New regiospecific catalytic approaches to 4,5-dihydroisoxazoles and 2,5-dihydroisoxazoles from o-propargylic hydroxylamines

Article abstract of DOI:10.1055/S-0029-1219365

Unprotected O-propargylic hydroxylamines undergo generally essentially quantitative cyclisations when exposed briefly to silver nitrate adsorbed onto silica gel to give 4,5-dihydroisox-azoles [2-isoxazolines], while N-protected derivatives give the corres

Full text of DOI:10.1055/S-0029-1219365

628
LETTER
New Regiospecific Catalytic Approaches to 4,5-Dihydroisoxazoles and
2,5-Dihydroisoxazoles from O-Propargylic Hydroxylamines
A
pproaches to 4,5-
a
D
ihydroiso
v
xazoles and
i
2,5-Dih
d
ydroisoxazoles W. Knight,*a Anthony J. Proctor,a John M. Cloughb
a
School of Chemistry, Cardiff University, Main College, Park Place, Cardiff, CF10 3AT, UK
Fax +44(2920)874030; E-mail: knightdw@cf.ac.uk
b
Syngenta, Research Chemistry, Jealott’s Hill International Research Centre, Bracknell, Berkshire, RG42 6EY, UK
Received 9 November 2009
Dedicated with all best wishes and gratitude for all the training and advice to Professor Gerry Pattenden on the occasion of his
70th birthday from two of his young apprentices, DWK and JMC.
metric versions, especially because of their potential to
contribute to the asymmetric synthesis of the valuable
degradation products mentioned above.6 An entirely dif-
ferent strategy features various intramolecular cyclisa-
Abstract: Unprotected O-propargylic hydroxylamines undergo
generally essentially quantitative cyclisations when exposed briefly
to silver nitrate adsorbed onto silica gel to give 4,5-dihydroisox-
azoles [2-isoxazolines], while N-protected derivatives give the cor-
responding 2,5-dihydroisoxazoles [3-isoxazolines] in similarly tions, the simplest of which are base-induced cyclisations
of allylic oximes7 or, in a reverse sense, of propargylic hy-
excellent yields, given that an appropriate functionality on nitrogen
droxylamines to give 4,5-dihydroisoxazoles.8 Other meth-
is used.
Key words: 4,5- and 2,5-dihydroisoxazoles, O-propargylic hy-
droxylamines, cyclisations, silver nitrate on silica gel
ods have been adapted for inducing cyclisations of allylic
oximes
including
palladium-catalysed
alkene
carbonylation9 and an application of bromoetherifica-
tion.10 A dual iron–gold catalyst has been found to be ca-
The three isomeric dihydroisoxazoles,1 of which two (1 pable of both introducing a hydroxylamine function at a
and 2, Figure 1) form the subject of this paper, are signif- propargylic alcohol site by hydroxy displacement and of
icant chemical entities for a number of reasons. Firstly, inducing subsequent cyclisation to similar 4,5-dihy-
some display biological activities in their own right2 and droisoxazoles.11 In related studies, cationic gold(I) com-
secondly they can act as precursors to heteroaromatic plexes have been shown capable of causing cyclisations of
isoxazoles.1 However, perhaps equally important roles are O-allenylmethyl hydroxylamines to give such heterocy-
offered by their decomposition chemistry: for example, cles in a stereocontrolled fashion,12 which can also be ob-
relatively facile reductive cleavage of the weak N–O bond tained starting with enones by sequential Michael addition
of 4,5-dihydroisoxazoles 1 leads to either b-hydroxy ke- of thiophenol, oximation, S-methylation, and internal dis-
tones (aldols) or g-amino alcohols, depending upon the placement of sulfur.13 Examples of spiro-substituted de-
exact conditions used.1,3 Clearly, this is a most useful ad- rivatives have been prepared by thermal rearrangements
ditional feature associated with these heterocycles as it of- of various N-nitroso-4,5-dihydropyrazoles.14
fers completely different tactics for the synthesis of these
valuable compound types.
Isomeric 2,5-dihydroisoxazoles 2 can likewise be ob-
tained usually in decent yields from intramolecular cycli-
sations of O-propargylic hydroxylamines, given the use of
N
NR4
a suitable protecting group (Boc) on nitrogen, by employ-
ing a mixed Au/Ag catalyst system15 as well as by 5-endo-
dig iodocyclisations16 of similar precursors, generally
protected at nitrogen by a sulfonyl group.17 This type of
heterocycle can also be obtained in a so far limited way by
selenoetherifications of O-allyl oximes followed by boro-
O
O
R3
R3
R1
R1
R2
R2
1
2
Figure 1
hydride reductions of the resulting iminium species.18
A
particularly good way to make examples of the third
member of this group, the 2,3-dihydroisoxazoles, is by
zinc(II)-catalysed 5-endo-dig cyclisations of the corre-
sponding acetylenic hydroxylamines;19 such heterocycles
are useful as precursors of both b-amino ketones and g-
amino alcohols.20
In terms of their synthesis, there are a few classical meth-
ods for the elaboration of these structural types,1 one of
the most useful being the synthesis of 4,5-dihydroisox-
azoles 1 using [1,3]-dipolar cycloadditions between nitrile
oxides and alkenes.4,5 Well-established protocols are
available for carrying out this methodology, and current
interest is very much centred on the development of asym-
It was against this background that we speculated that the
exceptionally efficient furan synthesis that we have re-
cently developed could perhaps also be used to form these
useful types of heterocycles in a very clean and simple
manner.21 This methodology (Scheme 1) employs as the
catalyst silver(I) nitrate adsorbed on silica gel, a material
SYNLETT 2010, No. 4, pp 0628–0632
0
1.
0
3.
2
0
1
0
Advanced online publication: 02.02.2010
DOI: 10.1055/s-0029-1219365; Art ID: D31809ST
© Georg Thieme Verlag Stuttgart · New York

LETTER
Approaches to 4,5-Dihydroisoxazoles and 2,5-Dihydroisoxazoles
629
more usually associated with use as a stationary phase for sure to aqueous methylamine in diethyl ether and the de-
alkene isomer separation. The precursor 3-alkyne-1,2-di- sired products 12 isolated by cooling the resulting mixture
ols 3 are converted into the furans 4 in essentially quanti- to 0 °C, filtering off the precipitated phthalic diamide and
tative yields under virtually neutral conditions and at solvent evaporation.
ambient temperature in a matter of a few hours. The meth-
od is also amenable to adaption to flow methodology, and
the catalyst, being heterogeneous, can readily be recov-
O
O
N
10
HO
ered and recycled. However, there was certainly an ele-
ment of doubt in this present idea as Dalla and Pale have
shown that hydroxy groups positioned adjacent to an
alkyne group greatly facilitate the binding of silver(I) ions
to the acetylenic function.22
N
O
OH
O
Ph3P, DIAD
THF, 0 °C
16 h, 69–89%
O
R1
R1
20 °C
R2
R2
9
11
40% aq MeNH2 (2.1 equiv)
Et2O, 20 °C, 4 h
90–98%
R2
HO
R1
R2
R3
i
+ H2O
R1
R3
O
OH
NH2
O
4
3
R1
Scheme 1 Reagents and conditions: i) 10% w/w AgNO3–SiO2 (10
R2
mol%), CH2Cl2, 20 °C, ca. 3 h, >97%.
12
Scheme 3
Of course, this is exactly the set up in the case of the furan
precursors 3, but would not be the case in the projected
precursors 5 and 7 which we hoped could be converted
into the corresponding dihydroisoxazole isomers 6 and 8
(Scheme 2) in much the same manner. Herein, we report
that both types of cyclisations are indeed viable but, in the
latter case, only with a limited range of nitrogen protect-
ing groups.
The unprotected O-propargylic hydroxylamines 12 turned
out to be relatively sensitive but, fortunately, were isolat-
ed in a sufficiently pure state as to be suitable for attempt-
ing the key step in this sequence. We were delighted to
find that 10% w/w silver nitrate–silica gel was indeed ca-
pable of catalysing their conversion into the correspond-
ing 4,5-dihydroisoxazoles 6 under very mild conditions,
despite the absence of a propargylic hydroxy group as dis-
cussed above. After some optimisation, we found the best
conditions for most substrates involved simply stirring a
suspension of 0.05 equivalents of the catalyst with the pre-
cursor 12 in dry dichloromethane for fifteen minutes. Fil-
tration through a plug of either Celite or silica gel,
washing the solid with fresh dichloromethane and evapo-
ration of the combined filtrates left the essentially pure di-
hydroisoxazoles 6 in excellent yields (Table 1).
NHR3
NH2
N
NR3
O
O
O
O
?
?
R2
R1
R2
R1
R1
R1
R2
R2
5
6
7
8
Scheme 2
The synthesis of suitable precursors proved to be relative-
ly straightforward, but did stimulate the development of a
new, chromatography-free method for working up a Mit-
sunobu reaction.23 A series of secondary propargylic alco-
hols 9, generated by condensations of aldehydes with
lithio acetylides, were subjected to a Mitsunobu displace-
ment with N-hydroxyphthalimide 10 (Scheme 3).24 These
all proceeded to give respectable yields of the expected
phthalimides 11 without the need for conventional col-
umn chromatography; for a full description of this meth-
od, see reference 23 and for definitions of the nature of the
R1 and R2 substituents, see Table 1 below. The only ex-
ception was the diphenyl example [11; R1 = R2 = Ph], in
which case the product was exceptionally sensitive to
elimination and decomposition, presumably by reason of
the facile generation of a highly stabilised secondary ben-
zylic and propargylic carbenium ion when exposed to any
type of acid. Only when the Mitsunobu reagents were add-
ed sequentially without any intermediate periods of
stirring23 could a 41% yield be realised. The phthalimide
group was then removed from all such products by expo-
The only detectable impurities were very small traces of
the corresponding isoxazole oxidation products, the
amount of which increased if more silver nitrate was used.
Under the optimised conditions, levels of these oxidation
products were usually around 2–3%. Subsequent ongoing
studies have indicated that such byproducts may well not
arise by simple oxidation of the dihydroisoxazoles 6.25
Notable features of this chemistry are that both benzyl and
especially silyl protecting groups survive unmolested (en-
tries 3 and 4) and that a highly hindered example (entry 6)
works equally well, as did the diphenyl derivative (entry
7), which was more difficult to prepare (see above). Ex-
amples of terminal alkynes (entries 8 and 9) proved slow-
er to react but, by using twice as much catalyst, equivalent
yields were obtained after reaction times of one hour. A fi-
nal example (entry 10) demonstrated, perhaps surprising-
ly, that electron-poor alkynes react just as easily.
We presume that such cyclisations proceed via the corre-
sponding enamines (2,5-dihydroisoxazoles), although at-
Synlett 2010, No. 4, 628–632 © Thieme Stuttgart · New York

630
D. W. Knight et al.
LETTER
to 0.1 equivalents of 10% w/w silver nitrate–silica gel
resulted in a relatively slow conversion but into the 4,5-
dihydroisoxazole 6a (Scheme 5)! By contrast, the corre-
sponding phenyl derivative 13b was converted very effi-
ciently into the expected 2,5-dihydro-isomer 14. The
related 4-nitrophenylsulfonyl (nosyl) derivative 13c, also
having two alkyl substituents, was similarly but more rap-
idly converted, with loss of the sulfonyl group, into the
same 4,5-dihydroisoxazole 6a. These surprisingly facile
hydrolyses suggest that there may be more than meets the
eye to these types of cyclisation, especially in view of the
very contrasting result obtained from the phenyl-substi-
tuted derivative 13b. Some related and also unexplained
limitations have been observed in related gold-catalysed
cyclisations involving sulfonamides.15
Table 1 Synthesis of 4,5-Dihydroisoxazoles 6 from Hydroxyl-
amines 12
NH2
N
O
10% w/w AgNO3–SiO2 (0.05 equiv)
CH2Cl2, 20 °C, 0.25 h
O
R2
R1
R1
6
R2
12
Entry
12
R1
R2
Yield (%)a
1
2
a
b
c
d
e
f
i-Bu
i-Bu
i-Bu
i-Bu
i-Bu
i-Bu
Ph
Bu
96
90
92
91
91
96
89
96b
96b
88
Ph
3
CH2OBn
4
(CH2)2OTBS
5
4-MeOC6H4
NHTs
N
O
O
6
t-Bu
Bu
10% w/w AgNO3–SiO2 (0.1 equiv)
CH2Cl2, 20 °C, 4 h
81%
7
g
h
i
Ph
Bu
Ph
Bu
8
i-Bu
Ph
H
13a
NHTs
6a
9
H
Ts
N
O
O
O
10c
j
i-Bu
4-O2NC6H4
Ph
10% w/w AgNO3–SiO2 (0.1 equiv)
CH2Cl2, 20 °C, 4 h
97%
a Overall yields from the phthalimides 11.
b Terminal alkynes required exposure to 0.1 equiv of Ag(I) for 1 h for
complete reaction.
13b
14
c The starting material was obtained by a Sonogashira coupling be-
tween the precursor alkyne in entry 8 and 4-iodonitrobenzene.
NHNs
N
O
Bu
10% w/w AgNO3–SiO2 (0.1 equiv)
1
tempts to observe such species by H NMR proved
CH2Cl2, 20 °C, 1 h
89%
fruitless. We therefore turned to a study of how to make
such isomeric heterocycles by adding a suitable protecting
group to the amine nucleophile in the hydroxylamines 12.
In view of the very mild nature of the cyclisations, it
seemed highly likely that this would be an easily achieved
aim but, as so often happens, this turned out to be an erro-
neous assumption.
13c
6a
Scheme 5
This spelled the end to our work on sulfonamide deriva-
tives as we turned to various N-acyl analogues with a view
to achieving more predictable and general success in such
cyclisations leading to the 2,5-dihydro-isomers; we chose
to start with carbamate-like derivatives (Table 2). Once
again, the synthesis of these suffered from overreaction
with the acylating reagent, meaning that all of the initial
products were contaminated with the corresponding bis-
acyl analogues. This was, however, easily solved in all
cases except one by a simple ambient temperature hydro-
lysis of one of two such groups.
We chose to use sulfonamide derivatives first of all as
these, especially the N-tosyl derivatives, were expected to
be very stable. Three examples 13a–c were prepared, but
under carefully controlled conditions in order to avoid bis-
sulfonylation of the rather nucleophilic nitrogen. This was
achieved in good yields (Scheme 4) using strictly one
equivalent of the sulfonyl chloride and mixing the reac-
tants at low temperature prior to warming to ambient.
NH2
NHSO2R2
Our first observation was that the ‘methoxycarbonyl’ de-
rivative 15a seemed to be a rather unstable moiety, the
ONHCO2Me group, reflected both by the very poor yield
obtained in making it and also by its unexpected and com-
plete decomposition when exposed to silver nitrate–silica
gel. By contrast, the corresponding ‘N-Boc’ derivative
15b was much more stable but unfortunately resistant to
silver-catalysed cyclisation. Hence, such ‘carbamate’ de-
rivatives did not seem well suited to this type of cyclisa-
tion, in contrast to the related gold-catalysed chemistry.15
Similarly, the N-trifluoromethyl derivative 15c also failed
O
O
R2SO2Cl, py
DMAP (cat.)
CH2Cl2, –78 °C
to 0 °C, 16 h
i-Bu
i-Bu
R1
R1
13a R1 = Bu; R2 = p-MeC6H4
13b R1 = Ph; R2 = p-MeC6H4
13c R1 = Bu; R2 = p-O2NC6H4
12a R1 = Bu
12b R1 = Ph
~ 85%
Scheme 4
In view of the usual stability of tosylamides, we were very
surprised to find that exposure of the tosyl derivative 13a
Synlett 2010, No. 4, 628–632 © Thieme Stuttgart · New York

LETTER
Approaches to 4,5-Dihydroisoxazoles and 2,5-Dihydroisoxazoles
631
ly suitable; it is therefore very likely that a range of other
such acyl derivatives could also be used successfully. Fur-
thermore, in the last two examples 16d,e, there was no in-
terference from the pendant silyloxy or hydroxy groups
which, potentially, could undergo competing 5-endo-dig
cyclisations. Overall, it was indeed fortunate that these cy-
clisations were so clean as the products 16 are quite sensi-
tive to chromatographic purification, which can result in
considerable losses. In the present cases, this was not nec-
essary.
Table 2 Synthesis of N-Acyl Derivatives 15
NHCOR2
NH2
O
O
i) R2COCl (1.5 equiv), py (1.5 equiv)
DMAP (cat.), CH2Cl2, 0–20 °C, 4 h
R1
R1
ii) aq 2 M NaOH, MeOH
20 °C, 4 h
12
15
Entry
12
a
R1
Bu
Bu
Bu
Bu
Ph
H
R2
15
a
b
c
Yield over 2 steps (%)
1
2
3
4
5
6
OMe
Ot-Bu
CF3
Me
17
81
85
92
90
88
We also found it possible to conduct such cyclisations in
a flow system, which consisted of an HPLC column
packed with 10% w/w silver nitrate on silica gel. A col-
umn measuring 15 × 1 cm was capable of cyclising pre-
cursors 15d and 15e (Table 3; entries 1 and 2) completely
to the expected products 16a,b at a flow rate of 0.3 mmol
per minute using a 0.1 M solution in dichloromethane.
Identical results were obtained using representative exam-
ples (entries 1–3) from Table 1, when returns of the corre-
sponding 4,5-dihydroisoxazoles 6 were also essentially
quantitative. Hence this could represent a viable tactic for
scale-up of these reactions.
a
a
a
d
e
b
h
Me
Me
f
to undergo cyclisation, perhaps because of the reduced
nucleophilicity of the nitrogen. The acetyl derivatives
15d–f also turned out to be relatively sensitive. For exam-
ple, hindered rotamer interconversion was apparent from
inspection of their room-temperature proton spectra in
deuteriochloroform but when attempts were made to se-
cure more informative spectra in warm DMSO-d6, exten-
Finally, as outlined above, it has been reported that if
hydrochloride salts of representative O-propargylic
hydroxylamines 12 (Scheme 3) are heated with potassium
carbonate in methanol or neutralised with sodium hydrox-
sive decomposition set in. However, despite this thermal ide, then cyclisation to give 4,5-dihydroisoxazoles 6 oc-
curs smoothly.8 In the present work, we observed that
simply heating examples of the ‘free’ hydroxylamines 12
in methanol failed to produce any of the heterocycles 6.
The reasons behind this remain a mystery.
instability, cyclisations at ambient temperature using the
same silver catalyst gave excellent returns of the hoped-
for 2,5-dihydroisoxazoles 16. These and related results
are collected in Table 3.
In summary, this approach represents, overall, a relatively
brief and certainly highly efficient route to both 4,5- and
2,5-dihydroisoxazoles. The methodology naturally is
regiospecific and hence delivers single isomers of these
sensitive heterocycles. The main limitation in synthesis-
ing the 2,5-isomers by this method is that the amino group
should, in general, be protected as an acyl derivative and
not with a sulfonyl or alkoxycarbonyl function. Given that
the initial precursors, secondary propargylic alcohols, can
be obtained as single enantiomers using a number of ap-
proaches, then this chemistry should be amenable to the
synthesis of optically pure products of both types.
Table 3 Synthesis of 2,5-Dihydroisoxazoles 16 from Acylated Hy-
droxylamines 15
COR2
NHCOR2
N
O
O
10% w/w AgNO3–SiO2 (0.1 equiv)
CH2Cl2, 20 °C, 1 h
R1
R1
15
16
Entry
15
d
e
R1
R2
16 Yield (%)
1
2
3
4
5
Bu
Me
Me
Me
Ph
a
b
c
98
97
0
Ph
f
H
Acknowledgment
We thank Syngenta and the EPSRC for financial support.
(CH2)2OTBS
(CH2)2OH
d
e
96
81
Ph
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Synlett 2010, No. 4, 628–632 © Thieme Stuttgart · New York

632
D. W. Knight et al.
LETTER
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