F3B · OEt2-promoted intramolecular Si to C phenyl group migration: A highly diastereoselective synthesis of (4S,5S)-4-hydroxy-5-phenyl-2-pyrrolidinone

Article abstract of DOI:10.1080/00397910500503538

A highly diastereoselective approach to N-protected cis-(4S,5S)-4-hydroxy- 5-phenyl-2-pyrrolidinone is reported. The key step is a boron trifluoride etherate-promoted chemoselective intramolecular Si → C phenyl group migration. Copyright Taylor & Francis

Full text of DOI:10.1080/00397910500503538

Synthetic Communicationsw, 36: 1131–1139, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910500503538
.
F3B OEt2-Promoted Intramolecular Si to C
Phenyl Group Migration: A Highly
Diastereoselective Synthesis of (4S,5S)-4-
Hydroxy-5-phenyl-2-pyrrolidinone
Liang-Xian Liu
Department of Chemistry and Biology, Ganan Teachers’ College,
Ganzhou, Jiangxi, China and Department of Chemistry and the Key
Laboratory for Chemical Biology of Fujian Province, College of
Chemistry and Chemical Engineering, Xiamen University, Xiamen,
Fujian, China
Pei-Qiang Huang
Department of Chemistry and the Key Laboratory for Chemical Biology
of Fujian Province, College of Chemistry and Chemical Engineering,
Xiamen University, Xiamen, Fujian, China
Abstract: A highly diastereoselective approach to N-protected cis-(4S,5S)-4-hydroxy-
5-phenyl-2-pyrrolidinone is reported. The key step is a boron trifluoride etherate–
promoted chemoselective intramolecular Si ! C phenyl group migration.
Keywords: Boron trifluoride etherate, N-acyliminium, phenyl migration, 2-
pyrrolidinone
Cis-2-substituted 3-pyrrolidinols and syn-(3S,4S)-g-alkyl-b-hydroxy-g-amino
acids are structural features found in many natural products and bioactive
compounds. For example, 2,3-disubstituted pyrrolidine 1 (Figure 1), as its
higher homologous L-733,060 (2), is a selective and potent neurokinin
substance P-receptor antagonist,[1] D-cis-3-hydroxystachydrine (3) was
Received in Japan July 28, 2005
Address correspondence to Pei-Qiang Huang, Department of Chemistry and the
Key Laboratory for Chemical Biology of Fujian Province, College of Chemistry
and Chemical Engineering, Xiamen University, Xiamen, Fujian 361005, China.
E-mail: pqhuang@xmu.edu.cn
1131

1132
L.-X. Liu and P.-Q. Huang
Figure 1. 3-Hydroxyprolines and related compound.
isolated from the fruit of Courbonia virgata,[2] and (2S,3R)-3-hydroxyproline
(cis-3-hydroxy-L-proline) (4) is a common component of a number of
bioactive natural peptides such as telomycin antibiotics[3] and DNA gyrase
inhibitors cyclothialidines.[4] In addition, (3S,4S)-statine (5) and (3S,4S)-4-
amino-3-hydroxy-5-phenylpentanoic acid (AHPPA) (6) are important
peptide mimetics.[5]
Previously, we have developed a flexible approach to trans-(4S,5R)-5-
substituted 4-hydroxy-2-pyrrolidinones[6–8] from which trans-2-substituted
3-pyrrolidinols[7] and anti-(3S,4R)-g-substituted-b-hydroxy-g-amino acids[8]
can be synthesized. Although a combination of this method and that of
Ohta et al.[9] would provide an approach to cis-(4S,5S)-5-substituted
4-hydroxy-2-pyrrolidinones and thus syn-(3S,4S)-g-substituted b-hydroxy-
g-amino acids, the modest yield obtained in the reductive deoxygenation of
an N,O-acetal intermediate[9] prompted us to search for a new approach to
cis-(4S,5S)-5-substituted 4-hydroxy-2-pyrrolidinones. Recently, inspired by
the work of Tomooka[10] on the cis-phenyl delivery method, we have
developed a new method for the asymmetric synthesis of cis-(5S,6S)-5-
hydroxy-6-phenyl-2-piperidinone, which led to the asymmetric synthesis of
L-733, 060.[11] We now report an extension of this method to the asymmetric
synthesis of cis-(4S,5S)-l-benzyl-4-hydroxy-5-phenyl-2-pyrrolidinone 12.
The synthesis started with the known N-benzylmalimide (S)-7, easily
available from (S)-malic acid in two pots.[7] Treatment of (S)-7 with t-butyl-
diphenylsilyl chloride in CH2C12 gave O-silylated malimide (S)-8 in 95%
yield (Scheme 1). Controlled partial reduction of (S)-8 with sodium boro-
hydride (MeOH, CH2Cl2, 0.5 h, 2158C) led to the formation of both C-2
and C-5 partially reduced products 9 and 10 in an 87 : 13 ratio with a
combined yield of 89% (based on the recovered starting material, 20%).
The diastereomeric ratio of the C-2 regioisomer 9 was 92 : 8 according to chro-
matographic separation. The cis/trans stereochemistries of the diastereomeric
9 were assigned according to the observed vicinal coupling constants[6–8]
(J4,5 ¼ 7.7 Hz for cis-9 and J4,5 ¼ 4.8 Hz for trans-9). With the desired

Si to C Phenyl Group Migration
1133
Scheme 1.
N,O-semiacetals cis/trans-9 in hand, the key Si ! C phenyl group migration
was investigated. Thus, according to the conditions defined for the higher
homologous,[11] the major diastereomer cis-9 was treated with boron trifluor-
ide etherate for 3 days at rt, which smoothly gave the desired phenyl group
migrated product 11 as a diastereomeric mixture in 85% yield. 19F NMR
analysis of the crude product showed that the diastereomeric ratio is 1.7 : 1.
Although the two diastereomers were difficult to separate by column chrom-
atography on silica gel, a sample of the major diastereomer (more polar)
was isolated for characterization, to which cis-stereochemistry was attributed
on the basis of the vicinal coupling constant[6–8] (J4,5 ¼ 6.0 Hz). The observed
low diastereoselectivity in the phenyl group migration reaction allowed us to
assume that the low diastereoselectivity was at the Si atom, instead of at the
C-5. Indeed, cleavage of the sily group in 11 (a diastereomeric mixture) led
to only one diastereomer cis-12 in quantitative yield. The stereochemistry
of cis-12 was assigned, once again, according to the vicinal coupling
constants[6–8] (J4,5 ¼ 6.2 Hz). Noteworthy is that compound 11, being
stable during the purification procedure, is more stable than its higher homolo-
gous 13.[11]
The reaction is thought to proceed through an N-acyliminium ion[12]
A
(Scheme 2), which contains two types of nucleophilic phenyl groups,
namely, those at the Si atom and that presented in the benzyl group. That
no cyclized product[13,14] was isolated means that with BF3 OEt2 as an
.
efficient fluoride source, the phenyl group at the silicon atom is more nucleo-
philic. Thus the Si to C phenyl group delivery (via B) is electronically
favorable compared with the alternative N-benzyl phenyl group cyclization.
In summary, a new highly diastereoselective and high-yielding approach
to cis-(4S,5S)-4-hydroxy-5-phenyl-2-pyrrolidinone 12 is developed, which
might serve as a key intermediate in the asymmetric synthesis of 1. In

1134
L.-X. Liu and P.-Q. Huang
Scheme 2.
addition, because the phenyl group can be oxidized to give a carboxyl
group,[15] 12 could also be used in the asymmetric synthesis of 3 and 4.[16]
The successful phenyl group delivery is of interest to the synthetic
community because of the difficulties encountered in the attempts to
perform the analogous allyl group delivery.[17] Moreover the use of
[18]
.
BF3 OEt2 as a fluoride source is of current interest.
EXPERIMENTAL
Melting points were determined on a Yanaco M-500 micro-melting-point
apparatus and were uncorrected. IR spectra were obtained on a Nicholet
1
Avatar 360 FT-IR spectrophotometer. H, 13C, and 19F NMR spectra were
taken on a Varian Unity þ500 spectrometer. Mass spectra were recorded on
a Bruker Dalton ESquire 3000 plus LC-MS apparatus (direct injection).
Optical rotations were measured with a Perkin-Elmer 341 automatic polari-
meter. Flash column chromatography was carried out with silica gel (300–
400 mesh). Dichloromethane was distilled over P2O5. THF was distilled
over sodium benzophenone ketyl under N2.
(S)-l-Benzyl-3-[(tert-butyldiphenylsilyl)oxy]-2,5-pyrrolidinedione (8)
A solution of tert-butyldiphenylchlorosilane (2.3 mL, 8.84 mmol) in CH2Cl2
(5 mL) was addded, at 0 8C and under N2 atmosphere, to a mixture of the

Si to C Phenyl Group Migration
1135
known malimide 7[6] (1.50 g, 7.32 mmol), imidazole (1.00 g, 14.71 mmol), and
a catalytic amount of DMAP in anhydrous CH2Cl2 (25 mL). The mixture was
stirred overnight at room temperature. Water (20 mL) was added, and the
mixture was extracted with CH2Cl2 (2 ꢀ 20 mL).
The combined organic phases were washed with brine (10 mL), dried over
anhydrous Na2SO4, filtered, and concentrated under reduced pressure. Flash
chromatographic purification (eluent : EtOAc/PE ¼ 1 : 8) of the residue
afforded 8 (3.08 g, yield: 95%) as a colorless oil. [a]1D3 243.3 (c 1.1,
CHCl3). IR (film) nmax: 3070, 2931, 1716, 1429, 1397, 1141, 1113 cm21
.
1H NMR (500 MHz, CDCl3): d 7.80 (d, J ¼ 1.2 Hz, 2H, Ar-H), 7.65
(d, J ¼ 1.2 Hz, 2H, Ar-H), 7.40 (m, 7H, Ar-H), 7.25 (m, 4H, Ar-H), 4.65
(d, J ¼ 14.1 Hz, 1H, NCH2), 4.61 (d, J ¼ 14.1 Hz, 1H, NCH2), 4.46 (dd,
J ¼ 7.9, 5.0 Hz, 1H, H-3), 2.65 (dd, J ¼ 17.9, 7.9 Hz, 1H, H-4), 2.56 (dd,
J ¼ 17.9, 5.0 Hz, lH, H-4), 1.10 (s, 9H, 3 ꢀ CH3). 13C NMR (125 MHz,
CDCl3): d 175.99 (C55O), 173.63 (C55O), 135.97 (2C), 135.70 (2C),
135.48, 132.79, 131.87, 130.24, 130.14 (2C), 128.91 (2C), 128.66 (2C),
127.96 (2C), 127.86 (2C), 68.39 (C-3), 42.32 (NCH2), 38.75 (C-4), 26.67
(3 ꢀ CH3), 19.18 (Si-C). MS (ESI) (m/z): 444 (M þ Hþ, 100). Anal. calcd.
for C27H29NO3Si: C, 73.10; H, 6.59; N, 3.16. Found: C, 73.29; H, 6.69;
N, 3.09.
(4S,5RS)-1-Benzyl-4-[(tert-butyldiphenylsilyl)-5-hydroxy-2-
pyrrolidinoe (9)
NaBH4 (1.03 g, 27.09 mmol) was added at 2208C to a solution of 8 (3.00 g,
6.77 mmol) in MeOH (20 mL) and CH2Cl2 (5 mL). The mixture was stirred
at that temperature for 30 min., then quenched with cold saturated aqueous
NaHCO3 (10 mL) and brine (10 mL) and diluted with cold CH2Cl2 (20 mL).
The aqueous phase was extracted with CH2Cl2 (3 ꢀ 15 mL). The combined
organic phases were washed with brine (5 mL), dried over anhydrous
Na2SO4, filtered, and concentrated under reduced pressure. After flash chro-
matographic purification (eluent : EtOAc/PE ¼ 1 : 4), cis-9, (1.65 g), trans-9
(0.15 g, cis-9: trans-9 ¼ 92: 8), and 10 (0.28 g) were obtained with a
combined yield of 69%. In addition, 0.61 g of the starting material 8 (20%)
was recovered.
cis-9: white crystals. Mp 96–97 8C (Et2O/PE ¼ 3:1). [a]2D3, 229.3 (c 1.0,
CHCl3). IR (film) nmax: 3380, 2931, 2858, 1686, 1427, 1113 cm21 1H
.
NMR (500 MHz, CDCl3): d 7.62 (d, J ¼ 1.0 Hz, 2H, Ar-H), 7.60
(d, J ¼ 1.0 Hz, 2H, Ar-H), 7.40 (m, 6H, Ar-H), 7.25 (m, 5H, Ar-H), 4.85
(d, J ¼ 14.6 Hz, 1H, NCH2), 4.83 (s, 1H, OH), 4.31 (ddd, J ¼ 7.7, 7.0,
4.4 Hz, 1H, H-4), 4.22 (d, J ¼ 14.6 Hz, 1H, NCH2), 3.73 (d, J ¼ 7.7 Hz, 1H,
H-5), 2.38 (dd, J ¼ 17.2, 4.4 Hz, 1H, H-3), 2.29 (dd, J ¼ 17.2, 7.0 Hz, 1H,
H-3), 1.10 (s, 9H, 3 ꢀ CH3). 13C NMR (125 MHz, CDCl3): d 171.24

1136
L.-X. Liu and P.-Q. Huang
(C55O), 136.53, 135.66 (2C), 135.52 (2C), 132.24, 131.93, 130.45, 130.30,
128.60 (2C), 128.38 (2C), 128.05 (3C), 127.51 (2C), 81.86 (C-5), 67.07 (C-
4), 43.61 (NCH2), 38.59 (C-3), 26.85 (3 ꢀ CH3), 19.11 (Si-C). MS (ESI)
(m/z): 446 (M þ Hþ, 100), 468 (M þ Naþ, 35). Anal. calcd. for
C27H31NO3Si: C, 72.77; H, 7.01; N, 3.14. Found: C, 72.48; H, 7.12; N, 3.08.
Trans-9 (minor isomer): [a]D20 270.9 (c 1.0, CHCl3). Mp 99–100 8C (Et2O).
1
IR (film) nmax: 3411, 2931, 1686, 1427, 1112 cm21. H NMR (500 MHz,
CDCl3): d 7.65 (m, 4H, Ar-H), 7.50–7.40 (m, 6H, Ar-H), 7.10 (m, 3H,
Ar-H), 6.60 (d, J ¼ 1.0 Hz, 2H, Ar-H), 5.10 (d, J ¼ 4.8 Hz, 1H, H-5), 4.69
(d, J ¼ 15.4 Hz, 1H, NCH2), 4.11 (ddd, J ¼ 7.5, 7.3, 4.8 Hz, 1H, H-4), 3.41
(d, J ¼ 15.4 Hz, 1H, NCH2), 2.68 (dd, J ¼ 16.6, 7.5 Hz, 1H, H-3), 2.62
(d, J ¼ 9.2 Hz, 1H, OH, D2O exchangeable), 2.60 (dd, J ¼ 16.6, 7.5 Hz,
1H, H-3), 1.10 (s, 9H, 3 ꢀ CH3). 13C NMR (125 MHz, CDCl3): d 172.64
(C55O), 136.43, 135.91 (2C), 135.66 (2C), 132.12, 131.42, 130.58,
130.43, 128.39 (2C), 128.32 (2C), 128.11 (2C), 127.49 (2C), 127.24, 83.55
(C-5), 67.41 (C-3), 43.59 (NCH2), 37.74 (C-4), 26.83 (3 ꢀ CH3), 19.58
(Si-C). MS (ESI) (m/z): 446 (M þ Hþ, 100), 468 (M þ Naþ, 25). Anal.
calcd. for C27H31NO3Si: C, 72.77; H, 7.01; N, 3.14. Found: C, 71.92; H,
7.12; N, 3.13.
Regioisomer 10 (more polar diastereomer): [a]2D0 þ 13.1 (c 1.0, CHCl3). IR
(film) nmax: 3327, 2931, 2858, 1674, 1428, 1112, 1084 cm21 1H NMR
.
(500 MHz, CDCl3): d 7.60–7.50 (m, 4H, Ar-H), 7.45–7.25 (m, 11H, Ar-H),
4.91 (d, J ¼ 15.2 Hz, 1H, NCH2), 4.76 (dd, J ¼ 6.0, 0.8 Hz, 1H, H-3), 4.13
(dd, J ¼ 5.5, 1.2 Hz, 1H, H-5), 4.09 (d, J ¼ 15.2 Hz, 1H, NCH2), 2.67
(m, 1H, H-4), 2.34 (m, 1H, H-4), 1.62 (s br, 1H, OH, superposed with the
peak of H2O in CDCl3, D2O exchangeable), 1.00 (s, 9H, 3 ꢀ CH3). 13C
NMR (125 MHz, CDCl3): d 173.63 (C55O), 136.07, 135.51 (4C), 132.87,
129.93, 129.86, 128.56 (2C), 127.77 (4C), 127.71 (3C), 127.38, 88.63 (C-
5), 72.85 (C-3), 42.85 (NCH2), 38.87 (C-4), 26.65 (3 ꢀ CH3), 18.87 (Si-C).
MS (ESI) (m/z): 446 (M þ Hþ, 100), 468 (M þ Naþ, 40).
(4S,5S)-1-Benzyl-4-[(RS)-(tert-butylphenylfluorosilyl)oxy]-5-
phenyl-2-pyrrolidinone (11)
.
BF3 OEt2 (0.42 mL, 3.40 mmol) was slowly added to a cooled (0 ꢀ 5 8C)
solution of cis-9 (302 mg, 0.67 mmol) in dry CH2Cl2 (4 mL) under N2 atmos-
phere. The mixture was stirred for 72 h at room temperature. Saturated
aqueous NaHCO3 (5 mL) was added at 0 8C, and the aqueous phase was
extracted with CH2Cl2 (3 ꢀ 5 mL). The combined organic phases were
washed with brine (5 mL), dried over anhydrous Na2SO4, filtered, and
concentrated under reduced pressure. Flash chromatographic purification
(eluent : EtOAc/PE ¼ 1 : 8) of the residue gave 11 (256 mg, yield: 85%) as
a diastereomeric mixture at the silicon. The diastereomeric ratio at the

Si to C Phenyl Group Migration
1137
silicon is 1.7 : 1 as determined by 19F NMR analysis. A sample of the major
diastereomer of 11 was obtained by further flash chromatographic purification.
The major diastereomer of 11 (more polar): colorless oil. [a]1D3 234.0
(c 1.4, CHCl3). IR (film) nmax: 3031, 1699, 1478, 1429, 1417, 1363, 1255,
1
1155, 1123, 1087 cm21. H NMR (500 MHz, CDCl3): d 7.45–7.35 (m 4H,
Ar-H), 7.28–7.16 (m, 7H, Ar-H), 7.08–6.98 (m, 4H, Ar-H), 5.21
(d, J ¼ 14.5 Hz, 1H, NCH2), 4.52 (ddd, J ¼ 10.9, 6.0, 1.0 Hz, 1H, H-4),
4.44 (d, J ¼ 6.0 Hz, 1H, H-5), 3.57 (d, J ¼ 14.5 Hz, 1H, NCH2), 2.72
(dd, J ¼ 15.9, 10.9 Hz, 1H, H-3), 2.68 (dd, J ¼ 15.9, 1.0 Hz, 1H, H-3), 0.78
(s, 9H, 3 ꢀ CH3). 13C NMR (125 MHz, CDCl3): d 173.31 (C55O), 136.00,
134.57 (2C), 134.28, 130.79, 129.26 (2C), 128.58 (3C), 128.36, 128.29
(3C), 128.20, 127.88 (2C), 127.61, 67.67, 66.17, 44.36 (NCH2), 40.56
(C-3), 24.86 (3 ꢀ CH3), 17.55 (Si-C). 19F NMR (471 MHz, CDCl3, TFA as
external standard: 277 ppm): d 2156.36 (major isomer), 2151.51 (minor
isomer). The diastereomeric ratio at the silicon is 1.7:1 as determined by
19F NMR analysis. MS (ESI) (m/z): 448 (M þ Hþ, 100). Anal. calcd. for
C27H30FNO2Si: C, 72.45; H, 6.76; N, 3.13. Found: C, 72.20; H, 6.63; N, 3.21.
(4S,5S)-1-Benzyl-4-hydroxy-5-phenyl-2-pyrrolidinone (12)
TBAF (1 M in THF, 0.58 mL) was added to a solution of diastereomeric
mixture of 11 (130 mg, 0.29 mmol) in dry THF (3 mL) at room temperature.
The solution was allowed to stir overnight. After quenching with water
(5 mL), the mixture was extracted with CH2Cl2 (3 ꢀ 5 mL), and the
combined organic layers were washed with brine (3 mL), dried over
anhydrous Na2SO4, filtered, and concentrated under reduced pressure. The
residue was chromatographed on silica gel (eluent: EtOAc/PE ¼ 1:2) to
yield 12 (77 mg) in quantitative yield. Colorless oil. [a]1D5 217.9 (c 0.4,
CHCl3). IR (film) nmax: 3382, 3025, 1673, 1420, 1255 cm21 1H NMR
.
(500 MHz, CDCl3): d 7.50–7.40 (m, 3H, Ar-H), 7.30–7.20 (m, 5H, Ar-H),
7.05 (m, 2H, Ar-H), 5.23 (d, J ¼ 14.5 Hz, 1H, NCH2), 4.54 (d, J ¼ 6.2 Hz,
1H, H-5), 4.49 (ddd, J ¼ 6.9, 6.2, 4.5 Hz, 1H, H-4), 3.57 (d, J ¼ 14.5 Hz,
1H, NCH2), 2.78 (dd, J ¼ 17.2, 6.9 Hz, lH, H-3), 2.58 (dd, J ¼ 17.2, 4.5 Hz,
lH, H-3), 1.60 (s, lH, OH). 13C NMR (125 MHz, CDCl3): d 173.51 (C55O),
135.88, 133.30, 128.98 (3C), 128.84, 128.61, 128.58, 128.52 (2C), 127.63
(2C), 66.79, 66.02, 44.43 (NCH2), 39.35 (C-3). MS (ESI) (m/z): 268
(M þ Hþ, 100), 290 (M þ Naþ, 30). Anal. calcd. for C17H17NO2: C, 76.38;
H, 6.41; N, 5.24. Found: C, 76.53; H, 6.61; N, 4.94.
ACKNOWLEDGMENT
The authors are grateful to the Natural Science Foundation (NSF) of China
(20272048; 203900505) and the Ministry of Education (Key Project
104201) for financial support.

1138
L.-X. Liu and P.-Q. Huang
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