A simplified synthesis of the diastereomers of Levuglandin E2

Article abstract of DOI:10.1081/SCC-200048945

We report a synthesis of Levuglandin E₂, as a mixture of easily separable diastereomers (15-E₂-isoketals). The key feature is the protection of its reactive aldehyde as dimethyl acetal, which is introduced with 2,2-dimethoxyethanal and removed in minutes with Montmorillonite K-10. The synthesis could be modified to the preparation of 15-E₂-isoketals with stable isotopes or with radiolabels. Copyright Taylor & Francis, Inc.

Full text of DOI:10.1081/SCC-200048945

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A Simplified Synthesis of the
Diastereomers of Levuglandin
E2
Venkataraman Amarnath a , Kalyani Amarnath a
, Tina Masterson a , Sean Davies a & L. Jackson
Roberts II a
a Departments of Pathology and Pharmacology,
Vanderbilt University Medical Center, Nashville, TN,
37232, USA
Published online: 28 Jul 2007.
To cite this article: Venkataraman Amarnath , Kalyani Amarnath , Tina Masterson ,
Sean Davies & L. Jackson Roberts II (2005) A Simplified Synthesis of the Diastereomers
of Levuglandin E2 , Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 35:3, 397-408, DOI: 10.1081/
SCC-200048945
To link to this article: http://dx.doi.org/10.1081/SCC-200048945
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Synthetic Communicationsw, 35: 397–408, 2005
Copyright # Taylor & Francis, Inc.
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1081/SCC-200048945
A Simplified Synthesis of the Diastereomers
of Levuglandin E2
Venkataraman Amarnath, Kalyani Amarnath, Tina Masterson,
Sean Davies, and L. Jackson Roberts II
Departments of Pathology and Pharmacology, Vanderbilt University
Medical Center, Nashville, TN, USA
Abstract: We report a synthesis of Levuglandin E2, as a mixture of easily separable
diastereomers (15-E2-isoketals). The key feature is the protection of its reactive
aldehyde as dimethyl acetal, which is introduced with 2,2-dimethoxyethanal and
removed in minutes with Montmorillonite K-10. The synthesis could be modified to
the preparation of 15-E2-isoketals with stable isotopes or with radiolabels.
Keywords: Levuglandin E2, dimethyl acetal of reactive aldehyde, montmorillonite
deprotection
INTRODUCTION
Levuglandin E2 (1), formed by the rearrangement of PGH2,[1] and its related
stereo-isomers 15-E2-isoketal, generated by free radical mediated oxidation of
arachidonic acid,[2] have attracted recent attention for their high reactivity
with proteins.[3,4] Our continuing studies on the effects of protein
adduction[5] caused by these 4-ketoaldehydes required their ready availability.
Because it is difficult to isolate pure 1 from natural sources, we turned to its
synthesis. Chemical preparation also has the advantage of providing 1 with
stable or radio isotopes. The most important consideration during the
synthesis of 1 (Scheme 1) is given to the protection of the reactive aldehyde
Received in the USA October 24, 2004
Address correspondence to Venkataraman Amarnath, Departments of Pathology,
Vanderbilt University Medical Center, Nashville, TN 37232, USA. Fax: 615-343-
9825; E-mail: venkataraman.amarnath@vanderbilt.edu

398
V. Amarnath et al.
Scheme 1. Retrosynthetic analysis of Levuglandin E2.
group. In the pioneering work of Salomon[6] this was accomplished with a
vicinal diol (2), which in turn was blocked with an isopropylidene group
(4). During the synthesis, the dioxolane ring was introduced with isopropyli-
dene-L-glyceraldehyde (6) that also provided control over the stereochemistry
across the 8- and 9-carbons. The generation of 1 from the stable precursor 4
involved the hydrolysis of the dioxolane ring with acetic acid, oxidation of
the resulting diol with NaIO4, destroying the excess reagent with ethylene
glycol, and the complete removal of acetic acid. This time-consuming depro-
tection hindered performing further studies on 1. Because our interest is
centered on 1 formed from free radical induced oxidation, an easier
synthesis giving a mixture of stereoisomers of 15-E2-isoketal seemed
adequate. In any case, the stereochemistry of the 8- and 9-carbon atoms is
lost during rapid reaction of 1 with the amino groups of proteins to form
pyrroles. Therefore, a simpler synthesis incorporating the protection of the
aldehyde as dimethyl acetal[7] was designed. A single step hydrolysis of
3 would readily provide 1. In addition, during the new synthesis, the

Synthesis of Levuglandin E2
399
protected aldehyde function and the 9-carbon could be introduced with the
commercially available 2,2-dimethoxyethanal (5). In contrast, 6 has to be
freshly obtained using a labor-intensive preparation.[8]
RESULTS AND DISCUSSION
The seven-carbon chain of 1 having the carboxyl group is built with methyl
7-bromo-5(Z)-heptenoate (11) while the other side chain is provided by
3-[(tert-butyldimethylsilyl)oxy]-1-(tributylstannyl)-1(E)-octene (15). Among
the synthetic methods available for the bromoester 11, we chose the one
used by Taber[9] and improved it by combining the hydrolysis of the tetrahy-
dropyranyl group with esterification to produce the hydroxy ester 10
(Scheme 2). The silyl derivative 14 of oct-1-yn-3-ol (13) was heated with
tributyltinhydride to obtain 15 (Scheme 3). For radioactive labeling of
these key intermediates, the chloroalkyne 7 was treated with Na14CN and
oct-1-yn-3-one (12) was reduced with NaBT4. The labeled products were
incorporated in the rest of the synthetic sequence.
Diethyl 2-oxopropylphosphonate (16) required in the next step is readily
obtained from triethyl phosphonoacetate and acetyl chloride.[10] This reaction
step was also useful for introducing 13C labels, as in 16-13C3. The latter led to
the preparation of 1 with 3 additional mass units, which could be used as an
internal standard during measurements by mass spectrometry. After conden-
sing 16 and 11, the protected aldehyde function is introduced using 5 in the
last step of Scheme 4. The enone 18 was found to be mostly a single isomer
and a comparison of the chemical shifts of the proton directly attached to
the double bond and the carbonyl carbon with the published values of the
isomers[6] suggested the E configuration.
The fully protected LGE2 (19), assembled from the enone 18 and the silyl-
stannane 15 (Scheme 5), was a mixture of diastereomers a and b (disregarding
the orientation of carbon-12). The isomers could be easily separated by flash
Scheme 2. Synthesis of methyl 7-bromo-5(Z)-heptenoate.

400
V. Amarnath et al.
Scheme 3. Synthesis of stannane 15.
chromatography. From their NMR spectra, the faster moving isomers (19a)
appeared to be erythro (SR and RS) and the others (19b) threo (SS and
RR).[6] The silyl group in 19 was removed with N-bromosuccimide in moist
DMSO[11] and the ester was hydrolyzed with LiOH[12] to give the dimethyl
acetal 3 as the immediate precursor of 1, and it could be stored under Ar at
–208C for extended periods. The diastereomers of 20 and 3 could also be
easily separated if the stereochemical purity of 1 is desired.
Although the hydrolysis of the acetal has been accomplished with a
variety of mineral or Lewis acids,[13] the use of Montmorillonite K-10
Scheme 4. Synthesis of enone 18.

Synthesis of Levuglandin E2
401
Scheme 5. Synthesis of stereomers of Levuglandin E2.
attracted our attention.[14] Treatment with mildly acidic clay in dichloro-
methane offered the following advantages: (1) The deprotection of related
1,1-dimethoxy compounds was complete in minutes at room temperature.
(2) The heterogeneous K-10 could be easily removed by filtration, after the
release of the reactive aldehyde is complete, minimizing side reactions. (3)
The deprotection could be scaled down to small-scale preparations. When a
solution of 3a in dichloromethane was stirred with K-10, the conversion to
1 was found by HPLC to be complete within 10 min. The clay was
separated by centrifugation and the solvent could be removed under N2 to
recover LGE2. This simple deprotection step provides 15-E2-isoketal
whenever needed and is amenable to handling labeled derivatives in mg quan-
tities, as shown by the hydrolysis of 3-13C3 and other radio-labeled acetals.
EXPERIMENTAL
General
Chromatography was run using EM aluminum plates coated with silica or on
columns of silica gel. Thin-layer plates were visualized using a UV lamp as

402
V. Amarnath et al.
well as exposing to vapors of iodine. Hewlett-Packard GC 5890 with FID or
mass selective detector was fitted with Phenomenex Zebron ZB-wax column
(15 m ꢀ 0.53 mm ꢀ 1 mm) for GC analysis. NMR spectra in CDCl3 were
accumulated using a Bruker DPX-300 and MS measurements were
performed on a Thermo-Finnigan TSQ700 mass spectrometer.
Methyl 7-Bromo-5(Z)-heptenoate (11)
A solution of 6-[(tetrahydropyran-2-yl)oxy]-4-hexynenitrile (8, 10.9 g,
50 mmol) in methanol (120 mL) containing 10 M NaOH (30 mL) was heated
to reflux for 20 h and concentrated to remove methanol. The aqueous
solution was washed with ether (50 mL), cooled in ice, acidified to pH 3
(with H3PO4 and 6 M HCl), and extracted with CHCl3 (3 ꢀ 40 mL). The
combined extracts were washed with brine (50 mL), dried, and evaporated.
The acid 9 (11 g) was dissolved in methanol (75 mL), stirred with BF3-
methanol (25 mL; 1.6 M) at room temperature for 4 h and evaporated. The
residue was dissolved in ethyl acetate (50 mL), washed with 10 M NaHCO3
(2 ꢀ 25 mL) and brine (20 mL), dried, and evaporated. The crude hydroxy
ester 10 was purified by distillation; 80–858C/0.6 Torr; 6.3 g (80% from 8).
It was selectively reduced with hydrogen over Lindler’s catalyst at 08C[15]
and converted to the bromo ester 11 as described previously.[9] The crude
ester was purified by column chromatography using 20 : 1 hexane-ethyl
acetate (60% from 10).
A solution of 1-(tetrahydropyran-2-yloxy)-6-chloro-2-hexyne (7, 0.22 g,
1 mmol) in DMSO (1 mL) was added to a vial containing Na14CN (5 mCi,
55 mCi/mmol) and heated at 458C with stirring for 4 h. The reaction
mixture was transferred to a flask containing the chlorohexyne 7 (4.6 g,
21 mmol) and NaCN (0.98 g, 20 mmol) in DMSO (20 mL), and heating was
continued for 12 h. After the workup[9] [1-14C]-6-[(tetrahydropyranyl)oxy]-
4-hexynenitrile was obtained in 90% radiochemical yield.
3-[(tert-Butyldimethylsilyl)oxy]-1-(tributylstannyl)-1(E)-octene (15)
Tributyltin hydride (freshly distilled; 4.0 mL, 15 mmol) was added drop-wise
to a mixture of 3-(tert-butyldimethylsilyloxy-1-octyne (14, 3.6 g, 14 mmol)
and azo-bis-isobutyronitrile (20 mg) kept under Ar. The mixture was stirred
and heated at 110–1158C for 2 h. The presence of the addition product was
checked by GC-MS (peaks clustered around m/z 475 (M 2 57); only a trace
of the tin hydride was detected and the liquid was used directly in the next
1
step. A sample was distilled (110–1208/0.1 Torr) for NMR; H NMR same
as reported;[15] 13C NMR d 152.1, 126.3 and signals for aliphatic carbons.

Synthesis of Levuglandin E2
403
Oct-1-yn-3-one (12, 0.5 g, 4 mmol) in methanol (10 mL) was stirred with
NaB3H4 (40 mg, 0.5 Ci) for 30 min and NaBH4 (20 mg) was added. After 15 m,
the reaction mixture was evaporated. The residue was mixed with 50 mM HCl
(20 mL) and ethyl acetate (20 mL). The layers were separated and the aqueous
layer was extracted with ethyl acetate (2 ꢀ 20 mL). The organic layers were
combined, dried, and evaporated to get 13-T in quantitative yield, which
was silylated[15] and converted to the stannane 15-T as described above.
Diethyl [1,2,3-13C3]-2-oxopropylphosphonate (16-13C3)
A suspension of anhydrous magnesium chloride (1.35 g, 13.5 mmol) in dry
dichloromethane (10 mL) was stirred under Ar and a solution of triethyl phos-
phono[2-13C]-acetate (3 g, 13.3 mmol) in dichloromethane (5 mL) was intro-
duced followed by triethylamine (3.8 mL, 27 mmol). After 30 min at room
temperature the flask was cooled to 58C and [1,2-13C2]-acetyl chloride (1 g,
12.5 mmol) in 2 mL of dichloromethane was injected. After 2 h at room temp-
erature, the reaction was quenched with 1 M HCl (35 mL). The phases were
separated and the aqueous layer was extracted with dichloromethane
(2 ꢀ 10 mL). The organic layers were combined, dried, and evaporated. The
residue was purified by column chromatography (3 : 1 hexane-ethyl acetate)
to get diethyl [1,2,3-13C3]-1-ethoxycarbonyl-2-oxopropylphosphonate as an
oil; 2.8 g. It was mixed with water (0.4 mL, 22 mmol) and heated at
130 + 58C for 2 h. After cooling, water (10 mL) was added, and the product
was extracted with dichloromethane (3 ꢀ 10 mL), dried, concentrated, and
purified by flash chromatography (ethyl acetate); bp 60–658C/0.25 Torr;
2 g (80%); GC-MS m/z 197 (Mþ); 13C NMR clusters at d 199.8 (C55O),
44.2 and 42.5 (CH2P), and 31.3 (CH3CO). The unlabeled phosphonate 16
was similarly prepared on a larger scale.
Methyl 8-(diethylphosphono)-9-oxodec-5(Z)-enoate (17)
The workup of the reaction mixture starting from sodium hydride (1.2 g of
60% suspension), diethyl 2-oxopropyl-phosphonate (4.85 g, 25 mmol) and
11 (4.85 g, 22 mmol)[6] involved addition of NaH2PO4 (2.8 g), evaporation,
and mixing with brine (10 mL). The combined ethyl acetate extracts
(6 ꢀ 30 mL) were dried, evaporated, and purified by column chromatography
(hexane containing 35–80 % ethyl acetate) to obtain 17 (5.1 g, 70%).
Methyl 8-acetyl-10,10-dimethoxy-5(Z),8-decadienoate (18)
Sodium hydride (500 mg of a 60% suspension, 12.5 mmol) was stirred
in anhydrous THF (60 mL) under dry Ar and the flask was cooled in dry

404
V. Amarnath et al.
ice-isopropanol bath (2208C). The ketophosphonate 17 (3.3 g, 10 mmol) in
dry THF (15 mL) was added over 10 min and the reaction mixture was
stirred at 258C for 2 h. The temperature of the bath was lowered to 2788C
and a solution of 2,2-dimethoxyethanal in methyl-t-butyl ether (1.40 g,
13.3 mmol) and THF (10 mL) was introduced. The reaction mixture was
allowed to warm slowly to 2258C when it became a clear solution. After
stirring at that temperature for 20 min, a mixture of brine (20 mL) and
NaH2PO4 (2 g) in water (20 mL) was slowly added. The layers were
separated and the aqueous layer was extracted with ethyl acetate
(2 ꢀ 50 mL). The organic layers were combined, washed with water
(30 mL), dried, and concentrated. The crude product was purified by
column chromatography (hexane-ethyl acetate 5 : 1, 4 : 1, and 3 : 1) to a
clear liquid; 2.1 g (74%); GC-MS; MS m/z 284 (1, Mþ), 252 (20,
1
M 2 H 2 OCH3), 151 (100); H NMR d 6.34 (d, 1H, J ¼ 6 Hz, C9-H), 5.11
and 5.26 (m, 1H each, C5-H and C6-H), 5.07 (d, 1H, J ¼ 6 Hz, C10-H), 3.55
(s, 3H, CO2CH3), 3.26 (s, 6H, OCH3), 3.08 (d, 2H, J ¼ 7 Hz, C7-H), 2.27
(t, 2H, J ¼ 7.2 Hz, C2-H), 2.23 (s, 3H, CH3CO), 2.10 (apparent q, 2H,
J ¼ 7.2 Hz, C4-H), 1.64 (quintet, 2H, J ¼ 7.4 Hz, C3-H); 13C NMR d 198.8
(CO), 173.8 (CO2), 143.3 (C-8), 137.2 (C-9), 126.9 and 129.6 (C-5 and
C-6), 99.6 (C-10), 52.6 (OCH3), 51.2 (CO2CH3), 33.3 (C-2), 26.4, 25.7
(CH3CO), 25.0, 24.3. Anal. calcd for C15H24O5 (284.36): C, 63.33; H, 8.51;
found: C, 62.87; H, 8.64.
Methyl
8-[13C2]-acetyl-[8-13C]-10,10-dimethoxy-5(Z),8-decadienoate
(18-13C3) was similarly prepared in two steps from 16-13C3; MS m/z 287
(1, Mþ), 255 (20, M–H–OCH3), 154 (100); 13C NMR clusters at d 25.6,
143.3, and 198.8.
Methyl 8-acetyl-9-dimethoxymethyl-12-(t-butyldimethylsilyloxy)
heptadeca-5(Z),10(E)-dienoate (19)[16,17]
Dry CuCN (270 mg, 3 mmol) was suspended in THF (2 mL) under dry Ar and
cooled to –308C. Methyllithium (3.2 mL of 2 M solution; 6.4 mmol) was
added drop-wise with good mixing to get a tan solution. Vinylstannane 15
(1.95 g, 81% pure, 3 mmol) in THF (4 mL) was slowly added and the
reaction mixture was stirred at room temperature for 1.5 h when it turned
deep yellow. The flask was cooled to 2788C and enone 18 (0.57 g, 2 mmol)
in THF (2 mL) was added with a syringe. The reaction mixture was slowly
warmed to –308C, stirred at that temperature for 25 min, and quenched by
adding satd NH4Cl, (18 mL), NH4OH (2 mL), brine (20 mL), and ether
(40 mL). The resulting mixture was stirred for 15 min, when it turned deep
blue, and filtered. The aqueous layer of the filtrate was extracted with ethyl
acetate (3 ꢀ 25 mL). The organic layers were combined, washed with brine
(2 ꢀ 20 mL), dried, and evaporated. The residue was purified by column

Synthesis of Levuglandin E2
405
chromatography (hexane-ethyl acetate 9 : 1, 7 : 1, 6 : 1, 5 : 1) to purify and
separate the two diastereomers (RF 0.45 and 0.40 in 5 : 1 hexane-ethyl
acetate) from the unreacted 18 (150 mg); 19a (110 mg; 20%); MS m/z 526
1
(Mþ); H NMR d 5.28–5.47 (m, 4H, vinyl H’s), 4.20 (m, 1H, C12-H), 4.05
(m, 1H, (CH3O)2C-H), 3.61 (s, 3H, CO2CH3), 3.25 and 3.27 (s, 3H
each, (OCH3)2), 2.8 (m, 1H, C8-H), 2.6 (m, 1H, C9-H), 2.25 (t, 2H,
J ¼ 7.5 Hz, C2-H2), 2.0–2.2 (2H, C7-H2), 2.05 (s, 3H, COCH3), 2.01
(m, 2H, C4-H2), 1.58–1.61 (m, 2H, C3-H2), 1.3–1.4 (2H, C13-H2), 1.22 (bs,
6H, C14-H2, C15-H2, C16-H2), 0.84 (s, 9H, C(CH3)3), 0.82 (t, 3H,
J ¼ 5.4 Hz, C17-H3), 20.01 (s, 6H, Si(CH3)2); 13C NMR d 211.3
(carbonyl), 173.8 (C-1), 137.8 (C-11), 130.5, 127.5, 125.7, 105.7 (acetal),
73.5 (C-12), 54.6 (C-9), 53.7 (OCH3), 51.6 (C-8), 51.3 (CO2CH3), 38.3,
33.3, 31.9 (CH3CO), 31.5, 28.0, 26.0, 25.7 (C(CH3)3), 24.8, 24.6, 22.5,
1
18.1, 13.9 (C-17), 24.5, and – 5.0; 19b (135 mg; 25%) H NMR d 5.47
(m, 1H, C11-H), 5.24–5.29 (m, 3H, rest of the vinyl H’s), 4.03 (m, 2H, C12-H
and (CH3O)2C-H), 3.61 (s, 3H, CO2CH3), 3.24 and 3.15 (s, 3H
each, (OCH3)2), 2.63–2.64 (m, 2H, C8-H and C9-H), 2.22 (t, 2H, J ¼ 7.5 Hz,
C2-H2), 2.0–2.2 (2H, C7-H2), 2.07 (s, 3H, COCH3), 2.01 (apparent q, 2H,
J ¼ 7.7 Hz, C4-H2), 1.62–1.64 (m, 2H, C3-H2), 1.3–1.4 (2H, C13-H2), 1.21
(bs, 6H, C14-H2, C15-H2, C16-H2), 0.82 (s, 9H, C(CH3)3), 0.80 (t, 3H,
J ¼ 5.4 Hz, C17-H3), 20.02 (s, 6H, Si(CH3)2); 13C NMR d 211.6 (carbonyl),
173.8 (C-1), 137.9 (C-11), 130.4, 127.2, 125.9, 106.9 (acetal), 72.8
(C-12), 55.0 (C-9), 53.2 (OCH3), 51.3 (CO2CH3), 51.1 (C-8), 38.3, 33.3, 31.6
(CH3CO), 31.5, 28.8, 26.4, 25.7 (C(CH3)3), 24.8, 24.6, 22.5, 18.1, 13.9
(C-17), 24.6, and – 5.0. Anal. calcd for C29H54O6Si.CH3CO2C2H5 (614.94):
C, 64.46; H, 10.16; found: C, 64.75; H, 10.36. Labeled 19-13C3 was obtained
using 18-13C3 in place of 18; 13C NMR d 211.24, 51.6, and 31.9 for 19a-13C3
and 211.63, 51.1, and 31.5 for 19b-13C3.
8-Acetyl-9-dimethoxymethyl-12-hydroxyheptadeca-5(Z),10(E)-
dienoic acid (3)
A cold mixture of N-bromosuccinimde (200 mg), DMSO (4.5 mL), and water
(0.25 mL) was added to 19 (0.51 g, 1 mmol) and the solution was stirred under
Ar for 3.5 h. Ethyl acetate (50 mL) was added and the organic layer was
washed with brine (2 ꢀ 10 mL). The washings were extracted with ethyl
acetate (2 ꢀ 10 mL). The organic phases were combined, dried, and evapor-
ated. The residue was purified by flash chromatography (hexane-ethyl
acetate 2 : 1, 1 : 1, and 1 : 2) to get pure 20a and 20b (RF 0.5 and 0.3 in 3 : 2
1
hexane-ethyl acetate); 20b MS m/z 412 (Mþ); H NMR d 5.52 (m, 1H,
C11-H), 5.27–5.32 (m, 3H, rest of the vinyl H’s), 4.13 (m, 2H, C12-H and
(CH3O)2C-H), 3.64 (s, 3H, CO2CH3), 3.30 and 3.22 (s, 3H each, (OCH3)2),
2.6–2.7 (m, 2H, C8-H and C9-H), 2.26 (t, 2H, J ¼ 7.2 Hz, C2-H2), 2.0–2.2

406
V. Amarnath et al.
(2H, C7-H2), 2.07 (s, 3H, COCH3), 2.01 (apparent q, 2H, J ¼ 7.7 Hz, C4-H2),
1.62–1.64 (m, 2H, C3-H2), 1.3–1.4 (2H, C13-H2), 1.27 (bs, 6H, C14-H2,
C15-H2, C16-H2), 0.85 (t, 3H, J ¼ 5.4 Hz, C17-H3); 13C NMR d 211.7
(carbonyl), 173.8 (C-1), 137.7 (C-11), 130.7, 127.2, 127.0, 106.5 (acetal),
72.5 (C-12), 55.3 (C-9), 53.7 (OCH3), 51.3 (CO2CH3), 51.1 (C-8), 37.0,
33.3, 31.7, 31.5, 28.6, 26.4, 25.1, 24.6, 22.6, 14.0 (C-17); 20a d 106.4
(acetal), 72.8 (C-12), 54.6 (C-9), 51.5 (C-8); the rest of the NMR signals
were essentially the same as given for 20b.
The alkyne 8 with 14C-labeled nitrile (20 mmol) was incorporated in the
reaction sequence to obtain 20 with the radiolabel at the carboxyl group;
3 mmol (250 mCi/mmol, 15% yield from 8). The stannane 15 carrying
tritium was used for obtaining 20 with radiolabel in the side chain bearing
the hydroxyl group.
The ester 20 from the previous step was taken in THF (10 mL) and stirred
with a solution of LiOH.H2O (210 mg) in water (20 mL) for 1 h. The reaction
mixture was washed with ether (10 mL), cooled in ice, and pH was lowered to
6 with 0.5% H3PO4 and to 4 with 0.1 M HCl. The solution was saturated with
NaCl and extracted with ethyl acetate (3 ꢀ 10 mL). The extracts were
combined, dried, evaporated, and purified by column chromatography (ethyl
acetate; RF 0.30 and 0.25 for 3a and 3b, respectively); 0.26 g (65% from
20); MS m/z 397 (M – 1)–. The NMR spectra were similar to those of 20
1
except the absence of singlet at d 3.64 in H NMR and the signal at d 51.3
in 13C NMR. Anal. calcd for C22H38O6.CH3CO2C2H5 (486.65): C, 64.17;
H, 9.53; found: C, 63.90; H, 9.66.
The acetal 3-13C3 with 13C labels at the acetyl group and at the 8-position
was similarly obtained by deprotecting 19-13C3; MS m/z 400 (M – 1)–.
LGE2 Isomers
A 10 mM solution of 3 in dichloromethane (100 mL) was gently agitated with
Montmorillonite K-10 (10 mg) to keep the clay in suspension. After 15 min
the clay was separated by centrifugation and rinsed with the same solvent
(2 ꢀ 50 mL). The supernatants were combined with DMSO (200 mL) and
dichloromethane was removed under a stream of N2; MS m/z 351 (M – 1)2.
1
The ratio of H NMR signals at d 9.6 and 2.2 is used to estimate the purity
that was greater than 60%. LGE2 diastereomers with three 13C labels,
obtained from hydrolyzing 3-13C3, exhibited a MS with m/z 354 (M–1)2
and three clusters of signals in the 13C NMR at d 30.5, 49.2, and 215.7 ppm.
CONCLUSION
We have presented a new strategy of generating a highly reactive aldehyde (1)
from its dimethyl acetal using Montmorillonite K-10. This strategy also

Synthesis of Levuglandin E2
407
proved beneficial for preparing 4-oxo-2(E)-nonenal[18] and is expected to be
useful for the synthesis of other reactive aldehydes.
ACKNOWLEDGMENT
The work was supported by the National Institutes of Health Grant GM 42056.
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