Synthesis of a tricyclic lactam via Beckmann rearrangement and ring-rearrangement metathesis as key steps

Article abstract of DOI:10.3762/bjoc.11.163

A tricyclic lactam is reported in a four step synthesis sequence via Beckmann rearrangement and ring-rearrangement metathesis as key steps. Here, we used a simple starting material such as dicyclopentadiene.

Full text of DOI:10.3762/bjoc.11.163

Synthesis of a tricyclic lactam via Beckmann rearrangement
and ring-rearrangement metathesis as key steps
Sambasivarao Kotha*, Ongolu Ravikumar and Jadab Majhi
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2015, 11, 1503–1508.
Department of Chemistry, Indian Institute of Technology-Bombay,
Powai, Mumbai-400 076, India, Fax: 022-25767152
doi:10.3762/bjoc.11.163
Received: 12 June 2015
Accepted: 12 August 2015
Published: 27 August 2015
Email:
Sambasivarao Kotha* - srk@chem.iitb.ac.in
* Corresponding author
This article is part of the Thematic Series "Progress in metathesis
chemistry II".
Keywords:
allylation; Beckmann rearrangement; lactams; oximes;
ring-rearrangement metathesis
Guest Editor: K. Grela
© 2015 Kotha et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
A tricyclic lactam is reported in a four step synthesis sequence via Beckmann rearrangement and ring-rearrangement metathesis as
key steps. Here, we used a simple starting material such as dicyclopentadiene.
Introduction
The Beckmann rearrangement (BR), a well-known protocol for several metathetic transformations such as ring-closing
the conversion of ketoxime to an amide in the presence of acid metathesis (RCM) and ring-opening metathesis (ROM). The
was discovered in 1886. This rearrangement involves the migra- RRM has emerged as a powerful tool in organic synthesis
tion of a group anti to the leaving group on the nitrogen atom. because of its ability to transform simple starting materials into
The BR has widely been used in synthetic organic chemistry, complex targets involving an ingenious design. The retrosyn-
for example, a large-scale production of Nylon-6 is based on the thetic strategy to the target molecule 1 is shown in Figure 1.
synthesis of ε-caprolactam from cyclohexanone oxime RRM of the tricyclic allylated compound 2 can deliver the
involving the BR. The activation energy for the BR is almost target lactam 1. The key synthon 2 can be derived by allylation
the same as that of the nucleophilic substitution at sp2 nitrogen. of lactam 3, which in turn can be prepared via BR starting with
To synthesize various aza-arenes and cyclic imines, such as the known enone 4 [25-27], derived from dicyclopentadiene (5)
quinolines, aza-spiro compounds and dihydropyrroles, the [28-30].
intramolecular SN2-type reaction at the oxime nitrogen is useful
[1-6]. Here, we plan to use the BR in combination with a ring- Results and Discussion
rearrangement metathesis (RRM) [7-24] to generate lactam To begin with, the oxidation of dicyclopentadiene (5) in the
derivative 1. The RRM protocol involves a tandem process with presence of SeO2 gave 1α-dicyclopentadienol (6), which on
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Beilstein J. Org. Chem. 2015, 11, 1503–1508.
Figure 1: Retrosynthetic analysis of tricyclic amide 1.
treatment with pyridinium chlorochromate (PCC) [31] deliv- different reaction conditions. These include: (a) NaN3, heat
ered the known tricyclic enone 4. Selective reduction of enone 4 1 day in TFA (b) NaN3, FeCl3 in DCE at rt and reflux, 1 day
with Zn in AcOH/EtOH under reflux conditions gave the satu- and (c) TMSN3, FeCl3 in DCE, 1 day. Surprisingly, the desired
rated ketone 7 [32] (Scheme 1).
lactam 9a was not formed. Interestingly, when the tricyclic
ketone 7 was treated with hydroxylamine-O-sulfonic acid
Later, tricyclic ketone 7 was reacted with NH2OH·HCl in the (NH2OSO3H) in glacial AcOH under reflux conditions, the
presence of NaOAc in dry MeOH at rt to give a mixture of lactams 9a and 9b were obtained in 48% yield (9a:9b = 2:1) the
oximes 8a and 8b and this mixture was subjected to a BR under ratio of oximes 9a and 9b was calculated based on 1H NMR
different reaction conditions, like (a) p-TsCl, rt, 15 h, CH3CN spectral data (Scheme 3).
(b) p-TsCl, reflux, 15 h, CH3CN (c) PPA, reflux for 20 min.
Surprisingly, in all these instances no rearrangement product
was observed. Interestingly, when the mixture of oximes 8a and
8b was treated with TsCl in the presence of NaOH at rt lactams
9a and 9b were obtained in 66% combined yield for two steps
(9a:9b = 2:1) (Scheme 2) but the products were inseparable by
column chromatography. Next, we attempted to separate the
Scheme 3: Beckmann rearrangement reaction in a single step.
mixture of these isomers (9a and 9b) by selective crystalliza-
tion using different solvent systems. Finally, one of the lactam
derivative 9a (δ = 3.86, dd, J = 5.8, 2.9 Hz, 1H) was isolated in
Having prepared the lactams 9a and 9b, the allylation reaction
was attempted with the lactam mixture in the presence of NaH/
allyl bromide in dry DMF to generate allyl derivatives 10a and
10b in 84% yield. Later, without separation of allyl lactams 10a
and 10b, RRM was attempted with the lactam mixture under
different catalyst conditions. For example, reaction conditions
pure form from ethanol in 20% yield over two steps.
Subsequently, we attempted to synthesize the desired lactam 9a
via Schmidt reaction or BR of the keto derivative 7 in a single
step. In this regard, the tricyclic ketone 7 was treated under
Scheme 1: Synthesis of tricyclic ketone 4.
Scheme 2: Beckmann rearrangement of oximes 8a and 8b.
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Beilstein J. Org. Chem. 2015, 11, 1503–1508.
such as: (a) G-I in dry CH2Cl2, under ethylene atmosphere at rt;
(b) G-II in dry CH2Cl2, under ethylene atmosphere at rt and
(c) G-I and G-II in dry toluene under ethylene atmosphere did
not deliver the desired RRM product 1a (Scheme 4).
Separation of the required isomer from the mixture of oximes
8a and 8b or the lactams 9a and 9b was not possible by column
chromatography because of the same Rf value of the individual
compounds. Finally, isolation of the required lactam 9a from
the mixture was accomplished by using crystallization. Since
this method is cumbersome, an alternate method was attempted.
To this end, we changed our synthetic route and tried to use the
unsaturated ketone 4 and hoped for a different outcome during
the BR. In this content, oximation of the enone 4 was carried
out with NH2OH·HCl in the presence of NaOAc in dry MeOH.
The stereoisomeric oximes, i.e., (E)-11b and (Z)-11a were sep-
arated by silica gel column chromatography to deliver 47% and
23% yields, respectively (Scheme 5).
Scheme 5: Synthesis of Beckmann rearrangement precursors.
When the oxime 11a was treated with TsCl in the presence of
NaOH in dioxane/H2O (3:4 v/v) at rt lactam 12 was formed in
34% yield. However, the oxime 11b did not give the rearranged
product under the same reaction conditions, which clearly indi-
cates that the oxime 11b is unreactive towards BR (Scheme 6).
The stereostructure of the oxime 11b has been determined by
single crystal X-ray diffraction data (Figure 2) [33].
Figure 2: Molecular crystal structure of compound 11b.
Allylation of lactam 12 in the presence of NaH/allyl bromide in
dry DMF gave the allyl derivative 2 in 80% yield. Finally, the
RRM of compound 2 was accomplished with G-II catalyst in
dry CH2Cl2, under ethylene atmosphere at rt in the presence of
Ti(OiPr)4 to deliver the tricyclic compound 1 in 90% yield
(Scheme 7). Its structure has been established on the basis of
1H NMR and 13C NMR spectral data and further supported by
HRMS data.
Scheme 4: Synthesis of ring-rearrangement precursors.
Scheme 6: Beckmann rearrangement of oxime isomers 11a and 11b.
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Scheme 7: Synthesis of aza tricyclic compound 1 by RRM.
Conclusion
v/v); IR (neat): 3195 (m), 3067 (w), 2938 (s), 2868 (m), 1674
In summary, we have demonstrated the RRM strategy with the (s), 1627 (m), 1452 (w), 1434 (w), 1410 (m), 1333 (m), 1252
norbornene derivative 2 to deliver the tricyclic compound 1 (w), 1201 (m), 1031 (w), 783 (m), 541 (m) cm−1; 1H NMR (400
involving a short synthetic sequence. However, a similar com- MHz, CDCl3) δ 6.31 (s, 1H), 6.22 (dd, J = 5.8, 3.0 Hz, 1H),
pound 10a did not deliver the RRM product. For the first time, 6.10 (dd, J = 5.8, 3.0 Hz, 1H), 3.86, (dd, J = 5.8, 2.9 Hz, 1H),
we have demonstrated that BR in combination with RRM is a 2.97 (s, 1H), 2.88 (s, 2H), 2.48–2.40 (m, 1H), 2.13–2.05
useful strategy to generate azacyclic compounds. Here we have (m,1H), 1.94–1.87 (m, 1H), 1.56 (dt, J = 8.8, 1.8 Hz, 1H), 1.42
used an inexpensive starting material such as dicyclopentadiene (d, J = 8.8 Hz, 1H), 1.23–1.13 (m, 1H) ppm; 13C NMR (100
(5).
Hz, CDCl3) δ 175.5, 137.6, 134.2, 54.8, 48.0, 47.8, 46.5, 39.49,
31.4, 23.2 ppm.
Experimental
Synthesis of compounds 9a and 9b
Synthesis of compounds 11a and 11b
Method 1: Analogously as described in [4], a mixture of 7 (2 g, Analogously as described in [4], a mixture of 4 (9 g,
13.51 mmol), hydroxylamine hydrochloride (1.41 g, 61.64 mmol), hydroxylamine hydrochloride (6.41 g,
20.27 mmol), NaOAc (1.66 g, 20.27 mmol) and methanol 92.34 mmol), NaOAc (7.58 g, 92.49 mmol) and methanol
(50 mL) was stirred at rt for 1 h. The residue after evaporation (225 mL) were stirred at rt for 1 h. The residue after evapor-
of the solvent was diluted with water and extracted with ether. ation of the solvent was diluted with water and extracted with
Removal of ether furnished the crude oxime (2.4 g). p-Toluene- diethyl ether. Removal of ether furnished the crude oxime
sulfonyl chloride (6.15 g, 32.28 mmol) was added portion-wise which was purified by silica gel column chromatography by
over 15 min to a stirred solution of the crude oxime (2.4 g) and eluting appropriate mixture of ethyl acetate/petroleum ether to
NaOH (2.97 g. 74.44 mmol) in 150 mL dioxane/water 3:4 at afford compounds 11a (2.29 g, 23%) and 11b (4.61 g, 47%) as
5 °C. The mixture was stirred at rt for 15 h and dioxane was colourless solids.
removed in vacuo. The residue was dissolved in CH2Cl2 and
washed with brine. Removal of the solvent and column 11a: Rf = 0.29 (EtOAc/pentane 2:8 v/v); IR (neat): 3325 (m),
chromatography gave a mixture of amide isomers (9a, 9b) 3013 (m), 2400 (w), 1725 (w), 1337 (w), 1216 (m), 927 (m),
(1.45 g, 66%). The amide mixture was crystalized in different 759 (s) cm−1; 1H NMR (500 MHz, CDCl3) δ 9.15 (s, 1H), 6.54
solvents and finally one of isomer 9a was isolated from ethanol (dd, J = 5.8, 1.3 Hz, 1H), 6.38 (dd, J = 5.8, 2.5 Hz, 1H), 5.97
20%.
(dd, J = 5.6, 2.8 Hz, 1H), 5.77 (dd, J = 5.6, 2.9 Hz, 1H), 3.32
(m, 1H), 3.18 (dd, J = 10.7, 4.5 Hz, 1H), 3.16 (s, 1H), 2.28 (s,
Method 2: A mixture of 7 (100 mg, 0.68 mmol) and hydroxyl- 1H), 2.90 (s, 1H), 1.61 (d, J = 8.3 Hz, 1H), 1.47 (d, J = 8.3 Hz,
amine-O-sulfonic acid (113 mg, 1.0 mmol) in AcOH (5 mL) 1H) ppm; 13C NMR (125 Hz, CDCl3) δ 165.1, 149.1, 133.3,
was heated at reflux conditions for 4 h under nitrogen. After 133.1, 126.4, 51.0, 50.5, 46.1, 45.9, 44.1 ppm; HRMS (Q-Tof)
completion of the reaction (TLC monitoring), the reaction m/z: [M + Na]+ calcd for C10H11NNaO, 184.0733; found,
mixture was basified with 3 N NaOH solution and the 184.0734.
organic layer was extracted with CH2Cl2, washed with
water, brine and dried by Na2SO4. The combined organic layer 11b: mp = 89–91 °C; Rf = 0.30 (EtOAc/petroleum ether 2:8
was concentrated under reduced pressure and column chroma- v/v); IR (neat): 3322 (m), 3020 (m), 2396 (w), 2125 (w), 1705
tography gave a mixture of amide isomers 9a and 9b (1.06 g, (m), 1217 (m), 926 (m), 759 (s) cm−1; 1H NMR (500 MHz,
48%). The amide mixture was crystalized in different solvents CDCl3) δ 8.71 (s, 1H), 6.30 (dd, J = 5.8, 2.5 Hz, 1H), 6.00 (dd,
and finally isomer 9a was isolated from ethanol. White solid 9a; J = 5.7, 1.3 Hz, 1H), 5.90 (dd, J = 5.6, 3.0 Hz, 1H), 5.76 (dd, J
mp = 150–155 °C; yield 15%: Rf = 0.30 (EtOAc/pentane 1:1 = 5.6, 2.9 Hz, 1H), 3.43 (s, 1H), 3.35 (dd, J = 6.1, 4.2 Hz, 1H),
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Beilstein J. Org. Chem. 2015, 11, 1503–1508.
3.30 (m, 1H), 2.90 (s, 1H), 1.64 (d, J = 8.3 Hz, 1H), 1.50 (d, J = Synthesis of compound 1
8.3, 1H) ppm; 13C NMR (125 Hz, CDCl3) δ 168.1, 147.0, Analogously as described in [8], to a stirred solution of com-
133.1, 132.9, 131.1, 51.9, 50.8, 45.1, 45.0, 44.1 ppm; HRMS pound 2 (20 mg, 0.099 mmol) in dry CH2Cl2 (20 mL) degassed
(Q-Tof) m/z: [M + Na]+ calcd for C10H11NNaO, 184.0733; with nitrogen for 10 minutes, purged with ethylene gas for
found, 184.0737.
10 minutes was then added Ti(OiPr)4 and Grubbs-II catalyst
(8.4 mg, 10 mol %) and stirred for 5 h at reflux conditions
under ethylene atmosphere. After completion of the reaction
Synthesis of compound 12
Analogously as described in [4], p-toluenesulfonyl chloride (TLC monitoring) the solvent was removed on a rotavapor
(2.36 g, 12.42 mmol) was added portionwise over 15 min to a under reduced pressure and purified by silica gel column chro-
stirred solution of oxime 11a (1.0 g, 6.21 mmol) and NaOH matography by eluting with an appropriate mixture of ethyl
(1.24 g. 31.05 mmol) in 100 mL dioxane/water 3:4 at 5 °C. The acetate/petroleum ether to afford 1 as a brown coloured semi
mixture was stirred at rt for 15 h and the dioxane was removed solid (18 mg, 90%). IR (neat): 3020 (m), 2927 (m), 2861 (m),
in vacuo. The residue was dissolved in CH2Cl2 and washed 2396 (w), 1727 (w), 1608 (w), 1461 (w), 1216 (m), 929 (w),
with the brine. Removal of solvent and column chromatog- 762 (s) cm−1; 1H NMR (500 MHz, CDCl3) δ 6.35–6.27 (m,
raphy using an appropriate mixture of ethyl acetate/petroleum 1H), 6.05–5.89 (m, 1H), 5.88–5.83 (m, 1H), 5.75–5.72 (m, 1H),
ether gave the pure lactam 12 (0.33 g, 34%) as a semi solid. IR 5.63 (dt, J = 16.0, 9.7 Hz, 1H), 5.02–4.91 (m, 2H), 4.64–4.57
(neat): 3020 (m), 2400 (w), 2125 (w), 1678 (w), 1422 (w), 1216 (m, 1H), 4.07–4.03 (m , 1H), 3.50–3.42 (m, 1H), 3.19–3.14 (m,
(m), 1049 (w), 1022 (w), 929 (w), 759 (s) cm−1; 1H NMR (500 1H), 3.12–2.94 (m, 1H), 2.62–2.55 (m, 1H), 2.21–2.03 (m, 1H),
MHz, CDCl3) δ 6.36–6.34 (m, 1H), 6.15 (dd, J = 5.5, 3 Hz, 1.62–1.53 (m, 1H) ppm; 13C NMR (125 Hz, CDCl3) δ 164.2,
1H), 6.07 (dd, J = 5.5, 3 Hz, 1H), 5.96 (bs, 1H), 5.63 (dt, J = 139.6, 139.6, 125.7, 123.5, 123.2, 115.5, 59.0, 58.8, 49.1, 42.3,
8.5, 2 Hz, 1H), 4.12–4.08 (m, 1H), 3.10 (t, J = 0.5 Hz, 1H), 3.06 40.9, 39.6 ppm; HRMS (Q-Tof) m/z: [M + Na]+ calcd for
(d, J = 0.5 Hz, 1H), 2.99–2.95 (m, 1H), 1.44 (dt, J = 8.5, 2Hz, C13H15NNaO, 224.1046; found, 224.1041.
1H), 1.25–1.22 (m, 1H) pmm; 13C NMR (125 Hz, CDCl3) δ
164.4, 142.4, 136.9, 134.5, 122.4, 54.9, 49.8, 47.8, 44.5, 39.3
Supporting Information
ppm; HRMS (Q-Tof) m/z: [M + Na]+ calcd for C10H11NNaO,
184.0733; found, 184.0733.
Supporting Information File 1
NMR spectra of synthesized compounds and X-ray data of
Synthesis of compound 2
compound 11b.
Analogously as described in [8], a suspension of NaH (20 mg,
0.83 mmol) in dry DMF (5mL), was added to compound 12
(70 mg, 0.43 mmol) in dry DMF (5 mL) and allyl bromide
(57 mg, 0.47 mmol) at 0 °C under nitrogen and it was stirred for
20 minutes at 0 °C. After completion of the reaction (TLC
monitoring) the reaction mixture was acidified with saturated
ammonium chloride and extracted with ethyl acetate. The
combined organic layer was washed with water and brine and
then dried over sodium sulfate. Later, the organic layer was
concentrated under reduced pressure and purified by silica gel
column chromatography by eluting with an appropriate mixture
of ethyl acetate/petroleum ether to afford compound 2 as a
brown liquid (87 mg, 80%). IR (neat): 3370 (s), 2945 (m), 2832
(m), 2532 (w), 2044 (w), 1662 (w), 1450 (m), 1114 (m), 1030
(s), 770 (m) cm−1; 1H NMR (500 MHz, CDCl3) δ 6.25–6.23
(m, 1H), 6.05–6.01 (m, 2H), 5.85–5.77 (m, 1H), 5.67 (dd, J =
10, 2 Hz, 1H), 5.26–5.22 (m, 2H), 4.47–4.46 (m, 1H), 4.02 (dd,
J = 10, 3.5 Hz, 1H), 3.65–3.60 (m, 1H), 3.29 (s, 1H), 3.08 (s,
1H), 3.01–2.97 (m, 1H), 1.45 (dt, J = 9, 2 Hz, 1H), 1.21–1.24
(m, 1H) ppm; 13C NMR (125 Hz, CDCl3) δ 162.5, 140.1, 137.1,
133.8, 133.6, 123.1, 117.7, 59.43, 48.4, 47.4, 47.3, 44.7, 40.0
ppm; HRMS (Q-Tof) m/z: [M + Na]+ calcd for C13H15NNaO,
224.1046; found, 224.1041.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-11-163-S1.pdf]
Acknowledgements
We thank the Department of Science and Technology (DST),
New Delhi for the financial support and the Sophisticated
Analytical Instrument Facility (SAIF), IIT-Bombay for
recording spectral data and also thank Gaddamedi Sreevani and
Darshan Mhatre for their help in collecting the X-ray data and
structure refinement. S. K. thanks the Department of Science
and Technology for the award of a J. C. Bose fellowship. O. R.
thanks the University Grants Commission, New Delhi for the
award of a research fellowship. J. M thanks DST for the award
of inspire fellowship.
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