Total synthesis of leopolic acid A, a natural 2,3-pyrrolidinedione with antimicrobial activity

Article abstract of DOI:10.3762/bjoc.12.159

The first total synthesis of leopolic acid A, a fungal metabolite with a rare 2,3-pyrrolidinedione nucleus linked to an ureido dipeptide, was designed and carried out. Crucial steps for the strategy include a Dieckmann cyclization to obtain the 2,3-pyrrolidinedione ring and a Wittig olefination to install the polymethylene chain. An oxazolidinone-containing leopolic acid A analogue was also synthesized. The antibacterial activity showed by both compounds suggests that they could be considered as promising candidates for future developments.

Full text of DOI:10.3762/bjoc.12.159

Total synthesis of leopolic acid A, a natural 2,3-pyrrolidine-
dione with antimicrobial activity
Atul A. Dhavan1, Rahul D. Kaduskar1, Loana Musso1, Leonardo Scaglioni1,
Piera Anna Martino2 and Sabrina Dallavalle*1,§
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2016, 12, 1624–1628.
1Department of Food, Environmental and Nutritional Sciences,
Division of Chemistry and Molecular Biology, Università degli Studi di
Milano, via Celoria 2, I-20133 Milano, Italy and 2Department of
Veterinary Medicine - Microbiology and Immunology, Università degli
Studi di Milano, via Celoria 10, I-20133 Milano, Italy
doi:10.3762/bjoc.12.159
Received: 01 June 2016
Accepted: 14 July 2016
Published: 29 July 2016
Email:
Associate Editor: J. S. Dickschat
Sabrina Dallavalle* - sabrina.dallavalle@unimi.it
© 2016 Dhavan et al.; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
§ Tel. +39 0250316818; Fax +39 0250316801
Keywords:
antimicrobial; heterocyclic compounds; natural products;
pyrrolidinedione; total synthesis
Abstract
The first total synthesis of leopolic acid A, a fungal metabolite with a rare 2,3-pyrrolidinedione nucleus linked to an ureido dipep-
tide, was designed and carried out. Crucial steps for the strategy include a Dieckmann cyclization to obtain the 2,3-pyrrolidine-
dione ring and a Wittig olefination to install the polymethylene chain. An oxazolidinone-containing leopolic acid A analogue was
also synthesized. The antibacterial activity showed by both compounds suggests that they could be considered as promising candi-
dates for future developments.
Introduction
Great concern has recently been expressed about the dimin- yielded a new metabolite, leopolic acid A (1, Figure 1) [2].
ishing efficacy of current antibiotic therapies and the emer- Leopolic acid has unprecedented structural features consisting
gence of multidrug resistant strains. Novel potent compounds of an aliphatic side chain attached to a 2,3-pyrrolidinedione
endowed with new mechanisms of action are urgently needed. residue, which in turn, is connected to the ureido dipeptide
In this regard, natural products continue to be a rich source of L-Phe-L-Val. The compound showed antifungal and antibacte-
biologically validated structures, which can be modified and op- rial activity against Mucor hiemalis and Staphylococcus aureus
timized in drug discovery [1].
with a MIC of 32 and 16 μg/mL, respectively [2].
Chemical analysis of a terrestrial-derived Streptomyces sp. iso- Compounds containing the isomeric 2,4-pyrrolidinedione ring
lated from the rhizosphere of the plant Juniperus excelsa system (tetramic acids) are widespread among the fungal
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The unique structure and bioactivity of leopolic acid prompted
us to plan a synthesis which might be used in the preparation of
various analogues. A retrosynthetic disconnection of 1 at the
amide bond gave two fragments, the ureido dipeptide L-Phe-L-
Val and 4-decyl-3-hydroxy-1,5-dihydropyrrol-2-one, which
appeared well suited for a convergent synthetic approach
(Figure 1).
Figure 1: Structure of leopolic acid A.
Results and Discussion
A straightforward route to the intriguing 2,3-pyrrolidinedione
metabolites and show a number of biological activities, i.e., system appeared to be the Michael addition of a suitable amine
antibacterial, antiviral, antifungal and anticancer. More than one to ethyl acrylate, followed by a Dieckmann cyclization with
hundred of them have been isolated from a variety of natural diethyl oxalate [10,11]. We chose the p-methoxybenzyl (PMB)
sources [3].
protecting group for the amine, because of its facile cleavage
with cerium ammonium nitrate (CAN) or 2,3-dichloro-5,6-
Conversely, natural compounds with a simple 2,3-pyrrrolidine- dicyanobenzoquinone (DDQ). Thus, 2,3-pyrrolidinedione 3 was
dione nucleus are indeed very few [4-6]. Even though this core obtained by the reaction of ethyl acrylate with p-methoxy-
can be considered an attractive target, synthetic studies directed benzylamine, followed by treatment with diethyl oxalate
to find new biologically active compounds are scarce as well (Scheme 1) [12]. On the basis of NMR data, the compound
[7-9].
exists as an enol tautomer (see Supporting Information File 1).
Scheme 1: Synthesis of leopolic acid A. Reagents and conditions: a) p-methoxybenzylamine, EtOH, rt, 12 h, 98%; b) diethyl oxalate, NaOEt, EtOH,
reflux, 3 h, 83%; c) BnBr, K2CO3, DMF, 0 °C to rt, 1 h, 50%; d) DIBAL-H, CH2Cl2, −78 °C, 2 h, 56%; e) PPh3, CBr4, CH2Cl2, rt, 5 h, 52%; f) PPh3,
toluene, reflux, 5 h, 75%; g) n-nonanal, LiHMDS, THF, −78 °C to rt, 5 h, 52%; h) CAN, CH3CN:H2O, 0 °C to rt, 3 h, 73%; i) 2-tert-butoxycarbonyl-
amino-3-methylbutyric acid pentafluorophenyl ester, n-BuLi, THF, −78 °C, 0.5 h, 71%; j) DIBAL-H, CH2Cl2, −78 °C, 2 h, 30%; k) PCC, CH2Cl2, 0 °C to
rt, 12 h, 46%; l) n-nonyltriphenylphosphonium bromide, n-BuLi, THF, −78 °C to 0 °C, 2 h; m) TFA, CH2Cl2, 0 °C to rt, 1 h; n) L-phenylalanine benzyl
ester DIEA, triphosgene, CH2Cl2, rt, 1 h, 60% over three steps; o) H2, Pd/C, EtOAc, rt, 2 h, 77%.
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Beilstein J. Org. Chem. 2016, 12, 1624–1628.
Indeed, perusal of the literature indicated that apparently all mation of the oxazolidinone 14 (Scheme 2). We assume that the
4-monosubstituted 2,3-pyrrolidinediones are highly enolized reaction occurred via inversion of the configuration at the
[13-15]. Protection of the enolic OH with a benzyl group, using hydroxylated carbon, based on an intramolecular SN2 cyclocar-
BnBr and K2CO3, gave compound 4. The reduction of 4 with bamation by the Boc group [20,21]. The small coupling con-
DIBAL-H gave the corresponding primary alcohol, which was stant (J = 2.6 Hz) between the two hydrogens on the oxazolidi-
converted into bromide 5 by Appel reaction with PPh3 and none ring confirmed that the compound was the anti derivative
CBr4. The phosphonium salt obtained from this bromide was [16,17,22,23].
subjected to a Wittig reaction with nonanal, to afford com-
pound 6 [12].
Attempts to remove the PMB protecting group (CAN, DDQ,
TFA or hydrogenation) from 6 or from the intermediate alcohol
and bromide resulted in the decomposition of the products. The
instability of these and other derivatives of the N-unsubstituted
ring, already observed by others [11], made the synthetic task
more troublesome than expected.
The NH-free pyrrolidinedione scaffold was found to be stable
only in the presence of the ester group in position 4. In fact, a
deprotected compound 7 could only be obtained by treatment of
4 with CAN.
This result prompted us to install the amino acid fragment on
compound 7 before constructing the chain in position 4, so that
it could also act as a NH-protecting group in the following
steps. However, as the ureido fragment would interfere with the
reduction of the ester group on the nucleus, we opted for
installing sequentially the valine, then the phenylalanine
residue.
Scheme 2: Synthesis of compound 17. Reagents and conditions:
a) Oxalyl chloride, DMSO, CH2Cl2, TEA, −78 °C to 0 °C, 2 h, 60%;
Thus, compound 7 was coupled with pentafluorophenyl ester-
activated N-Boc-valine to obtain compound 8 (Scheme 1).
Treatment with DIBAL-H led to the reduction of both the ester
and the CO group of the valine moiety (9). Compound 9 was
obtained as a 9:1 mixture of diastereomers, the syn (S,S) dia-
stereomer being the major one. The selectivity of the reduction
is in agreement with previous results and is consistent with the
Cram model for a chelation controlled reduction [16-18]. Both
OH groups of compound 9 were oxidized by PCC to obtain
b) n-nonyltriphenylphosphonium bromide, n-BuLi, THF, −78 °C to 0 °C,
2 h, 74%; c) L-phenylalanine benzyl ester, DIEA, triphosgene, CH2Cl2,
rt, 72 h, 49%; d) H2, Pd/C, EtOAc, rt, 2 h, 50%.
The oxazolidinone ring is rare among natural products, but there
are successful examples in medicinal chemistry of drugs con-
taining this skeleton, such as the antibiotic linezolid [24].
aldehyde 10. The long carbon chain was incorporated into 10 by As the core structure of compound 14 represents a new scaffold,
Wittig olefination with n-nonyltriphenylphosphonium bromide never isolated or synthesized before, we decided to incorporate
and n-BuLi at −78 °C (11). Deprotection and treatment with the structural features found in leopolic acid into this novel
triphosgene and phenylalanine benzyl ester at rt allowed the for- heterocyclic system. Accordingly, Wittig olefination of 14, fol-
mation of compound 13. Finally, catalytic hydrogenation lowed by coupling with L-phenylalanine benzyl ester, and
afforded the desired leopolic acid A (1), whose spectroscopic subsequent catalytic hydrogenation in EtOAc of 16 afforded the
data completely matched with those reported in the literature [1] analogue of leopolic acid 17 in 50% yield.
(see Supporting Information File 2 for NMR spectra).
Compounds 1 and 17 were subjected to a preliminary study of
Interestingly, while searching the best conditions to obtain com- the antimicrobial activity against Staphylococcus and
pound 10, we treated 9 with oxalyl chloride and DMSO, Escherichia coli (Staphylococcus pseudintermedius [25],
following the Swern protocol [19]. This treatment led to the for- 25 strains; Escherichia coli, 20 strains. QC: S. aureus ATCC
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File 2 for details). The antibacterial activity shown by both
compounds suggests that they can be considered as promising
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