Synthesis of novel N-cyclopentenyl-lactams using the Aubé reaction

Article abstract of DOI:10.3762/bjoc.11.119

A novel and convenient method utilizing the Aubé reaction to access a new class of compounds that are similar to carbocyclic nucleosides is reported. The azido alcohol derived from Vince lactam undergoes the Aubé reaction with various cyclic ketones to gi

Full text of DOI:10.3762/bjoc.11.119

Synthesis of novel N-cyclopentenyl-lactams using the
Aubé reaction
Madhuri V. Shinde1, Rohini S. Ople1, Ekta Sangtani2, Rajesh Gonnade2
and D. Srinivasa Reddy*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2015, 11, 1060–1067.
1Division of Organic Chemistry, CSIR-National Chemical Laboratory,
Dr. Homi Bhabha Road, Pune, 411008, India and 2Center for Material
Characterization, CSIR-National Chemical Laboratory, Dr. Homi
Bhabha Road, Pune, 411008, India
doi:10.3762/bjoc.11.119
Received: 06 April 2015
Accepted: 03 June 2015
Published: 23 June 2015
Email:
D. Srinivasa Reddy* - ds.reddy@ncl.res.in
Associate Editor: R. Sarpong
* Corresponding author
© 2015 Shinde et al; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
Aubé reaction; biological activity; carbocyclic nucleosides;
cyclopentenylated lactams; cyclopentylated lactams
Abstract
A novel and convenient method utilizing the Aubé reaction to access a new class of compounds that are similar to carbocyclic
nucleosides is reported. The azido alcohol derived from Vince lactam undergoes the Aubé reaction with various cyclic ketones to
give cyclopentenyl-substituted lactams. Upon dihydroxylation, this affords the N-cyclopentenyl-lactam compounds in racemic
form. Given the numerous uses of nucleosides and related compounds, we were interested in the synthesis of carbocylic nucleoside
mimics. The attempts and results are described herein.
Introduction
One popular method for the synthesis of N-substituted lactams This helps the azide addition in an intramolecular fashion,
is the Aubé reaction [1-4]. Over the last few decades, this reac- followed by ring expansion (Scheme 1) [1-4].
tion has gained popularity and resulted in the production of a
variety of chemical structures and generated new techniques in Recently, we applied this reaction for the synthesis of
chemistry [5-10]. Additionally, it was applied to the synthesis sugar–lactam conjugates starting from an azido-alcohol
of different natural products of biological importance [11-15]. embedded in sugar derivatives and cyclic ketones [16]. In
In the Aubé reaction, an intermolecular reaction takes place continuation of this work (and also to expand the potential of
between an azido alcohol and a ketone to provide lactams this chemistry), a new class of compounds were prepared.
through an in situ-generated hemiacetal as a temporary tether. These cyclopentenoid–lactams, which look like carbocyclic
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Scheme 1: The Aubé reaction and its selected applications.
nucleosides, were prepared using the Aubé reaction where it Results and Discussion
was planned to use an azido alcohol embedded in a cyclopen- To our knowledge, there are no reports of the synthesis of
tenoid system. It is well established in the literature that carbo- cyclopentenoid lactams, likely due to the lack of suitable
cyclic nucleosides and related compounds are important to methods to synthesize these compounds. This gives the oppor-
pharmaceuticals and these compounds have been the focus of tunity to explore the Aubé reaction, which evolves an entirely
many studies and a number of reported syntheses [17-30]. Some new family of compounds by using simple chemistry. In this
selected compounds and their important associated activities are paper, efforts towards target molecules and the details of new
highlighted in Figure 1.
chemistry developed during this process are discussed. To begin
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Figure 1: Selective carbocyclic nucleoside analogues from the literature and our initial designs.
with, the azido alcohol 3 required for the present work was 2D NMR analysis), as shown in Scheme 2. These two experi-
envisaged from Vince lactam (±)-1. The known amino alcohol ments proved that the major azido alcohol is the desired 1,3-
[31] (prepared from Vince lactam by following literature pro- substituted cyclopentenyl derivative (±)-3, with the minor being
cedures) was treated with triflyl azide or imidazole sulfonyl (±)-3’.
azide [32,33] using standard conditions to furnish the desired
azido alcohol (±)-3 as an 8:1 regioisomeric mixture. However, As we could not separate the isomers of azido alcohol (±)-3
we could neither purify nor isolate compound (±)-3 in pure [36], we decided to proceed with the mixture. The Aubé reac-
form, which was always accompanied by an undesired isomeric tion was performed under similar conditions as the previous
compound. To confirm whether it is a diastereomer or regio- work [16]. Cyclohexanone was reacted with azido alcohol (±)-3
isomer, we performed two derivatization reactions. Firstly, in dichloromethane with varying amounts of BF3·OEt2 and the
reduction of the azido functionality in compound (±)-3 to the results are summarized in Scheme 3 and Table 1. We were able
corresponding amine followed by Boc protection resulted in to achieve a moderate yield of cyclic lactam (±)-6 with excess
two compounds. Fortunately, these were separable by silica gel ketone, Lewis acid and longer reaction times. While this
column chromatography to give compound (±)-4 and (±)-4’ in research was underway, Aubé et al. proposed that an unfavor-
good yields. The major compound (±)-4 was known in the able catalyst–product interaction results in product inhibition,
literature and the data were compared with that reported [34]. thus deterring the progress of the reaction under catalytic condi-
The structure of the minor diastereomer (±)-4’ was assigned tions, which leads to the use of excess Lewis acid in super stoi-
based on 2D NMR correlations. Secondly, the azido alcohol chiometric amounts [37,38]. They reported that the use of hexa-
(±)-3 was subjected to a click reaction with phenylacetylene fluoroisopropanol (HFIP) can reduce the Lewis acid require-
under standard conditions and the resulting two compounds, ment. Accordingly, by varying a few conditions, we were able
(±)-5 and (±)-5’, were again separable by column chromatog- to optimize the reaction as summarized in Table 1. The opti-
raphy and subsequently characterized. The NMR analysis mized conditions are: 2.5 equiv of ketone, 4–5 equiv of HFIP
and comparison with literature data [35] confirmed the struc- and one equiv of BF3·OEt2 followed by hydrolysis with aq.
tures of the major (±)-5 and minor isomer (±)-5’ (based on KOH.
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Scheme 2: Top: synthesis of azido alcohol derivative 3 and bottom: structural elucidation of the minor diastereomer.
Scheme 3: Aubé reaction of cylopentenyl azido alcohol 3 with cyclohexanone.
Table 1: Optimization of Aubé reaction conditions.
With the optimized conditions in hand, we then tested the scope
of the reaction using different cyclic ketones. Thus, the reaction
of the azido alcohol (±)-3 with cyclobutanone and cyclopen-
tanone afforded the corresponding cyclopentene-substituted
lactams (±)-7 and (±)-8 in good yields. The 2D NMR analysis
and HMBC correlations for one of the products (±)-8 are shown
in Scheme 4 and confirmed the 1,2-substitution in the cyclopen-
tene ring of the product. Similarly, lactam (±)-9 was prepared
from 4-pyranone with (±)-3 and obtained in moderate yield.
BF3·Et2O
Solvent
Reaction time
Yield
0.2 equiv
1.5 equiv
2.5 equiv
0.2 equiv
0.5 equiv
1.0 equiv
CH2Cl2
CH2Cl2
CH2Cl2
HFIP
15 h
1 d
≈10%
20%
1 d
60%
15 h
7 h
≈10%
≈10%
72%
HFIP
HFIP
2 h
To further broaden the scope of this method, the reaction
Upon careful examination of the spectral data and on the basis was also performed with substituted cyclic ketones to give
of the recent results from the Aubé group [37,38], the initially corresponding lactam products in only moderate yields.
anticipated 1,3-substituted structure was revised to 1,2-substi- Unfortunately, in most cases, the desired lactams could not be
tuted cyclopentenoid lactams (±)-6, as drawn in Scheme 3.
isolated in pure form and were contaminated with cleaved sec-
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ring-expansion reaction of hydroxy azides with cyclic ketones.
This involves the Lewis acid-promoted formation of an
N-diazonium intermediate, which undergoes rearrangement to
give an iminium ether intermediate that can be hydrolyzed by a
base [8,41]. Along these lines we have proposed the mecha-
nism below based on products we isolated from the Aubé reac-
tion (Scheme 5).
Scheme 5: Proposed mechanism for the Aubé reaction for azido alco-
hols embedded in a cyclopentene system.
Scheme 4: Substrate scope of the reaction: preparation of cyclopen-
tene-substituted lactams and key NMR correlations (HMBC) of com-
pound (±)-8.
After having access to cyclopentenyl-substituted lactams, the
ondary amides, diastereomers, regioisomers, etc. Finally, the double bond present in these products was dihydroxylated under
reaction of the constrained symmetrical ketone 6,6-dimethyl-bi- standard conditions using catalytic osmium tetroxide and NMO
cyclo[3.1.0]hexan-3-one [39] with (±)-3 resulted in compound (Scheme 6) [42-44].
(±)-10 in 40% yield. Since all synthesized compounds contain a
1,2-substituted cyclopentene ring in their structures, it is The corresponding triols (±)-11 to (±)-15 were readily formed
suggested that the starting 1,3-azido alcohol 3 undergoes a 1,3- and obtained as single diastereoisomers, indicating that the
shift to give the corresponding 1,2-azido alcohol under the reaction proceeded in a highly selective manner. The triols
Lewis acid-mediated reaction conditions.
(±)-14 and (±)-15 also resulted from corresponding substituted
lactams. The observed selectivity of the dihydroxylation can be
We envisage that the 1,2-substituted cyclopentenoid lactams explained by the reagent approach from the opposite side of
obtained from the Aubé reaction result from the 1,3-allylic both substituents present on the cyclopentene ring. We have
rearrangement of compound (±)-3 to (±)-3’. There is a report of assigned the structure based on the spectral data (see Experi-
allylic azide shift in cyclic sytems by Carell et al. in 2007 [40]. mental). In addition, crystals of the triol (±)-12 were obtained
The work of Aubé on AAC (azide–alkyne cycloaddition) chem- and single crystal X-ray analysis further established the
istry also gives a good indication that equilibrating allylic azide assigned structure without any ambiguity, as shown in Figure 2
stereoisomers can selectively participate in reactions [37-45]. In and Supporting Information File 2 [46,47]. The synthesized
2000, Aubé et al. proposed a mechanism for the intermolecular trihydroxylated amides (±)-11–(±)-15 can be considered as
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Beilstein J. Org. Chem. 2015, 11, 1060–1067.
mimics of 1,2-carbocyclic nucleosides due to close structural
resemblance with that of molecules documented in the litera-
ture [29,30].
Conclusion
In conclusion, we have developed an efficient method to
synthesize a new class of racemic cyclopentylated lactams
using the Aubé reaction where the bases are replaced by lactam
rings. We originally planned to synthesize 1,3-substituted
cyclopentyl-substituted lactams; however, based on an
allyl–azide rearrangement, we obtained lactams with an 1,2-
substituted cyclopentane on the nitrogen instead. These com-
pounds can be useful tools as they are structurally close to
nucleosides and may have pharmaceutical relevance.
Experimental
General procedure for the Aubé reaction
using BF3·OEt2
Boron trifluoride etherate (0.07 mL, 0.71 mmol) was added to a
solution of azido alcohol 3 (0.10 g, 0.71 mmol) and ketones
(2.0 mmol, 3 equiv) in HFIP (0.2 mL) cooled to 0 °C. The mix-
ture was warmed to room temperature and stirred for 2–5 h.
Upon completion, the reaction was quenched with 15% aq.
KOH solution diluted with CH2Cl2 and the mixture was
extracted with CH2Cl2 (3 × 5 mL). The organic layer was
washed with brine, dried over anhydrous Na2SO4 and concen-
trated in vacuum. The obtained crude oil was subjected to silica
gel column chromatography using ethyl acetate/methanol 9:1 as
eluent to afford cyclic amides in 40–70% yield.
1-(5-(Hydroxymethyl)cyclopent-2-en-1-yl)azepan-2-
one (6)
IR νmax (film): 2930, 1618, 1476, 1444, 1039 cm−1; 1H NMR
(500 MHz, CDCl3) δ 6.15 (dd, J = 3.0, 4.5 Hz, 1H), 5.73 (d, J =
1.5 Hz, 1H), 5.34 (d, J = 8.0 Hz, 1H), 3.56–3.52 (m, 1H), 3.33
(t, J = 10.0 Hz, 1H), 3.21–3.12 (m, 1H), 3.02 (dd, J = 6.3,
14.0 Hz, 1H), 2.65–2.56 (series of multiplets, 3H), 2.46–2.39
(m, 1H), 1.93–1.79 (series of multiplets, 4H), 1.72–1.54 (m,
4H); 13C NMR (125 MHz, CDCl3) δ 178.5, 137.0, 129.6, 62.1,
60.8, 45.3, 45.1, 37.7, 34.4, 29.8, 29.0, 23.4; HRMS calculated
for C12H19NO2, [M + Na]+: 232.1308, found 232.1316.
Scheme 6: Hydroxylated cyclopentyl-substituted lactams.
General procedure for the dihydroxylation reaction
0.2 mL of OsO4 (2.5% in tert-butanol, 1 mol %) was added to a
solution of cyclic lactam (0.27 mmol) and NMO (0.3 mmol,
1.1 equiv) in tert-butanol at room temperature and stirred for
2–5 hours. Upon completion of the reaction, Na2S (diluted with
MeOH) was added, and the mixture was filtered and passed
through a short pad of celite. After concentration, the residue
was purified by silica gel column chromatography using ethyl
acetate/methanol 10:1 as eluent to afford triols in good yield.
Figure 2: ORTEP plot of triol (±)-12.
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IR νmax (film): 3385, 2927, 1623, 1567, 1450, 1358 cm−1;
1H NMR (400 MHz, CD3OD) δ 4.60–4.58 (m, 1H), 4.35 (dd,
J = 5.0, 10.0 Hz, 1H), 4.08–4.05 (m, 1H), 3.60 (dd, J = 7.0,
16.0 Hz, 1H), 3.50–3.36 (series of multiplets, 3H), 2.73–2.67
(m, 1H), 2.56–2.49 (m, 2H), 1.91–1.83 (series of multiplets,
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266.1363, found 266.1358.
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We thank CSIR, New Delhi for the support through CSIR-NCL
Joint Collaborative Program (BSC0124) under the XII five year
plan. MVS, RSO and ES thank CSIR, New Delhi for the RA
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