Synthesis of Highly Substituted 1,2-Diazetidin-3-ones, Small-Ring Scaffolds for Drug Discovery

Article abstract of DOI:10.1002/chem.201801309

1,2-Diazetidin-3-ones are readily accessible, small ring scaffolds that upon functionalization have the potential to produce diverse 3-dimensional structures for drug discovery. Thus, treatment of diazo hydrazides, obtained from simple hydrazides and malonyl half ester derivatives, followed by diazo transfer, with catalytic amounts of rhodium(II) acetate dimer results in intramolecular carbenoid N?H insertion to give 1,2-diazetidin-3-ones. Although subsequent functionalization reactions could be hampered by the lability of the 4-membered ring, a wide range of new derivatives was available by deprotection at N-1, and subsequent amide or urea formation. The structures of four four-membered rings was confirmed by X-ray crystallography; the compounds showed modest growth inhibitory activity in mammary carcinoma cells.

Full text of DOI:10.1002/chem.201801309

DOI: 10.1002/chem.201801309
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Heterocycles |Hot Paper|
Synthesis of Highly Substituted 1,2-Diazetidin-3-ones, Small-Ring
Scaffolds for Drug Discovery
Marilia S. Santos,[a] Andrew Nortcliffe,[a] William Lewis,[a] Tracey D. Bradshaw,[b] and
Christopher J. Moody*[a]
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Abstract: 1,2-Diazetidin-3-ones are readily accessible, small
ring scaffolds that upon functionalization have the potential
to produce diverse 3-dimensional structures for drug discov-
ery. Thus, treatment of diazo hydrazides, obtained from
simple hydrazides and malonyl half ester derivatives, fol-
lowed by diazo transfer, with catalytic amounts of rho-
dium(II) acetate dimer results in intramolecular carbenoid
NÀH insertion to give 1,2-diazetidin-3-ones. Although subse-
quent functionalization reactions could be hampered by the
lability of the 4-membered ring, a wide range of new deriva-
tives was available by deprotection at N-1, and subsequent
amide or urea formation. The structures of four four-mem-
bered rings was confirmed by X-ray crystallography; the
compounds showed modest growth inhibitory activity in
mammary carcinoma cells.
work[6] of the 1,2-diazetidinone core was not found as a sub-
structure in a random 2% sample of the ZINC database of
commercially available compounds.[7] Furthermore, examining
the molecular shape of the virtual library using a PMI plot indi-
cates that diverse three-dimensional molecular shape can be
achieved by functionalization of the 1,2-diazetidinone core
(Figure 2).
Introduction
Four-membered rings are increasingly prevalent as bioactive
scaffolds in drug discovery programmes.[1] Not only do they
add structural novelty in under-explored areas of chemical
space, they can also contribute to enhanced physicochemical
properties. This has been amply illustrated by the dramatic in-
crease in interest in oxetane ring systems in recent years.[2]
One four-membered ring that attracted our attention is the
1,2-diazetidin-3-one core structure (Figure 1).[3] Although
known for almost a 100 years, this
small ring system remains largely
unexplored in medicinal chemistry,
but has recently attracted renewed
attention following the discovery
Figure 1. The 1,2-diazetidin-
3-one scaffold.
that they act as potent inhibitors of
the serine hydrolase protein phos-
phatase methylesterase-1 by acyla-
tion of the key serine residue.[4] Not-
withstanding their potential liability toward ring opening,
these densely functionalized small rings possess well-defined
vectors, and form a worthwhile low molecular weight scaffold
for drug discovery.
To examine the potential molecular and structural diversity
of the 1,2-diazetidin-3-one scaffold a virtual library was enum-
erated using LLAMA (Lead-Likeness And Molecular Analysis).[5]
A virtual library of 177 compounds based on 3 core scaffolds
was shown to have suitable molecular properties as lead com-
pounds for drug discovery (RMM=369, ALogP=1.11, pTSA=
104 ꢁ2). Additionally, the 1,2-diazetidinone fragment is highly
distinctive from existing fragment databases. The Murko frame-
Figure 2. Principal moments of inertia (PMI) plot of the prepared scaffolds,
generated using LLAMA.
[a] M. S. Santos, Dr. A. Nortcliffe, Dr. W. Lewis, Prof. Dr. C. J. Moody
School of Chemistry
In addition to being an attractive scaffold for further func-
tionalization, 1,2-diazetidin-3-ones are aza-analogues of the
well known b-lactam antibiotics. Despite being discovered
90 years ago, b-lactam antibiotics continue to play a key role
in clinical practice and remain one of the best-investigated
families of antibiotics. Their mode of action and susceptibility
to resistance via b-lactamase enzymes have been widely stud-
ied, resulting in the chemical synthesis and biological evalua-
tion of numerous derivatives and analogues.[8] Much of the in-
terest in analogues has focused on the modification of the
ring system of naturally occurring b-lactams such as the peni-
University of Nottingham
University Park
Nottingham NG7 2RD (United Kingdom)
E-mail: c.j.moody@nottingham.ac.uk
[b] Dr. T. D. Bradshaw
School of Pharmacy
Centre for Biomolecular Science
University of Nottingham
University Park
Nottingham NG7 2RD (United Kingdom)
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under:
https://doi.org/10.1002/chem.201801309.
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cillins and cephalosporins. Such analogues often incorporate
an additional nitrogen atom in the ring, and include 2-azape-
nems,[9] 3-azacephams,[9b] and 1-azacephems (Figure 3).[10] 1,2-
Diazetidin-3-ones represent an alternative aza-b-lactam ana-
logue that places the additional nitrogen in the four-mem-
bered ring itself.
to 1,2-diazetidin-3-ones based on the cyclization of benzophe-
none chloroacetylhydrazone, followed by treatment with 4-tol-
uenesulfonic acid to give the parent ring system for further
elaboration (Scheme 1B).[13] Our own contributions centered
on the intramolecular NÀH insertion reaction of rhodium car-
bene intermediates derived from diazocarbonyl compounds
(Scheme 1C).[14]
Despite the development of successful synthetic routes,[3]
1,2-diazetidinones remain poorly investigated, and attempts to
further develop their chemistry have often foundered due to
their lability and inherent ring strain. We now report a detailed
study on the synthesis of a range of 1,2-diazetidin-3-one scaf-
folds using the versatile rhodium carbene methodology, fol-
lowed by their subsequent derivatization reactions, and pre-
liminary biological evaluation.
Results and Discussion
The synthesis began with the reaction between benzaldehyde
and tert-butyl carbazate followed by reduction with cyanobor-
ohydride to form the hydrazine 1a. Treatment with methyl ma-
lonyl chloride and triethylamine in dichloromethane at room
temperature gave the desired 1,3-dicarbonyl compound 2a in
70% yield (Scheme 2). Subsequent diazo group transfer reac-
tion using p-acetamidobenzenesulfonyl azide (p-ABSA)[15] in
the presence of DBU afforded diazocarbonyl compound 3a in
good yield. The diazocarbonyl compound represents the key
substrate for the overall strategy, and treatment with a catalyt-
ic amount of rhodium(II) acetate dimer (3 mol%) induced met-
allocarbene formation and intramolecular NÀH insertion pro-
viding the 1,2-diazetidin-3-one 4a as a single product in 79%
yield after chromatographic purification. The same synthetic
sequence was applied to the hydrazine 1b derived from furfu-
ral, and delivered the 1,2-diazetidin-3-one 4b in comparable
fashion. Likewise the corresponding benzyl ester 4c was pre-
pared as outlined in Scheme 2. As expected, the 1,2-diazetidin-
3-ones exhibited a high frequency carbonyl stretch at about
1800 cmÀ1 in their IR spectra.
Figure 3. Naturally occurring b-lactams and some aza-analogues.
1,2-Diazetidin-3-ones were originally prepared almost a cen-
tury ago by Staudinger by reaction of azo compounds with ke-
tenes.[11] The process has been subject of renewed interest
with the development of enantioselective versions using asym-
metric nucleophilic catalysis (Scheme 1A),[12] although it re-
mains somewhat limited in its substrate scope. In contrast, in
seminal work Taylor and co-workers described versatile routes
With the 4-membered heterocycle in hand, it was a priority
to establish its ability to participate in a range of functionaliza-
tion reactions. Initial attempts to remove the Boc-protecting
group under standard acidic conditions were unsatisfactory,
but exposure of aza-b-lactams 4a and 4b to boron trifluoride
etherate[16] resulted in clean removal of the nitrogen protecting
group to deliver the 1,2-diazetidin-3-ones 5 in which N-1 is
free for further elaboration. Exemplar acylation reactions at N-1
using acetyl chloride proceeded smoothly to provide deriva-
tives 6 (Scheme 2).
In parallel, and to expand the structural variation in the 1,2-
diazetidinones through functional group interconversion, we
attempted to hydrolyze the 4-carboxylate ester in 1,2-diazetidi-
none 4a to the corresponding carboxylic acid 7. Standard alka-
line hydrolysis conditions resulted in rapid consumption of
ester 4a, but no carboxylic acid 7 could be isolated. Likewise
use of Me3SiOK or Me3SnOH as reagents was unsuccessful. In
all cases, a complex mixture of products was observed, sug-
gesting that the 1,2-diazetidin-3-one ring is unstable under
Scheme 1. Routes to 1,2-diazetidin-3-ones.
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Scheme 2. Synthesis of 1-Boc-1,2-diazetidinones, and their deprotection and re-acylation.
such conditions. Therefore, we switched our attention to the
corresponding benzyl ester 4c in the expectation that it could
be cleaved under non-nucleophilic conditions by hydrogenoly-
sis. In the event, this was the case, and the carboxylic acid 7
could be isolated in 65% yield after hydrogenolysis over palla-
dium on charcoal (Scheme 3). However, the acid 7 proved to
chloride (DMTMM), prepared in situ from 2-chloro-4,6-dime-
thoxy-1,3,5-triazine (CDMT) and N-methylmorpholine (NMM),
and propylphosphonic anhydride (T3P) resulted in similar
yields for amide 8. Direct amidation attempts using the trime-
[17]
thylaluminum reagent, DABAL-Me3 also gave disappointing
results. Presumably the low yields in amide coupling reactions
again reflect the instability of 1,2-diazetidinone-4-carboxylic
acid.
Given the instability of the 1,2-diazetidinone-4-carboxylic
acid, and the resulting poor yields in amide formation, we in-
vestigated the alternative strategy in which the amide is
formed before the construction of the 4-membered ring. Thus,
hydrazine derivatives 2b, 2d and 2e (Scheme 2) were hydro-
lyzed to the corresponding acids in near quantitative yields,
and coupled to morpholine as a representative amine to give
the amides 9 (Scheme 4). Diazo transfer proceeded smoothly
using o-nitrobenzenesulfonyl azide (o-NBSA),[18] the original
conditions employing p-ABSA being unsatisfactory. The result-
ing diazocarbonyl compounds 10 underwent rhodium(II) cata-
lyzed NÀH insertion to give the 1,2-diazetidin-3-ones 11. As
before, treatment with boron trifluoride etherate successfully
removed the NÀ1 protecting group in good to excellent yield,
and the resulting amine 12 underwent amide coupling with a
range of representative acid chlorides to give amides 13a–f
(Scheme 4). The structures of amides 13a, 13b and 13d were
confirmed by X-ray crystallography (Figure 4). In a similar
manner, 1,2-diazetidinone 12a reacted with isocyanates to
give a range of ureas 14 (Scheme 5).
Scheme 3. Formation of 1,2-diazetidinone-4-carboxylic acid, and amide for-
mation; X-ray crystal structure of amide 8 (CCDC 1829702).
be unstable, and decomposed within a few hours of its prepa-
1
ration. H and 13C NMR spectroscopic analysis of the resulting
degradation products did not provide evidence for decomposi-
tion by a simple process such as decarboxylation. Nevertheless,
a range of amide coupling reactions was investigated using
benzylamine as a model amine nucleophile. The first amide
coupling agent evaluated was HATU which enabled the forma-
tion of amide 8, the structure of which was confirmed by X-ray
crystallography, in poor yield of 21%. Use of EDCI, CDI or
PyBOP was unsuccessful, whereas other coupling agents such
as 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methyl-morpholinium
With a range of novel 1,2-diazetidin-3-ones in hand, it re-
mained to carry out preliminary biological evaluation. The
compounds were tested against a range of bacterial and
fungal strains as part of the CO-ADD (Australia) antimicrobial
screen, but were for the most part inactive.[19] Com-
pounds 13e and 14b showed activity antifungal activity
against Cryptococcus neoformans var. grubii showing 91 and
51% inhibition of growth, respectively after 2 h incubation.
Further structure–activity relationship studies are ongoing.
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Scheme 5. Synthesis of 1,2-diazetidin-3-one ureas.
Table 1. Growth inhibitory activity of 1,2-diazetidin-3-ones in MCF-7 and
HCT-116 carcinoma cells lines.
Compound
GI50 meanÆSEM [mm]
MCF-7
HCT-116
5a
5b
6a
6b
12b
13a
13b
13c
13d
13e
13 f
14c
72.7Æ12.3
76.0Æ7.51
82.0Æ18.0
95.3Æ6.69
54.3Æ4.18
64.0Æ3.51
67.3Æ6.06
26.3Æ21.9
49.3Æ10.7
52.3Æ2.60
66.3Æ12.9
57.3Æ9.70
>100
>100
>100
>100
>100
>100
>100
41.3Æ23.9
>100
>100
>100
>100
MCF-7 breast cancer cells; HCT-116 colon cancer cells; cells were exposed
to experimental agents for 48 h before viability was assessed by MTT
assay. MeanÆSEM (standard error of the mean) of ꢀ3 independent trials
are shown; n=4 per concentration per trial.
Scheme 4. [Fur=2-furyl] Synthesis of 1,2-diazetidinone-4-carboxamides.
against colon carcinoma cells, but showed modest growth in-
hibitory activity in mammary carcinoma cells.
Conclusion
A range of 1,2-diazetidine-3-ones has been synthesized using
intramolecular NÀH insertion of a rhodium carbenoid as a key
step. Although some attempts at subsequent derivatization
could be thwarted by the lability of the 4-membered ring, a
range of derivatives was accessed by functionalization of the
pendant carboxylate group prior to cyclization. The com-
pounds show modest biological activity.
Figure 4. X-Ray crystal structures of 1,2-diazetidin-3-ones a) 13a, b) 13b and
c) 13d (CCDC 1829700, 1829701, 1829703 respectively).
Experimental Section
Full experimental details are given in the Supporting Information,
together with copies of NMR spectra.
The compounds were also screened against two cancer cell
lines: MCF-7 (mammary carcinoma) and HCT116 (colon carcino-
ma), using the MTT assay. The results are summarized in
Table 1, and show that the compounds were largely inactive
CCDC 1829702, 1829700, 1829701, and 1829703 contain the sup-
plementary crystallographic data for this paper. These data are pro-
vided free of charge by The Cambridge Crystallographic Data
Centre
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Acknowledgements
We thank CAPES (10719/2014-08) for financial support (to
M.S.S.). A.N.’s contribution was carried out within the European
Lead Factory and has received support from the Innovative
Medicines Initiative Joint Undertaking under grant agreement
no. 115489, resources of which are composed of financial con-
tribution from the European Union’s Seventh Framework Pro-
gramme (FP7/2007–2013) and EFPIA companies’ in-kind contri-
bution. Antimicrobial screening was performed by CO-ADD
(The Community for Antimicrobial Drug Discovery), funded by
the Wellcome Trust (UK) and The University of Queensland
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Conflict of interest
The authors declare no conflict of interest.
Keywords: 4-membered rings · carbene insertion · drug
discovery · heterocycles · lead-oriented synthesis
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Manuscript received: March 15, 2018
Accepted manuscript online: March 31, 2018
Version of record online: && &&, 0000
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FULL PAPER
Small rings to populate chemical
space: 1,2-Diazetidin-3-ones, readily ob-
tained through intramolecular rhodium
carbene NÀH insertion, are useful scaf-
folds for drug discovery.
&
Heterocycles
M. S. Santos, A. Nortcliffe, W. Lewis,
T. D. Bradshaw, C. J. Moody*
&& – &&
Synthesis of Highly Substituted 1,2-
Diazetidin-3-ones, Small-Ring
Scaffolds for Drug Discovery
1,2-Diazetidin-3-ones remain an under-investigated small-ring heterocycle.
Nevertheless their diverse structures offer the potential to explore chemical space as
illustrated by the principal moments of inertia “triangle” in rod-like (rocket), sphere-
like (planet) and disc-like (flying saucer) regions. Inspired by the unexpected three-
dimensionality of substituted 1,2-diazetidin-3-ones, the synthesis and functionalization
of a range of these strained four-membered rings are reported; for more details, see
Full Paper by C. J. Moody et al. on page && ff.
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