Total Synthesis of L-156,373 and an oxoPiz Analogue via a Submonomer Approach

Article abstract of DOI:10.1021/acs.orglett.8b00912

The first chemical synthesis of L-156,373 (1), a potent oxytocin receptor antagonist isolated from Streptomyces silvensis, is reported. Assembly of the unusual d-Piz-l-Piz dipeptide subunit was achieved through a sequential electrophilic amination-acylati

Full text of DOI:10.1021/acs.orglett.8b00912

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Total Synthesis of L‑156,373 and an oxoPiz Analogue via a
Submonomer Approach
Yassin M. Elbatrawi,† Chang Won Kang,† and Juan R. Del Valle*
Department of Chemistry, University of South Florida, Tampa, Florida 33620, United States
S
* Supporting Information
ABSTRACT: The first chemical synthesis of L-156,373 (1), a
potent oxytocin receptor antagonist isolated from Streptomyces
silvensis, is reported. Assembly of the unusual D-Piz-L-Piz dipeptide
subunit was achieved through a sequential electrophilic amina-
tion−acylation−deprotection strategy followed by late-stage Piz
ring formation. Synthesis and incorporation of a novel N-hydroxy-
L-isoleucine building block is also described. This submonomer
approach was further applied to the expedient synthesis of a di-δ-
oxopiperazic acid analogue of 1 starting from Fmoc-Glu(tBu)-OH
building blocks.
isolated from the fermentation broth of Strepomyces silvensis.4
he piperazic acid (Piz) residue and its derivatives are
Tcomponents in over 75 nonribosomal peptides isolated Compound 1 features an unusual D-Piz-L-Piz dipeptide subunit
primarily from Streptomyces bacteria.1 Piz natural products have
embedded within a hexapeptide macrocycle. Radioligand
binding assays showed that 1 possesses a 150 nM binding
affinity (Ki) for the oxytocin receptor (OTR) and approx-
imately 20-fold selectivity over the related arginine vasopressin
receptors AVPR-V1 and AVPR-V2. Semisynthetic modifications
of 1 led to a small set of analogues with enhanced biological
activity.4b,5 For example, treatment of a des-hydroxy derivative
of 1 with alkaline hydrogen peroxide led to 2, which exhibits a
Ki of 9.6 nM against OTR and over 160-fold selectivity over
AVPR subtypes. Compound 2 features both Δ-Piz and an
unprecedented δ-oxopiperazic acid (oxoPiz) residue. Although
these macrocycles proved to be useful leads for drug
development, optimization efforts initially relied on obtaining
small quantities of 1 through fermentation. To date, the
chemical synthesis of 1 has not been described.
Strategies for the synthesis of enantiopure Piz
monomers1a,b,2c typically rely on α-hydrazination of oxazolidi-
nones6 or aldehydes7 (C−Nα bond formation), chiral auxiliary-
based azo-Diels−Alder cycloaddition8 (C−N and C−C bond
formations), or electrophilic amination of amino acid
derivatives9 (N−N bond formation). A recent multicomponent
reaction approach to Piz-containing dipeptides has also been
described.10 To facilitate access to peptides featuring Piz and its
congeners, we sought to employ a sequential electrophilic
amination−acylation−deprotection strategy, followed by late-
stage Piz ring formation, as depicted in Figure 2. The efficiency
of this route relies on oxaziridine-mediated amination of
primary amines, a process often complicated by undesired
diamination or poor conversion when the nucleophile is not
employed in excess.9c,11 We were particularly interested in
garnered considerable interest from the synthetic chemistry
community owing to their important and diverse biological
activities. Synthetic access to the Piz family of cyclic α-
hydrazino acids has also enabled initial studies of their impact
on peptide conformation.2 The unusual propensity for Nα-
acylated derivatives of Piz and dehydropiperazic acid (Δ-Piz) to
exclusively adopt a trans amide conformation contrasts with
those of proline and pipecolic acid, which both exhibit
significant cis amide rotamer populations.3 Despite its
occurrence in nature and unique structural properties, Piz is
not commonly employed in residue scanning applications. This
is mainly due to the fact that suitably protected Piz building
blocks require multiple steps to prepare in enantiopure form
and often suffer from moderate coupling yields during peptide
elongation.1a,b,2c Methods to more efficiently incorporate Piz
into host structures may prompt its wider use in peptidomi-
metic drug design.
In the 1980s, Merck, Sharp, and Dohme Research
Laboratories initiated a medicinal chemistry campaign around
the Piz-containing natural product L-156,373 (1, Figure 1),
Received: March 21, 2018
Figure 1. L-156,373 (1) and a semisynthetic analogue (2).
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00912
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 2. Synthesis of N-Hydroxyisoleucine Derivative 11
Figure 2. Submonomer approach to di-Piz and di-oxoPiz.
targeting structures such as 1, which contain Piz-Piz dipeptide
subunits that can be difficult to prepare using preformed
building blocks due to steric and stereoelectronic factors.12
Here, we report the total synthesis of 1 and an unnatural di-
oxoPiz analogue via this submonomer approach.
The precursors to the L- and D-Piz residues in 1 were
envisioned to be protected δ-hydroxynorvaline derivatives that
would eventually undergo SN2 ring closure. The required
orthogonally protected building blocks were synthesized as
shown in Scheme 1. Glutamic acid benzyl esters (L and D) were
OH gave 10 in 62% yield. A Mitsunobu reaction with N,O-di-
Alloc-hydroxylamine was then followed by ester cleavage with
dilute TFA/DCM to afford 11 as a single isomer.
Scheme 1. Synthesis of δ-Hydroxynorvaline Building Blocks
With the requisite building blocks in hand, we carried out the
synthesis of 1, as depicted in Scheme 3. Tripeptide 12 was
obtained in 71% yield over three steps via HATU-mediated
coupling reactions. Deprotection of the Fmoc group was then
followed by N-amination using oxaziridine 1911a,b to provide
13. The poor nucleophilicity of the α nitrogen in N′-carbamate
protected hydrazino esters/amides is well documented and
typically requires the use of acid chlorides for successful
coupling. Incorporation of (R)-4 was thus achieved through
preactivation using the Ghosez’s reagent and treatment with a
mixture of 13 and NaHCO3 in DCM. Tetrapeptide 14 was
then aminated in the same manner as 12 to afford 15 in 84%
yield.
The (N-OH)-Ile building block 11 was similarly converted to
the acid chloride and reacted with 15 to provide the
intermediate pentapeptide. Removal of both Alloc groups was
followed by chemoselective N-acylation with the preformed
acid chloride of Cbz-Phe-OH to give 16 in 47% yield over three
steps. Pd/C-mediated hydrogenolysis of the terminal protecting
groups in 16 proceeded without appreciable N−O bond
cleavage. The crude zwitterion then underwent rapid macro-
cyclization in the presence of HATU/DIEA to provide 17 in
82% yield over two steps.
subjected to Fmoc-protection followed by reduction of their
mixed anhydride derivatives to give 3. Silyl protection and
hydrogenolysis then afforded orthogonally protected mono-
mers 4.
In addition to the Piz residues, 1 harbors the unusual
nonproteinogenic amino acid N-hydroxy-L-isoleucine (N−OH-
Ile). N-Hydroxy amino acids are components of various
nonribosomal peptides, often appearing alongside Piz and its
oxidized derivatives. β-Branched variants are found in only a
small subset of natural products, including the pargamicins,13
polyoxypeptins,14 pulcherrimin,15 and callyspongidipeptide A.16
The presence of N-OH-Ile in 1 results in five contiguous
tertiary amide linkages, three of which are N-heteroatom
substituted.
Scheme 2 depicts our efforts toward a suitably protected N-
OH-Ile building block for incorporation into 1. The additional
steric demand of β-branching became evident in our failed
attempts to prepare 6 via the triflate derived from α-hydroxy
ester 5. In contrast, the same SN2 reaction with O-benzyl-
hydroxylamine proceeded readily with triflates derived from
both leucine and phenylalanine methyl esters.17 Although a
Mitsunobu reaction between 5 and (N-Alloc)-O-benzyl-
hydroxylamine was unsuccessful, O-Boc-N-Alloc-hydroxylamine
was sufficiently nucleophilic to provide 8 as a single isomer in
moderate yield.18 However, alkaline hydrolysis or Lewis acid
mediated methyl ester rupture led to rapid elimination of tert-
butyl carbonic acid (across the C−N bond) or Boc
deprotection, respectively. The propensity for elimination
under mildly basic conditions was circumvented through the
use of a PMB ester in conjunction with N,O-di-Alloc
protection. Thus, diazotization and alkylation of H-D-allo-Ile-
Silyl ether cleavage of 17 and conversion to the dimesylate
was followed immediately by K2CO3-promoted cyclization to
form both Piz rings. Slow elimination of water at the sensitive
(N-OH)-Ile residue led to some erosion of product yield, but
was largely avoided through careful reaction monitoring.
Acidolysis of the Boc groups finally afforded 1 in 41% over
three steps. Synthetic 1 exhibits a well-resolved 1H NMR
spectrum devoid of any conformational isomers, despite the
1
presence of five tertiary amide bonds. The H NMR spectrum
of 1 taken in C6D6 was identical to that previously disclosed for
the natural product,4a thus confirming the structure of L-
156,373.
Inspired by the reported semisynthesis of 2, we sought to
apply our submonomer approach to an expedient synthesis of
peptides featuring oxoPiz residues. Thus, we selected an
oxidized variant of 2 as a synthetic target. As shown in Scheme
4, both L and D Fmoc-Glu(tBu)-OH were employed as oxoPiz
precursors. Macrocycle 20 was synthesized using a route
analogous to that described for the assembly of 17. Treatment
B
DOI: 10.1021/acs.orglett.8b00912
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
accomplished via N-amination and acid-promoted cyclization of
glutamic acid-γ-tert-butyl ester subunits. The submonomer
approach described herein represents a convenient method to
access an array of Piz-containing peptides and natural products,
including those harboring consecutive Piz residues. We
anticipate that these results will facilitate the use of Piz and
oxoPiz in peptide scanning applications and prompt further
studies on their utility as potential proline surrogates.
Scheme 3. Synthesis of L-156,373 (1)
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b00912.
Experimental procedures, characterization data, copies of
1H and 13C NMR spectra (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: delvalle@usf.edu.
ORCID
Juan R. Del Valle: 0000-0002-7964-8402
Author Contributions
†Y.M.E. and C.W.K. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Science Foundation
(CHE1709927). We thank the USF Interdisciplinary NMR
Facility and USF Mass Spectrometry Facility for assistance.
REFERENCES
■
(1) (a) Oelke, A. J.; France, D. J.; Hofmann, T.; Wuitschik, G.; Ley,
S. V. Nat. Prod. Rep. 2011, 28, 1445−1471. (b) Handy, E.; Sello, J.
́
Top. Heterocycl. Chem. 2015, 48, 97−124. (c) Dardic, D.; Lauro, G.;
Bifulco, G.; Laboudie, P.; Sakhaii, P.; Bauer, A.; Vilcinskas, A.;
Hammann, P. E.; Plaza, A. J. Org. Chem. 2017, 82, 6032−6043.
(d) Wang, J.; Cong, Z.; Huang, X.; Hou, C.; Chen, W.; Tu, Z.; Huang,
D.; Liu, Y. Org. Lett. 2018, 20, 1371−1374.
Scheme 4. Synthesis of Di-oxoPiz Analogue 21.
(2) (a) Xi, N.; Alemany, L. B.; Ciufolini, M. A. J. Am. Chem. Soc.
1998, 120, 80−86. (b) Ciufolini, M. A.; Xi, N. Chem. Soc. Rev. 1998,
27, 437−445. (c) Kuchenthal, C.-H.; Maison, W. Synthesis 2010, 2010,
̈
719−740.
(3) (a) MacArthur, M. W.; Thornton, J. M. J. Mol. Biol. 1991, 218,
397−412. (b) Cheng, H. N.; Bovey, F. A. Biopolymers 1977, 16, 1465−
1472. (c) Higashijima, T.; Tasumi, M.; Miyazawa, T. Biopolymers
1977, 16, 1259−1270.
(4) (a) Goetz, M. A.; Schwartz, C. D.; Koupal, L. R.; Liesch, J. M.;
Hensens, O. D.; Freidinger, R.; Anderson, P. S.; Pettibone, D. J.;
Woodruff, B. H. Eur. Patent 0256847, February 24, 1988.
(b) Pettibone, D. J.; Clineschmidt, B. V.; Anderson, P. S.;
Freidinger, R. M.; Lundell, G. F.; Koupal, L. R.; Schwartz, C. D.;
Williamson, J. M.; Goetz, M. A.; Hensens, O. D.; Liesch, J. M.;
Springer, J. P. Endocrinology 1989, 125, 217−222.
(5) (a) Bock, M. G.; DiPardo, R. M.; Williams, P. D.; Tung, R. D.;
Erb, J. M.; Gould, N. P.; Whitter, W. L.; Perlow, D. S.; Lundell, G. F.;
Ball, R. G.; Pettibone, D. J.; Clineschmidt, B. V.; Veber, D. F.;
Freidinger, R. M. In Chemical Aspects of Enzyme Biotechnology:
Fundamentals; Baldwin, T. O., Raushel, F. M., Scott, A. I., Eds.;
Springer US: Boston, MA, 1990; pp 123−134. (b) Perlow, D. S.; Erb,
J. M.; Gould, N. P.; Tung, R. D.; Freidinger, R. M.; Williams, P. D.;
of 20 with TFA/TES/DCM then led to rapid and high-yielding
ring closures attended by Boc deprotection. The facile synthesis
of analogue 21 using commercially available glutamate building
blocks highlights the ease with which oxoPiz can be introduced
into host peptides.
In summary, we have completed the first total synthesis of
the naturally occurring OTR antagonist L-156,373 (1). Key
features include the sequential electrophilic amination of
peptide N-terminal primary amines, stereospecific construction
and incorporation of a novel (N-OH)-Ile building block, and
tandem cyclization to form the Piz heterocycles. In addition,
incorporation of a novel oxoPiz motif into host structures was
C
DOI: 10.1021/acs.orglett.8b00912
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Veber, D. F. J. Org. Chem. 1992, 57, 4394−4400. (c) Williams, P. D.;
Bock, M. G.; Tung, R. D.; Garsky, V. M.; Perlow, D. S.; Erb, J. M.;
Lundell, G. F.; Gould, N. P.; Whitter, W. L.; Hoffman, J. B.; et al. J.
Med. Chem. 1992, 35, 3905−3918. (d) Bock, M. G.; DiPardo, R. M.;
Williams, P. D.; Pettibone, D. J.; Clineschmidt, B. V.; Ball, R. G.;
Veber, D. F.; Freidinger, R. M. J. Med. Chem. 1990, 33, 2321−2323.
(6) (a) Hale, K. J.; Delisser, V. M.; Manaviazar, S. Tetrahedron Lett.
1992, 33, 7613−7616. (b) Hale, K. J.; Jogiya, N.; Manaviazar, S.
Tetrahedron Lett. 1998, 39, 7163−7166.
(7) (a) List, B. J. Am. Chem. Soc. 2002, 124, 5656−5657. (b) Henmi,
Y.; Makino, K.; Yoshitomi, Y.; Hara, O.; Hamada, Y. Tetrahedron:
Asymmetry 2004, 15, 3477−3481. (c) Chen, Y.; Lu, Y.; Zou, Q.; Chen,
H.; Ma, D. Org. Process Res. Dev. 2013, 17, 1209−1213. (d) Kalch, D.;
Youcef, R. A.; Moreau, X.; Thomassigny, C.; Greck, C. Tetrahedron:
Asymmetry 2010, 21, 2302−2306.
(8) (a) Evans, D. A.; Chapman, K. T.; Hung, D. T.; Kawaguchi, A. T.
Angew. Chem., Int. Ed. Engl. 1987, 26, 1184−1186. (b) Aspinall, I. H.;
Cowley, P. M.; Mitchell, G.; Stoodley, R. J. J. Chem. Soc., Chem.
Commun. 1993, 1179−1180. (c) Liu, B.; Li, K.-N.; Luo, S.-W.; Huang,
J.-Z.; Pang, H.; Gong, L.-Z. J. Am. Chem. Soc. 2013, 135, 3323−3326.
(d) Liu, B.; Liu, T.-Y.; Luo, S.-W.; Gong, L.-Z. Org. Lett. 2014, 16,
6164−6167.
(9) (a) Ciufolini, M. A.; Xi, N. J. Org. Chem. 1997, 62, 2320−2321.
(b) Boger, D. L.; Schule, G. J. Org. Chem. 1998, 63, 6421−6424.
̈
(c) Hannachi, J. C.; Vidal, J.; Mulatier, J. C.; Collet, A. J. Org. Chem.
2004, 69, 2367−2373.
(10) Handy, E. L.; Totaro, K. A.; Lin, C. P.; Sello, J. K. Org. Lett.
2014, 16, 3488−3491.
(11) (a) Armstrong, A.; Jones, L. H.; Knight, J. D.; Kelsey, R. D. Org.
Lett. 2005, 7, 713−716. (b) Kang, C. W.; Sarnowski, M. P.; Elbatrawi,
Y. M.; Del Valle, J. R. J. Org. Chem. 2017, 82, 1833−1841. (c) Vidal, J.;
Damestoy, S.; Guy, L.; Hannachi, J.-C.; Aubry, A.; Collet, A.; Aubry, A.
Chem. - Eur. J. 1997, 3, 1691−1709.
(12) (a) Ciufolini, M. A.; Shimizu, T.; Swaminathan, S.; Xi, N.
Tetrahedron Lett. 1997, 38, 4947−4950. (b) Li, W.; Gan, J.; Ma, D.
Angew. Chem., Int. Ed. 2009, 48, 8891−8895. (c) Phillip Kennedy, J.;
Lindsley, C. W. Tetrahedron Lett. 2010, 51, 2493−2496. (d) Yoshida,
M.; Sekioka, N.; Izumikawa, M.; Kozone, I.; Takagi, M.; Shin-ya, K.;
Doi, T. Chem. - Eur. J. 2015, 21, 3031−3041.
(13) (a) Hashizume, H.; Sawa, R.; Yamashita, K.; Nishimura, Y.;
Igarashi, M. J. Antibiot. 2017, 70, 699. (b) Igarashi, M.; Sawa, R.;
Kinoshita, N.; Hashizume, H.; Nakagawa, N.; Homma, Y.; Nishimura,
Y.; Akamatsu, Y. J. Antibiot. 2008, 61, 387.
(14) (a) Umezawa, K.; Nakazawa, K.; Uemura, T.; Ikeda, Y.; Kondo,
S.; Naganawa, H.; Kinoshita, N.; Hashizume, H.; Hamada, M.;
Takeuchi, T.; Ohba, S. Tetrahedron Lett. 1998, 39, 1389−1392.
(b) Umezawa, K.; Nakazawa, K.; Ikeda, Y.; Naganawa, H.; Kondo, S. J.
Org. Chem. 1999, 64, 3034−3038.
(15) Kluyver, A. J.; van der Walt, J. P.; van Triet, A. J. Proc. Natl.
Acad. Sci. U. S. A. 1953, 39, 583.
(16) Yang, B.; Dong, J.; Zhou, X.; Yang, X.; Lee, K. J.; Wang, L.;
Zhang, S.; Liu, Y. Helv. Chim. Acta 2009, 92, 1112−1117.
(17) Data not shown. A related substitution using an α-bromoamide
derived from isoleucine was previously reported to lead to significant
Cα epimerization: Lin, D. W.; Masuda, T.; Biskup, M. B.; Nelson, J.
D.; Baran, P. S. J. Org. Chem. 2011, 76, 1013−1030.
(18) Hanessian, S.; Yang, R.-Y. Synlett 1995, 1995, 633−634.
D
DOI: 10.1021/acs.orglett.8b00912
Org. Lett. XXXX, XXX, XXX−XXX