Chiral C2-symmetric 2,4-disubstituted azetidines as chiral ligands in the addition of diethylzinc to aldehydes

Article abstract of DOI:10.1016/S0957-4166(99)00186-X

Chiral C₂-symmetric 2,4-disubstituted azetidine derivatives having a β-amino alcohol moiety have been successfully synthesized and their stereoselective catalytic abilities have been examined in the asymmetric addition of diethylzinc to aldehydes in hexane. When N-(2,2-diphenyl-2- hydroxyethyl)-(S,S)-2,4-bis(methoxymethyl)azetidine 12 was used as a catalytic chiral ligand, the production of sec-alcohols having an S-absolute configuration could be achieved in high chemical yields (92-99%) and high enantiomeric excesses (83-93% for arylaldehydes and 63-65% for aliphatic aldehydes). For some arylaldehydes, the enantioselectivities are higher than the corresponding chiral C₂-symmetric 2,5-disubstituted pyrrolidine ligands. However, when the chiral C₂-symmetric azetidine derivatives 13 and 14, which have bulky substituents on the 2,4-positions were used as the chiral ligands under the same reaction conditions, the enantiomeric excesses of the corresponding sec-alcohols decreased to 20-30%.

Full text of DOI:10.1016/S0957-4166(99)00186-X

Pergamon
Tetrahedron: Asymmetry 10 (1999) 1673–1679
Chiral C2-symmetric 2,4-disubstituted azetidines as chiral ligands
in the addition of diethylzinc to aldehydes
Min Shi ∗ and Jian-Kang Jiang
Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
354 Fenglin Lu, Shanghai 200032, China
Received 15 February 1999; accepted 12 May 1999
Abstract
Chiral C2-symmetric 2,4-disubstituted azetidine derivatives having a β-amino alcohol moiety have been success-
fully synthesized and their stereoselective catalytic abilities have been examined in the asymmetric addition of di-
ethylzinc to aldehydes in hexane. When N-(2,2-diphenyl-2-hydroxyethyl)-(S,S)-2,4-bis(methoxymethyl)azetidine
12 was used as a catalytic chiral ligand, the production of sec-alcohols having an S-absolute configuration could
be achieved in high chemical yields (92–99%) and high enantiomeric excesses (83–93% for arylaldehydes and
63–65% for aliphatic aldehydes). For some arylaldehydes, the enantioselectivities are higher than the corresponding
chiral C2-symmetric 2,5-disubstituted pyrrolidine ligands. However, when the chiral C2-symmetric azetidine
derivatives 13 and 14, which have bulky substituents on the 2,4-positions were used as the chiral ligands under
the same reaction conditions, the enantiomeric excesses of the corresponding sec-alcohols decreased to 20–30%.
© 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction
High efficiencies of C2-symmetric chiral reagents, including auxiliaries and catalyst ligands, in
asymmetric induction have attracted much attention in asymmetric synthesis.1 Previously, we reported
chiral C2-symmetric 2,5-disubstituted pyrrolidine derivatives as chiral catalyst ligands in the reaction of
diethylzinc with arylaldehydes.2 Very high yields and very high enantioselectivities have been achieved
in this asymmetric addition reaction.3 The relatively rigid C2-symmetric five-membered ring structure
is considered to be the determining factor for those ligands being so effective in asymmetric catalytic
reactions. According to this point of view, we tried to design and synthesize some novel chiral ligands
which have C2-symmetric four-membered rings since the backbone of a four-membered ring is totally
rigid, and consequently higher enantiomeric excesses (ees) might be achieved. Based on this concept, we
have started to synthesize some chiral C2-symmetric azetidine β-amino alcohols and have used them as
chiral ligands for catalytic asymmetric reactions.
∗
Corresponding author. E-mail: mshi@pub.sioc.ac.cn
0957-4166/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(99)00186-X

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2. Results and discussion
The optically active azetidine 2,4-dicarboxylic acid ester 1 could be synthesized by a similar method
to that reported for the preparation of chiral C2-symmetric pyrrolidine 2,5-dicarboxylic acid ester4,5 and
its diol 2 could be easily obtained as the key intermediate by reduction with lithium aluminum hydride
in THF. Thus, we can readily introduce a Me group, a TBDMS group (tert-butyldiphenylsilyl) or a
TBDPS group (tert-butyldiphenylsilyl) on the 2,4-position of the azetidine ring, respectively, to obtain
the compounds 3, 4 and 5 (Scheme 1).
Scheme 1. Conditions: (a) LiAlH4, THF, reflux, 5 h; 96%. (b) MeI, NaH, THF, rt-reflux, 89%. (c) TBDMSCl, imidazole,
CH2Cl2, rt 30 h; 76%. (d) TBDPSCl, imidazole, CH2Cl2, rt 30 h; 86%. (e) H2, Pd(OH)2, MeOH; 80% for 6, 90% for 7 and 90%
for 8. (f) BrCH2CO2Et, K2CO3, MeCN, 0°C–rt 24 h; 50% for 9, 99% for 10 and 90% for 11. (g) PhMgBr, THF, 0°C, 12 h; 75%
for 12, 85% for 13 and 70% for 14
The chiral auxiliary (2-phenethyl group) can be removed by catalytic hydrogenation over Pd(OH)2/C
(20%) to give the corresponding chiral C2-symmetric 2,4-disubstituted azetidines 6, 7 and 8, which
were subsequently treated with ethyl α-bromoacetate and then a Grignard reagent to afford the des-
ired β-amino alcohols 12, 13 and 14. All those synthetic reactions are very similar to the syn-
thesis of the corresponding 2,5-disubstituted pyrrolidine derivatives.3 Only in the preparation of N-
(ethoxycarbonylmethyl)-(S,S)-2,4-bis(methoxymethyl)azetidine 9, is the reaction somewhat different
from the corresponding 2,5-disubstituted pyrrolidine case. The yield of 9 is 50%, with the formation of
N,N-bis(ethoxycarbonylmethyl)-(S,S)-2,4-bis(methoxymethyl)azetidinium bromide 15 in 12% yield as a
by-product (Scheme 2). However, after carefully re-examining our previous results,3 we confirmed that
the formation of the corresponding pyrrolidinium bromide could not be observed at all in the preparation
of the corresponding N-(ethoxycarbonylmethyl)-(R,R)-2,5-bis(methoxymethyl)pyrrolidine.3 This result
strongly suggests that the nitrogen atom in the four-membered rings has higher nucleophilicity than that
in the five-membered rings; thus the tertiary amine could further react with another ethyl α-bromoacetate
to afford the ammonium salt.
It is well known that the reaction of aldehydes with diethylzinc giving the corresponding sec-alcohols
takes place in the presence of a catalytic amount of β-amino alcohol.6 Excellent chiral inductions

M. Shi, J.-K. Jiang / Tetrahedron: Asymmetry 10 (1999) 1673–1679
1675
Scheme 2.
including asymmetric amplifications by use of chiral β-amino alcohols in this reaction have been
reported.7 Now this interesting reaction has been widely utilized for the examination of the efficiencies
of chiral ligands.
In order to clarify the chiral induction abilities of these interesting C2-symmetric azetidine deriva-
tives and the differences between the four-membered ring and five-membered ring, we examined the
asymmetric addition reaction of diethylzinc with arylaldehydes and aliphatic aldehydes in the presence
of catalytic amounts (4 mol%) of chiral β-amino alcohols 12, 13 and 14. The results are summarized
in Table 1. The ees of the products were determined by HPLC analysis using a chiral stationary-
phase column (Chiralcel OD and OJ) and the absolute configurations of the major enantiomers were
assigned according to the sign of their specific rotations.8,9 As shown in Table 1, high yields and high
ees could be achieved by using the chiral C2-symmetric β-amino alcohol 12. For some arylaldehydes,
the ees are higher than the corresponding chiral C2-symmetric 2,5-disubstituted pyrrolidine cases. For
example, in the case of p-chlorobenzaldehyde, the ee of the product is only 76% in the presence of
5 mol% of N-(2,2-diphenyl-2-hydroxyethyl)-(R,R)-2,5-bis(methoxymethyl)pyrrolidine.2 But, using the
azetidine derivative, the ee can reach 92% (Table 1). Even in the presence of 0.8 mol% of 12, similar
results could be obtained under the same reaction conditions. On the other hand, for the aliphatic
aldehydes the ees decreased to 63–65%. This is simply due to the phenyl group being larger than the
normal aliphatic group. The bulky substrates usually give higher stereoselectivity. However, when the
chiral C2-symmetric azetidine derivatives having bulky substituents on the 2,4-position were used as the
chiral ligands under the same reaction conditions, the ees of the corresponding sec-alcohols decreased
to 20–30%. A similar tendency has also been observed in the same addition reaction using chiral C2-
symmetric 2,5-disubstituted pyrrolidine derivatives as the chiral ligands.2 It is common knowledge that
higher stereoselectivity can be achieved by introduction of sterically bulky substituents into the chiral
ligands or auxiliaries. However, our results showed that this is not always the case. This result suggests
that the flexible methoxy group on the side chain of 12 may provide an additional coordination to
the β-amino alcohol-chelated zinc atom7,10 and this totally rigid chiral Lewis acid affects the highly
stereoselective addition reaction,3 whereas the sterically bulky group on the 2,4-position would impede
this kind of coordination and influence both the approach of aldehyde and reaction transition state which
may cause the decrease of the enantioselectivity.
Based on these results, we illustrate the possible active species of 12 and the corresponding five-
membered β-amino alcohol in the above mentioned asymmetric addition reaction in Fig. 1 in which
complex A and complex B represent the chiral Lewis acids for the asymmetric reaction. We believe
the two active species are very similar. The only difference between them is the backbone structure of
the azetidine and pyrrolidine ring. The totally rigid azetidine complex A is responsible for the slightly
better chiral induction compared to the pyrrolidine case. In order to verify this speculation, we also tried
to synthesize the chiral C2-symmetric 2,6-disubstituted six-membered ring according to the literature.5
However, the separation of these diastereoisomers was very difficult. Until now, we could not obtain the
enantiomerically pure C2-symmetric 2,6-disubstituted piperidine.

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Table 1
Asymmetric addition reaction of diethylzinc with aldehydes in the presence of 5 mol% chiral
C2-symmetric 2,4-disubstituted azetidines 12, 13, and 14
Figure 1.
In conclusion, we have found that the totally rigid C2-symmetric four-membered ring is a good
potential chiral unit for the catalytic asymmetric induction reaction. Efforts are underway to elucidate
the mechanistic details of why N-(2,2-diphenyl-2-hydroxyethyl)-(S,S)-2,4-bis(methoxymethyl)azetidine
12 is such an effective catalyst ligand.
3. Experimental
Mps were obtained with a Yanagimoto micro melting point apparatus and are uncorrected. Optical
rotations were determined in a solution of CHCl3 and methanol at 20°C by using a Perkin–Elmer 241
1
MC digital polarimeter; [α]D values are given in units of 10−1 deg cm2 g−1. H NMR spectra were
determined for solutions in CDCl3 with tetramethylsilane (TMS) as internal standard on a Bruker AMX-
300 spectrometer; J values are in hertz. IR spectra were determined by a Perkin–Elmer 983 spectrometer.
Mass spectra were recorded with a HP-5989 instrument. High resolution mass spectra were recorded
on a Finnigan MA+ instrument. All solid compounds reported in this paper gave satisfactory CHN

M. Shi, J.-K. Jiang / Tetrahedron: Asymmetry 10 (1999) 1673–1679
1677
microanalyses with an Italian Carlo-Erba 1106 analyzer. Compounds 1, 2, 3 and 6 were prepared
according to the literature.4,5
3.1. Preparation of (S,S)-2,4-bis[(tert-butyldimethylsiloxy)methyl]-N-[(S)-1-phenylethyl]azetidine 4
To a solution of 2 (260 mg, 1.18 mmol) and imidazole (160 mg, 2.36 mmol) in dichloromethane (30
ml) was added tert-butyldimethylsilyl chloride (356 mg, 2.36 mmol) and the reaction mixture was stirred
for 36 h at room temperature. The mixture was washed with brine and dried over MgSO4. The solvent
was removed under reduced pressure and the residue was purified by flash chromatography to give 4 as a
colorless oil (eluent: ethyl acetate:petroleum ether=1:10). Yield: 400 mg, 76%; [α]D −60.1 (c 1, CHCl3);
1H NMR (CDCl3, 300 MHz) δ 0.08 (6H, s, SiMe), 0.10 (6H, s, SiMe), 0.96 (18H, s, Me3C), 1.12 (3H,
d, J=6.2, Me), 2.12 (2H, t, J=7.0), 3.50–3.70 (6H, m), 4.00–4.20 (1H, q, J=6.2), 7.10–7.50 (5H, m, Ar);
[HRMS (EI) found: 449.3157 (M+); C25H47NO2Si2 requires: 449.3145].
3.2. Preparation of (S,S)-2,4-bis[(tert-butyldiphenylsiloxy)methyl]-N-[(S)-1-phenylethyl]azetidine 5
This compound was prepared in the same manner as that described above. Yield: 658 mg, 86%; [α]D
+20.1 (c 1, CHCl3); 1H NMR (CDCl3, 300 MHz) δ 0.96 (18H, s, Me3C), 1.12 (3H, d, J=6.2, Me), 2.02
(2H, t, J=7.0), 3.10 (2H, dd, J=10.4, 3.8), 3.45 (2H, dd, J=10.4, 6.3), 3.70–3.85 (2H, m), 3.90 (1H, q,
J=6.2), 7.00–7.10 (3H, m, Ar), 7.15–7.22 (2H, m, Ar), 7.30–7.50 (12H, m, Ar), 7.51–7.70 (8H, m, Ar);
[HRMS (EI) found: 697.3756 (M+); C45H55NO2Si2 requires: 697.3771].
3.3. Preparation of (S,S)-2,4-bis[(tert-butyldimethylsiloxy)methyl]azetidine 7
To a solution of 4 (390 mg, 0.87 mmol) in methanol (40 ml) was added Pd(OH)2/C (20%, 100 mg) and
the reaction mixture was stirred at room temperature for 64 h under a hydrogen atmosphere. The solvent
was removed under reduced pressure and the residue was used for the next reaction without purification.
Yield: 273 mg, 90%; [α]D −1.1 (c 1, CHCl3); 1H NMR (CDCl3, 300 MHz) δ 0.10 (12H, s, SiMe), 0.96
(18H, s, Me3C), 2.16 (2H, t, J=7.0), 2.61 (1H, br, s, NH), 3.70–3.85 (4H, m), 3.90 (2H, m); [HRMS (EI)
found: 345.2536 (M+); C17H39NO2Si2 requires 345.2519].
3.4. Preparation of (S,S)-2,4-bis[(tert-butyldiphenylsiloxy)methyl]azetidine 8
This compound was prepared in the same manner as that described above. Yield: 273 mg, 90%; [α]D
+3.5 (c 1, CHCl3); 1H NMR (CDCl3, 300 MHz) δ 0.96 (18H, s, Me3C), 2.20 (2H, t, J=7.0), 2.70 (1H,
br, s, NH), 3.70–3.85 (4H, m), 3.90 (2H, m), 7.30–7.50 (12H, m, Ar), 7.51–7.70 (8H, m, Ar); [HRMS
(EI) found: 593.3155 (M+); C37H47NO2Si2 requires: 593.3145].
3.5. Preparation of N-(ethoxycarbonylmethyl)-(S,S)-2,4-bis(methoxymethyl)azetidine 9
To a solution of 6 (300 mg, 2.10 mmol) and potassium carbonate (428 mg, 3.10 mmol) in acetonitrile
(30 ml) was added ethyl α-bromoacetate (380 mg, 0.25 ml, 2.28 mmol) and the reaction mixture was
stirred at room temperature for 18 h. The solvent was removed under reduced pressure and the residue was
extracted with ether (3×30 ml). The organic layer was washed with water, brine and dried over NaSO4.
The solvent was removed under reduced pressure and the residue was purified by flash chromatography
to give 9 as a colorless oil (eluent: ethyl acetate:petroleum ether=1:1). Yield: 230 mg, 50%; [α]D −70.2
(c 0.64, CHCl3); IR (neat) ν 1740 (C_O) cm−1; 1H NMR (CDCl3, 300 MHz) δ 1.28 (3H, t, J=7.1, Me),

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M. Shi, J.-K. Jiang / Tetrahedron: Asymmetry 10 (1999) 1673–1679
2.06 (2H, t, J=7.1), 3.36 (6H, s, OMe), 3.42 (1H, d, J=17.3), 3.48 (2H, dd, J=10.4, 4.0), 3.54 (2H, dd,
J=10.4, 6.3), 3.63 (1H, d, J=17.3), 3.72–3.83 (2H, m), 4.20 (2H, q, J=7.1); [HRMS (EI) found: 231.1459
(M+); C11H21NO4 requires: 231.1471].
3.6. Preparation of N-(ethoxycarbonylmethyl)-(S,S)-2,4-bis[(tert-butyldimethylsiloxy)methyl]azetidine
10
To a solution of 7 (250 mg, 0.74 mmol) and potassium carbonate (112 mg, 0.80 mmol) in acetonitrile
(30 ml) was added ethyl α-bromoacetate (124 mg, 0.74 mmol) and the reaction mixture was stirred
at room temperature for 38 h. The solvent was removed under reduced pressure and the residue was
extracted with ethyl acetate. The organic layer was washed with water, brine and dried over MgSO4. The
solvent was removed under reduced pressure and the residue was purified by flash chromatography to
give 10 as a colorless oil (eluent: ethyl acetate:petroleum ether=1:10). Yield: 319 mg, 99%; [α]D −28.1
(c 1.25, CHCl3); IR (neat) ν 1740 (C_O) cm−1; 1H NMR (CDCl3, 300 MHz) δ 0.08 (6H, s, SiMe), 0.10
(6H, s, SiMe), 0.96 (18H, s, Me3C), 1.28 (3H, t, J=5.9, Me), 2.02 (2H, t, J=7.0), 3.40 (1H, d, J=18.0),
3.50–3.73 (4H, m, CH2), 3.67 (1H, d, J=18.0), 4.00–4.20 (4H, m); [HRMS (EI) found: 431.2870 (M+);
C21H45NO4Si2 requires: 431.2887].
3.7. Preparation of N-(ethoxycarbonylmethyl)-(S,S)-2,4-bis[(tert-butyldiphenylsiloxy)methyl]azetidine
11
This compound was prepared in the same manner as that described above. Yield: 273 mg, 90%; [α]D
1
+12.2 (c 1.0, CHCl3); IR (neat) ν 1740 (C_O) cm−1; H NMR (CDCl3, 300 MHz) δ 1.00 (18H, s,
Me3C), 1.32 (3H, t, J=5.9, Me), 2.14 (2H, t, J=7.0), 3.40 (1H, d, J=18.0), 3.50–3.73 (4H, m, CH2), 3.67
(1H, d, J=18.0), 4.00–4.20 (4H, m), 7.30–7.50 (12H, m, Ar), 7.51–7.70 (8H, m, Ar); [HRMS (EI) found:
679.3502 (M+); C41H53NO4Si2 requires: 679.3513].
3.8. Preparation of N-(2,2-diphenyl-2-hydroxyethyl)-(S,S)-2,4-bis(methoxymethyl)azetidine 12
To a solution of phenylmagnesium bromide prepared from the reaction of bromobenzene (1.5 g, 9.55
mmol) and magnesium (250 mg, 10.42 mmol) in THF (10 ml) was added 9 (220 mg, 0.95 mmol) at
0°C and the reaction mixture was stirred for 24 h. The reaction was quenched with saturated NH4Cl
aqueous solution and extracted with ether (3×25 ml). The organic layer was washed with brine and dried
over MgSO4. The solvent was removed under reduced pressure and the residue was purified by flash
chromatography to give 12 as a colorless oil (eluent: ethyl acetate:petroleum ether=1:3). Yield: 242 mg,
75%; [α]D −16.0 (c 1.0, CHCl3); 1H NMR (CDCl3, 300 MHz) δ 2.02 (2H, t, J=7.0), 3.12 (1H, d, J=13.6),
3.17–3.26 (4H, m, CH2), 3.30 (6H, s, OMe), 3.40–3.60 (2H, m), 3.75 (1H, d, J=13.6), 5.40 (1H, br, s,
OH), 7.10–7.30 (6H, m, Ar), 7.40–7.50 (4H, m, Ar); [HRMS (EI) found: 341.1979 (M+); C21H27NO3
requires: 341.1991].
3.9. Preparation of N-(2,2-diphenyl-2-hydroxyethyl)-(S,S)-2,4-bis[(tert-butyldimethylsiloxy)methyl]-
azetidine 13
This compound was prepared in the same manner as that described above. Yield: 437 mg, 85%; [α]D
−11.0 (c 1.07, CHCl3); 1H NMR (CDCl3, 300 MHz) δ 0.08 (12H, s, SiMe), 0.96 (18H, s, Me3C), 2.02
(2H, t, J=7.0), 3.12 (1H, d, J=13.6), 3.20–3.50 (6H, m, CH2), 3.85 (1H, d, J=13.6), 5.40 (1H, br, s, OH),

M. Shi, J.-K. Jiang / Tetrahedron: Asymmetry 10 (1999) 1673–1679
1679
7.10–7.40 (6H, m, Ar), 7.40–7.60 (4H, m, Ar); [HRMS (EI) found: 523.3315 (M+−H2O); C31H49NO2Si2
requires: 523.3302].
3.10. Preparation of N-(2,2-diphenyl-2-hydroxyethyl)-(S,S)-2,4-bis[(tert-butyldiphenylsiloxy)methyl]-
azetidine 14
This compound was prepared in the same manner as that described above. Yield: 525 mg, 70%; [α]D
1
+22.3 (c 1.0, CHCl3); H NMR (CDCl3, 300 MHz) δ 0.96 (18H, s, Me3C), 2.12 (2H, t, J=7.0), 3.16
(1H, d, J=13.8), 3.20–3.60 (6H, m, CH2), 3.85 (1H, d, J=13.8), 5.40 (1H, br, s, OH), 7.10–7.40 (6H, m,
Ar), 7.40–7.52 (16H, m, Ar), 7.53–7.70 (8H, m, Ar); [HRMS (EI) found: 789.4029 (M+); C51H59NO3Si2
requires: 789.4033].
3.11. The formation of N,N-bis(ethoxycarbonylmethyl)-(S,S)-2,4-bis(methoxymethyl)azetidinium
bromide 15
This compound was obtained in the preparation of 9. Yield: 90 mg, 12%; [α]D +1.5 (c 0.95, CHCl3);
1H NMR (CDCl3, 300 MHz) δ 1.25 (6H, t, J=7.1, Me), 2.05 (1H, dt, J=14.7, 7.1), 2.13 (1H, dt, J=14.7,
7.1), 3.00–3.20 (1H, m), 3.18 (3H, s, OMe), 3.46 (3H, s, OMe), 3.45–3.60 (2H, m, CH2), 3.66 (4H, d,
J=1.3, CH2), 3.56–3.80 (2H, m, CH2), 4.18 (4H, q, J=7.1, CH2), 4.31 (1H, m); MS (EI) m/z 397, 399
(M+); [HRMS (EI) found: 397.1307 (M+); C15H28BrNO6 requires: 397.1300].
Typical reaction procedure: to a suspension of β-amino alcohol 12 (8.5 mg, 0.025 mmol) in hexane
(1.0 ml), diethylzinc (1.1 mmol, 1.1 ml of 1 M hexane solution) was added at 0°C. After stirring for 0.5
h, benzaldehyde (53 mg, 1.0 mmol) was added and the reaction mixture was stirred for 24 h at 0°C. The
reaction was quenched by 3% HCl aqueous solution and the organic product was extracted with ethyl
acetate. The extract was dried over MgSO4 and the solvent was evaporated under reduced pressure. The
residue was purified by silica gel TLC to give the optically active 1-phenylpropanol (67 mg, 99%).
Acknowledgements
We thank the National Natural Sciences Foundation of China for financial support.
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