5.82 (1H, ddt, J ) 17.1, 10.3, 6.1 Hz), 5.07 (1H, dq, J ) 17.1, 1.5
Hz), 5.00 (1H, dq, J ) 10.3, 1.5 Hz), 4.19 (2H, mq, J ) 7.2 Hz),
2.39 (2H, m), 2.32 (2H, m), 2.12 (1H, ddd, J ) 25.2, 15.2, 5.2
Hz), 2.05 (1H, ddd, J ) 19.2, 15.2, 8.4 Hz), 1.42 (3H, d, J ) 21.6
Hz), 1.40 (3H, d, J ) 21.5 Hz), 1.28 (3H, t, J ) 7.0 Hz). 13C NMR
(100 MHz, CDCl3): δ 172.5, 172.3, 137.1, 115.8, 95.6 (d, J )
165.6 Hz), 61.8, 50.1, 42.3 (d, J ) 21.4 Hz), 35.8, 29.5, 27.8 (d,
J ) 24.3 Hz), 26.4 (d, J ) 24.7 Hz), 14.3. 19F NMR (376 MHz,
CDCl3): δ -136.6. IR (NaCl thin film): 3297, 3079, 2982, 2937,
SCHEME 5. Optimized Conditions for Generation
of γ-Fluoroleucine‚H2SO4
1745, 1652, 1540, 1443, 1375, 1193, 1027, 915, 801, 665 cm-1
.
HRMS (TOF) calcd for [C13H22FNO3 + Na] 282.1482, found
282.1483.
Preparation of (S)-γ-Fluoroleucine Ethyl Ester‚H2SO4:
Method B (direct isolation). To a solution of the amide (31 g,
119.7 mmol, 1 equiv) in MTBE (620 mL, 20 mL/g) was added
H2O (2.2 mL, 119.7 mmol, 1 equiv), TFA (26.7 mL, 359.1 mmol,
3 equiv), and DBDMH (19 g, 71.8 mmol, 0.6 equiv) at rt. The
resulting mixture was stirred at rt for 4 h, at which no more
starting material nor imidate ester intermediate were observed.
The reaction was then concentrated to half its volume (10
mL/g) and then treated with neat H2SO4 (1 equiv) at rt. The re-
sulting suspension was then cooled to 0 °C, aged for 2 h, and
filtered and the wet cake was washed with cold MTBE:iPAc (1:
1) and dried in vacuo under a stream of N2. The product was
obtained in 75% corrected yield (91 wt %, 9 wt % dimethylhy-
dantoin, 27.2 g) and 97% ee and, if desired, can be recrystallized
from MeCN to afford pure compound in 94% recovery (70%
overall yield and >99% ee). The enantiomeric excess value of
the product was determined from its free base (liberated with
K2CO3/H2O and extracted with MTBE) using chiral GC spec-
troscopy [Restek Rt-âDEX sa column, 30 m × 0.32 mm i.d. ×
0.25 µm df; Method: initial T ) 100 °C (30 min), ramp at 20
°C/min to 230 °C (10 min), column pressure ) 10.07 psi (1.7
mL/min), inlet T ) 200 °C, inlet pressure ) 10 psi, total flow )
46.4 mL/min; retention time for (S)-ent (desired) 28.86 min, for
(R)-ent 30.38 min]. [R]25D +9.8 (c 0.52, EtOH). Mp: 105-106 °C.
1H NMR (400 MHz, d6-DMSO): δ 8.63 (3H, br), 4.18 (2H, q, J )
7.1 Hz), 4.11 (1H, app t, J ) 6.8 Hz), 2.20 (1H, ddd, J ) 28.9,
14.9, 6.8 Hz), 2.12 (1H, ddd, J ) 28.9, 14.9, 6.8 Hz), 1.39 (3H, d,
J ) 21.6 Hz), 1.38 (3H, d, J ) 21.6 Hz), 1.23 (3H, t, J ) 7.1 Hz).
13C NMR (100 MHz, d6-DMSO): δ 169.4, 94.5 (d, J ) 165.6 Hz),
61.9, 49.1, 41.0 (d, J ) 21.8 Hz), 26.6 (d, J ) 23.9 Hz), 26.2 (d,
J ) 23.7 Hz), 13.7. 19F NMR (376 MHz, CDCl3): δ -137.0. IR
(NaCl thin film): 2984 (br), 2937, 1748, 1598, 1520, 1377, 1290,
(S)-6b in MeCN to DBDMH in the presence of H2O (2
equiv) and TFA (2 equiv) at rt for 12 h, followed by treat-
ment of the resulting free amine in MTBE or iPAc with
H2SO4 (1 equiv) gave (S)-1‚H2SO4 in 80% yield and 97%
enantiopurity (Scheme 5). Alternatively, the reaction
could be carried out in MTBE in the presence of H2O (1
equiv) and TFA (3 equiv) at rt for 2 h, at which a complete
hydrolysis of the ketoimidate ester 18b was obtained.
Addition of H2SO4 directly to the reaction mixture at rt,
followed by cooling to 0 °C for 2 h, resulted in crystalli-
zation of the bisulfate salt, which was isolated in 75%
yield and 97% enantiopurity (Scheme 5). The product iso-
lated from the latter procedure, however, was typically
contaminated with 6-8 wt % of dimethylhydantoin by-
products, which can be easily removed by a single recrys-
tallization from MeCN to afford pure (S)-1‚H2SO4 in 94%
recovery (70% overall yield) and >99% enantiomeric
excess.23
In summary, we have developed a novel route to (S)-
γ-fluoroleucine ethyl ester 1 via a dynamic kinetic resolu-
tion of azlactone 7b catalyzed by immobilized lipase B
(Novozyme-435). Such transformation allows for the reac-
tion to be performed in organic media and provides ease
of product isolation. In this regard, we demonstrated that
the lipase-catalyzed ring-opening of 3-butenylazlactone
7b afforded the N-pentenamide ester 6b in 80% yield and
84% ee. Considering the tendency of the molecule to un-
dergo defluoro-lactonization under acidic conditions, the
choice of the R group in 7 was determined such that the
amide functionality in the ring-opened product 6 can be
removed under mild conditions. Hence, subsequent treat-
ment with DBMDH, TFA, and H2O in MeCN or MTBE
liberated the free amine, which was isolated as its hydro-
gen sulfate ((S)-1‚H2SO4) in >97% ee and 75-80% yield.
1205, 1173, 1045, 884 cm-1
.
(S)-γ-Fluoroleucine ethyl ester free base: [R]25D +15.9 (c
0.54, EtOH (lit.4b +9, c 0.5, EtOH)). 1H NMR (400 MHz,
CDCl3): δ 4.13 (2H, q, J ) 7.2 Hz), 3.62 (1H, dd, J ) 7.9, 5.0
Hz), 2.09 (1H, ddd, J ) 23.4, 14.6, 5.0 Hz), 1.80 (1H, ddd, J )
19.0, 14.6, 7.9 Hz), 1.57 (2H, br), 1.39 (3H, d, J ) 21.6 Hz), 1.38
(3H, d, J ) 21.5 Hz), 1.23 (3H, t, J ) 7.2 Hz). 13C NMR (100
MHz, CDCl3): δ 175.7, 95.2 (d, J ) 166.2 Hz), 61.1, 51.7 (d, J )
2.8 Hz), 45.8 (d, J ) 21.6 Hz), 27.5 (d, J ) 24.7 Hz), 27.0 (d, J
) 24.6 Hz), 14.3. 19F NMR (376 MHz, CDCl3): δ -138.0. IR
(NaCl thin film): 3382, 3315, 2982, 2939, 1732, 1467, 1375, 1282,
Experimental Section
1186, 1032, 862 cm-1
.
Enzymatic Dynamic Kinetic Ethanolysis of Azlactone
7b. To a solution of the azlactone 7b (4 kg, 18.8 mol, 1 equiv) in
MTBE (20 L, 5 mL/g) was added EtOH (5.5 L, 93.8 mol, 5 equiv),
Et3N (524 mL, 3.76 mol, 0.2 equiv), and immobilized lipase B
(Novozyme-435) (4 kg, 100 wt % equiv). The resulting suspension
was aged at rt for 4 h, at which a complete consumption of
starting material was observed. The suspension was then heated
to 35 °C for 0.5 h and then filtered. The enzyme was washed
with MTBE until the filtrate turned colorless. The filtrate was
then successively washed with 1 N aqueous HCl (10L), saturated
aqueous NaHCO3 (10 L), and brine, and then concentrated in
vacuo to give crude ester, which can be deprotected directly
without further purification (4 kg, 82%). If desired, the crude
oil can be purified by SiO2 gel chromatography (3:1 hexanes:
MTBE) to give pure product as a white solid in 87% ee as
analyzed by chiral HPLC spectroscopy (Chiralcel OD-H, 4% iPA/
hexanes, 1 mL/min, 35 °C, 210 nm, retention time for (S)-ent
Acknowledgment. We acknowledge Pete Dormer
and Lisa dimichele for their assistance with the NMR
experiments, as well as Mirlinda Biba for chiral assay
developments and Thomas J. Novak with the HRMS
experiments.
Supporting Information Available: Experimental pro-
cedures and characterization of the prepared substrates,
including their NMR spectra. This material is available free
JO047918J
(23) If desired, dimethylhydantoin byproduct can be removed by
neutralizing the salt with aqueous K2CO3 or K3PO4 and the free base
extracted with iPAc or MTBE and retreated with H2SO4. Alternatively,
the salt can be recrystallized from MeCN to afford pure compound in
a 94% recovery and >99% ee.
13.68 min, and (R)-ent 7.42 min). [R]25 -32.9 (c 0.56, EtOH).
D
Mp: 35-37 °C. 1H NMR (400 MHz, CDCl3): δ 6.16 (1H, br),
J. Org. Chem, Vol. 70, No. 6, 2005 2375