Facile Synthesis of Terminal 1,2-Diols with High Optical Purity via Oxazaborolidine-Catalyzed Asymmetric Borane Reduction

Full text of DOI:10.1021/jo980455v

5280
J . Org. Chem. 1998, 63, 5280-5282
Sch em e 1
F a cile Syn th esis of Ter m in a l 1,2-Diols w ith
High Op tica l P u r ity via
Oxa za bor olid in e-Ca ta lyzed Asym m etr ic
Bor a n e Red u ction
Byung Tae Cho* and Yu Sung Chun
Departmet of Chemistry, Hallym University, Chunchon,
Kangwondo 200-702, Republic of Korea
Received March 10, 1998
Optically active terminal 1,2-diols have found wide-
spread use as chiral building blocks1 and numerous
applications as chiral auxiliaries or ligands2 for asym-
metric synthesis. Accordingly, the development of a
general methodology for the asymmetric synthesis of
these compounds, such as the catalytic asymmetric
dihydroxylation of olefins,3 the catalytic enantioselective
addition of dialkylzincs to R-silyloxy aldehyde deriva-
tives,4 and the enzymatic hydrolysis of diol monoacetates5
has been extensively investigated. In fact, 1,2-chiral diols
have been prepared from the asymmetric reduction of
R-hydroxy ketones via chiral boranes6 and chiral diphos-
phine-catalyzed hydrosilylations.7 Recently, a number
of asymmetric borane reductions of functionalized prochiral
ketones catalyzed by oxazaborolidines has been reported
by Corey and others.8,9 The reductions of R-halo ketones,9c
2-acyl-1,3-dithianes,10 R-ketophosphonates,11 and other
prochiral ketones containing heteroatoms12 have been
reported to give excellent enantioselectivities. We re-
cently have reported asymmetric reduction of function-
alized ketones, such as R-amino ketones13 and R-keto
acetals.14 The extension of this methodology for the
asymmetric synthesis of terminal 1,2-diols via asym-
metric reduction of R-hydroxy ketone derivatives, how-
ever, has not been reported to date.15 We describe here
a simple and convenient procedure for obtaining terminal
1,2-diols with high optical purity by employing the
oxazaborolidine-catalyzed asymmetric borane reduction
of R-[(triorganosilyl)oxy] ketones.
The R-[(triorganosilyl)oxy] ketone derivatives (6a -n )
used as substrates were prepared from the silylation of
R-hydroxy ketones16 with 1.2 equiv of triorganosilyl
chloride in the presence of 1.5 equiv of imidazole in
dichloromethane at room temperature in 62-96% yields,
according to the literature procedure.17
* To whom correspondence should be addressed. Tel.: +82-361-240-
1423. Fax: +82-361-242-4572. E-mail: btcho@sun.hallym.ac.kr.
(1) Hanessian, H. Total Synthesis of Natural Products: the “Chiron”
Approach; Pergamon Press: Oxford, 1983.
(2) Seyden-Penn, J . Chiral Auxiliaries and Ligands in Asymmetric
Synthesis; J ohn Wiley & Sons, Inc.: New York, 1995.
(3) Kolb, H. C.; VanNieuwenhze, M. S.; Sharpless, K. B. Chem. Rev.
1994, 94, 2483.
(4) Eisenberg, C.; Knochel, P. J . Org. Chem. 1994, 59, 3760.
(5) Wong, C.-H.; Whitesides, G. M. Enzymes in Synthetic Organic
Chemistry; Pergamon Press: Oxford, 1994.
(6) Ramachandran, P. V.; Lu, Z.-H.; Brown, H. C. Tetrahedron Lett.
1997, 38, 761.
Initially, five chiral oxazaborolidines (1,18 2,19 3,20 4,21
and 522) prepared from commercially available materials
were chosen as representative catalysts for the reduction
of 2-[(tert-butyldimethylsilyl)oxy]acetophenone (6a). Thus,
slow addition of 6a over 1 h to a solution of 0.6 equiv of
borane-THF in the presence of 10 mol % of each ox-
azaborolidine in THF at 25 °C (Scheme 1) provided
(7) Burk, M. J .; Feaster, J . E. Tetrahedron Lett. 1992, 33, 2099.
(8) For general reviews on the oxazaborolidine-catalyzed reductions,
see: (a) Sing, V. K. Synthesis 1992, 605. (b) Wallum, S.; Martens, J .
Tetrahedron: Asymmetry 1992, 3, 1475.
(9) (a) Corey, E. J .; Bakshi, R. K.; Shibata, S.; Chen, C.-P.; Singh,
V. K. J . Am. Chem. Soc. 1987, 109, 7925. (b) Corey, E. J .; Bakshi, R.
K. Tetrahedron Lett. 1990, 30, 611. (c) Corey, E. J .; Link, J . O. J . Org.
Chem. 1991, 56, 442.
(10) DeNinno, M. P.; Perner, R. J .; Lijewski, L. Tetrahedron Lett.
1990, 31, 7415.
(16) Levene, P. A.; Walti, A. Organic Synthesis; J ohn Wiley &
Sons: New York, 1943; Collect. Vol. II, pp 5-6.
(17) Corey, E. J .; Venkateswarlu, A. J . Am. Chem. Soc. 1972, 94,
6190.
(18) Itsuno, S.; Nakano, M.; Miyazaki, K.; Masuda. H.; Itoh, K.;
Hirao, A.; Nakahama, S. J . Chem. Soc., Perkin Trans. 1 1985, 2039.
(19) Corey, E. J .; Bakshi, R. K.; Shibata, S. J . Am. Chem. Soc. 1987,
109, 5551.
(20) Berenguer, R.; Garcia, J .; Vilarrasa, J . Tetrahedron: Asymmetry
1994, 5, 165.
(21) Qallich, G. J .; Blake, J . F.; Woodall, T. M. J . Am. Chem. Soc.
1994, 116, 851.
(11) Meier, C.; Laux, W. H. G. Tetrahedron: Asymmetry 1995, 6,
1089.
(12) Qallich, G. J .; Woodall, T. M. Tetrahedron Lett. 1993, 34, 785.
(13) Cho, B. T.; Chun, Y. S. Tetrahedron: Asymmetry 1992, 3, 341.
(14) Cho, B. T.; Chun, Y. S. Tetrahedron: Asymmetry 1994, 5, 1147.
(15) (a) For a catalytic asymmetric borane reduction of benzils, see:
Prasad, K. R. K.; J oshi, N. N. J . Org. Chem. 1996, 61, 3888. (b) For an
enzymatic reduction of 1-hydroxy ketones, see: Lee, L. G.; Whitesides,
G. M. J . Org. Chem. 1986, 51, 25.
(22) Yaping, H.; Gao, Y.; Nie, X.; Zepp, C. M. Tetrahedron Lett. 1994,
35, 6631.
S0022-3263(98)00455-1 CCC: $15.00 © 1998 American Chemical Society
Published on Web 06/27/1998

Notes
J . Org. Chem., Vol. 63, No. 15, 1998 5281
Ta ble 1. Asym m etr ic Bor a n e Red u ction of r-[(Tr ior ga n osilyl)oxy] Keton es (6) in th e P r esen ce of 10 m ol % of
Oxa za bor olid in es in THF a t 25 °Ca
yield,b
%
[R]25
D
entry
compd
cat.
diol
(EtOH)
% ee
config
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
6a
6a
6a
6a
6a
6b
6c
6d
6e
6f
6g
6h
6i
6j
6k
6l
1
2
3
4
5
2
2
2
2
2
2
2
2
2
2
2
2
2
8
8
8
8
8
8
8
8
8
90
94
85
84
88
94
95
92
90
92
99
93
98
94
93
82
80
84
+35.91 (c 4.24)
+38.90 (c 3.61)
e
e
e
91c
Sd
Sd
Sd
Sd
Sd
Sd
Sd
Sd
Sd
Sh
Sh
Sj
99c(100)d
52c
74c
84c
+38.91 (c 3.81)
+38.91 (c 3.59)
+34.71 (c 3.81)
+32.11 (c 3.99)
+80.59 (c 1.01)g
+53.66 (c 1.08)g
+18.81 (c 2.48)
+33.67 (c 1.98)k
+32.60 (c 1.02)
+32.52 (c 1.04)
-9.40 (c 2.59)
+7.90 (c 1.98)g
+4.94 (c 1.05)g
99c (100)d
99c (100)d
88c
77c
9
9
87f (87)h
58f
10
11
12
12
13
14
15
>99i
>99f (100)l
96f (100)m
96f (100)m
73n
Sl
Sm
Sm
So
Sp
Sq
6m
6n
75n
96n (94)q
a
[6]:[BH3]:[cat] ) 1:0.6:0.1. Cat. ) oxazaborolidine. [6] ) 0.3 M. The reaction was complete within 10 min to give the monosilylated
b
product alcohol (7). Isolated and purified yield obtained from reduction of 6, followed by desilylation with n-Bu4NF (see the Experimental
Section). c Determined by HPLC analysis using a Daicel Chiralcel OB chiral column; hexane/i-PrOH ) 9/1. Based on [R]23 -38.4 (c
d
D
1.12, EtOH), 99% ee, R ref 23. e Not measured. f Determined by HPLC analysis using a Daicel Chiralcel OD chiral column; hexane/i-
PrOH ) 9/1. Measured in CHCl3. Based on [R]22 -70.3 (c 0.91, CHCl3), 76% ee, R ref 24. Determined by HPLC analysis using a
g
h
i
D
j
Daicel Chiralcel OB chiral column; hexane/i-PrOH ) 96/4. The absolute configuration is not known, but probably S, based on the elution
k
l
m
order and the sign of rotation value of the product diol 10. Measured in acetone. Based on [R]D -33.5 (c 2.0, acetone), R ref 25. Based
on [R]23 -31.2 (c 0.997, EtOH), 99.5% ee, R ref 23. Determined by GC analysis of its bis(trifluoromethyl) acetate using a Chiraldex
n
D
G-TA column (Astec Inc). o Based on [R]25 -12.87 (c 2.5, EtOH), S ref 26. Based on [R]20 +10.3 (c 1.15, CHCl3), S ref 27. Based on
p
q
D
D
[R]23 -4.8 (c 0.922, CHCl3), 91% ee, R ref 23.
D
Sch em e 2
1-[(tert-butyldimethylsilyl)oxy]-2-phenylethanol (7a) within
10 min. This was easily converted to 1-phenyl-1,2-
ethanediol (8) by the treatment of n-Bu4NF at room
temperature in almost quantitative yields. The desilyl-
ation could also be accomplished by the addition of
methanol to destroy excess hydride, followed by direct
treatment of n-Bu4NF to give 8 directly. The enantio-
meric excess of the diol 8 was determined by HPLC
analysis using a Chiralcel OB column (eluent: hexane/
i-PrOH ) 9:1). Among the catalysts examined, Corey’s
CBS reagent (2) provided the best enantioselectivity
approaching 100% ee (Table 1, entries 1-5). Subse-
quently, the influence of different silyl groups on the
selectivity of the asymmetric borane reduction of the
silyloxy derivatives (6a -e) of 2-hydroxyacetophenone
was examined. Of the silyloxy ketones tested, compounds
6a-c (tert-butyldimethylsilyl (TBDMS), triethylsilyl (TES),
and triisopropylsilyl (TIPS) derivatives) produced 8 with
optical purities approaching 100% ee (Table 1, entries 2,
6, and 7). In contrast, the more sterically bulky silyl
groups, such as thexyldimethylsilyl (TDS) and tert-
butyldiphenylsilyl (TBDPS) groups, resulted in decreased
enantioselectivity, 88% ee and 77% ee, respectively (Table
1, entries 8 and 9). Interestingly, the reduction of
2-[(triisopropylsilyl)oxy]-4′-methylacetophenone (6h ) pro-
vided the corresponding diol in >99% ee in contrast to
58% ee for 2-[(triisopropylsilyl)oxy]-2′-methylacetophe-
none (6g) (Table 1, entries 11 and 12). These results
indicate that the asymmetric induction was sensitive to
steric effects of the substituent proximal to the carbonyl
group. This is a common phenomenon in oxazaboroli-
dine-catalyzed reductions.28 For other aromatic ana-
logues 6i-k bearing p-bromophenyl and 2-naphthyl
groups, we also obtained the product diols in 100% ee
(Table 1, entries 13-15). In the case of aliphatic ana-
logues, the asymmetric reduction of the silyloxy ketones
6l,m afforded somewhat lower enantioselectivities com-
pared with those obtained from aromatic analogues
(Table 1, entries 16 and 17). However, the silyloxy
ketone 6n having a cyclohexyl group again produced very
high enantioselectivity (Table 1, entry 18). Moreover, in
all the cases examined, it is noteworthy that the product
diols (8-15) obtained are consistently enriched in the
S-enantiomers. The stereochemical course of the asym-
metric reduction can be explained by the proposed
mechanism involving a transition state 16, where the
R-silyloxy ketones are attacked by hydride on their Re
faces to provide (S)-diols (Scheme 2).19,29
In summary, we have established a convenient and
simple procedure for the preparation of terminal 1,2-diols
with high optical purity via the oxazaborolidine-catalyzed
asymmetric borane reduction of R-[(triorganosilyl)oxy]
ketone derivatives. Among the oxazaborolidines exam-
ined, catalyst 2 provided the best result to give 1,2-diols
(26) Mori, K.; Saaki, M.; Tamada, S.; Suguro, T.; Masuda, S.
Tetrahedron 1979, 35, 1601.
(23) Becker, H.; King, S. B.; Taniguchi, M.; Vanhessche, K. P. M.;
Sharpless, K. B. J . Org. Chem. 1995, 60, 3940.
(24) Lohray, B. B.; Bhshan, V. Tetrahedron Lett. 1992, 33, 5113.
(25) Ferraboshi, P.; Grisenti, P.; Manzocchi, A.; Santaniello, E.
Tetrahedron 1994, 50, 10539.
(27) Bach, J .; Berenguer, R.; Farras, J .; Garcia, J .; Meseguer, J .;
Vilarrasa, J . Tetrahedron: Asymmetry 1995, 6, 2683.
(28) Corey, E. J .; Helal, C. J . Tetrahedron Lett. 1996, 37, 5675.
(29) J ones, D. K.; Liotta, D. C.; Shinkai, I.; Mathre, D. J . J . Org.
Chem. 1993, 58, 799.

5282 J . Org. Chem., Vol. 63, No. 15, 1998
Notes
the reaction mixture was stirred for 10 min, quenched cautiously
with optical purity approaching 100% ee for most aro-
matic analogues. This procedure can be used as an
excellent alternative to synthesis of optically active
terminal 1,2-diols. Further applications using this meth-
odology are now under investigation.
with methanol (0.5 mL), and stirred for additional 30 min.
A
solution of tetrabutylammonium fluoride (1.5 mmol; 1.0 M, 1.5
mL) in THF was added, and the resulting mixture was stirred
for 1 h at room temperature. The solvent was evaporated under
reduced pressure, and the residue was directly chromatographed
using the appropriate solvents to obtain the chemically pure
corresponding diols 8-15.
Exp er im en ta l Section
(S)-(+)-1-(o-Tolu yl)-1,2-eth an ediol (9): Rf 0.51 (ethyl acetate/
hexane 2:1); mp 98-100 °C; IR (KBr, cm-1) 3274, 3031, 2863,
1485, 1362, 1084, 757; 1H NMR (400 MHz) δ 2.25 (br s, 1H),
2.35 (s, 3H), 2.52 (br s, 1H), 3.59-3.64 (m, 1H), 3.73 (d, 1H, J )
11.1 Hz), 5.07 (dd, 1H, J ) 8.0, 3.0 Hz), 7.14-7.26 (m, 3H), 7.50
(d, 1H, J ) 7.2 Hz); 13C NMR (100 MHz) δ 138.44, 134.78, 130.46,
127.77, 126.33, 126.65, 71.44, 66.93, 19.03. Anal. Calcd for
C9H12O2: C, 71.03; H, 7.94. Found: C, 70.98; H, 8.03.
(S)-(+)-1-(p-Br om op h en yl)-1,2-eth a n ed iol (11): Rf 0.50
(ethyl acetate/hexane 4:1); mp 100-102 °C; IR (KBr, cm-1) 3406,
3032, 2872, 1487, 1069, 1011, 829; 1H NMR (400 MHz) δ 2.17
(br s, 1H), 2.64 (br s, 1H), 3.60-3.64 (m, 1H), 3.75 (d, 1H, J )
11.3 Hz), 4.80 (dd, 1H, J ) 8.0, 3.2 Hz), 7.25 (d, 2H, J ) 8.1
Hz), 7.49 (d, 2H, J ) 8.2 Hz); 13C NMR (100 MHz) δ 139.45,
131.66, 127.79, 121.88, 74.02, 67.90. Anal. Calcd for C8H9-
BrO2: C, 44.27; H, 4.18; Br, 36.81. Found: C, 44.22; H, 4.30;
Br, 36.67.
(S)-(+)-1-Cycloh exyl-1,2-eth a n ed iol (15): Rf 0.50 (ethyl
acetate/hexane 2:1); mp 50-52 °C; IR (neat, cm-1) 3346, 2874,
1466, 1379, 1072, 1044; 1H NMR (400 MHz) δ 0.94-1.01 (m,
2H), 1.06-1.19 (m, 3H), 1.32-1.34 (m, 1H), 1.55-1.71 (m, 4H),
1.78-1.81 (m, 1H), 2.65 (br s, 2H), 3.34-3.38 (m, 1H), 3.45 (dd,
1H, J ) 10.8, 8.2 Hz), 3.62 (dd, 1H, J ) 10.9, 2.1 Hz); 13C NMR
(100 MHz) δ 63.77, 39.69, 27.94, 27.65, 25.36, 25.06. Anal.
Calcd for C8H16O2: C, 66.63; H, 11.18. Found: C, 66.66; H,
11.03.
Gen er a l Meth od s. All operations with air-sensitive materi-
als were carried out under a nitrogen atmosphere with oven-
dried glassware. Liquid materials were transferred with a
double-ended needle. The reactions were monitored by TLC
using silica gel plates, and the products were purified by flash
column chromatography on silica gel (Merck; 230-400 mesh).
NMR spectra were recorded at 200 or 400 MHz for 1H and 100
MHz for 13C using Me4Si as the internal standard in CDCl3.
Optical rotations were measured with a high-resolution digital
polarimeter. Melting points were uncorrected. Enantiomeric
excesses (% ee’s) of the product diols were determined with a
GC apparatus equipped with a 20 m Chiraldex G-TA chiral
capillary column or with a HPLC apparatus fitted with a 25 cm
Chiralcel OB or OD column.
Ma ter ia ls. Most of the organic compounds utilized in this
study were commercial products of the highest purity. They
were further purified by distillation when necessary. THF was
distilled over sodium benzophenone ketyl and stored in ampules
under nitrogen atmosphere. BH3-THF, triethylsilyl chloride,
tert-butyldimethylsilyl choride, triisopropylsilyl chloride, thexyl-
dimethylsilyl chloride, tert-butyldiphenylsilyl chloride, (S)-R,R-
diphenyl-2-pyrrolidinemethanol, (1S,2R)-cis-1-amino-2-indanol,
and (1R,2S)-2-amino-1,2-diphenylethanol were purchased from
the Aldrich Chemical Co. The chiral oxazaborolidines 1-5 were
prepared from the treatment of the corresponding amino alcohols
with BH3-THF according to the known procedure.18-22 R-[(Tri-
organosilyl)oxy] ketone derivatives 6a -n were prepared from
the reaction of the corresponding R-hydroxy ketones16 with
triorganosilyl chloride in the presence of imidazole in dichlo-
romethane according to a known procedure17 (see the Supporting
Information).
Ack n ow led gm en t. We are grateful to the Organic
Chemistry Research Center sponsored by the Korea
Science Engineering Foundation for financial support.
P r ep a r a tion of Ch ir a l Ter m in a l 1,2-Diols by Asym -
m etr ic Bor a n e Red u ction of r-[(Tr ior ga n osilyl)oxy] k e-
ton es (6) Ca ta lyzed by Oxa za bor olid in es. The following
procedure for reduction of 6 catalyzed by 2 is representative.
To a solution of 2 (0.1 mmol; 0.2 M, 0.5 mL) in THF was added
a solution of BH3-THF (0.64 mmol; 0.8 M, 0.8 mL) in THF. To
this was added slowly 2 mL of THF solution of 6 (1 mmol) over
a period of 1 h using a syringe pump at 25 °C. After the addition,
Su p p or tin g In for m a tion Ava ila ble: Experimental, spec-
tral, and elemental analysis data for compounds 6-8, 10, 12-
14 (11 pages). This material is contained in libraries on
microfiche, immediately follows this article in the microfilm
version of the journal, and can be ordered from the ACS; see
any current masthead page for ordering information.
J O980455V