Azidophosphonium salt-directed chemoselective synthesis of (E)/(Z)-cinnamyl-1H-Triazoles and regiospecific access to bromomethylcoumarins from Morita-Baylis-Hillman adducts

Article abstract of DOI:10.3762/BJOC.16.130

The direct transformation of Morita-Baylis-Hillman (MBH) adducts into molecules of interest is a crucial process wherein allylic hydroxy-protected or halogenated MBH adducts are commonly preferred. Herein, we report an azidophosphonium salt (AzPS)-catalys

Full text of DOI:10.3762/BJOC.16.130

Azidophosphonium salt-directed chemoselective synthesis of
(E)/(Z)-cinnamyl-1H-triazoles and regiospecific access to
bromomethylcoumarins from Morita–Baylis–Hillman adducts
Soundararajan Karthikeyan1, Radha Krishnan Shobana1,
Kamarajapurathu Raju Subimol2, J. Helen Ratna Monica1
and Ayyanoth Karthik Krishna Kumar*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2020, 16, 1579–1587.
1Organic & Material Chemistry Research Laboratory, Department of
Chemistry, The American College, Madurai, Tamil Nadu, India and
2Department of Chemistry, Fatima College, Madurai, Tamil Nadu,
India
doi:10.3762/bjoc.16.130
Received: 31 March 2020
Accepted: 22 June 2020
Published: 01 July 2020
Email:
Ayyanoth Karthik Krishna Kumar* - karthikkumar1265@gmail.com
Associate Editor: M. Rueping
* Corresponding author
© 2020 Karthikeyan et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
halomethylcoumarin; Morita–Baylis–Hillman adducts; organocatalyst;
phosphonium salt; triazolation
Abstract
The direct transformation of Morita–Baylis–Hillman (MBH) adducts into molecules of interest is a crucial process wherein allylic
hydroxy-protected or halogenated MBH adducts are commonly preferred. Herein, we report an azidophosphonium salt (AzPS)-cat-
alysed straight forward protocol for synthesising structurally demanding (E)/(Z)-cinnamyl-1H-1,2,3-triazoles and halomethyl-
coumarins from MBH adducts. The novel methodology, efficient catalyst, and direct utilization of MBH adducts under mild reac-
tion conditions qualify the reported procedures as powerful synthetic tools.
Introduction
The presence of versatile functional groups in close proximity cyclic frameworks [10]. However, most of the reported synthe-
classifies Morita–Baylis–Hillman adducts as privileged key tic transformations utilize either allylic hydroxy-protected or
scaffolds for synthetic organic chemists. Accordingly, MBH allyl halide-substituted MBH adducts [11-23].
adducts have been explored as strategic intermediates for the
synthesis of interesting molecules, such as carbamates of unsat- Among the known synthetic transformations using functionali-
urated β-amino acids [1], β-phenylsylfenyl-α-cyanohydrocin- zed MBH adducts, cycloaddition reactions are challenging and
namaldhydes [2], 2-alkylcarbonyl-1-indanols [3], dihydropyra- attractive for synthetic organic chemists. In this context,
zoles [4], tetrahydroacridines [5], γ-lactams [6], quinolin-5-ones acetate-functionalized Morita–Baylis–Hillman adducts have
[7], spirobisglutarimides [8], indolizines [9], and spiro carbo- been extensively utilized over other precursors. For example,
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heterocycles such as, pyrroles (e.g., IV) [24], keto pyrroles individual research groups have reported the multistep pathway
(e.g., V) [25], pyridines (e.g., VI) [26], pyrrolotriazoles (e.g., to access the cinnamyl-1H-1,2,3-triazole derivatives IX
VII) [27], and triazolobenzoxazonines (e.g., VIII) [28] result from acetates of MBH adducts (Scheme 2) [32,33]. The other
from MBH acetates (Scheme 1). From these synthetic elabora- preferable moiety for triazole transformations is the allyl halide
tions, three successive steps are universally utilized: (i) acetyla- of MBH adducts, however, the vicinity of its (E)- and (Z)-
tion, (ii) azidation, and (iii) cycloaddition to produce IV–VIII. isomers restricts their use as a favourable starting moiety [34].
In spite of the broad scope and synthetic utility, it is evident that After a careful bibliographic investigation, it became evident
the multistep synthetic methodology is the only existing module that there were no one-pot protocols for direct transformations
for cycloaddition reactions.
of MBH adducts to cinnamyl triazoles. The outcome of devel-
oping a one-pot synthetic strategy will be worthwhile for phar-
Our research group is focused on developing one-pot synthetic macologically important triazoles, such as isavuconazole,
transformations for complex molecules [29-31]. Two tazobactam, and ravuconazole [35].
Scheme 1: Literature-reported cycloaddition reactions of MBH acetates involving azides and alkynes [24-28].
Scheme 2: Synthetic methodologies for triazolations of MBH adducts. a) Literature-reported indirect triazolation of MBH adducts [32,33]. b) This work:
phosphonium salt-catalysed triazolation of MBH adducts.
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Results and Discussion
mation File 1 for the substituent patterns of the compounds
Initially, phosphonium salts were barely utilised or exploited in 1a–o). In this precedent reaction, the adduct 1a and propargyl
synthetic transformations. Later, in 2014, several organic trans- alcohol (2a) in THF were treated with the AzPS (1 equiv) and
formations employed quaternary phosphonium salts as CuI (3 mol %) at room temperature. To our expectations, the
favourable catalysts [36]. Their synthetic utility was not only reaction afforded the (E)-cinnamyl-1H-1,2,3-triazole in a low
confined to catalysis, but they were also used as intermediates yield of 24% (Table 1, entry 1). Thereby, we anticipated that an
for the synthesis of 1H-indazoles [37], as promoters for stereo- increase in the proportion of the AzPS would substantially
selective rearrangements [38], and as temporary protectors of increase the yield of 3a (Table 1, entries 2 and 3), but unexpect-
O,P-acetals [39], which branded them as promising motifs. The edly, the reaction demonstrated an unsatisfactory yield. There-
above reports and the Lewis acid character of quaternary phos- after, on attempting the reaction with an improved ratio of CuI
phonium salts (QPS) [40-48] qualifies them as reliable cata- (5 mol %) and AzPS (2 equiv), the expected product 3a was ob-
lysts for the proposed methodology. The most elaborate process tained in a moderate yield (71%, Table 1, entry 4). However, a
in the proposed methodology is the protection and elimination further increase in the AzPS ascertained a gradual decrease in
of the allylic hydroxy group. We believe that this crucial the yield of 3a (Table 1, entries 5 and 6). The outcome of this
strategy could be primarily resolved by a quaternary phos- analysis might have been due to the formation of large amounts
phonium salt. After the initial screening of various quaternary of the byproduct triphenylphosphine oxide, which impeded the
phosphonium salts, the azidophosphonium salt purification process and decreased the yield of 3a. Alternative
[Ph3P+CBr3]N3−, reported by Blanco and co-workers, was Cu(I) catalysts, CuCl and CuBr, were also used at 5 mol % with
opted to accomplish our goal [49-51]. The AzPS surprisingly the AzPS (2 equiv), however, the combination showed no
synchronised with the functional and structural requirements of potential increase in the yield of 3a (Table 1, entries 7 and 8).
the proposed work. The azidophosphonium salt was generated Comprehensive investigations on the proposed methodology
and purified according to a modified literature procedure [49]. revealed 2 equiv of the AzPS and 5 mol % of CuI as the opti-
mized catalytic combination. Further, the optimized reaction
The one-pot model reaction was investigated using the MBH was screened in presence of various solvents (Table 1, entries
adduct 1a (1 equiv) and propargyl alcohol (2a, 1.2 equiv) in 9–13), and the outcome revealed acetonitrile as the most prefer-
presence of the AzPS [Ph3P+CBr3]N3− (see Supporting Infor- able solvent, yielding 3a in 83% yield (Table 1, entry 11). Inter-
Table 1: Optimization of the triazolation of the MBH adduct 1a.
entry
equiv of AzPS
Cu(I) salt (mol %)
solvent
yield (%)
1
1
CuI (3)
CuI (3)
CuI (3)
CuI (5)
CuI (5)
CuI (5)
CuCl (5)
CuBr (5)
CuI (5)
CuI (5)
CuI (5)
CuI (5)
CuI (5)
THF
24
33
42
71
45
36
47
54
66
69
83
64
69
2
1.5
2
THF
3
THF
4
2
THF
5
3
THF
6
4
THF
7
2
THF
8
2
THF
9
2
EtOAc
acetone
CH3CN
DMF
DMSO
10
11
12
13
2
2
2
2
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estingly, the dilution of the reaction mixture did not alter the adducts (Scheme 3). The MBH adducts derived from methoxy
efficiency of this reaction.
and ethoxy acrylate stereochemically afforded the (E)-
cinnamyl-1,4-disubstituted 1,2,3-triazole derivatives 3a–d/g–k/
The substrate scope of the optimized reaction and its limita- m–q in a yield of 70–88%. Distinctively, the cyano acrylate-
tions were further extended to structurally distinct MBH substituted MBH adduct stereoselectively afforded the (Z)-
Scheme 3: Scope of the one-pot cascade reaction of the unprotected Morita–Baylis–Hillman adducts 3a–q.
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Beilstein J. Org. Chem. 2020, 16, 1579–1587.
cinnamyl-1,4-disubstituted 1,2,3-triazole derivatives 3e/f/l in a outcome of this process is the structurally relevant azido moiety
yield of 82–92%. Irrespective of the acetylene moiety, the MBH IIIa, which then undergoes a 1,3-dipolar cycloaddition with the
adducts derived from acrylonitrile comparatively afforded copper acetylide IVa to furnish the 1,4-disubstituted 1,2,3-tri-
cinnamyl-1,4-disubstituted 1,2,3-triazoles at an improved yield azoles Va (Figure 1).
compared to that of the methyl and ethyl counterparts. Notably,
the MBH adducts derived from the para-bromo-, para-chloro-, At this stage, we sought to analyse the outcome of the proposed
and para-nitrobenzaldehydes favourably assisted the formation reaction following a sequential addition of the reagents utilised
of the corresponding (E)-cinnamyl-1,4-disubstituted 1,2,3-tri- for the synthesis of AzPS. Therefore, a preliminary investiga-
azole derivatives 3g–m in a yield of 72–87%. Alternatively, the tion was attempted using the MBH adduct 1a (1 equiv) and
ortho- and meta-substituted aryl-MBH adducts were incompat- propargyl alcohol (2a,1.2 equiv) in the presence of CuI
ible with the optimized reaction conditions, and this was (5 mol %), triphenylphosphine (1 equiv), bromomethane
presumably due to the apparent steric hindrance. Similarly, the (1.1 equiv), and sodium azide (2 equiv). Unexpectedly, the
MBH adducts derived from aliphatic aldehydes, salicylalde- reaction yielded (Z)-methyl-2-(bromomethyl)-3-phenylacrylate
hydes, and methyl- or methoxy-substituted benzaldehydes were (58%) over the expected triazole. Similarly, the MBH adduct
also inert under the optimized reaction conditions. Therefore, it derived from furan, 1i, and phenylacetylene (2b) also yielded
is evident that the electronic variation of the substituents on the (Z)-methyl 2-(bromomethyl)-3-(furan-2-yl)acrylate (42%)
aromatic moiety of the MBH adducts played a crucial role in rather than the expected triazole (Scheme 4). Thereby, it was
determining the outcome of the optimized reactions. We further clearly evident that the addition of the individual reagents
extended the scope of this transformation to five-membered prevented the formation of complicated triazoles.
heterocyclic MBH adducts. To our delight, except pyrroles, the
proposed methodology was amenable to MBH adducts of furan Interestingly, the MBH adducts derived from salicylaldehydes
and thiophene (3n–q, 70–80%).
were inert to triazolations, surprisingly affords bromomethyl-
coumarin in the presence of AzPS and HBr. The reaction was
The mechanistic pathway for the triazolation proceeded via a optimized using salicylaldehyde (1 equiv) in the presence of
nucleophilic attack on the AzPS by the allylic alcohol of the AzPS (2 equiv) and HBr (2.0 equiv). The reaction afforded
MBH adduct Ia. Subsequently, the azide ion undergoes a 6-(bromomethyl)coumarin (4a) in a yield of 78% (Table 2,
nucleophilic attack on the allylic carbon atom of the oxyphos- entry 3). The synthetic utility of the reaction was further extend-
phonium intermediate IIa and generates the 2-azidoalkene IIIa. ed to ortho-vanillin and para-bromobenzaldehyde to afford the
Interestingly, the consecutive nucleophilic attack by the azido corresponding halomethylcoumarins (4b/c). However, this
ion smoothly initiates the allylic rearrangement and thereby regiospecific transformation was restricted only to the MBH
facilitates the removal of the crucial phosphonium oxide. The adducts derived from salicyladehydes and tert-butyl acrylate
Figure 1: Proposed mechanism for the synthesis of 1,4-disubstituted triazoles.
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Scheme 4: Comparative analysis of the sequential one-pot reaction.
Table 2: Optimization of the reaction conditions for 3-(bromomethyl)coumarins from MBH adducts.
entry
equiv of AzPS
equiv of HBr
solvent
yield 4a (%)
1
2
3
4
5
6
7
2
2
2
2
2
3
4
–
1
2
3
4
2
2
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
–
33
78
62
31
65
57
[52,53]. Among the reported methodologies on synthesis of bromine facilitated the nucleophilic attack at the vinylic centre
halomethylcoumarins [54,55], the present methodology was of Ib and the spontaneous removal of triphenylphosphine oxide
attractive due to its good yield and the simple reaction condi- to yield IIb. A consecutive intramolecular nucleophilic attack
tions.
of the hydroxy moiety at the carbonyl carbon of IIIb further
drove the cyclisation to afford the bromomethylcoumarin 4a.
As shown in Figure 2, the mechanistic pathway for 4a–c
progressed via treating the MBH adduct (1m) with AzPS and Conclusion
HBr. The outcome of this process was the phosphonium-pro- In summary, we reported the first protocol on the quaternary
tected MBH moiety Ib and hydrazoic acid. The counter ion phosphonium salt-mediated direct synthesis of cinnamyltria-
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Beilstein J. Org. Chem. 2020, 16, 1579–1587.
Figure 2: Proposed mechanism for the synthesis of 3-(bromomethyl)coumarins.
zoles and 3-(bromomethyl)coumarins from Morita–Baylis–Hill- were utilized for synthesising the quaternary phosphonium salt.
man adducts. In contrast to the contending reports on the syn- Initially, triphenylphosphine and sodium azide were stirred at
thesis of 1,2,3-triazoles and halomethylcoumarins from MBH 0 °C in dimethylformamide (5 mL) for 30 minutes. To the mix-
adducts, our studies report moderate reaction conditions with an ture, bromomethane in DMF was added slowly to avoid a
improved yield. The above investigation provides a useful syn- sudden increase in temperature. The reaction was slowly
thetic tool for synthetic organic chemists. The synthesis of bio- warmed to room temperature and stirred for another 30 minutes.
logically important triazoles using the reported methodology is Finally, the reaction was quenched by the addition of diethyl
underway in our laboratory.
ether. The filtration of insoluble inorganic salts resulted in a
transparent liquid, which, upon concentration by evaporation,
provided a crude oily residue. The residue was dissolved in
ethyl acetate, washed with brine, and dried over sodium
Experimental
General information
Chemicals were purchased from Sigma-Aldrich, Spectrochem sulphate to yield a clear oily residue of the quaternary phos-
(P) Ltd., Central Drug House (P) Ltd., and Rankem, India. All phonium salt.
chemicals were used without further purification. The solvents
were purified using standard procedures. 1H and 13C NMR Typical procedure for 1a–o
spectra were recorded on a Bruker Avance 300 MHz spectrom- As described in [52]. A mixture of benzaldehyde (1.1 g,
eter using CDCl3 and DMSO-d6 as the solvent. Tetramethyl- 1.14 mL, 0.01 mol), methyl acrylate (2.05 g, 2.15 mL,
silane (TMS) was used as an internal standard. Chemical shifts 0.023 mol) and DABCO (0.87 g, 0.0077 mol) in chloroform
are given in δ relative to TMS. High-resolution mass spectra (5 mL) was stirred at room temperature for 7 d. The reaction
were recorded on an Agilent Technologies 6540 UHD accurate mixture was quenched with 10% aqueous hydrochloric acid
mass Q-TOF LC–MS spectrometer. Melting points are uncor- (50 mL) and washed repeatedly with water. The chloroform
rected. The compounds were purified using column chromatog- extract was then dried, concentrated, and purified by column
raphy on silica gel (100–200 mesh) using hexane/ethyl acetate chromatography (hexane/EtOAc 8:2, v/v) to afford 1a as
and chloroform/methanol as eluent.
colourless oil (1.64 gm, 85%).
Typical procedure for quaternary
phosphonium salts
As described in [49]. Typically, triphenylphosphine, to a solution of the Morita–Baylis–Hillman adduct 1a (1 equiv)
Typical procedure for 3a–q
A solution of AzPS (2 equiv) in acetonitrile (5 mL) was added
bromomethane, and sodium azide at a molar ratio of 1.1:1.1:5 in acetonitrile (3 mL). The reaction mixture was then stirred for
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Beilstein J. Org. Chem. 2020, 16, 1579–1587.
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