
R
1
NH
3
+
Cl
c,d,f
R
1
HN
CCl
3
a-e
NH
3
+
Cl
R
COOH
R
R
COOH
11a
: R=Ph,
R
1
=H, 64%
11b
: R=Ph,
R
1
=Me, 62%
6a
: R=Ph, R
1
=H
6b
: R=Ph, R
1
=Me
6d
: R=CH
2
OTBS, R
1
=H
6e
: R=C
6
F
5
, R
1
=H
11a
: R=Ph, 73%
11d
: R=CH
2
OH,
64%
11e
: R=C
6
F
5
, 69%
a
Reagents and conditions: a. (1) NaOH; aq. EtOH, RT,
24 h. b. Boc
2
O; Et
2
O, RT, 3 h. c. (1) O
3
, CH
2
Cl
2
, –78 °C.
(2) Me
2
S, –78 °C
ŗ
RT, 3 h. d. NaClO
2
, 2-methylbut-2-ene,
NaH
2
PO
4
;
t-
BuOH, H
2
O, RT, 0.5 h. e. HCl; Et
2
O, RT, 0.5 h.
f. conc. aq. HCl; reflux, 1 h.
In conclusion, we have developed an efficient
protocol for the enantioselective synthesis of allylic
amines via asymmetric allylboration and Overman
rearrangement. We have demonstrated the utility of this
——————————————————
(17) (a)
5a
:
ŋ
2
D
= –84º (D
2
O, c=1.0),
ŋ
2
D
= –138º (10% aq. HCl,
c=0.5; 93% ee) (lit: Badorrey, R.; Cativiela, C.; Díaz-de-Villegas,
M.; Gálvez, J. A.
Tetrahedron
1997
,
53
, 1411.
ŋ
2
D
= –155° (1 M
HCl, c=1.0)); (b)
5b
:
ŋ
2
D
= –77º (10% aq. HCl, c=0.2; 91% ee) (lit:
Davis, F. A.; Lee, S.; Zhang, H.; Fanelli, D. L.
J. Org. Chem.
2000
,
65
, 8704.
ŋ
2
D
= –85° (1 M HCl, c=0.7)); (c)
5d
:
ŋ
2
D
= +10º (D
2
O,
c=0.4, 90% ee) (lit: Rose, J. E.; Leeson, P. D.; Gani, D.
J. Chem.
Soc. Perkin Trans. 1
1995
, 157: +11° (H
2
O, c=0.5); (d)
5e
:
ŋ
2
D
=
+36º (D
2
O, c=0.7).
3
pressure while it slowly warmed to RT. The product was
then extracted with Et
2
O (3×20 mL), washed with brine,
dried with MgSO
4
, and the solvent was removed under
reduced pressure, and the residue purified on silica gel
(flash; 200:1 hexanes:ethyl acetate) to furnish
5b
in 73%
yield (0.66 g, 3.5 mmol) and 91% ee as analysed by HPLC
using Chiracel OD-H column and hexanes / isopropanol as
the mobile phase.
1
H NMR (300 MHz, CDCl
3
,
Ō
): 1.95 (br
s, 1H), 2.13 (d,
J
= 1.32 Hz, 3H), 2.42 (t,
J
= 6.48 Hz, 2H),
4.63 (q,
J
= 7.05 Hz, 1H), 5.15-5.23 (m, 2H), 5.79-5.95 (m,
2H), 7.28-7.44 (m, 5H);
13
C NMR (75 MHz, CDCl
3
,
Ō
):
16.7, 42.4, 68.4, 118.5, 126.1, 127.6, 128.5, 130.2, 134.5,
137.6, 143.1.
Preparation of 2,2,2-trichloro-
N
-[(1
S
,2
E
)-1-methyl-
1-phenylhexa-2,5-dienyl]acetamide (6b):
To
5b
(0.5 g, 2.7
mmol), diluted with THF (13 mL) and cooled to –42 °C, was
added sodium bis(trimethylsilyl)amide (1 M in THF; 0.27
mL, 0.27 mmol) and the reaction was stirred for 0.5 h, when
trichloroacetonitrile (0.3 mL, 2.9 mmol) was added. The
reaction was then allowed to warm to RT and the solvent
was removed under reduced pressure. To the obtained crude
trichloroacetimidates, diluted with xylenes (6 mL), was
added potassium carbonate (0.4 g, 2.9 mmol) and the
mixture was stirred at reflux (150 °C) for 10 h. The mixture
was then filtered through Celite, concentrated under reduced
pressure, and purified on silica gel (flash; 99:1 hexanes:ethyl
acetate) to afford 0.76 g (2.6 mmol, 79% yield) of allylic
amide
6b
as an oily substance with 91% ee as analysed by
HPLC.
1
H NMR (300 MHz, CDCl
3
,
Ō
): 1.89 (s, 3H), 2.87 (t,
J
= 6.35 Hz, 2H), 5.03-5.09 (m, 2H), 5.58-5.68 (m, 1H),
5.78-5.97 (m, 2H), 6.96 (br s, 1H), 7.29-7.39 (m, 5H);
13
C
NMR (75 MHz, CDCl
3
,
Ō
): 26.4, 36.6, 61.2, 76.9, 93.6,
116.3, 125.7, 126.6, 127.9, 128.7, 129.3, 134.3, 136.4, 144.0,
160.1.
Preparation of (2
R
)-2-amino-2-phenylpropionic acid
hydrochloride (11b):
Ozone was passed through a solution
of
6b
(0.6 g, 1.8 mmol) in CH
2
Cl
2
(200 mL) and MeOH (200
mL) at –78 °C until it turned dark blue. The reaction was
then quenched with Me
2
S (1 mL) and stirred overnight while
it was warming to RT. After evaporation of the solvents, the
obtained product was purified on silica gel to give 0.5 g (1.7
mmol, 94% yield) of 2,2,2-trichloro-
N
-[(1
R
)-1-methyl-2-
oxo-1-phenylethyl]acetamide.
1
H NMR (300 MHz, CDCl
3
,
Ō
): 2.00 (s, 3H), 7.34-7.47 (m, 5H), 8.21 (br s, 1H), 9.21 (s,
1H);
13
C NMR (75 MHz, CDCl
3
,
Ō
): 17.9, 66.3, 92.8, 126.7,
129.3, 129.7, 134.7, 160.4, 194.3. To the obtained aldehyde,
diluted with 2-methylpropan-2-ol (25 mL) and 2-methylbut-
2-ene (5 mL), was added sodium chlorite (0.7 g, 7.7 mmol),
sodium phosphate monobasic (0.6 g, 4.3 mmol), and water
(5 mL). The mixture was stirred at RT for 0.5 h, the product
was extracted with ethyl acetate (3×30 mL) and filtered
through a short plug of silica gel. The solvent was removed
under reduced pressure and the obtained material was
refluxed with conc. HCl (6 mL) for 1 h. Following the
evaporation of HCl, the obtained solid was washed with
Et
2
O and dried to give
11b
in 62% yield (0.2 g, 1.0 mmol)
from
6b
.
1
H NMR (300 MHz, D
2
O,
Ō
): 2.00 (s, 3H), 7.48-
7.53 (m, 5H);
13
C NMR (75 MHz, D
2
O,
Ō
): 21.5, 69.1,
125.0, 127.0, 129.2, 129.4, 137.1, 175.8.
ŋ
2
D
= –77º (10% aq.
HCl, c= 0.2).
Acknowledgment.
Financial assistance from the
Herbert C. Brown Center for Borane Research
18
and Aldrich
Chemical Company is gratefully acknowledged.
Supporting Information Available
. The spectral data
(
1
H,
13
C, and
19
F NMR spectra) and other physical
characteristics of the compounds described (40 pages). This
material is available free of charge via the internet at
——————————————————
(18) Contribution number 35 from the Herbert C. Brown Center for
Borane Research.
4
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