Application of the N-dibenzyl protective group in the preparation of β-lactam pseudopeptides

Article abstract of DOI:10.3390/molecules24071261

Despite the great importance of β-lactam antibiotics, there is still a limited number of synthetic approaches for the formation of β-lactam–containing dipeptides. In this study, we report upon the stereoselective preparation of β-lactam–containing pseudopeptides, where different reaction conditions and NH₂ protective groups were tested to obtain compounds that contain 3-amino-azetidin-2-one. We demonstrate that the protective group is essential for the outcome of the reaction. Successful implementation of dibenzyl-protected serine-containing dipeptides through the Mitsunobu reaction can provide the desired products at high yields and stereoselectivity.

Full text of DOI:10.3390/molecules24071261

molecules
Article
Application of the N-Dibenzyl Protective Group in
the Preparation of β-Lactam Pseudopeptides
Rok Frlan , Martina Hrast and Stanislav Gobec *
The Chair of Pharmaceutical Chemistry, Faculty of Pharmacy, University of Ljubljana, Ašker cˇ eva 7,
1
*
000 Ljubljana, Slovenia; rok.frlan@ffa.uni-lj.si (R.F.); martina.hrast@ffa.uni-lj.si (M.H.)
Correspondence: stanislav.gobec@ffa.uni-lj.si; Tel.: +386-1-4769585; Fax: +386-1-4258031
ꢀ
ꢁꢂꢀꢃꢄꢅꢆꢇ
Academic Editor: Peter J. Rutledge
Received: 22 February 2019; Accepted: 27 March 2019; Published: 1 April 2019
ꢀ
ꢁꢂꢃꢄꢅꢆ
Abstract: Despite the great importance of
of synthetic approaches for the formation of
report upon the stereoselective preparation of
reaction conditions and NH protective groups were tested to obtain compounds that contain
β
-lactam antibiotics, there is still a limited number
-lactam–containing dipeptides. In this study, we
-lactam–containing pseudopeptides, where different
β
β
2
3
-amino-azetidin-2-one. We demonstrate that the protective group is essential for the outcome of the
reaction. Successful implementation of dibenzyl-protected serine-containing dipeptides through the
Mitsunobu reaction can provide the desired products at high yields and stereoselectivity.
Keywords: β-lactams; dibenzylamino; Mitsunobu reaction; nocardicin; pseudopeptides
1
. Introduction
The
agents such as the penams (penicillin), penems, carbapenems, cephems and monobactams. Moreover,
monocyclic -lactams are potent cholesterol absorption inhibitors, which act through the inhibition
of the enzyme cholesterol acyl transferase [ ]. They are also dopamine receptor antagonists [ ] and
vasopressin V1a antagonists [ ]. Spirocyclic -lactams have also been shown to be potent type II
-turn–inducing peptide mimetics [ ]. In addition, these heterocycles also have several other biological
activities, such as antitumor, antitubercular, hypoglycemic, and anti-inflammatory actions. Their
pharmacological and chemical activities greatly depend on the types of substitution at the -lactam
-lactam ring with the
β-lactam heterocycle is the main building block of the most broadly used antibacterial
β
1
2
3
β
β
4
β
ring. However, synthesis of compounds with an appropriately substituted
desired diastereoselectivity is a challenging task for medicinal chemists [5,6].
β
There are numerous approaches for the synthesis of the
applied synthetic strategies include cycloaddition reactions, such as the Staudinger or Gilman-Speeter
reactions, the Kinugasa reaction, or different cyclization reactions [ ]. These last reactions can occur
β-lactam ring. The most frequently
7,8
through formation of N1-C2, N1-C4, or C3-C4 bonds. Interestingly, the Mitsunobu reaction, which is
the subject of the present report, has rarely been applied for the formation of the N1-C4 bond [9,10].
In this study, we focused on N1-C4
There are only a few methodologies in the literature that describe the formation of
pseudopeptides without a substituent on the -lactam ring [10 19]. Versatile and efficient synthetic
routes for -lactam rings that contain dipeptides are thus needed. Recently, a new pathway for the
synthesis of these compounds was developed that involves N-alkylation of the -lactam ring with a
haloacetate electrophile followed by the introduction of a side chain in the -position with respect to
β-lactam ring closure with less common reactants: dipeptides.
β-lactam
β
–
β
β
α
the ester carbonyl group (Scheme 1a) [15]. However, despite a number of different compounds being
synthesized by this methodology, it was not possible to form the desired monoalkylated products
stereoselectively. In addition, there were also limitations in the selection of the electrophiles used
Molecules 2019, 24, 1261; doi:10.3390/molecules24071261
www.mdpi.com/journal/molecules

Molecules 2019, 24, 1261
2 of 15
for alkylation, which affected the scope of the reaction [15]. A more versatile procedure is therefore
required to widen the chemical space of the structurally novel 3-amino-azetidin-2-ones.
Scheme 1. Formation of
of CH , ( ) phthalimide, Ox protective group, (
diethyl azodicarboxylate; DIAD = diisopropyl azodicarboxylate.
β
-lactam pseudopeptides without a C4 substituent. Previous work: (
a) alkylation
b
c
) Cbz, Boc protective group and ( ) this work. DEAD =
d
2
Previous reports of the preparation of nocardicins (Figure 1) have indicated N-phthalimido–
protected serylphenylglycine dipeptides cyclized into azetidin-2-ones under the classical Mitsunobu
reaction conditions (Scheme 1b). However, the dehydropeptide product and epimerization at the
exocyclic chiral center (C
was demonstrated to be strongly dependent on the acidity of the C
acidity of the N-H bond. This was rationalized by the hypothesis that a cyclization intermediate is
formed in which the ’-C is sp2-hybridized and this increases the acidity of the nitrogen center. In
general, increased acidity of C ’-H facilitates the formation of -lactams, and lowers the amount of
dehydropeptides in the reaction mixture [16 17]. Simple serylphenylglycine dipeptides thus gave
α
’-H) were commonly observed [10]. Interestingly, the cyclization reaction
α
’-H bond, rather than on the
α
α
β
,
much lower yields compared to those obtained by cyclization of their phenylglycine analogs [17].
Direct application of synthetic methodologies developed for the synthesis of nocardicins and their
analogs is therefore not directly applicable to the preparation of β-lactam phenylalanine derivatives.

Molecules 2019, 24, 1261
3 of 15
Figure 1. Nocardicin A, a representative β-lactam antibiotic of the nocardicins subgroup.
For the successful reaction here, protection of both of the hydrogens of the primary amine
functionality in the serine was demonstrated to be of vital importance. When the carboxybenzyl (Cbz)
or tert-butyloxycarbonyl (Boc) protective groups were applied, the reaction resulted in only a mixture
of aziridine and dehydropeptide (Scheme 1c, Cbz, Boc, respectively). The use of phthalimide (Pht)
or 4,5-diphenyloxazoline-2-one (Ox) protective groups is thus essential for the successful reaction
(Scheme 1b) [16,17,20].
2
. Results and Discussion
During an ongoing project that is devoted to the preparation of pharmacologically interesting
monocyclic
β-lactam analogs (e.g., Figure 1, the nocardicins), we initially faced the task of synthesizing
β
-lactam phenylalanine derivatives that differ from nocardicins in their additional CH group between
2
the phenyl ring and the exocyclic Cα’-H group.
The initial aim of this study was to find the optimal reaction conditions to prepare
β-lactam
pseudopeptides by the N1-C4 cyclization of serylphenylalanine derivatives, such as 1a (Scheme 2).
The effects of various N-protecting groups on the reaction outcome, and the application of these
findings to other chemically similar dipeptides under different reaction conditions, were first screened.
A hypothesis was defined such that because of the steric and electronic properties of a protective
group, it would be essential to the formation of both the
dehydrated products.
β-lactam ring and the amounts of the
Scheme 2. N1-C4 cyclization of dipeptide 1a–1c. Conditions: (a) KOH, pTolSO Cl, CH Cl , reflux; (b)
2
2
2
◦
◦
SO Cl , Et N, toluene, −50 C to rt; (c) CH SO Cl, Et N, DMAP, CH Cl ; (d) DIAD, PPh , THF, 0 C
2
2
3
3
2
3
2
2
3
to rt, 16 h, 15% yield or DIAD, P(OEt) , THF, rt, 16 h, 0% yield.
3

Molecules 2019, 24, 1261
4 of 15
Three very different protective groups were applied on the primary serine NH group, each
2
with very distinctive electron-withdrawing properties that could potentially influence the reaction
outcome: phthalimide (1a), trityl (1b) and dibenzyl (1c) protective groups. Both, phthalimide (1a) and
dibenzyl (1c) protective groups were used to mask hydrogens of the primary NH in order to prevent
2
the well-documented risk of aziridine formation in the cyclodehydration step [20]. In contrast, the
application of the trityl protection was rationalized by the idea that its unmasked N-H group would
not participate in the following reactions, because of its low acidity and the steric hindrance of the
triphenylmethane moiety. In addition, both trityl (1b) and dibenzyl (1c) protective group also have
lower electron withdrawing properties compared to the phthalimide group (1a).
Next, each of the three compounds 1a
idea was also to avoid high reaction temperatures and strong bases, such as lithium diisopropylamide
LDA) or NaH, which have otherwise been used in other -lactam ring preparations [11 21]. Firstly,
–1c was treated under different reaction conditions. The
(
β
,
the conversion of an alcoholic OH group of serine to a sulfonate group was tested because it is
widely used in aziridine ring formation, in which carbamate or trityl nitrogen is used as the nitrogen
source [22
,
23]. Furthermore, the activation of a serine OH group via conversion to an imidazolyl
25]. It was therefore postulated
sulfonate has been reported to yield 3-aminonocardicinic acids [24
,
that each of the derivatives 1a–1c will react with the amide nitrogen instead. For this purpose, three
reagents were used: p-toluenesulfonyl chloride (condition a); sulfuryl chloride (condition b); and
methanesulfonyl chloride (condition c). Unfortunately, all three reactions by which a sulfonate was
formed (i.e., conditions a, b, c) as an intermediate were not suitable for
conditions yielded only a product of ester hydrolysis , the aziridine
b in a ratio of 1:1 (which we were not able to separate) (Scheme 2).
Secondly, compound 1a was treated under Mitsunobu reaction conditions, which is commonly
applied for the preparation of nocardicin derivatives. However, to the best of our knowledge, there are
no previous reports on the preparation of -lactam phenylalanine derivatives using the Mitsunobu
reaction. Furthermore, attempts to prepare less reactive glycine derivatives using the Mitsunobu
reaction have given the corresponding -lactam ring at poor yields. In addition, the mixtures obtained
were difficult to separate, which was explained by the lower acidity of the exocyclic C ’-H group which
β-lactam ring formation. These
4
5 or a mixture of chlorides 6a and
6
β
β
α
consequently decreases the yield of a reaction and increases the number of side products [16]. It was
therefore not surprising that our first attempts to cyclize 1a (Scheme 2) under the classical Mitsunobu
conditions using PPh /DIAD or DEAD gave only low yields of
2
β
-lactam 3a and dehydropeptide
, in nearly equimolar ratios. These are difficult to separate in larger quantities. In addition, the
application of P(OEt) instead of PPh in the Mitsunobu reaction, which has been successfully applied
3
3
3
in the synthesis of nocardicin analogs, resulted in no products whatsoever [13,18]. Our findings were
therefore in agreement with the literature reports that have described the influence of the C
α’-H group
acidity on the reaction outcome (Scheme 1) [16].
Further optimization of the reaction conditions through changing the solvent and temperature
increased the yield of -lactam 3a to 45%, but the ratio of dehydrodipeptide to 3a remained at
:1. A modified procedure that would give larger quantities of -lactam using simple cyclization
β
2
1
β
without the formation of dehydrodipeptide was therefore required. Compounds 1b and 1c with trityl
and dibenzyl protection were then treated under the same classical Mitsunobu reaction conditions
as 1a. Even though the tritylated compound 1b gave no product whatsoever, a combination of the
classical Mitsunobu reaction and dibenzyl protection, gave positive results, whereby dipeptide 1c
was transformed into the desired SS diastereoisomer 3c at 76% isolated yield. Epimerization at the
exocyclic C
α’-H stereogenic center was also seen, with the diastereoisomeric ratio (dr) of SS to SR of
7:1; these were easy to separate. Importantly, no dehydrodipeptide side products were seen, and the
yields were much higher when compared to the results of the Mitsunobu reaction for phthalimide
protected dipeptide 1a. Furthermore, the outcome of the reaction gave higher isolated yields when
compared to the less reactive phthalimide-protected glycine [16] and cyclopropyl derivatives [17] that
have been reported in the literature.

Molecules 2019, 24, 1261
5 of 15
The reaction conditions for both of the reaction steps were then further tested on analogous
serylphenylalanine dipeptides 1d, 1e, which both gave cyclized products 3d 3e at 99% and 90%
,
yields, respectively (Scheme 3). These demonstrated the utility of the dibenzyl protective group in the
preparation of simple β-lactam phenylalanine derivatives.
Scheme 3. Formation of β-lactam pseudopeptides and their deprotection. Conditions: (a) DIAD, PPh3,
THF or DMF; (b) H -Pd/C, 5% EtOH/EtOAc.
2
However, there are also limitations when the acidity of the Cα stereogenic center is higher, in
terms of the dr and the optimal solvent. For example, phenylalanine 3c, benzyl 3d, and acetyl-protected
tyrosine dipeptide 3f were all prepared at high dr in THF. In contrast, cyclization of the phenylglycine
derivative 1e in which a direct link between C
α
’-H and electron-withdrawing phenyl ring makes
1d and 1f, proceeded only in DMF, to
C
α
’-H more acidic compared to the phenylalanine analogues 1c,
give 3e at high yields, but with almost equimolar dr of the SS and SR isomer. It appears that the basic
nitrogen of the dibenzylamino moiety in combination with DMF as a solvent induces epimerization at
the exocyclic Cα’-H when the basic dibenzylamino moiety is applied. This is in agreement with a report
by Miller, who observed epimerization to thermodynamic equilibrium when traces of trimethylamine
were added to pure phenylglycine derivatives [13].
Removal of the dibenzyl protecting group from 3b–3e preceded the next reaction step. Pd-C in a
mixture of 5% EtOH in ethyl acetate was preferred to other combinations of catalysts and solvents
because it gave significantly higher yields compared to the reactions in EtOH, MeOH or THF. Here,
the percentage of EtOH and the traces of DIAD in the reaction mixtures had an enormous influence on
the yield and the general outcome of the reactions. In the presence of DIAD as an impurity, higher
amounts of EtOH had to be used to obtain full conversion of 3b
–3f to yield the corresponding products
7b–
7f. However, higher proportions of EtOH also resulted in lower yields, with higher amounts of
side products. The purities of all of the compounds 3b–3f were thus of uttermost importance for the
successful reaction (Scheme 3).
Compounds 3c
–3e were deprotected with 100% conversion to yield debenzylated products 7b,
7
d, and 7e at 60%, 80%, and 78% isolated yields, respectively (Scheme 3). Many other side products

Molecules 2019, 24, 1261
6 of 15
were also observed, among which L-Phe-OCH was isolated at 10% yield when 3b was treated under
3
these conditions. Unfortunately, the acetyl protective group on phenol 3f proved to be too labile
to enable successful deprotection to 7f, giving instead only a mixture of unidentifiable acetylated
products. However, it was easy to selectively remove acetyl protection from the tyrosine OH group
prior to debenzylation of 3e. For this purpose, 0.1 M LiOH in methanol was used, to obtain phenol
8
at quantitative yields, with dr of 10:1. This product offered the possibility of future preparation of
O-derivatized analogs. This compound was then easily debenzylated to 7d at 85% yield.
Because these compounds contain a primary amine and an ester functional group they have a
potential to oligomerize after prolonged storage. We would therefore advise the reader to transform
them into some form of a salt if stored for a prolonged period of time.
3
. Materials and Methods
3
.1. Materials and Instruments
The reactions were monitored by TLC carried out on Merck silica gel (60 F254, Darmstadt,
Germany) by using UV light as visualizing agent, KMnO in water, phosphomolybdic acid in ethanol
4
and ninhydrine in ethanol and heat as developing agents. Column chromatography was performed
1
using Merck Silica Gel 60. Proton nuclear magnetic resonance spectra ( H-NMR) were obtained at
4
00 MHz on Bruker 400 spectrometers (Billerica, MA, USA). Spectra were recorded in CDCl , MeOD
3
and DMSO-d solutions. Chemical shifts are reported in ppm, referenced to tetramethylsilane (TMS)
6
13
as the external reference. Carbon-13 nuclear magnetic resonance spectra ( C-NMR) were obtained at
00 MHz on Bruker 400 spectrometer. Chemical shifts are reported in ppm, referenced to the solvent
1
peak of CDCl . Low-resolution mass spectra were obtained with a Shimadzu GC-MS-QP2010 mass
3
spectrometer (Kyoto, Japan). High resolution mass spectra (HRMS) were recorded on Q Executive
Plus LC-MS/MS system (Thermo Scientific, Waltham, MA, USA).
3
.2. Synthesis and Characterization
3
.2.1. Procedure for Synthesis of Precursors 1a and 1b (Scheme 2)
(
S)-Methyl ((S)-2-((tert-butoxycarbonyl)amino)-3-hydroxypropanamido)-3-phenylpropanoate (S3) [26]. To a
suspension of Boc-L-Ser (S1) (1.00 g, 4.9 mmol, 1.0 eq) in CH Cl (50 mL, 0.1 M), N-methylmorpholine
2
2
was added (1.6 mL, 14.6 mmol, 3.0 eq), followed by the addition of L-Phe-OCH hydrochloride (S2
)
3
◦
(1.05 g, 4.9 mmol, 1.0 eq). After 0.5 h, the solution was cooled to 0 C and HOBt (658 mg, 4.9 mmol,
1
.0 eq) and EDC (1.02 g, 5.4 mmol, 1 eq) were added and the reaction mixture was stirred for 18 h at
room temperature. A solution was dissolved in ethyl acetate (100 mL) and extracted with 10% citric
acid (2 20 mL), saturated solution NaHCO (2 20 mL), brine (20 mL) and dried over anhydrous
Na SO . Organic solvent was evaporated under reduced pressure and the crude product was purified
×
×
3
2
4
by flash column chromatography in Hex/EtOAc = 1/2 as eluent. White solid (446 mg, 1.2 mmol, 25%
◦
1
yield) [26]; R (Hexane/ethyl acetate = 1/2) = 0.25; mp = 92–93 C; H-NMR (400 MHz, DMSO-d ):
δ
f
6
8.14 (d, J = 7.8 Hz, 1H), 7.31–7.14 (m, 5H), 6.65 (d, J = 8.3 Hz, 1H), 4.80 (t, J = 5.8 Hz, 1H), 4.49 (ddd,
J = 8.0, 7.8, 5.8 Hz, 1H), 3.99 (ddd, J = 8.3, 6.8, 4.7 Hz, 1H), 3.58 (s, 3H), 3.50, 3.43 (ABXY, J = 11.2, 5.8, 4.7,
1
3
6
1
2
.8 Hz, 2H), 3.01, 2.94 (ABX, dd, J =13.5, 8.0, 5.8 Hz, 2H) ppm; C-NMR (100 MHz, CDCl ):
δ
172.0,
3
71.2, 135.9, 129.4, 128.8, 127.4, 80.6, 63.1, 55.2, 53.6, 52.7, 37.9, 28.4 ppm; MS (ESI ): m/z 389 [M + Na]+,
+
−
−
89 (100); HRMS (ESI ): Calc mass for C H N O [M − H] : 365.1713; measured: 365.1721; IR
18
25
2
6
(
ATR): 3521.6, 3338.0, 2977.8, 1736.1, 1668.1, 1649.2, 1540.3, 1519.9, 1307.3, 1266.4, 1223.9, 1008.3, 698.4,
-1
25
D
6
16.3 cm ; [α] = +33.7 (0.56, MeOH).
(
S)-Methyl 2-((S)-2-amino-3-hydroxypropanamido)-3-phenylpropanoate hydrochloride (S4) [26]. Compound
S4 was synthesized according to the literature procedure [26]. To a solution of S3 (446 mg, 1.2 mmol)
◦
in CH Cl (5 mL) was added CF COOH (1 mL) dropwise at 0 C, and the solution was stirred at rt for
2
2
3
5
h. Solvent was evaporated and the white residue was dissolved in diethyl ether (10 mL) to which 2 M

Molecules 2019, 24, 1261
7 of 15
HCl in diethyl ether (3.0 mL, 6.1 mmol, 5 eq) was added and filtered to afford the product S4. White
◦
1
needles (306 mg, 1.0 mmol, 83% yield); mp = 149–150 C; H-NMR (400 MHz, MeOD):
δ 7.36–7.19 (m,
5
H), 4.75 (dd, J = 8.7, 5.5 Hz, 1H), 4.04–3.87 (m, 2H), 3.79 (dd, J = 10.3, 5.9 Hz, 1H), 3.73 (s, 3H), 3.23
13
(dd, J = 14.2, 5.4 Hz, 1H), 3.04 (dd, J = 14.2, 8.7 Hz, 1H) ppm; C-NMR (100 MHz, MeOD):
δ
+
173.1,
+
1
2
6
68.4, 138.1, 130.4, 129.7, 128.2, 61.8, 56.3, 55.7, 53.0, 38.3 ppm; MS (ESI ): m/z 289 ([M + Na] , 100),
67 ([M + H] , 80); IR (ATR): 3325.0, 3245.5, 1731.4, 1663.4, 1535.3, 1445.8, 1363.2, 1225.6, 1049.1, 699.2,
17.6 cm ; [α]D = +86.0 (0.233, MeOH).
+
−
1
25
(S)-Methyl ((S)-2-(3-(1,3-dioxoisoindolin-2-yl)-2-oxoazetidin-1-yl)-3-phenylpropanoate (1a, Scheme 2). To
a solution of S4 (1.1 g, 3.63 mmol, 1 eq) in DMF (20 mL) phthalanhydride (0.59 g, 3.99 mmol, 1.1 eq)
◦
and triethylamine (0.53 mL, 3.81 mmol, 1.05 eq) were added, and the solution was stirred at 100 C
for 18 h. After the mixture was cooled to room temperature, DMF was evaporated under reduced
pressure and the oily residue was dissolved in EtOAc (50 mL). Organic phase was washed with water
(
50 mL), 1 M HCl (50 mL), saturated solution NaHCO (50 mL) and brine (50 mL) and dried over
3
anhydrous Na SO . Organic solvent was evaporated under reduced pressure yielding a white solid.
2
4
◦
White solid (1.14 g, 2.9 mmol, 80% yield); R (hexane/ethyl acetate = 1/1) = 0.11; mp = 93–96 C;
H-NMR (400 MHz, CDCl3):
f
1
δ 7.85 (dd, J = 5.5, 3.1 Hz, 2H), 7.73 (dd, J = 5.5, 3.1 Hz, 2H), 7.33–7.37
(
1
3
1
m, 2H), 7.27–7.30 (m, 1H), 7.22–7.24 (m, 2H), 5.25 (dd, J = 5.6, 3.0 Hz, 1H,), 4.87 (dd, J = 9.3, 6.2 Hz,
H), 3.90 (dd, J = 5.4, 3.0 Hz, 1H), 3.83 (s, 3H), 3.64 (t, J = 5.6 Hz, 1H), 3.29 (dd, J = 14.3, 6.2 Hz, 1H),
.08 (dd, J = 14.3, 9.3 Hz, 1H) ppm; 13C-NMR (100 MHz, CDCl ): 135.8, 134.4, 131.7, 128.9, 128.7, 127.7,
3
+
+
+
23.3, 55.0, 53.1, 52.6, 45.2, 35.8 ppm; MS (ESI ): m/z 419 [M + Na, 100] ; HRMS (ESI ): Calc mass for
+
C H O N : 397.1394 [M + H] ; measured: 397.1406; IR (ATR): 1711.4, 1675.8, 1529.2, 1467.8, 1437.5,
1
[
21
21
6
2
−
1
386.1, 1215.2, 1115.5, 1079.7, 1030.3, 983.2, 911.3, 877.3, 789.1, 719.7, 702.4, 648.9, 531.4, 504.7 cm
;
25
α] = −213.0 (0.424, MeOH).
D
Methyl trityl-L-seryl-L-phenylalaninate (1b, Scheme 2). To a solution of dipeptide S4 (500 mg, 1.7 mmol,
1
.0 eq) in chloroform (5.5 mL, 0.3 M), trimethylamine (690
µ
L, 5.0 mmol, 3 eq) was added. After 5 min,
◦
the solution was cooled to 0 C followed by the slow addition of trityl chloride (576 mg, 2.0 mmol,
1
◦
.2 eq) at 0 C. The reaction mixture was stirred for 18 h at room temperature. Ethyl acetate (50 mL)
was added and the mixture was extracted with 10% citric acid (2
×
10 mL), saturated solution NaHCO3
(
2
×
10 mL), brine (10 mL) and dried over anhydrous Na SO . Organic solvent was evaporated under
2
4
reduced pressure and the crude product was recrystallized from diethyl ether. White crystals (220 mg,
◦
32 mmol, 26% yield); R (hexane/ethyl acetate = 1/1) = 0.29; mp = 118–120 C; H-NMR (400 MHz,
f
1
4
CDCl3):
δ
7.79 (d, J = 8.5 Hz, 1H), 7.47–7.12 (m, 20H), 4.85 (ddd, J = 8.5, 6.3, 6.0 Hz, 1H), 3.79 (s, 3H),
3
2
.48, 2.32 (AMXY, J = 11.1, 5.5, 2.8, 5.7 Hz, 2H), 3.21 (bs, 1H), 3.16, 3.13 (ABX, J = 14.6, 6.0, 6.3 Hz,
H), 2.80 (bs, 1H), 1.67 (t, J = 5.5 Hz, 1H) ppm; 13C-NMR (100 MHz, CDCl3):
δ
173.7, 172.2, 146.0,
+
136.1, 129.5, 129.0, 128.8, 128.3, 127.5, 126.9, 71.7, 63.4, 59.1, 53.0, 52.8, 38.1 ppm; MS (ESI ): m/z 531
+
−
−
H] , 100): 507.2284; measured:
([M + Na] , 100); HRMS (ESI ): Calc mass for C H N O ([M
−
32
31
2
4
−
1
5
07.2277; IR (ATR): 3593.4, 3468.1, 332.0, 2948.7, 1739.8, 1668.0, 1491.1, 1198.9, 1016.0, 743.6, 706.1 cm
;
2
5
[
α] = +198.4 (0.310, MeOH).
D
3
.2.2. Procedure for Synthesis of Precursor 1c (Scheme 2)
Dibenzyl-L-serine (S6) [27]. Dibenzyl-L-serine (S6) was synthesized according to the modified literature
procedure [27]. L-Ser (20.0 g, 190 mmol, 1 eq) and tetrabutylammonium iodide (7.0 g, 19 mmol, 0.1 eq)
were dissolved in a solution of KOH (74.7 g, 1.3 mol, 7 eq, 3 M) in a mixture of water and ethanol (1:1,
440 mL). Benzyl chloride (131 mL, 1.14 mmol, 6 eq) was added dropwise and the mixture was stirred
under reflux for 1 h. KOH (10.7 g, 190 mmol, 1 eq) was added again and the mixture was stirred further
for another hour. After the mixture was cooled down, the white precipitate was dissolved by the
addition of water. The solution was extracted with toluene (5 × 100 mL), combined organic fractions
were washed with brine (50 mL) and dried over anhydrous Na SO . Organic solvent was evaporated
2
4
under reduced pressure and the crude product was purified by flash column chromatography. The

Molecules 2019, 24, 1261
8 of 15
crude product was dissolved in water (150 mL), the pH value was adjusted to 5 with 1 M HCl and the
mixture was left standing over 2 days in a refrigerator to obtain white crystals. White crystals (41.2 g,
◦
1
1
44 mmol, 76% yield); mp = 144–146 C; H-NMR (400 MHz, DMSO-d ):
δ
7.40–7.14 (m, 10H), 3.81,
6
3.62 (AB, J = 13.9 Hz, 2H), 3.78, 3.64 (ABX, J = 11.0, 7.2, 6.0 Hz, 2H), 3.25 (dd, J = 7.2, 6.0 Hz, 1H) ppm.
Methyl dibenzyl-L-seryl-L-phenylalaninate (1c, Scheme 2). Dipeptide methyl dibenzyl-L-seryl-L-
phenylalaninate (1b) was synthesized in a coupling procedure similar to the synthesis of S3 starting
from dibenzyl-L-serine (S6) (1.10 g, 3.9 mmol, 1.0 eq). The crude product was purified by flash column
chromatography on silica using a gradient of hexane/ethyl acetate starting from 3/1 to 1/1. Colorless
1
liquid (1.62 g, 3.6 mmol, 94% yield); R (Hexane/ethyl acetate = 3/1) = 0.18; H-NMR (400 MHz,
f
CDCl3):
H), 4.03, 3.95 (ABX, J = 11.3, 6.9, 4.4 Hz, 2H), 3.78 (s, 3H), 3.73, 3.49 (AB, J = 13.7 Hz, 4H), 3.31 (dd,
J = 6.9, 4.4 Hz, 1H), 3.26, 3.08 (AB, J = 14.0, 5.3, 6.2 Hz, 2H) ppm; 13C-NMR (100 MHz, CDCl3):
δ 7.94 (d, J = 7.7 Hz, 1H), 7.33–7.13 (m, 10H), 7.13–6.95 (m, 5H), 4.83 (ddd, J = 7.7, 5.3, 6.2 Hz,
1
δ
1
5
73.6, 171.9, 138.4, 135.7, 129.2, 128.7 (2 signals overlapping), 128.6, 127.5, 127.2, 62.4, 58.1, 54.9, 53.3,
+
+
−
2.6, 38.1 ppm; MS (ESI ): m/z 469 [M + Na] ; HRMS (ESI ): Calc. Mass for C H N O : 445.2127
2
7
29
2
4
−
([
M − H] , 100), measured: 445.2130; IR (ATR): 3372.9, 3028.5, 1742.6, 1655.4, 1494.6, 1216.0, 1028.0,
−
1
25
6
97.8 cm ; [α]D = +133.6 (0.44, MeOH).
3
.2.3. Synthesis of Trityl Protected Dipeptide 4 and Aziridine 5 (Scheme 2)
Trityl-L-seryl-L-phenylalanine (4, Scheme 2). To a solution of trityl protected dipeptide 1b (0.1 g, 0.2 mmol,
1
eq) in CH Cl (10 mL), TsCl (1.1 eq) and KOH (2.5 eq) were added and the reaction mixture was stirred
2
2
◦
for 4 h at 50 C. The solution was cooled to room temperature, organic solvent was evaporated and the
crude mixture was purified by flash column chromatography on silica using ethyl acetate/methanol =
5
=
/1 as eluent. White solid (45 mg, 0.09 mmol, 46% yield); R (Hexane/ethyl acetate = 1/1) = 0; mp
f
◦
71–37 C; H-NMR (400 MHz, CDCl ): δ 7.47–7.12 (m, 21H), 4.62–4.42 (m, 1H), 3.52, 2.50 (m, 2H),
3
1
13
3.23–3.13 (m, 1H), 3.12–2.94 (m, 2H) ppm; C-NMR (100 MHz, CDCl3):
δ
−
175.8, 174.8, 145.9, 136.3,
+
1
29.5, 129.0,128.8, 128.2, 127.4, 126.9, 71.6, 63.6, 58.9, 54.0, 37.3 ppm; MS (ESI ): m/z 493 [M
−
H, 100] ;
−
−
HRMS (ESI ): Calc mass for C H N O [M
−
H] : 493.2133; measured: 493.2135; IR (ATR): 3327.3,
31
29
2
4
−
1
2
928.2, 1723.9, 1649.4, 1520.1, 1492.8, 1447.5, 1187.6, 1123.2, 1030.1, 903.6, 745.7, 698.3, 633.4, 523.1 cm
.
Methyl ((S)-1-tritylaziridine-2-carbonyl)-L-phenylalaninate (
5, Scheme 2) [28]. Procedure A: To a solution
of dipeptide (0.1 g, 0.2 mmol, 1 eq) in anhydrous toluene, triethylamine (68
µ
L, 0.5 mmol, 2.5 eq) was
◦
50 C and SO Cl was added
2 2
added under argon atmosphere. The reaction mixture was cooled to
−
◦
dropwise. The mixture was stirred at
evaporated and the crude product was purified by flash chromatography. Procedure B: To a solution
−
50 C for 2 h, then 18 h at room temperature. The solvent was
◦
of dipeptide (0.1 g, 0.2 mmol, 1 eq) in CH Cl (10 mL) cooled to 0 C, triethylamine (68
µL, 0.49 mmol,
2
2
2
.5 eq) was added, followed by MsCl (17 µL, 0.21 mmol, 1.1 eq). The reaction mixture was stirred at
room temperature for 16 h. The solvent was evaporated and the crude product was purified by flash
chromatography using a gradient of hexane/ethyl acetate starting from 4/1 to 1/1. White solid (25 mg,
◦
.05 mmol, 26% yield); R (Hexane/ethyl acetate = 1/1) = 0.35; mp = 53–55 C; H-NMR (400 MHz,
f
1
0
CDCl3):
δ
7.34–7.37 (m, 7H), 7.16–7.29 (m, 13H), 4.90–4.96 (m, 1H), 3.80 (s, 3H), 3.20 (J = 6.1, 1.8 Hz,
2
H), 1.95 (dd, J = 6.6, 2.7 Hz, 1H), 1.69 (dd, J = 2.6, 0.7 Hz, 1H), 1.37 (dd, J = 6.6, 0.7 Hz, 1H) ppm;
13
C-NMR (100 MHz, CDCl ): δ 171.0, 170.6, 143.4, 136.0, 129.5 (two overlapping signals), 128.9, 127.9,
3
+
+
−
1
4
27.5, 127.3, 74.7, 52.7, 52.1, 38.1, 34.0, 30.1 ppm; MS (ESI ): m/z 513 [M + Na, 100] ; MS (ESI ): m/z
89 [M
−
H, 100] ; HRMS (ESI ): Calc mass for C H O N [M + H] : 491.2329; measured: 491.2332;
32 31 3 2
+
+
−
IR (ATR): 3367.6, 3058.4, 1743.5, 1677.2, 1511.4, 1446.8, 1351.4, 1216.6, 1080.7, 1009.2, 902.8, 745.1, 705.9,
−
1
25
6
33.4 cm ; [α]D = −332.3 (0.546, MeOH).
3
.2.4. General Procedure for the Synthesis of 2, 3a, 3c via Mitsunobu Reaction (Schemes 2 and 3)
To a solution of dipeptide 1a or 1c (0.2 mmol, 1.0 eq) and PPh (1.5 eq) in anhydrous tetrahydrofuran
3
◦
DIAD (1.5 eq) was added drop-wise at 0 C under argon atmosphere. After the solution was stirred

Molecules 2019, 24, 1261
9 of 15
for 18 h at room temperature the solvent was evaporated and the crude product was purified by
flash chromatography.
Methyl (2-(1,3-dioxoisoindolin-2-yl)acryloyl)-L-phenylalaninate (
2, Scheme 2). White solid (20 mg, 0.052 mmol,
◦
1
2
(
1% yield; R (Hexane/ethyl acetate = 1/2) = 0.51; mp = 63–65 C; H-NMR (400 MHz, CDCl ):
δ
7.91
dd, J = 5.5, 3.1 Hz, 2H), 7.78 (dd, J = 5.5, 3.1 Hz, 2H), 7.21–7.30 (m, 3H), 7.15–7.17 (m, 5H), 6.48 (d,
J = 7.6 Hz, 1H), 6.08 (d, J = 1,4 Hz, 1H), 5.82 (d, J = 1,4 Hz, 1H), 4.96 (dt, J = 7.6, 5.3 Hz, 1H), 3.76 (s, 3H),
.22 (t, J = 5.2 Hz, 2H) ppm; 13C-NMR (100 MHz, CDCl3):
171.6, 166.5, 162.6, 135.7, 134.7, 132.7, 131.8,
f
3
3
δ
+
+
−
1
28.7, 128.6, 127.3, 124.1, 120.5, 53.7, 52.6, 37.7 ppm; MS (ESI ): m/z 401 [M + Na] ; MS (ESI ): m/z 377
+
+
+
[M − H] ; HRMS (ESI ): Calc mass for C H O N [M + H] 379.1288; measured: 379.1287; IR (ATR):
21
19
5
2
3
299.1, 1789.4, 1722.6, 1656.8, 1624.7, 1536.3, 1454.7, 1378.1, 1303.2, 1273.6, 1207.7, 1172.2, 1116.3, 1083.1,
−1
25
D
1031.3, 995.4, 950.6, 920.1, 883.4, 788.1, 735.2, 701.3, 630.1, 572.4, 532.5 cm
;
[α] = −585,9 (0.375, MeOH)
.
Methyl (S)-2-((S)-3-(1,3-dioxoisoindolin-2-yl)-2-oxoazetidin-1-yl)-3-phenylpropanoate (3a, Scheme 2). White
◦
solid (14 mg, 0.037 mmol, 15% yield); R (chloroform/acetone = 100/5) = 0.39; mp =122–124 C;
H-NMR (400 MHz, CDCl3):
f
1
δ 7.88 (dd, J = 5.5, 3.0 Hz, 2H), 7.77 (dd, J = 5.5, 3.0 Hz, 2H), 7.16–7.20 (m,
3
(
H), 7.05–7.07 (m, 2H), 6.80 (d, J = 7.7 Hz, 1H), 4.86–4.91 (m, 2H), 4.40 (dd, J = 11.5, 6.8 Hz, 1H), 4.03
dd, J= 11.5, 5.3 Hz, 1H), 3.73 (s, 3H), 3.13 (dd, J = 5.7, 4.1 Hz, 2H) ppm; 1 C-NMR (100 MHz, CDCl3):
3
δ
171.7, 168.0, 167.4, 135.6, 134.5, 131.7, 129.3, 128.6, 127.2, 123.7, 60.9, 54.5, 53.51, 52.5, 37.7 ppm; MS
−
−
H] ; MS (ESI ) m/z 400.6 [M + Na] ; HRMS (ESI ): Calc mass for C H O N
21 19 5 2
+
+
+
(
ESI ): m/z 376.5 [M
−
+
[M + H] : 379.1288; measured: 379.1298; IR (ATR): 1763.4, 1740.4, 1717.2, 1391.3, 1205.2, 1115.8, 1025.9,
−
1
25
7
16.9, 701.6 cm ; [α]D = −507.8 (0.546, MeOH).
Methyl (S)-2-((S)-3-(dibenzylamino)-2-oxoazetidin-1-yl)-3-phenylpropanoate (3cS, Scheme 2). Yellow oil
1
(
76% yield); R (hexane/ethyl acetate = 6/1) = 0.19; H-NMR (400 MHz, CDCl ):
δ
7.39–7.11 (m, 16H),
f
3
4
.70 (dd, J = 10.0, 5.6 Hz, 1H), 4.13 (dd, J = 5.2, 2.5 Hz, 1H), 3.74, 3.64 (AB, J = 13.7 Hz, 4H), 3.39, 3.18
(
(
AMX, J = 5.6, 5.2, 2.5 Hz, 2H), 3.71 (s, 3H), 3.22, 3.00 (ABX, J = 13.4, 10.0, 5.6 Hz, 2H) ppm; 13C-NMR
100 MHz, CDCl3):
δ 170.8, 169.7, 138.6, 136.4, 129.1, 128.9, 128.8, 128.5, 127.4, 127.3, 68.5, 54.8, 54.7, 52.7,
+
+
−
−
4
4
3.2, 35.7 ppm; MS (ESI ): m/z 429 [M + H] ; HRMS (ESI ): Calc mass for C H O N [M − H] :
27
29
3
2
−
1
29.2173; measured: 429.2170; IR (ATR): 3028.4, 2952.4, 1744.9, 1447.4, 1220.3, 744.2, 700.3 cm
;
25
[
α] = −34.5 (0.38, MeOH).
D
Methyl (R)-2-((S)-3-(dibenzylamino)-2-oxoazetidin-1-yl)-3-phenylpropanoate (3cR, Scheme 2). Yellow oil (9%
1
yield); R (hexane/ethyl acetate = 6/1) = 0.13; H-NMR (400 MHz, CDCl ):
δ 7.38–7.08 (m, 15H), 4.85
f
3
(
3
dd, J = 11.3, 5.1 Hz, 1H), 4.25 (dd, J = 5.0, 2.4 Hz, 1H), 3.72 (s, 3H), 3.40, 3.24 (AB, J = 14.0 Hz, 4H), 3.34,
.11 (AMX, J = 5.5, 5.0, 2.4 Hz, 1H), 3.31, 2.95 (AMX, J = 14.6, 5.1, 11.3 Hz 2H) ppm; 13C-NMR (100 MHz,
CDCl3): 170.7, 169.5, 138.5, 136.2, 129.0 (2 signals), 128.9, 128.5, 127.4, 127.3, 68.7, 54.4, 53.8, 52.7,
2.7, 36.0 ppm; HRMS (ESI ): Calc mass for C H O N [M
δ
−
−
H] : 429.2173; measured: 429.2172; IR
4
(
−
27
29
3
2
−
1
ATR): 3332.8, 3027.8, 2952.5, 1736.2, 1453.9, 1259.3, 1221.4, 1096.2, 1027.9, 735.4, 697.0 cm
.
3
.2.5. Synthesis of Chlorides 6a and 6b (Scheme 2)
Compounds 6a and 6b were synthesized according to the procedures described for the preparation
using SO Cl or CH SO Cl to obtain a crude mixture, which was purified with flash column
of
5
2
2
3
2
chromatography. However, a mixture of both compounds 6a and 6b in equimolar ratio was obtained,
which was further purified with preparative HPLC. A separation was possible, but the compounds
were transformed back upon treatment to give mixture of 6a:6b in a ratio 1:1 again. The reported data
are given for a mixture of both compounds. Colorless liquid (60% yield); R (hexane/diethyl ether =
f
1
3
/1) = 0.36; H-NMR (400 MHz, acetone-d ):
δ
7.84 (d, J = 7.9 Hz, 1H), 7.65 (d, J = 8.0 Hz, 1H), 4.82
6
(
(
(
ddd, J = 8.8, 8.0, 5.1 Hz, 1H), 4.72 (ddd, J = 8.1, 7.9, 5.6 Hz, 1H), 4.47 (dd, J = 6.9, 6.4 Hz, 1H), 4.00, 3.88
ABX, J = 11.3, 6.8, 6.7 Hz, 2H), 3.75, 3.53 (AX, J = 13.9 Hz, 4H), 3.71, 3.58 (AB, J = 13.7 Hz, 4H), 3.50
dd, J = 6.7, 6.8 Hz, 1H), 3.28, 3.08 (AMX, J = 14.0, 8.8, 5.1 Hz, 2H), 3.18, 3.07 (ABX, J = 13.9, 5.6, 8.1 Hz,
2
H), 3.12, 2.83 (AMX, J = 13.5, 6.9, 6.4 Hz, 2H,) ppm; 13C-NMR (100 MHz, acetone-d6):
δ 172.6, 172.2,

Molecules 2019, 24, 1261
10 of 15
1
5
70.0, 168.6, 140.1, 139.8, 138.0, 137.8, 130.3, 129.9, 129.7, 129.4, 129.3, 129.2, 129.1, 128.0, 127.7, 64.6, 58.9,
+
+
+
8.3, 56.6, 55.1, 55.0, 54.4, 52.3, 52.5, 42.3, 38.2, 38.0 ppm; MS (ESI ): m/z 429 [M + H] ; HRMS (ESI ):
+
Calc mass for C H ClNO [M + H] : 465.1939; measured: 465.1928; IR (ATR): 3028.6, 1737.1, 1603.2,
1
27
29
3
−
1
494.4, 1453.1, 1377.9, 1256.4, 1221.8, 1174.0, 1128.7, 1026.1, 915.4, 745.9, 698.7 cm
3
.2.6. Synthesis of Tyrosine Derivatives 1d–1f (Scheme 3)
Methyl (R)-3-(4-(benzyloxy)phenyl)-2-((tert-butoxycarbonyl)amino)propanoate (S8a) [29]. Benzyl protected
compound S8a was synthesized according to the modified literature procedure [29] from methyl
(tert-butoxycarbonyl)-D-tyrosinate (S7) (8.80 g, 29.8 mmol, 1 eq) which was dissolved in acetone
(
160 mL). Potassium carbonate (4.12 g, 29.8 mmol, 1 eq) was added and the mixture obtained was
◦
cooled to 0 C followed by dropwise addition of benzyl bromide (3.9 mL, 32.9 mmol, 1.1 eq) and the
reaction mixture was stirred for 24 h at room temperature. Ethyl acetate (200 mL) was then added
and the solution was washed with water (100 mL). Water layer was then extracted with ethyl acetate
(
2 × 50 mL) and the combined organic fractions were washed with brine, dried over anhydrous sodium
sulfate, filtered and evaporated. A crude mixture was purified with flash column chromatography
using hexane/ethyl acetate = 3:1 as eluent. White amorphous solid (10.77 g, 27.9 mmol, 94% yield); mp
=
◦
83–86 C; R (hexane:ethyl acetate = 3:1) = 0.38; H-NMR (400 MHz, CDCl ): δ 7.45–7.28 (m, 5H),
f
3
1
7
6
.03 (d, J = 8.6 Hz, 2H), 6.90 (d, J = 8.6 Hz, 2H), 5.03 (s, 2H), 4.98 (d, J = 8.0 Hz, 1H), 4.54 (ddd, J = 8.0,
.1, 5.7 Hz, 1H), 3.70 (s, 3H), 3.05, 2.99 (ABX, J = 14.1, 6.1, 5.7 Hz, 2H), 1.42 (s, 9H) ppm; 13C-NMR
(
5
1
100 MHz, CDCl3): δ 172.5, 157.9, 155.2, 137.1, 130.4, 128.6, 128.3, 128.0, 127.5, 114.9, 79.9, 70.0, 54.6,
+
+
2.2, 37.4, 28.4 ppm; MS (ESI ): 408 [M + Na] ; IR (ATR): 3367.2, 2977.4, 1742.6, 1710.4, 1509.8, 1365.4,
−
1
25
241.0, 1161.1, 1016.9, 732.9 cm ; [α]D = −4.3 (0.66, MeOH).
Methyl (R)-3-(4-acetoxyphenyl)-2-((tert-butoxycarbonyl)amino)propanoate (S8b
)
[30]. Acetyl protected
compound S8b was synthesized from methyl (tert-butoxycarbonyl)-D-tyrosinate (S7) (8.02 g, 27.2 mmol,
◦
1
eq) which was dissolved in pyridine (150 mL) and the mixture obtained was cooled to 0 C followed by
dropwise addition of acetanhydride (12.7 mL, 136 mmol, 5 eq) and the reaction mixture was stirred for
4 h at room temperature. Ethyl acetate (200 mL) was then added and the solution was washed with 1 M
HCl (2 50 mL), saturated NaHCO (2 50 mL) and brine (50 mL). Organic fraction was dried over
anhydrous sodium sulfate, filtered and evaporated. A crude mixture was purified by crystallization
2
×
×
3
◦
1
from diethyl ether. White amorphous solid (8.65 g, 25.5 mmol, 94% yield); mp = 54–56 C; H-NMR
400 MHz, DMSO-d6): 7.26 (d, J = 8.4 Hz, 2H), 7.03 (d, J = 8.4 Hz, 2H), 4.17 (ddd, J = 10.2, 8.1, 5.0 Hz,
H), 3.61 (s, 3H), 2.99, 2.85 (ABX, J = 13.8, 10.4, 5.0 Hz, 2H), 2.25 (s, 3H), 1.32 (s, 9H) ppm; 13C-NMR
(
δ
1
+
(100 MHz, CDCl ):
δ
173.0, 169.7, 155.9, 149.6, 130.5, 122.0, 78.8, 55.6, 52.3, 36.2, 28.6, 21.3 ppm; MS (ESI ):
3
+
−1
3
59.9 [M + Na] ; IR (ATR): 3375.4, 2980.2, 1748.4, 1684.6, 1502.1, 1368.3, 1216.5, 1163.9, 1022.4 cm
;
25
[
α] = +5.3 (0.56, 8.5 mg CH CN).
D
3
Methyl (R)-2-(4-(benzyloxy)phenyl)-2-((tert-butoxycarbonyl)amino)acetate (S8c) [31]. Benzyl protected
compound S8c was synthesized from methyl (R)-2-(4-(benzyloxy)phenyl)-2-((tert-butoxycarbonyl)
amino)acetate (S7c) by the literature described procedure [31]. Yelow oil (8.735 g, 23.5 mmol, 98%
1
yield); R (Hexane/ethyl acetate = 1/1) = 0.64; H-NMR (400 MHz, CDCl ) δ 1.43 (s, 9H), 3.71 (s, 3H),
f
3
5
6
.05 (s, 2H), 5.25 (d, J = 7,1 Hz, 1H), 6.95 (d, J = 8.7 Hz, 2H), 7.27 (d, J = 8.7 Hz, 2H), 7.33 7.43 (m,
H) ppm.
−
(R)-3-(4-(Benzyloxy)phenyl)-1-methoxy-1-oxopropan-2-aminium trifluoroacetate (S9a) [31]. Amine S9a was
synthesized according to the modified literature procedure for S enantiomer [32] from S8a (10.6 g,
2
4
7.5 mmol, 1 eq), which was dissolved in CH Cl (90 mL). Trifluoroacetic acid (8.4 mL, 110 mmol,
2 2
eq) was added and after 18 h of stirring overnight, the solution was evaporated and crystalized
◦
1
from diethyl ether. White amorphous solid (9.58 g, 24.0 mmol, 87% yield); mp = 117–119 C; H-NMR
400 MHz, DMSO-d6): 8.45 (s, 3H), 7.47–7.30 (m, 5H), 7.14 (d, J = 8.6 Hz, 2H), 6.99 (d, J = 8.6 Hz,
H), 5.09 (s, 2H), 4.54 (t, J = 6.5 Hz, 1H), 3.69 (s, 3H), 3.01, 3.06 (ABX, J = 14.2, 6.5 Hz, 2H) ppm; MS
(
2
δ

Molecules 2019, 24, 1261
11 of 15
+
+
(
8
ESI ): 286 [M + H] ; IR (ATR): 3034.0, 1742.1, 1662.6, 1610.9, 1513.1, 1449.0, 1243.1, 1179.3, 1135.1,
−
1
25
36.8, 721.2 cm ; [α]D = −15.4 (0.45, MeOH).
(R)-3-(4-Acetoxyphenyl)-1-methoxy-1-oxopropan-2-aminium trifluoroacetate (S9b) [30]. Amine S9b was
synthesized according to the modified literature procedure for S enantiomer [30] from S8b (10.6 g,
2
4
7.5 mmol, 1 eq), which was dissolved in CH Cl (90 mL). Trifluoroacetic acid (8.4 mL, 110 mmol,
2 2
eq) was added and after 18 h of stirring overnight, the solution was evaporated and used in further
reactions without purification.
(R)-1-(4-(Benzyloxy)phenyl)-2-methoxy-2-oxoethan-1-aminium chloride (S9c) [33]. Amine S9c was synthesized
according to the modified literature procedure [33] from S8c. White powder (6.869 g, 23.5 mmol, 93%
1
yield); R (Hexane/ethyl acetate = 1/1) = 0.07; H-NMR (400 MHz, DMSO-d ):
δ
3.38 (s, 3H), 3.70 (s, 3H),
f
6
5.14 (s, 2H), 5.20 (s, 1H), 7.08 (d, J = 8.8 Hz, 2H), 7.27−7.45 (m, 7H) ppm.
Methyl (R)-3-(4-(benzyloxy)phenyl)-2-((S)-2-(dibenzylamino)-3-hydroxypropanamido) propanoate (1d, Scheme 3).
Dipeptide methyl dibenzyl-L-seryl-L-phenylalaninate (1d) was synthesized in a coupling procedure
similar to the synthesis of S3 starting from dibenzyl-L-serine (S6) (7.02 g, 24.6 mmol, 1.05 eq) and S9
(
9.33 g, 23.4 mmol, 1.00 eq). Yellow liquid (7.53 g, 77% yield); R (Hexane:ethyl acetate = 1:2) = 0.45;
f
1
H-NMR (400 MHz, CDCl3):
H), 7.19–7.11 (m, 4H), 6.99–6.90 (m, 4H), 5.08, 5.04 (AB, J = 11.8 Hz, 2H), 4.85 (ddd, J = 8.9, 5.3, 5.5 Hz,
H), 4.11, 3.97 (ABX, J = 11.4, 7.5, 4.0 Hz, 2H), 3.84, 3.43 (AB, J = 13.3 Hz, 4H), 3.62 (s, 3H), 3.30 (dd, J = 7.5,
.0 Hz, 1H), 3.12, 2.98 (ABX, J = 13.7, 5.6, 5.2 Hz, 2H) ppm; 13C-NMR (100 MHz, CDCl3):
174.1, 171.8,
δ 7.74 (d, J = 8.9 Hz, 1H), 7.49–7.42 (m, 2H), 7.41–7.34 (m, 2H), 7.34–7.20 (m,
7
1
4
δ
1
5
58.3, 138.5, 136.9, 130.5, 128.9, 128.8, 128.6, 128.2, 127.8, 127.6 (2 peaks), 115.2, 70.1, 61.7, 57.4, 54.7,52.6,
+
+
−
+
−
2.4, 36.8 ppm; MS (ESI ): m/z 575 [M + Na] ; MS (ESI ): m/z 551 [M
−
H] ; HRMS (ESI ): Calc mass
−
for C H N O [M
−
H] : 551.2546; measured: 551.2542; IR (ATR): 3367.6, 3029.7, 1741.7, 1662.7, 1509.0,
34
35
2
5
−1
25
1241.3, 1176.2, 1026.8, 909.0, 729.8, 696.3 cm ; [α]D = +17.4 (c 1.0, MeOH).
Methyl (S)-2-(4-(benzyloxy)phenyl)-2-(2-(dibenzylamino)-3-hydroxypropanamido)acetate (1e, Scheme 3).
Dipeptide methyl (S)-2-(4-(benzyloxy)phenyl)-2-(2-(dibenzylamino)-3-hydroxypropanamido) acetate
(1e) was synthesized as a mixture of diastereoisomers in a coupling procedure similar to the synthesis
of S3 starting from dibenzyl-L-serine (S6) (10.0 g, 35.0 mmol, 1 eq) and S9c (12.96 g, 42.0 mmol, 1.20
1
eq). Orange oil (10.368 g, 19.3 mmol, 55% yield); R (Hexane/ethyl acetate = 1/1) = 0.31; H-NMR
f
(
400 MHz, CDCl3):
δ
8.46 (d, J = 7.1 Hz, 0.5H), 8.37 (d, J = 7.4 Hz, 0.5H), 7.27 7.42 (m, 15H), 7.20 (d,
−
J = 8.7 Hz, 1H), 7.13 (d, J = 8.7 Hz, 1H), 6.89 (dd, J = 13.2, 8.7 Hz, 2H), 5.44 (d, J = 2.2 Hz, 0.5H), 5.42
(
2
1
d, J = 2.2 Hz, 0.5H), 5.05 (s, 1H), 5.02 (s, 1H), 4.14
H), 3.84 (d, J = 13.6 Hz, 1H), 3.75 (s, 1.5H), 3.73 (s, 1.5H), 3.57 (d, J = 4.1 Hz, 1H), 3.53 (d, J = 4.5 Hz,
H), 3.40 (dd, J = 7.5, 4.0 Hz, 0.5H), 3.34 (dd, J = 7.5, 3.7 Hz, 0.5H), 3.25
−
4.17 (m, 0,5H), 4.04
−
4.08 (m, 0.5H), 3.93 4.03 (m,
−
−
3.31 (m, 1H) ppm; 13C-NMR
(
100 MHz, MeOD):
δ
173.6; 171.2; 158.9; 138.5; 136.7; 128.9; 128.7; 128.5; 128.4; 128.3; 128.1; 127.6; 127.5;
+
+
− −
1
15.3; 70.1; 62.1; 57.8; 55.7; 54.8; 52.9 ppm; MS (ESI ): 561 [M + Na] ; 537.2 [M
−
H] ; HRMS (ESI ):
−
Calc mass for C H N O [M
1
6
−
H] : 537.2395; measured: 537.2399; IR (ATR): 3480, 3363, 3032, 2951,
3
3
33
2
5
728, 1666, 1608, 1505, 1453, 1435, 1383, 1302, 1247, 1173, 1138, 1079, 1012, 992, 905, 855, 806, 743, 700,
−
1
46, 609, 575, 524, 505 cm
.
Methyl (R)-3-(4-acetoxyphenyl)-2-((S)-2-(dibenzylamino)-3-hydroxypropanamido)propanoate (1f, Scheme 3).
Dipeptide methyl dibenzyl-L-seryl-L-phenylalaninate (1f) was synthesized in a coupling procedure
similar to the synthesis of S3 starting from dibenzyl-L-serine (S6) (12.14 g, 24.6 mmol, 1.3 eq) and
◦
S9b (8.50 g, 32.7 mmol, 1.00 eq) [30]. White crystals (8.32 g, 52% yield); mp =116–117 C; R (Hexane:
f
1
ethyl acetate = 1:1) = 0.32; H-NMR (400 MHz, CDCl ):
δ 7.76 (d, J = 8.8 Hz, 1H), 7.35–7.14 (m, 10H),
3
7
.04–6.98 (m, 4H), 4.88 (ddd, J = 8.8, 5.6, 5.6 Hz, 1H), 4.14, 4.01 (ABX, J = 11.4, 7.5, 4.6 Hz, 2H), 3.88, 3.51.
(
2
1
AB, J = 13.4 Hz, 4H), 3.66 (s, 3H), 3.34 (dd, J = 7.4, 3.9 Hz, 1H), 3.13, 3.09 (ABX, J = 14.0, 5.8, 5.4 Hz, 2H),
.35 (s, 3H) ppm; 13C-NMR (100 MHz, CDCl3):
δ
174.0, 171.6, 169.5, 150.0, 138.5, 133.6, 130.4, 128.9,
+
28.7, 127.6, 122.0, 61.8, 57.5, 54.7, 52.6, 52.5, 37.3, 21.3 ppm; HRMS (ESI ): Calc mass for C H N O
29
33
2
6

Molecules 2019, 24, 1261
12 of 15
+
5
1
05.2333 [M + H] ; measured: 505.2335; IR (ATR): 3460.3, 3328.0, 1758.3, 1738.2, 1669.4, 1669.4, 1503.7,
−
1
25
215.6, 1194.9, 914.7, 755.4, 701.8, 585.5 cm ; [α]D = −99.0 (c 0.546, MeOH).
Methyl (R)-3-(4-(benzyloxy)phenyl)-2-((S)-3-(dibenzylamino)-2-oxoazetidin-1-yl)propanoate (3d, Scheme 3).
-Lactam 3d was synthesized from dipeptide 1d (7.37 g, 13.3 mmol) according to the procedure
β
described for the preparation of 3c using PPh and DIAD in THF to obtain a crude mixture, which
3
was purified with flash column chromatography on silica using petroleum ether:acetone = 10:3 as
◦
eluent. White crystals (7.06 g, 99% yield); R (petroleum ether:acetone = 10:3) = 0.22; mp = 92–93 C,
H-NMR (400 MHz, CDCl3)
f
1
δ 7.41–7.17 (m, 15H), 7.13 (d, J = 8.7 Hz, 2H), 6.82 (d, J = 8.7 Hz, 2H), 4.84
(dd, J = 11.5, 5.1 Hz, 1H), 4.72, 4.69 (AB, J = 11.7 Hz, 2H), 4.25 (dd, J = 5.1, 2.3 Hz, 1H), 3.72 (s, 3H,
3
.38, 3.22 (AB, J = 13.7 Hz, 4H), 3.36, 3.10 (AMX, J = 5.6, 5.1, 2.3 Hz, 2H), 3.26, 2.89 (AMX, J = 14.7, 5.1,
1
1
1.5 Hz, 2H) ppm; 13C-NMR (100 MHz, CDCl3):
δ
170.7, 169.4, 158.0, 138.4, 136.9, 129.8, 128.9, 128.6,
+
28.3, 128.2, 128.0, 127.5, 127.2, 115.1, 69.7, 68.7, 54.2, 53.5, 52.5, 42.5, 35.1 ppm; HRMS (ESI ): Calc
+
mass for C H N O : 535.2597; measured: 535.2596 [M + H] ; IR (ATR): 3443.2, 1743.8, 1665.6, 1496.5,
34
35
2
4
−
1
25
1
195.6, 746.4, 699.5 cm ; [α]D = +59.5 (c 0.347, MeOH).
Methyl (R)-2-(4-(benzyloxy)phenyl)-2-(3-(dibenzylamino)-2-oxoazetidin-1-yl)acetate (3e
-Lactam 3e was synthesized from dipeptide 1e (9.12 g, 34.8 mmol) according to the procedure
,
Scheme 3).
β
described for the preparation of 3c using PPh and DIAD in THF to obtain a crude mixture, which was
3
purified with flash column chromatography on silica using hexane/ethyl acetate = 2/1 as eluent
to obtain diastereoisomeric mixture of products. Yellow gel (7.30 g, 14.0 mmol, 32% yield), R
f
1
(
(
3
Hexane/ethyl acetate = 2/1) = 0.26; H-NMR (400 MHz, MeOD)
δ
7.17–7.43 (m, 17H), 6.99–7.02
m, 2H), 5.54 (s, 0.4H), 5.53 (s, 0.6H), 4.31 (dd, J = 5.0, 2.4 Hz, 0.4H), 4.19 (dd, J = 5.1, 2.4 Hz, 0.6H),
.77–3.80 (m, 2H), 3.71–3.72 (m, 3H), 3.63–3.69 (m, 2H), 3.55 (dd, J = 5.7, 2.3 Hz, 0.6H), 3.51 (d,
J = 13.5 Hz, 2H), 3.46 (t, J =5.2 Hz, 0.4H), 3.02 (dd, J = 5.9, 2.4 Hz, 0.4H), 2.92 (t, J = 5.4 Hz, 0.6H) ppm;
1
3
C-NMR (100 MHz, MeOD):
δ 171.5; 171.1; 160.6; 139.6; 139.4; 138.4; 130.8; 130.2; 130.2; 129.6; 129.4;
1
4
29.0; 128.6; 128.4; 128.4; 127.0; 126.6; 116.5; 116.5; 71.1; 69.2; 61.6; 58.5; 58.4; 55.7; 55.5; 53.2; 44.19;
+
+
3.8 ppm; MS (ESI ): 542.7 [M + (Na)] ; IR (ATR): 3245.3, 3033.1, 2983.6, 1734.7, 1688.2, 1610.7, 1513.3,
−
1
1
454.1, 1378.2, 1255.5, 1177.6, 1109.4, 1052.1, 1023.3, 925.3, 852.2, 793.7, 743.8, 699.4, 634.2, 597.1 cm
;
+
+
HRMS (ESI ): Calc mass for C H N O [M + H] : 521.2435; measured: 521.2432.
33
33
2
4
Methyl (R)-3-(4-acetoxyphenyl)-2-((S)-3-(dibenzylamino)-2-oxoazetidin-1-yl)propanoate (3f, Scheme 3).
-Lactam 3f was synthesized from dipeptide 1f (91 mg, 180 mol) according to the procedure described
for the preparation of 3c using PPh and DIAD in THF to obtain a crude mixture, which was purified
β
µ
3
with flash column chromatography on silica using CH Cl :acetone = 50:1 as eluent. Viscous oil (139 mg,
2
2
1
9
0% yield); R (CH Cl :acetone = 50:1) = 0.17; H-NMR (400 MHz, CDCl ):
δ
7.34–7.18 (m, 12H), 6.99
f
2
2
3
(d, J = 8.5 Hz, 2H), 4.77 (dd, J = 10.7, 5.4 Hz, 1H), 4.25 (dd, J = 5.1, 2.4 Hz, 1H), 3.70 (s, 3H), 3.48,
3
5
.34 (AB, J = 13.4 Hz, 4H), 3.32, 3.10 (AMX, J = 5.5, 5.1, 2.5 Hz, 2H), 3.27, 2.98 (AMX, J = 14.7, 10.8,
.5 Hz, 2H) ppm; 13C-NMR (100 MHz, CDCl3):
δ
170.4, 169.4, 169.3 (2 signals overlapping), 149.8,
+
138.3, 133.7, 129.8, 129.0, 128.4, 127.2, 121.9, 68.7, 54.5, 54.0, 52.6, 42.9, 35.2 ppm; HRMS (ESI ): Calc
+
mass for C H O N [M + H] : 487.2227, measured: 487.2221; IR (ATR): 2952.4, 1735.1, 1508.6, 1369.8,
29
31
5
2
−
1
25
1
369.8, 1193.5, 1104.6, 1018.3, 748.1, 699.0 cm ; [α]D = +61.2 (0.623, MeOH).
Methyl (R)-2-((S)-3-(dibenzylamino)-2-oxoazetidin-1-yl)-3-(4-hydroxyphenyl)propanoate (
-Lactam was synthesized from acetate 3f (4.30 g, 8.84 mmol) in MeOH (88 mL) to which a 1 M
aqueous solution of LiOH (10.60 mL, 10.6 mmol, 1.2 eq) was added dropwise at 0 C during 3 h.
8
, Scheme 3).
β
8
◦
The solution was neutralized with acetic acid and evaporated. Viscous oil (3.53 g, 90% yield); R
f
1
(
CH Cl :acetone = 30:1) = 0.33; H-NMR (400 MHz, CDCl ):
δ
7.31–7.24 (m, 8H, Ar-H), 7.23–7.17 (m,
2
2
3
2H, Ar-H), 6.99 (d, J = 8.41 Hz, 2H), 6.66 (d, J = 8.6 Hz, 2H), 4.78 (dd, J = 11.1, 5.3 Hz, 1H), 4.26 (dd,
J = 5.0, 2.4 Hz, 1H), 3.69 (s, 3H), 3.45, 3.32 (AB, J = 13.4 Hz, 4H), 3.36, 3.16 (AMX, J = 5.5, 5.0, 2.4 Hz,
H), 3.21, 2.87 (AMX, J = 14.6, 11.1, 5.3 Hz, 2H) ppm; 13C-NMR (100 MHz, CDCl3):
170.6, 170.2, 155.6,
38.2, 129.7, 128.8, 128.3, 127.2, 127.0, 115.9, 68.2, 54.3 (2 signals overlapping), 52.6, 43.0, 35.0 ppm;
2
δ
1

Molecules 2019, 24, 1261
13 of 15
+
+
HRMS (ESI ): Calc mass for C H O N : 445.2122, measured: 445.2113 [M + H] ; IR (ATR): 3308.9,
3
27
29
4
2
−
1
25
027.4, 2953.18, 1724.3, 1516.6, 1221.8, 1173.9, 731.7, 697.7 cm ; [α]D = +56.2 (c 0.623, MeOH).
3
.2.7. Deprotection of Dibenzylamine from 3c–3f and 8 for the Preparation of 7c–7e (Scheme 3)
Methyl (S)-2-((S)-3-amino-2-oxoazetidin-1-yl)-3-phenylpropanoate (7c, Scheme 3). Dibenzylamine 3c
450 mg, 1.05 mmol) was dissolved in a mixture of ethyl acetate and ethanol (95:5) and the solution was
(
flushed with argon. Palladium on activated carbon (5% Pd, 90 mg) was added and the suspension was
slowly flushed with hydrogen for 30 min. The mixture was stirred under hydrogen for 24 h, palladium
was then removed by filtration through a pad of Celite and the solution obtained evaporated. The
crude product was purified with flash column chromatography on silica using CH Cl :MeOH = 20:1
2
2
◦
as eluent. White crystals (156 mg, 0.63 mmol, 60% yield); mp = 274–276 C; R (CH Cl :MeOH = 20:1)
f
2
2
1
=
0.17; H-NMR (400 MHz, CDCl ): δ 7.34–7.15 (m, 5H), 4.85 (dd, J = 10.0, 5.4 Hz, 1H), 3.98 (dd, J = 5.1,
3
2
2
5
2
.4 Hz, 1H), 3.74 (s, 3H), 3.11, 3.15 (AMX, J = 5.6, 5.1, 2.4 Hz, 2H), 3.24, 3.00 (AMX, J = 14.4, 10.0, 5.4 Hz
H), 1.86 (bs, 2H) ppm; 13C-NMR (100 MHz, CDCl ):
δ 171.2, 170.5, 136.2, 128.8, 128.7, 127.3, 59.5,
3
+
+
+
4.8, 52.6, 49.4, 35.7 ppm; MS (ESI ): m/z 271.2 [M + Na] ; HRMS (ESI ): Calc mass for C H N O :
13
17
2
3
+
49,1234; measured: 249.1232 [M + H] ; IR (ATR): 3325.0, 2954.3, 1731.2, 1631.2, 1554.1, 1380.2, 1340.3,
−
1
25
1
222.7, 1174.7, 744.3, 698.7 cm ; [α]D = −41.8 (1.874, MeOH).
Methyl (R)-2-((S)-3-amino-2-oxoazetidin-1-yl)-3-(4-hydroxyphenyl)propanoate (7d, Scheme 3). Compound
was synthesized from 3d (4.30 g, 6.4 mmol) or (2.00 g, 3.60 mmol) according to the procedure
described for the preparation of 7c and purified with flash column chromatography on silica using
CH Cl :MeOH = 9:1 as eluent. Tetrahedral white crystals (from 3d: 1.37 g, 5.2 mmol, 80% yield, from
7
d
8
2
2
◦
: 808 mg, 3.1 mmol, 85% yield); mp = 94–95 C; R (CH Cl :MeOH = 9:1) = 0.21; H-NMR (400 MHz,
f
2 2
1
8
CDCl3):
δ 7.07 (d, J = 8.5 Hz, 2H), 6.72 (d, J = 8.5 Hz, 2H), 4.74 (dd, J = 11.1, 4.9 Hz, 1H), 4.09 (dd, J = 5.1,
2
1
5
.3 Hz, 1H), 3.85 (bs, 2H), 3.76 (s, 3H), 3.72, 3.01 (AMX, J = 5.8, 5.1, 2.3 Hz, 2H), 3.23, 2.89 (AMX, J = 14.5,
1.1, 4.8 Hz 2H) ppm; 13C-NMR (100 MHz, CDCl ):
δ
171.4, 170.5, 156.1, 130.1, 126.9, 115.9, 59.2, 54.7,
3
+
+
2.7, 49.2, 35.3 ppm; HRMS (ESI ): Calc mass for C H N O [M + H] : 265.1188; measured: 265.1184;
1
3
17
2
4
−
1
25
IR (ATR): 3260.4, 1731.1, 1649.4, 1514.8, 1229.5, 1019.9, 825.1 cm ; [α]D = +52.6 (0.61, MeOH).
Methyl (R)-2-(3-amino-2-oxoazetidin-1-yl)-2-(4-hydroxyphenyl)acetate (7e, Scheme 3). Compound 7e was
synthesized from 1e (3.85 g, 7.4 mmol) according to the procedure described for the preparation of 7c
and purified with flash column chromatography on silica using CH Cl :MeOH = 9:1 as eluent. Yellow
2
2
1
gel (1.444 g, 5.77 mmol, 78% yield); R (DKM/MeOH= 9/1) = 0.29; H-NMR (400 MHz, MeOD):
δ
f
7
0
.13–7.17 (m, 2H), 6.81–6.84 (m, 2H), 5.48 (s, 1H), 4.18 (dd, J = 5.3, 2.5 Hz, 0.5H), 4.06 (dd, J = 4.9, 2.5 Hz,
.5H), 3.82 (t, J = 5.3 Hz, 0.5H), 3.75 (s, 3H), 3.37–3.39 (m, 1H), 2.85–2.88 (m, 0.5H) ppm; 13C-NMR
+
(
100 MHz, MeOD):
δ
172.0; 171.7; 159.3; 130.8; 130.6; 125.2; 116.8; 59.6; 58.5; 53.1; 50.3 ppm; MS (ESI ):
+
+
+
2
51.1 [M + H] ; HRMS (ESI ): Calc mass for C H O N : 251.1032, measured: 251.1038 [M + H] ; IR
12 14 4 2
(
5
ATR): 2959.1, 1732.4, 1648.3, 1611.9, 1515.5, 1451.6, 1384.3, 1236.7, 1175.2, 1106.2, 1011.1, 833.4, 757.5,
−
1
−
−
28.9 cm ; HRMS (ESI ): Calc mass for C H N O [M − H] : 249.0881; measured: 249.0884.
12
13
2
4
4
. Conclusions
There are only a limited number of methodologies used for the preparation of C4-
β-lactam–
containing dipeptides without a substituent on C4. A study was conducted here of the different
reaction conditions on serine dipeptides that were N-protected with a phthalimide, trityl, or dibenzyl
group. This demonstrated that the protective group and the choice of correct reaction conditions are
very important for the successful formation of the β-lactam ring, and for the extent of side-product
formation. Across all of the reaction conditions that were investigated, the use of the Mitsunobu
reaction on dibenzyl-protected dipeptides gave the best results. Importantly, the dibenzyl group was
easily removed from the β-lactam phenylalanine derivatives, and products with a free amino group
were obtained. The present approach adds to the utility of the N-dibenzyl protective group for the
synthesis of β-lactam phenylalanine-containing dipeptides.

Molecules 2019, 24, 1261
14 of 15
Author Contributions: R.F. and M.H. conceived, designed end performed the experiments. R.F. and M.H. wrote
the manuscript with the help of S.G., who also supervised the research activity. All authors read and approved
the final manuscript.
Funding: This research was funded by Slovenian Research Agency (ARRS), through research core funding
◦
N P1-0208.
Acknowledgments: The authors thank Izidor Sosi cˇ for fruitful discussions and for critical reading of the
manuscript. We also thank Christopher Berrie for proofreading of the manuscript.
Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the
study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to
publish the results.
References
1
.
Augelliszafran, C.E.; Blankley, C.J.; Roth, B.D.; Trivedi, B.K.; Bousley, R.F.; Essenburg, A.D.; Hamelehle, K.L.;
Krause, B.R.; Stanfield, R.L. Inhibitors of acyl-CoA:cholesterol acyltransferase. 5. Identification and structure-
activity relationships of novel .beta.-keto amides as hypocholesterolemic agents. J. Med. Chem. 1993, 36,
2
943–2949. [CrossRef]
Sreenivasan, U.; Mishra, R.K.; Johnson, R.L. Synthesis and dopamine receptor modulating activity of lactam
conformationally constrained analogs of Pro-Leu-Gly-NH . J. Med. Chem. 1993, 36, 256–263. [CrossRef]
2
3
.
.
2
[
PubMed]
Guillon, C.D.; Koppel, G.A.; Brownstein, M.J.; Chaney, M.O.; Ferris, C.F.; Lu, S.-F.; Fabio, K.M.; Miller, M.J.;
Heindel, N.D.; Hunden, D.C.; et al. Azetidinones as vasopressin V1a antagonists. Bioorg. Med. Chem. 2007
5, 2054–2080. [CrossRef] [PubMed]
Bittermann, H.; Gmeiner, P. Chirospecific Synthesis of Spirocyclic
Potent Type II β-Turn Inducing Peptide Mimetics. J. Org. Chem. 2006, 71, 97–102. [CrossRef] [PubMed]
Decuyper, L.; Jukic, M.; Sosic, I.; Zula, A.; D’Hooghe, M.; Gobec, S. Antibacterial and -Lactamase Inhibitory
Activity of Monocyclic β-Lactams. Med. Res. Rev. 2018, 38, 426–503. [CrossRef] [PubMed]
,
1
4
5
6
.
.
.
β-Lactams and Their Characterization as
β
Vangala, V.B.; Maddani, M.R.; Hindupur, R.M.; Pati, H.N. Chemistry of Macrocyclic β-Lactam: An Overview.
Heterocycles 2015, 91, 701–717.
7
8
.
.
Singh, G.S.; Sudheesh, S. Advances in synthesis of monocyclic beta-lactams. Arkivoc 2014, SI, 337–385.
Pitts, C.R.; Lectka, T. Chemical Synthesis of beta-Lactams: Asymmetric Catalysis and Other Recent Advances.
Chem. Rev. 2014, 11, 7930–7953. [CrossRef]
9
1
1
1
1
1
1
.
Deketelaere, S.; Nguyen, T.V.; Stevens, C.V.; D’Hooghe, M. Synthetic Approaches toward Monocyclic
3
-Amino-beta-lactams. Chemistryopen 2017, 6, 301–319. [CrossRef]
0. Wisniewski, K.; Koldziejczyk, A.S.; Falkiewicz, B. Applications of the Mitsunobu reaction in peptide
chemistry. J. Pept. Sci. 1998, 4, 1–14. [CrossRef]
1. Wasserman, H.H.; Hlasta, D.J.; Tremper, A.W.; Wu, J.S. Application of new beta-lactam syntheses to the
preparation of (+/−)-3-aminonocardicinic acid. J. Org. Chem. 1981, 46, 2999–3011. [CrossRef]
2. Salituro, G.M.; Townsend, C.A. Total syntheses of ( )-nocardicins a-g—A biogenetic approach. J. Am. Chem.
−
Soc. 1990, 112, 760–770. [CrossRef]
3. Townsend, C.A.; Brown, A.M.; Nguyen, L.T. Nocardicin A—Stereochemical and biomimetic studies of
monocyclic beta-lactam formation. J. Am. Chem. Soc. 1983, 105, 919–927. [CrossRef]
4. Townsend, C.A.; Nguyen, L.T. Asymmetric, biogenetically modeled synthesis of ( )-3-aminonocardicinic
−
acid. J. Am. Chem. Soc. 1981, 103, 4582–4583. [CrossRef]
5. Decuyper, L.; Deketelaere, S.; Vanparys, L.; Jukic, M.; Sosic, I.; Sauvage, E.; Amoroso, A.M.; Verlaine, O.;
Joris, B.; Gobec, S.; et al. In Silico Design and Enantioselective Synthesis of Functionalized Monocyclic
3-Amino-1-carboxymethyl-beta-lactams as Inhibitors of Penicillin-Binding Proteins of Resistant Bacteria.
Chem. Eur. J. 2018, 24, 15254–15266. [CrossRef] [PubMed]
1
1
1
6. Miller, M.J.; Mattingly, P.G. The direct chemical conversion of peptides to beta-lactams. Tetrahedron 1983, 39,
2
563–2570. [CrossRef]
7. Farouzgrant, F.; Miller, M.J. Syntheses of novel 1-aminocyclopropane-1-carboxylic acid (acc)-containing
beta-lactams. Bioorg. Med. Chem. Lett. 1993, 3, 2423–2428. [CrossRef]
8. Gaudelli, N.M.; Townsend, C.A. Stereocontrolled Syntheses of Peptide Thioesters Containing Modified Seryl
Residues as Probes of Antibiotic Biosynthesis. J. Org. Chem. 2013, 78, 11115. [CrossRef]

Molecules 2019, 24, 1261
15 of 15
1
2
2
9. Kamiya, T.; Hashimoto, M.; Nakaguch, O.; Oku, T. Total synthesis of monocyclic
nocardicin-A and nocardicin-D. Tetrahedron 1979, 35, 323–328. [CrossRef]
0. Bose, A.K.; Manhas, M.S.; Sahu, D.P.; Hegde, V.R. Stereospecific cyclization of β-hydroxy aryl amides to
β
-lactam antibiotics,
β-lactams. Can. J. Chem. 1984, 62, 2498–2505. [CrossRef]
1. Nakatsuka, S.; Tanino, H.; Kishi, Y. Biogenetic-type synthesis of penicillin-cephalosporin antibiotics. 1.
Stereocontrolled synthesis of penam-ring and cephem-ring systems from an acyclic tripeptide equivalent.
J. Am. Chem. Soc. 1975, 97, 5008–5010. [CrossRef] [PubMed]
2
2. Chaudhari, P.; Bari, S. Efficient synthesis of N-sulfonyl β-arylmethylalaninates from serine via ring opening
of N-sulfonyl aziridine-2-carboxylate. Synth. Commun. 2015, 45, 401–412. [CrossRef]
2
3. Foley, D.; Pieri, M.; Pettecrew, R.; Price, R.; Miles, S.; Lam, H.K.; Bailey, P.; Meredith, D. The in vitro
transport of model thiodipeptide prodrugs designed to target the intestinal oligopeptide transporter, PepT1.
Org. Biomol. Chem. 2009, 7, 3652–3656. [CrossRef] [PubMed]
2
4. Hanessian, S.; Couture, C.; Wyss, H. Design and reactivity of organic functional groups-utility of
imidazolylsulfonates in the synthesis of monobactams and 3-amino nocardicinic acid. Can. J. Chem. 1985, 63,
3
613–3617. [CrossRef]
2
2
2
5. Hanessian, S.; Sahoo, S.P.; Couture, C.; Wyss, H.B. Novel synthetic approaches to monocyclic beta-lactam
antibiotics. Soc. Chim. Belg. 1984, 93, 571–578. [CrossRef]
6. Wang, P.; Zhang, S.M.; Meng, Q.Y.; Liu, Y.; Shang, L.Q.; Yin, Z. Site-Specific Chemical Modification of Peptide
and Protein by Thiazolidinediones. Org. Lett. 2015, 17, 1361–1364. [CrossRef] [PubMed]
7. Wyszogrodzka, M.; Mows, K.; Kamlage, S.; Wodzifiska, J.; Plietker, B.; Haag, R. New approaches towards
monoamino polyglycerol dendrons and dendritic triblock amphiphiles. Eur. J. Org. Chem. 2008, 53–63.
[CrossRef]
2
2
8. Schwab, R.S.; Schneider, P.H. Straightforward synthesis of non-natural chalcogen peptides via ring opening
of aziridines. Tetrahedron 2012, 68, 10449–10455. [CrossRef]
9. Desjardine, K.; Pereira, A.; Wright, H.; Matainaho, T.; Kelly, M.; Andersen, R.J. Tauramamide, a lipopeptide
antibiotic produced in culture by Brevibacillus laterosporus isolated from a marine habitat: Structure
elucidation and synthesis. J. Nat. Prod. 2007, 70, 1850–1853. [CrossRef]
3
0. Ebright, R.H.; Ebright, Y.W.; Mandal, S.; Wilde, R.; Li, S. Antibacterial agents: n(alpha)-aroyl-n-aryl-
phenylalaninamides. U.S. Patent 20160347708 (A1), 1 December 2016.
3
1. Yang, X.Y.; Phipps, R.J.; Toste, F.D. Asymmetric Fluorination of alpha-Branched Cyclohexanones Enabled
by a Combination of Chiral Anion Phase-Transfer Catalysis and Enamine Catalysis using Protected Amino
Acids. J. Am. Chem. Soc. 2014, 136, 5225–5228. [CrossRef]
3
2. Cheng, X.C.; Wang, R.L.; Dong, Z.K.; Li, J.; Li, Y.Y.; Li, R.R. Design, synthesis and evaluation of novel
metalloproteinase inhibitors based on L-tyrosine scaffold. Bioorg. Med. Chem. 2012, 20, 5738–5744. [CrossRef]
[
PubMed]
3. Bello, C.; Cea, M.; Dal Bello, G.; Garuti, A.; Rocco, I.; Cirmena, G.; Moran, E.; Nahimana, A.; Duchosal, M.A.;
Fruscione, F.; et al. Novel 2-[(benzylamino)methyl]pyrrolidine-3,4-diol derivatives as -mannosidase
inhibitors and with antitumor activities against hematological and solid malignancies. Bioorg. Med. Chem.
010, 18, 3320–3334. [CrossRef] [PubMed]
3
α
2
©
2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).