Aluminium-Catalysed Oxazolidinone Synthesis and their Conversion into Functional Non-Symmetrical Ureas

Article abstract of DOI:10.1002/adsc.201500635

An efficient and practical aluminium-catalysed approach towards a range of functional oxazolidinones is reported. The method is based on cheap and readily available starting materials including terminal and internal (bicyclic) epoxides and phenyl carbamate. The oxazolidinones serve as highly useful synthons for the high yield preparation of non-symmetrical ureas by nucleophilic ring-opening affording the targeted urea compounds with excellent functional group diversity, high regioselectivity and isolated yields up to >99%.

Full text of DOI:10.1002/adsc.201500635

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DOI: 10.1002/adsc.201500635
Very Important Publication
Aluminium-Catalysed Oxazolidinone Synthesis and their
Conversion into Functional Non-Symmetrical Ureas
Victor Laserna,a Wusheng Guo,a and Arjan W. Kleija,b,*
a
Institute of Chemical Research of Catalonia (ICIQ), Av. Països Catalans 16, 43007 Tarragona, Spain
Fax: (+34)-977-920-823; phone: (+34)-977-920-247; e-mail: akleij@iciq.es
Catalan Institute of Research and Advanced Studies (ICREA), Pg. Lluís Companys 23, 08010 Barcelona, Spain
b
Received: July 1, 2015; Revised: July 24, 2015; Published online: August 18, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500635.
Abstract: An efficient and practical aluminium-cata- metrical ureas by nucleophilic ring-opening affording
lysed approach towards a range of functional oxazo- the targeted urea compounds with excellent func-
lidinones is reported. The method is based on cheap tional group diversity, high regioselectivity and iso-
and readily available starting materials including ter- lated yields up to >99%.
minal and internal (bicyclic) epoxides and phenyl
carbamate. The oxazolidinones serve as highly useful Keywords: aluminium; homogeneous catalysis; N,O
synthons for the high yield preparation of non-sym- ligands; oxazolidinones; ureas
Introduction
tion partners (see Scheme 1). The approach uses oxa-
zolidinones (cyclic carbamates) as intermediates that
Organic ureas are ubiquitous structures in organic allow for nucleophilic ring-opening by suitable
chemistry and their re-emerged application potential amines. Such an approach is not completely without
in various areas of chemistry has been recently high- precedence,[16,17] however so far oxazolidinone type in-
lighted.[1] Prominent fields where ureas have gained termediates have been rarely used for the preparation
importance include (asymmetric) organocatalysis[2,3] of non-symmetrical ureas with wide scope[18–20] and
and supramolecular chemistry.[4] Organocatalytic acti- thus a more general methodology based on these pre-
vation of organic substrates by means of hydrogen- cursors derived from readily available, cheap epoxides
bonding patterns that consist of the two urea NH and phenyl carbamate (Scheme 1) would provide an
groups has been recognised as a powerful tool to ori- attractive and simple route towards highly functional
entate carbonyl fragments in order to increase the ef- ureas.
ficiency of various organic conversions. Asymmetric
Jacobsen et al. recently disclosed a method for the
ureas have also been developed successfully and ap- formation of chiral amino alcohols through a Co(sa-
plied as enantio-controlling mediators in, for instance, len) mediated enantioselective conversion of meso-
the asymmetric Strecker reaction.[5] In the context of
supramolecular applications, N,N’-disubstituted urea
derivatives have proven to be versatile building
blocks for the preparation of hydrogen-bonded supra-
molecular polymers,[6,7] helical type foldamers[8] and
anion transporting molecules.[9]
There exist various synthetic methods towards non-
symmetrical ureas although in most cases reactive
species such as carbonylimidazolide[10] or (in situ pre-
pared) isocyanate reagents[11–13] are required together
with air-sensitive/toxic additives[14,15] and/or expensive
metal catalysts/ligands.[12] Therefore, we set out to ex-
plore a practical synthesis of non-symmetrical ureas
from readily available and cheap starting materials
combined with a highly modular nature of the reac- ureas through oxazolidinone intermediates.
Scheme 1. A modular approach towards non-symmetrical
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Victor Laserna et al.
Table 1. Screening of catalyst structures A–C and reaction
conditions in the synthesis of oxazolidinone 1 from cyclo-
hexene oxide and phenyl carbamate.[a]
cyclic epoxides in the presence of phenyl carbamate
as nucleophile.[21] This approach demonstrated the use
of phenyl carbamate as a cheap and readily available
reagent in the formal transfer of an “amide” unit to
an epoxide giving enantioenriched oxazolidinones as
intermediates towards the synthesis of chiral amino
alcohols. However, the reported method was restrict-
ed to a limited number of cyclic epoxides. In order to
be able to develop a more general method towards
oxazolidinone precursors en route to non-symmetrical
ureas, we envisioned that our previously reported
Lewis acidic Al(aminotriphenolate) catalysts would
present an alternative catalyst for oxazolidinone for-
mation (Scheme 1). Recently we showed that both in-
ternal as well as terminal epoxides are easily activated
towards nucleophilic ring-opening by these catalyst
systems to form highly functional cyclic carbonates in
the presence of carbon dioxide.[22–24] In this work we
will show that these Al catalysts provide easy access
to a wide range of oxazolidinone precursors useful for
a modular approach towards highly functionalised
ureas that are generally obtained in excellent yield
and regioselectivity.
Entry
Cat.
Amount
[mol%]
CHO
[mmol]
Temp.
[8C]
Conv.[b]
[%]
1
2
3
4
5
6
7
8
–
–
–
–
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.5
0.75
2.0
1.0
1.0
50
70
50
50
70
70
70
70
50
50
50
50
50
70
70
0
17[c]
41
A
A
A
A
B
B
C
C
C
C
C
C
C
2.5
5.0
2.5
5.0
2.5
5.0
2.5
5.0
2.5
2.5
2.5
1.0
2.5
47
94[c]
99[c]
10[c]
15[c]
95 (89)[d]
99
9
10
11
12
13
14
15
81
84
99
99[c]
99 (71)[c,d]
Results and Discussion
We first set out to test a small series of Al(aminotri-
phenolate) complexes A–C (Figure 1) in the synthesis
of oxazolidinone 1 under various conditions (Table 1)
using the coupling of cyclohexene oxide (CHO) and
phenyl carbamate as a benchmark reaction. The reac-
tion without the addition of catalyst does not proceed
at 508C (entry 1), and at 708C only low conversion of
the phenyl carbamate reagent is noted. In general we
[a]
General conditions: 0.5 mmol phenylcarbamate, catalyst
A–C (0–5 mol%), 18 h, CH3CN.
[b]
Conversion determined by 1H NMR (CDCl3) based on
phenol formation.
[c]
[d]
White precipitate noted.
Isolated yield of oxazolidinone product in brackets.
found that those reactions that were carried out at levels at 508C (entries 3 and 4). Much better activity
this latter temperature (entries 2, 5–8 and 14–15) was noted for the chloride-substituted Al complex C
caused (partial) decomposition of the carbamate re- for which a loading of 2.5 mol% and an equimolar
agent and the white precipitates noted could be isolat- amount of epoxide (entry 11) already provided 81%
ed and assigned to the formation of 1,3,5-triazine- conversion. Further increasing the epoxide/carbamate
2,4,6(1H, 3H, 5H)-trione, a cyclic structure supported ratio to 2 led to nearly full conversion (95%) with
by 1 H, 15N, 13C NMR and IR analysis (see the Sup- a high isolated yield (89%) for the oxazolidinone
porting Information).
product (entry 9). Therefore, the reaction conditions
Therefore, we decided to optimise further the catal- of this latter entry seem to be rather ideal for the syn-
ysis conditions at 508C using various catalysts, catalyst thesis of oxazolidinone 1.
loadings and amount of the epoxide (CHO). Al cata-
Once we had discovered the best conditions for the
lyst A performs much better than B (cf., entries 5–8) conversion of CHO into product 1 using the most ef-
but its use results in moderate substrate conversion fective Al catalyst (i.e., C), we then started to investi-
gate the scope of this process (Scheme 2). In general,
this Al-mediated synthesis allows for a wide range of
(functional) groups to be present in the epoxide sub-
strate including alkene (2 and 4), alkyl halide (5), ep-
oxidic (8), aryl bromide (9), alkyne (13) and ether (10
and 13–15) groups. Of further note are the syntheses
of oxazolidinones 11 and 16 and compounds 1–3 that
consist of an (a)cyclic dialkyl-substituted pattern. The
Figure 1. Al catalysts A–C used in this work.
alternative formation of acyclic and cyclic 4,5- and
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Aluminium-Catalysed Oxazolidinone Synthesis
Scheme 2. Synthesis of oxazolidinones 1–16 from various epoxides and phenyl carbamate catalysed by Al-aminotriphenolate
complex C (2 mol%). Reported here are isolated yields after column purification; selectivities refer to regioisomers formed
in the case of terminal epoxide conversion (note: only the major isomer is shown).
5,5’-disubstituted oxazolidinone regioisomers from
aziridenes/CO2,[25–27] epoxide/isocyanate[28,29] or prop-
[30]
argylic amine/CO2 substrate combinations typically
require harsher reaction conditions, a higher loading
of catalyst and/or the use of stoichiometric/expensive
additives and more elaborate procedures. The method
disclosed here towards oxazolidinones 1–16 is opera-
tionally simple and allows for high conversion under
attractive reaction conditions (508C, 2 mol% of cata-
lyst). The majority of the oxazolidinone compounds
could be isolated in good yields (up to 91%) and high
regioselectivity for the 5-isomer was noted with some
exceptions (cf., the synthesis of 9, 12 and 16). Since
these reactions occur with inversion of configuration
Scheme 3. Proposed mechanism leading to formal inversion
in cis-2,3-dimethyloxirane.
at the carbon centre at the initial stage of the reaction
(i.e., nucleophilic attack of the carbamate reagent character: the aminotriphenolate ligand acts here as
onto the epoxide-Al complex), those reactions that in- a proton relay mediator increasing the nucleophilic
volve cis-configured carbon centres in the oxirane character of the alkoxide which allows a more effi-
unit give rise to oxazolidinones with a trans disposi- cient ring-closing to occur releasing the final (configu-
tion (cf., syntheses of 1–3 and 11a). Thus these con- rationally inverted) product and a phenoxide as a leav-
versions proceed with formal inversion unlike the re- ing group.[31] The formation of phenol, as a result of
actions between isocyanates and epoxides.[29] This in- the phenoxide abstracting a proton, is easily recog-
version pathway was further substantiated by the syn- nised in the reaction mixture and thus allows for fol-
thesis of 11b (cis diastereoisomer of 11a; dr >99%) lowing the course of the process (see Table 1,
that was prepared from the trans-epoxide.
Scheme 3).
The envisioned mechanism involves a nucleophilic
The oxazolidinones 1–16 serve as useful precursors
attack of the phenyl carbamate on the coordinated towards the formation of highly functional, non-sym-
epoxide producing a reactive alkoxide which is stabi- metrical ureas as shown in Scheme 4. Treatment of
lised by the Al complex (Scheme 3) displaying dual the respective oxazolidinone precursor with a suitable
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Victor Laserna et al.
Scheme 4. Synthesis of non-symmetrical ureas 17–32 from oxazolidinone precurcors and primary/secondary amines. Report-
ed yields are isolated ones after column purification/crystallisation.
amine (1.2–4 equiv,) allows for isolation of the urea cyclic (24, 25, 27, 31 and 32), alkylamine (28) and vici-
product in high yield and chemoselectivity. The ami- nal diol (26) groups. Note that urea 26 is derived
nolysis of the oxazolidinone occurs with exclusive from the oxazolidinone 5 which is based on the initial
À
preference for scission of the NH C=O bond. The use of epichlorohydrin. The longer reaction time
formation of ureas 17–24, 27 and 28 from bicyclic ox- needed to convert 5 into 26 resulted in effective hy-
azolidinones proceeds generally under mild reaction drolysis of the alkyl chloride fragment which is in line
conditions in the presence of a small excess of amine with the recorded NMR data and mass spectrometric
reagent in CH3CN solvent. This procedure is attrac- analysis (see the Supporting Information).
tive, practical and proceeds with full conversion of
Both primary as well as secondary amines (cf., syn-
the starting materials. In the case where oxazolidi- theses of 17, 22, 27 and 32) react smoothly with the
nones derived from terminal epoxides are used as oxazolidinone precursors further amplifying the po-
starting materials (i.e., 25, 26 and 29–32), the aminol- tential scope of this urea formation reaction. The mo-
ysis reaction required generally higher temperature, lecular structure of urea 27 (Figure 2) was determined
longer reaction times and a larger excess of amine re- by X-ray diffraction. Compound 27 was derived from
agent (4 equiv.) under solvent-free conditions. In the oxazolidinone 1. Thus, the relative configuration of
latter cases, somewhat lower isolated yields are noted the urea and the free alcohol groups (trans) is a fur-
(up to 73%). ther testament for the proposed inversion mechanism
The scope demonstrated in Scheme 3 illustrates as depicted in Scheme 3.
that highly functionalised ureas can be prepared in-
The diversity of functional groups in the presented
cluding those incorporating pyrrolidine (17), alkene scope of urea products has potential in post-synthetic
(19, 20, 21, 23, 25 and 29), alkyne (20 and 31), hetero- modifications as shown in Scheme 5. To demonstrate
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Aluminium-Catalysed Oxazolidinone Synthesis
scope in ureas that can be attained. Further to this,
these functional ureas may serve as suitable scaffolds
in organic synthesis which can be post-modified using
“click” type reactions as demonstrated herein.
Experimental Section
Oxazolidinone Synthesis
Typically, phenyl carbamate (1 mmol), the epoxide
(2 mmol), the aluminium catalyst (2 mol%) and acetonitrile
(2 mL) were introduced into a glass vial (5 mL) equipped
with a stirring bar. The vial was closed and introduced into
a silicon oil bath preheated at the desired reaction tempera-
ture (50–708C) and the mixture was stirred for 18 h. Once
the reaction mixture had cooled down to room temperature
the product was purified by flash-column chromatography
and analysed. Full characterisation data and copies of rele-
vant spectra are provided in the Supporting Information.
Figure 2. X-ray molecular structure determined for urea 27.
Selected interatomic distances (Š) and angles (8) with esds
À
À
in parentheses: N(1A) C(1A)=1.366(5), N(2A) C(1A)=
À
À
1.356(5),
C(1A) O(1A)=1.248(5),
N(2A) C(6A)=
À
À
À
À
1.449(5); N(1A) C(1A) N(2A)=117.8(4), N(1A) C(1A)
À
À
O(1A)=120.4(4), N(2A) CC(1A) CO(1A)=121.8(4).
Urea Synthesis
Typically, the oxazolidinone (0.5 mmol) was introduced into
an HPLC vial equipped with a stirring bar. For the oxazoli-
dinones based on bicyclic scaffolds, the amine reagents were
introduced (usually 1.2 equiv. but was increased to 1.8 equiv.
when dealing with volatile amines) with addition of acetoni-
trile (0.2 mL) and heated to the desired temperature (508C)
for 18 h. Once the reaction mixture had cooled down to
room temperature the pure product was isolated by evapo-
ration of the excess of amine. The crude ureas were recrys-
tallised from dichloromethane (ureas 20, 21, 23, 24, 27 and
28) or purified by flash column chromatography. The oxazo-
lidinones with monocyclic scaffolds were treated with the re-
spective amine reagent (4 equiv.) under neat conditions and
heated to 708C for 66 h. The products were then isolated by
recrystallisation from dichloromethane (for urea 29) or puri-
fied by flash column chromatography. Full characterisation
data and copies of relevant spectra are provided in the Sup-
porting Information.
Scheme 5. Synthetic potential of urea 13.
further use of these scaffolds, urea 13 was subjected
to “click” chemistry conditions using benzyl azide as
coupling partner: this afforded the triazole 34
smoothly in 91% yield.[32] The use of click reactions
to functionalise polymer supports or other macromo-
lecular structures is interesting for catalytic[33] and
supramolecular applications[34,35] and therefore this
chemistry combined with suitable urea synthons
opens up new opportunities in the aforementioned
areas.
Crystallographic Studies
The measured crystal was stable under atmospheric condi-
tions; nevertheless it was treated under inert conditions im-
mersed in perfluoro-polyether as protecting oil for manipu-
lation. Data collection: measurements were made on
a Bruker-Nonius diffractometer equipped with an APPEX
II 4 K CCD area detector, a FR591 rotating anode with Mo
Ka radiation, Montel mirrors and a Kryoflex low tempera-
ture device (T=À1738C). Full-sphere data collection was
used with w and f scans. Programs used: data collection
Apex2V2011.3 (Bruker-Nonius 2008), data reduction
Saint+Version 7.60A (Bruker AXS 2008) and absorption
correction SADABS V. 2008-1 (2008). Structure solution:
SHELXTL Version 6.10 (Sheldrick, 2000) was used.[36] Struc-
ture refinement: SHELXTL-97-UNIX VERSION.
Conclusions
We here disclose a simple but effective two-step
method for the formation of highly functional, non-
symmetrical ureas from easy to prepare oxazolidi-
nones. The cyclic carbamates are derived from cheap
and readily available epoxides and phenyl carbamate
mediated through Al catalysis. The method is further
Crystal data for 27: C11H20N2O3, Mr =228.29, triclinic, P-1,
a=6.4520(13) Š, b=9.1250(18) Š, c=20.031(4) Š, a=
92.008, b=92.908, g=90.068, V=1177.1(4) Š3, Z=4, 1=
characterised by its operational simplicity and wide 1.288 mgMÀ3, m=0.094 mmÀ1, l=0.71073 Š, T=100(2) K,
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Victor Laserna et al.
F(000)=496, crystal size=0.15”0.04”0.01 mm, q(min)=
1.0188, q(max)=27.678, 5421 reflections collected, 5421 re-
flections unique, GoF=1.061, R1 =0.0812 and wR2 =0.1951
[I> 2s(I)], R1 =0.1438 and wR2 =0.2367 (all indices), min/
max residual density=À0.716/0.718 [e·ŠÀ3]. Completeness
to q(27.678)=98.9%. CCDC 1405849 contains the supple-
mentary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/
cif.
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Acknowledgements
We thank ICIQ, ICREA and the Spanish Ministerio de Econ-
omía y Competitividad (MINECO) through Severo Ochoa
Excellence Accreditation 2014–2018 (SEV-2013-0319) and
project CTQ2014-60419-R for support. W.G. thanks the
CELLEX foundation for financial support and V. L. wishes
to acknowledge the Generalitat de Catalunya for an FI fel-
lowship. Eduardo C. Escudero-Adµn and Dr. Eddy Martin
are thanked for the X-ray crystallographic analysis.
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