Montmorillonite K10 catalyzed highly regioselective azidolysis of epoxides: A short and efficient synthesis of phenylglycine

Article abstract of DOI:10.1080/00397911.2018.1524494

A series of β‐hydroxyazides were effectively synthesized from the regioselective ring opening of epoxides by sodium azide using montmorillonite K10 as a novel heterogeneous catalyst in aqueous acetonitrile in good to excellent yields. The utility of this method has been demonstrated by achieving a short synthesis of phenylglycine in 33.5% overall yield.

Full text of DOI:10.1080/00397911.2018.1524494

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: http://www.tandfonline.com/loi/lsyc20
Montmorillonite K10 catalyzed highly
regioselective azidolysis of epoxides: A short and
efficient synthesis of phenylglycine
Keshab Ch Ghosh, Isita Banerjee & Surajit Sinha
To cite this article: Keshab Ch Ghosh, Isita Banerjee & Surajit Sinha (2018): Montmorillonite
K10 catalyzed highly regioselective azidolysis of epoxides: A short and efficient synthesis of
phenylglycine, Synthetic Communications, DOI: 10.1080/00397911.2018.1524494
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https://doi.org/10.1080/00397911.2018.1524494
Montmorillonite K10 catalyzed highly regioselective
azidolysis of epoxides: A short and efficient synthesis of
phenylglycine
Keshab Ch Ghosh, Isita Banerjee, and Surajit Sinha
Department of Organic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, India
ABSTRACT
ARTICLE HISTORY
Received 3 August 2018
A series of b-hydroxyazides were effectively synthesized from the
regioselective ring opening of epoxides by sodium azide using
montmorillonite K10 as a novel heterogeneous catalyst in aqueous
acetonitrile in good to excellent yields. The utility of this method has
been demonstrated by achieving a short synthesis of phenylglycine
in 33.5% overall yield.
KEYWORDS
Azidolysis; b-azido alcohols;
epoxide; montmorillonite
K10; phenylglycine
GRAPHICAL ABSTRACT
Introduction
Epoxides are the most useful synthetic intermediates in organic synthesis.[1] Because of
their ring strain and high reactivity, their reactions with various nucleophiles lead to
high regio and stereoselective ring-opening products.[2] Among them, the azidolysis of
epoxides are the most important for preparation of azido alcohols.[3] 1,2-Azido alcohols
are very important precursors of b-amino alcohols and vicinal diamines, which are pre-
sent in various natural products and different bioactive compounds.[4] Furthermore,
b-amino alcohols have played significant roles in the chemistry of carbohydrates and
nucleosides.[5] a-Amino acids can be prepared from corresponding b-amino alcohols by
oxidation of alcohol group. Conventionally, epoxide ring opening with azides is out
using NaN3 as reagent and NH4Cl[4a,4c–f,6] as coordinating salt in alcohol-water at
65–80 ꢀC. Under these conditions, azidolysis is generally carried out over a long reaction
time (12–48 h), and the azidohydrin is often accompanied by isomerization, epimeriza-
tion, and rearrangement products.[3,4a] Beside this, the azidolysis of epoxide is promoted
by homogeneous systems as metal chlorides,[7] salts,[8] and alkyl metal azides[9]
some heterogeneous catalysts, such as ammonium-12-molybdophosphate,[10] amberlite
CONTACT Surajit Sinha
ocss5@iacs.res.in
Department of Organic Chemistry, Indian Association for the
Cultivation of Science, Jadavpur 700032, India.
Supplemental data (full experimental details, spectral, and analytical data of synthesized compounds, and copies of
1H and 13C NMR) can be accessed on the publisher’s website.
ß 2018 Taylor & Francis Group, LLC

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K. CH GHOSH ET AL.
Table 1. Optimization of azidolysis of cyclohexene oxide.
Entry
Azide
Catalyst (mol%)
Solvent
Temp (ꢀC)
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
NaN3
NaN3
NaN3
NaN3
NaN3
NaN3
NaN3
NaN3
NaN3
NaN3
NaN3
TMSN3
40
40
40
40
40
20
10
10
5
DMF
DMF
H2O
CH3CN
CH3CN
CH3CN/H2O (8:2)
CH3CN/H2O (8:2)
CH3CN/H2O (8:2)
CH3CN/H2O (8:2)
CH3CN/H2O (8:2)
CH3CN/H2O (8:2)
CH3CN/H2O (8:2)
RT
90
RT
RT
80
80
80
80
80
80
RT
80
24
24
24
24
18
10
10
8
00
10
10
00
31
80
81
82
85
60
61
39
9
8
10
11
12a
0
5
5
48
36
36
The reaction was conducted with 1 eqv. of substrate and 1.5 eqv. of Azide.
a3 eqv. TMSN3 was used.
IRA-400 supported azide,[11] oxone,[12] sodium azide supported on Zeolite CaY,[13] qua-
ternized ammonium salt,[14] and quaternized amino functionalized cross-linked poly-
acrylamide[15] have been reported. Ring opening of epoxides with NaN3 by controlling
the pH in water has also been reported.[16] The combination of trimethylsilyl azide with
BF3.OEt2 has been used for ring opening of vinyl epoxides.[17] All the above methods
and procedures have their own advantages, but many of these methodologies suffer
from one or more different disadvantages such as unsatisfactory yields, strong acidic
conditions, hygroscopic in nature, lack of regioselectivity, cost, and stability of
the reagent.
Montmorillonite K10 is a commercially available cheap solid acid, considered as an
enviornmentally benign catalysts for electrophilic activation in organic reaction[18]
including epoxide ring opening with amines.[18b,c] Herein, we report the use of mont-
morillonite K10 as an efficient catalyst for regioselective azidolysis of epoxides with
NaN3 in aqueous acetonitrile and synthesis of phenylglycine using this methodology as
a key step.
Results and discussion
To standardize the reaction conditions for screening the catalyst, a series of experiments
were carried out for a representative reaction of cyclohexene oxide (Table 1).
Different solvents like DMF, H2O, CH3CN, and mixed solvent CH3CN–H2O were
screened for the azidolysis of cyclohexene oxide by treating with 1.5 mmol of sodium
azide in presence of 5–40 mol% of catalyst in different reaction conditions. Among
these, only mixed solvent, CH3CN–H2O was found to be suitable for this reaction and
5 mol% catalyst was effective to give maximum yield when the reaction was carried out
at 80 ꢀC (Table 1, Entry 9). When NaN3 was replaced by TMSN3, the yield was poor
(Table 1, Entry 12).
To generalize the validity of the protocol, a wide range of substituted epoxides were
treated with NaN3 under the standardized conditions. The result is summarized in

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3
Table 2 showing that b-azidoalcohols were obtained in excellent yields with high or
exclusive regioselectivity. In contrast to the earlier report[18b] on ring opening of epox-
ide of epichlorohydrin by aniline in 69% yield, we got very clean azido alcohol 2a in
94% yield (Table 2, Entry 2). Compound 2a could be useful for making aziridine ring.
2-Aryl epoxides afforded 2-azidoalcohols as major product by the attack of azide
nucleophile at the benzylic position due to the formation of the stabilized benzylic cat-
ion (Table 2, Entry 3–4) which is consistent with the previously reported result-
s[7a–c,10,12] and the expected outcome of the regioselectivity of ring opening of styrene
oxide by azide was discussed earlier by Sarangi et al.[7c] This is a suitable substrate for
the preparation of phenylglycine (vide infra). In the case of monoalkyl-substituted epox-
ides, desired b-azido alcohols were obtained with a reversal of regioselectivity because
of the predominant attack of azide ion on the less hindered carbon of the epoxide
(Table 2, Entry 5–13). All the terminal epoxides gave highly regioselective azidohydrins
in almost quantitative yields. We applied this methodology on enantiomerically pure
functionalized cyclohexene oxide 14. As a part of our research,[19] 14 was synthesized
according to Scheme 1 via the formation of lactone 15.[20] 15 on treatment with
NaOMe in MeOH gave methyl ester 16. After mesylation followed by ketal deprotection
yielded free diol 18 in excellent yield. 18 was treated with DBU to obtain the epoxide
19 in 74% yield. After TBDPS protection, the desired epoxide 14 was obtained which
was subjected to montmorillonite-K10 mediated ring opening to get a mixture of prod-
ucts in which 14a was the major product due to the attack of azide ion on the less hin-
dered carbon center. Reaction occurred smoothly in SN2 fashion and only trans product
1
was obtained (Table 2, Entry 1, 14), confirmed from corresponding H NMR spectrum.
These highly functionalized stereoselective cyclohexane derivatives could be useful as a
building block for organic synthesis.[21]
Plaussible mechanism in the ring opening reaction by montmorillonite K10 is shown
in Scheme 2. For 2-aryl epoxides, oxygen of epoxide interacts with the Mont K10 cata-
lyst and generates a stable benzylic cation. Finally, azide ion attack to the positive car-
bon center via SN1 pathway to form terminal alcohol. For alkyl substituted epoxides,
oxygen atom interacts with the Mont K10 and azide ion attack from the less hindered
carbon center to form terminal azide.
Encouraged by these results, we then decided to employ this method for the synthesis
of phenyl glycine. Arylglycines are the important nonproteinogenic class of amino
acids.[22] The isolation of arylglycines from natural sources is rare but has increased in
frequency over the past 40 years. For example, 3-hydroxy and 3,5-dihydroxyphenylgly-
cine were isolated from latex.[23] Aryl glycines are found in numerous glycopeptide anti-
biotics,[24] such as in vancomycin, teicoplanin. Vancomycin consists of a heptapeptide
in which three of the amino acid residues are functionalized arylglycines. Several substi-
tuted phenylglycine derivatives, including 3-hydroxyphenylglycine, have been described
as potent and selective agonists or antagonists of glutamate receptors of the central ner-
vous system (CNS).[25] Another natural source of arylglycines is the family of monocyc-
lic b-lactam antibiotics known as the nocardicins. Nocardicins contain two
p-hydroxyphenylglycine derivatives which are thought to serve as the starting material
for nocardicin biosynthetically.[26] Beside from the naturally occurring arylglycines,
there are also a number of unique synthetic arylglycines. D-arylglycines are used as a

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K. CH GHOSH ET AL.
Table 2. Ring-opening of epoxides with NaN3 in CH3CN–H2O (8:2) using Montmorillonite-K10
(5 mol%) at 80 ꢀC.
Entry
1
Epoxide
Producta
Time (h)
8
Yieldb (%)
85
2
3
7
8
94
75(15)
4
8
67(17)
5
6
8
8
91
95
7
8
8
8
7
7
86(5)
96
9
98
10
98
11
8
95
12
13
8
7
92
97
14
8
39(22)
a The products obtained were characterized by 1H ,13C NMR, IR and mass spectra.
b isolated pure product, yields in parantheses corresponds to the other isomer.

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Scheme 1. Synthesis of compound 14.
Scheme 2. Tentative mechanistic pathway of the epoxide ring opening reaction.
side chain moiety of semisynthetic penicillins and cephalosporins. For example, phenyl-
glycine present as a side-chain constituent in synthetic antibiotic cephalexin[27] and p-
hydroxyphenylglycine is used as a side-chain moiety in the antibiotics cefadroxil and
amoxicillin.[28] Because of their significant biological importance, lot of attention has
been given to their synthesis. We started our synthesis from 2-azido-2-phenylethanol
3a. Initially, we tried to oxidize the azido alcohol to azido acid using Jones reagent,[29]
but we got the desired azido acid 20 as a minor product (22%) along with benzonitrile
21 as a major product (Scheme 3). Similar result was observed during RuCl3 catalyzed
oxidation[30] of azido alcohol 3a.

6
K. CH GHOSH ET AL.
Scheme 3. Oxidation of azido alcohol.
Scheme 4. Synthesis of phenylglycine.
So, we modified our synthetic route. The 2-azido alcohol 3a was reduced to 2-amino
alcohol 22 via hydrogenation.[31] Amine of 22 was protected with Cbz-chloride to afford
protected[32] amino alcohol 23. Here, we decided to follow stepwise conversion of alco-
hol to acid via formation of aldehyde. Accordingly, alcohol 23 was treated with IBX to
form aldehyde which was oxidized to acid under Pinnick condition.[33] Interestingly,
two steps oxidation gave better yield in compare to Jones oxidation. The acid on treat-
ment with diazomethane furnished methyl ester 24.[34] Finally, hydrogenation of 24
afforded the free amino ester 25 in 33.5% yield from 3a (Scheme 4). It is worth to men-
tion here that synthesis of phenylglycine was not possible using previously reported
amine-mediated ring opening of epoxides by K-10[18b,c] as the generation of free amine
was impossible.
Conclusion
We have developed a mild and simple method for the preparation of 1,2-azidoalcohols
by ring-opening of epoxides with NaN3 using only 5 mol% of montmorillonite K10 as a
heterogeneous catalyst. The advantages of the present method such as simplicity in
operation, the low cost of reagents, high yields of products, excellent regioselectivity
make it a valuable alternative to the existing methods reported in the literature.
Furthermore, we have described an efficient synthesis of phenyl glycine using this meth-
odology as a key step.
Experimental section
All reagents were purchased from commercial sources and used without further
purification unless otherwise stated. Petroleum ether (PE) refers to the fraction
of petroleum boiling between 60 ꢀC and 80 ꢀC. The following abbreviations are

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7
used
for
CbzCl ¼ Benzyl
chloroformate,
MsCl ¼ Methanesulfonyl
chloride,
TBDPSCl ¼ tert-
DBU ¼ 1,8-Diazabicyclo[5.4.0]undec-7-ene,
MeOH ¼ methanol,
Butyldiphenylchlorosilane, DMAP ¼ 4-Dimethylaminopyridine, IBX =2-iodoxybenzoic
acid, THF ¼ Tetrahydrofuran, DMF ¼ Dimethylformamide, DCM ¼ dichloromethane,
EtOAc ¼ ethyl acetate. All the reactions were carried out in oven-dried glassware under
an argon atmosphere using anhydrous solvents, standard syringe, and septum techni-
ques unless otherwise indicated. Organic extracts were dried over anhydrous Na2SO4
and then filtered prior to removal of all volatiles under reduced pressure on rotary
evaporation. Chromatographic purification of products was accomplished using column
chromatography on silica gels (mesh 100–200). Thin-layer chromatography (TLC) was
carried out on aluminum sheets, Silica Gel 60 F254 (Merck; layer thickness 0.25 mm).
Visualization of the developed chromatogram was performed using UV light and/or
1
ninhydrin, CAM stains. IR spectra were recorded as thin films (for liquids). H and 13C
NMR spectra were recorded at 300, 400, or 500 MHz and 75, 100, or 125 MHz, respect-
ively using CDCl3 as solvent. Chemical shifts (d) are given in ppm relative to the solv-
ent residual peak or TMS as internal standard. The following abbreviations are used
for multiplicity of NMR signals: s ¼ singlet, d ¼ doublet, t ¼ triplet, m ¼ multiplet,
br ¼ broad. High Resolution Mass Spectra (HRMS) were measured in a QTOF I (quad-
rupole-hexapole-TOF) mass spectrometer with an orthogonal Z-spray-electrospray inter-
face on Micro (YA-263) mass spectrometer (Manchester, UK).
General method for epoxide ring opening of styrene oxide
To a mixture of epoxide (1 equiv.) and NaN3 (1.5 equiv.) in CH3CN–H2O (8:2, 10 mL)
Montmorillonite K 10 (5% w/w) was added. The reaction mixture was stirred at 80 ꢀC
for a specified time as required to complete the reaction. After completion as indicated
using TLC, the reaction mixture was then filtered, the filter pad was washed with
CH3CN, and the combined filtrates were evaporated. The reaction mixture was
extracted with EtOAc, the combined organic layers were washed with H2O and brine,
dried over anhydrous Na2SO4, and evaporated under reduced pressure. The crude prod-
uct was then purified using column chromatography over silica gel using EtOAc/petrol-
eum ether as eluent to provide the pure azido alcohol.
2 -Azido-2-phenylethanol (3a)
1
Isolated yield: 520 mg (75%); colorless oil; H NMR (300 MHz, CDCl3) d 7.44–7.31 (m,
5H), 4.68 (t, J ¼ 6.3 Hz, 1H), 3.75 (t, J ¼ 6.3 Hz, 2H), 2.20 (d, J ¼ 6.3 Hz, 1H); 13C NMR
(75 MHz, CDCl3) d 136.4, 129.1, 128.8, 127.3, 68.0, 66.6; IR (Neat) ꢀmax 3411, 3014,
2918, 2106, 1454 cmꢁ1 [7a]
.
2 -Amino-2-phenylethanol (22)
A solution of 2-Azido-2-phenylethanol 3a (540 mg, 3.31 mmol) in MeOH (10 mL) was
degassed with Argon. 10% Pd/C (35 mg) was added and the mixture was stirred under
H2 for 24 h at room temperature. After filtration over celite, the solvent was removed

8
K. CH GHOSH ET AL.
under reduced pressure to give the title compound 22 as slightly yellow oil (450 mg,
99%). The crude product was used in the next step without further purification.
1H NMR (500 MHz, CDCl3) d 7.35–7.26 (m, 5H), 4.03 (m, 1H), 3.71 (m, 1H), 3.55
(t, J ¼ 9.5 Hz, 1H), 2.49–2.34 (m, 3H); 13C NMR (125 MHz, CDCl3) d 142.7, 128.7,
127.6, 126.6, 68.1, 57.5; IR (Neat) ꢀmax 3367, 3064, 1602, 1492, 1215 cmꢁ1; HRMS (ESI):
m/z [M þ H]þ calcd for C8H12NO: 138.0913; found: 138.0911.[32]
Benzyl (2-hydroxy-1-phenylethyl)carbamate (23)
To a solution of 22 (778 mg, 5.67 mmol) and DMAP (7 mg, 0.056 mmol) in DCM
(20 mL) was added dropwise benzyl chloroformate (971 lL, 60 mmol) at 0 ꢀC, followed
by Et3N (1.98 mL, 14.2 mmol). The resulting mixture was stirred at room temperature
for overnight. The solvent was evaporated. The reaction mixture was extracted with
EtOAc and the combined organic layers were washed with H2O and brine, dried over
anhydrous Na2SO4, and evaporated under reduced pressure. Crude product was purified
by column chromatography using EtOAc-pet ether (3:7) on silica gel to afford 23 as a
1
colorless oil (1.16 g, 76% yield). H NMR (300 MHz, CDCl3) d 7.32–7.24 (m, 10H), 5.75
(s, 1H), 5.07 (s, 2H), 4.81 (s, 1H), 3.77 (s, 2H), 2.77 (br s, 1H); 13C NMR (75 MHz,
CDCl3) d 156.6, 139.3, 136.4, 128.9, 128.6, 128.3, 127.9, 126.7, 67.1, 66.4, 57.2; IR (Neat)
ꢀmax 3365, 2931, 2106, 1734, 1109 cmꢁ1; HRMS (ESI): m/z [M þ Na]þ calcd for
C16H17NO3Na: 294.1106; found: 294.1107.[32]
Methyl 2-(((benzyloxy)carbonyl)amino)-2-phenylacetate (24)
To a solution of the alcohol 23 (310 mg, 1.14 mmol) in EtOAc (15 mL) was added 2-
iodoxybenzoic acid (IBX) (640 mg, 2.29 mmol) portion wise, and it was refluxed for 2 h
in open atmosphere. After completion of the reaction (TLC), the mixture was cooled to
room temperature and filtered through a celite pad. Solvent removal under reduced
pressure afforded the crude aldehyde (307 mg) which was used in the next step without
further purification.
To a solution of the above crude aldehyde in t-BuOH (6 mL) and 2-methyl-2-butene
(3 mL) was added a solution of NaClO2 (465 mg, 5.14 mmol) and NaH2PO4 (475 mg,
3.43 mmol) in H2O (3 mL) at 0 ꢀC. The reaction mixture was stirred for 2 h at room
temperature and diluted with saturated aqueous NH4Cl solution (1 mL). The organic
layer was extracted with EtOAc (3 ꢂ 5 mL). The combined organic layers were dried, fil-
tered, and concentrated in vacuo to give the crude product which was purified by silica
gel column chromatography, eluting with DCM-MeOH (97:3), to afford the correspond-
ing acid as white foam (291 mg). Product was confirmed by mass spectrometry. HRMS
(ESI): m/z [M þ Na]þ calcd for C16H15NO4Na: 308.0899; found: 308.0898.
The free acid (291 mg, 1.02 mmol) was then dissolved in diethyl ether (5 mL, min-
imum volume). Diazomethane in diethyl ether was added slowly at 0 ꢀC until a yellow
color persisted (monitored by TLC). Removal of ether followed by silica gel column
chromatography purification using EtOAc—petroleum ether (1:4) gave the product
24 as colorless to faint yellow oil (185 mg, 55%, after 3 steps from amino alcohol 23).
1H NMR (300 MHz, CDCl3) d 7.39–7.31 (m, 10H), 5.99 (d, J ¼ 7.5 Hz, 1H), 5.42

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SYNTHETIC COMMUNICATIONSV
9
(d, J ¼ 7.5 Hz, 1H), 5.11 (d, J ¼ 3.9 Hz, 2H), 3.71 (s, 3H); 13C NMR (75 MHz, CDCl3) d
171.4, 155.4, 136.6, 136.2, 129.0, 128.6, 128.5, 128.2, 127.2, 67.1, 58.0, 52.8; IR (Neat)
ꢀmax 3338, 2945, 1739, 1707, 1512, 1234 cmꢁ1; HRMS (ESI): m/z [M þ Na]þ calcd for
C17H17NO4Na: 322.1055; found: 322.1056.
Methyl 2-amino-2-phenylacetate (25)
A solution of compound 24 (135 mg, 0.45 mmol) in MeOH (5 mL) was degassed with
Argon. 10% Pd/C (15 mg) was added, and the mixture was stirred under H2 for 20 h at
room temperature. After filtration over celite, the solvent was removed under reduced
pressure to give the title compound which was purified by column chromatography
using EtOAc/pet ether (2:3) on silica gel to afford 25 as a slightly yellow oil (60 mg,
1
81% yield). H NMR (300 MHz, CDCl3) d 7.39–7.26 (m, 5H), 4.61 (s, 1H), 3.69 (s, 3H),
1.93 (s, 2H); 13C NMR (75 MHz, CDCl3) d 174.6, 140.4, 128.9, 128.1, 126.9, 58.8, 52.4;
IR (Neat) ꢀmax 3377, 3029, 2952, 1735, 1454, 1186 cmꢁ1; HRMS (ESI): m/z [M þ H]þ
calcd for C9H12NO2: 166.0863; found: 166.0865.
Acknowledgments
K. C. G and I. B. are thankful to CSIR, New Delhi for their fellowships.
Funding
S.S. thanks Indo-French Centre for Promotion of Advanced Research (CEFIPRA) for financial
support by a grant [IFC/A/5105-2/2014/1060].
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