Discovery of a series of pyrrolidine-based endothelin receptor antagonists with enhanced ET(A) receptor selectivity

Article abstract of DOI:10.1016/S0968-0896(99)00022-X

Endothelins, ET-1, ET-2, and ET-3 are potent vasoconstricting and mitogenic 21-amino acid bicyclic peptides, which exert their effects upon binding to the ET(A) and ET(B) receptors. The ET(A) receptor mediates vasoconstriction and smooth muscle cell proliferation, and the ET(B) receptor mediates different effects in different tissues, including nitric oxide release from endothelial cells, and vasoconstriction in certain vascular cell types. Selective antagonists of endothelin receptor subtypes may prove useful in determining the role of endothelin in various tissue types and disease states, and hence as therapeutic agents for such diseases. The pyrrolidine carboxylic acid A-127722 has been disclosed as a potent and ET(A)-selective antagonist, and is currently undergoing clinical trials. In our efforts to find antagonists with altered selectivity (ET(A)-selective, ET(B)-selective, or nonselective), we investigated the SAR of the 2-substituent on the pyrrolidine. Compounds with alkyl groups at the 2-position possessed ET(A) selectivity improved over A-127722 (1400-fold selective), with the best of these compounds showing nearly 19,000-fold selectivity. Copyright (C) 1999 Elsevier Science Ltd.

Full text of DOI:10.1016/S0968-0896(99)00022-X

Bioorganic & Medicinal Chemistry 7 (1999) 991±1002
Discovery of a Series of Pyrrolidine-based Endothelin Receptor
Antagonists with Enhanced ETA Receptor Selectivity
Steven A. Boyd,a,* Robert A. Mantei,a Andrew S. Tasker,a Gang Liu,a
Bryan K. Sorensen,a Kenneth J. Henry Jr,a Thomas W. von Geldern,a Martin Winn,a
Jinshyun R. Wu-Wong,a William J. Chiou,a Douglas B. Dixon,a Charles W. Hutchins,b
Kennan C. Marsh,c Bach Nguyenc and Terry J. Opgenortha
aMetabolic Disease Research, Abbott Laboratories, Pharmaceutical Products Division, 100 Abbott Park Road, Building AP10,
Abbott Park, IL 60064-6101, USA
bAdvanced Technologies Research, Abbott Laboratories, Pharmaceutical Products Division, 100 Abbott Park Road, Building AP10,
Abbott Park, IL 60064-6068, USA
cExploratory Science, Abbott Laboratories, Pharmaceutical Products Division, 100 Abbott Park Road, Building AP9, Abbott Park,
IL 60064-6122, USA
Received 29 June 1998; accepted 14 September 1998
AbstractÐEndothelins, ET-1, ET-2, and ET-3 are potent vasoconstricting and mitogenic 21-amino acid bicyclic peptides, which
exert their e€ects upon binding to the ETA and ETB receptors. The ETA receptor mediates vasoconstriction and smooth muscle cell
proliferation, and the ETB receptor mediates di€erent e€ects in di€erent tissues, including nitric oxide release from endothelial cells,
and vasoconstriction in certain vascular cell types. Selective antagonists of endothelin receptor subtypes may prove useful in
determining the role of endothelin in various tissue types and disease states, and hence as therapeutic agents for such diseases. The
pyrrolidine carboxylic acid A-127722 has been disclosed as a potent and ETA-selective antagonist, and is currently undergoing
clinical trials. In our e€orts to ®nd antagonists with altered selectivity (ETA-selective, ETB-selective, or nonselective), we investi-
gated the SAR of the 2-substituent on the pyrrolidine. Compounds with alkyl groups at the 2-position possessed ETA selectivity
improved over A-127722 (1400-fold selective), with the best of these compounds showing nearly 19,000-fold selectivity. # 1999
Elsevier Science Ltd. All rights reserved.
Introduction
<1 nM)6,7 and activates a complex intracellular signal-
ling cascade.8,9 ET-1 acts as a growth factor in certain
tissues, including vascular smooth muscle, and is syner-
gistic with other growth factors (PDGF, bFGF, TGF,
EGF, insulin).10 Endothelins may play a role in vascular
diseases, including hypertension, congestive heart fail-
ure, vasospasm, restenosis following angioplasty
(PCTA), subarachnoid hemorrhage, ischemia, pulmon-
ary hypertension, and renal failure.1
Endothelins ET-1, ET-2, and ET-3, are 21-amino acid,
bicyclic vasoconstrictor peptides, with a conserved hexa-
peptide tail (Fig. 1).1±3 The endothelins are produced by
a number of cell types and appear to act as paracrine
factors through binding to cell surface receptors. The
two known mammalian endothelin receptor subtypes,
ETA and ETB, are G-protein-coupled cell surface
receptors containing seven transmembrane regions. ETA
binds selectively to ET-1, and ETB is nonselective.4
Endothelin receptors are widely distributed in human
tissues.5 For instance, ETA is present primarily on vas-
cular smooth muscle, lung, aorta and heart, and ETB is
present on endothelium, cerebral cortex, cerebellum,
liver, kidney, lung and placenta. In the vasculature, ET-1
is produced primarily by endothelial cells. ET-1 binding
to ETA produces contraction of smooth muscle (EC50
While ETA appears to play a major role in the vaso-
constrictor and mitogenic e€ects of ET-1,1c,10 binding of
the endothelins to ETB is responsible for the vasodila-
tory response to endothelins, via release of nitric
oxide.11 In addition, ETB appears to play a role as a
clearance receptor for the endothelins through the
internalization of the hormone±receptor complex.12
Thus, a case can be made for highly selective ETA
receptor antagonists as therapeutic agents, whereby the
potentially positive e€ects of the ETB receptor are not
a€ected. The early peptidic endothelin receptor antago-
nists exhibited high selectivity for the ETA receptor, as
Key words: Endothelin; ETA-selectivity; pyrrolidine-3-carboxylic acid;
vasodilation; anti-mitogenic.
*Corresponding author. Tel.: 847-937-0423; fax: 847-938-1674.
0968-0896/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.
PII: S0968-0896(99)00022-X

992
S. A. Boyd et al. / Bioorg. Med. Chem. 7 (1999) 991±1002
Figure 1. Structures of the endothelin isoforms.
evidenced by BQ-12313 (818-fold) and FR13931714
(8770-fold). The peptidic ETB antagonist BQ-78815 was
also very selective (1080-fold for ETB), indicating that
high levels of selectivity could be achieved. However,
the subsequently disclosed nonpeptide antagonists were
relatively nonselective. For example, the ETA-selective
oral agents bosentan16 (20-fold), SB21724217 (100-fold),
and L-754,14218 (38-fold) are considerably less selective
than the better peptidic agents. More recently, we19 and
others20 have reported compounds with ETA selectiv-
ities in the 1000-fold range. We desired antagonists with
higher selectivity, thus we set about the task of re-eval-
uating the SAR of our pyrrolidine-3-carboxylic acid
derivative ABT-627.19
decarboxylation.21 Condensation of the ketoesters 2 with
nitrostyrene 3 provided the adducts 4. Upon reduction
of the nitro group, the amino group cyclized onto the
ketone to form the imines 5. The imines were further
reduced to a mixture of diastereomeric pyrrolidine
esters 6. These crude mixtures of diastereomers were
generally isomerized to a mixture of only the cis,trans
and trans,trans pyrrolidines by base-catalyzed equili-
bration with sodium ethoxide. N-Alkylation of the pyr-
rolidines 6 with bromoacetamide 7 provided the
elaborated pyrrolidines 8. Alternatively, the pyrrolidine
esters 8 could be equilibrated at this stage using the
same conditions. In contrast to the 2-aryl compounds,
many of the 2-alkyl diastereomers 8 could be separated
by column chromatography. Basic hydrolysis of the
esters with either NaOH or LiOH a€orded the target
pyrrolidine-3-carboxylic acids. Fortuitously, the cis,-
trans-isomers were not hydrolyzed under these condi-
tions, allowing their removal from the product. Scheme 2
shows the straightforward hydrogenolysis of the benzyl-
oxymethyl-bearing pyrrolidine, to provide 2-hydroxy-
methyl antagonist 16. The 2-unsubstituted compound
was produced by an azomethine ylide reaction of ethyl
cinnamate 40 with silylmethyl aminal 41 according to
the procedure of Cottrell et al. (Scheme 3).22 The
methylbenzyl pyrrolidine 42, obtained as a 1.5:1 mixture
of diastereomers, was hydrogenolyzed to reveal the
unprotected pyrrolidine 6 (R=H) by standard condi-
tions. Pyrrolidine 6 was then alkylated and hydrolyzed
as above to give carboxylic acid 27.
Results and Discussion
Our plan was to investigate the role of the pyrrolidine
C-2 substituent in receptor subtype selectivity. Our
homology-based computer models of the endothelin
receptors indicated di€erences between the receptor
subtypes in this binding region. We had the following
questions: (1) was an aryl group required, (2) was an
ether oxygen required, and (3) could the anity for the
ETB receptor be drastically a€ected by modi®cation of
the group at C-2 of the pyrrolidine?
Synthetic Chemistry
The synthesis of the antagonists followed our previously
reported route,20 in which the pyrrolidine ring is formed
by a nitroalkene-b-ketoester condensation, followed by
a two-step reductive cyclization (Scheme 1). The requi-
site b-ketoesters 2 were formed from the corresponding
carboxylic acids by acylation of their imidazolides
with ethyl magnesium malonate, followed by in situ
In Vitro Structure±Activity Studies
The in vitro receptor binding activities of the antagonists
are listed in Tables 1±3. Table 1 lists data for a series of
para-substituted 2-phenylpyrrolidines. The ether oxygen
atom was not required for potent antagonism of ETA,

S. A. Boyd et al. / Bioorg. Med. Chem. 7 (1999) 991±1002
993
Scheme 1.
since p-tolyl (9) and p-ethylphenyl (11) were equipotent
with p-methoxyphenyl A-127722 (1). A preference for
small p-alkyl groups on the phenyl ring was apparent, as
the larger t-butyl was considerably less potent than
methyl or ethyl. Interestingly, the alkyl groups were
relatively less active against the ETB receptor compared
to 1. The potency data for a series of alkyl ether-bearing
antagonists is listed in Table 2. The cis-cyclohexyl
methylether analogue of A-127722 (15) retains con-
siderable potency, but is an order of magnitude less
potent than the parent. The trans analogue 14 was
relatively inactive, indicating a rather close contact with
the receptor in this binding region. Several ether deri-
vatives of the 2-hydroxymethyl group were investigated
to examine the e€ect of various-sized hydrophobic
moieties. Hydroxymethyl derivative 16 itself was not
very potent, however the addition of a butyl (19),
phenyl (23), and benzyl (24) provided single-digit nano-
molar potencies and a decrease in anity for the ETB
receptor.

994
S. A. Boyd et al. / Bioorg. Med. Chem. 7 (1999) 991±1002
Scheme 2.
Scheme 3.
Table 1. In vitro receptor antagonism for ETA- and ETB-rich tissue preparations: pyrrolidine-C-2 aryl modi®cations
IC50(mM)
Compound
R
ETA
(MMQ cell membranes)
ETB
(porcine cerebellum)
A-127722 (1)
9
10
11
12
13
OCH3
CH3
CF3
Et
Pr
tBu
0.00036
0.00012
0.0033
0.00017
0.0011
0.0347
0.515
2.14
14.3
1.30
1.13
14.4

S. A. Boyd et al. / Bioorg. Med. Chem. 7 (1999) 991±1002
995
Table 2. In vitro receptor antagonism for ETA- and ETB-rich tissue preparations: pyrrolidine-C-2 alkyl ether derivatives
IC50(mM)
Compound
R
ETA
(MMQ cell membranes)
ETB
(porcine cerebellum)
14
15
16
17
18
19
20
21
22
23
24
25
26
trans-4-CH3O-cyclohexyl
cis-4-CH3O-cyclohexyl
CH2OH
0.377
0.0092
0.541
0.0228
0.0169
0.0041
0.0246
0.0878
0.292
>100
12.0
>100
79.5
98.3
CH2OCH2CH3
CH2OPr
CH2OBu
12.0
CH2OCH2CH2OCH3
CH2OtBu
CH2OC(CH3)2CH2CH3
CH2OPh
CH2OCH2Ph
>100
79.4
86.6
18.0
9.9
>100
>100
0.0045
0.0068
0.214
CH2CH2OCH3
CH2CH2OCH2CH3
0.044
Table 3. In vitro receptor antagonism for ETA- and ETB-rich tissue preparations: pyrrolidine-C-2 alkyl derivatives
IC50 (mM)
Compound
R
ETA
(MMQ cell membranes)
ETB
(porcine cerebellum)
27
28
29
30
31
32
33
34
35
36
37
38
39
H
Pr
Bu
C5H11
C6H13
C7H15
iPr
18.6
>100
>100
>100
47.3
18.7
22.9
>100
79.2
0.0646
0.0637
0.0025
0.0056
0.0923
0.135
0.0174
0.0965
0.0098
0.106
CH2CH(CH3)CH2CH3
CH2CH2CH(CH3)2
4-CH3-cyclohexyl
PhCH2
cyclohexyl-CH2
4-CH3-cyclohexyl-CH2
>87
>100
79.0
0.0882
0.0036
83.7
9.19
A series of analogues with simple alkyl chains at the
pyrrolidine C-2 was prepared to further explore the
nature of this hydrophobic pocket, and the data are
shown in Table 3. The unsubstituted analogue 27, with
an IC50 of 18.6 mM, clearly shows the requirement for a
group in this position. Methylcyclohexyl compound 39 is
less potent than the analogous p-tolyl derivative 9, but is
more selective for ETA than lead compound 1. A range
of simple hydrocarbon substituents showed improved
selectivity for ETA. Branching was tolerated at the 2-
position of the chain: the 2-methylbutyl derivative 34
was potent and selective for ETA, but the 3-methylpropyl
derivative 35 had lower anity for both receptor sub-
types. From the group of compounds presented here,
the optimum potency and selectivity was represented by
30, which bears a pentyl chain. Longer straight-chain
aliphatic groups, and the larger benzyl and cyclohexyl-
methyl analogues, led to diminished potency. Com-
pound 30 displays a 2.5 nM antagonism for ETA, and
has an 18,900-fold selectivity versus the ETB receptor.

996
S. A. Boyd et al. / Bioorg. Med. Chem. 7 (1999) 991±1002
Figure 2. Plasma concentrations of 31 after a 5 (iv, n=1) or 10 (po, n=3) mg/kg dose in rat (mean‹SEM).
Preliminary Pharmacokinetic Analysis
Ethyl 4-t-butylbenzoylacetate was prepared by the
method of Krapcho et al.23 Ethyl (3,4-methylenedioxy)
cinnamate was produced from piperonal by the method
of Blanchette et al.24 N-(Trimethylsilylmethyl)-N-meth-
oxymethyl-1-methylbenzylamine was made by the
method of Cottrell et al.22 The endothelin receptor
binding assays and pharmacokinetic assay in rat were
performed as described previously.19
We determined pharmacokinetic properties of one mem-
ber of this C-2 alkyl class of antagonists, n-hexyl derivative
31. Figure 2 shows the drug concentrations following oral
dosing of 10 mg/kg in rats, as determined by HPLC ana-
lysis of the plasma samples taken over time. The antagonist
displayed rapid absorption into the circulation, with an
average half-life of 2.3 h, Cmax of 0.73 mg/mL, and an oral
bioavailability (compared to a 5 mg/kg iv dose) of 38.9%.
(‹)-Ethyl 3-methylhexanoate. To a slurry of 60%
sodium hydride (2.26 g, 57 mmol) in 10 mL of hexane
and 100 mL of diethyl ether was added triethylphos-
phonoacetate (10.3 mL, 52 mmol). Once gas evolution
ceased, 2-pentanone (6.0 mL, 64 mmol) was added.
After 3 h at room temperature, the reaction was quen-
ched with water and partitioned into ether. The organic
layer was washed with water and brine, dried with
anhydrous sodium sulfate, ®ltered, and the solvent was
removed under reduced pressure. The residue was dis-
solved in 50 mL of ethanol and 10% palladium on car-
bon (6.0 g) was added. The vessel was pressurized to
4 atm of hydrogen and was shaken at room temperature
for 3 h. The reaction was ®ltered and the solvent was
removed under reduced pressure to give 3.0 g of the title
compound, isolated as a 3.6:1 mixture of geometrical
Conclusion
We have developed a highly selective ETA receptor
antagonist, based on our pyrrolidine-3-carboxylic acid
template. Aliphatic groups at the 2-position of the
pyrrolidine were required for this high level of selectiv-
ity, with the best examples (30, 31, and 36) having
>10,000-fold selectivity for ETA. Introducing such
lipophilic groups did not have a large negative impact
on oral bioavailability, as evidenced by the 39 F% for
compound 31. The further extension of compounds
with alternative functionality at the 2-position, as well
as the e€ect of high ETA-selectivity on the pharmacol-
ogy of these agents, is currently under investigation and
will be reported in the near future.
1
isomers. Major isomer: H NMR (CDCl3, 300 MHz) d
5.66 (m, 1H), 4.20±4.08 (m, 2H), 2.14 (d, J=1.2 Hz,
3H), 2.12 (d, J=6.9 Hz, 1H), 2.10 (dd, J=7.8, 1.2 Hz,
1H), 1.59±1.43 (m, 2H), 1.31±1.24 (m, 3H), 0.98±0.88
(m, 3H). Minor isomer (partial data): 1H NMR (CDCl3,
300 MHz) d 2.63±2.57 (m, 2 H), 1.87 (d, J=1.5 Hz, 3H).
Experimental
General
Methyl 3-oxo-6-octenoate and ethyl butyrylacetate were
purchased commercially and used without puri®cation.
(‹)-Ethyl 5-methyl-3-oxooctanoate (2, R=CH2CH(CH3)-
CH2CH2CH3). To a solution of ethyl 3-methylhexan-

S. A. Boyd et al. / Bioorg. Med. Chem. 7 (1999) 991±1002
997
oate in 150 mL of ethanol was added sodium hydroxide
(2.3 g, 57.6 mmol). After 48 h at room temperature, the
solvent was removed under reduced pressure, and the
residue was dissolved in 150 mL of water. The solution
was extracted with ether, then the aqueous phase was
acidi®ed with concentrated hydrochloric acid and
extracted with dichloromethane. The organic layer was
dried with anhydrous magnesium sulfate, ®ltered, and
the solvent was removed under reduced pressure to give
3.74 (s, 1.5H), 3.97 (d, J=7.5 Hz, 0.5H) and 4.05 (d,
J=8 Hz, 0.5H), 4.10±4.20 (m, 1H), 4.68±4.82 (m, 2H),
5.06±5.52 (m, 2H), 5.95 (2 s, 2H), 6.65 (m, 1H), 6.68 (br
s, 1H), 6.75 (d, J=7.5 Hz, 1H; MS (DCI/NH3) m/z 381
(M+NH4)+. Anal. calcd for C18H21NO7: C, 59.50; H,
5.82; N, 3.85. Found: C, 59.32; H, 5.71; N, 3.72.
trans,trans-Methyl 2-pentyl-4-(1,3-benzodioxol-5-yl)-pyr-
rolidine-3-carboxylate (8, R=pentyl). Part A. Nitro
ketone 4 (R=3-pentenyl) (849 mg, 2.34 mmol) was dis-
solved in 100 mL EtOAc, then Raney nickel (2.55 g,
washed 3Â with EtOH) was added. The mixture was
shaken under 4 atm H2 for 21 h, then ®ltered and con-
centrated in vacuo to provide the crude pyrroline 5
(R=pentyl) as a pale red oil (702 mg, 2.21 mmol, 95%
crude): TLC (EtOAc:hexane, 1:3) Rf 0.34; MS (DCI/
NH3) m/z 318 (M+H)+.
1
2.7 g of (‹) 3-methylhexanoic acid: H NMR (CDCl3,
300 MHz) d 2.35 (dd, J=15, 6 Hz, 1H), 2.14 (dd, J=15,
8.1 Hz, 1H), 2.04±1.91 (m, 1H), 1.42±1.16 (m, 4H), 0.97
(d, J=6.6 Hz, 3H), 0.90 (t, J=6.9 Hz, 3H).
(‹) 3-Methylhexanoic acid was treated according to the
method of Bram and Vilkas25 to provide 3.9 g of the
1
title ketoester: H NMR (300 MHz, CDCl3) d 4.19 (q,
J=7.2 Hz, 2H), 3.41 (s, 2H), 2.52 (dd, J=16.8, 6 Hz,
1H), 2.33 (dd, J=16.2, 7.8 Hz, 1H), 2.10±1.91 (m, 1H),
1.38±1.12 (m, 4H), 1.28 (t, J=7.2 Hz, 3H), 0.92±0.86
(m, 6H). Enol form is present to the extent of 7% in
CDCl3. Enol form (partial data): H NMR (300 MHz,
CDCl3) d 12.09 (s, 1H), 4.96 (s, 1H).
Part B. Crude pyrroline
5 (R=pentyl) (692 mg,
2.18 mmol) was dissolved in 4 mL EtOH and 4 mL
THF, then 1 mg of bromocresol green indicator and
sodium cyanoborohydride (154 mg, 2.45 mmol) were
added. The blue mixture was rapidly stirred and treated
dropwise with a solution of concentrated aqueous hydro-
chloric acid in EtOH (1:2 v/v), maintaining a yellow-
green color. Once the color stayed yellow, the reaction
was stirred an additional 15 min, at which time TLC
(2% MeOH±CH2Cl2) indicated complete conversion to
a low Rf spot. The mixture was made acidic with two
drops of ethanolic hydrochloric acid, then it was con-
centrated in vacuo. Water (20 mL) and CH2Cl2 (20 mL)
were added to the resulting residue, and the aqueous
phase was adjusted to pH 10 with 1 M aqueous
Na2CO3. The layers were shaken and separated, and the
aqueous phase was extracted with 2Â15 mL CH2Cl2.
The combined organic extract was washed with 25 mL
brine, dried (Na2SO4), ®ltered, rotoevaporated and the
residue placed under high vacuum to provide 665 mg
(95% crude) of the crude pyrrolidine ester 6 (pale yellow
oil) as a mixture of diastereomers. This mixture of iso-
mers (including the cis,cis isomer) was epimerized to a
mixture of trans,trans and cis,trans isomers under the
following conditions. A solution of the crude compound
(660 mg, 2.07 mmol) in 3 mL methanol was treated with
a solution of sodium methoxide (made from the addi-
tion of sodium metal (14 mg, 0.61 mmol) to 1 mL
methanol). The resultant solution was heated at re¯ux
for 18 h. The reaction was concentrated under reduced
pressure, and the residue was partitioned between
25 mL saturated NaHCO3 diluted with 10 mL water,
and 30 mL CH2Cl2. The aqueous phase was extracted
(2Â30 mL CH2Cl2), then the combined organic phases
were washed with 20 mL brine, dried (Na2SO4), ®ltered
and the ®ltrate concentrated under reduced pressure to
a€ord the crude product. Puri®cation by ¯ash chroma-
tography (3.5% methanol±CH2Cl2) gave 336 mg (57%)
of pure trans,trans-pyrrolidine ester 6 (R=pentyl) as a
1
Ethyl 4-benzyloxy-3-oxobutyrate (2, R=CH2OBn). A
solution of EtOAc (1.28 mL, 13.1 mmol, distilled from
CaH2 under Ar) in 10 mL THF was added dropwise
over 5 min to a 75 ꢀC solution of lithium bis(tri-
methylsilyl)amide (13.0 mL, 1.0 M in THF, 13.0 mmol),
maintaining internal temperature < 50 ꢀC. The solu-
tion was stirred at 70±75 ꢀC for 30 min, then a solution
of N-methyl-N-methoxy benzyloxyacetamide (2.48 g,
11.9 mmol) in 10 mL THF was added dropwise via syr-
inge over 5 min, maintaining internal temperature
< 65 ꢀC. The reaction was stirred at 78 ꢀC for 3 h,
then it was transferred via cannula to a 0 ꢀC mixture of
30 mL of 10% aqueous citric acid and 100 mL EtOAc.
The mixture was stirred at 0 ꢀC for 5 min, then the mix-
ture was separated, and the aqueous phase was extrac-
ted with 2Â25 mL EtOAc. The combined organic phase
was washed with 50 mL brine, dried (Na2SO4), ®ltered
and concentrated in vacuo to a yellow oil. Vacuum dis-
tillation (bp 105±112 ꢀC, 0.005 mmHg) produced 1.91 g
(8.07 mmol, 68%) of the ketoester as a colorless liquid:
1H NMR (CDCl3, 300 MHz) d 1.25 (t, J=7 Hz, 3H),
3.54 (s, 2H), 4.14 (s, 2H), 4.17 (q, J=7 Hz, 2H), 4.60 (s,
2H), 7.28±7.42 (m, 5H); enol forms also present: 1.30 (t,
J=7 Hz), 3.64 (s), 4.07 (m), 4.19 (q), 4.68 (s), 5.36 (t,
J=1 Hz), 11.99 (s); MS (DCI/NH3) m/z 254 (M+H)+.
Methyl 2-(4-hexenoyl)-4-nitro-3-(1,3-benzodioxole-5-yl)-
butyrate (4, R=3-pentenyl). A solution of methyl 3-
oxo-6-octenoate (502 mg, 2.95 mmol) in 10 mL iso-
propanol was added to a solution of 5-(2-nitrovinyl)-
1,3-benzodioxole (712 mg, 3.69 mmol) in 10 mL THF,
then DBu (22 mL, 0.15 mmol) was added. The resulting
reddish solution was stirred at room temperature for
20 min. The solution was concentrated in vacuo and
¯ash chromatographed (18% ethyl acetate±hexane), to
produce 879 mg (2.42 mmol, 82%) of ester as a mixture
of diastereomers in a 1:1 ratio. 1H NMR (CDCl3,
300 MHz) d 1.55±1.66 (m, 3H), 2.02±2.17 (br m, 1H),
2.20±2.37 (m, 1.5H), 2.49±2.76 (m, 1.5H), 3.57 (s, 1.5H),
1
yellow oil: H NMR (CDCl3, 300 MHz) d 0.90 (br t,
3H), 1.25±1.70 (br m, 8H), 1.83±2.02 (br s, 2H), 2.58
(dd, J=8, 9 Hz, 1H), 2.99 (dd, J=8, 14 Hz, 1H), 3.34±
3.45 (m, 2H), 3.53 (q, J=9 Hz, 1H), 3.66 (s, 3H), 5.94 (s,
2H), 6.65±6.75 (m, 3H); MS (DCI/NH3) m/z 320
(M+H)+. Anal. (C18H25NO4) C, H, N.

998
S. A. Boyd et al. / Bioorg. Med. Chem. 7 (1999) 991±1002
Part C. Pyrrolidine ester 6 (R=pentyl) (42.4 mg,
0.133 mmol) and N,N-dibutyl bromoacetamide (35.5 mg,
0.142 mmol) were combined in 0.5 mL CH3CN, then
ethyldiisopropylamine (46 mL, 0.26 mmol) was added.
The reaction was stirred at room temperature for 20 h,
then an additional portion of N,N-dibutyl bromoaceta-
mide20 (5 mg, 0.02 mmol) was added and the reaction
allowed to proceed an additional 30 h. The reaction was
partitioned between 20 mL of 5% aqueous citric acid and
25 mL EtOAc. The aqueous phase was extracted with
20 mL EtOAc, and the combined organic phase was
washed with 15 mL brine, dried (Na2SO4), ®ltered,
rotoevaporated and the residue placed under high
vacuum to provide the crude product as a foam. The
crude was puri®ed by column chromatography on silica
gel (1% MeOH±CH2Cl2) to give 53.4 mg (0.109 mmol,
82%) of acetamidopyrrolidine ester 8 as a yellow oil:
trans,trans-Ethyl 2-(hydroxymethyl)-4-(1,3-benzodioxol-
5-yl)-1-[[(dibutylamino)carbonyl]methyl]pyrrolidine-3-carb-
oxylate (8, R=CH2OH). Ester
8 (R=CH2OBn)
(128 mg, 0.232 mmol) and 25 mg of 20% Pd(OH)2 on
charcoal in 7 mL EtOH was stirred under 1 atm hydro-
gen for 48 h. The mixture was ®ltered through a plug of
Celite, and the catalyst was washed with 2Â10 mL
EtOH, then the combined ®ltrate and washes were con-
centrated under reduced pressure to a€ord 107 mg of
the crude alcohol. Puri®cation by ¯ash chromatography
(30% EtOAc±hexane) provided 13 mg (12%) of pyrro-
lidine ester 8 (R=CH2OH) as pale-yellow wax: TLC
(30% EtOAc±hexane) Rf 0.23; 1H NMR (CDCl3,
300 MHz) d 0.92 (t, J=7 Hz) and 0.97 (t, J=7 Hz, 6H
total), 1.21 (t, J=8 Hz, 3H), 1.20±1.43 (br m, 4H), 1.44±
1.6 (br m, 4H), 1.6±2.5 (vbr s, 1H plus H2O), 3.00±3.09
(m, 1H), 3.10±3.40 (m, 6H), 3.52 (AB, 2H), 3.56±3.67
(m, 3H), 4.13 (q, J=8 Hz, 2H), 5.92 (m, 2H), 6.71 (d,
J=8 Hz, 1H), 6.80 (dd, J=2, 8 Hz, 1H), 6.89 (d,
J=2 Hz, 1H); MS (DCI/NH3) m/z 463 (M+H)+.
1
TLC (1% MeOH±CH2Cl2) Rf 0.15; H NMR (CDCl3,
300 MHz) d 0.84±0.94 (m, 6H), 0.97 (t, J=7 Hz, 3H),
1.20±1.40 (br m, 8H), 1.40±1.80 (br m, 8H), 2.70 (dd,
J=6.5, 9 Hz, 1H), 2.85±3.00 (m, 3H), 3.04±3.39 (m, 4H),
3.40±3.67 (m, 3H), 3.68 (s, 3H), 5.92 (q, J=1.5 Hz, 2H),
6.70 (d, J=8 Hz, 1H), 6.74 (dd, J=1, 8 Hz, 1H), 6.87 (d,
J=1 Hz, 1H); MS (DCI/NH3) m/z 489 (M+H)+.
(10S,3R*,4S*)-Ethyl 1-(2-methylbenzyl)-4-(1,3-benzodi-
oxol-5-yl)pyrrolidine-3-carboxylate (41). N-(Trimethylsi-
lylmethyl)-N-methoxymethyl-1-methylbenzylamine 40
(638 mg, 2.54 mmol) and ethyl (3,4-methylenedioxy)-
cinnamate (399 mg, 1.94 mmol) in 3 mL CH2Cl2 at
0 ꢀC was treated with tri¯uoroacetic acid (0.15 mL,
1.94 mmol). The reaction was stirred for 3 h at 0 ꢀC,
then an additional portion of silyl aminal 40 (107 mg,
0.426 mmol) was added, and the reaction stirred at
ambient temperature for 16 h. TLC (10% EtOAc±hex-
ane) indicated incomplete reaction. In an e€ort to
induce a more complete reaction, a third portion of
aminal 40 (115 mg, 0.457 mmol) and tri¯uoroacetic acid
(0.03 mL, 0.39 mmol) were added to the reaction at
room temperature, and the solution was stirred for 5 h.
TLC (10% EtOAc±hexane) indicated nearly complete
consumption of the cinnamate. The reaction was
partitioned between 15 mL CH2Cl2 and 20 mL of M aq
Na2CO3, and the aqueous phase was extracted with
2Â15 mL CH2Cl2. The combined organic extract was
washed with 20 mL brine, dried (Na2SO4), ®ltered
and concentrated under reduced pressure to provide a
yellow oil (936 mg). The crude product was puri®ed by
¯ash chromatography on silica gel (10% EtOAc±
hexane) to a€ord 353 mg (0.959 mmol, 49%) of the
mixture of title compounds as a yellow oil which solidi-
trans,trans-2-Pentyl-4-(1,3-benzodioxol-5-yl)-1-[[(dibutyl-
amino)carbonyl]methyl]pyrrolidine-3-carboxylic acid (30).
Ester 8 (R=pentyl) (50.0 mg, 0.102 mmol) was dissolved
in 0.6 mL EtOH, and a solution of lithium hydroxide
(9 mg, 0.22 mmol) in 3 mL water was added. The reaction
was sealed and stirred at room temperature for 16 h. The
mixture was concentrated in vacuo and partitioned
between 5 mL CH2Cl2 and 3 mL of 10% aqueous citric
acid. The organic extract was washed with 2 mL brine,
dried (Na2SO4), ®ltered, rotoevaporated and the residue
placed under high vacuum to provide the crude acid as a
yellow foam (46.3 mg, 96% crude yield). Puri®cation by
¯ash chromatography on silica gel (3.5% MeOH/0.5%
HOAc/CH2Cl2) produced 29.6 mg (0.062 mmol, 61%) of
pyrrolidine carboxylic acid: 1H NMR (CDCl3, 300 MHz)
d 0.87 (br t) and 0.89 (br t, 6H total), 0.97 (t, J=7.5 Hz,
3H), 1.21±1.42 (br m, 10), 1.43±1.78 (br m, 6H), 2.76 (t,
J=7 Hz, 1H), 3.02±3.30 (br m, 6H), 3.40±3.60 (m, 3H),
3.73 (d, J=14 Hz, 1H), 5.98 (AB, 2H), 6.70 (d, J=7 Hz,
1H), 6.77 (dd, J=1.5, 7 Hz, 1H), 6.89 (d, J=1.5 Hz, 1H);
MS
(C27H42N2O5 0.5H2O) C, H, N.
(DCI/NH3)
.
m/z
475
(M+H)+.
Anal.
1
®ed upon standing: H NMR (CDCl3, 300 MHz) d 1.19
trans,trans-Ethyl 2-(benzyloxymethyl)-4-(1,3-benzodioxol-
5-yl)-1-[[(dibutylamino)carbonyl]methyl]pyrrolidine-3-carb-
oxylate (8, R=CH2OBn). The procedure described
above for 4 and 8 (R=pentyl) was followed, with the
substitution of ethyl 4-benzyloxy-3-oxobutyrate for 4-
methoxybenzoylacetate, to a€ord the title compound as
(t, J=7 Hz) and 1.21 (t, J=7 Hz, 3H total), 1.37
(d, J=7 Hz) and 1.40 (d, J=7 Hz, 3H total), 1.44±1.60
(m, 4H), 2.56 (dd, J=6, 9 Hz, 0.5H), 2.68±2.79 (br m,
1H), 2.80±2.87 (m, 1H), 2.88±3.04 (br m, 2H), 3.12 (t,
J=8 Hz, 0.5H), 3.54 (br q, J=7 Hz, 1H), 4.03 (m, 2H),
5.92 (s) and 5.93 (s, 2H total), 6.68 (m) and 6.77 (dd,
J=2, 7 Hz, 2H total), 6.85 (d, J=1 Hz) and 6.92 (d,
J=2 Hz, 1H), 7.19±7.40 (br m, 5H); MS (DCI/NH3) m/
z 368 (M+H)+. Anal. (C21H23NO4) C, H, N.
1
a colorless oil: TLC (30% EtOAc±hexane) Rf 0.18; H
NMR (CDCl3, 300 MHz) d 0.88 (t, J=7 Hz, 6H), 1.17
(t, J=7Hz, 3H), 1.20±1.34 (br m, 4H), 1.40±1.56 (br m,
3H), 2.85 (t, J=8 Hz, 1H), 2.98±3.30 (m, 5H), 3.39±3.60
(m, 3H), 3.64±3.75 (m, 2H), 3.92 (d, J=14 Hz, 1H), 4.10
(two overlapping q, J=6.5 Hz, 2H), 4.53 (s, 2H), 5.91
(m, 2H), 6.69 (d, J=9 Hz, 1H), 6.77 (dd, J=1.5, 9 Hz,
1H), 6.91 (d, J=1.5 Hz, 1H); MS (DCI/NH3) m/z 553
(M+H)+.
trans-Ethyl 4-(1,3-benzodioxol-5-yl)pyrrolidine-3-carboxyl-
ate (8, R=H). A solution of benzyl pyrrolidine 41
(235 mg, 0.61 mmol) in 5 mL MeOH was treated with
20% palladium on carbon (65 mg) and stirred under
1 atm H2 (balloon) for 4 h, then the reaction was ®ltered

S. A. Boyd et al. / Bioorg. Med. Chem. 7 (1999) 991±1002
999
through Celite. The catalyst was washed with 50 mL
MeOH, and the combined ®ltrate was concentrated
under reduced pressure to provide 189 mg (0.641 mmol,
100%) of the product as a colorless oil: 1H NMR
(CDCl3, 300 MHz) d 1.24 (t, J=8 Hz, 3H), 1.89 (br s,
1H plus H2O), 2.80±3.05 (m, 2H), 3.25±3.37 (m, 2H),
3.37±3.50 (br m, 2H), 4.14 (q, J=7 Hz, 2H), 5.94 (s,
2H), 6.70 (dd, J=2, 9 Hz, 1H), 6.72±6.79 (m, 2H); MS
(DCI/NH3) m/z 264 (M+H)+.
(t, J=7 Hz, 3H), 0.86 (t, J=7 Hz, 3H), 100 (m, 2H),
1.29 (s, 13H), 1.45 (m, 2H), 2.73 (d, J=14 Hz, 1H),
3.15±2.85 (m, 4H), 3.65±3.25 (m, 5H), 3.77 (d, J=14 Hz,
1H), 5.92 (d, J=4 Hz, 1H), 5.94 (d, J=4 Hz, 1H), 6.74
(d, J=9 Hz, 1H), 6.87 (dd, J=3, 8 Hz, 1H), 7.04 (d,
J=2 Hz, 1H), 7.32 (d, J=3 Hz, 4H); MS (DCI/NH3)
m/z 537 (M+H)+. Anal. (C32H44N2O5) C, H, N.
trans,trans-2-(trans-4-Methoxycyclohexyl)-4-(1,3-benzo-
dioxol-5-yl)-1-[[(dibutylamino)carbonyl]methyl]pyrrolidine-
1
The following compounds were made in analogy to the
procedure described for compound 8.
3-carboxylate (14). H NMR (CDCl3, 300 MHz,) d 0.92
(t, J=7 Hz, 3H), 0.97 (t, J=7 Hz, 3H), 1.33 (m, 9H),
1.62 (m, 7H), 1.95 (m, 2H), 2.88 (m, 1H), 3.00±3.25 (m,
3H), 3.26 (s, 3H), 3.27±3.50 (m, 5H), 3.69 (m, 2H), 5.92
(s, 2H), 6.61 (d, J=8 Hz, 1H), 6.76 (dd, J=7, 2 Hz, 1H),
trans,trans-2-(4-Methylphenyl)-4-(1,3-benzodioxol-5-yl)-
1-[[(dibutylamino)carbonyl]methyl]pyrrolidine-3-carboxyl-
ate (9). 1H NMR (CD3OD, 100 MHz) d 0.80 (t,
J=7 Hz, 3H), 0.88 (t, J=7 Hz, 3H), 1.05 (hex, J=7 Hz,
2H), 1.27 (hex, J=7 Hz, 2H), 1.3±1.5 (m, 4H), 2.33 (s,
3H), 2.9±3.2 (m, 5H), 3.3±3.5 (m, 3H), 3.54 (d,
J=15 Hz, 1H), 3.63 (ddd, J=5, 8, 9 Hz, 1H), 3.90 (d,
J=10 Hz, 1H), 5.92 (m, 2H), 6.75 (d, J=8 Hz, 1H), 6.86
(dd, J=2, 8 Hz, 1H), 7.04 (d, J=2 Hz, 1H), 7.19 (d,
6.84 (d, J=2 Hz, 1H); MS (DCI/NH3) m/z 517
+
.
(M+H) . Anal. (C29H44N2O6 0.50H2O) C, H, N.
trans,trans-2-(cis-4-Methoxycyclohexyl)-4-(1,3-benzodi-
oxol-5-yl)-1-[[(dibutylamino)carbonyl]methyl]pyrrolidine-
1
3-carboxylate (15). H NMR (CDCl3, 300 MHz) d 0.91
(t, J=7 Hz, 3H), 0.96 (t, J=7 Hz, 3H), 1.00±1.20 (m,
4H), 1.33 (m, 4H), 1.52 (m, 6H), 1.83 (m, 2H), 2.10 (m,
2H), 2.82 (m, 1H), 3.00±3.30 (m, 6H), 3.32 (s, 3H), 3.42
(m, 2H), 3.65 (m, 2H), 5.92 (s, 2H), 6.61 (d, J=8 Hz,
J=8 Hz, 2H), 7.32 (d, J=8 Hz, 2H). MS (APCI+) m/z
+
.
495 (M+H) . Anal. (C29H38N2O5 0.3 H2O) C, H, N.
trans,trans-2-[4-(Tri¯uromethyl)phenyl]-4-(1,3-benzodioxol-
5-yl)-1-[[(dibutylamino)carbonyl]methyl]pyrrolidine-3-carb-
oxylate (10). H (CD3OD, 100 MHz) d 0.87 (t, J=7 Hz,
1H), 6.77 (dd, J=2, Hz, 1H), 6.84 (d, J=2 Hz, 1H); MS
+
.
(DCI/NH3) m/z 517 (M+H) . Anal. (C29H44N2O6 0.30
H2O) C, H, N.
1
3H), 0.98 (t, J=7 Hz, 3H), 1.02 (hex, J=7 Hz, 2H), 1.27
(hex, J=7 Hz, 2H), 1.3±1.5 (m, 4H), 2.9±3.2 (m, 5H),
3.3±3.5 (m, 4H), 3.65 (ddd, J=5, 8, 9 Hz, 1H), 3.94 (d,
J=10 Hz, 1H), 5.93 (m, 2H), 6.76 (d, J=8 Hz, 1H), 6.87
(dd, J=2, 8 Hz, 1H), 7.05 (d, J=2 Hz, 1H), 7.66 (m,
4H). MS (APCI) m/z 549 (M+H)+. Anal.
(C29H35N2O5F3) C, H, N.
trans,trans-2-(Hydroxymethyl)-4-(1,3-benzodioxol-5-yl)-1-
[[(dibutylamino)carbonyl]methyl]pyrrolidine-3-carboxylate
(16). TLC (10% MeOH±CH2Cl2) Rf 0.20; 1H NMR
(CDCl3, 300 MHz, two rotomeric forms) d 0.73 (t,
J=7 Hz) and 0.86±1.02 (m, 6H total), 1.10±1.70 (br m,
8 H), 2.50±4.00 (several br m, 13H plus H2O), 4.07±4.14
(br m) and 4.59 (br d, 1H total), 5.89 (s) and 5.91 (s, 2H
total), 6.60±6.75 (br m), 6.74±6.83 (m) and 6.90 (br s,
3H total); HRMS (FAB) calcd for C23H35N2O6
(M+H)+ 435.2495, found 435.2491.
trans,trans-2-(4-Ethylphenyl)-4-(1,3-benzodioxol-5-yl)-1-
[[(dibutylamino)carbonyl]methyl]pyrrolidine-3-carboxylate
(11). 1H (CDCl3, 300 MHz) d 0.78 (t, J=7 Hz, 3H), 0.87
(t, J=7 Hz, 3H), 1.02 (m, 2H), 1.10±1.40 (m, 6 H), 1.45
(m, 2H), 2.26 (q, J=9 Hz, 2H), 2.73 (d, J=14 Hz, 1H),
2.90±3.13 (m, 4H), 3.23±3.53 (m, 5H), 3.60 (m, 1H), 3.77
(d, J=9 Hz, 1H), 5.92 (d, J=4 Hz, 1H), 5.94 (d,
J=4 Hz, 1H), 6.73 (d, J=9 Hz, 1H), 6.86 (dd, J=3,
8 Hz, 1H), 7.03 (d, J=3 Hz, 1H), 7.16 (d, J=8 Hz, 2H),
7.31 (d, J=8 Hz, 2H); MS (DCI/NH3) m/z 509
(M+H)+. Anal. (C30H40N2O5) C, H, N.
trans,trans-2-(Ethoxymethyl)-4-(1,3-benzodioxol-5-yl)-1-
[[(dibutylamino)carbonyl]methyl]pyrrolidine-3-carboxylate
(17). Isolated as a colorless glass; TLC (10% MeOH±
CH2Cl2) Rf 0.53; 1H NMR (CDCl3, 300 MHz, roto-
meric forms) d 0.70 (t, J=7 Hz), 0.80 (t, J=7 Hz) and
0.96±1.04 (m, 6 H total), 1.04±1.75 (m, 11H), 1.34±1.53
(br m, 4H), 2.65 (AB) and 2.80±3.08 (m, 2H total),
3.10±3.82 (br m, 12H), 4.03 (m) and 4.22±4.45 (br m,
2H total), 5.90 (s) and 5.91 (s, 2H total), 6.65±6.84
(m) and 6.93 (m) and 6.99 (m, 3H total); MS (FAB)
trans,trans-2-(4-propylphenyl)-4-(1,3-benzodioxol-5-yl)-1-
[[(dibutylamino)carbonyl]methyl]pyrrolidine-3-carboxylate
(12). 1H (CDCl3, 300 MHz,) d 0.78 (t, J=7 Hz, 3H),
0.85 (t, J=7 Hz, 3H), 0.92 (t, J=7 Hz, 3H), 1.00±1.50 (m,
8H), 1.62 (q, J=8 Hz, 2H), 2.55 (t, J=8 Hz, 2H), 2.75 (d,
J=15 Hz, 1H), 2.88±3.13 (m, 4H), 3.20±3.55 (m, 4H),
3.60 (m, 1H), 3.76 (d, J=10 Hz, 1H), 5.93 (d, J=2 Hz,
1H), 5.95 (d, J=2 Hz, 1H), 6.73 (d, J=9 Hz, 1H), 6.84
(dd, J=6, 2 Hz, 1H), 7.03 (d, J=2, 1H), 7.13 (d, J=9 Hz,
+
.
m/z 463 (M+H) . Anal. (C25H38N2O6 1.5H2O) C,
H, N.
trans,trans-2-Propoxymethyl-4-(1,3-benzodioxol-5-yl)-1-
[[(dibutylamino)carbonyl]methyl]pyrrolidine-3-carboxylate
1
(18). H NMR (CDCl3, 300 MHz) d 0.87±0.98 (m, 9H),
1.21±1.39 (m, 4H), 1.43±1.57 (m, 4H), 1.58±1.70 (m,
2H), 3.13±3.29 (m, 4H), 3.34±3.43 (m, 3H), 3.45±3.55
(m, 3H), 3.69 (dd, J=10.2, 4.5 Hz, 1H), 3.80±4.20 (m,
4H), 5.93 (s, 2H), 6.73 (d, J=7.8 Hz, 1H), 6.84 (dd,
2H), 7.31 (d, J=9 Hz, 2H); MS (DCI/NH3) m/z 523
+
.
(M+H) . Anal (C31H42N2O5 0.25H2O) C, H, N.
trans,trans-2-[4-(1,1-dimethylethyl)phenyl]-4-(1,3-benzodi-
oxol-5-yl)-1-[[(dibutylamino)carbonyl]methyl]pyrrolidine-
3-carboxylate (13). H NMR (CDCl3, 300 MHz) d 0.76
J=8.2, 1.7 Hz, 1H), 6.93 (d, J=1.7 Hz, 1H); MS (DCI/
+
.
NH3) m/z 477 (M+H) . Anal. (C26H40N2O6 0.50TFA)
C, H, N.
1

1000
S. A. Boyd et al. / Bioorg. Med. Chem. 7 (1999) 991±1002
trans,trans-2-(Butoxymethyl)-4-(1,3-benzodioxol-5-yl)-1-
[[(dibutylamino)carbonyl]methyl]pyrrolidine-3-carboxylate
(19). H (CDCl3, 300 MHz,) d 0.87±0.98 (m, 9H), 1.23±
1.42 (m, 6H), 1.42±1.66 (m, 6H), 3.12±3.60 (m, 10H),
3.81±4.07 (m, 4H), 4.54 (br s, 1H), 4.70 (br s, 1H), 5.94
(s, 2H), 6.75 (d, J=8.1 Hz, 1H), 6.86 (dd, J=8.1,
trans,trans-2-(Methoxyethyl)-4-(1,3-benzodioxol-5-yl)-1-
[[(dibutylamino)carbonyl]methyl]pyrrolidine-3-carboxylate
(25). TLC (2.5%MeOH-0.5%HOAc-CH2Cl2) Rf 0.25;
1H NMR (CDCl3, 300 MHz) d 0.91 (t, J=7 Hz) and
0.95 (t, J=7 Hz, 6H total), 1.28±1.41 (br m, 4H), 1.45±
1.63 (br m, 4H), 2.00±2.20 (br m, 2H), 3.06 (br t,
J=9 Hz, 1H), 3.30 (s) and 3.20±3.68 (br m, 11H total),
3.72±4.10 (br m, 4H), 5.92 (s, 2H), 6.72 (d, J=8.5 Hz,
1H), 6.82 (dd, J=1.5, 8.5 Hz, 1H), 6.91 (d, J=1.5 Hz,
1H); MS (FAB) m/z 463 (M+H)+. Anal.
1
1.8 Hz, 1H), 6.91 (d, J=1.8 Hz, 1H); MS (DCI/NH3)
+
.
m/z 491 (M+H) . Anal. (C27H42N2O6 1.15H2O 0.80
.
TFA) C, H, N.
.
trans,trans-2-(Methoxyethoxymethyl)-4-(1,3-benzodioxol-
5-yl)-1-[[(dibutylamino)carbonyl]methyl]pyrrolidine-3-carb-
(C25H38N2O5 H2O) C, H, N.
1
oxylate (20). H (CDCl3, 300 MHz,) d 0.90 (t, J=7 Hz,
trans,trans-2-(2-Ethoxyethyl)-4-(1,3-benzodioxol-5-yl)-1-
[[(dibutylamino)carbonyl]methyl]pyrrolidine-3-carboxylate
3H), 0.96 (t, J=7 Hz, 3H), 1.22±1.42 (m, 4H), 1.44±1.63
(br m, 4H), 2.88 (overlapping dd, J=6.5, 7 Hz, 1H),
3.08 (dd, J=8, 9 Hz, 1H), 3.15±3.50 (br m) and 3.35 (s,
10H total), 3.50±3.56 (m, 2H), 3.57±3.68 (m, 3H), 3.69±
3.84 (m, 3H), 5.91 (AB, 2H), 6.70 (d, J=8 Hz, 1H), 6.81
(dd, J=2, 8 Hz, 1H), 6.94 (d, J=2 Hz, 1H). Anal.
(C26H40N2O7) C, H, N.
1
(26). H NMR (CDCl3, 300 MHz) d 0.91 (t, J=7.4 Hz,
3H), 0.94 (t, J=7.4 Hz, 3H), 1.19 (t, J=7.0 Hz, 3H),
1.24±1.38 (m, 5H), 1.46±1.60 (m, 4H), 2.03±2.12 (m,
2H), 3.07 (t, J=8.0 Hz, 1H), 3.07±3.34 (m, 6H),
3.43±3.52 (m, 3H), 3.59±3.74 (m, 3H), 3.80±4.01 (m,
2H), 5.93 (s, 2H), 6.72 (d, J=8.1 Hz, 1H), 6.79 (dd,
J=1.7, 8.2 Hz, 1H), 6.87 (d, J=1.7 Hz, 1H); MS
(DCI/NH3) m/z 477 (M+H)+. Anal. (C26H40N2
trans,trans-2-(tert-Butoxymethyl)-4-(1,3-benzodioxol-5-yl)-1-
[[(dibutylamino)carbonyl]methyl]pyrrolidine-3-carboxylate
.
O6 0.4TFA) C, H, N.
1
(21). H NMR (300 MHz, CDCl3) d 0.92 (q, J=7.2 Hz,
6H), 1.20±1.35 (m, 4H), 1.28 (s, 9H), 1.44±1.60 (m, 4H),
3.13±3.60 (m, 6H), 3.76±4.07 (m, 4H), 4.50±4.74 (m,
4H), 5.95 (s, 2H), 6.77 (d, J=8.1 Hz, 1H), 6.88 (dd,
trans,trans-4-(1,3-Benzodioxol-5-yl)-1-[[(dibutylamino)-
carbonyl]methyl]pyrrolidine-3-carboxylate (27). 1H (CDCl3,
300 MHz,) d 0.91 (t, J=7 Hz, 3H), 0.96 (t, J=8 Hz, 3H),
1.22±1.40 (m, 4H), 1.43±1.60 (m, 4H), 2.96±3.05 (m, 1H),
3.09±3.40 (br m, 6H), 3,54±3.69 (m, 2H), 3.70 (d,
J=2 Hz) and 3.73 (d, J=4 Hz, 2H total), 3.76±3.87 (m,
1H), 5.92 (s, 2H), 6.0±6.6 (v br s, 1H plus H2O), 6.72 (d,
J=8 Hz, 1H), 6.79 (dd, J=1, 8 Hz, 1H), 6.85 (d,
J=1 Hz, 1H); MS (DCI/NH3) m/z 405 (M+H)+. Anal.
J=8.1, 1.8 Hz, 1H), 6.93 (d, J=1.8 Hz, 1H); MS (DCI/
+
.
NH3) m/z 491 (M+H) . Anal. (C27H42N2O6 0.15
.
H2O 1.10TFA) C, H, N.
trans,trans-2-[(1,1-Dimethylpropoxy)methyl]-4-(1,3-benzo-
dioxol-5-yl)-1-[[(dibutylamino)carbonyl]methyl]pyrrolidine-
3-carboxylate (22). 1H NMR (300 MHz, CDCl3) d
0.84±0.95 (m, 9H), 1.20 (s, 6H), 1.21±1.36 (m, 4H),
1.43±1.60 (m, 4H), 3.12±3.63 (m, 8H), 3.74±3.88 (m,
2H), 3.90±4.06 (m, 2H), 4.55±4.72 (m, 4H), 5.96 (s, 2H),
6.77 (d, J=8.1 Hz, 1H), 6.86 (dd, J=8.1, 1.8 Hz, 1H),
.
(C22H32N2O5 0.75 H2O) C, H, N.
trans,trans-2-(Propyl)-4-(1,3-benzodioxol-5-yl)-1-[[(dibutyl-
amino)carbonyl]methyl]pyrrolidine-3-carboxylate
(28).
1H NMR (CDCl3, 300 MHz) d 0.89 (t, J=7.5 Hz), 0.92
(t, J=7.5 Hz), and 0.97 (t, J=7.5H, 9H total), 1.22±1.80
(br m, 12H), 2.83 (t, J=7.5 Hz, 1H), 3.40±3.55 (br m,
2H), 3.55±3.68 (m, 1H), 3.78 (d, J=15 Hz, 1H), 5.92 (q,
J=1 Hz, 2H), 6.70 (d, J=8 Hz, 1H), 6.79 (dd, J=1,
6.92 (d, J=1.8 Hz, 1H); MS (DCI/NH3) m/z 505
+
.
.
(M+H) . Anal. (C28H44N2O6 0.5H2O 1.1TFA) C, H,
N.
trans,trans-2-(Phenoxymethyl)-4-(1,3-benzodioxol-5-yl)-1-
[[(dibutylamino)carbonyl]methyl]pyrrolidine-3-carboxylate
(23). H (CDCl3, 300 MHz,) d 0.80 (t, J=7.2 Hz, 3H),
8 Hz, 1H), 6.90 (d, J=1 Hz, 1H). MS (DCI/NH3) m/z
+
.
447 (M+H) . Anal. (C25H38N2O5 0.5H2O) C, H, N.
1
0.87 (t, J=7.2 Hz, 3H), 1.13±1.30 (m, 4H), 1.38±1.52
(m, 4H), 3.09±3.66 (m, 8H), 3.82±3.96 (m, 2H), 4.16±
4.40 (m, 4H), 5.94 (s, 2H), 6.74 (d, J=8.1 Hz, 1H), 6.83±
trans,trans-2-Butyl-4-(1,3-benzodioxol-5-yl)-1-[[(dibutyl-
amino)carbonyl]methyl]pyrrolidine-3-carboxylate
(29).
1
TLC (10% MeOH±CH2Cl2) Rf 0.37; H NMR (CDCl3,
300 MHz, rotomeric forms) d 0.71 (t, J=7 Hz) and
0.77±1.05 (m, 9H total), 1.05±1.20 (m, 2H), 1.20±1.72
(br m, 13H), 2.48±2.52 (m, 1H), 2.87±3.00 (m, 1H),
3.05±3.60 (m, 5H), 3.60±3.80 (br m, 2H), 3.88±4.05 (br
m, 1H), 4.28 (br d, J=15 Hz, 1H total), 5.90 (s) and 5.92
6.92 (m, 3H), 6.96±7.03 (m, 2H), 7.62±7.33 (m, 2H); MS
+
.
(ACPI) m/z 511 (M+H) . Anal. (C29H38N2O6 0.1
.
H2O 0.45TFA) C, H, N.
trans,trans-2-(Phenylmethoxymethyl)-4-(1,3-benzodioxol-
5-yl)-1-[[(dibutylamino)carbonyl]methyl]pyrrolidine-3-car-
(s, 2H total), 6.67±6.82 (m, 3H total); MS (FAB)
+
1
.
m/z 461 (M+H) . Anal. (C26H40N2O5 1.75H2O: C,
boxylate (24). TLC (5% MeOH±CH2Cl2) Rf 0.17; H
NMR (CDCl3, 300 MHz) d 0.87 (t, J=7 Hz) and 0.90 (t,
J=7 Hz, 6H total), 1.13±1.33 (br m, 4H), 1.34±1.53 (br
m, 4H), 2.95±3.12 (br m, 2H), 3.19 (br t, J=9Hz, 1H),
3.23±3.35 (br m, 2H), 3.45±3.76 (br m, 3H), 3.77±3.86
(m, 1H), 3.88±4.35 (br m, 6H), 4.56 (AB, 2H), 5.91 (s,
2H), 6.70 (d, J=9 Hz, 1H), 6.84 (dd, J=1, 8.5 Hz, 1H),
6.93 (d, J=1 Hz, 1H), 7.29±7.38 (m, 5H); MS (FAB)
m/z 525 (M+H)+.
63.45; H, 8.90; N, 5.69. Found: C, 63.18; H, 8.22; N,
5.60.
trans,trans-2-(Hexyl)-4-(1,3-benzodioxol-5-yl)-1-[[(dibutyl-
amino)carbonyl]methyl]pyrrolidine-3-carboxylate
(31).
1H NMR (CDCl3, 300 MHz) d 0.82±1.00 (m, 9H),
1.20±1.40 (m, 12H), 1.45±1.60 (m, 4H), 1.70±1.90 (br m,
2H), 3.10±3.46 (m, 6H), 3.65 (t, J=10.8 Hz, 1H), 3.76 (t,

S. A. Boyd et al. / Bioorg. Med. Chem. 7 (1999) 991±1002
1001
J=11.0 Hz, 1H), 3.92±4.06 (m, 2H), 4.14±4.34 (m, 2H),
5.94 (s, 2H), 6.73 (d, J=8.1 Hz, 1H), 6.79 (dd, J=8.1,
1.8 Hz, 1H), 6.87 (d, J=1.8 Hz, 1H); MS (DCI/NH3)
m/z 489 (M+H) . Anal. (C28H44N2O5 0.9 TFA) C, H,
N.
1H NMR (300 MHz, CDCl3) d 0.86 (t, J=6.9 Hz, 3H),
0.93 (t, J=6.9 Hz, 3H), 1.04±1.47 (m, 8 H), 2.48±2.60
(m, 1H), 2.60±2.75 (m, 1H), 3.07±3.46 (m, 4H), 3.55 (t,
J=10.8 Hz, 1H), 3.70 (t, J=10.8 Hz, 1H), 3.82±3.89 (m,
2H), 4.05±4.18 (m, 1H), 4.64±4.75 (m, 1H), 5.91 (s, 2H),
6.75 (d, J=8.1 Hz, 1H), 6.88 (dd, J=8.1, 1.8 Hz, 1H),
+
.
trans,trans-2-(Heptyl)-4-(1,3-benzodioxol-5-yl)-1-[[(dibutyl-
6.96 (d, J=1.8 Hz, 1H), 7.20±7.36 (m, 5H); MS
+
.
(APCI+) m/z 495 (M+H) . Anal. (C29H38N2O5 TFA)
C, H, N.
amino)carbonyl]methyl]pyrrolidine-3-carboxylate
(32).
1H NMR (CDCl3, 300 MHz) d 0.83±0.98 (s, 9H), 1.18±
1.40 (m, 14H), 1.44±1.60 (m, 4H), 1.72±1.96 (br m, 2H),
3.12±3.45 (m, 6H), 3.65 (t, J=10.5 Hz, 1H), 3.76 (t,
J=11.2 Hz, 1H), 3.90±4.06 (m, 2H), 4.13±4.33 (m, 2H),
5.93 (s, 2H), 6.73 (d, J=7.8 Hz, 1H), 6.79 (dd, J=7.8,
trans,trans-2-(Cyclohexylmethyl)-4-(1,3-benzodioxol-5-yl)-
1-[[(dibutylamino)carbonyl]methyl]pyrrolidine-3-carboxylate
1
(38). H (CDCl3, 300 MHz) d 0.83 (d, J=6.9 Hz, 3H),
1.7 Hz, 1H), 6.87 (d, J=1.7 Hz, 1H); MS (DCI/NH3)
+
0.84 (d, J=6.9 Hz, 3H), 0.91 (t, J=7.5 Hz, 3H), 0.96 (t,
J=7.5 Hz, 3H), 1.13±1.90 (m, 15H), 3.00±4.20 (m, 12H),
5.93 (s, 2H), 6.74 (d, J=8.1 Hz, 1H), 6.78 (dd, J=8.1,
.
m/z 503 (M+H) . Anal. (C29H46N2O5 0.75 TFA) C, H,
N.
1.8 Hz, 1H), 6.87 (d, J=1.8 Hz, 1H); MS (APCI+) m/z
+
.
489 (M+H) . Anal. (C28H44N2O5 0.65 TFA) C, H, N.
trans,trans-2-Isopropyl-4-(1,3-benzodioxol-5-yl)-1-[[(dibutyl-
amino)carbonyl]methyl]pyrrolidine-3-carboxylate
(33).
1H NMR (CDCl3, 300 MHz) d 0.92 (m, 12H), 1.32 (m,
4H), 1.55 (m, 4H), 1.88 (m, 1H), 2.83 (t, J=7 Hz, 1H),
3.11 (m, 2H), 3.25 (m, 3H), 3.42 (m, 3H), 3.66 (q,
J=7 Hz, 1H), 3.75 (d, J=14 Hz, 1H), 5.92 (s, 2H), 6.71
(d, J=8 Hz, 1H), 6.76 (dd, J=6, 2 Hz, 1H), 6.85 (d,
J=2 Hz, 1H); MS (DCI/NH3) m/z 447 (M+H)+. Anal.
trans,trans-2-[(4-Methylcyclohexyl)methyl]-4-(1,3-benzo-
dioxol-5-yl)-1-[[(dibutylamino)carbonyl]methyl]pyrrolidine-
3-carboxylate (39). Colorless wax; 1H NMR (CDCl3,
300 MHz, 1:1 mixture of cis, trans isomers) d 0.84 (d,
J=6.9 Hz, 2H), 0.89 (d, J=7.5 Hz, 3H), 0.93 (t,
J=7.5 Hz, 3H), 0.96 (t, J=7.5 Hz, 3H), 1.10±1.40 (m,
9H), 1.40±1.60 (m, 6H), 1.60±1.81 (m, 3H), 3.11±3.22
(m, 2H), 3.26 (d, J=9.0 Hz, 1H), 3.35 (br t, J=9.0 Hz,
2H), 3.63±3.77 (m, 1H), 3.80±3.92 (m, 1H), 3.98±4.15
(m, 2H), 4.27±4.47 (m, 2H), 5.94 (s, 2H), 6.74 (d,
J=8.1 Hz, 1H), 6.80 (d, J=1.8, 8.1 Hz, 1H), 6.87 (d,
J=1.8 Hz, 1H). MS (APCI+) m/e 515 (M+H)+. Anal.
.
(C25H38N2O5 0.50 H2O) C, H, N. 65.91; H, 8.63; N,
6.15. Found: C, 66.07; H, 8.10; N, 6.03.
trans,trans-2-(2-Methylbutyl)-4-(1,3-benzodioxol-5-yl)-1-
[[(dibutylamino)carbonyl]methyl]pyrrolidine-3-carboxylate
(34). TLC (10% MeOH±CH2Cl2) Rf 0.49; 1H NMR
(CDCl3, 300 MHz, rotomeric forms and mixture of dia-
stereomers) d 0.69 (br t, J=7 Hz) and 0.75±2.15 (several
br m, approximately 26H total), 2.48±2.65 (br m, 1H),
2.87±3.01 (br m, 1H), 3.06±3.82 (br m, 7H), 3.90±4.40
(br m, 2H), 5.90 (s) and 5.92 (s, 2H total), 6.67±6.90 (m,
3H total); MS (FAB) m/z 475 (M+H)+.
.
(C30H46N2O5 1.15TFA.0.5 hexane) C, H, N.
References
1. For recent reviews on the endothelin system, see: (a) Doh-
erty, A. M. J. Med. Chem. 1992, 35, 1493. (b) Shiosaki, K.
Exp. Opin. Ther. Patents 1994, 4, 1361. (c) Opgenorth, T. J.
Adv. Pharmacol. 1995, 33, 1. (d) Williams, D. L.; Walsh, T. F.
In Antihypertensive Drugs; van Zwieten, P. A., Greenlee, W. J.,
Eds.; Harwood Academic: Amsterdam, 1997; pp 213±280. (e)
Lago, M. A.; Luengo, J. I. Annu. Rep. Med. Chem. 1996, 31, 81.
2. Yanagisawa, M.; Kurihara, H.; Kimura, S.; Tomobe, Y.;
Kobayashi, M.; Mitsui, Y.; Yazaki, Y.; Goto, K.; and Masaki,
T. Nature (London) 1988, 332, 411.
trans,trans-2-(3-Methylbutyl)-4-(1,3-benzodioxol-5-yl)-1-
[[(dibutylamino)carbonyl]methyl]pyrrolidine-3-carboxylate
(35). TLC (10% MeOH±CH2Cl2) Rf 0.41; 1H NMR
(CDCl3, 300 MHz, rotomeric forms) d 0.73 (t, J=7 Hz)
and 0.77±1.05 (m, 12H total), 1.07±1.75 (m, approxi-
mately 14H plus H2O), 2.48±2.63 (m, 1H), 2.87±3.05 (m,
1H), 3.05±3.60 (several br m, 5H), 3.62±4.02 (br m, 2H),
4.29 (br d, J=15 Hz, 1H), 5.89 (s) and 5.93 (s, 2H total),
6.65±6.90 (m, 3H total); MS (FAB) m/z 475 (M+H)+.
3. Inoue, A.; Yanagisawa, M.; Kimura, S.; Kasya, Y.;
Miyauchi, T.; Goto, K.; Masaki, T. Proc. Natl. Acad. Sci.
USA 1989, 86, 2863.
4. Arai, H.; Hori, S.; Aramori, I.; Ohkubo, H.; Nakanishi, S.
Nature (London) 1990, 348, 730.
5. Maskai, T. Endocr. Rev. 1993, 14, 256.
6. Miyauchi, T.; Tomobe, Y.; Shiba, R.; Ishikawa, T.; Yana-
gisawa, M.; Kimura, S.; Sugishita, Y.; Ito, I.; Goto, K.;
Masaki, T. Circulation 1990, 81, 1874.
7. Masaki, T.; Yanagisawa, M. Cardiovasc. Drug Rev. 1990, 8,
373.
8. Takuwa, Y.; Kasuya, Y.; Takuwa, N.; Kudo, M.; Yanagi-
sawa, M.; Goto, K.; Masaki, T.; Yamashita, K. J. Clin. Invest.
1990, 85, 653.
trans,trans-2-(4-Methylcyclohexyl)-4-(1,3-benzodioxol-5-
yl)-1-[[(dibutylamino)carbonyl]methyl]pyrrolidine-3-carb-
oxylate (36). Colorless wax; 1H NMR (CDCl3,
300 MHz) d 0.73±0.90 (m, 2H), 0.91 (t, J=7.5 Hz, 3H),
0.96 (t, J=7.5 Hz, 3H), 1.05±1.20 (m, 3H), 1.24±1.41
(m, 5H), 1.45±1.82 (m, 11H), 3.03 (brt, J=7.5 Hz, 1H),
3.15 (dd, J=6.6, 9.0 Hz, 1H), 3.21 (dd, J=6.9, 8.1 Hz,
1H), 3.24±3.60 (m, 4H), 3.63±4.0 (brm, 2H), 3.84 (brd,
J=8.4 Hz, 1H), 4.05 (brd, J=15.0 Hz, 1H), 5.93 (s, 2H),
6.72 (d, J=8.1 Hz, 1H), 6.79 (dd, J=1.8, 8.1 Hz, 1H),
9. (a) Simonson, M. S.; Dunn, M. J. FASEB J. 1990, 4, 2989±
3000. (b) Simonson, M. S. Physiol. Rev. 1990, 73, 375.
10. Battistini, B.; Chailler, P.; D'Orleans-Juste, P.; Briere, N.;
Sirois, P. Peptides 1993, 14, 385.
6.87 (d, J=1.8 Hz, 1H). MS (APCI+) m/z 501
+
.
(M+H) . Anal. (C29H44N2O5 0.60 TFA) C, H, N.
trans,trans-2-Benzyl-4-(1,3-benzodioxol-5-yl)-1-[[(dibutyl-
amino)carbonyl]methyl]pyrrolidine-3-carboxylate
11. Clozel, M.; Gray, G. A.; Breu, V.; Lo‚er, B.-M.; Oster-
walder, R. Biochem. Biophys. Res. Commun. 1992, 186, 867.
(37).

1002
S. A. Boyd et al. / Bioorg. Med. Chem. 7 (1999) 991±1002
12. Fukuroda, T.; Fujikawa, T.; Ozaki, S.; Ishikawa, K.;
Yano, M.; Nishikibe, M. Biochem Biophys Res. Commun.
1994, 199, 1461.
13. Ishikawa, K.; Fukami, T.; Nagase, T.; Fujita, K.;
Hayama, T.; Niiyama, K.; Mase, T.; Ihara, M.; Yano, M. J.
Med. Chem. 1992, 35, 2139.
14. Sogabe, K.; Nirei, H.; Shoubo, M.; Hamada, K.;
Nomoto, A.; Henmi, K.; Notsu, Y.; Ono, T. J. Vasc. Res.
1992, 29, 201.
F. C.; Siegl, P. K. S.; Zhang, K.; Naue, J.; Vyas, K.; Walsh,
T. F.; Fitch, K. J.; Chakravarty, P. K.; Greenlee, W. J.;
Clineschmidt, B. V. J. Pharmacol. Exp. Ther. 1995, 275,
1518.
19. Winn, M.; von Geldern, T. W.; Opgenorth, T. J.; Jae,
H.-S.; Tasker, A. S.; Boyd, S. A.; Kester, J. A.; Mantei, R. A.;
Bal, R.; Sorensen, B. K.; Wu-Wong, J. R.; Chiou, W. J.;
Dixon, D. B.; Novosad, E. I.; Hernandez, L.; Marsh, K. C. J.
Med. Chem. 1996, 39, 1039.
15. Ishikawa, K.; Ihara, M.; Noguchi, K.; Mase, T.; Mino,
N.; Saeki, T.; Fukuroda, T.; Fukami, T.; Ozaki, S.; Nagase,
T.; Nishikibe, M; Yano, M. Proc. Natl. Acad. Sci. USA 1994,
91, 4892.
16. Clozel, M.; Breu, V.; Burri, K.; Cassal, J.-M.; Fishli, W.;
Gray, G. A.; Hirth, G.; Lo‚er, B.-M.; Muller, M.; Neidhart,
W.; Ramuz, H. Nature (London) 1993, 365, 759.
17. Elliott, J. D.; Lago, M. A.; Cousins, R. D.; Gao, A.;
Leber, J. D.; Erhard, K. F.; Nambi, P.; Elshourbagy, M. A.;
Kumar, C.; Lee, J. A.; Bean, J. W. ; DeBrosse, C. W.; Eggle-
ston, D. S.; Brooks, D. P.; Feuerstein, G.; Ru€olo, Jr., R. R.;
Weinstock, J.; Gleason, J. G.; Peisho€, C. E.; Ohlstein, E. H.
J. Med. Chem. 1994, 37, 1553.
20. Doherty, A. M.; Patt, W. C.; Edmunds, J. J.; Berryman,
K. A.; Reisdorph, B. R.; Plummer, M. S.; Shahripour, A.; Lee,
C.; Cheng, X.-M.; Walker, D. M.; Haleen, S. J.; Keiser, J. A.;
Flynn, M. A.; Welch, K. M.; Hallak, H.; Taylor, D. G.; Rey-
nolds, E. E. J. Med. Chem. 1995, 38, 1259.
21. Brooks, D. W.; Lu, L. D.-L.; Masamune, S. Angew.
Chem., Int. Ed. Engl. 1979, 18, 72.
22. Cottrell, I. F.; Hands, D.; Kennedy, D. J.; Paul, K. J.;
Wright, S. H. B.; Hoogsteen, K. J. Chem. Soc. Perkin Trans. 1
1991, 5, 1091.
23. Krapcho, A. P.; Diamanti, J.; Cayen, C.; Bingham, R.
Org. Synth 1967, 47, 20.
24. Blanchette, M. A.; Choy, W.; Davis, J. T.; Essenfeld, A.
P.; Masamune, S.; Roush, W. R.; Sakai, T. Tetrahedron Lett.
1984, 25, 2183.
18. Williams, Jr., D. L.; Murphy, K. L.; Nolan, N. A.;
O'Brien, J. A.; Pettibone, D. J.; Kivlighn, S. D.; Krause,
S. M.; Lis, Jr., E. V.; Zingaro, G. J.; Gabel, R. A.; Clayton,
25. Bram, G.; Vilkas, M. Bull. Chim. Soc. Fr. 1964, 945.