A chiral ligand-mediated asymmetric addition of a lithium BHA ester enolate to an aldehyde

Article abstract of DOI:10.1248/cpb.50.1131

The asymmetric reaction of a lithium enolate generated from a BHA (2,6-di-tert-buty-4-methoxyphenyl) propanoate was allowed to react with benzaldehyde in the presence of a diether-type chiral ligand affording the corresponding anti-aldol product in a moderate enantioselectivity. A tetradentate ligand induced better enantioselectivity albeit relative loss of anti-selectivity. A variation of lithiating amide agent affected the selectivity, indicating involvement of an amine as a component of the mixed aggregate. Absolute configuration of some of the aldol products was determined by standard transformations.

Full text of DOI:10.1248/cpb.50.1131

August 2002
Notes
Chem. Pharm. Bull. 50(8) 1131—1134 (2002)
1131
A Chiral Ligand-Mediated Asymmetric Addition of a Lithium BHA Ester
Enolate to an Aldehyde1)
a
b
b
,b
*
Yumiko NOMURA, Mayu IGUCHI, Hirohisa DOI, and Kiyoshi TOMIOKA
a Faculty of Pharmaceutical Sciences, The University of Tokyo; Hongo, Bunkyo-ku, Tokyo 113–0033, Japan: and
b Graduate School of Pharmaceutical Sciences, Kyoto University; Yoshida, Sakyo-ku, Kyoto 606–8501, Japan.
Received March 19, 2002; accepted May 31, 2002; released June 5, 2002
The asymmetric reaction of a lithium enolate generated from a BHA (2,6-di-tert-buty-4-methoxyphenyl)
propanoate was allowed to react with benzaldehyde in the presence of a diether-type chiral ligand affording the
corresponding anti-aldol product in a moderate enantioselectivity. A tetradentate ligand induced better enantios-
electivity albeit relative loss of anti-selectivity. A variation of lithiating amide agent affected the selectivity, indi-
cating involvement of an amine as a component of the mixed aggregate. Absolute configuration of some of the
aldol products was determined by standard transformations.
Key words aldol reaction; asymmetric reaction; lithium enolate; ligand; propanoate
An asymmetric addition reaction of an ester enolate with a presence of 1.3 eq of 1 in toluene, and following treatment
carbonyl compound is one of the important and fundamental with 7 at Ϫ78 °C for 5 min to give an almost diastereomeri-
carbon–carbon bond forming reactions.2—4) We have been in- cally pure anti-12. The aldol product was immediately sub-
volved in the chiral ligand-mediated asymmetric reactions of jected to acetylation with acetic anhydride–triethylamine–
highly reactive metalated nucleophiles.5—8) Particularly, the DMAP in methylene chloride to afford the corresponding ac-
reaction of a lithium ester enolate with an imine represents a etate anti-13 as a major product of a 7 : 93 mixture (Entry 2).
successful entry to a carbon–carbon bond forming asymmet- Enantioselectivity of anti-13 was determined to be 32% by a
ric reaction, which is mediated by a chiral diether 1 and a chiral stationary phase HPLC analysis. Improvement of the
chiral aminoether affording the corresponding b-lactam in enantioselectivity was possible by using LDA as a base af-
a satisfactorily high enantioselectivity and high chemical fording anti-13 in 80% yield and 50% ee (Entry 3). A
yield.9—13) Straightforward extension of the imine condensa- ternary complex reagent12) was proved not to be beneficial
tion to an aldol-type reaction with an aldehyde as a reaction providing anti-13 in 40% yield and 15% ee (Entry 4). The
partner provides a formidable challenge, because the reactiv- ether ligand 2 having an ethoxy group was not a choice, giv-
ity of an aldehyde itself is high enough to an extent that does ing poorer selectivity of 9% (Entry 5). The best ligand
not need any activation for the reaction with a lithium enolate among examined was a tetradentate 3, which mediated the
(Fig. 1).14,15) We describe herein that the reaction of a lithium reaction to afford anti-13 in 61% ee. However, the syn/anti
BHA ester enolate with an aldehyde is mediated by a chiral selectivity was lost to afford syn-13 as a major product in
ligand giving a moderate enantioselectivity.16)
35% ee (Entry 6). The present procedure was applicable in
Asymmetric Reaction of Propanoate We began our the reaction with pivalaldehyde 8 giving the corresponding
studies with the reaction of tert-butyl propanoate 4 with ben- aldol product anti-15 in 56% ee by 1, in 49% ee by 3 (Entries
zaldehyde 7 (Fig. 2). Treatment of 4 with LDA in the pres-
ence of a chiral diether ligand 1 in toluene and then with 7
gave the target 10 as a 27 : 73 mixture of syn- and anti-aldol
products (Table 1, Entry 1). However, the enantioselectivity
of the acetate 11 was poor. It has been reported by Heathcock
that a lithium enolate generated from BHA (2,6-di-tert-buty-
4-methoxyphenyl) propanoate 5 in THF reacted with 7 to
give stereoselectively anti-aldol product 12.17) Since a BHA
ester methodology has been our favourite,18) we applied the
Heathcock procedure in our asymmetric aldol-type reaction
in toluene, instead of a THF solvent.
Generation of a lithium enolate from BHA ester 5 was
possible by butyllithium treatment at Ϫ78 °C for 1 h in the
Fig. 1. Asymmetric Aldol Reaction
Fig. 2. Ligands, Esters, Aldehydes, and Products
© 2002 Pharmaceutical Society of Japan
To whom correspondence should be addressed. e-mail: tomioka@pharm.kyoto-u.ac.jp

1132
Vol. 50, No. 8
Table 1. A Chiral Ligand-Mediated Asymmetric Addition of a Lithium Enolate to an Aldehyde
Entry
Ester
Ligand
Base
eq
Aldehyde
Product
Yield %
syn : anti
syn ee %
anti ee %
1
2
3
4
5
6
7
8
4
5
5
5
5
5
5
5
1
1
1
1
2
3
1
3
LDA
BuLi
LDA
LDA
LDA
LDA
LDA
LDA
1.2
1.2
1.2
2.4
1.2
1.3
1.2
1.2
7
7
7
7
7
7
8
8
11
13
13
13
13
13
15
15
64
99
78
40
70
88
76
60
27 : 73
7 : 93
1 : 99
1 : 99
1 : 99
77 : 23
1 : 99
82 : 18
9
11
32
50
15
9
35
35
ent-61
56
ent-49
Table 2. A Chiral Ligand-Mediated Asymmetric Addition of a Lithium Enolate of 6 to an Aldehyde
Entry
Ligand
Base
BuLi
LDA
LDA
LICA
LTMP
BuLi
LDA
LDA-LiBr
LDA
eq
Aldehyde
Product
Yield %
ee %
1
2
3
4
5
6
7
8
9
10
11
12
13
1
1
1
1
1
3
3
3
3
1
3
1
3
1.0
1.2
2.4
1.2
1.3
1.0
1.3
1.3
2.4
1.0
1.0
1.0
1.0
7
7
7
7
7
7
7
7
7
8
8
9
9
17
17
17
17
17
17
17
17
17
19
19
21
21
79
57
69
46
40
44
68
23
79
56
26
71
57
35
20
11
ent-19
10
ent-64
ent-69
ent-44
ent-63
7
BuLi
LDA
BuLi
LDA
16
27
49
7, 8).
Generation of a lithium enolate from propanoate 5 in-
volves the E- and Z-problem, which makes difficult the
analysis of the reaction. Although a BHA ester produces Z-
enolate due to its steric bulk in a THF solvent, there is no
guarantee that selective formation of the fixed geometry of
an enolate is possible even in a toluene solvent. Then, we
turned our attention to 2-methylpropanoate 6 that does not
produce such a stereochemical problem.
Asymmetric Reaction of BHA 2-Methylpropanoate
Generation of an enolate from BHA 2-methylpropanoate 6
with butyllithium in the presence of 1 in toluene and follow-
ing treatment with benzaldehyde 7 provided 16, which was
then converted to an acetate 17 in 79% yield and 35% ee
(Table 2, Entry 1). LDA treatment and ternary complex
reagent12) did not provide any improvement to afford 17 in
20% ee and 11% ee (Entries 2, 3). The efficiency was depen-
dent on the lithiation agent; lithium cyclohexylisopropyl-
amide produced enantiomeric 17 in 19% ee and lithium
tetramethylpiperidide afforded 17 in 10% ee (Entries 4, 5).
These influences by lithiation agents apparently suggest an
involvement of amine and amide moiety in an aggregate of a
lithium enolate.
Improvement was realized by using a tetradentate ligand 3
as a controlling agent. The similar and relatively high enan-
tioselectivity was observed regardless to lithiation reagents.
Butyllithium and LDA treatment of 6, and ternary complex
with LDA13) in the presence of 3 gave ent-17 in the almost
similar level of selectivity 63—69% ee (Entries 6, 7, 9).
Presence of lithium bromide affected on chemical yield and
enantioselectivity (Entry 8). However, the reaction with pi-
valaldehyde 8 proceeded unsatisfactorily to afford 19 in ut-
Fig. 3. Conversion to the Established Compounds
most 16% ee (Entries 10, 11). The reaction with cinnamalde-
hyde 9 gave the adduct 21 in 49% ee with an aid of 3.
Determination of the Absolute Configuration of the
Aldol Products Absolute configuration of some of the
aldol products was determined as shown in Fig. 3. The ac-
etate 13 was treated with cerium ammonium nitrate in aque-
ous acetonitrile to afford a carboxylic acid 22, which was
then converted to a methyl ester 23 of the established stereo-
chemistry. The pivaloyl adduct 15 was also treated similarly
to afford 25. The 2-methylpropanoate adduct ent-17 was con-
verted to a carboxylic acid 26.

August 2002
1133
Conclusion
(Table 2, Entry 13) 21: Solid of of [a]D25 ϩ5.2° (cϭ1.02, CHCl3) and mp
138—146 °C (mp 147 °C (hexane) for racemic 21). 49% ee (Optipak XC,
hexane/2-propanolϭ60/1, 254 nm, 0.5 ml/min, minor 14 min, major 16 min).
1H-NMR (CDCl3): 1.34 and 1.36 (each 9H, s), 1.49 and 1.57 (each 3H, s),
2.03 (3H, s), 3.79 (3H, s), 5.74 (1H, d, Jϭ8 Hz), 6.29 (1H, dd, Jϭ8, 16 Hz),
6.69 (1H, d, Jϭ16 Hz), 6.88 (2H, s), 7.3 (5H, m). IR (KBr): 1720 cmϪ1. MS
m/z: 480 (Mϩ). Anal. Calcd for C30H40O5: C 74.97, H 8.39. Found: C 74.87,
H 8.47.
The extension of an imine-enolate condensation reaction
to an aldol reaction was examined using BHA propanoates.
A chiral diether ligand was found to induce moderate enan-
tioselectivity and high anti-selectivity. A tetradentate ligand
induced better enantioselectivity albeit loss of anti-selectiv-
ity. These clearly indicate that more sophisticated devises are
required for extension of the imine-enolate condensation
technology to an aldol-type reaction with a carbonyl com-
pound.
Determination of the Absolute Configuration of 13 A mixture of (Ϫ)-
25
365
13 (18% ee, [a] Ϫ4.1° (cϭ1.00, CHCl3). 0.28 g, 0.7 mmol) and cerium
(IV) diammonium nitrate (984 g, 1.8 mmol) in a mixture of water (1.5 ml)
and acetonitrile (1.4 ml) was stirred at rt for 1 h under sonication. Mannitol
(0.33 g, 1.9 mmol) was added to a dark red solution above giving a yellow
solution, which was stirred for another 0.5 h. After addition of water, the
mixture was extracted with methylene chloride. The organic layer was ex-
tracted with 5% sodium hydroxide, which was extracted back with methyl-
ene chloride after acidification with c. HCl. The organic layer was washed
with brine and concentrated to give the corresponding carboxylic acid 22
(0.1 g). An ether (1 ml) solution of the acid was then treated with dia-
zomethane (prepared from nitrosomethylurea (0.29 g, 2.8 mmol)) and diluted
with methylene chloride after addition of acetic acid. The organic layer was
washed with satd sodium bicarbonate, brine, and then dried over magnesium
sulfate. Concentration and silica gel column chromatography (hexane–
etherϭ4/1) gave the corresponding methyl ester (2R,3S)-23 (50 mg, 36%) as
a solid of mp 48—51 °C and [a]D20 Ϫ11.2° (cϭ1.41, CHCl3). The absolute
configuration was determined as above by comparing specific rotation with
the established (2S,3R)-(ϩ)-23.19) 1H-NMR (CDCl3): 0.99 (3H, d, Jϭ7 Hz),
2.79 (1H, dq, Jϭ7, 8 Hz), 3.00 (1H, br s), 3.71 (3H, s), 4.73 (1H, d, Jϭ8 Hz),
7.32 (5H, m). IR (neat): 1740 cmϪ1. MS m/z: 194 (Mϩ). Ee was confirmed
by HPLC (Chiralpak XC, hexane/2-propanolϭ9/1, 254 nm, 0.5 ml/min,
major 16 min, minor 25 min).
Experimental
BHA Propanoate 5 A THF (60 ml) solution of 2,6-di-tert-butyl-4-
methoxyphenol (18.9 g, 80 mmol) was added to a suspension of NaH (88
mmol) in THF (120 ml) at 0 °C over 5 min. After 1 h stirring at room temper-
ature (rt), propionyl chloride (10.4 ml, 120 mmol) was added, and the mix-
ture was stirred at rt for 2 h before addition of 15% NaOH (100 ml). The
mixture was stirred at rt for 0.5 h and then diluted with water (100 ml). The
mixture was extracted with AcOEt. The organic layer was washed with
brine, and then dried over magnesium sulfate. Concentration and silica gel
column chromatography (benzene/hexaneϭ1/4) gave 5 (19.7 g, 84%) as col-
1
orless prisms (hexane) of mp 53 °C. H-NMR (CDCl3): 1.00 (18H, s), 1.33
(3H, t, Jϭ8 Hz), 2.60 (2H, q, Jϭ8 Hz), 3.83 (3H, s), 7.77 (2H, s). IR (nujol):
1770, 1600 cmϪ1. MS m/z: 292 (Mϩ). Anal. Calcd for C18H28O3: C 73.93, H
9.65. Found: C 73.86, H 9.80.
BHA 2-Methylpropanoate 6 Prepared by the same procedure for 5 in
90% yield as colorless prisms (hexane) of mp 44 °C. 1H-NMR (CDCl3): 1.31
(18H, s), 1.35 (6H, d, Jϭ8 Hz), 2.82 (1H, q, Jϭ8 Hz), 3.75 (3H, s), 6.84
(2H, s). IR (nujol): 1750, 1590 cmϪ1. MS m/z: 306 (Mϩ). Anal. Calcd for
C19H30O3: C 74.47, H 9.87. Found: C 74.45, H 10.15.
Determination of the Absolute Configuration of 15 A mixture of
(ϩ)-15 ([a]D25 ϩ15.9° (cϭ1.01, CHCl3), 0.64 g, 1.5 mmol) and cerium (IV)
diammonium nitrate (2.19 g, 4 mmol) in a mixture of water (3 ml) and ace-
tonitrile (3 ml) was stirred at rt for 1 h under sonication. Mannitol (0.75 g,
4 mmol) was added to a dark red solution above giving a yellow solution,
which was stirred for another 0.5 h. After addition of water, the mixture was
extracted with methylene chloride. The organic layer was extracted with 5%
sodium hydroxide, which was extracted back with methylene chloride after
acidification with c. HCl. The organic layer was washed with brine and con-
centrated to give the corresponding carboxylic acid 24 (0.18 g). An ether
(2 ml) solution of the acid was treated with diazomethane (prepared from ni-
trosomethylurea (0.46 g, 4.5 mmol)) and diluted with methylene chloride
after addition of acetic acid. The organic layer was washed with satd sodium
bicarbonate, brine, and then dried over magnesium sulfate. Concentration
and silica gel column chromatography (hexane/etherϭ4/1) gave the corre-
sponding methyl ester (2R,3S)-25 (113 mg, 35%) as a pale yellow oil of
[a]D25 Ϫ16.5° (cϭ0.96, CHCl3). The absolute configuration was determined
as above by comparing specific rotation with the established (2R,3S)-(Ϫ)-
25.20) 1H-NMR (CDCl3): 0.89 (9H, s), 1.34 (3H, d, Jϭ7 Hz), 2.79 (1H, dq,
Jϭ1, 8 Hz), 3.17 (1H, d, Jϭ1 Hz), 3.68 (3H, s). IR (neat): 1740 cmϪ1. MS
m/z: 174 (Mϩ).
Typical Procedure for Asymmetric Aldol Reaction (Table 1, Entry 3)
A hexane solution of n-BuLi (1.2 mmol, 0.85 ml) was added to a solution of
diisopropylamine (1.2 mmol, 0.17 ml) in toluene (10 ml) at Ϫ78 °C. After
stirring for 10 min, a solution of a chiral diether 1 (440 mg, 1.8 mmol) and
BHA propanoate 5 (292 mg, 1 mmol) in toluene (10 ml) was added. The
mixture was stirred at Ϫ78 °C for 1 h, and then was added by benzaldehyde
(0.11 ml, 1 mmol). After 5 min, the mixture was treated with satd ammonium
chloride and extracted with AcOEt. The organic layer was washed with 10%
HCl, water, satd sodium bicarbonate, and brine, and then dried over magne-
sium sulfate. Concentration gave 12 as a yellow oil, which was diluted with
methylene chloride (5 ml). Acetic anhydride (0.22 ml, 2.4 mmol) and triethy-
lamine (0.42 ml, 3 mmol) was added at Ϫ20 °C. After addition of 4-dimethy-
laminopyridine (DMAP) (70 mg, 0.6 mmol), the whole was stirred at rt for
20 min. The mixture was then diluted with methylene chloride and washed
with 10% HCl, satd sodium bicarbonate, and brine, and then dried over
sodium sulfate. Concentration and silica gel column chromatography (ben-
zene/etherϭ20/1) gave 13 (0.34 g, 78%) as a solid of mp 100—126 °C (mp
25
365
of racemic 13: 133—135 °C) and [a] Ϫ11.5° (cϭ0.996, CHCl3). 50% ee
(Optipak TC, hexane/2-propanolϭ100/1, 254 nm, 1 ml/min, minor 8.7 min,
1
major 10.8 min). H-NMR (CDCl3): 1.31 (18H, s), 1.86 (3H, s), 3.22 (1H,
dq, Jϭ8, 9 Hz), 3.69 (3H, s), 5.98 (1H, d, Jϭ9 Hz), 6.76, 6.80 (2H, s), 7.28
(5H, m). IR (nujol): 1760 cmϪ1. MS m/z: 440 (Mϩ). Anal. Calcd for
C27H36O5: C 73.61, H 8.24. Found: C 73.67, H 8.37.
Determination of the Absolute Configuration of 17 A mixture of (Ϫ)-
25
365
17 (69% ee, [a] Ϫ1.7° (cϭ1.20, CHCl3). 0.4 g, 0.9 mmol) and cerium
(IV) diammonium nitrate (CAN) (1.88 g, 3.4 mmol) in a mixture of water
(2 ml) and acetonitrile (2 ml) was stirred at rt for 2 h under sonication. Man-
nitol (1.07 g, 5.9 mmol) was added to a dark red solution above giving a yel-
low solution, which was stirred for another 0.5 h. After addition of water, the
mixture was extracted with methylene chloride. The organic layer was ex-
tracted with 5% sodium hydroxide, which was extracted back with methyl-
ene chloride after acidification with c. HCl. The organic layer was washed
with brine and concentrated to give an oil. Silica gel column chromatogra-
phy (methylene chloride/methanolϭ9/1) gave the corresponding methyl
ester (S)-26 (102 mg, 59%) as solid of mp 140—152 °C and [a]D25 ϩ5.0°
(cϭ0.88, MeOH). The absolute configuration was determined as above by
comparing specific rotation with the established (S)-(ϩ)-25.21) 1H-NMR
(CDCl3): 0.99 and 1.06 (each 3H, s), 4.92 (1H, s), 5.06 (2H, br s), 7.2 (5H,
m). IR (nujol): 1750 cmϪ1. MS m/z: 194 (Mϩ).
(Table 1, Entry 7) 15: An oil of [a]D25 ϩ15.9° (cϭ1.01, CHCl3). 56% ee
(Chiralpak AD, hexane/2-propanolϭ100/1, 254 nm, 0.5 ml/min, minor
9.9 min, major 24 min). 1H-NMR (CDCl3): 1.02 (9H, s), 1.32 and 1.36 (each
9H, s), 1.61 (3H, d, Jϭ8 Hz), 1.97 (3H, s), 3.16 (1H, dq, Jϭ6, 8 Hz), 3.78
(3H, s), 4.95 (1H, d, Jϭ6 Hz), 6.85 (2H, s). IR (neat): 1750 cmϪ1. MS m/z:
420 (Mϩ). Anal. Calcd for C24H40O5: C 71.39, H 9.59. Found: C 71.11, H
9.59.
(Table 2, Entry 2) 17: Colorless prisms (hexane) of mp 113—114 °C and
25
[a] ϩ0.9° (cϭ5.64, CHCl3). 20% ee (Optipak TC, hexane/2-propanolϭ
365
1
30/1, 254 nm, 0.5 ml/min, minor 12 min, major 16 min). H-NMR (CDCl3):
1.28 and 1.40 (each 9H, s), 1.30 and 1.57 (each 3H, s), 1.99 (3H, s), 3.78
(3H, s), 6.22 (1H, s), 6.87 (2H, m), 7.3 (5H, m). IR (KBr): 1750 cmϪ1. MS
m/z: 454 (Mϩ). Anal. Calcd for C28H38O5: C 73.98, H 98.42. Found: C
73.89, H 8.53.
(Table 2, Entry 11) 19: Colorless oil with 16% ee (Optipak XC, hexane/2-
propanolϭ100/1, 254 nm, 0.5 ml/min, minor 10 min, major 14 min). 1H-
NMR (CDCl3): 1.10 (9H, s), 1.34 (18H, s), 1.58 and 1.62 (each 3H, s), 3.5
(1H, br s), 3.72 (1H, s), 3.79 (3H, s), 6.88 (2H, s). IR (neat): 1720 cmϪ1. MS
m/z: 393 (Mϩ).
Acknowledgments This research was supported by Takeda Science
Foundation and a Grant-in-Aid for Scientific Research on Priority Areas (A)
“Exploitation of Multi-Element Cyclic Molecules” from the Ministry of Ed-
ucation, Culture, Sports, Science and Technology, Japan.

1134
Vol. 50, No. 8
References and Notes
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