Rapid synthesis of substituted pyrrolines and pyrrolidines by nucleophilic ring closure at activated oximes

Article abstract of DOI:10.1039/c2ob26423d

Substituted pyrrolines are available by ring closure initiated by direct nucleophilic attack of stabilized enolates at the nitrogen of oximes activated with a leaving group, in a process which effectively out-competes the more usual Beckmann rearrangement. Subsequent reduction provides diastereoselective access to the corresponding pyrrolidines. This provides a rapid route to saturated heterocyclic systems from readily available precursors.

Full text of DOI:10.1039/c2ob26423d

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COMMUNICATION
Rapid synthesis of substituted pyrrolines and pyrrolidines by nucleophilic
ring closure at activated oximes†
Nandkishor Chandan, Amber L. Thompson and Mark G. Moloney*
Received 20th July 2012, Accepted 15th August 2012
DOI: 10.1039/c2ob26423d
Substituted pyrrolines are available by ring closure initiated
by direct nucleophilic attack of stabilized enolates at the
nitrogen of oximes activated with a leaving group, in a
process which effectively out-competes the more usual
Beckmann rearrangement. Subsequent reduction provides
diastereoselective access to the corresponding pyrrolidines.
This provides a rapid route to saturated heterocyclic systems
from readily available precursors.
access substituted pyrrolines and pyrrolidines; this approach was
particularly attractive, since it neatly avoided the difficult
electron-deficient enamine reduction in the final step of our
previously published approaches.27–29 Noteworthy is that this
approach is distinct from the alternative better known rearrange-
ment-cyclisation pathway involving intramolecular interception
of the intermediate nitrilium ion generated under the convention-
al Beckmann reaction.25 Of interest is that intramolecular cycli-
sations by nucleophilic substitutions leading to tetrahydrofuran
derivatives30 and pyrroline synthesis by zirconocene-mediated
cyclisation of unsaturated oximes31 have been recently reported.
The required substrates 3a–r for this work were directly pre-
pared by conjugate addition of malonates 1 with α,β-unsaturated
aldehydes and ketones 2 using mild base conditions in very
good yield (Scheme 1 and Table 1); substrates 3b–e, g–h were
used in racemic form, and the remaining substrates were used as
diastereomeric mixtures. These adducts were readily converted
to the corresponding oximes 4a–r (as syn-/anti-mixtures) under
standard conditions, again in high yield. These oximes could be
easily O-arylated using NaH/ethanol/2,4-dinitrofluorobenzene to
give oxime ethers 5 or tosylated using triethylamine/tosyl
chloride to give tosyl oximes 6 again as inseparable syn-/anti-
mixtures (Scheme 1 and Table 1). However, the latter approach
generally proved to be simpler to execute, requiring only room
temperature stirring with the indicated reagents, and giving pro-
ducts which were easily purified, since the possibility of trans-
esterification of substrates 4 with ethanol, which proved
problematic with the former approach, was avoided. If these
compounds 5 and 6 were treated with sodium hydride in dry
THF at room temperature, rapid ring closure followed, giving the
Δ4,5-pyrroline products 7a–h, k–m, generally in excellent yield
(Table 1); again, the tosyl esters 6a–h, k–m proved to be the
better intermediate in this process, since the resulting products
were more easily isolated by chromatography. Bulky esters were
tolerated, giving consistently good yields (e.g. compounds 7c,f,
k, Table 1). Noteworthy is that the structure of these products
Functionalised pyrrolidines continue to attract interest, not only
for their synthetic challenge,1,2 but also their value in synthetic
chemistry3 as well as their diverse biological properties.4
Although the manipulation of pre-existing nitrogen heterocycles
has been a very successful strategy to access functionalised
pyrrolidin(on)es,5–7 effective metal-mediated8–13 and organo-
catalytic14–16 ring closing strategies have been developed in
recent years. However, it is very unusual to see amongst these
cyclisation strategies, sequences which proceed via closure on
nitrogen (for a discussion of the utilisation of N–X containing
heterocycles in synthesis, see Minakaka17). Although it had been
reported some time ago that ring closure onto a nitrogen bearing
a good leaving group provided rapid and direct access to
pyrrolidines,18–22 this strategy has not been widely adopted, no
doubt because of the potential for competitive Beckmann- and
Neber-type rearrangement processes.23–25 However, Narasaka
and Nakamura reported that a ring-closure reaction proceeding
by SN2-type process onto the sp2 hybridised nitrogen atom of
oxime derivatives by the attack of a carbon nucleophile of an
aromatic moiety should be energically feasible by ab initio
calculation,22 and went on to demonstrate that such a process
was in fact high yielding. The identity of the cyclisation products
was, however, found to be solvent and reagent dependant.22 The
viability of this approach was further recently demonstrated by
Black,26 who elegantly exploited a base-mediated ring closure of
activated indole ketoximes to access pyrazoles. We report here
our investigation to develop the utility of this process further to
1
was readily confirmed from the H chemical shift values for the
CH2CH2 couple, typically around 2.4 and 2.6 ppm, which is
fully consistent with the proposed structure, and quite different
from the products which might arise from more conventional
Beckman rearrangement-intramolecular trapping. Of particular
interest is that in the case of the adduct derived from acrolein
(2, R2 = R3 = H), arylether 5g could not be isolated, and
The Department of Chemistry, Chemistry Research Laboratory, The
University of Oxford, 12 Mansfield Road, Oxford, OX1 3TA, UK.
E-mail: mark.moloney@chem.ox.ac.uk
†Electronic supplementary information (ESI) available. CCDC
891076–891078. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c2ob26423d
This journal is © The Royal Society of Chemistry 2012
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Scheme 1
Table 1 Yields of compounds prepared as indicated in Schemes 1 and 2
Yield (%)
R1
Compound
R2
R3
Adduct 3
Oxime 4
Oxime ether 5
Tosyl oxime 6
Pyrroline 7
Pyrrolidine 9
a
b
c
d
e
C(O)OEt
H
H
H
H
H
H
H
Ph
Ph
Ph
Ph
H
Me
Me
Me
Me
Me
Me
H
Ph
Ph
Ph
Ph
Me
Me
Ph
Ph
Me
Ph
90
91
90
89
84
98
85
86
81
88
83
89
66
92
92
80
93
89
86
93
93
90
92
87
90
97
85
86
90
92
93
88
95
98
58
66
68
—
—
—
—
52
—
75
—
—
—
37
—
—
72
65
72
67
94
92
97
86
78
84
32
54
—
—
40
77
83
42
—
—
—
—
54d; 80e
—d; 65e
—d; 69e
—d; 63e
—d; 56e
—d; 70e
45d; —e
78d; 63e
(8i, 67)c
(8j, 81)c
—d; 69e
43d; 65e
—d; 73d,e
(8n, 72)c
(8o, 78)c
—
89a (93b)
C(O)OMe
C(O)Ot–Bu
C(O)OBn
C(O)OAllyl
C(O)OMenthyl
C(O)OEt
C(O)OEt
C(O)OMe
C(O)OBn
C(O)Ot–Bu
C(O)NHBn
C(O)NHCH(Me)Ph
C(O)NHBn
C(O)NHCH(Me)Ph
NO2
91b
92b
88b
92b
f
90a
g
h
I
j
k
l
m
n
o
p
q
r
74a
(11, 96a) (96b)
—
—
94b
97b
90b
—
H
Ph
Ph
H
Ph
H
—
—
—
—
CN
Ph2CvN
—
—
Me
a Reduction by H-Cube. b Reduction by sodium cyanoborohydride. c Product of Beckmann rearrangement. d Via phenyl ether cyclisation. e Via tosyl
oxime cyclisation.
spontaneous cyclisation to pyrroline 7g was observed, in good
yield (45%); however, synthesis of the corresponding tosyloxime
in this case was low yielding (32%) and accompanied by the for-
mation of the nitrile elimination compound 10 (56%) as the
major product. For the more highly substituted systems derived
from the aryl-substituted chalcone substrates (2, R2 = R3 = Ph),
an efficient competing Beckmann rearrangement emerged,
giving amides 8i and 8j in 67 and 81% yield respectively when
oximes 6i,j were treated with tosyl chloride/triethylamine. None-
theless, it was found in this case that the more highly substituted
system 4k could be converted to tosyl oxime 6k in 40% yield
along with pyrroline 7k in 24% yield; if the pure tosyl oxime 6k
was treated with sodium hydride in THF, pyrroline 7k was
obtained in 69% yield. Reduction (H-Cube hydrogenation
followed by hydrogenolysis, giving amino acid 11 in excellent
yield (96%).
This sequence could be similarly applied to malonamides;
thus, synthesis of the half ester/amide analogues 3l–o proceeded
efficiently (Scheme 1, Table 1), although this reaction was com-
plicated by double Michael addition leading to products of type
12 which made for difficult purification. Application of the
above activation–cyclisation sequence led to the corresponding
oximes 4l–o generally in good yield (Table 1) and conversion to
tosyl oximes 6l,m followed by cyclisation as before gave access
to pyrrolines 7l,m. Of interest is that reaction of oxime 6l with
2,4-dinitrofluorobenzene gave not only oxime ether 7l in low
yield (43%) but also cyclised pyrroline 9l in 23% yield. Cyclisa-
tion of 6l,m was also found to be possible using DBU in dry
dichloromethane (53, 57% yield respectively), acetonitrile (61,
49% yield respectively), or anhydrous Cs2CO3 in dry dichloro-
methane (46, 38% yield respectively) and these conditions are
preferable since although sodium hydride gave acceptable yields
(Pd/C)29,32 or NaBH3CN (2 M HCl in MeOH)33) of the Δ4,5
-
pyrroline products 7a–h, k–m gave the substituted pyrrolidines
9a–g, k–m. Noteworthy is that in the case of 7h, formation of
9h under catalytic hydrogenation conditions was immediately
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Fig. 1 NOE correlations.
Scheme 2
at small scale, reaction yields dropped significantly on scale-up.
Pyrrolines 7l,m were treated with NaBH3CN and gave the
reduced product as 2,2,5-trisubstituted pyrrolidines 9l,m in
excellent yields. However, when this sequence was applied to
the diphenyl substituted derivatives 4n,o, although the activated
tosyl oximes 6n and 5o were readily obtained, only the products
of Beckmann fragmentation 8n,o were obtained upon subsequent
base treatment. The structure of 8o was unequivocally estab-
lished by single crystal X-ray analysis.34
The establishment of the diastereomeric purity of the products
9a–g, k–m, and the assignment of their stereochemistry, was
difficult, due to 5-membered ring conformational, rotameric and
diastereotopic behaviour, which could not be simplified even
with high field NMR spectroscopic analysis; this was especially
true of amide products 9l,m, and we have frequently observed
such behaviour in related pyrrolidine systems.28,32 With the
exception of 9e which was isolated as a single compound, all
products were obtained as inseparable mixtures of diastereomers,
and in the case of 9d, nOe analysis at 700 MHz established the
major one to be that shown in Fig. 1, consistent with hydride
entry to the less hindered face. Substrates 7f,m,o, all with chiral
ester or amide appendages, which were prepared in order to
either bias the diastereoselectivity of the process, or at least
facilitate separation of diastereomeric products once formed, did
not permit the isolation of pure products. However, for products
9h,k, only a single diastereomer was obtained, the cis-3,5-stereo-
chemistry of which was established by nOe analysis (Fig. 1),
and corresponding to Si facial selectivity for the entry of hydride
or hydrogen. For 9k, the NOE analysis did not permit assign-
ment of the carbon bearing the two esters.
Successful cyclisations could be extended to unsymmetrical
Weinreb-modified malonamides (Scheme 2); thus, 13a was
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Scheme 3
reacted with methyl vinyl ketone in the presence of base
(anhydrous K2CO3 in dry dichloromethane at rt) and Michael
adduct 14a was obtained in a very good yield. This was readily
converted into the corresponding oxime 14b, and thence to
mesyl oxime 14c, also in a very good yield (Scheme 2), both of
were inseparable as syn-/anti-mixtures. Mesyl oxime 14c was
treated with base DBU in acetonitrile at 0 °C and gave the ring
closure product pyrroline 15 in 69% yield, reduction of which with
NaBH3CN gave the pyrrolidine 16 in quantitative yield and as a
single diastereomer, but whose stereochemistry could not be
unequivocally assigned by NOE analysis. Despite this success, a
surprising outcome was found in the case of nitro, imine and
nitrile activating groups in place of the malonate component.
Although the required ketones 3p,q,r, oximes 4p,q,r and aryl ether
5p,q or tosyl oxime 6r were readily prepared in excellent yield
using the same approach outlined above (Scheme 1 and Table 1),
no cyclisation could be achieved under the standard conditions, or
variants thereof. In the formation of oxime 6q, prepared from
nitrile 13b in the standard manner (Scheme 2), cyclised product 17
was also formed, the structure of which was established by single
crystal X-ray analysis.34 This product arose by initial attack of
hydroxylamine at the nitrile centre followed by ring closure.
This sequence was also applicable to Meldrum’s acid
(Scheme 3); thus, reaction with methyl vinyl ketone gave deriva-
tive 18a, and conversion to the oxime 18b was efficient (77%
yield). The crude oxime was used directly, and treatment with
p-toluenesulfonyl chloride and base (Et3N in dry CH2Cl2) gave
not the expected tosyl oxime 18c but instead the crystalline
spirocyclic Δ4,5-pyrroline product 19 directly, identical to that
reported by Danheiser.35 The structure of this compound was
confirmed by single-crystal X-ray crystallographic analysis,34
which clearly demonstrated the fully orthogonal nature of the
two heterocyclic rings. This structural result corroborated the
NMR assignment of the ring closure pathway alluded to above.
Attempted reduction of 19 led to rapid loss of starting material,
but no product could be isolated or identified.
was examined (Scheme 3); thus, diethyl 2-methylmalonate on
reaction with methyl vinyl ketone in presence of mild base gave
Michael adduct 20a, which when treated with alcoholic KOH in
ethanol gave the decarboxylated product 20b directly in 91%
yield. This ketoester 20b was then readily converted into the
oxime 21a and thence aryl ether 21b. However, when this com-
pound was then subjected to various conditions (NaH, THF;
KH, THF; LIHMDS, THF; LDA, THF) in an attempt to obtain
the desired cyclised product pyrroline, although starting material
was consumed, no desired products could be identified. Simi-
larly, of interest was the suitability of diethyl glutaconate derived
systems (Scheme 3), in which an SN′-type process might be
expected to lead to ring closure. Diethyl glutaconate was used
for conjugate addition reaction with methyl vinyl ketone in the
presence of base, and gave adduct 22a. Adduct 22a was readily
converted into the corresponding oxime 22b using the standard
conditions and thence to the tosyl oxime 22c by treatment with
p-toluenesulfonyl chloride and base in a good yield of 76%.
However, reaction with NaH in dry THF heated under reflux for
30 min led to consumption of starting material but no trace of
the ring closure product pyrroline nor the rearranged product
could be found. These results clearly indicated the need for a
stabilised malonate or equivalent system for successful ring
closure.
That the observed cyclisation pathway leading to pyrrolines
was a delicate balance of potentially competing reaction path-
ways was further demonstrated using cyclohexenone as a sub-
strate. Application of the standard oxime-tosyl oxime-cyclisation
sequence via intermediates 23a–c (Scheme 4) gave cyclopropyl
product 24 in excellent yield (74%) arising by Beckmann-type
fragmentation of tosyl oxime 23c, rather than pyrroline 25
(although carbon–carbon double bonds at a bridgehead cannot
exist for small ring size,36,37 there are some reports of the iso-
lation of anti-Bredt rule compounds,38–42 and in particular of
anti-Bredt bicyclic imines43).
We have shown that conversion of activated oximes by direct
ring closure onto an activated oxime of pendant dicarbonyl resi-
dues leads to the efficient formation of pyrrolines, which may be
In order to probe the need for the highly stabilized malonate-
derived enolate system, synthesis of a monoactivated analogue
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Scheme 4
Oxime ethers 5 from oximes 426,52
readily reduced to the corresponding pyrrolidines. In view of the
recent development of enantioselective organocatalysed pro-
cesses providing access to the starting substrates of type 3,44 this
route is likely to become applicable to the rapid synthesis of
enantiopure functionalised pyrrolidines.
The oxime (1.0 eq.) from procedure C was dissolved in EtOH
and sodium metal (1.1 eq.) was added. The mixture was stirred
for 30 min at rt to ensure complete reaction. The reaction
mixture was cooled to 0 °C before 2,4-dinitrofluorobenzene
(1.5 eq.) was added slowly, during which the reaction colour
suddenly changes to deep yellow-orange. The reaction was
stirred till completion by TLC. After completion of the reaction,
EtOH was removed under reduced pressure and the residue was
dissolved in dilute HCl. The oxime ether was extracted by
EtOAc, washed with saturated NaHCO3, water and brine, dried
over Na2SO4 and on concentration gave the product, which was
purified by flash column chromatography (eluting with
EtOAc : petrol).
Experimental: general procedures
Michael addition (methyl vinyl ketone) giving adducts 345–51
Methyl vinyl ketone (1.2 eq.) was added dropwise to a well-
stirred mixture of malonate (1.0 eq.) and anhydrous K2CO3
(1.5 eq.) in dry CH2Cl2 and the mixture stirred for 15 h at rt or
till completion of the reaction by TLC. Water and EtOAc was
added sequentially and the organic layer was separated, washed
with 1 M HCl, water, brine and dried over Na2SO4. Concen-
tration of the organic layer gave the crude product, which was
purified by flash column chromatography (eluting with EtOAc :
petrol) to afford the Michael adduct.
Tosyl or mesyl oximes from oximes 453
To a stirred solution of oxime (1.0 eq.) from procedure C in dry
CH2Cl2 at 0 °C was treated with Et3N or pyridine (2.0 eq.) fol-
lowed by slow addition of p-toluenesulfonyl chloride or
methanesulfonyl chloride (1.2 eq.). The reaction was further
stirred for 2 h while progress was monitored by TLC, and after
completion of reaction, HCl (1 M) was added, the product was
extracted by CH2Cl2, and the extracts were washed with
saturated NaHCO3 and dried over Na2SO4. Concentration of
CH2Cl2 gave crude product which was purified by flash column
chromatography (eluting with EtOAc : petrol).
Michael addition (chalcone) giving adducts 345–51
Chalcone (1.1 eq.) in dry CH2Cl2 was added dropwise to a well-
stirred mixture of malonate (1.0 eq.) and anhydrous K2CO3
(1.5 eq.) in dry CH2Cl2 and the mixture was heated to reflux for
15 h or till completion of the reaction by TLC. H2O and EtOAc
were added sequentially; the organic layer was separated,
washed with 1 M HCl, water, brine and dried over Na2SO4. Con-
centration of the organic layer gave the crude product, which
was purified by flash column chromatography (eluting with
EtOAc : petrol) to afford the Michael adduct.
Ring closure reaction giving pyrrolines 726,52
To a solution of oxime ether or tosyl oxime (1.0 eq.) from pro-
cedure D or E in dry THF at rt, a dispersion of NaH (3.0 eq.
60% dispersion in mineral oil) was added and heated to reflux
for 30 min to 1 h and the progress of the reaction was monitored
by TLC. After completion of the reaction, saturated NH4Cl was
added and the pyrroline product was extracted with EtOAc. The
extracts were dried over Na2SO4, and concentration of reaction
mixture gave the crude product which was purified by flash
column chromatography (eluting with EtOAc : petrol).
Oximes 4 from Michael adducts 326,52
To a solution of the Michael adduct from procedure A or B (1.0
eq.) in EtOH, was added NH2OH·HCl (2.0 eq.) and Et3N
(2.5 eq.) and the reaction mixture was heated to reflux for 2 h
(7 h in case of chalcone adduct). The progress of the reaction
was monitored by TLC, and on completion, the reaction mixture
was allowed to cool to rt. EtOH was removed under reduced
pressure and the residue was dissolved in EtOAc and H2O
added. The organic layer was separated, washed with brine and
dried over Na2SO4. Concentration of the organic layer gave the
crude product, which was purified by flash column chromato-
graphy (eluting with EtOAc : petrol) to afford the oxime product.
Reduction giving pyrrolidines 9 (H-Cube hydrogenation)
The solution of the cyclised product pyrroline (1.0 eq.) in EtOH
was passed through the capillary of a H-Cube hydrogenation
plant (0.5 mL min−1) at 50 °C, using 10% Pd/C as catalyst.
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EtOH was removed under reduced pressure and the crude
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with EtOAc : petrol).
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Reduction giving pyrrolidines 9 (NaBH3CN)33
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23 The Beckmann and Related Reaction, ed. D. Craig, Pergamon Press,
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27 N. Chandan and M. G. Moloney, Org. Biomol. Chem., 2008, 6,
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28 S. R. Hussaini and M. G. Moloney, Org. Biomol. Chem., 2003, 1,
1838–1841.
To a solution of pyrroline product (1.0 eq.) in MeOH was added
2 M HCl in MeOH, followed by NaBH3CN (3.0 eq.) and
additional 2 M HCl in MeOH was added periodically to main-
tain the acidic pH of the reaction mixture. The progress of the
reaction was monitored by TLC, and after 30–60 min, MeOH
was removed and the residue extracted with dilute NaHCO3 and
EtOAc, washed with water, brine, dried over Na2SO4 and the
organic layer concentrated and purified by flash column
chromatography (eluting with EtOAc : petrol).
29 S. R. Hussaini and M. G. Moloney, Synth. Commun., 2005, 35,
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Experimental procedures: details
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31 M. Kitamura, Y. Shintaku, D. Kido and T. Okauchi, Tetrahedron Lett.,
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Full details of experimental procedures, compound characteriz-
ation and spectra are included in ESI.†
32 S. R. Hussaini and M. G. Moloney, Tetrahedron Lett., 2004, 45,
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33 J. S. Petersen, G. Fels and H. Rapoport, J. Am. Chem. Soc., 1984, 106,
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34 Crystallographic data (excluding structure factors) have been deposited
with the Cambridge Crystallographic Data Centre (CCDC
891076–891078).
Acknowledgements
NC gratefully acknowledges receipt of a National Overseas
Scholarship from the Indian Ministry of Social Justice and
Empowerment (GF6-352746).
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