Pyrrolo[3,2-e][1,4]diazepin-2-one synthesis: A head-to-head comparison of soluble versus insoluble supports

Article abstract of DOI:10.1021/jo200424q

Aryldiazepin-2-ones are known as "privileged structures", because they bind to multiple receptor types with high affinity. Toward the development of a novel class of aryldiazepin-2-one scaffolds, the synthesis of pyrrolo[3,2-e][1,4]diazepin-2-ones on a support was explored starting from N-(PhF)-4-hydroxyproline and featuring an acid-catalyzed Pictet-Spengler reaction to form the diazepine ring. Three supports [Wang resin, tetraarylphosphonium (TAP) soluble support, and Merrifield resin] were examined in the synthesis of the heterocycle and exhibited different advantages and disadvantages. Wang resin proved effective for exploratory optimization of the synthesis by identification of intermediates after resin cleavage under mild conditions; however, the acidic conditions of the Pictet-Spengler reaction caused premature loss of resin-bound material. Direct monitoring of reactions by TLC, RP-HPLC-MS, and in certain cases NMR spectroscopy was possible with the TAP support, which facilitated purification of intermediates by precipitation; however, incomplete precipitation of material led to overall yields lower than those from solid-phase approaches on resin. Merrifield resin proved stable to the conditions for the synthesis of the pyrrolo[3,2-e][1,4]diazepin-2-one targets and would be amenable to "split-and-mix" chemistry; however, relatively harsh conditions were necessary for final product cleavage. Perspective for the application of different solid-phase approaches in heterocycle library synthesis was thus obtained by demonstration of the respective utility of the three supports for preparation of pyrrolo[3,2-e][1,4] diazepin-2-one.

Full text of DOI:10.1021/jo200424q

ARTICLE
pubs.acs.org/joc
Pyrrolo[3,2-e][1,4]diazepin-2-one Synthesis: A Head-to-Head
Comparison of Soluble versus Insoluble Supports
Nicolas Boutard, Julien Dufour-Gallant, Philippe Deaudelin,† and William D. Lubell*
Chemistry Department, Universitꢀe de Montrꢀeal, C.P. 6128, Succursale Centre-Ville, Montreal, Quebec H3C 3J7, Canada
S Supporting Information
b
ABSTRACT:
Aryldiazepin-2-ones are known as “privileged structures”, because they bind to multiple receptor types with high affinity. Toward the
development of a novel class of aryldiazepin-2-one scaffolds, the synthesis of pyrrolo[3,2-e][1,4]diazepin-2-ones on a support was
explored starting from N-(PhF)-4-hydroxyproline and featuring an acid-catalyzed PictetꢀSpengler reaction to form the diazepine
ring. Three supports [Wang resin, tetraarylphosphonium (TAP) soluble support, and Merrifield resin] were examined in the
synthesis of the heterocycle and exhibited different advantages and disadvantages. Wang resin proved effective for exploratory
optimization of the synthesis by identification of intermediates after resin cleavage under mild conditions; however, the acidic
conditions of the PictetꢀSpengler reaction caused premature loss of resin-bound material. Direct monitoring of reactions by TLC,
RP-HPLC-MS, and in certain cases NMR spectroscopy was possible with the TAP support, which facilitated purification of
intermediates by precipitation; however, incomplete precipitation of material led to overall yields lower than those from solid-phase
approaches on resin. Merrifield resin proved stable to the conditions for the synthesis of the pyrrolo[3,2-e][1,4]diazepin-2-one
targets and would be amenable to “split-and-mix” chemistry; however, relatively harsh conditions were necessary for final product
cleavage. Perspective for the application of different solid-phase approaches in heterocycle library synthesis was thus obtained by
demonstration of the respective utility of the three supports for preparation of pyrrolo[3,2-e][1,4]diazepin-2-one.
’ INTRODUCTION
literature to exhibit, respectively, antimuscarinic10 and GABA
antagonist properties.11
Aryldiazepin-2-ones, such as [1,4]benzodiazepin-2-ones, are
considered as “privileged structures”,1 because of their affinity for
multiple receptor targets. Their ability to bind protein receptors
and elicit interesting biological activity may be due to their
potential to mimic peptide turn secondary structures.2 The
aryldiazepin-2-one scaffold3 achieved popularity first in the
1960s as a component of GABA receptor antagonists (i.e.,
diazepam (Valium))3d,e for treating CNS-related conditions.
The platform was later shown to interact with various hormone
receptors1b,4 and has been used in enzyme inhibitors5 as well as in
inhibitors of proteinꢀDNA interactions.6
Their notable activities and opportunity for novel structural
diversity have made the development of methods for construct-
ing and functionalizing pyrrolodiazepinone scaffolds a promising
subject for medicinal chemistry research. Recent examples have
key steps featuring the use of the multicomponent Ugi-type
condensation,12 phenyliodine(III)bis-(trifluoroacetate)-mediated in-
tramolecular N-acylnitrenium ion reaction,13 and cyclization via
Schmidt thermolysis of acylazides.14 In addition, the intramole-
cular PaalꢀKnorr reaction has provided pyrrolo[1,2-d][1,4]di-
azepin-4-ones 7 (Figure 1).2b
A solution-phase synthesis of the pyrrolo[3,2-e][1,4]diazepin-
2-one scaffold 13 was recently achieved starting from (2S,4R)-4-
hydroxproline (Scheme 1).15 The key for introducing struc-
tural diversity in this approach was the synthesis of 4-N-
substituted-aminopyrrole-2-carboxylates 10 upon treatment
Pyrrolodiazepin-2-ones have been reported less often in the
literature; however, they similarly display remarkable biological
activities. For example, the DNA-binding pyrrolo[2,1-c][1,4]be-
nzodiazepine natural product, anthramycin (2, Figure 1), is an
antitumor antibiotic.7 Pyrrolodiazepin-2-ones 3 and 4 are, re-
spectively, inhibitors of angiotensin converting enzyme (ACE)8
and HIV reverse transcriptase (HIV-RT).9 In addition, pyrrolo-
diazepin-2-one scaffolds 5 and 6 have been reported in the patent
Received: February 24, 2011
Published: April 14, 2011
r
2011 American Chemical Society
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Figure 2. Supports used for the synthesis of pyrrolo[3,2-e]-
[1,4]diazepin-2-one.
and deprotection with piperidine. Ring closure was accomplished
by PictetꢀSpengler condensation with aldehydes, giving the
pyrrolo[3,2-e][1,4]diazepin-2-ones with a cis relative stereo-
chemistry for the major compounds. The stereoselective out-
come of the reaction favored an endo attack of the E iminium ion
in a transition state having the amino acid side chain in an
equatorial orientation. Aromatic aldehydes, especially those
bearing electron-withdrawing groups, gave generally better yields
than aliphatic aldehydes.
Figure 1. Representative pyrrolodiazepinones.
In addition to offering a modular approach for making the
bicyclic ring system, crystals of pyrrolo[3,2-e][1,4]diazepin-2-
one 13 (Scheme 1, R1 = Bn, R2 = (S)-s-Bu, R3 = Ph) were iso-
lated and subjected to X-ray analysis, which demonstrated that
the dihedral angles of the diazepinone R-amino acid residue
(φ = 71°, ψ = ꢀ93°) compared favorably with those of an ideal
reverse γ-turn (φ = 60° to 70°, ψ = ꢀ70° to ꢀ85°).15
Scheme 1. Solution-Phase Synthesis of Pyrrolo[3,2-e]-
[1,4]diazepin-2-ones.15
Considering the importance of γ-turn conformations for the
biological activity of various peptides, such as enkephalin,18
angiotensin,19 bradykinin,20 vasopressin,21 substance P,22
somatostatin,23 and oxytocin,24 we are developing a diversity-
oriented methodology for modular construction of pyrrolo[3,2-e]-
[1,4]diazepin-2-one libraries. Herein is reported the examination
of three different supports to provide the desired scaffold without
purification of intermediates by chromatography.
Wang resin (Figure 2) was tried because of previous success in
the solid-phase synthesis of 3-aminopyrrole-2,5-dicarboxylate
analogues;25 however, its labile nature under acidic conditions
meant cleavage of product in the PictetꢀSpengler cyclization,
inhibiting further modification of the pyrrolo[3,2-e][1,4]diaze-
pin-2-one scaffold on the support. Merrifield resin was effective
for synthesizing the pyrrolo[3,2-e][1,4]diazepin-2-one scaffold
without resin cleavage, offering an opportunity for further
modification of the heterocycle; however, resin cleavage necessi-
tated relatively harsh conditions. The (40-(bromomethyl)-[1,10-
biphenyl]-4-yl)triphenylphosphonium salt 1426 (Figure 2), a
soluble variant of Merrifield resin, was chosen as a solubility
control group (SCG) for its advantages in the preparation of
small molecules and peptides,26,27 because reactions can be
performed under homogeneous conditions in polar solvent
systems prior to purification by precipitation from solvents of
low polarity. Precipitation of the tetraarylphosphonium (TAP)
substrates from Et2O has been shown to be counterion dependent
of methyl (S)-4-oxo-1-(9-phenyl-9H-fluoren-9-yl)prolinate 9 with
different primary amines in the presence of catalytic p-TsOH,
which caused pyrrole formation and concomitant loss of the
9-phenyl-9H-fluorene (PhF-H) group.16,17 Compounds 11 were
obtained by 4-aminoacylation with various Fmoc-amino acids
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ARTICLE
(Brꢀ < ClO4ꢀ < PF6ꢀ);27b thus, the inexpensive perchlorate salt
was chosen for our study. In contrast to resins, which required
cleavage prior to reaction analysis, substrates were examined
directly on the TAP support by TLC, RP-HPLC-MS, and NMR
spectroscopy. The head-to-head comparison of these three
supports in the context of the synthesis of pyrrolo[3,2-e]-
[1,4]diazepin-2-one has revealed their strengths and limitations
and is guiding efforts toward the development of an effective
method for diversity-oriented library synthesis.
Scheme 2. Supported Synthesis of 4-Aminopyrrole-2-
carboxylates
’ RESULTS
4-Aminopyrrole-2-carboxylate Synthesis. In the synthesis
of pyrrolo[3,2-e][1,4]diazepin-2-one, the initial unit of diversity
was added in the synthesis of the 4-aminopyrrole-2-carboxylate.
In previous syntheses of aminopyrrole on solid support, an ester
linkage to the Wang resin was used to prepare 4-aminopyrrole-2-
carboxylates with tertiary 4-amino groups,25 and a cysteinamine
linker to the Merrifield resin was used to attach the aminopyrrole
to the resin as a secondary amine, which was employed in the
synthesis of pyrrolopyrimidines.28 In both cases, a 4-oxo-
N-(PhF)proline-2-carboxylate served as precursor in the amino-
pyrrole forming reaction, which has proven applicable using a
wide variety of primary and secondary amines in solution.17
In light of this precedent, benzylamine was the only example
employed to form 4-aminopyrrole-2-carboxylate in the com-
parative study.
To link (2S,4R)-4-hydroxy-N-(PhF)proline (15) to the sup-
port, the corresponding cesium salt 16 was prepared and reacted
with the Wang bromide resin [4-(bromomethyl)phenoxymethyl
polystyrene, prepared from Wang resin, Figure 2] according to
previously published procedures, and loading was ascertained as
previously described (Scheme 2).25,29 Merrifield resin and its
soluble TAP variant 14 were employed as obtained from the
manufacturer.
To avoid experimental bias during comparisons of the solid
supports, Wang and Merrifield resins were placed in IRORI
MacroKans and reacted simultaneously together in the same pot,
using magnetic stirring to agitate the Kans without crushing the
resin. Cesium salt 16 (150 mol %) was reacted with Wang
bromide and Merrifield resins in the presence of dibenzo-18-
crown-6 in DMF at 70 °C for 48 h.30 Anchoring efficiency was
estimated to be quantitative based on the increase of the resin
mass. Moreover, IR spectroscopy indicated disappearance of the
CꢀBr band at 592 cmꢀ1 and appearance of bands at 3448 and
1737 cmꢀ1 for the OH and CdO stretches of resin-bound
(2S,4R)-4-hydroxy-N-(PhF)proline 17.
Soluble support14 was reacted with cesium salt 16 (110 mol %)
in the presence of catalytic potassium iodide in DMF at 60 °C for
3 h, which gave quantitative conversion to the TAP-bound
4-hydroxy-N-(PhF)proline 19 as indicated by TLC (Rf = 0.51,
7.5% MeOH in DCM).31 After reactions on the TAP support,
workup was completed by a saturated aqueous LiClO4 wash to
ensure the presence of the counteranion. Partial purification of
TAP-supported material was achieved by dissolving in a mini-
mum volume of CH2Cl2 and precipition using a 5-fold volume of
Et2O. For characterization purposes, analytical samples of TAP-
supported intermediates were purified by radial chromatography
(see Supporting Information).32
treatment with DIEA. In both cases, the oxidation was performed
twice for complete conversion to ketone, which was ascertained
by the appearance of a second carbonyl band at 1758 cmꢀ1 and
disappearance of the OH band around 3440ꢀ3460 cmꢀ1 in the
IR spectra of resins 20 and 21. On TAP support, complete
oxidation of alcohol 19 was achieved using similar conditions and
less reagent in a single reaction as comfirmed by RP-HPLC-MS.
Ketone 22 exhibited low stability and was best employed
immediately in the following step.
Resin-bound 4-aminopyrroles 23 and 24 were, respectively,
prepared by treatment of ketones 20 and 21 with an excess of
benzylamine (2300 mol %), in the presence of catalytic p-TsOH
(0.1 equiv) in dry THF. In individual cases, 4-benzylaminopyr-
role loading was evaluated by the amount of PhF-H recovered
after filtration of the resin, washings, evaporation of the filtrate,
and column chromatography (2% Et2O/hexanes). 4-Aminopyr-
role-2-carboxylate could not be detected by RP-LCMS after resin
cleavage with acid (TFA/DCM 1/1) or methoxide (0.44 M
NaOCH3 in THF/MeOH). After N-acylation, 4-amidopyrroles
(see below, Table 1) could however be detected by RP-HPLC-
MS analysis after cleavage with sodium methoxide, which caused
concomitant Fmoc removal. Accordingly, the PhF group was
observed to be retained on 10 to 30% of pyrrole, because of
premature aromatization before PhF-H extrusion.17 This side
reaction was presumed to be due to residual oxygen and avoided
by degassing the resin suspended in THF using freezeꢀthaw
cycles of exposure to high vacuum followed by flushing argon and
4-Oxo-N-(PhF)prolines 20 and 21 were obtained, respec-
tively, from oxidation of resin-bound alcohols 17 and 18 using
oxalyl chloride and DMSO at ꢀ78 °C for 4 h, followed by
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Table 1. Aminoacylation of 4-Benzylaminopyrrole 23
Scheme 3. Supported Aminoacylation of 4-Aminopyrrole-2-
carboxylates
purity after cleavage
entry
acylating acid
30 (tR; [M þ H]þ) 31 (tR; [M þ H]þ)b
a
b
c
d
e
f
Fmoc-Phe-OH
Fmoc-Leu-OH
Fmoc-Asp(OMe)-OH 76% (16.0; 346.2)a NDd
82% (18.4; 364.2)a 85% (16.6; 378.2)b
81% (17.2; 330.2)a 93% (15.1; 344.2)a
Fmoc-His(Trt)-OH
Fmoc-Ser(OtBu)-OH 87% (11.4; 304.2)b,c 83% (16.4; 374.2)b
90% (11.9; 354.2)a 75% (21.2; 610.2)b
Fmoc-Lys(Boc)-OH
Fmoc-Cys(Trt)-OH
Fmoc-Met-OH
82% (12.5; 345.2)a 91% (17.6; 459.2)b
84% (12.8; 320.0)b,c 60% (21.2; 576.2)b
68% (16.7; 348.2)a 69% (14.9; 362.2)b
g
h
i
Fmoc-Glu(OtBu)-OH 74% (12.9; 346.2)a ND
j
Fmoc-Pro-OH
65% (15.8; 314.2)a 52% (13.7; 328.2)b
k
l
Fmoc-Orn(Boc)-OH ND
82% (15.1; 445.2)b
33 (16.2; 273.2)a
Fmoc-Aib-OH
32 (11.7; 259.1)a
m
p-NO2PhCO2H
82% (21.9; 366.2)b 37% (23.1; 380.0)b
a Determined by RP-HPLC-MS [C18 column 150 mm ꢁ 4.6 mm, 5 μm,
5ꢀ90% MeOH in H2O in 20 min and then 90% MeOH for 15 min, 0.1%
FA, UV: λ = 254 nm, retention time (tR) expressed in min.]. b Deter-
mined by RP-HPLC-MS (C18 column 150 mm ꢁ 4.6 mm, 5 μm,
5ꢀ80% MeOH in H2O in 20 min and then 90% MeOH for 15 min, 0.1%
c
FA, UV: λ = 254 nm). Several conformers were detected. d ND: not
detected.
addition of predried MgSO4 (2-fold the weight of dry resin),
benzylamine, and p-TsOH, followed by further degassing by
argon bubbling for 30 min before heating the reaction at 50 °C
for 15 h. Under these optimized conditions, complete conversion
to 4-benzylaminopyrroles 23 and 24 was achieved as indicated by
the quantitative amounts of PhFH recovered, as well as by RP-
HPLC-MS analysis of 4-amidopyrrole 26a after resin cleavage
with sodium methoxide.
4-Benzylaminopyrrole 25 was prepared on the TAP support in
acetonitrile instead of THF, because of limited solubility in the
latter. Quantitative conversion of TAP-supported ketone 22 to
aminopyrrole 25 was indicated by RP-HPLC-MS analysis in the
same amount of time (15 h) as on resin using less N-benzylamine
(400 mol %).
Aminoacylation. The scope of N-(Fmoc)amino acids which
could be employed in the acylation of 2-aminopyrroles was
examined on Wang resin 23 on 0.03 mmol scale using 12
N-(Fmoc)amino acids (listed in Table 1) and p-nitrobenzoic
acid (500 mol %, Scheme 3). Triphosgene (BTC) was employed
as coupling agent in the presence of 2,4,6-collidine in dry THF
for 15 h. Efficiency of coupling was ascertained on two resin
aliquots, which were first both treated with excess acetyl chloride
and DIEA in DMF for 2 h, followed by 20% piperidine in DMF
to, respectively, cap excess aminopyrrole and remove the Fmoc
group. Subsequently, the resins were, respectively, cleaved with
TFA/DCM 1:1 for 1 h to give the corresponding acids, or with
0.44 M NaOCH3 in 9:1 THF/MeOH for 3 h to furnish the ester
counterparts. Cleaved material was analyzed by RP-HPLC-MS
without isolation on a 0.01 mmol scale. Although tertiary amide
isomers complicated certain chromatographic analyses, the de-
tection of residual aminopyrrole 23 was facilitated by conversion
to the corresponding acetamide.
of the aminopyrrole starting material. The steric bulk of
N-(Fmoc)Aib was presumed to inhibit coupling, which also
failed using its symmetric anhydride prepared with DIC. Purities
were generally higher after the acidic cleavages, which failed only
to detect the His and Orn analogues 26d and 26k that were
observed after the methoxide cleavage. The latter, however, was
unable to detect the dicarboxylate Asp and Glu analogues 26c
and 26i.
Phenylalanine was deemed the amino acid of choice for
comparative analyses with the other supports, because of its
effective coupling as determined by both cleavage methods and
utility in subsequent PictetꢀSpengler chemistry as described
below. On the Merrifield resin, the BTC-mediated coupling of
Fmoc-Phe (500 mol %) gave amide 34 in 85% purity, as
determined by the methoxide cleavage analysis. On the TAP
support, a DCM/THF 8:2 mixture enhanced solubility in
Except for N-(Fmoc)amino isobutyramide 26l, for which only
the cleaved capped starting material 32 and 33 were detected, all
desired amides 26 were produced, with complete consumption
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ARTICLE
coupling of Fmoc-Phe (300 mol %) with BTC and 2,4,6-collidine
to give amide 35. Subsequent Fmoc removal was performed,
respectively, with 20% piperidine in DMF twice for 20 min, and
with 50% piperidine in DCM once for 10 min on the resin-bound
and TAP-supported Fmoc-Phe substrates.
PictetꢀSpengler Reaction and Pyrrolo[3,2-e][1,4]diaze-
pin-2-one Cleavage. The conditions for the PictetꢀSpengler
cyclization were initially examined on Wang resin, because
parallel chemistry could be conveniently performed and assessed
using this support as demonstrated in the amino acylation
study above. Conjecturing that the use of strong Brønsted acids
might cause premature cleavage of substrate, milder proto-
cols were first explored using B(OMe)3,33 Yb(OTf)3,34 LiCl in
DCM/hexafluoropropanol,35 and iodine;36 however, all had no
effect and left the starting material unchanged. A PictetꢀSpengler
protocol was attempted using an N-acyliminium ion (Scheme 4),37
which have previously enhanced such iminium-mediated Man-
nich-type reactions.38 On 0.01 mmol scale, ornithine derivative
29k was arbitrarily chosen as substrate and reacted, respectively,
with benzaldehyde and isobutyraldehyde in the presence of
methyl orthoformate, as water scavenger, in dry DCM to give
imines 36 and 37, which were treated with p-nitrophenyl
chloroformate in pyridine to induce PictetꢀSpengler cyclization
by way of the respective N-acyliminiums 38 and 39. After 18 h at
room temperature, resin cleavage with 50% TFA in DCM and RP-
HPLC-MS analysis revealed diazepinones 40 and 41 as minor
components contaminated with carbamate 42 as major product.
Previous success, in the solution-phase synthesis of pyrrolo-
[3,2-e][1,4]diazepin-2-ones using trifluoroacetic acid as catalyst
in the PictetꢀSpengler cyclization, compelled study of protic acid
conditions on resin. Optimization was performed on Leu amide
29b on 0.01 mmol scale using isobutyraldehyde (1000 mol %) as
a challenging substrate, by varying the solvent (2 mL), tempera-
ture, mode of heating, and TFA concentration (Scheme 5).
Reaction efficiency was evaluated after resin cleavage with
50% TFA in DCM and RP-HPLC-MS analysis, without product
isolation. The conditions were optimized to produce the high-
est amount of the postulated major stereoisomer (3S,5R)-44b
and minimum side products and starting material (Table 2).
Unsaturated diazepinone side-product 45 was soon recognized
as an inherent product in the synthesis of pyrrolo[3,2-e]-
[1,4]diazepin-2-one, which is presumed to be formed by a
mechanism as that described for the formation of similar
unsaturated carbolines39 and could not be avoided by careful
degassing using freezeꢀthaw cycles under vacuum. Moreover,
pyrrolo[3,2-e][1,4]diazepin-2-ones (3S,5R)-44b and (3S,5S)-
44b were observed to convert to their unsaturated counterpart
45b upon standing over time.
Scheme 4. N-Acyliminium PictetꢀSpengler Reaction on
Compound 29k
Treating Wang resin 29b with 1% TFA in DCM and TFE/
DCM 1:1 at rt for 24 h led, respectively, to 10% and 15%
conversion to (3S,5R)-44b (Table 2).40 Heating 29b with 1%
TFA in DCM and TFE/DCM 1:1 at 70 °C in a sealed tube for
24 h increased conversion to 29%, albeit with formation of almost
equal amounts of dehydro analogue 45b and some (3S,5S)-
diastereoisomer. Microwave heating at 70 °C in a sealed reactor
in a 4:1 DCM/TFE mixture reduced the reaction time to 2 h,
minimized formation of 45b, and improved conversion to
(3S,5R)-44b to 73%. Microwave heating at lower temperature
(60 °C) decreased the rate of cyclization, and higher temperature
(80 °C) promoted formation of (3S,5S)-44b and 45b. More
polar solvents such as THF or CHCl3 increased the amount of
45b and caused formation of unidentified impurities. Toluene
gave similar results as DCM/TFE 4:1. Benzene gave the best
results with 83% conversion to (3S,5S)-44b with minimal
amounts of (3S,5S)-44b and 45b (6% and 8%). Further attempts
to reduce the amount of TFA in benzene caused a drop in the
Scheme 5. PictetꢀSpengler Cyclization of Pyrrole 29b
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Table 2. Optimization of the PictetꢀSpengler Cyclization on Compound 29b
entry
TFA, %
T
t, h
solvent
43b,a %
(3S,5R)-44b,a %
(3S,5R)-44b,a %
45b,a %
1
1
RT
24
24
24
2
DCM
90
85
42
67
17
11
8c
13c
17
3
10
15
29
23
73
65
55
9
NDb
ND
5
ND
ND
24
7
2
1
RT
DCM/TFE 1/1
DCM/TFE 1/1
DCM/TFE 4/1
DCM/TFE 4/1
DCM/TFE 4/1
CHCl3
3
1
70 °C
4
1
60 °C microwave
70 °C microwave
80 °C microwave
70 °C microwave
70 °C microwave
70 °C microwave
70 °C microwave
70 °C microwave
3
5
1
2
3
7
6
1
2
9
15
18
61
5
7
1
2
4
9
1
2
THF
ND
4
9
1
2
toluene
74
83
45
10
11
1
2
benzene
6
8
0.1
2
benzene
23
6
26
a Determined by RP-HPLC-MS (C18 column 150 mm ꢁ 4.6 mm, 5 μm, 0ꢀ90% MeOH in H2O in 20 min and then 90% MeOH for 15 min, 0.1% FA,
UV: λ = 254 nm). b ND: not detected. c In these cases, unknown impurities (15ꢀ20%) were also detected.
Table 3. PictetꢀSpengler Cyclization with Diverse Amino Acid-Derived Amidopyrroles 29
entry
R2
43,a % (tR; [M þ H]þ)
(3S,5R)-44,a % (tR; [M þ H]þ)
(3S,5S)-44,a % (tR; [M þ H]þ)
45,a % (tR; [M þ H]þ)
a
Bn
27 (12.2; 364.0)
12 (17.3; 330.2)
9 (10.7; 346.2)
17 (8.8; 354.2)
29 (10.7; 304.1)
76 (9.0; 345.2)
ND
73 (15.1; 418.2)
72 (20.8; 384.4)
63 (14.7; 400.2)
54 (11.8; 408.3)
38 (12.8; 358.2)
24 (10.5; 399.2)
ND
NDb
ND
b
i-Pr
5 (19.8; 384.4)
8 (13.4; 400.2)
29 (11.7; 408.3)
8 (12.2; 358.2)
ND
11 (22.6; 382.3)
c
(CH2)2CO2CH3
4-CH2-1H-imidazole
CH2OH
20 (15.9; 398.3)
dc
e
ND
25 (13.3; 356.1)
f
(CH2)4NH2
ND
ND
ND
ND
ND
gc
hd
ic
jc
CH2SH
ND
(CH2)2SCH3
(CH2)3CO2H
pyrrolidine (from Pro)
NDe
ND
ND
ND
ND
ND
78 (11.1; 314.2)
ND
ND
a Determined by RP-HPLC-MS (C18 column 150 mm ꢁ 4.6 mm, 5 μm, 0ꢀ90% MeOH in H2O in 20 min and then 90% MeOH for 15 min, 0.1% FA,
UV: λ = 254 nm). b ND: not detected. c In these cases, unknown impurities were also detected. d In this case, two isomers with very close retention time
were observed. e 43h, 44h, and 45h products bearing one extra oxygen atom were detected.
Scheme 6. One-Pot PictetꢀSpengler Cyclization of Pyrroles 29aꢀj
formation of (3S,5S)-44b and increased the proportion of 45b
(entry 11, Table 2).
furnished the corresponding cyclized product (3S,5R)-44f, albeit
in low conversion (24%) with considerable product derived from
residual starting material. No desired products were detected
from reactions of N-(cysteinyl), N-(methioninyl), N-(glutamyl),
and N-(prolyl)amidopyrroles 29gꢀj; however, analysis of the
cleaved product from reaction of Met analogue 29h indicated
molecular ions for product plus 16 and 32 mass units, suggesting
sulfur oxidation under the conditions of the PictetꢀSpengler
reaction.
The synthesis of phenylalanine-derived amidopyrrole 29a was
repeated on larger scale using 300 mg of starting Wang bromide
resin (0.66 mmol) employing IRORI MacroKans in parallel
reactions with its Merrifield resin counterpart 34 (0.37 mmol,
420 mg of resin) in the same pot. For the PictetꢀSpengler
reaction, resins 29a and 34 were separately transferred into three
IRORI MiniKans (100 mg of resin capacity), which could be fit
With optimized PictetꢀSpengler conditions in hand, we
employed a “one-pot” procedure: isobutyraldehyde was reacted
with resins 29aꢀj (0.01 mmol scale) packed in IRORI Micro-
Kans, and substrates were cleaved separately using 50% TFA in
DCM containing 1% of triethylsilane as scavenger and analyzed
by RP-HPLC-MS, without isolation (Table 3, Scheme 6).
Substrates bearing simpler alkyl R2 groups (i.e., i-Pr and Bn)
gave better conversion than their heteroalkyl counterparts. Supported
N-(γ-methyl aspartyl)-, N-(histidyl)-, and N-(serinyl)amidopy-
rroles 29cꢀe gave the corresponding (3S,5S)-pyrrolodiazepi-
none 44 in reasonable proportions. In the case of 29d, RP-
HPLC-MS analysis indicated peaks with retention times and
molecular ions that suggested a side product from PictetꢀSpen-
gler reaction with the imidazole ring. N-(Lysyl)amidopyrrole 29f
4538
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ARTICLE
Scheme 7. Large-Scale PictetꢀSpengler Cyclization on
Scheme 8. PictetꢀSpengler Cyclization on Merrifield Resin
Wang Resin
and TAP Soluble Support
into the 20 mL Biotage microwave vessel and, respectively,
reacted with p-nitrobenzaldehyde, which had given better yields
of diazepinone in the solution-phase synthesis,15 using optimized
conditions (1% TFA in benzene, 1000 mol % aldehyde, 70 °C)
for an extended 3 h reaction time to complete conversion
(Schemes 7 and 8).
Pyrrolo[3,2-e][1,4]diazepin-2-one 47 was prone to oxidation
even when stored under argon at ꢀ10 °C which led to a 1:1
mixture of 47 and unsaturated analogue 48 after a few hours.
Compared with the isopropyl analogue 44b, the presence of the
electron-withdrawing nitrophenyl group may account for the low
stability of 47, because of the increased acidic character of the
5-position proton of the diazepinone ring. Analytical samples
were purified by reverse phase preparative HPLC, yielding pure
48 and a 9/1 47:48 mixture after freeze-drying.
Attempts to cleave pyrrolo[3,2-e][1,4]diazepin-2-one from
10 mg aliquots of Merrifield resin using TFA under microwave
heating (TFA/TES/H2O 95:2.5:2.5, 2 mL at 50 °C for 30 min)42
gave only enough material for RP-HPLC-MS analysis, which
indicated 48 to be 35% pure, contaminated with other degrada-
tion products, with neither a trace of its diastereoisomer nor
dehydro counterpart 48 nor starting material (Scheme 9).
On the TAP support, the PictetꢀSpengler reaction was per-
formed using 1% TFA in 1,2-dichloroethane, instead of benzene,
to ensure solubility of amidopyrrole 35 during microwave
irradiation at 70 °C for 3 h (Scheme 8). Analysis by RP-
HPLC-MS, after precipitation, indicated no cleavage products
in the decanted reaction solution and that the TAP-supported
In the case of Wang resin, after the microwave reaction, the
resin and liquid phase were filtered, the resin was washed, and the
filtrate and washings were evaporated. The resin was cleaved
using TFA/DCM 1:1 containing 1% of triethylsilane, filtered,
and washed, and the filtrate and washings were similarly evapo-
rated. Residues from evaporation of the filtrate after Pictetꢀ
Spengler reaction and cleavage were separately purified by
flash column chromatography41 (AcOEt/MeOH/MeCN/H2O
70:10:10:10 as eluent) to yield inseparable 30:70 mixtures of
hexahydropyrrolodiazepinone 47 and tetrahydropyrrolodiazepi-
none 48 in 29% (from the PictetꢀSpengler cyclization) and 3%
yields (from resin cleavage). The premature cleavage of around
90% of the PictetꢀSpengler cyclization product suggested that
substrate may have been cleaved and reacted both in the solid and
solution phases during the reaction; moreover, it disqualified the
use of Wang resin for further modification of the pyrrolo-
[3,2-e][1,4]diazepin-2-one scaffold on support. On the contrary,
a similar analysis of the residue from the liquid phase of the
PictetꢀSpengler reaction on Merrifield resin 34 by RP-HPLC-
MS revealed the presence of residual p-nitrobenzaldehyde
without cleaved starting material and only trace amounts of
product 48 (less than 1% of the UV signal at λ = 280 nm).
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ARTICLE
Scheme 9. Microwave-Assisted TFA Cleavage of Compound
49
the supported substrates, with those of lower mass having less
solubility in diethyl ether, leading to higher recovery. The
monitoring of TAP-supported substrates during reactions was
facilitated by RP-HPLC-MS, because of the relatively high UV
absorbance and ion detection of the tetrarylphosphonium group;
however, impurities not linked to the TAP support were better
detected by 1H NMR spectroscopy. A single purification by silica
gel column chromatography of the final pyrrolo[3,2-e][1,4]-
diazepin-2-one was performed after seven steps to isolate pure
final ester 53 in 22% overall yield. Although the TAP support
was not deemed convenient for “split-and-mix” methodology,
the potential for performing synthesis in solution and for
examining reactions directly on the support made this method
useful for preparing a specific pyrrolo[3,2-e][1,4]diazepin-2-one
target.
The solid support Wang and Merrifield resins and the soluble
tetraarylphosphonium support exhibited complementary utilities
in the synthesis of pyrrolo[3,2-e][1,4]diazepin-2-one. With
respect to overall yield, the Wang resin provided pyrrolodiaze-
pinone in a 32% combined yield for 47 and 48, albeit product was
prematurely cleaved under the acidic conditions used for the
PictetꢀSpengler cyclization. The overall yields for Merrifield
resin and the TAP soluble support 14 were somewhat lower,
likely because of the need for vigorous conditions in the final
cleavage of the former and incomplete mass recoveries after
precipitation of the latter support; however, both were resistant
to the acidic conditions of the PictetꢀSpengler reaction and
could in principle be used for further modification of the scaffold
on the support. Considering the applicability of the former for
split-and-mix chemistry in IRORI Kans, an investigation is
currently underway for the construction of pyrrolo[3,2-e]-
[1,4]diazepin-2-ones using Merrifield resin and alternative clea-
vage methods, which will be reported in due course.
material consisted of 92% (3S,5R)-50, 2% (3S,5S)-51, and 6%
dehydrogenated product 52 without any starting material 35.
Cleavage of pyrrolo[3,2-e][1,4]diazepin-2-one from both
Merrifield resin and the TAP support was effectively accom-
plished using 0.44 M NaOCH3 in 9:1 THF/MeOH for, respec-
tively, 4 and 1 h. Treating the Merrifield resin substrate with
methoxide for longer reaction times resulted in significant
amounts of degradation. Cleavage provided only tetrahydro
diazepinone methyl ester 53 in, respectively, 22% and 17%
overall yields from the Merrifield and TAP supports, after
preparative RP-HPLC.
’ DISCUSSION AND CONCLUSIONS
All three supports served to provide pyrrolo[3,2-e][1,4]dia-
zepin-2-one products. None proved, however, to be ideal for the
most significant criteria of (1) effective monitoring of reaction
mixtures, (2) balancing between stability for broad applications
and facile cleavage, (3) capacity to minimize the use of excess
reagents for obtaining high yield, and (4) utility in split-and-
mix43 experiments. Nevertheless, each support had benefits. For
example, the solid supports, Wang and Merrifield resins were
particularly useful for parallel experiments, because the cross-
linked polymers could be conveniently split into independent
reactors, which could be employed together in exploratory
chemistry in the same pot, before separate cleavages and analyses.
The labile nature of Wang resin under both the acid and
methoxide cleavage conditions facilitated subsequent analysis
by LCMS. The stability of Merrifield resin offers the potential for
further modification of the pyrrolo[3,2-e][1,4]diazepin-2-one
scaffold after the PictetꢀSpengler cyclization. The soluble TAP
support offered the practical advantage of employing homoge-
neous conditions, which resulted in the use of less reagents for
driving reactions to completion and quicker reaction times;
moreover, chemistry on the soluble TAP support could be
conveniently monitored by TLC (using MeOH/CH2Cl2
eluents) and RP-HPLC-MS, in addition to, in certain cases,
NMR spectroscopy. Although the chemistry on both the solid
and soluble supports was similar, in some cases, solvents were
changed to comply with the solubility requirements of the TAP-
supported compounds.
’ EXPERIMENTAL SECTION
General. Wang resin SS (75ꢀ100 mesh, 1.2 mmol/g) and Merri-
field resin SS (70ꢀ90 mesh, 1.3 mmol/g) were purchased from
Advanced Chemtech (Louisville, KY). Wang bromide resin was pre-
pared from Wang resin according to the literature method.29 TAP
soluble support (bromomethylphenyltriphenylphosphonium) was ob-
tained from Soluphase Inc. (Montreal, Quebec, Canada). IRORI
MacroKans and MiniKans were from Nexus Biosystems, Inc. (Poway,
CA). (2S,4R)-4-Hydroxy-N-1-(PhF)proline was prepared from
(2S,4R)-4-hydroxy-N-1-(PhF)proline methyl ester according to refs
25 and 16. Fmoc-Orn(Boc) and Fmoc-Asp(OMe) were synthesized
according to known procedures.44 Anhydrous THF, MeCN, DCM,
MeOH, and DMF were obtained from a commercial drying and filtrating
system; DMSO was dried over activated 4 Å molecular sieves 24 h prior
to use and stored in a tightly sealed bottle under an argon atmosphere.45
Reactions requiring anhydrous conditions were performed under an
atmosphere of dry argon; glassware was flame-dried immediately prior
to use and allowed to cool under argon atmosphere. Liquid reagents,
solutions, and solvents were added via syringe through rubber septa.
Flash column chromatography41 was performed using SiliaFlashF60
silica (Silicycle, Quebec City). Glass-backed plates precoated with silica
gel (SiliaPlate TLC Extra Hard Layer 60 Angstrom F-254, Silicycle,
Quebec City) were used for thin layer chromatography (TLC) and were
visualized by UV fluorescence or staining with a ninhydrin or a
phosphomolybdic acid solution in EtOH prior to heating. Purification
by radial chromatography32 was performed using 2 mm sorbent layers.
Sorbent consists in a 2.5/1 mix of silica gel (TLC standard grade,
2ꢀ25 μm, pore size 60 Å with fluorescent indicator)/calcium sulfate
The recovery of the TAP support by precipitation showed
variable efficiency (based on theoretical mass) during the course
of the synthesis, which amounted to a 43% overall recovery of
supported material after six steps. Attempts to enhance recovery
by additional precipitation of material from filtrates often caused
the coprecipitatation of unwanted impurities. For example, after
the preparation of aminopyrrole 25, precipitation gave a 122%
mass recovery due to coprecipitation of impurities, which were
not eliminated by additional precipitation. In general, preciptita-
tion performances were contingent on the molecular weight of
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ARTICLE
hemihydrate. Plates were prepared according to ref 32, and elution flow
varied between 6 and 8 mL/min. 1H NMR and 13C NMR spectra were
recorded on 300, 400, and 500 MHz spectrometers with chemical shift
values in ppm relative to residual chloroform (δH 7.26 and δC 77.2),
acetone (δH 2.05 and δC 29.84), or DMSO (δH 2.50 and δC 39.52) as
standards. Coupling constant J values are given in hertz, with splitting
described as s (singlet), d (doublet), t (triplet), q (quartet), m
(multiplet). Specific rotation, [R]D values are given in units 10ꢀ1 deg
was added (COCl)2 (2.20 mL, 25.80 mmol, 12 equiv) dropwise to a
solution of DMSO (3.70 mL, 51.60 mmol, 24 equiv) in DCM (50 mL) at ꢀ
78 °C, and the mixture was stirred for 30 min. Into a 200 mL round-
bottom flask containing, respectively, IRORI MacroKans possessing
(2S,4R)-4-hydroxy-N-(PhF)proline Wang (17, 0,406 g ꢁ 2, 0.80 mmol/
g, 0.65 mmol) and Merrifield (18, 0.425 g ꢁ 4, 0.88 mmol/g,
1.50 mmol) resins swollen in dry DCM (50 mL) at ꢀ78 °C, the DMSO
mixture was transferred by a double-tipped needle, and the Kans were
agitated gently with magnetic stirring for 4 h at ꢀ78 °C. The resins
became a light green color, which turned progressively to light brown
within 1 h. After 4 h, the resins at ꢀ78 °C were treated dropwise with
DIEA (13.50 mL, 77.40 mmol, 36 equiv) over 1 h, after which time the
resins appeared dark brown. The suspension containing the Kans was
allowed to warm to room temperature over 1 h and filtered. The Kans
were transferred into a 250 mL bottle and washed sequentially with
100 mL volumes of DCM (ꢁ2), MeOH (ꢁ2), DCM (ꢁ2), MeOH
(ꢁ2), and DCM (ꢁ2) as described above. The resins in Kans were dried
in vacuo, and the reaction was repeated a second time to ensure
complete alcohol oxidation as ascertained by the disappearance of the
hydroxyl absorption around 3440ꢀ3460 cmꢀ1 in the IR spectrum. The
beige resin was usually stored in desiccators under vacuum or under
argon in the refrigerator. 20: IR (KBr, cmꢀ1): 1732 and 1758 (CdO).
21: IR (KBr, cmꢀ1): 1732 and 1758 (CdO).
3
3
cm2 ꢀ1. Microwave-assisted reactions were performed in a Biotage
g
3
Initiator microwave apparatus (Biotage AB, Uppsala, Sweden) in glass
Biotage reaction vessels containing a magnetic stir bar and sealed with a
septum cap. Analytical RP-HPLC analyses were performed on a C18
column 50 mm ꢁ 4.6 mm, 5 μm (column A) or on a C18 column 150
mm ꢁ 4.6 mm, 5 μm (column B) with a flow rate of 0.5 mL/min using a
linear gradient of MeOH (0.1% formic acid (FA)) in water (0.1% FA).
Retention times (tR) from analytical RP-HPLC are reported in minutes.
Indicated compounds were purified with a C18 column 250 mm ꢁ
21 mm, 5 μm (column C), using a specified linear gradient of MeOH or
MeCN (0.1% FA) in water (0.1% FA), with a flow rate of 10.6 mL/min.
Resin Swelling in IRORI Kans. Under a stream of argon, resins in
Kans and a stir bar were placed in a flame-dried round-bottom flask,
which was exposed to three cycles of vacuum (20 mmHg) followed by
argon, prior to addition of the specified amount of the appropriate dry
solvent. The flask was purged with argon and sealed using a rubber
septum, and the resin was allowed to swell for 30 min under inert
atmosphere.
4-Benzylamino-1H-pyrrole-2-carboxylate Wang Resin
(23). In a 100 mL round-bottom flask, two IRORI MacroKans contain-
ing (2S)-4-oxo-N-(PhF)proline Wang resin 20 (0.405 g ꢁ 2, 0.80
mmol/g, 0.65 mmol) were swollen in THF (40 mL). The mixture was
degassed by three freezeꢀthaw cycles, covered with a blanket of argon,
and treated with anhydrous MgSO4 (0.8 g, dried overnight at 100 °C
Resin Washing in IRORI Kans. The Kans were placed together in
a glass bottle equipped with a stir bar and treated with solvent. The bottle
was sealed with a screw cap and shaken by hand for 10 s. The Kans were
next magnetically stirred for 5 min. The solvent was decanted through a
perforated screw cap, and then the bottle was shaken vigorously by hand,
head down, for 10 s. This operation was repeated for as many times and
solvents as specified. At the end of the process, residual solvent in the
Kans was removed by centrifugation or by subjecting the head of each
Kan to vacuum for 1 min using a water aspirator.
prior to use), followed by p-TsOH H2O (25 mg, 0.13 mmol, 0.2 equiv)
3
and benzylamine (1.60 mL, 14.95 mmol, 23 equiv). The mixture was
purged with argon bubbles for 30 min and stirred for 18 h at 50 °C in an
oil bath under an argon atmosphere. The Kans containing the green
resin were transferred to a bottle and washed sequentially with 50 mL
volumes of THF, DCM, MeOH, H2O, DCM, MeOH, H2O, MeOH
(ꢁ2), and DCM (ꢁ3). The light green resin (total weight: 697 mg) was
usually stored in desiccators under vacuum or under argon in the
refrigerator. The filtrate from the reaction mixture and all washings
was combined and evaporated in vacuo to a residue, which was
partitioned between water (20 mL) and EtOAc (20 mL). The aqueous
layer was extracted with EtOAc (2 ꢁ 20 mL). The combined organic
layers were washed with 10% HCl (2 ꢁ 30 mL), satd NaHCO3 (30 mL),
and brine (30 mL), dried over sodium sulfate, filtered, and evaporated to
a residue, which was purified by column chromatography (2% Et2O/
hexanes). Combination of the collected fractions and evaporation of the
volatiles yield 130 mg of 9,9-phenylfluorene (0.54 mmol, PhF-H, mp
146 °C, lit.25 mp 148 °C) corresponding to a 0.77 mmol/g loading of
aminopyrrole on Wang resin.
(2S,4R)-4-Hydroxy-N-(PhF)proline Wang and Merrifield
Resins (17, 18). In a 200 mL round-bottom flask, a solution of
N-(PhF)hydroxyproline 15 (1.65 g, 4.44 mmol, 200 mol %, prepared
according to ref 25) in MeOH (9 mL) was treated with 1 mL of water,
titrated with 20% Cs2CO3 to pH 7 (about 2.5 mL), evaporated to
dryness, twice suspended in toluene, and evaporated. The residue was
ground into a powder by scraping the wall of the flask with a spatula. The
round-bottom flask was charged with two IRORI MacroKans containing
Wang bromide resin29 (0.30 g ꢁ 2, 1.1 mmol/g, 0.66 mmol) and four
IRORI MacroKans containing Merrifield resin (0.30 g, ꢁ 4, 1.3 mmol/g,
1.56 mmol). The resins were swollen, and cesium salt 16 was dissolved in
DMF (100 mL). After treatment with dibenzo-18-crown-6 (1.2 g, 3.33
mmol, 1.5 equiv), the flask was purged with argon,46 and the reaction
mixture was heated to 70 °C with an oil bath and agitated gently with
magnetic stirring for 48 h. The Kans containing resins were transferred
together into a 250 mL bottle and washed sequentially with 100 mL
volumes of DCM, MeOH, H2O, DCM, MeOH, H2O, MeOH (3ꢁ), and
DCM (3ꢁ). The beige resins in the Kans were dried in vacuo and usually
stored in desiccators under vacuum or under argon in the refrigerator.
Anchoring efficiency of respective resins was evaluated by measuring
their mass increase after the reaction. Excess (2S,4R)-4-hydroxy-
N-(PhF)proline was recovered as previously described.25 17: IR
(KBr, cmꢀ1): 3448 (OH), 1737 (CdO). The mass increase after
reaction was 216 mg and corresponded to a loading of 0.80 mmol/g
(98% conversion). 18: IR (KBr, cmꢀ1): 3457 (OH), 1732 (CdO). The
mass increase after reaction was 501 mg and corresponded to a loading
of 0.88 mmol/g (96% conversion).
4-Benzylamino-1H-pyrrole-2-carboxylate Merrifield Re-
sin (24). 4-Benzylamino-1H-pyrrole-2-carboxylate Merrifield resin 24
was synthesized as described above for 4-benzylamino-1H-pyrrole-2-
carboxylate Wang resin 23 from 21 (0.425 g ꢁ 4, 0.88 mmol/g,
1.50 mmol) using 4 IRORI MacroKans, 80 mL of THF, 3.77 mL of
benzylamine (34.5 mmol, 23 equiv), 57 mg of p-TsOH H2O (0.3 mmol,
3
0.2 equiv) and 1.2 g of dry MgSO4. After reaction, the Kans containing
brownish green resin (total weight: 1.467 g) were usually stored in
desiccators under vacuum or under argon in the refrigerator. Purification
of the residue from resin filtration and washings as described above
yielded 359 mg (1.48 mmol) of PhF-H, indicative of an aminopyrrole
loading on Merrifield resin of 1.01 mmol/g.
4-[N-(9-Fluorenylmethoxycarbonyl)-L-phenylalaninyl]be-
nzylamino-1H-pyrrole-2-carboxylate Wang and Merrifield
Resins (26a, 27). In a 100 mL round-bottom flask under argon
atmosphere, Fmoc-Phe (3.971 g, 10.25 mmol, 5 equiv) was mixed with
(2S)-4-Oxo-N-(PhF)proline Wang and Merrifield resins
(20, 21). To a 100 mL round-bottom flask, under argon atmosphere,
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ARTICLE
bis(trichloromethyl) carbonate (0.912 g, 3.38 mmol, 1.65 equiv) in THF
(50 mL) at 0 °C and treated dropwise with 2,4,6-collidine (3.8 mL, 28.70
mmol, 14 equiv) over 5 min. The white suspension was stirred for
10 min at 0 °C and transferred by syringe to a 200 mL round-bottom
flask containing IRORI MacroKans filled, respectively, with 4-benzyla-
mino-1H-pyrrole-2-carboxylate Wang (23, 0.348 g ꢁ 2, 0.77 mmol/g,
0.54 mmol) and Merrifield (24, 0.368 g ꢁ 4, 1.01 mmol/g, 1.48 mmol)
resins swollen in THF (50 mL) at 0 °C. The mixture was allowed to
warm to room temperature with stirring for 15 h. The Kans containing
resin were transferred to a bottle and washed with 100 mL volumes of
DMF (ꢁ3), MeOH (ꢁ3), and DCM (ꢁ3) as described above. The dark
green resins (Wang: 0.899 g; Merrifield: 2.005 g) were usually stored in
desiccators under vacuum or under argon in the refrigerator. Aliquots
(15 mg) of each resin were removed from the Kans, partitioned into two
2 mL plastic filtration tubes equipped with polyethylene frits, swollen,
respectively, in DMF for 20 min, treated twice with a 20% solution of
piperidine in DMF for 20 min, and filtered. The resins were washed
successively with agitation for 1 min and filtered from DMF (3 ꢁ 1 mL),
MeOH (3 ꢁ 1 mL), and DCM (3 ꢁ 1 mL). A positive Kaiser test47
indicated qualitatively the presence of free amine. The resins were,
respectively, treated with acetic anhydride (37 μL, 0.39 mmol, 20 equiv)
and DIEA (85 μL, 0.49 mmol, 25 equiv) in DMF (1 mL) for 2 h, washed
as described above. Afterward, a negative Kaiser test indicated quanti-
tative acylation. In the case of 4-[N-(9-fluorenylmethoxycarbonyl)-L-
phenylalaninyl]benzylamino-1H-pyrrole-2-carboxylate Wang resin 26a,
the acylated material was cleaved from the resin using 50% TFA in DCM
(1 mL) for 1 h and the filtrate was evaporated to dryness to a residue,
which was subjected to RP-LCMS analysis (column A, 0ꢀ80% MeOH
in H2O for 10 min then 90% MeOH for 5 min, 0.1% FA, UV: λ =
280 nm, tR 8.76), which indicated >90% purity, with no sign of acetylated
starting material. In the case of 4-[N-(9-fluorenylmethoxycarbonyl)-L-
phenylalaninyl]benzylamino-1H-pyrrole-2-carboxylate Merrifield resin
27, cleavage was performed using a 0.44 M MeONa solution in THF/
MeOH (9:1, 1 mL)48 for 2 h. The sequence of filtration, washing of the
resin with EtOAc, and partitioning of the filtrate between EtOAc and
saturated aqueous NH4Cl solution gave an organic layer, which was
filtered through a Pasteur pipet containing a 5 mm layer of Na2SO4 and
evaporated to dryness. Analysis of the residue by RP-LCMS (column B,
0ꢀ80% MeOH over H2O in 20 min and then 90% MeOH for 15 min,
0.1% FA, UV: λ = 280 nm, tR 26.34) demonstrated >82% purity, with no
sign of acetylated starting material.
4-(L-Phenylalaninyl)benzylamino-1H-pyrrole-2-carboxyl-
ate Wang and Merrifield Resins (29a, 34). In a 250 mL bottle,
4-[N-(9-fluorenylmethoxycarbonyl)-L-phenylalaninyl]benzylamino-
1H-pyrrole-2-carboxylate Wang (26a, 0.442 g ꢁ 2, 0.60 mmol/g,
0.53 mmol) and Merrifield (27, 0.497 g ꢁ 4, 0.74 mmol/g, 1.47 mmol)
resins in IRORI MacroKans were swollen in DMF (100 mL) for 0.5 h,
filtered, and exposed to a freshly prepared 20% piperidine in DMF
solution (100 mL) with gentle magnetic stirring for 20 min. The
reaction mixture was decanted, and the resin Kans were retreated with
the piperidine in DMF solution as performed previously. The resins in
Kans were washed with 100 mL volumes of DMF (ꢁ3), MeOH (ꢁ3),
and DCM (ꢁ3) as described above. A positive Kaiser test47 indicated
qualitatively the presence of free amines. The resins (29a: 0.754 g, light
brownish orange, 34: 1.682 g, light brownish green) were usually stored
in desiccators under vacuum or under argon in the refrigerator.
(S)-1,3-Dibenzyl-5-(4-nitrophenyl)-2-oxo-1,2,3,6-tetrahy-
dropyrrolo[3,2-e][1,4]diazepine-7-carboxylic Acid (46). The
contents from one MacroKan containing 4-(L-phenylalaninyl)ben-
zylamino-1H-pyrrole-2-carboxylate Wang resin (29a, 0.377 g, 0.69
mmol/g, 0.26 mmol) were split into three IRORI MiniKans, which
were treated with benzene (15 mL) and placed into a 20 mL Biotage
microwave vessel.49 The vessel was sealed with a conventional rubber
septum and placed under argon atmosphere. After being swollen for
20 min, the resin was treated with p-nitrobenzaldehyde (0.390 g, 2.58
mmol, 10 equiv), followed by TFA (150 μL), and stirred with heating
under microwave irradiation at 75 °C for 3.5 h. The MiniKans contain-
ing resin were transferred to a bottle and sequentially washed with
50 mL volumes of THF (ꢁ3), MeOH (ꢁ3), and DCM (ꢁ3) as
described above. The filtrates were evaporated to dryness, and the
residue (450 mg) was purified by flash silica gel column chromatogra-
phy using EtOAc/MeOH/MeCN/H2O (70:10:10:10) as eluent to
yield two different fractions, the first one consisting of an inseparable
30/70 mixture of hexahydropyrrolodiazepinone 47 and tetrahydropyr-
rolodiazepinone 48 (33 mg), and the second one (4 mg) consisting in a
8/92 mixture of 47 and 48 (29% global yield). The resin in the Kans was
treated with TFA/TES (95:5, 6 mL) for 4 h, filtered, and washed with
50 mL volumes of THF (ꢁ3), MeOH (ꢁ3), and DCM (ꢁ3) as
described above. The filtrate and washings were combined and evapo-
rated to a residue (9 mg), which was purified by chromatography using
AcOEt/MeOH/MeCN/H2O (70:10:10:10) as eluent. Evaporation of
the collected fractions gave the same yellow oil consisting of an in-
separable 30/70 mixture of hexahydro pyrrolodiazepinone 47 (4 mg,
3% global yield). Pure 48 (2 mg) was obtained by preparative HPLC
(column C, 60ꢀ90% MeOH in H2O over 30 min.): [R]21 = ꢀ47.5
D
(c = 0.75); 1H NMR (CD3OD, 300 MHz) δ 3.55 (dd, J = 8.0, 13.6 Hz,
1H), 3.68 (dd, J = 6.0, 13.8 Hz, 1H), 4.08 (dd, J = 8.0, 6.0 Hz, 1H), 5.01
(d, J = 15.4 Hz, 1H), 5.31 (d, J = 15.4 Hz, 1H), 6.69 (s, 1H), 7.09 (dd, J =
2.1, 3.5 Hz, 2H), 7.19 (m, H, 4H), 7.30 (t, J = 7.3 Hz, 2H), 7.37 (d, J =
7.2 Hz, 2H), 7.62 (d, J = 8.7 Hz, 2H), 8.23 (d, J = 8.7 Hz, 2H), 8.50 (s,
1H); 13C NMR (125 MHz, acetone-d6) δ 38.7 (CH2), 49.5 (CH2), 67.0
(CH), 102.7 (CH), 120.2 (C), 123.1 (3CH), 126.0 (C), 126.7 (2CH),
126.9 (CH), 128.0 (2CH), 128.3 (2CH), 129.9 (2CH), 130.1 (2CH),
133.5 (C), 137.6 (C), 139.9 (C), 143.5 (C),148.7 (C), 158.9 (CdO þ
CdN), 165.5 (CdO); HRMS calcd for C28H23N4O5 [M þ H]þ:
495.1665, found: 495.1663; IR υmax/cmꢀ1 (NaCl): 3424 (N-H), 3154
(OH), 3057, 3036, 2942 (CꢀH), 1674 (CdO acid), 1655 (CdO
amide), 1603 (CdCarom), 1553 (CdCarom), 1520 (Ar-NO2), 1494
(CdCarom), 1453 (CdCarom), 1434 (CdCarom), 1346 (CꢀH), 1314
(CꢀO), 1116.
(S)-Methyl 1,3-Dibenzyl-5-(4-nitrophenyl)-2-oxo-1,2,3,6-
tetrahydropyrrolo[3,2-e][1,4]diazepine-7-carboxylate from
Merrifield Resin (53). In IRORI Kans, the PictetꢀSpengler reaction
was performed on 4-(L-phenylalaninyl)benzylamino-1H-pyrrole-2-car-
boxylate Merrifield resin 34 using microwave heating as described above
for Wang resin 29a. Resin 34 (0.420 g, 0.88 mmol/g, 0.37 mmol) from
one MacroKan was split into three IRORI MiniKans, swollen in benzene
(15 mL), treated with p-nitrobenzaldehyde (0.559 g, 3.70 mmol,
10 equiv) and TFA (150 μL), and heated with microwave irradiation.
The resin in MiniKans was sequentially washed with 50 mL volumes of
THF (ꢁ3), MeOH (ꢁ3), and DCM (ꢁ3) as described above. Evapora-
tion of the combined washing solutions gave a trace amount of (3S,5R)-
1,3-dibenzyl-5-(4-nitrophenyl)-2-oxo-1,2,3,4,5,6-hexahydropyrrolo[3,2-e]-
[1,4]diazepine-7-carboxylic acid 48 as analyzed by RP-HPLC analysis
(column A, 10ꢀ90% MeOH in H2O for 20 min then 90% MeOH for 15
min, 0.1% FA, UV: λ = 280 nm, tR 24.34, accounting for less than 1% of
the UV signal) along with p-nitrobenzaldehyde. An aliquot of resin 49
(10 mg) was suspended in TFA/TES/H2O (95:2.5:2.5, 2 mL) and
treated under microwave-assisted cleavage conditions42a at 50 °C for 30
min in a V-shaped 2 mL Biotage vessel with a triangular stir bar.
Filtration of the resin and evaporation of the filtrate gave a residue,
which was analyzed by RP-HPLC (column A, 0ꢀ80% MeOH in
H2O over 10 min then 90% MeOH for 5 min, 0.1% FA, UV: λ = 280 nm)
and found to contain (3S,5R)-1,3-dibenzyl-5-(4-nitrophenyl)-2-oxo-
1,2,3,4,5,6-hexahydropyrrolo[3,2-e][1,4]diazepine-7-carboxylic acid 48
of 35% purity (UV signal at λ = 280 nm, tR 11.08) with no trace of
starting 4-(L-phenylalaninyl)benzylamino-1H-pyrrole-2-carboxylic acid
(tR 7.05). Product cleavage was performed in a 60 mL filtration tube with
4542
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The Journal of Organic Chemistry
ARTICLE
a polyethylene frit, by shaking the remaining resin suspended in a 0.44 M
MeONa solution in THF/MeOH (9:1, 30 mL) at room temperature for
4 h. The resin was filtered and washed with EtOAc (3 ꢁ 30 mL). The
filtrate and washings were combined and washed with saturated aqueous
NH4Cl (50 mL). The aqueous phase was extracted with EtOAc (2 ꢁ
20 mL), and the combined organic layers were washed with brine
(50 mL), dried over anhydrous Na2SO4, and evaporated to dryness.
Purification of the residue was performed by flash chromatography on a
silica gel column (15 to 20% gradient of EtOAc in hexanes) to yield (S)-
methyl 1,3-dibenzyl-5-(4-nitrophenyl)-2-oxo-1,2,3,6-tetrahydropyrrolo-
[3,2-e][1,4]diazepine-7-carboxylate53 as a yellow oil (42 mg, 0.084 mmol,
22%). [R]21D = ꢀ46.4 (c = 0.33,); 1H NMR (CDCl3, 300 MHz) δ 3.68
(dd, J = 8.0, 13.5 Hz, 1H), 3.75 (dd, J = 5.7, 13.5 Hz, 1H), 3.82 (s, 3H),
4.08 (dd, J = 8.0, 5.7 Hz, 1H), 4.96 (d, J = 15.4 Hz, 1H), 5.28 (d, J = 15.4
Hz, 1H), 6.74 (d, J = 2.4 Hz, 1H), 7.08 (dd, J = 2.2, 3.5 Hz, 2H), 7.23 (m,
4H), 7.34 (t, J = 7.2 Hz, 2H), 7.4 (d, J = 7.2 Hz, 2H), 7.62 (d, J = 8.8 Hz,
2H), 8.23 (d, J = 8.8 Hz, 2H), 8.92 (s, 1H); 13C NMR (125 MHz,
CDCl3) δ 38.7 (CH2), 50.4 (CH2), 52.5 (CH3), 67.4 (CH), 106.2
(CH), 122.9 (C), 124.0 (2CH), 124.9 (C), 126.5 (CH), 127.1 (2 CH),
127.6 (CH), 128.4 (2CH), 128.8 (2CH), 130.0 (2CH), 130.1 (2CH),
134.4 (C), 136.6 (C), 139.2 (C), 142.6 (C),149.3 (C),158.5
(CdO),160.8 (CdN),165.6 (CdO); HRMS calcd for C29H25N4O5
[M þ H]þ: 509.1819, found: 509.1822; IR υmax/cmꢀ1 (NaCl): 3423
(NꢀH), 3064, 3030, 2954, 2929 (CꢀH), 1718 (CdO ester), 1674
(CdO amide), 1582 (CdCarom), 1556 (CdCarom), 1521 (arom-NO2),
1494 (CdCarom), 1453 (CdCarom), 1434 (CdCarom), 1347 (Cꢀ
Hmethyl), 1307, 1270 (CꢀO), 1224 (CꢀO), 1163, 1107, 1008.
(s, 1CH), 65.5 (s, 1CH2), 70.1 (s, 1CH), 76.2 (s, 1C), 115.6 (d, J = 91.5
Hz, 1C), 117.7 (d, J = 89.5 Hz, 3C), 119.9 (s, 2CH), 120.2 (s, 2CH*),
126.7 (s, 2CH), 127.2 (s, 1CH*), 127.37 (s, 2CH), 127.41 (s, 2CH),
127.67 (s, 2CH*), 127.75 (s, 2CH*), 128.4 (s, 2CH),128.6 (s, 2CH),
128.7 (s, 4CH), 128.8 (s, 2CH*), 129.1 (s, 2CH), 129.3 (s, 2CH*), 130.8
(d, J = 12.9 Hz, 6CH), 134.5 (d, J = 10.3 Hz, 6CH), 135.1 (d, J = 10.7 Hz,
2CH), 135.8 (d, J = 3.0 Hz, 3CH), 137.1 (s, 1C), 137.9 (d, J = 1.5 Hz,
1C), 140.0 (s, 1C), 141.4 (s, 2C), 142.8 (s, 2C*), 146.1 (s, 2C), 147.5 (s,
2C*), 147.86 (s, 1C), 147.9 (s, 1C*), 175.4 (s, CdO); HRMS calcd for
C55H45NO3P [M]þ: 798.3140, found: 798.3131; IR υmax/cmꢀ1
(NaCl): 3494 (N-H), 3072, 3020, 2927 (CꢀH), 1736 (CdO ester),
1596 (CdCarom), 1486 (CdCarom), 1439 (CdCarom), 1272 (CꢀO),
1216 (CꢀO), 1150 (arom), 1094 (arom).
4-Benzylamino-1H-pyrrole-2-carboxylate TAP (25). A stir-
red solution of oxalyl chloride (0.42 mL, 5.01 mmol, 1.5 equiv) in DCM
(6 mL) at ꢀ78 °C was treated with dry DMSO (0.47 mL, 6.68 mmol, 2
equiv) in DCM (6 mL). After being stirred for 20 min, a solution of 19
(3.0 g, 3.34 mmol, 1 equiv) in DCM (18 mL) was added dropwise to the
mixture, which was stirred for 4 h at ꢀ78 °C, treated with DIEA (3.5 mL,
20.04 mmol, 6 equiv) over 20 min, stirred at ꢀ78 °C for 1 h, and allowed
to warm to room temperature for 1 h. The mixture was treated with
KH2PO4 1 M (60 mL). The layers were separated, and the aqueous
phase was extracted with DCM (2 ꢁ 30 mL). The organic layers were
combined, washed with water (90 mL), aqueous saturated LiClO4 (2 ꢁ
90 mL), and water (2 ꢁ 90 mL), dried (Na2SO4), and evaporated to a
residue, which was redissolved in DCM (10 mL) and precipitated in ice-
cold Et2O (50 mL). The white solid was filtered off, washed with ice-cold
Et2O (2 ꢁ 20 mL), and dried in vacuo to give 22 as a white powder
(2.81 g, 94% recovery). Ketone 22 was directly used in the next step. A
stirred solution of 22 (2.81 g, 3.53 mmol, 1 equiv) in dry MeCN (80 mL)
was degassed by three freezeꢀthaw cycles. The mixture was treated with
MgSO4 (dried overnight prior use, 3 g). The vessel was purged with
argon, the mixture was treated with benzylamine (1.5 mL, 14.12 mmol,
4 equiv) and p-TsOH (95 mg, 0.35 mmol, 10 mol %), and degassed with
bubbling of argon for 30 min. The reaction mixture was warmed to
50 °C, stirred overnight at this temperature, and evaporated to a residue,
which was partitioned between DCM (60 mL) and water (60 mL). The
layers were separated. The aqueous layer was extracted with DCM (2 ꢁ
30 mL). The organic layers were combined, washed with aqueous
saturated NaHCO3 (2 ꢁ 60 mL), water (90 mL), aqueous saturated
LiClO4 (2 ꢁ 90 mL), and water (2 ꢁ 90 mL), dried (Na2SO4), and
evaporated to a residue, which was redissolved in DCM (10 mL) and
precipitated in ice-cold Et2O (50 mL). The white solid was filtered off,
washed with ice-cold Et2O (2 ꢁ 20 mL), dried in vacuo to a white
powder, and stored under argon in the refrigerator (3.20 g, 122%
recovery). An analytical sample (100 mg) was purified by radial
preparative chromatography (gradient: 0 to 5% MeOH in DCM). After
evaporation of the collected fractions, the residue was redissolved in
DCM (2 mL) and precipitated in ice-cold Et2O (10 mL). The white
solid was filtered off, washed with ice-cold Et2O (2 ꢁ 5 mL), and dried in
vacuo to give 25 as a white powder (74 mg). Rf: 0.46 (7.5% MeOH in
DCM); 1H NMR (CDCl3, 400 MHz) δ 4.14 (s, 2H), 5.27 (s, 2H), 6.45
(t, J = 1.9 Hz, 1H), 6.51 (t, J = 1.9 Hz, 1H), 7.23 (t, J = 7.0 Hz, 1H),
7.29ꢀ7.37 (m, 4H), 7.50 (d, J = 8.2 Hz, 2H), 7.60ꢀ7.72 (m, 10H), 7.77
(td, J = 3.6, 7.8 Hz, 6H), 7.88 (dt, J = 1.9, 7.8 Hz, 3H), 7.93 (dd, J = 3.1,
8.4 Hz, 2H); 13C NMR (75 MHz, CDCl3): δ 52.3 (s, 1CH2), 65.9 (s,
1CH2), 105.4 (s, 1C), 110.0 (s, 1C), 116.4 (d, J = 91.6 Hz, 1C), 118.4 (d,
J = 88.9 Hz, 1C), 121.0 (s, 1C), 127.9 (s, 1C), 128.49 (s, 2CH), 128.51
(s, 2CH), 129.3 (s, 2CH), 129.6 (s, 2CH), 129.9 (d, J = 13.3 Hz, 2CH),
131.6 (d, J = 12.8 Hz, 6CH), 135.2 (d, J = 12.8 Hz, 6CH), 135.2 (d, J =
10.7 Hz, 2CH), 136.6 (d, J = 2.9 Hz, 3CH), 137.9 (s, 1C), 138.4 (s, 1C),
138.7 (d, J = 1.5 Hz, 1C),140.6 (s, 1C), 148.6 (d, J = 3.1 Hz, 1C), 161.4
(CdO); HRMS calcd for C43H36N2O2P [M]þ: 643.2517, found:
643.2509; IR υmax/cmꢀ1 (NaCl): 3447 (N-H), 3065, 3021, 2925,
(2S,4R)-4-Hydroxy-N-(PhF)proline TAP (19). In a 100 mL
round-bottom flask, a solution of N-(PhF)hydroxyproline 15 (2 g,
5.39 mmol, 1 equiv, prepared according to reference25) in MeOH
(10 mL) was treated with 1 mL of water, titrated with 20% Cs2CO3 to
pH 7 (about 3 mL), evaporated to dryness, and twice suspended in
toluene and evaporated. The residue was ground into a powder by
scraping the wall of the flask with a spatula. The round-bottom flask was
charged with (40-(bromomethyl)-[1,10-biphenyl]-4-yl)triphenylpho-
sphonium perchlorate 14 (TAP-Br, 2.95 g, 4.85 mmol, 0.9 equiv),
DMF (50 mL), and potassium iodide (90 mg, 0.54 mmol, 10 mol %).
The flask was purged with argon,47 and the reaction mixture was heated
to 60 °C with an oil bath and agitated with magnetic stirring for 3 h at
which point TLC showed no remaining TAP-Br starting material (Rf =
0.64). The volatiles were removed by purging with air while stirring for
3 h, and the residue was dissolved in DCM (50 mL). The solution was
washed with aqueous saturated NaHCO3 (2 ꢁ 30 mL), water (30 mL),
aqueous saturated LiClO4 (2 ꢁ 30 mL), and water (2 ꢁ 30 mL), dried
(Na2SO4), and evaporated to a residue which was redissolved in DCM
(10 mL) and precipitated in ice-cold Et2O (50 mL). The white solid was
filtered off, washed with ice-cold Et2O (2 ꢁ 20 mL), and dried in vacuo
to give a white powder, which was stored under argon in the refrigerator
(3.09 g, 91% recovery). An analytical sample (90 mg) was purified by
radial chromatography (gradient: 0 to 5% MeOH in DCM). After
evaporation, the residue was redissolved in DCM (2 mL) and pre-
cipitated in ice-cold Et2O (10 mL). The white solid was filtered off,
washed with ice-cold Et2O (2 ꢁ 5 mL), dried in vacuo to give 19 as a
white powder (82 mg). Rf = 0.51 (7.5% MeOH in DCM); [R]21
1
= ꢀ13.5 (c = 0.9,); H NMR (CDCl3, 400 MHz) δ 1.80 (m, 2H),
D
1.99 (dt, J = 12.5, 5.5 Hz, 1H), 2.92 (dd, J = 5.4, 9.7 Hz, 1H), 3.37 (dd, J =
4.7, 9.1 Hz, 1H), 3.59 (dd, J = 5.4, 9.7 Hz, 1H), 4.55 (dt, J = 5.8, 11.6 Hz,
1H), 4.61 (d, J = 12.9 Hz, 1H), 4.81 (d, J = 12.9 Hz, 1H), 7.12 (td, J = 1.1,
7.5 Hz, 1H), 7.22 (m, 4H), 7.29 (m, 2H), 7.32 (dd, J = 1.1, 7.5 Hz, 1H),
7.36 (dt, J = 7.5, 0.7 Hz, 1H), 7.42 (td, J = 1.1, 7.5 Hz, 1H), 7.56 (m, 3H),
7.61ꢀ7.75 (m, 12H), 7.75ꢀ7.81 (m, 6H), 7.89 (ttd, J = 1.2, 2.0, 6.8 Hz,
3H), 7.97 (ddt, J = 1.9, 3.2, 8.5 Hz, 2H); 13C NMR (75 MHz, CDCl3):
two conformers are observed, signals given by the second conformer are
signaled with an asterisk (*), δ 39.9 (s, 1CH2), 56.7 (s, 1CH2), 59.5
4543
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The Journal of Organic Chemistry
ARTICLE
2855 (CꢀH), 1698 (CdO ester), 1596 (CdCarom), 1483 (CdCarom),
1439 (CꢀP), 1393 (CꢀN), 1094 (arom), 998 (CꢀP).
Present Addresses
†Galderma Production Canada Inc., 19400 Route Transcanadi-
enne, Baie d’Urfꢀe, Quebec, H9X 3S4, Canada.
(S)-Methyl 1,3-dibenzyl-5-(4-nitrophenyl)-2-oxo-1,2,3,6-
tetrahydropyrrolo[3,2-e][1,4]diazepine-7-carboxylate from
TAP Support (53). A solution of N-(Fmoc)-Phe (4.9 g, 12.52 mmol,
3 equiv) and bis(trichloromethyl) carbonate (BTC, 1.1 g, 4.18 mmol,
1 equiv) in 1.7:1 DCM/THF (72 mL) at 0 °C was treated slowly with
2,4,6-collidine (4.7 mL, 35.51 mmol, 8.5 equiv), yielding a white
suspension. After 1ꢀ3 min, a solution of aminopyrrole 25 in DCM
(45 mL) was added to the mixture containing the freshly formed acid
chloride. The reaction mixture was stirred for 15 h under argon atmo-
sphere and treated with 1 N HCl (100 mL). The organic layer was
separated and washed with saturated aqueous NaHCO3 (2 ꢁ 90 mL),
water (90 mL), aqueous saturated LiClO4 (2 ꢁ 90 mL), and water (2 ꢁ
90 mL), dried (Na2SO4), and evaporated to a residue, which was
redissolved in DCM (10 mL) and precipitated in ice-cold Et2O
(50 mL). The white solid was filtered off, washed with ice-cold Et2O
(2 ꢁ 10 mL), and dried in vacuo to give 28 as a beige powder which was
stored under argon in the refrigerator [3.7 g, 87% recovery, Rf: 0.45 (5%
MeOH in DCM), HRMS: calcd for C67H55N3O5P [M]þ: 1012.3874,
found: 1012.3894]. Acylaminopyrrole 28 (0.7 g, 0.63 mmol, 1 equiv)
was dissolved in DCM (5 mL), treated with piperidine (5 mL), stirred
for 10 min, and added dropwise to a stirring mixture of Et2O and hexane
1/1 (100 mL). The precipitate was collected by filtration, washed with
ice-cold Et2O (2 ꢁ 10 mL), and dried in vacuo. The beige solid 35
(499 mg, 89% recovery) was dissolved in dichloroethane (20 mL),
transferred to a 20 mL Biotage microwave vessel, treated with trifluor-
oacetic acid (0.2 mL) and 4-nitrobenzaldehyde (1.2 g, 6.29 mmol, 10
equiv), sealed, and heated at 70 °C under microwave for 3 h. The vessel
was cooled to room temperature and treated with aqueous saturated
NaHCO3 (20 mL). The phases were separated, and the aqueous layer
was extracted with DCM (3 ꢁ 10 mL). The combined organic layers
were washed with aqueous saturated LiClO4 (2 ꢁ 50 mL) and water
(50 mL), dried (Na2SO4), and evaporated to a residue, which was
redissolved in DCM (10 mL) and precipitated in ice-cold Et2O (50 mL).
The white solid was filtered off, washed with ice-cold Et2O (2 ꢁ 10 mL),
and dried in vacuo to give 51 as a yellow powder [0.41 g, 64% recovery,
Rf: 0.43 (5% MeOH in DCM), HRMS: calcd for C59H48N4O5P [M]þ:
923.3357, found: 923.3374]. Pyrrolodiazepinone 51 was dissolved in a
0.44 M MeONa solution in THF/MeOH (9:1, 25 mL) at room
temperature under argon, and the solution was stirred for 1 h and
then quenched with saturated aqueous NH4Cl (50 mL). The aqueous
phase was extracted with EtOAc (2 ꢁ 50 mL), and the combined
organic layers were washed with brine (50 mL), dried (Na2SO4), and
evaporated to a residue which was purified by flash chromatography
on a silica gel column (15 to 20% gradient of EtOAc in hexanes)
to yield (S)-methyl 1,3-dibenzyl-5-(4-nitrophenyl)-2-oxo-1,2,3,6-
tetrahydropyrrolo[3,2-e][1,4]diazepine-7-carboxylate 53 as a yellow
oil (13 mg, 39% yield, 17% overall yield). All the characterization
data were identical to those of compound 53 obtained from
Merrifield resin.
’ ACKNOWLEDGMENT
The authors acknowledge financial support from the Natural
Sciences and Engineering Research Council of Canada (NSERC),
Universitꢀe de Montrꢀeal, Canadian Institutes of Health Research,
CIHR-Team in GPCR allosteric regulation (CTiGAR), The Fonds
Quꢀebꢀecois de la Recherche sur la Nature et les Technologies
ꢀ
(FQRNT), The Ministꢁere du Dꢀeveloppement Economique, de
l0Innovation et de l0Exportation du Quꢀebec, and funds for equip-
ment were provided by the Canadian Foundation for Innovation.
We thank Marie-Noelle Roy of Soluphase Inc. (Montreal, Canada)
for fruitful discussions. We are grateful to Marie- Christine Tang and
Karine Venne from the Universitꢀe de Montrꢀeal Mass Spectrometry
Facility for MS analyses, and to Cꢀedric Malveau from the Universitꢀe
de Montrꢀeal High Field NMR Regional Laboratory for 500 MHz
NMR analyses.
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’ ASSOCIATED CONTENT
Supporting Information. 1H and 13C NMR spectra of
S
b
isolated compounds (48, 53, 19, and 25), RP-HPLC-MS traces
for aminoacylation and PictetꢀSpengler cyclization compounds
prepared on Wang resin. This material is available free of charge
via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: william.lubell@umontreal.ca.
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The Journal of Organic Chemistry
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