Ring-deactivated hydroxymethylpyrroles as inhibitors of α-chymotrypsin

Article abstract of DOI:10.1016/S0968-0896(00)00274-1

N-Acyl and N-sulfonylhydroxymetyhylpyrroles have been synthesised and shown to inhibit α-chymotrypsin. A hydrophobic group in the N-substituent has been shown to be required for activity. Copyright

Full text of DOI:10.1016/S0968-0896(00)00274-1

Bioorganic & Medicinal Chemistry 9 (2001) 621±628
Ring-deactivated Hydroxymethylpyrroles as Inhibitors of
ꢀ-Chymotrypsin
Andrew D. Abell* and Brent K. Nabbsy
Department of Chemistry, University of Canterbury, Christchurch, New Zealand
Received 28 June 2000; accepted 10 October 2000
AbstractÐN-Acyl and N-sulfonylhydroxymetyhylpyrroles have been synthesised and shown to inhibit a-chymotrypsin. A hydro-
phobic group in the N-substituent has been shown to be required for activity. # 2001 Elsevier Science Ltd. All rights reserved.
Introduction
We have an ongoing interest in the design of inhibitors
of serine proteases based on simple heterocycles that
possess well-de®ned chemical reactivity. To this end we
reported that substitution of a hydroxymethylpyrrole
with an electron withdrawing group (EWG) on nitro-
gen, as in depicted in 1, results in deactivation of the
pyrrole ring and hence modulation of its chemical reac-
tivity.1 The net result is suppression of azafulvenium 2
formation, and hence its subsequent reactions. We also
demonstrated that hydrolytic removal of the EWG from
these pyrroles (route b, Scheme 1) unmasks the latent
reactivity to allow formation of a highly electrophilic
azafulvene 5. This species then reacts readily with an
available nucleophile to give 3b.2 A sequence of reac-
Scheme 1.
tions of this type (i.e., the conversion of 4 to 3b, via 5) is
central to a number of synthetic methods3,4 and also to
the biosynthesis of porphyrins and corrins.5 Given this
well-de®ned chemistry, we postulate that reaction
sequence b (Scheme 1) would form the basis of an inhi-
bitor of serine proteases6 if the enzyme were able to cata-
lyse removal of the EWG (e.g. deacylation) via its normal
catalytic action. The resulting hydroxymethylpyrrole
would yield an azafulvene capable of inhibiting the
Figure 1.
enzyme via covalent attachment (cf. conversion of 5 into
3a where NuÀ is the enzyme). Figure 1 shows the rela-
tionship between a natural substrate of a serine protease
(the site of hydrolysis is shown by an arrow) and the pro-
posed inhibitors. In this paper, we report the synthesis of
some ring-deactivated hydroxymethylpyrroles and the
initial results of the screening of these compounds
against a-chymotrypsin using a simple 96-well plate
inhibitory assay.
Results and Discussion
*Corresponding author. Tel.: +64-3-364-2818; fax: +64-3-364-
2110; e-mail: a.abell@chem.canterbury.ac.nz
yCurrent address: University Chemical Laboratory, Lens®eld Road,
Cambridge CB2 1EW, UK..
We reasoned that the EWG group in 1 would need to
mimic the P1 substituent of a natural peptidic substrate of
the target serine protease (e.g., a-chymotrypsin) in order
0968-0896/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S0968-0896(00)00274-1

622
A.D. Abell, B.K. Nabbs / Bioorg. Med. Chem. 9 (2001) 621±628
aminopyridine (DMAP) and N,N-diisopropylethyl-
amine (DIPEA) as the coupling agents. The N-protected
2-formylpyrroles were then simply reduced to the
desired hydroxymethylpyrroles 8 using zinc borohy-
dride. Compounds 9 were conveniently prepared from
8d (Scheme 2). Treatment of 8d with mesyl chloride in
the presence of DIPEA gave 9a while an analogous
reaction using N-phthalyl-l-leucine acid ¯uoride and
DIPEA/DMAP gave 9b.
The side-chain modi®ed derivatives 14, 15, 20, 21 and 27
were prepared as detailed in Schemes 3 and 4. The exten-
ded side chain of 14, 15, 20 and 21 was introduced by
acylating the Grignard derivative of pyrrole 10 with
methyl succinyl chloride to give the key derivative 11
(Scheme 3). Direct sulfonylation of 11 then gave 12 and 13
that yielded the desired hydroxymethylpyrroles 14 and 15,
respectively, on reduction of the ketone with sodium bor-
ohydride. The methyl ester of 11 was also hydrolysed
and the corresponding acid 16 was coupled with the
methyl ester of l-leucine using standard peptide coupling
conditions, to give 17. Sulfonylation of 17, followed by
reduction, gave 20 and 21 as mixtures of epimers. In the
last series of reactions, the Grignard derivative of pyrrole
10 was reacted with ethyl bromoactetate to give 22
(Scheme 4). Compound 22 was then a-formylated to give
23, the ester of which was hydrolyzed. The resulting free
acid 24 was then coupled to l-leucine methyl ester to
give 25 which was N-mesylated to give 26. The desired
compound 27 was ®nally prepared by reduction of the
formyl group of 26 using zinc borohydride. Compounds
8a±f, 14, 15, 20, 21 and 27 were assayed against a-chymo-
trypsin using a simple and convenient 96-well-based assay
system. We were interested in a rapid screening of activity
and as such inhibition data was conveniently determined
at two concentrations, that is, 12.5 and 125 mg/mL (see
Table 1). With the exception of 8c, all the simple N-acyl
and N-sulfonyl hydroxymethylpyrrole derivatives of
type 8 inhibited a-chymotrypsin. The most potent com-
pound inhibited a-chymotrypsin to the extent of 20% at
12.5 mg/mL (53 mM). Of particular interest is the fact
that inhibitory activities of the N-sulfonyl derivatives
(8d±f) are similar to the N-acyl derivatives (8a±b), at
Scheme 2. Reagents and conditions: (i) NaH then either N-Pht-l-Leu-
F (87%) or MeSO2Cl (93%) or PhSO2Cl (97%) or PhCH:CHSO2Cl
(56%) or Ph(CH2)2SO2Cl (84%); (ii) DMAP, DIPEA, CH2Cl2 then
Ph(CH2)2COCl (79%); (iii) Zn(BH4)2, Et2O, 0 ꢀC (88±94%); (iv)
DIPEA, MeSO2Cl then 10% aq HCl (9a, quant); (v) DMAP, DIPEA,
N-Pht-l-Leu-F, CH2Cl2 (9b, 90%).
to obtain an e€ective inhibitor of the enzyme (see Fig. 1).7
Only then would it bind e€ectively to the enzyme. With
this in mind we initially prepared derivative 8a8 in which
the N-acyl group is an amino acid (Scheme 2). A num-
ber of other N-substituted ring-deactivated hydroxy-
methylpyrroles (8b±f, 14, 15, 20, 21 and 27)9 were then
prepared in order to establish which N-substituents
provide inhibitors of a-chymotrypsin (Schemes 2±4).
We then carried out some preliminary studies on mod-
ifying the leaving group (see structures 9, Scheme 2).
The target derivatives were prepared as detailed in
Schemes 2±4. Treatment of 2-formylpyrrole 6 with sodium
hydride followed by the addition of either N-phthalyl-l-
leucine acid ¯uoride10 or the respective sulfonyl chlorides
gave compounds 7a,c±f in good yields. Compound 7b
was prepared under milder conditions using 4-dimethyl-
Scheme 3. Reagents and conditions: (i) MeMgI, Et2O, re¯ux then MeO2C(CH2)2COCl, re¯ux (49%); (ii) NaH, THF then either MesO2Cl (12, 86%:
18, 50%) or PhSO2Cl (13, 43%); (iii) NaBH4, MeOH, 0 ꢀC (14, 85%), (15, 90%), (20, 89%), (21, 89% for two steps); (iv) NaOH, MeOH±H2O (16,
78%); (v) l-LeuOMe.HCl, EDCI, HOBt, DIPEA (93%).

A.D. Abell, B.K. Nabbs / Bioorg. Med. Chem. 9 (2001) 621±628
623
both concentrations studied. The inactivity of 8c sup-
ports the notion11 that a large hydrophobic group is
required on nitrogen to promote binding to a-chymo-
trypsinÐthis group would correspond to the P1 residue1
of the natural substrate of a-chymotrypsin (see Fig. 1).
the side chain in the C-direction to make it better
resemble a peptidic substrate of the enzyme. However,
the results for these compounds do support the earlier
observation that a phenyl sulfonyl group is favored over
an N-methyl sulfonyl group for inhibition (compare
results for 14/15 and 20/21).
Having established that an N-phenylsulfonyl group (as
in 8d) provides active compounds, we turned our atten-
tion to determining which group at the 2-methyl posi-
tion was optimal for enzyme inhibition. The results for
9a and 9b show that chloro and N-phthalyl-l-leucinyl
groups are less favored for enzyme inhibition than the
hydroxyl group of 8. This suggests that potency is not
simply enhanced by an increase in leaving group ability
of this substituent (cf. 9a and 8d in Table 1) or its ability
to mimic a natural peptidic substrate of the enzyme (see
9b).5 The last series of compounds tested, namely 14, 15,
20, 21 and 27 show that there is little gain in extending
Conclusion
In summary, we have synthesised a range of N-acyl and
N-sulfonylhydroxymetyhylpyrroles and shown them to
provide a new class of inhibitor of a-chymotrypsin using
a quick and convenient screening assay. The require-
ment, for inhibition activity, of a hydrophobic group in
the N-substituent has also been demonstrated. We have
yet to establish the mechanism by which this class of
compound inhibit a-chymotrypsin, however, we postu-
late that it may occur via sequence b (Scheme 1) were
NuÀ is an enzyme active site residue. What we do know
is that compounds 4 (the postulated inactivating spe-
cies) readily react with a nucleophile and are as such
capable of inhibition. However, compounds 4 are too
reactive to assay directly. Further work is in progress to
characterise the exact mode of inhibition of these com-
pounds and to better understand those structural factors
that in¯uence potency.
Experimental
Proton detected NMR spectra were obtained on either a
Varian Unity 3000 spectrometer or a Varian XL300 spec-
trometer, both operating at 300 MHz. Carbon detected
NMR were obtained on the XL300 spectrometer operat-
ing at 75MHz. IR spectra were obtained using a Shi-
madzu 8201PC series FTIR. Mass spectrometry was
performed on either a Kratos MS80 Mass Spectrometer
or a Micromass LCT operating in Electrospray (ES) mode
with 50:50 acetonitrile water as solvent. a-Chymotrypsin
assays were carried out in microtitre plates (NUNC ¯at
bottomed plates, GIBCO). Compounds 7b,12 7c,1a 8a,12
8b,12 8c,1a and 2213 were prepared by literature methods.
Scheme 4. Reagents and conditions: (i) EtMgBr, THF, À10 ꢀC to rt
then BrCH2CO2Et, À10 ꢀC to rt then aq NH4Cl (61%); (ii) POCl3,
.
DMF, 1,2-DCE, re¯ux then NaOAc 3H2O, re¯ux (56%); (iii) NaOH,
MeOH±H2O (84%); (iv) l-LeuOMe.HCl, EDCI, HOBt, DIPEA
(85%). (v) NaH, THF then MeSO2Cl, À10 ꢀC to rt (70%); (vi)
Zn(BH4)2, Et2O, 0 ꢀC (83%).
ꢀ-Chymotrypsin assay
Solutions of the test compounds 8a±f, 9a±b, 14, 15, 20,
21 and 27 were made to 125 and 12.5 mg mLÀ1 in metha-
nol. Tris±HCl (50 mL of 0.4 M solution in water, pH 7.6),
distilled water (50 mL), test solution (50 mL) and a-chy-
motrypsin (50 mL, Sigma ex-Bovine pancreas, 9 units
mLÀ1 in 50 mM Tris±HCl bu€er, pH 7.6) were added to
each well of the microtitre plate. Incubation at 37 ꢀC for
30 min was followed by the addition of N-succinyl-l-
phenylalanine-4-nitroanilide (100 mL, 1 mg mLÀ1 solu-
tion in 50 mM Tris±HCl bu€er, pH 7.6). The absorbance
was read at 405 nm at t=0, and over approximately 3 h
period of incubation at 37 ꢀC when signi®cant color
change had taken place. Each sample was assayed in tri-
plicate and average absorbances were used to calculate the
percentage (%) inhibition. Sample blanks in which 50mM
Tris±HCl bu€er, pH 7.6, replaced a-chymotrypsin were
run concurrently.
Table 1. Inhibition of a-chymotrypsin
Compound
% Inhibition at:
12.5 mg mLÀ1
125 mg mLÀ1
8a
8b
8c
8d
8e
8f
9a
9b
14
15
20
21
27
10
10
0
20
15
10
5
0
5
10
0
15
5
0
10
0
5
5
5
0
0
0
0
0
10
0

624
A.D. Abell, B.K. Nabbs / Bioorg. Med. Chem. 9 (2001) 621±628
General procedure A: N-Acylation of pyrroles using
sodium hydride
H2SO4. The resulting solution was extracted with ether
(3Â15 mL), and the combined ethereal extracts were
washed with saturated aqueous brine (10 mL), dried and
evaporated under reduced pressure. See Experimental
for details.
To a stirred suspension of NaH (typically 1.26mmol,
80% suspension in oil, washed twice with petroleum ether,
1.2 equiv) in THF (6mL) under N2 was added the pyrrole
(typically 1.09mmol) dissolved in THF (2mL). After stir-
ring at rt for 15min, the electrophile (typically 1.2±
1.4 equiv) in THF (2 mL) was slowly added and stirring
was continued for 1 h at rt. Water (10 mL) was added, the
THF was removed under reduced pressure and the aqu-
eous residue was extracted with dichloromethane (DCM)
(3Â15 mL). The combined organic phases were washed
with saturated aqueous NaHCO3 (10 mL), water
(10 mL), saturated aqueous brine (10 mL), dried and
concentrated under reduced pressure. The residue was
puri®ed by ¯ash chromatography on silica. See Experi-
mental for details.
General procedure E: Peptide couplings using EDCI
To a stirred solution (ꢁ0.1 M) of the pyrrole carboxylic
acid (typically 1.14 mmol) and the l-amino acid ester
hydrochloride (1.1 equiv) in DCM (ꢁ11 mL) under N2 at
rt were added EDCI (1.3 equiv) and HOBt (1.5 equiv).
DIPEA (1.1 equiv) was added and the reaction mixture
was stirred for 16h. The solution was then diluted with
DCM (10 mL), washed with 3 M aqueous HCl (2Â10
mL), water (2Â10mL), dried and evaporated under
reduced pressure. The resultant oil was puri®ed by ¯ash
chromatography on silica. See Experimental for details.
1-(N-Phthalyl-L-leucinyl)pyrrole-2-carboxaldehyde (7a).
To a stirred suspension of NaH (7mg, 0.23mmol in 80%
suspension in oil, washed twice with petroleum ether,
1.1 equiv) in THF (5 mL) under N2 was added 2-formyl-
pyrrole 6 (20 mg, 0.21mmol) dissolved in THF (1 mL). To
the ice-cooled mixture was added a solution of N-phthalyl-
l-leucine acid ¯uoride14 (61 mg, 0.23 mmol, 1.1 equiv) in
THF (4 mL) over 15 min. The mixture was left to stir at
0 ꢀC for 30 min, and at rt overnight. The mixture was then
diluted with EtOAc (10 mL), washed with 2 M aq KHSO4
(10mL), water (10 mL), dried and evaporated under
reduced pressure. Flash chromatography on silica
(EtOAc:petroleum ether, 1:4) gave 7a (62 mg, 87%) as a
pale-yellow oil. Spectral data as reported.12
Modi®cation to general procedure A
The electrophile (typically 1.2±1.4 equiv) in THF (2 mL)
was slowly added to a cooled mixture (À10 to 0 ꢀC) of the
pyrrolyl anion, prepared as described above. Stirring was
continued at À10 to 0 ꢀC for 30 min and then for 1 h at
rt, before the reaction mixture was worked up using the
same method as described above.
General procedure B: Zinc borohydride reductions
To
a solution of the pyrrole aldehyde (typically
0.16mmol) in ether (10 mL) at 0ꢀC under N2 was added
Zn(BH4)2 (1equiv of 0.14 M solution in ether). The result-
ing solution was stirred at 0 ꢀC for 30min, then water
(2 mL) and 10% aqueous glacial acetic acid (2 mL) were
carefully added to quench the reaction. The separated
aqueous phase was extracted with DCM (2Â10 mL) and
the combined organic phases were washed with water
(2Â10 mL), saturated aqueous brine (10 mL), dried and
concentrated under reduced pressure. The residue was
puri®ed by ¯ash chromatography on silica. See Experi-
mental for details.
1-(Phenylsulfonyl)pyrrole-2-carboxaldehyde (7d). Gen-
eral procedure A was carried out using 2-formylpyrrole 6
(50 mg, 0.53mmol) and phenylsulfonyl chloride (81 mL,
0.63mmol, 1.2 equiv). Flash chromatography on silica
(EtOAc:petroleum ether, 1:3) gave 7d (121 mg, 97%) as
a white solid: mp 81 ꢀC (lit.15 79.5±80.5 ꢀC).
1-(2-Phenylethenyl)pyrrole-2-carboxaldehyde (7e). The
modi®ed general procedure A was carried out using
2-formylpyrrole 6 (50 mg, 0.53mmol) and b-trans-styr-
enesulfonyl chloride (128mg, 0.63mmol, 1.2 equiv). Flash
chromatography on silica (EtOAc:petroleum ether, 1:4)
gave 7e (77 mg, 56%) as a pale-yellow solid. An analytical
sample was obtained by recrystallisation from EtOAc/
petroleum ether to give a white crystalline solid: mp
88 ꢀC. 1H NMR (CDCl3) d 6.41 (m, 1H, pyrrole H4), 7.21
(m, 1H, pyrrole H3), 7.40±7.47 (m, 4H, CHPh and arom),
7.54 (m, 2H, arom), 7.62 (m, 1H, pyrrole H5), 7.81 (d,
J=15.6 Hz, 1H, SO2CH), 9.79 (s, 1H, CHO); 13C NMR
(CDCl3) d 111.6, 123.5, 127.5, 128.9, 129.1, 129.6, 131.4,
132.0, 133.1, 146.0, 178.2. IR (CHCl3) 2837, 1682,
1612cmÀ1. Anal. calcd for C13H11NO3S: C, 59.76; H,
4.24; N, 5.36. Found: C, 59.74; H, 4.16; N, 5.30.
General procedure C: Sodium borohydride reductions
To a solution of the pyrrole ketone (typically 0.20 mmol)
in MeOH (10 mL) at 0 ꢀC under N2 was added NaBH4
(15 equiv). The resulting solution was stirred at 0 ꢀC for
30 min, then diluted with DCM (10 mL), washed with
water (2Â10 mL), and the organic layer dried and eva-
porated under reduced pressure. The residue was puri®ed
by ¯ash chromatography on silica. See experimental for
details.
General procedure D: Hydrolysis of pyrrole esters using
sodium hydroxide
A solution of the pyrrole ester (typically 1.22 mmol) in
50% aq MeOH (4 mL) containing NaOH (2.5 equiv)
was stirred at 40 ꢀC for 1 h. Water (10 mL) was then
added, and the MeOH was removed under reduced
pressure. The aqueous solution was washed with ether
(10 mL), cooled to 5 ꢀC and acidi®ed with 50% aqueous
1-(2-Phenylethanesulfonyl)pyrrole-2-carboxaldehyde (7f).
The modi®ed general procedure A was carried out using
2-formylpyrrole 6 (50 mg, 0.53 mmol) and 2-phenyle-
thanesulfonyl chloride (129mg, 0.63mmol, 1.2 equiv).
Flash chromatography on silica (EtOAc:petroleum ether,

A.D. Abell, B.K. Nabbs / Bioorg. Med. Chem. 9 (2001) 621±628
625
1:4) gave 7f (116mg, 84%) as a pale-pink solid: mp 61±
63ꢀC. 1H NMR (CDCl3) d 3.08 (m, 2H, CH2Ph), 4.10 (m,
2H, SO2CH2), 6.38 (m, 1H, pyrrole H4), 7.12±7.30 (m,
6H, pyrrole H3 and arom), 7.56 (m, 1H, pyrrole H5),
9.68 (s, 1H, CHO); 13C NMR (CDCl3) d 29.3, 55.8,
111.5, 127.1, 128.2, 128.8, 129.0, 131.0, 133.1, 136.2,
178.2; IR (CHCl3) 1680 cmÀ1. HRMS calcd for C13H13
NO (M-SO2) 199.0997, found 199.0994.
2H, arom); 13C NMR (CDCl3) d 37.2, 111.8, 117.1,
124.5, 127.1, 129.3, 134.0, 138.8, 142.6. HRMS calcd for
C11H10NO2S (M-Cl) 220.0432, found 220.0435.
N-Phthalyl-L-leucine pyrrol-2-ylmethyl ester (9b). The
hydroxymethylpyrrole 8d (30 mg, 0.13mmol), DMAP
(15 mg, 0.13mmol, 1 equiv) and DIPEA (26 mL, 0.15
mmol, 1.2 equiv) were dissolved in DCM (8mL) at rt. N-
Phthalyl-l-leucine acid ¯uoride10 (40 mg, 0.15mmol,
1.2 equiv) dissolved in DCM (2 mL) was added and the
resultant solution was stirred for 24 h under N2. EtOAc
(10 mL) was added and the mixture was washed with
10% aqueous citric acid (10 mL), water (2Â10 mL),
dried and evaporated under reduced pressure. Flash
chromatography on silica (EtOAc:petroleum ether, 1:2)
gave 9b (55 mg, 90%) as a colourless oil which solidi®ed
at 0 ꢀC. An analytical sample was obtained by recrys-
tallisation froꢀm EtOAc/petroleum ether to give a white
solid: mp 112 C. 1H NMR (CDCl3) d 0.88 (d, J=6.3 Hz,
3H, CHMe2), 0.89 (d, J=6.3 Hz, 3H, CHMe2), 1.43 (m,
1H, CHMe2) 1.75 (m, 1H, CH2CHMe2), 2.29 (m, 1H,
CH2CHMe2), 4.80 (m, 1H, aH), 5.30 (m, 2H, CH2O),
6.25 (m, 1H, pyrrole H4), 6.37 (m, 1H, pyrrole H3), 7.32
(m, 1H, pyrrole H5), 7.52±7.87 (m, 9H, Pht Hs and
arom); 13C NMR (CDCl3) d 20.9, 23.1, 25.0, 37.0, 50.7,
58.8, 111.8, 117.9, 123.5, 124.3, 126.7, 128.3, 129.5,
131.8, 134.0, 134.1, 139.2, 167.7, 169.4. IR (CHCl3) 1776,
1742, 1717 cmÀ1. Anal. calcd for C25H24N2O6S: C, 62.49;
H, 5.03; N, 5.83. Found: C, 62.50; H, 5.33; N, 5.96.
2-Hydroxymethyl-1-(phenylsulfonyl)pyrrole (8d). The
N-phenylsulfonylpyrrole 7d (50 mg, 0.21 mmol) was
reduced with Zn(BH4)2 by general procedure B. Flash
chromatography on silica (EtOAc:petroleum ether, 1:2)
gave 8d (46 mg, 92%) as a pale-pink solid: mp 41±42ꢀC.
1H NMR (CDCl3) d 2.57 (bs, 1H, OH), 4.61 (s, 2H,
CH2OH), 6.24±6.28 (m, 2H, pyrrole H4 andpyrrole H3),
7.28 (m, 1H, pyrrole H5), 7.49±7.65 (m, 3H, arom), 7.82
(m, 2H, arom); 13C NMR (CDCl3) d 56.8, 112.0, 115.3,
123.6, 126.6, 129.5, 134.0, 134.6, 139.0. IR (CHCl3) 3578,
1369 cmÀ1. HRMS calcd for C11H11NO3S 237.0460,
found 237.0458.
2-Hydroxymethyl-1-(2-phenylethenylsulfonyl)pyrrole (8e).
The N-styrenesulfonylpyrrole 7e (50 mg, 0.19 mmol) was
reduced with Zn(BH4)2 by general procedure B. Flash
chromatography on silica (EtOAc:petroleum ether, 1:3)
gave 8e (48 mg, 95%) as a pink solid: mp 76±77 ꢀC; H
1
NMR (CDCl3) d 2.64 (bs, 1H, OH), 4.73 (s, 2H,
CH2OH), 6.24 (m, 1H, pyrrole H4), 6.30 (m, 1H, pyrrole
H3), 7.02 (d, J=15.6 Hz, 1H, CHPh), 7.18 (m, 1H, pyr-
role H5), 7.36±7.48 (m, 5H, arom), 7.63 (d, J=15.6 Hz,
1H, SO2CH); 13C NMR (CDCl3) d 56.7, 111.6, 115.2,
123.0, 124.3, 128.7, 129.1, 131.5, 131.7, 134.1, 143.4; IR
(CHCl3) 3585, 3026, 1614 cmÀ1. HRMS calcd for
C13H13NO3S 263.0616, found 263.0616.
Methyl 4-oxo-4-(1H-pyrrol-2-yl)butanoate (11). A solu-
tion of pyrrole (1.50 g, 22.4 mmol) in ether (9 mL) was
added dropwise to MeMgI (11.74 mL of 2.0 M solution
in ether, 23.5 mmol, 1.05 equiv) under N2 so as to cause
slight re¯ux. The solution was then heated at re¯ux for
an additional 30 min. After cooling to rt, methyl suc-
cinyl chloride (2.75 mL, 22.4 mmol, 1 equiv) in ether
(6 mL) was added, and the resulting mixture was heated
at re¯ux for 1 h. The reaction was left overnight, and
then poured into cold water. The aqueous and ether
layers were separated, and the aqueous phase was
extracted with ether (3Â10 mL). The combined ethereal
layers were washed with water (2Â10 mL), dried and
evaporated under reduced pressure. The resulting resi-
due was distilled at reduced pressure (oil pump) to give
11 (1.97 g, 49%) as a yellow solid. An analytical sample
was obtained by recrystallisation from petroleum ether
to give white crystals: mp 49 ꢀC. 1H NMR (CDCl3)
2-Hydroxymethyl-1-(2-phenylethanesulfonyl)pyrrole (8f).
The N-phenylethanesulfonylpyrrole 7f (50 mg, 0.19mmol)
was reduced with Zn(BH4)2 by general procedure B. Flash
chromatography on silica (EtOAc:petroleum ether, 1:3)
gave 8f (47 mg, 94%) as a colourless oil: 1H NMR
(CDCl3) d 2.47 (bs, 1H, OH), 2.99 (m, 2H, CH2Ph), 3.70
(m, 2H, SO2CH2), 4.76 (s, 2H, CH2OH), 6.23 (m, 1H,
pyrrole H4), 6.32 (m, 1H, pyrrole H3), 7.11±7.31 (m, 6H,
pyrrole H5 and arom); 13C NMR (CDCl3) d 29.3, 56.7
(2ÂC), 111.1, 115.6, 124.0, 127.2, 128.3, 128.9, 133.8,
136.6. IR (CHCl3) 3578, 1369 cmÀ1. HRMS calcd for
C13H15NO3S 265.0773, found 265.0771.
d
2.74 (t, J=6.8 Hz, 2H, CH2CO2Me), 3.16 (t,
2 - Chloromethyl - 1 - (phenylsulfonyl)pyrrole (9a). Mesyl
chloride (1.5 equiv) was added to an ice cooled solution
of 8d (30 mg, 0.13 mmol) in DCM (2 mL) containing
DIPEA (1.5 equiv). Stirring was continued at 0 ꢀC for
20 min and for a further 30 min at rt. The solution was
diluted with DCM (10 mL), washed with ice-cold water
(10 mL), cold 10% aq HCl (10 mL), and saturated aqu-
eous NaHCO3 (10 mL). The organic phase was then dried
and evaporated under reduced pressure. The residual oil
was eluted through a short plug of silica (EtOAc:pe-
troleum ether, 1:1) to give 9a (quantitative) as a white
solid: mp 66ꢀC. 1H NMR (CDCl3) d 4.84 (s, 2H, CH2Cl),
6.26 (m, 1H, pyrrole H4), 6.38 (m, 1H, pyrrole H3), 7.34
(m, 1H, pyrrole H5), 7.48±7.64 (m, 3H, arom), 7.89 (m,
J=6.8 Hz, 2H, COCH2), 3.69 (s, 3H, CO2Me), 6.27 (m,
1H, pyrrole H4), 6.99 (m, 1H, pyrrole H3), 7.05 (m, 1H,
pyrrole H5), 10.20 (bs, 1H, NH); 13C NMR (CDCl3, 75
MHz) d 28.2, 32.4, 51.7, 110.6, 116.6, 125.1, 131.3,
173.3, 188.5. IR (CHCl3) 3452, 3285, 1736, 1647 cmÀ1
.
Anal. calcd for C9H11NO3: C, 59.66; H, 6.12; N, 7.73.
Found: C, 59.62; H, 6.07; N, 7.58.
Methyl 4-oxo-4-[1-(methanesulfonyl)pyrrol-2-yl]butanoate
(12). General procedure A was carried out using the
pyrrole 11 (60 mg, 0.33 mmol) and mesyl chloride (37 mL,
0.46 mmol, 1.4 equiv). Flash chromatography on silica
(EtOAc:petroleum ether, 1:2), followed by recrystallisa-
tion from petroleum ether, gave 12 (74 mg, 86%) as ®ne

626
A.D. Abell, B.K. Nabbs / Bioorg. Med. Chem. 9 (2001) 621±628
white needles: mp 92 ꢀC. H NMR (CDCl3) d 2.75 (t,
J=6.6 Hz, 2H, CH2CO2Me), 3.18 (t, J=6.6 Hz, 2H,
COCH2), 3.70 (s, 3H, CO2Me), 3.72 (s, 3H, SO2Me),
6.31 (m, 1H, pyrrole H4), 7.21 (m, 1H, pyrrole H3), 7.57
(m, 1H, pyrrole H5); 13C NMR (CDCl3) d 27.6, 33.6,
42.8, 51.6, 110.0, 124.1, 129.6, 132.3, 172.9, 187.6. IR
(CHCl3) 1734, 1676 cmÀ1. HRMS calcd for C10H13
NO5S 259.0515, found 259.0511.
matography on silica (EtOAc:petroleum ether, 1:1) gave
17 (312 mg, 93%) as a mauve solid. An analytical sam-
ple was obtained by recrystallisation from EtOAc/pet-
roleum ether to give colorless crystals: mp 100±101 ꢀC.
1H NMR (CDCl3) d 0.91 (d, J=5.9 Hz, 6H, CHMe2),
1.51±1.67 (m, 3H, CH2CHMe2), 2.68 (m, 2H, CH2
CONH), 3.18 (m, 2H, COCH2), 3.72 (s, 3H, CO2Me),
4.66 (m, 1H, aH), 6.27 (m, 1H, pyrrole H4), 6.77 (d,
J=7.8 Hz, 1H, CONH), 6.98 (m, 1H, pyrrole H3 ), 7.04
(m, 1H, pyrrole H5), 10.02 (bs, 1H, NH); 13C NMR
(CDCl3) d 21.9, 22.7, 24.8, 30.3, 33.0, 41.7, 50.6, 52.2,
110.6, 116.8, 124.9, 131.4, 172.0, 173.9, 189.2. IR
(CHCl3) 3450, 3344, 1740, 1672, 1645 cmÀ1. Anal. calcd
for C15H22N2O4: C, 61.21; H, 7.53; N, 9.52. Found: C,
61.09; H, 7.60; N, 9.46.
1
Methyl 4-oxo-4-[1-(phenylsulfonyl)pyrrol-2-yl]butanoate
(13). General procedure A was carried out using the pyr-
role 11 (60 mg, 0.33mmol) and phenylsulfonyl chloride
(51 mL, 0.40mmol, 1.2 equiv). Flash chromatography on
silica (EtOAc:petroleum ether, 1:2), followed by recrys-
tallisation from EtOAc/petroleum ether, gave 13 (46 mg,
43%) as ®ne white crystals: mp 139±140 C; H NMR
(CDCl3) d 2.61 (t, J=6.8 Hz, 2H, CH2CO2Me), 3.03 (t,
J=6.8 Hz, 2H, COCH2), 3.61 (s, 3H, CO2Me), 6.35 (m,
1H, pyrrole H4), 7.12 (m, 1H, pyrrole H3), 7.48±7.62
(m, 3H, arom), 7.80 (m, 1H, pyrrole H5), 7.97 (m, 2H,
arom); 13C NMR (CDCl3) d 27.9, 33.8, 51.7, 110.5,
123.6, 128.1, 128.7, 130.2, 132.8, 133.6, 138.8, 142.7; IR
(CHCl3) 1734, 1682 cmÀ1; HRMS calcd for C15H15
NO5S 321.0671, found 321.0679.
ꢀ
1
N-{4-Oxo-4-[1-(methanesulfonyl)pyrrol-2-yl]}butanoyl-L-
leucine methyl ester (18). General procedure A was car-
ried out using 17 (60 mg, 0.20 mmol) and mesyl chloride
(23 mL, 0.29 mmol, 1.4 equiv). Flash chromatography on
silica (EtOAc:petroleum ether, 2:1) gave 18 (38 mg, 50%)
as a white solid: mp 106±107 ꢀC. 1H NMR (CDCl3)
d 0.92 (d, J=5.9 Hz, 3H, CHMe2), 0.93 (d, J=6.3 Hz,
3H, CHMe2), 1.51±1.71 (m, 3H, CH2CHMe2), 2.65 (m,
2H, CH2CONH), 3.22 (m, 2H, COCH2), 3.71 (s, 3H,
CO2Me), 3.72 (s, 3H, SO2Me), 4.61 (m, 1H, aH), 6.11
(d, J=8.3 Hz, 1H, CONH), 6.30 (m, 1H, pyrrole H4),
7.22 (m, 1H, pyrrole H3), 7.57 (m, 1H, pyrrole H5); 13C
NMR (CDCl3) d 21.9, 22.7, 24.8, 29.9, 34.3, 41.6, 43.1,
50.8, 52.2, 110.3, 124.5, 129.9, 132.5, 171.5, 173.4, 188.4;
IR (CHCl3) 3435, 1740, 1672 cmÀ1. HRMS calcd for
C16H24N2O6S 372.1355, found 372.1347.
Methyl 4-hydroxy-4-[1-(methanesulfonyl)pyrrol-2-yl]bu-
tanoate (14). The N-mesylpyrrole 12 (51 mg, 0.20 mmol)
was reduced with NaBH4 by general procedure C. Flash
chromatography on silica (EtOAc:petroleum ether, 1:1)
gave 14 (44 mg, 85%) as a colourless oil which solidi®ed at
0 ꢀC. An analytical sample was obtained by recrystallisa-
tion from petroleum ether to give white needles: mp 70ꢀC.
1H NMR (CDCl3) d 2.24 (m, 2H, CHOHCH2), 2.57 (m,
2H, CH2CO2Me), 2.98 (bs, 1H, OH), 3.29 (s, 3H,
SO2Me), 3.70 (s, 3H, CO2Me), 5.06 (t, J=6.6 Hz, 1H,
CHOH), 6.25 (m, 1H, pyrrole H4), 6.34 (m, 1H, pyrrole
H3), 7.15 (m, 1H, pyrrole H5); 13C NMR (CDCl3)
d 30.2, 30.7, 43.0, 51.8, 64.8, 111.2, 112.2, 123.1, 136.7,
174.2. IR (CHCl3) 3580, 1732 cmÀ1. HRMS calcd for
C10H15NO5S 261.0671, found 261.0671.
N-{4-Hydroxy-4-[1-(methanesulfonyl)pyrrol-2-yl]}buta-
noyl-L-leucine methyl esters (20). The N-mesylpyrrole
18 (54 mg, 0.14 mmol) was reduced with NaBH4 by gen-
eral procedure C. Flash chromatography on silica
(EtOAc:petroleum ether, 5:2) gave 20 (48 mg, 89%) as a
mixture of epimers (2:1 by 1H NMR), as an oil. 1H NMR
(CDCl3) of major isomer, d 0.94 (d, J=6.3 Hz, 6H,
CHMe2), 1.51±1.68 (m, 3H, CH2CHMe2), 2.24 (m, 2H,
CHOHCH2), 2.51 (m, 2H, CH2CONH), 3.32 (s, 3H,
SO2Me), 3.74 (s, 3H, CO2Me), 4.62 (m, 1H, aH), 5.07 (m,
1H, CHOH), 6.21±6.25 (m, 2H, CONH and pyrrole
H4), 6.34 (m, 1H, pyrrole H3), 7.14 (m, 1H, pyrrole
Methyl 4-hydroxy-4-[1-(phenylsulfonyl)pyrrol-2-yl]bu-
tanoate (15). The N-phenylsulfonylpyrrole 13 (25 mg,
0.08mmol) was reduced with NaBH4 by general procedure
C. Flash chromatography on silica (EtOAc:petroleum
1
1
ether, 2:3) gave 15 (23 mg, 90%) as a pale-purple oil. H
H5); selected H NMR of minor isomer, d 3.31 (s, 3H,
NMR (CDCl3) d 2.06 (m, 2H, CHOHCH2), 2.40 (m,
2H, CH2CO2Me), 3.59 (s, 3H, CO2Me), 4.84 (m, 1H,
CHOH), 6.19 (m, 1H, pyrrole H4), 6.25 (m, 1H, pyrrole
H3), 7.22 (m, 1H, pyrrole H5), 7.42±7.57 (m, 3H, arom),
7.71 (m, 2H, arom); 13C NMR (CDCl3) d 30.2, 30.6, 51.6,
64.7, 111.9, 112.6, 123.6, 126.5, 129.5, 134.0, 137.6, 139.1,
173.9. IR (CHCl3) 3578, 1776, 1732cmÀ1. HRMS calcd
for C15H15NO4S (MÀH2O) 305.0722, found 305.0723.
SO2Me); 13C NMR (CDCl3) of major isomer, d 21.9, 22.7,
24.8, 30.6, 32.9, 41.4, 43.0, 50.8, 52.3, 64.8, 111.1, 112.1,
122.9, 137.0, 173.1, 173.6; selected 13C NMR of minor
isomer, d 30.7, 33.0, 41.5, 50.8, 65.1, 173.1, 173.5. IR
(CHCl3) 3570, 3433, 3306, 1740, 1666cmÀ1; HRMS calcd
for C16H24N2O5S (MÀH2O) 356.1406, found 356.1407.
N-{4-Hydroxy-4-[1-(phenylsulfonyl)pyrrol-2-yl]}buta-
noyl-L-leucine methyl ester (21). General procedure A
was carried out using the pyrrole 17 (50 mg, 0.17mmol)
and phenylsulfonyl chloride (52mL, 0.41mmol, 2.4 equiv).
Flash chromatography on silica (EtOAc:petroleum ether,
1:1) gave an inseparable mixture of 17 and 19 (1:5 by 1H
NMR) which was used in the next step without further
N-[4-Oxo-4-(1H-pyrrol-2-yl)]butanoyl-L-leucine methyl
ester (17). The pyrrole ester 11 (300 mg, 1.66 mmol)
was hydrolysed with NaOH by general procedure D to
give 16 (217 mg, 78%) as a yellow solid, which was not
puri®ed further: mp 141 ꢀC (lit.16 140±141 ꢀC). A sample
of this material (190mg, 1.14mmol) was coupled with
l-leucine methyl ester hydrochloride (227mg, 1.25mmol,
1.1 equiv) according to general procedure E. Flash chro-
1
puri®cation. H NMR (CDCl3) of 19 assigned from the
mixture, d 0.88 (m, 6H, CHMe2), 1.42±1.62 (m, 3H,
CH2CHMe2), 2.51 (m, 2H, CH2CONH), 3.07 (m, 2H,

A.D. Abell, B.K. Nabbs / Bioorg. Med. Chem. 9 (2001) 621±628
627
COCH2), 3.68 (s, 3H, CO2Me), 4.52 (m, 1H, aH), 6.03 (d,
J=8.3Hz, 1H, CONH), 6.34 (m, 1H, pyrrole H4), 7.13
(m, 1H, pyrrole H3 ), 7.47±7.61 (m, 3H, arom), 7.79 (m,
1H, pyrrole H5), 7.96 (m, 2H, arom). The preceeding
sample of N-phenylsulfonylpyrrole 19 (48 mg, 0.11
mmol) was reduced with NaBH4 by general procedure
C. Flash chromatography on silica (EtOAc:petroleum
ether, 2:1) gave 21 (28 mg, 38% overall for both steps)
as a mixture of epimers (2:1 by 1H NMR). An analytical
sample was obtained by recrystallisation from EtOAc/
petroleum ether to give colorless crystals: mp 99±101 ꢀC.
1H NMR (CDCl3) of major isomer, d 0.94 (d, J=5.9 Hz,
6H, CHMe2), 1.51±1.67 (m, 3H, CH2CHMe2), 2.15 (m,
2H, CHOHCH2), 2.42 (m, 2H, CH2CONH), 3.75 (s, 3H,
CO2Me), 4.63 (m, 1H, aH), 4.96 (m, 1H, CHOH), 6.10
(d, J=7.3 Hz, 1H, CONH), 6.25 (m, 1H, pyrrole H4),
6.33 (m, 1H, pyrrole H3), 7.26 (m, 1H, pyrrole H5), 7.48±
according to general procedure E. Flash chromato-
graphy on silica (EtOAc:petroleum ether, 2:1) gave 25
(326 mg, 85%) as a pale-orange solid: mp 54 ꢀC. 1H
NMR (CDCl3) d 0.88 (d, J=5.9 Hz, 6H, CHMe2), 1.48±
1.64 (m, 3H, CH2CHMe2), 3.69 (s, 2H, CH2CO2), 3.72
(s, 3H, CO2Me), 4.63 (m, 1H, aH), 6.24 (dd, J=3.9,
2.4 Hz, 1H, pyrrole H3), 6.79 (d, J=7.8 Hz, 1H, CONH),
6.95 (dd, J=3.9, 2.4 Hz, 1H, pyrrole H4), 9.38 (s, 1H,
CHO), 10.89 (bs, 1H, NH); 13C NMR (CDCl3) d 21.6,
22.5, 24.7, 35.4, 40.9, 50.9, 52.2, 111.2, 122.9, 132.4,
135.5, 168.8, 173.6, 178.7. IR (CHCl3) 3422, 3298,
1744 cmÀ1. Anal. calcd for C14H20N2O4: C, 59.99; H,
7.19; N, 9.99. Found: C, 60.16; H, 7.20; N, 9.84.
N-{2-[5-Formyl-1-(methanesulfonyl)pyrrol-2-yl]}ethanoyl-
L-leucine methyl ester (26). The modi®ed general pro-
cedure A was carried out using pyrrole 25 (50 mg, 0.18
mmol) and mesyl chloride (17 mL, 0.21mmol, 1.2 equiv).
Flash chromatography on silica (EtOAc:petroleum ether,
1:1) gave 26 (45 mg, 70%) as a pale-yellow oil. 1H NMR
(CDCl3) d 0.95 (d, J=6.3 Hz, 6H, CHMe2), 1.53±1.80
(m, 3H, CH2CHMe2), 3.65 (s, 3H, SO2Me), 3.74 (s, 3H,
CO2Me), 3.93 and 4.00 (ABq, J=16.6 Hz, 2H, CH2CO),
4.64 (m, 1H, aH), 6.19 (d, J=8.3 Hz, 1H, CONH), 6.25
(d, J=3.9 Hz, 1H, pyrrole H3), 7.12 (d, J=3.9 Hz, 1H,
pyrrole H4), 9.70 (s, 1H, CHO); 13C NMR (CDCl3) d
21.8, 22.8, 24.6, 36.2, 41.6, 42.4, 50.9, 52.4, 115.1, 126.7,
135.4, 138.6, 168.6, 173.3, 178.2. IR (CHCl3) 3429, 3327,
1740 cmÀ1. HRMS calcd for C15H22N2O6S 358.1199,
found 358.1190.
1
7.63 (m, 3H, arom), 7.78 (m, 2H, arom); selected H
NMR of minor isomer, d 3.73 (s, 3H, CO2Me); 13C
NMR (CDCl3) of major isomer, d 21.9, 22.7, 24.8, 31.0,
32.9, 41.6, 50.8, 52.3, 65.3, 112.0, 112.7, 123.4, 126.5,
129.5, 134.0, 138.0, 139.1, 172.9, 173.5; selected 13C
NMR of minor isomer, d 31.0, 65.4, 126.5. IR (CHCl3)
3568, 3433, 3317, 1740, 1670 cmÀ1. Anal. calcd for
C21H28N2O6S: C, 57.78; H, 6.47; N, 6.42. Found: C,
57.86; H, 6.25; N, 6.51.
Ethyl 2-(5-formyl-1H-pyrrol-2-yl)ethanoate (23). POCl3
(1.1 equiv) was added dropwise over 15 min to a solu-
tion of DMF (1.1 equiv) cooled to 10±20 ꢀC under N2.
1,2-Dichloroethane (8 mL) was added, followed by a
solution of the pyrrole 229 (2.19 g, 14.3 mmol) in 1,2-
dichloroethane (8 mL) over 1 h. The mixture was heated
at re¯ux for 15 min, cooled to 20 ꢀC, treated with a
solution of NaOAc.3H2O (5 equiv) in water (30 mL) and
heated at re¯ux for a further 15 min. The 1,2-dichloro-
ethane layer was separated, and the aqueous phase was
extracted with ether (3Â15 mL). The combined organic
phases were washed with saturated aqueous NaHCO3
(3Â15 mL), dried and evaporated under reduced pres-
sure. The residue was distilled under high vacuum to give
23 (1.45 g, 56%) as a pale-orange solid. An analytical
sample was obtained by sublimation under reduced
pressure to give a white crystalline solid: mp 73±75 ꢀC.
1H NMR (CDCl3) d 1.29 (t, J=7.3 Hz, 3H, CO2
CH2CH3), 3.72 (s, 2H, CH2CO2), 4.22 (q, J=7.3 Hz, 2H,
CO2CH2CH3), 6.18 (m, 1H, pyrrole H3), 6.90 (m, 1H,
pyrrole H4), 9.45 (s, 1H, CHO), 9.88 (bs, 1H, NH); 13C
NMR (75 MHz, CDCl3) d 14.0, 33.4, 61.4, 111.0, 122.2,
132.6, 133.8, 169.6, 178.7. IR (CHCl3) 3433, 1732,
1651 cmÀ1. Anal. calcd for C9H11NO3: C, 59.66; H,
6.12; N, 7.73. Found: C, 59.89; H, 5.99, N, 7.71.
N-{2-[5-Hydroxymethyl-1-(methanesulfonyl)pyrrol-2-yl]}
ethanoyl-L-leucine methyl ester (27). The N-mesylpyr-
role 26 (35 mg, 0.10 mmol) was reduced with Zn(BH4)2
by general procedure B. Flash chromatography on silica
(EtOAc:petroleum ether, 2:1) gave 27 (29 mg, 83%) as
an orange solid. An analytical sample was obtained by
recrystallisation from methanol to give thin colourless
ꢀ
1
needles: mp 150±151 C. H NMR (CD3OD) d 0.94 (m,
6H, CHMe2), 1.55±1.81 (m, 3H, CH2CHMe2), 3.40 (s,
3H, SO2Me), 3.70 (s, 3H, CO2Me), 3.80 and 3.89 (ABq,
J=17.1 Hz, 2H, CH2CO), 4.47 (m, 1H, aH), 4.67 and
4.74 (ABq, J=13.2 Hz, 2H, CH2OH), 6.09 (d, J=3.4 Hz,
1H, pyrrole H3), 6.22 (d, J=3.4 Hz, 1H, pyrrole H4), 8.30
(d, J=7.8 Hz, 1H, CONH); 13C NMR (CD3OD) d 22.1,
23.6, 26.1, 36.7, 41.8, 42.8, 52.6, 53.0, 58.4, 114.7, 115.1,
132.3, 137.5, 173.2, 175.1. IR (CHCl3) 1740, 1676 cmÀ1
.
Anal. calcd for C15H24N2O6S: C, 49.99; H, 6.71; N,
7.77. Found: C, 49.70; H, 6.93; N, 7.50.
Acknowledgements
N-[2-(5-Formyl-1H-pyrrol-2-yl)]ethanoyl-L-leucine methyl
ester (25). The pyrrole ester 23 (300 mg, 1.66 mmol)
was hydrolysed with NaOH by general procedure D to
give 24 (213 mg, 84%) as a yellow solid that was not
puri®ed further: mp 123±124 ꢀC. 1H NMR (acetone-d6) d
3.86 (s, 2H, CH2CO2), 6.33 (dd, J=3.4, 2.4 Hz, 1H, pyr-
role H3), 7.08 (dd, J=3.4, 2.4 Hz, 1H, pyrrole H4), 9.43
(s, 1H, CHO), 11.49 (bs, 1H, NH). Thus obtained 24
(210 mg, 1.37 mmol) was coupled with l-leucine methyl
ester hydrochloride (274 mg, 1.51 mmol, 1.1 equiv)
The work was supported by a Royal Society of New
Zealand Marsden grant. We thank the University of
Canterbury for ®nancial assistance (PhD scholarship to
BKN).
References and Notes
1. (a) Abell, A. D.; Nabbs, B. K.; Battersby, A. R. J. Am.
Chem. Soc. 1998, 120, 1741. (b) Abell, A. D.; Nabbs, B. K.;

628
A.D. Abell, B.K. Nabbs / Bioorg. Med. Chem. 9 (2001) 621±628
Battersby, A. R. J. Org. Chem. 1998, 63, 8163. (c) Abell, A.
D.; Nabbs, B. K. Org. Lett. 1999, 1, 1403. (d) Abell, A. D.;
Litten, J. C. Tetrahedron Lett. 1992, 3005.
carboxyl side of this residue (for a de®nition of this nomen-
clature see: Schechter, I.; Berger, A. Biochem. Biophys. Res.
Commun. 1967, 27, 157).
2. Abell, A. D.; Litten, J. C.; Nabbs, B. K. J. Chem. Soc.,
Chem. Commun. 1998, 919.
3. Jones, A. R., Bean, G. P., Eds.; The Chemistry of Pyrroles
Academic: London, 1977, pp 356±357. Katritzky, A. R., Rees,
C. W., Eds.; Comprehensive Heterocyclic Chemistry; Perga-
mon: Oxford, 1984, Vol. 4.
4. Wallace, D. M.; Leung, S. H.; Smith, K. M. J. Org. Chem.
1993, 58, 7245. Tietze, L. F.; Kettschau, G.; Heitmann, K.
Synthesis 1996, 851.
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Trans 1 1982, 2427. Battersby, A. R., Leeper, F. J. Chem. Rev.
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J. M. Ed.; JAI: Connecticut, 1992; pp 189±212.
8. The choice of protecting group (N-phthaloyl) and the
amino acid (leucine) were dictated by ease of synthesis
(unpublished results).
9. We have previously shown that an N-sulfonyl group is
superior to an N-acyl group at deactivating a hydroxy-
methylpyrrole (see ref 1a). As a consequence these compounds
tend to be more stable and are in many cases crystalline solids.
10. The use of the acid ¯uoride, rather than the acid chloride,
gave a much better yield of the desired product. Acid ¯uorides
have also been reported to minimise epimerisation at the
amino acid stereogenic centre in reactions of this type (see ref
14).
11. Fersht, A. Enzyme Structure and Mechanism, 2nd ed.; W.
H. Freeman: New York, 1985; pp 17±22.
12. Abell, A. D.; Litten, J. C. Aust. J. Chem. 1993, 46, 1473.
13. Schloemer, G. C.; Greenhouse, R.; Muchowski, J. M. J.
Org. Chem. 1994, 59, 5230.
14. Savrda, J.; Chertanova, L.; Wakselman, M. Tetrahedron
1992, 33, 3005.
15. Hinz, W.; Jones, R. A.; Anderson, T. Synthesis 1986, 620.
16. Eraksina, V. N.; Ivanova, T. M.; Babushkina, T. A.;
Vasil'ev, A. M.; Suvorov, N. N. Khim. Geterotsikl. Soedin
1979, 7, 916.
6. Silverman, R. B. In Mechanism-Based Enzyme Inacti-
vation: Chemistry and Enzymology; CRC, Florida, 1988; Vols
1 and 2. Abell, A. D. In Detailed Reaction Mechanisms:
Mechanisms of Biological Importance; Coxon, J., Ed.; JAI:
London, 1992; Vol. 2, pp 243±260. Rando, R. R. Science 1974,
185, 320.
7. The P1 amino acid de®nes the point of cleavage of the sub-
strate peptide where, by de®nition, hydrolysis occurs on the