Oxazoline derivatives tagged with tosylated amino acids as recyclable organocatalysts for enantioselective allylation of aldehydes

Article abstract of DOI:10.1039/c3ra47424k

A series of amino acid-based oxazoline compounds have been prepared and successfully applied to the enantioselective allylation reaction of aldehydes. The fine-tuning of the structure of the oxazolines led to (S,S)-4 as an efficient organocatalyst which gave homoallyl alcohols in good yield (up to 90%) and excellent ee (up to 99%) for a wide range of substrates including aromatic, hetero-aromatic and α,β-unsaturated aldehydes. The chiral organocatalyst was synthesized in three easy steps with an overall 88% yield and successfully recycled for up to three cycles. On the basis of the experimental observations and NMR studies, a probable mechanism was proposed for this reaction.

Full text of DOI:10.1039/c3ra47424k

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Oxazoline derivatives tagged with tosylated amino
acids as recyclable organocatalysts for
Cite this: RSC Adv., 2014, 4, 12257
enantioselective allylation of aldehydes†
a
a
ab
Debashis Ghosh,ab Arghya Sadhukhan, Nabin Ch. Maity, Sayed H. R. Abdi,
*
Noor-ul H. Khan,ab Rukhsana I. Kureshyab and Hari C. Bajajab
A series of amino acid-based oxazoline compounds have been prepared and successfully applied to
the enantioselective allylation reaction of aldehydes. The fine-tuning of the structure of the oxazolines
led to (S,S)-4 as an efficient organocatalyst which gave homoallyl alcohols in good yield (up to 90%) and
excellent ee (up to 99%) for a wide range of substrates including aromatic, hetero-aromatic and a,b-
unsaturated aldehydes. The chiral organocatalyst was synthesized in three easy steps with an overall 88%
yield and successfully recycled for up to three cycles. On the basis of the experimental observations and
NMR studies, a probable mechanism was proposed for this reaction.
Received 9th December 2013
Accepted 16th January 2014
DOI: 10.1039/c3ra47424k
www.rsc.org/advances
used as organocatalysts for the allylation of aldehydes. Subse-
quently, chiral oxazolines appended with chiral sulfoxide14 and
N-oxides4k have also been registered for their potential as
Introduction
The application of organocatalysts in asymmetric C–C bond
formation reactions, especially in asymmetric allylation reac-
tions, is oen preferred over metal-based catalysts, particularly
in the pharmaceutical sector due to health, environmental, cost
and efficiency concerns. For the development of new organo-
catalysts in asymmetric allylation reactions, various natural and
synthetic amino acids as chiral building blocks seem to be most
suitable due to their cost-effectiveness and easy availability.
Chiral allylation products, i.e. homoallyl alcohols and homoallyl
amines, are important building blocks for the construction of
many biologically active compounds.1,2 For the production of
homoallyl alcohol, allyltrialkylsilanes2a,c and stannanes3 react
readily with aldehydes upon the activation of the carbonyl
group by a Lewis acid.2,3 By contrast, allyltrichlorosilane
requires activation with a Lewis base that coordinates to the
silicon atom. Lewis bases reported in this respect are N-oxides,4
formamides,5 bisformamides,6 phosphine oxide,7 phosphina-
mides,8 urea derivatives9 and catecholates.10 Oxazoline deriva-
tives as efficient ligands in metal-catalyzed asymmetric
allylation reactions has been well documented,2c,11,12 however,
Barrett et al.13 (1997) for the rst time demonstrated that
bidentate pyridine-linked oxazoline compounds can directly be
organocatalysts for the asymmetric allylation of aldehydes.
However, there are nagging issues, such as the extremely low
reaction temperatures, moderate yield and ee, and non-
recyclability of these catalysts, which need to be addressed.
Bearing in mind the value of oxazolines, overall stability of the
catalyst and our previous experience with tosylated amino
acids15 as organocatalysts in the enantioselective allylation
reaction of aldehydes, herein we have synthesized a series of
oxazoline-based organocatalysts,16,17 (S,S)-1 to (S,S,R)-5,
featuring sulfonamide groups with varying steric features and
two to three chiral centers. Chiral centers in organocatalysts
featuring congurations with different permutations and
combinations were prepared for their possible role in inu-
encing the product enantioselectivity. In order to know the
specic role of the sulfonamide moiety in the organocatalyst on
the activity and enantioselectivity of the allylation reaction, we
also synthesized
a
Boc-protected organocatalyst, (S,S)-6.18
Among these organocatalysts, (S,S)-4 (10 mol%) was found to be
the most promising, robust and recyclable (3 times) catalyst that
gave allylation products in 52–90% yield and 65–99% ee with
various substituted aldehydes at 0 ꢀC.
aDiscipline of Inorganic Materials and Catalysis, CSIR-Central Salt and Marine
Chemicals Research Institute (CSIR-CSMCRI), G. B. Marg, Bhavnagar-364 021,
Gujarat, India. E-mail: shrabdi@csmcri.org; Fax: +91-0278-2566970
bAcademy of Scientic and Innovative Research, CSIR-Central Salt and Marine
Chemicals Research Institute (CSIR-CSMCRI), Council of Scientic & Industrial
Research (CSIR), G. B. Marg, Bhavnagar-364 021, Gujarat, India
† Electronic supplementary information (ESI) available: 1H and 13C spectra for all
new and the few known compounds, and HPLC chromatograms for the homoallyl
alcohols described herein. See DOI: 10.1039/c3ra47424k
Results and discussion
Recently we have shown that tosylated phenylalanine-based
amides are efficient organocatalysts for the enantioselective
allylation of aldehydes. Therefore, instinctively we thought of
combining tosylated phenylalanine and oxazoline and synthe-
sising organocatalyst (S,S)-1 in three simple synthetic steps.
The same strategy was used to prepare the remaining
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 12257–12265 | 12257

View Article Online
RSC Advances
Paper
organocatalysts 1–5 with varied chirality (Scheme 1).15,17 We also
To verify this, we prepared catalyst (S,S)-6 by replacing the
synthesized catalyst (S,S)-6 (Scheme 2) by replacing the tosyl tosyl part of catalyst (S,S)-4 by Boc (Scheme 2), and observed a
part of the catalyst (S,S)-4 with Boc to ascertain the role of the signicant drop in both the product yield (55%) and ee (45%)
tosyl group in the allylation reaction.
(entry 9).
Catalyst (S,S)-4, which was the best performer, was subjected
To begin with, catalyst (S,S)-1 was evaluated for its efficacy in
the asymmetric allylation of aldehydes by using 4-methoxy to optimization of the catalytic reaction conditions to further
benzaldehyde (0.5 mmol) as a model substrate with allyltri- improve the yield and enantioselectivity of the allylation
chlorosilane (1.2 equivalent) as an allylating agent in CH2Cl2 at product. The role of additives in the asymmetric allylation
RT. In all of the catalytic reactions, diisopropylethyl amine reaction is well documented,2 therefore we rst scrutinized the
(DIPEA) as a base additive (2 equiv. with respect to the effect of different additives (mostly Lewis bases), viz., DIPEA,
substrate) was used to facilitate the allylation reaction. Catalyst Et3N, 1,8-diazabicycloundec-7-ene (DBU) and tetrabutylammo-
(S,S)-1 showed some hopeful results (Table 1, entry 1; yield 60%, nium iodide (TBAI) (Table 2, entries 2–5), on this reaction by
ee 54%). Catalyst (R,S)-1 with mismatched chirality gave a keeping the other parameters constant. It is to be noted that in
product with signicantly lower yield (56%) and ee (40%) the absence of an appropriate additive, the catalyst efficiency
(entry 2).
was signicantly lower (Table 2, entry 1). DIPEA was found to be
The opposite enantiomeric catalyst (S,R)-1 gave similar the most suitable (entry 2) among the additives used here.
activity (yield 55%) and enantioselectivity (ee 40%) but with the
opposite conguration of the allylation product (entry 3).
Even the amount of DIPEA with respect to the substrate
amount was found to be crucial for the desired outcome of the
Based on these results, it can be concluded that the enan- reaction, as is evident from the experiments carried out over a
tioinduction in the product is largely governed by the chirality range of 1–3 equivalents of DIPEA (entries 2, 6 and 7), with 2
originating from the phenylalaninol part of the catalyst. In equivalents (entry 2) found to be the optimum. The temperature
order to improve the efficiency of the organocatalyst, we effect (Table 2, entries 8–10) on this reaction, studied from ꢁ40
replaced the (S)-phenylalaninol moiety from catalyst (S,S)-1 with ꢀC to RT, revealed that at 0 ꢀC the product ee (97%) was highest
(S)-phenylglycinol and synthesized catalyst (S,S)-2. Unfortu- with 67% yield in 24 h (entry 8).
nately, there was a signicant drop in the product yield (40%) as
Further, the catalyst loading of 10 mol%, which was used in
well as the ee (17%) (entry 4). Catalyst (S,S)-3, which was the foregoing experiments, was found to be optimum as it was
prepared by replacing the (S)-phenylglycinol moiety from cata- observed that by decreasing the catalyst loading (5 mol%) the
lyst (S,S)-2 with (S)-tert-leucinol, showed relatively better product yield (50%) dropped signicantly. On the other hand,
performance (entry 5: yield, 65%; ee, 60%). To our pleasant with an increase in catalyst loading (15 mol%), the reaction was
surprise, substitution of tosylated (S)-phenylalanine from cata- faster but with a marginal drop in the ee (90%) while the
lyst (S,S)-3 with tosylated (S)-tert-leucine to obtain catalyst (S,S)- product yield remained similar (Table 3, entry 3). Next, solvent
4 resulted in a substantial improvement in the product yield variation studies (Table 3, entries 2, 4–7) showed that CH2Cl2
(72%) and ee (70%) (entry 6). For further improvement in the was the most suitable solvent among the solvents studied here
catalyst performance, we varied the steric features in the oxa- (Table 3, entry 2).
zoline part of the catalyst as in catalysts (S,R,S)-5 (entry 7) and
Aer achieving the optimum reaction conditions (Table 3,
(S,S,R)-5 (entry 8), but these could not match the performance entry 2) for the enantioselective allylation reaction, we investi-
of catalyst (S,S)-4. The sulfonamide moiety in organocatalyst gated the efficacy of the catalyst (S,S)-4 for various aromatic,
(S,S)-4 has a specic role, possibly in activating the silicon atom hetero-aromatic and aliphatic aldehydes as substrates (Table 4).
of allyltrichlorosilane through coordination of its sulfonamide
oxygen.
Among the different aromatic aldehydes, m/p-substituted
benzaldehydes (Table 4, entries 2–9) gave better ees as
Scheme 1 Synthesis of tosyl-protected organocatalysts.
12258 | RSC Adv., 2014, 4, 12257–12265
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
Scheme 2 Synthesis of Boc-protected organocatalyst.
compared to their o-counterparts (Table 4, entries 10 and 11). heteroaromatic aldehyde ,thiophene-2-carboxaldehyde (Table 4,
The enantioselectivity was comparatively better for substrates entry 13) behaved just like benzaldehyde by giving only a
with electron donating substituents (Table 4, entries 2–4) than marginally higher yield (82%) and comparable ee (92%). a,b-
electron withdrawing substituents (Table 4, entries 6–8). The Unsaturated aldehydes, viz. trans-cinnamaldehyde and trans-a-
bulkier aldehydes such as 2-naphthaldehyde, were found to be methyl cinnamaldehyde, as substrates gave products with good
more reactive in the present catalytic system, which gave a yields and good to excellent ees (Table 4, entries 14 and 15), but
product with higher yield, but with a signicantly lower ee with an inverted conguration (R) as compared to the products
(Table 4, entry 12) than benzaldehyde (Table 4, entry 1). The (conguration S) obtained with the rest of the aldehydes used in
Table 1 Screening of chiral organocatalysts (S,S)-1 to (S,S)-6 for the enantioselective allylation reactiona
Entry
Yieldb [%]
eec [%]
Cong.d
Entry
Yieldb [%]
eec [%]
Cong.d
Entry
Yieldb [%]
eec [%]
Cong.d
1
60
54
S
2
56
40
S
3
55
40
R
Entry
Yieldb [%]
eec [%]
Cong.d
Entry
Yieldb [%]
eec [%]
Cong.d
Entry
Yieldb [%]
eec [%]
Cong.d
4
40
17
S
5
65
60
S
6
72
70
S
Entry
Yieldb [%]
eec [%]
Cong.d
Entry
Yieldb [%]
eec [%]
Cong.d
Entry
Yieldb [%]
eec [%]
Cong.d
7
60
20
R
8
63
35
S
9
55
45
S
a
All the reactions were carried out by using the substrate 4-methoxy benzaldehyde (0.5 mmol), allyltrichlorosilane (0.6 mmol), diisopropylethyl
b
c
amine (1 mmol) and catalyst (10 mol%) in CH2Cl2 at RT. Isolated yields aer column chromatography. ee determined by chiral HPLC using
a Daicel Chiralcel OD-H column. Absolute congurations were assigned by comparing both the retention time and optical rotation with the
d
reported reliable data.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 12257–12265 | 12259

View Article Online
RSC Advances
Paper
Table 2 Effect of additives and temperature on the asymmetric ally- Table 4 Enantioselective allylation of various aldehydes using catalyst
lation reactiona
(S,S)-4a
Entry Additive [equiv.] Temp. [ꢀC] Time [h] Yieldb [%] eec [%]
Time Yieldb eec
Entry Substrate
[h]
[%]
[%] Cong.d
1
2
3
4
5
6
7
8
9
—
RT
RT
RT
RT
RT
RT
RT
0
24
20
20
20
20
20
20
24
24
36
40
72
77
68
50
55
71
67
62
60
58
70
50
41
30
68
70
97
96
90
DIPEA (2.0)
Et3N (2.0)
DBU (2.0)
TBAI (2.0)
DIPEA (1.0)
DIPEA (3.0)
DIPEA (2.0)
DIPEA (2.0)
DIPEA (2.0)
1
C6H5CHO (1a)
18
24
24
24
24
20
22
20
20
20
22
14
14
75
67
68
70
65
70
78
75
77
84
74
85
82
93
97
S
S
2
3
4
5
6
7
4-MeO–C6H4CHO (1b)
4-(C6H5CH2O)–C6H4CHO (1c)
4-(CH3)3C–C6H4CHO (1d)
4-Me–C6H4CHO (1e)
4-NO2–C 6H4CHO (1f)
4-F–C6H4CHO (1g)
>99 (ꢁ)e
97
86
90
75
77
93
64
69
81
92
S
S
S
S
S
S
S
S
S
S
ꢁ20
8
9
10
11
12
13
4-CF3–C6H4CHO (1h)
3-MeO–C6H4CHO (1i)
2-MeO–C6H4CHO (1j)
2-F–C6H4CHO (1k)
2-Naphthaldehyde (1l)
Thiophene-2-carbaldehyde
(1m)
10
ꢁ40
a
b
Reaction conditions as per Table 1. Isolated yield aer column
c
chromatography. ee determined by chiral HPLC using a Daicel
Chiralcel OD-H column.
14
15
16
(E)C6H5CHCHCHO (1n)
(E)C6H5CHC(CH3)CHO (1o)
C6H5CH2CHO (1p)
15
15
24
77
90
52
80
93
67
R
R
R
Table 3 Optimization of catalyst loading and effect of the solvent on
the asymmetric allylation reactiona
a
b
Reaction conditions as per Table 1. Isolated yields aer column
c
chromatography. ee determined by chiral HPLC using Daicel
Chiralcel and Chiralpak OD-H, AS-H, IA, IB and IC columns according
d
to reported procedures. Absolute congurations were assigned by
comparing both the retention time and optical rotation with the
reported reliable data. e Optical rotation for product 2c was negative.
Cat. loading
[mol%]
Time
[h]
Yieldb
[%]
eec
[%]
Entry
Solvent
1
2
3
4
5
5
10
15
10
10
CH2Cl2
CH2Cl2
CH2Cl2
THF
CH2Cl2 : THF
(1 : 1)
24
24
17
30
24
50
67
68
52
55
96
97
90
67
60
6
7
10
10
CHCl3
Toluene
20
36
60
40
70
55
a
b
Reaction conditions as per Table 1. Isolated yield aer column
c
chromatography. ee determined by chiral HPLC using a Daicel
Chiralcel OD-H column.
the present study. Similarly, the inversion of conguration was
also observed for the product (conguration R) obtained with
phenylacetaldehyde, but with a signicantly lower yield (52%)
and ee (67%).
Fig. 1 13C NMR spectra were taken in CDCl3: (a) benzaldehyde; (b)
catalyst (S,S)-4; (c) 13C spectrum of catalyst (S,S)-4 after interaction
with allyltrichlorosilane; (d) 13C spectrum of catalyst (S,S)-4 after
interaction with allyltrichlorosilane and benzaldehyde; (e) 13C spec-
trum of catalyst (S,S)-4 after interaction with allyltrichlorosilane,
benzaldehyde and DIPEA.
Reaction mechanism: NMR experiments
A series of NMR experiments were performed for understanding
of the mechanism of the allylation reaction operating in the
present catalytic system. For this, we looked for any observable shied downeld to 179.02 ppm upon the addition of allyltri-
changes in the NMR spectra of catalyst (S,S)-4 aer the chlorosilane. This large shiing (15.2 ppm) indicates that there
sequential addition of allyltrichlorosilane, substrate (benzal- is a strong interaction between the oxazoline nitrogen and
dehyde) and DIPEA. The 13C NMR spectra (Fig. 1) showed that silicon of allyltrichlorosilane to form an intermediate I-1
the imine carbon peak (163.82 ppm) of catalyst (S,S)-4 was (Scheme 3).4l,7c
12260 | RSC Adv., 2014, 4, 12257–12265
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
Aer the addition of the substrate, this peak (179.02 ppm)
only marginally shied to appear at 178.93 ppm (spectrum (d),
Fig. 1). It is to be noted that on adding DIPEA, product forma-
tion takes place (full spectrum in ESI†) with concomitant
release of the catalyst7c,15 (Scheme 3), as evident from spectrum
(e) (Fig. 1), where the oxazoline imine carbon peak of the free
catalyst at 163.42 ppm was restored (full spectrum in ESI†).
The product formation in the above reaction was followed by
1H NMR experiments. The reaction was conducted in CDCl3
with 10 mol% of catalyst (S,S)-4 for the allylation of benzalde-
hyde at RT, and 1H NMR spectra were recorded at 0, 4, 8 and 12
h intervals (Fig. 2). The allylation product peaks at 5.05–5.08
ppm grew over a period of time and became sufficiently visible
at ꢂ60% conversion (full spectra in ESI†).
A catalytic cycle (Scheme 3) based on the results obtained
during the catalyst screening experiments (Table 1) and NMR
study has been proposed here. Accordingly, the catalyst rst
interacts with allyltrichlorosilane to form an intermediate
I-1.4l,7c The intermediate I-1 then reacts with the substrate and
probably forms intermediate I-2,4l,7c which reacts with DIPEA to
give the desired product and releases the catalyst for the next
cycle.
Scheme 3 Proposed catalytic cycle for the enantioselective allylation
reaction catalysed by (S,S)-4.
Recyclability of the catalyst
The main problem with homogeneous catalysts is their sepa-
ration and reuse from the reaction medium in post-catalysis
workup. Nevertheless, we have attempted the recycling of
catalyst (S,S)-4 in order to make the present catalytic protocol
more cost-effective and also to demonstrate the robust nature of
the catalyst under the allylation reaction conditions used. The
reuse experiments were conducted using 4-methoxy benzalde-
hyde as a representative substrate with catalyst (S,S)-4 and
ꢀ
allyltrichlorosilane as the allyl source at 0 C in CH2Cl2 in the
presence of DIPEA. Aer the rst catalytic run, the amount of
solvent was reduced, and the organocatalyst (S,S)-4 was
precipitated by the addition of an excess amount of hexane. The
precipitate contained both organocatalyst (S,S)-4 and molecular
sieves. It was then thoroughly washed with hexane and dis-
solved in CH2Cl2 and immediately ltered. The ltrate was then
evaporated under vacuum and dried under an inert atmo-
sphere. It was then used for the subsequent catalytic run
without further purication. The recovered catalyst worked well
for the three cycles studied (Fig. 3), with only a marginal loss in
yield, which is attributed to the physical loss of the catalyst
Fig. 2 1H NMR spectra were recorded in CDCl3 following the general
allylation reaction procedure as described in the experimental section,
with 0.5 mmol substrate (benzaldehyde), 0.6 mmol allyltrichlorosilane,
1 mmol DIPEA and 10 mol% (S,S)-4 organocatalyst.
1
during the recovery process. Moreover, the H NMR spectra of
the isolated catalyst (see ESI†) showed no change or additional
peaks as compared to the fresh catalyst.
Conclusions
A series of oxazoline-based chiral organocatalysts were devel-
oped, among which the catalyst (S,S)-4 showed very good cata-
lytic efficiency in the asymmetric allylation reaction of a wide
range of substrates such as aromatic, hetero-aromatic and a,b-
unsaturated aldehydes, with allyltrichlorosilane as the allyl
source, under mild reaction conditions. The aromatic and
Fig. 3 Recyclability of catalyst (S,S)-4 using 4-methoxy benzaldehyde
as the representative substrate.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 12257–12265 | 12261

View Article Online
RSC Advances
Paper
hetero-aromatic aldehydes, in particular, gave allylation prod- precipitate was washed twice with hexane, ltered and dried
ucts with good yield (up to 90%) and excellent ee (up to 99%). completely to give III as a white solid (Scheme 1).
The catalyst was found to be robust and showed excellent
The synthesis of N-tosylated oxazolines ((S,S)-1 to (S,S,R)-
recyclability. Also, the mechanism of the asymmetric allylation 5).16,17 Tosylated chiral amide III (2 mmol) was dissolved in
reaction was proposed based on the experimental observations dichloromethane (10 mL), and then Et3N (4 mmol) and DMAP
and evidences obtained from the NMR studies.
(N,N-dimethylaminopyridine) (0.2 mmol) were added respec-
tively. To the clear solution, tosyl chloride (3.5 mmol dissolved in
3 mL CH2Cl2) was added slowly and the reaction was allowed to
stir for 48 h at RT. The solution containing a crystalline solid was
diluted with 10 mL of CH2Cl2, and upon washing with 20 mL of
saturated aqueous NH4Cl, a white solid formed in the aqueous
layer. Water (50 mL) was added, the layers were separated, and
the aqueous layer was back-extracted with CH2Cl2 (3 ꢃ 20 mL).
The combined organic extracts were washed with 15 mL of
saturated aqueous NaHCO3. The aqueous layer was back-
extracted with CH2Cl2 (3 ꢃ 20 mL). The combined organic
extracts were dried over Na2SO4, ltered through cotton and
concentrated under vacuum to give a yellow-white solid. The
resulting pale yellow solid was dissolved in the minimum
amount of dichloromethane and an excess amount of hexane
was added to precipitate out the desired product. The precipitate
was washed twice with hexane, ltered and dried completely to
give catalysts (S,S)-1 to (S,S,R)-5 as white solids (Scheme 1).
Experimental section
The different aldehydes and reagents were used as received. All
of the solvents were dried using standard procedures,19 and
were distilled and stored under activated molecular sieves. NMR
spectra were obtained with a Bruker F113V spectrometer (200
and 500 MHz) and were referenced internally with tetrame-
thylsilane (TMS). Splitting patterns were reported as s, singlet;
d, doublet; dd, doublet of doublets; t, triplet; q, quartet; m,
multiplet; br, broad. Enantiomeric excess (ee) values were
determined by HPLC (Shimadzu SCL-10AVP) using Daicel
Chiralpak OD-H, AS-H, IA, IB and IC chiral columns with 2-
propanol–hexane as the eluent. For the product purication,
ash chromatography was performed using silica gel 100–200
mesh.
General procedure for the preparation of catalysts (S,S)-1 to
(S,S,R)-5
Typical procedure for preparation of catalyst (S,S)-6
The synthesis of tosylated amino acids (II).15,17,20 To a solu-
The synthesis of N-Boc-amino acid (V).18 To a stirred solution
tion of chiral amino acid (6.05 mmol) dissolved in 15 mL of 1.5 of L-tert-leucine IV (6 mmol, 787.02 mg) in THF–H2O (5 mL of
N NaOH, p-toluenesulfonyl chloride (7.26 mmol) in diethyl each solvent) in room temperature was added NaOH (13.2
ether (10 mL) was added at room temperature. Aer 10 h of mmol, 528 mg), followed by Boc2O (6.6 mmol, 1440.5 mg), and
stirring, the ether layer was separated and the aqueous layer was the resulting mixture was stirred for 24 h. THF was removed
acidied with conc. HCl up to acidic pH. A white precipitate was under vacuum and the aqueous layer was extracted with CH2Cl2
thus obtained. Ethyl acetate was then added into the reaction (20 mL). The aqueous layer was acidied with HCl (1 M) to pH ꢂ
mixture and the aqueous layer was extracted twice with ethyl 4 and then extracted with CH2Cl2 (4 ꢃ 15 mL). The organic
acetate. In the case of phenylalanine, conc. HCl was added to phase was dried over anhydrous Na2SO4 and the solvent was
the reaction mixture until it became homogeneous. The crude evaporated under vacuum. The resulting crude product of N-
product was recrystallized from an ether and ethanol mixture to Boc-L-tert-leucine V was used without further purication in the
give pure compound II as white crystals (Scheme 1).
next step (Scheme 2).
The synthesis of amides (III).15,17,20 To the tosyl-protected
The synthesis of N-Boc-amide (VI).18 To the solution of N-
amino acid II (3.13 mmol) in dry CHCl3, freshly distilled thi- Boc-L-tert-leucine V (5.5 mmol, 1.29 g) in freshly dried THF (10
onyl chloride (341 mL, 4.70 mmol) was added dropwise and the mL) at ꢁ15 ꢀC, N-methylmorpholine (NMM, 6.6 mmol, 725 mL)
resulting mixture was stirred at room temperature for 1 h. and ethylchloroformate (6.6 mmol, 628 mL) were slowly added (a
Subsequently, the solution was reuxed for a further 2 h. The white solid was formed during the addition of EtOCOCl). The
clear yellow solution thus obtained was distilled out completely reaction mixture was stirred for 45 min at ꢁ15 ꢀC, and then
to give a quantitative yield of acid chloride as a yellow solid. The L-tert-leucinol (5.5 mmol, 644.5 mg) was added and the resulting
resultant acid chloride was then immediately dissolved in dry mixture was stirred at room temperature for another 3 h. The
CH2Cl2 under a nitrogen atmosphere and cooled to 0 ꢀC. To the mixture was ltered through silica gel (5 cm ꢃ 5 cm) and eluted
cooled solution, chiral amino alcohol (3.09 mmol) in CH2Cl2 with ethyl acetate (100 mL). The solvent was concentrated under
was added dropwise under a nitrogen atmosphere. Aer stirring vacuum and the resulting solid was recrystallized from CH2Cl2–
for 2 h, the reaction temperature was gradually increased to n-pentane, giving 1.64 g of the pure amide VI (Scheme 2).
room temperature and the mixture was allowed to stir at room
The synthesis of N-Boc-oxazoline ((S,S)-6). N-Boc-amide VI
temperature for 12 h. Then, the solution was extracted with (2 mmol) was dissolved in dichloromethane (10 mL) and then
water (50 mL ꢃ 5) and dried over anhydrous Na2SO4. Conse- Et3N (4 mmol) and DMAP (0.2 mmol) were added sequentially.
quently, the solvent was removed under reduced pressure, the To the clear solution, tosyl chloride (3.5 mmol dissolved in 3 mL
resulting pale yellow solid was dissolved in the minimum CH2Cl2) was added slowly and the reaction was allowed to stir
amount of dichloromethane, and an excess amount of hexane for 48 h at RT. The solution containing a crystalline solid was
was added to precipitate out the desired product. The diluted with 10 mL of CH2Cl2, and upon washing with 20 mL of
12262 | RSC Adv., 2014, 4, 12257–12265
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
saturated aqueous NH4Cl, a white solid formed in the aqueous 3033, 2947, 1650, 1541, 1450, 1370, 1327, 1160, 1087, 948,
layer. Water (50 mL) was added, the layers were separated, and 815, 741, 699, 667 cmꢁ1
;
1H NMR (500 MHz, DMSO-d6): d ¼
the aqueous layer was back-extracted with CH2Cl2 (3 ꢃ 20 mL). 2.33 (s, 3H), 2.42–2.46 (m, 2H), 2.76–2.80 (m, 1H), 3.05–3.10
The combined organic extracts were washed with 15 mL of (m, H), 3.16–3.20 (m, 1H), 3.67–3.73 (m, 1H), 3.92–3.93 (m,
saturated aqueous NaHCO3. The aqueous layer was back- 1H), 4.76 (t, J ¼ 5 Hz, 1H), 7.05–7.06 (m, 2H), 7.13–7.23 (m,
extracted with CH2Cl2 (3 ꢃ 20 mL). The combined organic 10H), 7.42 (d, J ¼ 8.5 Hz, 2H), 7.78 (d, J ¼ 8 Hz, 1H), 7.89 (d,
extracts were dried over Na2SO4, ltered through cotton and J ¼ 8 Hz, 1H); 13C NMR (125 MHz, DMSO-d6): d ¼ 21.40,
concentrated under vacuum to give a yellow-white solid. The 29.40, 37.69, 38.28, 53.80, 57.6, 65.83, 125.47, 126.63, 126.71,
resulting pale yellow solid was dissolved in the minimum 127.09, 128.40, 128.62, 128.70, 129.56, 129.73, 129.81, 129.93,
amount of dichloromethane and an excess amount of hexane 130.19, 136.79, 137.74, 138.29, 138.89, 142.98, 170.83. Anal.
was added to precipitate out the desired product. The precipi- calcd for C25H28N2O4S: C, 66.35; H, 6.24; N, 6.19; S, 7.09; O,
tate was washed twice with hexane, ltered and dried 14.14. Found: C, 66.30; H, 6.30; N, 6.25; S, 7.19; O, 14.44.
completely to give catalyst (S,S)-6 as a white solid (Scheme 2).
TOF-MS (ESI+) calcd for (C25H28N2O4S–H+): 451.18, found:
451.20.
Pre catalyst of (S,S)-6. Yield 1.62 g, 89%; white crystals; m.p.
General procedure for the catalytic asymmetric allylation of
aldehydes with allyltrichlorosilane using (S,S)-4 as the
organocatalyst
27
ꢀ
155 C; [a]D ¼ ꢁ70.2 (c ¼ 0.2, CHCl3); FTIR 3379, 3299, 3088,
2966, 1685, 1654, 1556, 1479, 1392, 1369, 1258, 1176, 1056,
1010, 933, 888, 854, 772, 706, 658 cmꢁ1 1H NMR (500 MHz,
;
In a N2 atmosphere glove box, 18.32 mg (0.05 mmol) of catalyst CDCl3): d ¼ 0.94 (s, 9H), 1.04 (s, 9H), 1.43 (s, 9H), 3.5–3.54 (m,
(S,S)-4 was weighed out into an oven-dried 5 mL reactor. To this, 1H), 3.81–3.87 (m, 3H), 5.20 (d, J ¼ 7.5 Hz, 1H), 5.97 (d, J ¼ 7.5
15 mg of powdered activated 4 A molecular sieves was imme- Hz, 1H); 13C NMR (125 MHz, CDCl3): d ¼ 26.65, 26.86, 28.24,
˚
diately added. Next, 1 mL of dry CH2Cl2 was added. The reactor 33.40, 33.76, 59.67, 62.93, 63.33, 80.17, 156.12, 171.95. Anal.
was sealed with a septum and Teon tape and taken out of the calcd for C17H34N2O4: C, 61.79; H, 10.37; N, 8.48; O, 19.37.
glove box. It was then cooled to 0 ꢀC. To this, allyltrichlorosilane Found: C, 61.71; H, 10.33; N, 8.53; O, 19.48. TOF-MS (ESI+) calcd
(1.2 equiv. with respect to the aldehyde) was added dropwise. for (C17H34N2O4): 330.25, found: 330.4.
Aer 2 h, the aldehyde (0.5 mmol) was added over 30 minutes
(S,S)-1. All of the characterization data for catalyst (S,S)-1
followed by DIPEA (2 equiv. with respect to the aldehyde) was were well matched with the literature data.17
added and the reaction was allowed to stir for 24 h at this
(R,S)-1. Yield 756 mg, 87%; white crystals; m.p. 118 ꢀC;
temperature. The reaction was quenched with aqueous NaHCO3 [a]2D7 ¼ ꢁ180.3 (c ¼ 0.2, MeOH); FTIR 3379, 3299, 3088, 2966,
(2 mL) and extracted with dichloromethane (3 ꢃ 15 mL). The 1685, 1654, 1556, 1479, 1392, 1369, 1258, 1176, 1056, 1010, 933,
1
organic layers were washed with brine and dried over Na2SO4. 888, 854, 772, 706, 658 cmꢁ1; H NMR (200 MHz, CDCl3): d ¼
The products were puried by ash chromatography using 2.39 (s, 3H), 2.43–2.46 (m, 2H), 2.93–3.07 (m, 3H), 3.85 (q, J ¼ 7
silica gel 100–200 mesh. The products which were conrmed by Hz, 1H), 4.12–4.23 (m, 1H), 5.05 (d, J ¼ 8 Hz, 1H), 7.06–7.11 (m,
NMR data corresponded to those published previously.
3H), 7.22–7.23 (m, 4H), 7.29–7.35 (m, 4H), 7.62 (d, J ¼ 8 Hz, 1H),
N-Boc-L-tert-leucine.18 Yield 1.27 g, 92%; White powder; m.p. 7.73–7.77 (m, 1H); 13C NMR (125 MHz, CDCl3): d ¼ 21.51, 22.65,
27
ꢀ
116 C; [a]D ¼ +22.8 (c ¼ 0.2, CHCl3); FTIR 3437, 3325, 3180, 29.66, 31.88, 36.91, 38.53, 45.77, 50.77, 57.81, 126.96, 127.12,
2975, 2551, 1744, 1710, 1650, 1513, 1415, 1234, 1162, 1058, 127.32, 128.74, 128.91, 129.13, 129.29, 129.81, 134.98, 135.73,
1009, 914, 847, 779, 690 cmꢁ1; 1H NMR (200 MHz, CDCl3): d ¼ 136.35, 143.91, 169.68. Anal. calcd for C25H26N2O3S: C, 69.1; H,
1.02 (s, 9H), 1.44 (s, 9H), 4.09 (d, J ¼ 9.6 Hz, 1H), 5.08 (d, J ¼ 8.4 6.03; N, 6.45; S, 7.38; O, 11.05. Found: C, 69.18; H, 6.15; N, 6.51;
Hz, 1H), 9.64 (br, 1H); 13C NMR (50 MHz, CDCl3): d ¼ 26.49, S, 7.32; O, 11.23. TOF-MS (ESI+) calcd for (C25H26N2O3S + H+):
28.26, 34.44, 61.61, 79.99, 155.63, 176.80; TOF-MS (ESIꢁ) calcd 435.17, found: 435.66.
for (C11H21NO4–H+): 230.15, found: 229.95.
(S,R)-1. Yield 773 mg, 89%; white crystal; m.p. 118 ꢀC;
1
Pre catalyst of (R,S)-1. Yield 1.27 g, 90%; white solid; m.p. [a]2D7 ¼ +185.5 (c ¼ 0.2, MeOH); H NMR (500 MHz, DMSO-d6):
30
135 C; [a]D ¼ ꢁ220 (c ¼ 0.2, MeOH);4 FTIR 3324, 3261, 3061, d ¼ 2.33 (s, 3H), 2.36–2.43 (m, 2H), 2.5–2.55 (m, 2H), 2.74–2.78
ꢀ
3032, 2949, 1648, 1539, 1451, 1369, 1328, 1158, 1089, 947, 814, (m, 1H), 2.92 (dd, J ¼ 10 Hz, 1H), 3.47–3.5 (m, 1H), 3.66 (dd, J ¼
743, 698, 669 cmꢁ1; 1H NMR (200 MHz, CDCl3): d ¼ 2.33 (s, 3H), 8 Hz, 1H), 3.95–4.00 (m, 1H), 6.98–6.99 (m, 2H), 7.12 (d, J ¼ 7
2.64 (m, 4H), 3.24 (d, J ¼ 4 Hz, 2H), 3.76 (q, J ¼ 7 Hz, 1H), 4.11– Hz, 2H), 7.16–7.23 (m, 5H), 7.44 (d, J ¼ 8 Hz, 2H), 7.51 (d, J ¼ 8
4.24 (m, 1H), 4.99 (d, J ¼ 7 Hz, 1H), 6.30 (d, J ¼ 8 Hz, 1H) 7.09– Hz, 2H), 7.83 (d, J ¼ 8 Hz, 1H), 8.34 (d, J ¼ 9.5 Hz, 1H); 13C NMR
7.1 (m, 2H), 6.85–6.87 (m, 2H) 7.10–7.24 (m, 10H), 7.47–7.51 (m, (125 MHz, DMSO-d6): d ¼ 20.92, 36.22, 37.84, 52.38, 57.70,
2H); 13C NMR (125 MHz, CDCl3): d ¼ 21.51, 36.66, 38.37, 53.17, 65.49, 125.97, 126.20, 126.38, 127.93, 128.15, 129.16, 129.26,
58.01, 63.34, 126.69, 127.14, 127.21, 128.63, 128.82, 129.19, 137.18, 138.26, 138.94, 142.11, 169.86. Anal. calcd for
129.28, 129.78, 135.29, 137.31, 143.9, 170.59. Anal. calcd for
C
C25H26N2O3S: C, 69.1; H, 6.03; N, 6.45; S, 7.38; O, 11.05. Found:
25H28N2O4S: C, 66.35; H, 6.24; N, 6.19; S, 7.09; O, 14.14. Found: C, 69.25; H, 6.08; N, 6.48; S, 7.35; O, 11.3. TOF-MS (ESI+) calcd
C, 66.29; H, 6.38; N, 6.35; S, 7.22; O, 14.40. TOF–MS (ESI+) calcd for (C25H26N2O3S): 434.17, found: 434.12.
for (C25H28N2O4S–H+): 451.18, found: 451.14.
Catalysts (S,S)-2, (S,S)-3, (S,S)-4, (S,R,S)-5 and (S,S,R)-5. All of
Pre catalyst of (S,R)-1. Yield 1.29 g, 91%; white solid; m.p. the characterization data for these catalysts were well matched
30
135 C; [a]D ¼ +210 (c ¼ 0.2, MeOH); FTIR 3323, 3262, 3059, with the literature data.17
ꢀ
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 12257–12265 | 12263

View Article Online
RSC Advances
Paper
27
ꢀ
3 (a) J. A. Marshall, Chem. Rev., 1996, 96, 31–48; (b)
J. A. Marshall, in Organometallics in Synthesis, ed. M.
Schlosser, J. Wiley and Sons, Chichester, U.K., 2002, p. 399;
(c) K. M. Waltz, J. Gavenonis and P. J. Walsh, Angew.
Chem., Int. Ed., 2002, 41, 3697–3699; (d) H. Hanawa,
T. Hashimoto and K. Maruoka, J. Am. Chem. Soc., 2003,
125, 1708–1709; (e) J. G. Kim, K. M. Waltz, I. F. Garcia,
D. Kwiatkowski and P. J. Walsh, J. Am. Chem. Soc., 2004,
126, 12580–12585; (f) Y.-C. Teo, K.-T. Tan and T.-P. Loh,
Chem. Commun., 2005, 1318–1320; (g) R. Takita, K. Yakura,
T. Ohshima and M. Shibasaki, J. Am. Chem. Soc., 2005, 127,
13760–13761; (h) X. Zhang, D. Chen, X. Liu and X. Feng,
J. Org. Chem., 2007, 72, 5227–5233; (i) K. Zheng, B. Qin,
X. Liu and X. Feng, J. Org. Chem., 2007, 72, 8478–8483; (j)
J. Huang, J. Wang, X. Chen, Y. Wen, X. Liu and X. Feng,
Adv. Synth. Catal., 2008, 350, 287–294; (k) I. S. Kim,
M.-Y. Ngai and M. J. Krische, J. Am. Chem. Soc., 2008, 130,
6340–6341; (l) Y. Kuang, X. Liu, L. Chang, M. Wang, L. Lin
and X. Feng, Org. Lett., 2011, 13, 3814–3817; (m) Z. Li,
B. Plancq and T. Ollevier, Chem. – Eur. J., 2012, 18, 3144–
3147.
(S,S)-6. Yield 1.32 g, 86%; white solid; m.p. 110 C; [a]D
¼
ꢁ115.5 (c ¼ 0.2, CHCl3); FTIR 3413, 2964, 2929, 1709, 1662,
1516, 1367, 1326, 1239, 1175, 1069, 982, 926, 864, 778, 649 cmꢁ1
;
1H NMR (200 MHz, CDCl3): d ¼ 0.88 (s, 9H), 0.98 (s, 9H), 1.44 (s,
9H), 3.84–3.89 (m, 1H), 4.07–4.21 (m, 3H), 5.13 (d, J ¼ 9.6 Hz,
1H); 13C NMR (50 MHz, CDCl3): d ¼ 25.84, 26.55, 28.28, 33.56,
34.66, 57.19, 68.50, 75.37, 79.30, 155.55, 166.00. Anal. calcd for
C
27H32N2O3: C, 65.35; H, 10.32; N, 8.97; O, 15.36. Found: C,
65.40; H, 10.42; N, 8.90; O, 15.47. TOF-MS (ESIꢁ) calcd for
(C17H32N2O3–H+): 311.24, found: 310.94.
Acknowledgements
Authors thank CSIR for the nancial support. Debashis Ghosh
is thankful to AcSIR for the Ph.D. enrolment. Analytical Disci-
pline and Centralized Instrumental Facility is gratefully
acknowledged for providing instrumental facilities.
Notes and references
1 For selected examples of asymmetric allylation reactions
¨
used in total synthesis, see: (a) A. Furstner and
4 (a) M. Nakajima, M. Saito, M. Shiro and S. Hashimoto, J. Am.
Chem. Soc., 1998, 120, 6419–6420; (b) T. Shimada, A. Kina,
S. Ikeda and T. Hayashi, Org. Lett., 2002, 4, 2799–2801; (c)
S. E. Denmark and J. Fu, Chem. Commun., 2003, 167–170;
(d) A. Kina, T. Shimada and T. Hayashi, Adv. Synth. Catal.,
2004, 346, 1169–1174; (e) W.-L. Wong, C.-S. Lee,
H.-K. Leung and H.-L. Kwong, Org. Biomol. Chem., 2004, 2,
1967–1969; (f) H. L. Kwong, H. L. Yeung, C. T. Yeung,
W. S. Lee, C. S. Lee and W. L. Wong, Coord. Chem. Rev.,
2007, 251, 2188; (g) A. V. Malkov, M. Bell, M. Orsini,
D. Pernazza, A. Massa, P. Herrmann, P. Meghani and
K. Langemann, J. Am. Chem. Soc., 1997, 119, 9130–9136; (b)
D. Meng, P. Bertinato, A. Balog, D.-S. Su, T. Kamenecka,
E. J. Sorensen and S. J. Danishefsky, J. Am. Chem. Soc.,
1997, 119, 10073–10092; (c) H. Nakamura, K. Nakamura
and Y. Yamamoto, J. Am. Chem. Soc., 1998, 120, 4242; (d)
G. E. Keck, C. A. Wager, T. T. Wager, K. A. Savin,
J. A. Covel, M. D. McLaws, D. Krishnamurthy and V. J. Cee,
Angew. Chem., Int. Ed., 2001, 113, 237–240; Angew. Chem.,
Int. Ed., 2001, 40, 231–234; (e) R. A. Fernandes, A. Stimac
and Y. Yamamoto, J. Am. Chem. Soc., 2003, 125, 14133; (f)
S. Kobayashi, C. Ogawa, H. Konishi and M. Sugiura, J. Am.
Chem. Soc., 2003, 125, 6610–6611; (g) A. B. Smith III,
W. Zhu, S. Shirakami, C. Sfouggatakis, V. A. Doughty,
C. S. Bennett and Y. Sakamoto, Org. Lett., 2003, 5, 761–764;
(h) P. A. Evans, J. Cui and S. J. Gharpure, Org. Lett., 2003, 5,
3883–3885; (i) T. Vilaivan, C. Winotapan, V. Banphavichit,
T. Shinada and Y. Ohfune, J. Org. Chem., 2005, 70, 3464–
3471; (j) G. E. Keck and A. P. Truong, Org. Lett., 2005, 7,
2153–2156; (k) S. E. Denmark and J. Fu, Org. Lett., 2002, 4,
1951–1953; (l) T. Itoh, M. Miyazaki, H. Fukuoka, K. Nagata
and A. Ohs, Org. Lett., 2006, 8, 1295–1297; (m) T. R. Wu
and J. M. Chong, J. Am. Chem. Soc., 2006, 128, 9646–9647;
(n) S. Lou, P. N. Moquist and S. E. Schaus, J. Am. Chem.
Soc., 2007, 129, 15398–15404; (o) S. E. Denmark,
C. S. Regens and T. Kobayashi, J. Am. Chem. Soc., 2007,
129, 2774–2776; (p) K. Maki, R. Motoki, K. Fujii, M. Kanai,
T. Kobayashi, S. Tamura and M. Shibasaki, J. Am. Chem.
Soc., 2005, 127, 17111–17117; (q) V. Rauniyar and
D. G. Hall, J. Org. Chem., 2009, 74, 4236–4241; (r) S. B. Han,
A. Hassan, I. S. Kim and M. J. Krische, J. Am. Chem. Soc.,
2010, 132, 15559–15561.
ˇ
´
P. Kocovsky, J. Org. Chem., 2003, 68, 9659–9668; (h)
´
ˇ
A. V. Malkov, L. Duova, L. Farrugia and P. Kocovsk
Angew. Chem., Int. Ed., 2003, 42, 3674–3677; (i)
J. F. Traverse, Y. Zhao, A. H. Hoveyda and M. L. Snapper,
Org. Lett., 2005, 7, 3151–3154; (j) Q. Chai, C. Song, Z. Sun,
Y. Ma, C. Ma, Y. Dai and M. B. Andrus, Tetrahedron Lett.,
´y,
´
2006, 47, 8611–8615; (k) A. V. Malkov, P. R. Lopez,
´
´ˇ
´
L. Biedermannova, L. Rulısek, L. Duova, M. Kotora,
ˇ
´
F. Zhu and P. Kocovsky, J. Am. Chem. Soc., 2008, 130, 5341–
ˇ´ ´ˇ
´
´
´
5348; (l) A. Kadlcıkova, I. Valterova, L. Duchackova,
´
J. Roithova and M. Kotora, Chem. – Eur. J., 2010, 16, 9442–
9445; (m) B. Bai, L. Shen, J. Ren and H. J. Zhu, Adv. Synth.
Catal., 2012, 354, 354–358.
5 (a) K. Iseki, S. Mizuno, Y. Kuroki and Y. Kobayashi,
Tetrahedron Lett., 1998, 39, 2767–2770; (b) C. Baudequin,
D. Chaturvedi and S. B. Tsogoeva, Eur. J. Org. Chem., 2007,
2623–2629.
6 S. B. Jagtap and S. B. Tsogoeva, Chem. Commun., 2006, 4747–
4749.
7 (a) S. E. Denmark, J. Fu, D. M. Coe, X. Su, N. E. Pratt and
B. D. Griedel, J. Org. Chem., 2006, 71, 1513–1522; (b)
M. Nakajima, S. Kotani, T. Ishizuka and S. Hashimoto,
Tetrahedron Lett., 2005, 46, 157–159; (c) S. Kotani,
S. Hashimoto and M. Nakajimaa, Tetrahedron, 2007, 63,
3122–3132.
2 (a) S. E. Denmark and J. Fu, Chem. Rev., 2003, 103, 2763–
2794; (b) J. W. J. Kennedy and D. G. Hall, Angew. Chem.,
Int. Ed., 2003, 42, 4732–4739; (c) M. Yus, J. C. G. Gomez
´
and F. Foubelo, Chem. Rev., 2011, 111, 7774–7854.
12264 | RSC Adv., 2014, 4, 12257–12265
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
8 (a) S. E. Denmark, J. Fu and M. J. Lawler, J. Org. Chem., 2006, 15 D. Ghosh, D. Sahu, S. Saravanan, S. H. R. Abdi, B. Ganguly,
71, 1523; (b) T. Nemoto, T. Hitomi, H. Nakamura, L. Jin,
K. Hatano and Y. Hamada, Tetrahedron: Asymmetry, 2007,
18, 1844.
9 I. Chataigner, U. Piarulli and C. Gennari, Tetrahedron Lett.,
1999, 40, 3633–3634.
N. H. Khan, R. I. Kureshy and H. C. Bajaj, Org. Biomol.
Chem., 2013, 11, 3451–3460.
16 D. A. Evans, G. S. Peterson, J. S. Johnson, D. M. Barnes,
K. R. Campos and K. A. Woerpel, J. Org. Chem., 1998, 63,
4541–4544.
10 M. Kira, K. Sato and H. Sakurai, J. Am. Chem. Soc., 1988, 110, 17 A. Sadhukhan, D. Sahu, B. Ganguly, N. H. Khan,
4599–4602.
R. I. Kureshy, S. H. R. Abdi, E. Suresh and H. C. Bajaj,
11 Z. Lu and S. Ma, Angew. Chem., Int. Ed., 2008, 47, 258–297.
Chem. – Eur. J., 2013, 19, 14224–14232.
´
¨
¨
12 E. Belanger, M.-F. Pouliot, M.-A. Courtemanche and 18 P. Vastila, I. M. Pastor and H. Adolfsson, J. Org. Chem., 2005,
J.-F. Paquin, J. Org. Chem., 2012, 77, 317–331. 70, 2921–2929.
13 R. M. Angell, A. G. M. Barrett, D. C. Braddock, S. Swallow and 19 D. D. Perrin, W. L. F. Armarego and D. R. Perrin, Purication
B. D. Vickery, Chem. Commun., 1997, 919–920. of Laboratory Chemicals, Pergamon, New York, 1981.
14 G. J. Rowlands and W. K. Barnes, Chem. Commun., 2003, 20 S. Saravanan, A. Sadhukhan, N. H. Khan, R. I. Kureshy,
2712–2713.
S. H. R. Abdi and H. C. Bajaj, J. Org. Chem., 2012, 77, 4375–4384.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 12257–12265 | 12265