Synthesis of chiral Nβ-protected amino diselenides from the corresponding amino alkyl iodides using NaBH2Se3 as a selenating reagent and their conversion to seleninic acids

Article abstract of DOI:10.1039/c5ra06147d

A convenient approach has been presented for the synthesis of N^β-protected amino diselenides from the corresponding amino alkyl iodides using in situ generated NaBH₂Se₃ as an efficient selenating reagent. All the diselenid

Full text of DOI:10.1039/c5ra06147d

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Graphical Abstract
β
Synthesis of chiral N -protected amino diselenides from the corresponding amino alkyl
iodides using NaBH Se as a selenating reagent and their conversion to seleninic acids
2
3
Nageswara Rao Panguluri, Veladi Panduranga, Girish Prabhu, Vishwanatha TM &
Vommina V Sureshbabu
#
109, Peptide Research Laboratory, Department of Studies in Chemistry, Central College
Campus, Bangalore University, Dr. B. R. Ambedkar Veedhi, Bangalore-560 001, India
Email: hariccb@gmail.com; hariccb@hotmail.com; sureshbabuvommina@rediffmail.com;
β
A convenient approach has been presented for the synthesis of N -protected amino diselenides
from the corresponding amino alkyl iodides using in situ generated NaBH Se as an efficient
2
3
selenating reagent. All the diselenides were obtained in good yields under very mild conditions,
short duration of time and the protocol is free from racemization. The methodology has been
effectively extended to the synthesis of N-protected L-selenocystine methyl ester. Clean
β
β
oxidation of N -protected amino diselenides to the N -protected amino seleninic acids using
3
5% aqueous H O has also been accomplished. The present protocol is compatible with all the
2
2
common urethane protecting groups.

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ARTICLE TYPE
β
Synthesis of chiral N -protected amino diselenides from the
corresponding amino alkyl iodides using NaBH Se as a selenating
2
3
reagent and their conversion to seleninic acids
Nageswara Rao Panguluri, Veladi Panduranga, Girish Prabhu, T. M. Vishwanatha and
Vommina V. Sureshbabu*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
β
A convenient approach has been presented for the synthesis of N ꢀprotected amino diselenides from the
corresponding amino alkyl iodides using in situ generated NaBH Se as an efficient selenating reagent.
2
3
1
0
All the diselenides are obtained in good yields under very mild conditions, short duration of time and the
protocol is free from racemization. The methodology has been effectively extended to the synthesis of Nꢀ
β
β
protected Lꢀselenocystine methyl ester. Clean oxidation of N ꢀprotected amino diselenides to the N ꢀ
protected amino seleninic acids using 35% aqueous H O2 has also been accomplished. The present
2
protocol is compatible with all the common urethane protecting groups.
1
9
oxazolidinones employing Li Se as a selenating reagent. But
1
5
Introduction
2
2
β
these protocols are limited to the synthesis of N ꢀBoc amino
diselenides, take more time and give moderate yields.
Chandrasekaran et al., reported the synthesis of NꢀTsꢀβꢀamino
Selenium is an essential trace element present in many different
compounds with unequivocal functions. Organoselenium
1
5
5
6
6
0
5
0
5
diselenides from aziridines using tetraethylammonium
2
20
compounds find applications in organic synthesis, ligand
tetraselenotungstate [(Et N) WSe ] as a selenating reagent.
4 2 4
β
3
4
Later
selenocystine derivatives from sulfamidates using potassium
selenocyanate (KSeCN) and benzyltriethylammonium
tetrathiomolbydate [(BnEt N) MoS ] have also been reported.
a variety of N ꢀprotected amino diselenides and
chemistry
and
in
biological
processes.
Selenium
2
2
3
3
0
5
0
5
supplementation plays an important role in disease prevention
5
ꢀ
like cancer, cardiovascular, neurodegenerative and Alzheimer’s.
21
3
2
4
8
The common dietary selenium compounds are selenite,
selenomethionine, methylselenocysteine and selenocysteine
However, longer reaction duration and use of expensive reagents
which decompose on prolonged exposure to air are some of the
disadvantages of these protocols. Cu(II) catalyzed synthesis of
dialkyl, aryl or heteroaryl diselenides has also been known but
the alkaline conditions employed in these protocols are not
compatible with Fmoc chemistry.
have also been reported for the synthesis of dialkyl, aryl or
heteroaryl diselenides from the corresponding halides.
method circumventing the use of aqueous alkali, compatible with
common urethane protecting groups (Boc/Cbz/Fmoc), using
easily prepared starting material is desirable for the synthesis of
(SeCys). In contrast to oxygen and sulphur, the selenium based
methodologies offer several unique features in organic
9
chemistry. Remarkably, diselenides have attracted much
22,23
A few other approaches
attention as intermediates in organic synthesis due to their higher
stability and easier handling relative to parent selenols. Most
importantly chiral diselenides have been employed as useful
ligands and catalysts in various asymmetric transformations such
2
4ꢀ27
Thus a
1
0
as diethyl zinc addition to aldehydes,
asymmetric
β
N ꢀprotected amino diselenides.
11
12
hydrosilylation, 1,4ꢀaddition of Grignard reagents to enones,
1
3
Seleninic acids are of synthetic importance due to their utility in
stereo selective ring opening of epoxides, palladiumꢀcatalyzed
28
the construction of seleninates, selenonates and other reaction
1
4
asymmetric allylic substitution and electrophilic selenenylation
29,30
7
7
8
0
5
0
intermediates.
They have also found widespread application
15
st
31,32
of alkenes. SeCys, recognized as the 21 proteinogenic amino
acid, often found in enzymatic active sites, where its known
function is either acting as a nucleophile, a metal ligand or a
redox element. In mammals, selenium is an integral part of
as catalysts in oxidation reactions.
most stable, among the four organic acids of selenium (selenol,
selenenic, seleninic, and selenonic acids). Kehler et al., reported
the synthesis of 3ꢀaminopropaneseleninic acid dihydrotosylate
and piperidineꢀ4ꢀseleninic acid dihydrotosylate from the
corresponding diselenides using pꢀtoluene sulfonic acid
monohydrate and 35% aqueous H O . Knapp et al., reported the
clean oxidation of selenoesters to the seleninic acids using
dimethyldioxirane (DMDO) in stoichiometric amounts.
Seleninic acid group is the
33
1
6
selenoproteins as selenocysteine. The derivatives of SeCys are
the convenient precursors for the synthesis of dehydroamino
acids which are useful in the preparation of peptide conjugates.
34
4
0
2
2
1
7
28
There are few methods reported for the synthesis of βꢀamino acid
derived diselenides, which are employed in the synthetic organic
chemistry. Braga et al., reported the synthesis of βꢀamino
35
Oxidative cleavage of organodiselenides by aqueous Br ,
2
36
37
aqueous H O2 or ozone has also been reported for the
2
synthesis of seleninic acids. In continuation of our efforts in
1
8
4
5
diselenides from NꢀBocꢀaziridines
as well as NꢀBocꢀ2ꢀ
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[journal], [year], [vol], 00–00 | 1

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3
8,39
organoselenium chemistry
and in order to overcome the
a series of Nꢀprotected amino alkyl iodides 1 to the corresponding
diselenides 2 (Table 2). The present methodology is an efficient
one due to the following attributes: compatibility with the
common urethane protecting groups like Boc/Cbz/Fmoc, starting
material can be prepared easily, monoselenides and triselenides
are not formed as byproducts, simple workup and purification
procedure.
drawbacks associated with the existing protocols, we envisaged
to devise a simple, mild and efficient protocol for the synthesis of
N ꢀprotected amino diselenides from the corresponding alkyl
iodides using in situ generated NaBH Se as a selenating reagent.
Further, the oxidation of N ꢀprotected amino diselenides to the
55
β
5
2
3
β
seleninic acids using 35% aqueous H O is delineated.
2
2
Results and discussion
The selenating reagent for the present study was prepared by the
1
0
5
method of Lalancette et al., which involved the treatment of one
o
mole of NaBH with three moles of selenium in THF at 0
C
4
4
0
under nitrogen atmosphere (Scheme 1). Hydrogen gas gets
evolved immediately and within 10 minutes the conversion of
black selenium powder into reddish suspension indicates the
60
β
Scheme 2 Synthesis of N ꢀprotected amino diselenides 2
4
1,42
β
1
formation of sodium selenoborate (NaBH Se ).
So far
Table 2 List of N ꢀprotected amino diselenides 2
2
3
NaBH Se has been employed as a reducing agent however its
selenating ability has not been explored.
2
3
4
3
Entry Amino alkyl
βꢀAmino diselenide 2
Yield
(%)
94
iodide 1
o
a
THF, 0 C
NaBH4
+
3 Se
NaBH Se + H2
2 3
1
0 min, N atm
2
b
91
Scheme 1 Preparation of sodium selenoborate (NaBH Se )
2
3
20
25
30
35
40
Our initial efforts were focused on the synthesis of BocꢀPheꢀCH2ꢀ
Seꢀ) (2c). In a typical reaction, to the freshly prepared NaBH Se
3
c
95
93
2
2
o
suspension in THF at 0 C in a twoꢀnecked roundꢀbottom flask
under nitrogen atmosphere, a solution of BocꢀPheꢀCH ꢀI (1c) in
2
o
THF was added drop wise using dropping funnel at 0 C. As
d
monitored by TLC, the desired BocꢀPheꢀCH ꢀSeꢀ) (2c) was
2
2
formed within 20 min. After completion of the reaction, the
reaction mixture was filtered through celite and washed with
THF. After simple workup, BocꢀPheꢀCH ꢀSeꢀ) (2c) was isolated
e
f
91
92
2
2
by column chromatography in 95% yield (Scheme 1). The
structure of BocꢀPheꢀCH ꢀSeꢀ) (2c) was confirmed by mass
2
2
1
spectral and NMR spectroscopic analysis [ H NMR δ : 3.16ꢀ3.26
1
3
g
h
i
94
92
94
(
m, 2H, ꢀCH ꢀSeꢀ); C NMR δ : 34.92 ppm for –CH ꢀSeꢀ carbon;
2
2
7
7
Se NMR δ 280.00 ppm]. Other selenating reagents such as
Li Se , (Et N) WSe and Na Se were found to be inefficient
1
4
21
44
2
2
4
2
4
2
2
Ph
in affording the BocꢀPheꢀCH ꢀSeꢀ) (2c) in good yield (Table 1).
2
2
Se
NHFmoc
Ph
FmocHN
Se
Moreover Na Se is not compatible with Fmoc chemistry and
2
2
large scale preparation of Li Se using super hydride (Liet BH) is
2
2
3
inconvenient due to the formation of pyrophoric byproducts.
Preparation of some of these selenating reagents (Table 1, entry
I
FmocHN
2
, 3) also take longer reaction duration.
Table 1 Screening of other selenating reagents for the synthesis of 2c
j
93
Using chiral HPLC, the racemization study of the prepared
compounds Fmocꢀ ꢀPheꢀCH ꢀSe) ꢀ 2h and Fmocꢀ ꢀPheꢀCH2ꢀ
Se) ꢀ 2h* was analyzed. The retention times were found to be at
R = 15.45 min for 2h and R = 17.54 min for 2h* respectively.
t t
Intentionally prepared equimolar mixture of 2h and 2h* showed
6
5
L
D
2
2
Entry
1
Selenating reagent
Time
2.5 h
Yield (%)
62
2
Li
N)
Na
2
Se
2
45
2
3
(Et
4
2
WSe
4
70 min
3 h
68
55
distinct peaks at R = 15.80 min and R = 17.38 min. This
t
t
2
Se
2
70
confirms that the present protocol is free from racemization.
ꢀSerine is the most widely used starting material for the
synthesis of ꢀselenocysteine among the amino acids (serine,
asparatic acid, glycine, cysteine) employed. The conversion of
serine to the ꢀselenocysteine can be achieved directly using
L
50
The milder condition, shorter reaction duration (20 min) and the
excellent yield obtained in case of 2c prompted us to examine the
scope and generality of the present protocol for the conversion of
L
Lꢀ
L
2
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β
Woollin’s reagent but the protocol is restricted to Nꢀacetyl
Table 3 List of N ꢀprotected amino seleninic acids 6
4
5
46
47,48,44
protection and the activation through tosylates, halides,
49
21
βꢀlactones, or sulfamidates using conventional selenating
reagents is also known. But these protocols often suffer from
several limitations, which include harsh reaction conditions,
longer reaction duration, use of expensive reagents, inseparable
byproducts formation, difficulty in workup, incompatibility with
base sensitive groups, etc. A protocol which circumvents some of
the drawbacks associated with reported protocols is desirable for
Entry
a
b
c
d
e
f
g
h
i
Pg
Boc
Boc
Boc
Boc
Cbz
Cbz
Cbz
Fmoc
Fmoc
R
Yield (%)
CH(CH
CH(CH )
2 3 2
3
)
2
90
90
5
0
5
0
5
CH
CH
2
C
6
H
5
91
89
92
90
92
90
91
6 5
C H
CH
CH(CH
3
3
)
2
CH
CH
CHCH
2
2
3
C
C
6
H
H
5
1
1
2
2
the synthesis of
In order to demonstrate the wider scope of this reagent, we
investigated NaBH Se for the synthesis of Nꢀprotected
Lꢀselenocysteine from
Lꢀserine.
6
5
2 3
CH CH
Lꢀ
50
2
3
selenocystine methyl ester 5 from Nꢀprotected
ester 3 through the activation to iodide 4 (Scheme 3). In brief,
commercially available Nꢀprotected ꢀserine methyl ester 3 was
converted to the Nꢀprotected ꢀserine iodide methyl ester 4 in
excellent yield using imidazole, PPh and I . The solution of 4 55 diselenides employing in situ generated NaBH
Lꢀserine methyl
Conclusion
L
In conclusion, an efficient protocol for the synthesis of Nꢀ
β
L
protected
L
ꢀselenocystine methyl ester, N ꢀprotected amino
Se as a selenating
reagent from the corresponding amino alkyl iodides is developed.
The preparation of NaBH Se is simple and fast, acts as a good
5
0
3
2
2
3
in THF was then treated with in situ generated NaBH Se3 to
2
afford the Nꢀprotected
Lꢀselenocystine methyl ester 5 in good
2
3
yield. Due to the instability and high reactivity of selenol (SeH)
functional group, Secys is usually prepared as diselenide, which
selenating reagent under mild conditions without any undesirable
side reaction. The N ꢀprotected amino seleninic acids were also
β
5
1,52
is reduced
methodology is more convenient than existing procedures for the
synthesis of Nꢀprotected ꢀselenocystine methyl ester 5 from Nꢀ
protected ꢀserine methyl ester 3.
in situ to the selenol when needed. The present 60 prepared by the oxidation of diselenides with 35% aqueous H O
2 2
o
at 0 C. All the diselenides and seleninic acids prepared during
the course of these investigations were found to be stable and
were characterized by mass spectrometry, and NMR analyses.
L
L
65
Experimental section
β
General procedure for the synthesis of N -protected amino
diselenides 2, 5
To a solution of sodium borohydride (NaBH , 10 mmol) in dry
THF, black selenium powder (30 mmol) was added at 0 C under
N atmosphere. The consumption of selenium powder in less than
4
Scheme 3 Synthesis of Nꢀprotected
L
ꢀselenocystine methyl ester
o
5
from Nꢀprotected
Lꢀserine methyl ester 3
70
2
β
Since there are no reports on the synthesis of N ꢀprotected amino
seleninic acids, in the next part of the work, we concentrated on
the synthesis of amino acid derived seleninic acids 6 through the
1
0 min lead to heterogeneous reddish suspension which indicated
the formation of NaBH Se . To the resulting NaBH Se
3
3
3
4
4
0
5
0
5
2
3
2
α
suspension, a THF solution containing N ꢀprotected amino alkyl
iodide 1 or 4 (10 mmol) was added drop wise at 0 C and the
o
oxidation of the diselenides 2 using mild H O (35% aqueous). In
2
2
a typical study, to a solution of BocꢀPheꢀCH ꢀSeꢀ) (2c) in DCM
2
2
75 stirring was continued for another 20 min. After completion of
the reaction (as monitored by TLC), the reaction mixture was
filtered through celite, washed with THF and the solvent was
removed under reduced pressure. The crude mass was diluted
with EtOAc, washed with water (2 x 20 mL), brine (2 x 20 mL),
°
at 0 C, 35% aqueous H O was added drop wise under vigorous
2
2
stirring till the completion of reaction (monitored by TLC,
reaction mixture became yellow to colorless). After 30 min, Bocꢀ
PheꢀCH ꢀSeO H (6c) was formed as white precipitate, which was
2
2
filtered off. A simple recrystalization from ethanol yielded Bocꢀ
PheꢀCH ꢀSeO H (6c) as white stable solid in 91% (Scheme 4).
8
8
9
9
0
5
0
5
dried over Na SO and the solvent was removed under reduced
2 4
pressure. The crude residue 2 or 5 was purified by column
2
2
Other solvents such as MeCN, THF, and 1,4ꢀdioxane were found
chromatography on silica gel with EtOAc/hexane (2:8).
β
to be inefficient in affording the desired product 6c in good yield.
General procedure for the synthesis of N -protected amino
β
The standardized reaction conditions provided all N ꢀprotected
seleninic acids 6
β
amino seleninic acids 6 in good yields (Table 3). The present
protocol is more convenient for the synthesis of seleninic acids
than reported synthesis from selenoesters and moreover handling
To a solution of N ꢀprotected amino diselenide 2 (10 mmol) in
o
DCM at 0 C, 35% aqueous H O (64 mmol) was added drop
2
2
wise under vigorous stirring till the completion of reaction
(monitored by TLC). The change in color from yellow to
colorless indicated the complete consumption of diselenide. After
30 min, the desired product seleninic acid 6 was formed as white
precipitate, which was filtered off and recrystalized using ethanol
as a solvent. All seleninic acids were isolated as stable solids.
2
8
of 35% aqueous H O is easier than DMDO.
2
2
Acknowledgements
β
Scheme 4 Synthesis of N ꢀprotected amino seleninic acid 6
We thank the Council of Scientific and Industrial Research
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(
CSIR), Government of India (No. 02(0149)/13/EMRꢀII) for the
24 R. MingꢀDe, Z. HuaꢀRong, F. WeiꢀQiang and Z. XunꢀJun, J.
Organomet. Chem., 1995, 485, 19ꢀ24.
financial assistance. Nageswara Rao Panguluri and Veladi
Panduranga are thankful to CSIR for the SRF fellowship.
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0
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021ꢀ1022.
5
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addresses:
sureshbabuvommina@rediffmail.com.
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