Asymmetric Synthesis of Homocitric Acid Lactone

Article abstract of DOI:10.1021/acs.joc.6b01997

A short, diastereoselective synthesis of homocitric acid lactone is described. The key step is a bioinspired aldol addition to set the stereogenic center in an intermediate that requires only modest oxidation state manipulation to complete the synthesis. This approach enables rapid generation of isotopomers in which carbon and hydrogen can be replaced by heavier nuclei at nearly every position.

Full text of DOI:10.1021/acs.joc.6b01997

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Note
Asymmetric Synthesis of Homocitric Acid Lactone
Leslie A. Nickerson, Valerie Huynh, Edward I. Balmond, Stephen P. Cramer, and Jared T. Shaw
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.6b01997 • Publication Date (Web): 29 Sep 2016
Downloaded from http://pubs.acs.org on September 30, 2016
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The Journal of Organic Chemistry
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Asymmetric Synthesis of Homocitric Acid Lactone
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Leslie A. Nickerson, Valerie Huynh, Edward I. Balmond, Stephen P. Cramer and Jared T. Shaw*
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Department of Chemistry, One Shields Ave, University of California, Davis, CA 95616
jtshaw@ucdavis.edu
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Received Date (will be automatically inserted after manuscript is accepted)
O
O
O
O
N
CH3
EtO
+
TiCl4
i-Pr2NEt
NMP
O
Bn
O
O
HO CO2Et
three
steps
HO2C
O
O
N
O
HO2C
Bn
homocitric acid
(HCA) lactone
Abstract A short, diastereoselective synthesis of homocitric acid lactone is described. The key step is a bio-inspired aldol
addition to set the stereogenic center in an intermediate that only requires modest oxidation state manipulation to complete
the synthesis. This approach enables rapid generation of isotopomers in which carbon and hydrogen can be replaced by
heavier nuclei at nearly every position.
Homocitric acid (HCA), which often occurs in its lactone form (1), is an important biosynthetic intermediate and a critical
co-factor for nitrogenase.1-5 The fleeting intermediacy of this compound makes it poorly available from natural sources and
has resulted in many approaches to its synthesis from simple starting materials.6-14 In order to enable the synthesis of
isotopomers for spectroscopic studies of nitrogenase, we developed a stereoselective synthesis from diethyl oxalate.15
Although this approach was reasonably streamlined, the number of steps makes this route cumbersome to larger scale
throughput. HCA is made biosynthetically by an aldol addition of acetyl coenzyme A (CoASAc) to alpha ketoglutarate
(Figure 1). Chen was able to capitalize on this approach synthetically in an efficient synthesis of (±)-1.16 Inspired by this
result and its potential to offer a short synthesis of (-)-1, we examined several approaches to absolute stereocontrol in the
aldol addition, culminating in a four-step synthesis.
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Fungi
1
2
3
4
5
6
O
O
H2O
HO2C
HO2C
CO2H
SCoA
HO2C
CH3
OH
O
O
HO2C
CoASH
CO2H
homocitric acid
(HCA)
HCA lactone
(1)
CO2H
7
8
9
Chen (2007)
O
O
1) LiHMDS
(±)-1
(-)-1
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EtO2C
2
CO2Et
2) hydrolysis
CH3
EtO
3
This Work
O
O
aldol
EtO2C
R
3 steps
X*
CH3
2 (R=CO2Et) or
4 (R=CHCH2)
X*=chiral
auxiliary
Figure 1. Biosynthesis of 1 by fungi and the biomimetic approaches to the chemical synthesis of 1.
We first attempted to develop an auxiliary controlled variant of Chen's aldol reaction. Although there are few examples of
ester-based chiral auxiliaries in aldol reactions, the works of Braun and Robin suggested that diastereocontrol was
possible.17,18 When the enolate derived from double deprotonation of the mono-acetate of (+)-1,1,2-triphenyl ethanol (5) was
treated with diethyl ketoglutarate (2), an appreciable amount of aldol product (69% conversion by NMR) was observed (eq
1). Selectivity was modest (61:39) and chromatographic separation proved to be prohibitively difficult.
We next turned our attention to the use of imide enolates, which offer reduced basicity and a broader array of substrates for
controlling asymmetry. Although these enolates have been in use for nearly four decades, there are few examples of high
levels of diastereocontrol from unsubstituted enolates derived from N-acetyl imides.19-25 In addition, imide-based aldol
additions rarely involve aldol additions to ketones26-29 and there are only three cases using α-keto esters.30-32 Mukaiyama
reported that N-acetyl thiazolidinethione (7) would add to dimethyl ketoglutarate (8) with >90% enantiomeric excess (ee)
when conducted with chiral amine 9 as a stoichiometric additive (Figure 2A).30 Later, Chamberlin observed good yield in the
addition of N-acetyl oxazolidinone (11) to benzyl lactate (12) with low diastereoselectivity (Figure 2B).31 Finally, Zanda has
reported that N-acetyl oxazolidinone (ent-11) will add to ethyl trifluorolactate (14) with diastereoselectivity nearly identical
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The Journal of Organic Chemistry
to Chamberlin's via the chlorotitanium enolate (Figure 2C).32 Importantly, in both cases where the chiral imide-based aldol
reactions lacked selectivity, the diastereomeric products were nearly always separable by silica gel chromatography.
1
2
3
S
4
O
A
5
S
N
CH3
6
7
8
9
N
HN
9
S
7
O
HO CO2CH3
R
CH3
+
O
S
N
N-ethylpiperidine
Sn(OTf)2, -78 °C
R
H3CO
O
10, >95% ee
50-60% yield
R=(CH2)2CO2CH3
O
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8
B
C
O
O
O
HO CO2Bn
CH3
O
LDA
CH3
O
N
CH3
O
N
BnO
-78 °C
O
Bn
11
Bn
O
13, 58:42 dr
99% yield
12
O
O
O
O
HO CO2CH3
CF3
TiCl4
CF3
O
N
CH3
O
N
EtO
i-Pr2NEt
-30 °C
O
Bn
Bn
15, 58:42 dr
91% yield
ent-11
14
Figure 2. Summary of acetate aldol reactions of α-keto esters.
Armed with these examples, we set out to find the best aldol conditions for the synthesis of an intermediate leading to 1. First
we attempted to reproduce the Mukaiyama example and no conversion to the desired aldol product was observed.30 Next,
following the other examples, attempted addition of the titanium enolates of both an oxazolidinone and a thiazolidinethione
to dimethyl ketoglutarate were unsuccessful.
Inspired by the success of the Evans aldol reaction of benzyl lactate previously investigated by Chamberlin, we reasoned that
ketoester 4 might be better behaved as the electrophile, perhaps because the extra ethoxycarbonyl group of 2 might be
leading to unproductive modes of coordination to titanium. We first tried the reaction conditions of Chamberlin using LDA,
but did not observe any aldol addition. We next turned to a titanium enolate reaction and were gratified to observe significant
conversion to product (Table 1). Unfortunately, the aldol addition products were accompanied by varying quantities of both
unreacted acetyloxazolidinone and unsaturated ester 17 resulting from aldol condensation (Table 1, entry 1). The alkene
configuration of 17 is assigned based on chemical shift correlation with related compounds.33 The configuration of the aldol
products was deduced after conversion of the major isomer to homocitric acid (Scheme 1, vide infra) and confirmed that the
modest preference for 16a matched the results of Chamberlin (Figure 2). The alkene side-product could be suppressed by
conducting the reaction at lower temperature at the expense of conversion (Table 1, entry 2). Although the aldol product
could be formed with little condensation by storing the reaction flask in a -80 °C freezer for 96 hours (Table 1, entry 3), the
time required and access to a -80 °C freezer made the method undesirable. The best results were obtained using conditions
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reported by Crimmins, in which N-methylpyrrolidinone (NMP) is used as a Lewis basic additive.34 Under these conditions,
yield is higher with shorter reaction times and elimination is suppressed at the expense of diastereoselectivity (Table 1 entries
4-7). Due to difficulties associated with obtaining the diastereomer ratio (dr) from NMR spectroscopy, dr was determined by
isolation in most cases. The accuracy of the yield-based dr was established by direct comparison to the ratio determined by
GCMS, which required the conversion of the aldol products to the corresponding TMS ethers to prevent decomposition by
retro-aldol reaction during gas chromatography (Table 1, entry 6). When this result was repeated at a shorter reaction time
(7.5 h), the isolated yield (41%; Table 1, entry 7) of aldol product 16a was comparable to the conversion observed by GCMS
(46%, Table 1, entry 6), with slight variation in the dr. The best results, in terms of generating the largest amount of the
major diastereomer, were realized at -40 °C with the use of excess enolate (Table 1, entry 8) or with excess ketoester (Table
1, entry 6). The ability to change the limiting reagent ensures minimum loss of starting materials in a synthesis involving
isotopomers. Finally, the use of several oxazolidinone auxiliaries derived from t-leucinol, 1-amino-2-indanol, and β-amino-
α,α-dimethyl-benzenepropanol (aka "superquat") all resulted in poorer conversion (not shown).22,25,35
1
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Table 1. Aldol addition reactions of 11 to 4.
O
O
HO CO2Et
O
O
O
O
N
O
N
CH3
16a (major)
Bn
Bn
11
TiCl4
i-Pr2NEt
NMP
O
O
HO CO2Et
+
N
CH2Cl2
O
16b (minor)
Bn
EtO
O
O
CO2Et
O
4
O
N
17
Bn
yield of
16a
(%)
aldol:elimb
(16a+16b):17
drc
16a:16b
NMP
(equiv.)
conv.
(%)b
entrya
temp.; time
1d
2d
3
4
5
6
7
8f
60
50
77
68
75
77
73
88
-78 to -60 °C; 5 h
-78 °C; 5 h
-78 °C; 96 h
-78 to 0 °C; 5 h
-78 to -40 °C; 8 h 2.0
-40 °C; 11 h
-40 °C; 7.5 h
-40 °C; 7.5 h
0
0
0
2.0
79:21
93:7
83:17
77:23
93:7
96:4
95:5
95:5
51:49b N/A
72:28
71:29
54:46
63:37
36
42
31
34
2.0
2.0
2.0
62:38e 46e
56:44
53:47
41
46
a) unless otherwise specified reactions were completed with 1 equiv.
of 11 (or ent-11) to 1.1 equiv. of 4. b) determined by integration in the
1H NMR spectrum. c) determined based on isolated yield, unless
otherwise noted. See experimental section for details. d) prepared
with ent-11. e) determined by GCMS after converting to TMS ether.
See supporting information. f) completed with 2 equiv. of 11 to 1
equiv. of 4.
Aldol addition product 16a could be easily converted into homocitric acid lactone (Scheme 1). Hydrolysis to the methyl ester
was achieved in high yield using sodium methoxide. Although oxidative cleavage was variable, this transformation produced
19 in at least 50% yield. Final hydrolysis to HCA was achieved in 71% yield.
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The Journal of Organic Chemistry
NaOCH3
CH3OH
O
HO CO2Et
18
RuCl3, NaIO4
1
2
16a
0 °C, 1h
98%
EtOAc/CH3CN/H2O
50%
H3CO
3
4
5
6
H3CO
O
TFA, H2O
EtO
reflux, 24h
O
(-)-1
O
71%
O
19
7
8
9
Scheme 1. Conversion of aldol product 16a to (-)-HCA (1).
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In conclusion, we have described a short asymmetric synthesis of homocitric acid lactone. This synthesis was inspired by the
biosynthetic pathway that produces natural homocitric acid, and minimizes the use of protecting groups and oxidation state
changes. Although the diastereoselectivity of the key step is modest, the ability to re-isolate the unreacted oxazolidinone,
paired with the small number of steps makes carrying out the synthesis on gram scale attainable. Based on recent examples
of multi-gram scale titanium-mediated aldol additions, accessing the aldol intermediate on a gram scale is feasible.36,37
Importantly, the modularity of the starting materials enables the installation of heavy carbon at many points in the molecule
to enable spectroscopy experiments that will discern the mechanistic role of this cofactor in nitrogen fixation by
nitrogenase.38
Experimental Section
General Information General Methods: Unless otherwise specified, all commercially available reagents were used as
received. All reactions using anhydrous solvents were carried out under an atmosphere of argon in flame-dried glassware
with magnetic stirring. Anhydrous solvent was dispensed from a solvent purification system that passes solvent through two
columns of dry neutral alumina. Purification of reaction products was carried out by flash chromatography using silica gel
F60 (230-400 mesh) or by automated chromatography with an UV/Vis detector. Analytical thin layer chromatography was
performed on 0.25 mm silica gel F-254 plates. Visualization was accomplished with UV light, KMnO4, or bromocresol green
followed by heating.
Instrumentation: 1H NMR spectra and proton-decoupled 13C NMR spectra were obtained on a 400, 600, or 800 MHz NMR
spectrometer. Chemical shifts (δ) are reported in parts per million (ppm) relative to internal standard (TMS, 0.00 ppm) or
residual solvent (CD3OD, 3.31 ppm). Multiplicities are given as: s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet),
or combinations of these signals. Infrared spectra were taken on a FTIR spectrometer. Gas chromatography-mass
spectrometry data was recorded on a spectrometer using electron impact ionization with an injection temperature of 250 °C
and a temperature ramp of 50 °C to 300 °C (20 min) with a hold at 300 °C (15 min). The column used had a diameter of 0.25
mm and was 30.0 m long and 0.25 μm thick.
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For AMM analysis, samples were analyzed by flow-injection analysis into an orbitrap mass spectrometer operated in the
1
2
centroided mode. Samples were injected into a mixture of 50% MeOH and 0.1% Formic Acid/H2O at a flow of 200 uL/min.
3
4
5
Source parameters were 5kV spray voltage, capillary temperature of 275̊ C and sheath gas setting of 20. Spectral data were
6
7
8
acquired at a resolution setting of 100,000 FWHM with the lockmass feature, which typically results in a mass accuracy
<2ppm.
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O
EtO
O
Ethyl 2-oxohex-5-enoate (4). Compound 4 was prepared according to the previously reported procedure.15 In a flask,
magnesium turnings (1.230 g, 50.60 mmol) were stirred vigorously overnight under argon. A solution of 4-bromo-1-butene
(2.20 mL, 21.7 mmol) in anhydrous THF (28 mL) was added in drop-wise aliquots over 15 min to the magnesium turnings
under argon, ensuring that the THF did not boil, and stirred for 10 min. The solution was carefully drawn-up in a syringe
leaving unreacted magnesium in the flask. The Grignard solution was added drop-wise to a flask at -78 °C containing diethyl
oxalate (2.44 mL, 18.0 mmol) dissolved in anhydrous Et2O (35 mL) and anhydrous THF (17.5) and allowed to stir for 4 h at
-78 °C. The reaction was quenched with sat. aq. NH4Cl (20 mL) at -78 °C. After quenching, EtOAc (20 mL) was added to
the flask and the flask was allowed to warm to room temperature. The two layers were separated and the aqueous layer was
extracted with EtOAc (2 x 30 mL). The combined organic layers were dried over Na2SO4, filtered, and concentrated in vacuo
with the rotary evaporator bath set to 40 °C and a vacuum that never pulled below 40 torr. The residue was purified by flash
1
chromatography (0:100 to 5:95 EtOAc/hexanes) to give 4 as a slightly yellow oil (3.108 g, 89%): H NMR (600 MHz,
CDCl3) δ 5.82 (ddt, J = 16.8, 10.3, 6.5 Hz, 1H), 5.07 (dd, J = 17.1, 1.6 Hz, 1H), 5.02 (dd, J = 10.3, 1.4 Hz, 1H), 4.32 (q, J =
7.2 Hz, 2H), 2.95 (t, J = 7.3 Hz, 2H), 2.44 – 2.36 (m, 2H), 1.37 (t, J = 7.1 Hz, 3H); 13C NMR (151 MHz, CDCl3) δ 193.9,
161.1, 136.2, 116.0, 62.6, 38.5, 27.1, 14.1. 1H and 13C NMR is consistent with published data.15
O
O
N
CH3
O
Bn
(R)-3-Acetyl-4-benzyloxazolidin-2-one (11). Compound 11 was prepared according to the previously reported procedure.39
To a flask containing (R)-4-benzyloxazolidin-2-one (5.00 g, 28.2 mmol) and DMAP (0.079 g, 0.65 mmol) in THF (20 mL)
was added Et3N (3.93 mL, 28.2 mmol). The reaction mixture was cooled to 0 °C and acetic anhydride (5.32 mL, 56.4 mmol)
was added drop-wise over a period of 5 min. The solution was allowed to warm to room temperature and stirred overnight.
The reaction mixture was diluted with EtOAc (50 mL) and washed with H2O (2 x 20 mL) followed by brine (2 x 20 mL).
The organic layer was dried with Na2SO4, filtered, and concentrated in vacuo. The crude product was purified by flash
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chromatography (0:100 to 50:50 EtOAc/hexanes) to give 11 as white crystals (6.002 g, 97%): 1H NMR (400 MHz, CDCl3) δ
7.37 – 7.31 (m, 2H), 7.31 – 7.27 (m, 1H), 7.23 – 7.19 (m, 2H), 4.73 – 4.62 (m, 1H), 4.25 – 4.14 (m, 2H), 3.31 (dd, J = 13.4,
3.4 Hz, 1H), 2.78 (dd, J = 13.4, 9.6 Hz, 1H), 2.56 (s, 3H); 13C NMR (151 MHz, CDCl3) δ 13C NMR (151 MHz, CDCl3) δ
1
2
3
4
5
6
•+
170.4, 153.8, 135.4, 129.6, 129.1, 127.5, 66.3, 55.1, 38.0, 24.0; GCMS (EI) m / z calcd for C12H13NO3 [M] •+ 219.1, found
7
8
9
219.1, RT = 9.66 min. [ꢀ]!!""#.! = −194.20 (c = 0.36, CH3OH). NMR data matches reported literature values.39
O
10
11
12
13
14
15
16
17
18
19
20
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50
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60
O
HO CO2Et
N
O
Bn
Ethyl (R)-2-(2-((R)-4-benzyl-2-oxooxazolidin-3-yl)-2-oxoethyl)-2-hydroxyhex-5-enoate (16a). In a flame-dried flask
under argon, TiCl4 (1.05 mL of 1.0 M in CH2Cl2, 1.05 mmol) was added drop-wise to a solution of 11 (0.219 g, 1.00 mmol)
in anhydrous CH2Cl2 (10 mL) at -40 °C, and was stirred for 15 min. i-Pr2NEt (0.19 mL, 1.1 mmol) was added drop-wise to
the TiCl4 solution, which resulted in an instant color change to dark red. The reaction mixture was stirred for a further 40 min
at -40 °C. N-Methyl-2-pyrrolidone (0.20 mL, 0.19 mmol) was added drop-wise to the solution and allowed to stir for 10 min
at -40 °C followed by the addition of 4 (0.078 g, 0.50 mmol) drop-wise. The reaction was stirred for 7.5 h at -40 °C. The
reaction was quenched drop-wise with sat. NH4Cl in CH3OH (10 mL) over 30 min at -40 °C and then allowed to warm to
room temperature. The biphasic mixture was diluted with CH2Cl2 (20 mL) and H2O (15 mL). The aqueous layer was
extracted with CH2Cl2 (3 x 20 mL). The combined organic layers were dried over Na2SO4, filtered and concentrated in vacuo.
The crude reaction mixture (or an aliquot) can be protected as a silyl ether using TMSCl (see Supporting Information) to
determine the product ratio by GC/MS. The crude mixture was purified by flash chromatography (0:100 to 30:70
EtOAc:hexanes) to give 16a as a clear oil (0.086 g, 46%).
Major diastereomer 16a: 1H NMR (600 MHz, CDCl3) δ 7.34 (t, J = 7.4 Hz, 2H), 7.30 – 7.26 (m, 1H), 7.19 (d, J = 7.4 Hz,
2H), 5.80 (ddt, J = 16.9, 10.3, 6.5 Hz, 1H), 5.04 (d, J = 17.0 Hz, 1H), 4.98 (d, J = 10.0 Hz, 1H), 4.69 – 4.63 (m, 1H), 4.35 –
4.25 (m, 2H), 4.22 (t, J = 8.4 Hz, 1H), 4.17 (dd, J = 9.2, 2.6 Hz, 1H), 3.70 (s, 1H), 3.48 (s, 2H), 3.22 (dd, J = 13.5, 2.9 Hz,
1H), 2.79 (dd, J = 13.5, 9.4 Hz, 1H), 2.31 – 2.23 (m, 1H), 2.05 – 1.96 (m, 1H), 1.89 – 1.80 (m, 2H), 1.32 (t, J = 7.1 Hz, 3H);
13C NMR (151 MHz, CDCl3) δ 175.2, 170.6, 153.2, 137.4, 134.9, 129.4, 129.0, 127.4, 115.1, 74.5, 66.3, 62.0, 55.0, 44.6,
38.5, 37.6, 27.3, 14.2; IR (thin film) 3514, 2929, 1782, 1737, 1700 cm-1; TLC (30:70 EtOAc:hexanes) RF = 0.38; AMM
+
+
!!.!
(ESI) m / z calcd for C20H26NO6 [M + H] 376.1755, found 376.1767. [ꢀ]
= −405.15 (c = 0.79, CH3OH). Diastereomer
!"#
ratio from NMR calculated using the peaks at 3.22 and 3.25 corresponding to major and minor diastereomers respectively.
Minor diastereomer 16b: 1H NMR (600 MHz, CDCl3) δ 7.32 (t, J = 7.5 Hz, 2H), 7.30 – 7.24 (m, 1H), 7.19 (d, J = 7.4 Hz,
2H), 5.80 (ddt, J = 16.8, 10.4, 6.5 Hz, 1H), 5.04 (d, J = 17.0 Hz, 1H), 4.97 (d, J = 10.1 Hz, 1H), 4.69 – 4.63 (m, 1H), 4.32 –
4.23 (m, 2H), 4.21 (t, J = 8.5 Hz, 1H), 4.17 (dd, J = 9.1, 3.2 Hz, 1H), 3.78 (s, 1H), 3.52 (d, J = 18.0 Hz, 1H), 3.43 (d, J = 18.0
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Hz, 1H), 3.25 (dd, J = 13.5, 3.0 Hz, 1H), 2.80 (dd, J = 13.5, 9.3 Hz, 1H), 2.33 – 2.24 (m, 1H), 2.03 – 1.95 (m, 1H), 1.89 –
1.79 (m, 2H), 1.31 (t, J = 7.1 Hz, 3H); 13C NMR (151 MHz, CDCl3) δ 175.3, 170.9, 153.4, 137.6, 135.0, 129.5, 129.1, 127.5,
115.2, 74.7, 66.4, 62.0, 54.9, 44.8, 38.6, 37.7, 27.5, 14.3; IR (thin film) 3525, 2981, 1782, 1737, 1700 cm-1; TLC (30:70
1
2
3
4
5
6
7
+
+
!".!
EtOAc:hexanes) RF = 0.50; AMM (ESI) m / z calcd for C20H26NO6 [M + H] 376.1755, found 376.1758. [ꢀ]
= 214.80
!"#
8
9
(c = 0.66, CH3OH).
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TMS
O
O
O
CO2Et
N
O
Bn
TMS ether of 16a. In a flame-dried flask under argon, imidizaole (0.294 g, 4.32 mmol) was added to the crude reaction
mixture (0.081 g) from the aldol reaction in CH2Cl2 (5.4 mL) followed by TMSCl (0.55 mL, 4.32 mmol) and stirred at room
temperature for 24 hours. The reaction was quenched with sat. aq. NaHCO3 (8 mL). The resulting solution was extracted
with CH2Cl2 (2 x 15 mL). The organic layer was washed with brine (10 mL) and dried over Na2SO4, filtered, and
concentrated in vacuo. The crude mixture was purified by flash chromatography (0:100 to 40:60 EtOAc:hexanes) to give the
TMS ether of 16a as a clear oil (0.038 g, 17% over two steps). GC/MS of the crude reaction mixture gave a product ratio of
(11:16a:16b = 26:46:28). 1H NMR (600 MHz, CDCl3) δ 7.35 – 7.31 (m, 2H), 7.29 – 7.25 (m, 1H), 7.22 – 7.19 (m, 2H), 5.81
(ddt, J = 16.8, 10.2, 6.5 Hz, 1H), 5.05 (dd, J = 17.2, 1.7 Hz, 1H), 4.98 (dd, J = 10.3, 1.6 Hz, 1H), 4.72 – 4.64 (m, 1H), 4.28 –
4.20 (m, 2H), 4.19 (d, J = 8.2 Hz, 1H), 4.15 (dd, J = 9.1, 3.2 Hz, 1H), 3.71 (d, J = 17.0 Hz, 1H), 3.29 – 3.21 (m, 2H), 2.82
(dd, J = 13.5, 9.1 Hz, 1H), 2.27 – 2.18 (m, 1H), 2.18 – 2.10 (m, 1H), 1.91 – 1.83 (m, 2H), 1.30 (t, J = 7.2 Hz, 3H), 0.18 (s,
9H); 13C NMR (151 MHz, CDCl3) δ 174.4, 169.8, 153.6, 137.9, 135.2, 129.7, 129.1, 127.5, 115.0, 77.8, 66.1, 61.3, 55.0,
44.6, 39.4, 37.7, 27.9, 14.3, 2.5; IR (thin film) 2981, 1782, 1752, 1707 cm-1; GCMS (EI) m / z calcd for C22H30NO6Si•+ [M-
CH3]•+ 432.2, found 432.3, RT = 14.10 min; AMM (ESI) m / z calcd for C23H33NO6Si+ [M + H]+ 448.2150, found 448.2149.
!!.!
[ꢀ]
= −247.72 (c = 0.80, CH3OH).
!"#
O
CO2Et
O
O
N
Bn
Ethyl (R,Z)-2-(2-(4-benzyl-2-oxooxazolidin-3-yl)-2-oxoethylidene)hex-5-enoate (17). Recovered during aldol reactions
with 4 and 11. 1H NMR (600 MHz, CDCl3) δ 7.34 (t, J = 7.4 Hz, 2H), 7.30 – 7.27 (m, 1H), 7.22 (d, J = 7.4 Hz, 2H), 6.98 (t,
J = 1.5 Hz, 1H), 5.83 (ddt, J = 16.8, 10.1, 6.4 Hz, 1H), 5.10 (dd, J = 17.1, 1.6 Hz, 1H), 5.04 (dd, J = 10.4, 1.5 Hz, 1H), 4.74 –
4.68 (m, 1H), 4.36 – 4.25 (m, 2H), 4.25 – 4.20 (m, 1H), 4.18 (dd, J = 9.1, 2.9 Hz, 1H), 3.35 (dd, J = 13.5, 3.3 Hz, 1H), 2.80
(dd, J = 13.5, 9.6 Hz, 1H), 2.57 – 2.50 (m, 2H), 2.37 – 2.29 (m, 2H), 1.33 (t, J = 7.0 Hz, 3H); 13C NMR (201 MHz, CDCl3) δ
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168.7, 164.3, 153.4, 148.5, 136.60, 135.3, 129.6, 129.1, 129.1, 121.8, 116.1, 66.5, 61.5, 55.2, 37.8, 33.8, 31.4, 14.2; IR (thin
1
2
film) 3029, 1782, 1730, 1689, 1640 cm-1; AMM (ESI) m / z calcd for C20H24NO5+ [M + H]+ 358.1649, found 358.1651.
3
4
O
HO CO2Et
5
H3CO
6
7
8
9
1-Ethyl 4-methyl (R)-2-(but-3-en-1-yl)-2-hydroxysuccinate (18). To a flame-dried flask under argon containing the major
diastereomer (16a) (0.150 g, 0.400 mmol) in CH3OH (6.0 mL) was added a solution of sodium methoxide (0.032 g, 0.592
mmol) in CH3OH (4.0 mL) drop-wise at 0 °C. The reaction mixture was stirred at 0 °C for 1.5 h and quenched with sat. aq.
NH4Cl (10 mL). The resulting solution was extracted with CH2Cl2 (3 x 20 mL). The organic layer was dried over Na2SO4,
filtered, and concentrated in vacuo. The crude mixture was purified by flash chromatography (0:100 to 30:70
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1
EtOAc:hexanes) to give 18 as a clear oil (0.092 g, 98%). H NMR (600 MHz, CDCl3) δ 5.77 (ddt, J = 16.9, 10.2, 6.5 Hz,
1H), 5.02 (dd, J = 17.2, 1.5 Hz, 1H), 4.96 (dd, J = 10.1, 1.6 Hz, 1H), 4.32 – 4.23 (m, 2H), 3.71 (s, 1H), 3.68 (s, 3H), 2.94 (d,
J = 16.1 Hz, 1H), 2.71 (d, J = 16.2 Hz, 1H), 2.28 – 2.19 (m, 1H), 1.99 – 1.90 (m, 1H), 1.84 – 1.74 (m, 2H), 1.31 (t, J = 7.1
Hz, 3H); 13C NMR (151 MHz, CDCl3) δ 174.8, 171.1, 137.4, 115.1, 74.7, 62.0, 51.8, 43.4, 38.3, 27.4, 14.1; IR (thin film)
+
+
3510, 2959, 1737, 1644 cm-1; AMM (ESI) m / z calcd for C11H19O5 [M + H] 231.1227, found 231.1228. [ꢀ]
= 10.78 (c
!".!
!"#
= 2.31, CH3OH).
CH3O2C
O
O
EtO2C
Ethyl (R)-2-(2-methoxy-2-oxoethyl)-5-oxotetrahydrofuran-2-carboxylate (19). To a flask containing 18 (0.167 g, 0.725
mmol) in EtOAc/CH3CN/H2O (5.3 mL, 2:2:3 by volume) was added NaIO4 (0.826 g, 3.86 mmol) followed by ruthenium(III)
chloride hydrate (4.4 mg, 0.021 mmol). The solution was stirred at room temperature for 1 h during which a light brown
precipitate formed. The reaction was quenched with i-PrOH (20 mL) and filtered through Celite. The filter pellet was washed
with additional i-PrOH (4 x 15 mL) and the filtrate was collected and concentrated in vacuo. The resulting oil was purified
1
by flash chromatography (10:90 to 100:0 EtOAc:hexanes) to give 19 as a clear oil (0.0926 g, 55%). H NMR (600 MHz,
CDCl3) δ 4.34 – 4.23 (m, 2H), 3.71 (s, 3H), 3.13 (d, J = 16.7 Hz, 1H), 2.98 (d, J = 16.8 Hz, 1H), 2.75 – 2.66 (m, 1H), 2.65 –
2.53 (m, 2H), 2.40 – 2.32 (m, 1H), 1.31 (t, J = 7.1 Hz, 3H); 13C NMR (151 MHz, CDCl3) δ 175.5, 170.5, 169.1, 83.0, 62.6,
+
52.2, 41.4, 31.3, 27.9, 14.1; IR (thin film) 2959, 1789, 1737, 1164 cm-1; AMM (ESI) m / z calcd for C10H15O6 [M + H]+
!!.!
231.0863, found 231.0885. [ꢀ]
= −37.11 (c = 0.28, CH3OH).
!"#
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CO2H
O
1
2
O
HO2C
3
4
(R)-(-)-2-(carboxymethyl)-5-oxotetrahydrofuran-2-carboxylic acid ((-)-1). 19 (0.0449 g, 0.195 mmol) was dissolved in
TFA/H2O (2 mL, 1:1 by volume) and heated to reflux for 24 h. The reaction mixture was concentrated and left on the high
vacuum (~ 1 torr) for 4 h. If necessary the product was purified on a short silica plug (0:100 to 40:60 acetone:hexanes) but
generally no further purification was necessary to give (-)-1 as an amorphous white solid (0.031 g, 85%). If a column was
deemed necessary fractions were assayed for product by spotting on a TLC and staining with bromocresol green, any
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fractions containing acidic products were collected. H NMR (600 MHz, CD3OD) δ 3.16 (d, J = 17.1 Hz, 1H), 2.94 (d, J =
17.1 Hz, 1H), 2.70 – 2.56 (m, 2H), 2.54 – 2.46 (m, 1H), 2.43 – 2.34 (m, 1H); 13C NMR (151 MHz, CD3OD) δ 178.7, 174.1,
172.4, 84.7, 42.1, 32.3, 28.7. [ꢀ]!!".! = −6.01 (c = 0.266, CH3OH). NMR data matches reported literature values.15
Acknowledgment LAN acknowledges UC Davis for support in the form of a Dow/Corson Fellowship. The authors thank
the NIH (GM-65440) for support of this project and the NSF for funds to support the 800 MHz NMR spectrometer (DBIO
722538). Michael T. Crimmins (UNC, Chapel Hill) is acknowledged for helpful discussions.
Supporting Information Available Copies of 1H and 13C NMR spectra, gas chromatograms, and mass spectra are available
free of charge via the Internet at http://pubs.acs.org.
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.All isotopically labeled starting materials are commercially available (Sigma-Aldrich product numbers:
diethyl oxalate-13C2, 595977; acetic anhydride-13C4, 487821; 4-bromo-1-butene-13C4, 604526) making the 13C-labeled
ketoester (see ref. 15) and acetyl oxazolidinone accessible to isotopically label every carbon of homocitric acid lactone if so
desired.
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