Diastereoselective alkylations of oxazolidinone vinylogous glycolates

Article abstract of DOI:10.1016/j.tetlet.2012.06.133

A highly Z-selective isomerization (double bond migration) was observed when oxazolidinone vinylogous glycolate was exposed to a strong base to give N-acyl oxazolidinone, bearing an electron rich olefin. The corresponding enolate was exposed to alkyl halides to provide alkylated compounds on the γ-position with respect to OBn group, with high regioselectivity and moderate diastereoselectivity. However, the nature of the chiral oxazolidinone leads to a significant increase in the reaction diastereoselectivity. A stereospecific formation of cis-olefin was also observed in these alkylated compounds.

Full text of DOI:10.1016/j.tetlet.2012.06.133

Tetrahedron Letters 53 (2012) 4775–4778
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Diastereoselective alkylations of oxazolidinone vinylogous glycolates
Jacqueline Jiménez, Rosa L. Meza-León, Fernando Sartillo-Piscil, Francisco J. Meléndez,
⇑
⇑
Estibaliz Sansinenea , Aurelio Ortiz
Facultad de Ciencias Químicas, Benemérita Universidad Autónoma de Puebla, Puebla 72570, Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
A highly Z-selective isomerization (double bond migration) was observed when oxazolidinone vinylogous
glycolate was exposed to a strong base to give N-acyl oxazolidinone, bearing an electron rich olefin. The
corresponding enolate was exposed to alkyl halides to provide alkylated compounds on the c-position
with respect to OBn group, with high regioselectivity and moderate diastereoselectivity. However, the
nature of the chiral oxazolidinone leads to a significant increase in the reaction diastereoselectivity. A ste-
reospecific formation of cis-olefin was also observed in these alkylated compounds.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 7 June 2012
Revised 26 June 2012
Accepted 28 June 2012
Available online 4 July 2012
Keywords:
Diastereoselective alkylations
Oxazolidinone vinylogous glycolate
Double bond migration
The metalation reaction of achiral hetero-substituted allylic
ethers has been extensively studied.1 These studies have focused
primarily in the regioselectivity of the allylic carbanion reactions
4 in 90% yield. This acid was treated with triethylamine and piva-
loyl chloride in dry THF,7 followed by addition of the oxazolidi-
none8 3a or 3b at 0 °C and after stirring for 18 h at room
temperature afforded 5a as a white solid in 60% yield and 5b as a
colorless liquid in 40% yield,9 as shown in Scheme 1.
with electrophiles, since it is possible to obtain
a and c-products
due to the nature of these anions.2 It has been found that this reg-
ioselectivity is insensitive to change in solvent or temperature but
is influenced by the ligand R (steric effects), substituent atom,
counterion and type of electrophile.
O
The hetero-substituted allylic anions generally react with alkyl
halides at the position while the allylic anions react with carbonyl
Xc
O
M
OBn
E
OBn
O
compounds at the
or boron ‘ate’ complexes,3 which direct both carbonyl compounds
and alkyl halides to the position with high regioselectivity.
It is noteworthy that the formation of the cis-enol ether stereo-
isomer on the product is highly stereospecific, suggesting that
a position, except when using allylic aluminum
OBn
E
base
γ-product
Xc
α
γ
Xc
O
a
E
Xc
Xc = Oxazolidin-2-one moiety
OBn
α-product
c
the alkylation reaction of allylic anions is carried out via internally
coordinated metallocycle (Fig. 1).2
Figure 1.
a
and c Alkylation products via metallation.
We report herein, a highly regioselective asymmetric alkylation
reaction carried out on chiral oxazolidinone vinylogous glycolates
with a strong base in the presence of alkyl halides. During this pro-
cess a highly stereospecific double-bond migration is observed.
The preparation of the chiral oxazolidinone vinylogous glyco-
lates 5a and b began with the synthesis of E-4-(benzyloxyl)-Z-
but-2-enoic acid 4,4,5 which was prepared from ethyl-2-but-2-yno-
O
O
BnOH, PPh3
AcOH
LiOH, THF/H2O
25 °C, 18 h
O
OBn
OBn
EtO
EtO
HO
Tol.,110 °C
18 h
1M HCl soln.
2
4
1
O
O
O
ate 1 by a c-addition reaction of benzyl alcohol with assistance of
Et3N
OBn
triphenyl phosphine6 to afford the ester 2 in 80% yield (Scheme 1).
O
N
O
NH
tert-BuCOCl,4
R
R2
+
R
R2
R1
A subsequent basic hydrolysis (LiOH, THF–H2O) delivered the acid
THF, 0-25 °C
18 h
R
R1
R
R = Ph R1 = H R2 = Ph
R = Ph R1 = H R2 = Ph
5a
5b
3a
3b
R = Me R1 = Pr R2 = H
i
R = Me R1 = Pr R2 = H
i
⇑
Corresponding authors. Tel.: +52 222 2295500x7518; fax: +52 222 2295584.
E-mail addresses: estisan@yahoo.com (E. Sansinenea), aurelioom@yahoo.com
Scheme 1.
(A. Ortiz).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.06.133

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J. Jiménez et al. / Tetrahedron Letters 53 (2012) 4775–4778
Table 2
6.3 Hz
LiCl
NaHMDS
O
O
H
Alkylation of oxazolidinone vinylogous glycolate 5b
OBn
H
Xc
Xc
O
1´
O
30 min.-60 °C
7h - 45 °C,
aq NH4Cl soln.
LiCl
Base
2´
OBn
OBn
4´
6
Xc
5a
Xc
3´
α
γ
R3 OBn
30 min.-60 °C
R3-X, -60 to -45 °C
7h, -45 °C
M
N
M
OBn
R3 = CH3
O
O
5b
9a
9b
R3 = CH2CH=CH2
aq NH4Cl
O
9c R3 = Bn
Ph
Ph
Xc = Oxazolidin-2-one moiety
Ph
Entry
Base
R3-X
Yield (%)a
d.r.b
Scheme 2. Double bond migration in N-enoyl oxazolidinone.
1
2
3
KHMDS
KHMDS
KHMDS
CH3I
AllylBr
BnBr
60
52
50
98/2
98/2
98/2
Table 1
a
Yield corresponding to purified diastereomer.
Diastereomeric ratios were determined by NMR of crude reaction mixture.
Alkylation of oxazolidinone vinylogous glycolate 5a
b
O
1´
O
LiCl
O
Base
2´
OBn
4´
Xc
Xc
Xc
3´
+
α
γ
30 min. -60 °C
R3 OBn
7
7a 8a
7b 8b
R3 OBn
8
R3-X, -60 to -45 °C
7h, -45 °C
O
O
O
5a
OBn
aq NH4Cl
=
=
R
3 = CH3
Xc
H2, Pd/C
Xc
R3 = CH2CH=CH2
OBn
EtOH, 16h,
25 °C
7c = 8c R3 = Bn
Xc = Oxazolidin-2-one moiety
7a
10
Xc = Oxazolidin-2-one moiety
LiOH, H2O2
OBn
OBn
Entry
Base
R3-X
Yielda (%)
d.r.b 7/8
HO
THF:H2O,0-25 °C
1
2
3
4
5
6
NaHMDS
KHMDS
LiHMDS
KHMDS
KHMDS
KHMDS
CH3I
CH3I
CH3I
AllylBr
BnBr
EtI
74
80
50
85
85
0
85/15
87/13
83/17
72/28
76/24
0
1.30 h, aq Na2SO3
1M HC lsoln.
11
10
HO
NaBH4
THF/H2O,
25 °C, 3h
12
a
Yield corresponding to purified mixture of diastereomers.
Diastereomeric ratios were determined by NMR of crude reaction mixture.
b
Scheme 3.
ꢀ78 °C. The alkylation proceeded well with some representative
alkyl halides (entries 2, 4–5), however, when ethyl iodide was em-
ployed as the electrophile, no alkylation product was observed
(entry 6), as shown in Table 1.
Nevertheless, the nature of the chiral oxazolidinone leads to a
significant change in the reaction diastereoselectivity. When the
compound 5b was exposed to the same alkylation reaction condi-
tion above described, furnished the compounds 9a–c in moderate
yield (50–60%) and with a high diastereoselectivity (98:2), as
shown in Table 2.
The double bond in oxazolidinone vinylogous glycolate 5a was
isomerized to the unconjugated cis double bond 6 (Scheme 2).
Therefore, the oxazolidinone vinylogous glycolate 5a was treated
with NaHMDS or KHMDS (1.5 equiv) in the presence of LiCl
(1 equiv) in dry THF at ꢀ60 °C. After 30 min the temperature was
increased at ꢀ45 oC and the reaction was stirred for 7 h. After the
reaction was quenched by the addition of a saturated solution of
NH4Cl to provide vinyl ether 6 in 95% yield.10 The isomer 6 was
achieved via a double bond migration,11 with a high Z-selectivity
(Z/E 99:1). The cis alkene geometry was established based on the
measurement of coupling constants of vinyl protons in the NMR
To determine the absolute configuration and relative stereo-
chemistry of the alkylation product, compound 7a was trans-
0
0
spectrum (JH3 –H4 = 6.3 Hz). The high selectivity implies that the
coordination of OBn group to the metal plays an important role
in the transition state, as shown in the Scheme 2.
formed into known carboxylic acid 11, via
a sequence of
reactions (Scheme 3). Hydrogenation of 7a with Pd-C in EtOH pro-
vided compound 10 in quantitative yield, removal of the chiral
auxiliary with LiOH, H2O2, delivered carboxylic acid 11 in 80%
The oxazolidinone vinylogous glycolate 5a was treated under
the reaction conditions described above to form the corresponding
enolate, followed by addition of excess alkyl halide (3 equiv) and
warming the reaction mixture to ꢀ45 °C (Table 1). After stirring
for 7 h at this temperature the alkylated compounds 7a–c and
O
8a–c,12 where alkylation took place on the
c-position with respect
LiOH, H2O2
O
R1
to OBn group, were achieved with high regioselectivity, moderate
diastereoselectivity (72/28-87/13), and moderate yields (50–85%).
In all cases, the compounds 7a–c always were predominant.
Stereospecific formation of cis-alkenes in all compounds 7a–c
and 8a–c was also observed. No changes in regioselectivity,
diastereoselectivity nor yield were observed when different metals
were used in metalation reaction, as shown in Table 1. The use of
KHMDS as base in the alkylation reaction of compound 5a provided
the best results (entry 2). Furthermore, lithium chloride (1 equiv)
plays an important role in the success of the alkylation because
its absence leads to a fast deacylation of the auxiliary even at
THF/H2O, 0-25 °C,
1.30 h, aq Na2SO3
R2
O
1 = OBn R2 = H
1 = H R2 = OBn
1M HCl soln.
13a
13b
R
R
Xc
OBn
7a
Xc = Oxazolidin-2-one moiety
O
LiOH, H2O2
HO
THF/H2O, 0-25 °C,
1.30 h, aq Na2SO3
aq KHSO4 soln.
OBn
14
Scheme 4.

J. Jiménez et al. / Tetrahedron Letters 53 (2012) 4775–4778
4777
Table 3
Calculated vicinal coupling constants (Hz) for the individual conformations A and B of the compounds 13a and 13b, as well as their comparison with experimental vicinal
coupling constants
H20
O
22
H16
O
H
O
O
H16
22
H
H20
BnO
cis-
BnO
Me
21
H
H21
γ
OBn Me
13a
-lactone-B
cis-
γ
-lactone-A
20 O
H
16
O
H
O
O
16
BnO
H
22
H
20 Me
H
21
H
22
H
Me
21
H
trans-
γ
-lactone-A
trans-γ-lactone-B
13b
b
c
-Lactone
Energy (u.a)
Dihedral angle
3JH–H calcd
3JH–H Karplus
3JH–H Expl
cis-A
13a
ꢀ691.4164
H
H
H
H
H
16–H20 19°
16–H21 104°
22–H20 23°
22–H21 96°
16–H20 19°
9.4
0.4
6.0
9.2
1.7
7.6
1.6
9.2
1.7
7.8
1.7
7.0
10.6
6.6
8.6
8.8
1.3
8.5
6.4
6.8
10.8
8.6
5.6
0.0
10.8
8.6
5.6
0.0
10.8
8.6
5.6
0.0
9.5
6.5
5.6
4.3
9.5
ꢀ0.4
9.4
cis-Aa
13a
ꢀ691.4256
H16–H20 19°
0.4
6.2
H22–H20 21°
H22–H21 98°
H16–H20 35°
H16–H21 157°
H22–H20 31°
H22–H21 150°
H16–H20 23°
H16–H21 99°
H22–H20 151°
H22–H21 32°
H16–H20 36°
ꢀ0.3
8.5
cis-B
13a
ꢀ691.4162
ꢀ691.4095
10.5
5.2
6.1
9.1
0.2
7.0
5.7
7.5
trans-A
13b
trans-B
13b
ꢀ691.4118
H16–H21 159°
H22–H20 90°
H
22–H21 30o
9.4
ꢀ0.4
4.6
10.8
1.2
6.6
6.5
5.6
4.3
a
The calculation was realized in solution (CHCl3).
Vicinal coupling constants have been calculated with a software which works using a generalized Karplus type equation.20
b
O
O
O
O
O
LiCl
OBn
O
N
KHMDS
LiOH, H2O2
O
N
HO
30 min. - 60 °C
CH3I, -60 to -45 °C
7h, -45 °C
OBn
OBn
THF/H2O, 0-25 °C
1.30 h, aq Na2SO3
aq KHSO4 soln.
5b
ent-14
70 % yield
9a
aq NH4Cl
60 % yield
d. r. 98:2
α
[
]
25= + 54.0
D
(c 0.7, CHCl3)
Scheme 5.
yield. By comparison with literature optical data,13 the absolute
stereochemistry of 7a was established as R. On the other hand,
when the compound 10 was treated with NaBH4 in THF/H2O14 pro-
vided the known alcohol15 12 in 83% yield as a colorless liquid, as
shown in Scheme 3.
is important to note that carboxylic acid 14 can be transformed to
the corresponding
days.
c-lactones 13a–b after its storage by several
To assign the relative configuration of the major diastereomer
c-lactones 13a and 13b, NOESY spectra were obtained; however,
of
Removal of the chiral auxiliary under Evans’ hydrolysis proce-
dure16 in the compound 7a, led to direct cyclization to lactones
13a–b as a mixture of diastereomers in 80% yield and with d.r. of
70:30, being 13a the major diastereomer. During this procedure
a 1 M HCl solution was used to adjust the acid pH, so under these
reaction conditions, the electron-rich alkene 7a underwent re-
moval of the chiral auxiliary and intramolecular cyclization reac-
in both cases it was not possible to observe any correlation. There-
fore, the relative configuration of 13a–b was assigned in base on
comparative analysis between calculated vicinal coupling con-
stants18 and experimental vicinal coupling constants. Conse-
quently, a geometry optimization for four conformers of
c-
lactones 13a–-b was realized in the gas phase.19 The results of
the calculated and experimental vicinal coupling constants are
shown in Table 3. Both conformers of cis-c-lactone 13a presented
tion to provide the
c
-lactones 13a–b. Carboxylic acid 1417 was
achieved in 80% yield, when the aqueous 1 M HCl solution was re-
placed with an aqueous solution of KHSO4 as shown in Scheme 4. It
similar energy values; however, a comparison between calculated
3
(3JH22-H20 = 6.0 Hz, JH22-H21 = 0.4 Hz) and experimental (3JH22–

4778
J. Jiménez et al. / Tetrahedron Letters 53 (2012) 4775–4778
3
the reaction was stirred for 7 h. After the reaction was quenched by the
addition of a saturated solution of NH4Cl (3 mL), THF was removed under
reduced pressure. The organic layer was extracted with CH2Cl2 (3 ꢁ 30 mL),
dried over Na2SO4, filtered and concentrated under reduced pressure. The oily
residue was purified by flash column chromatography eluting with hexane–
ethyl acetate (95:5) to give the compound(4R,Z)-N-(40-benzyloxy-but-30-
H20 = 5.6 Hz, JH22–H21 = 0 Hz) vicinal coupling constants, suggests
that cis- -lactone A 13a conformation is possibly more favored
than cis- -lactone-B 13a conformation. For trans- -lactone 13b,
c
c
c
both conformers also presented similar energy values and a com-
parison between calculated (3JH22-H2 = 7.0 Hz, JH22-H21 = 5.7 Hz)
3
enoyl)-4,5,5-triphenyloxazolidin-2-one 6; Yield 95%, ½a D25
ꢂ
+137 (c 2.0, CHCl3);
and experimental (3JH22-H20 = 5.6 Hz, 3JH22-H21 = 4.3 Hz) vicinal cou-
1H NMR (400 MHz, CDCl3) d: 7.63 (2H, dt, J = 7.0, 1.5 Hz, Ph), 7.44–7.25 (8H, m,
Ph), 7.10-6.98 (10H, m, Ph), 6.21 (1H, s, H-4), 6.12 (1H, dt, J = 6.3, 1.5 Hz, H-40),
4.74 (2H, s, CH2Ph), 4.58 (1H, dt, J = 6.6, 6.6 Hz, H-30), 3.77 (2H, m, CH2); 13C
NMR (100 MHz, CDCl3) d: 170.6 (C-10), 152.6 (C-2), 146.8 (C-40), 141.7, 138.0,
137.2, 135.7, 128.8, 128.7, 128.4, 128.3, 128.1, 128.0, 127.8, 127.6, 127.4, 127.3,
127.2, 126.1, 125.9 (Ph), 97.5 (C-30), 89.0 (C-5), 73.6 (CH2O), 66.0 (C-40), 31.5
pling constants, suggests that trans-
is more favored than trans- -lactone-B 13b conformation. This
analysis of the coupling constants allowed to assign the relative
configuration cis for -lactone 13a and trans for 13b.
A change of substituent and absolute configuration in the ring
of oxazolidinone provides a significant change in the diastereose-
lectivity of the alkylation of compound 5b (98:2). Removal of chiral
auxiliary under Evans’ hydrolysis modified procedure provides
ent-14 in 70% yield as shown in Scheme 5.
In conclusion, we described an unprecedented alkylation of chi-
ral oxazolidinone vinylogous glycolates. The compounds 5a–b un-
dergo stereo- and regioselective alkylation reactions with
concomitant migration of the double bond when exposed to a
c-lactone A 13b conformation
c
c
(C-20). IR max: 2923, 1788, 1712, 1496, 1450, 1364, 1347, 997, 845, 759 cmꢀ1
t ;
FAB-HRMS: calculated for (C32H28NO4), ([M+H])+ 490.2018; found, 490.2150.
11. Wille, A.; Tomm, S.; Frauenrath, H. Synthesis 1998, 305–308.
12. General Procedure for the alkylation reaction. To an oven-dried 100 mL round-
bottom equipped with magnetic stir bar was added anhydrous LiCl
a
(0.24 mmol, 10.3 mg) under an argon atmosphere followed of oxazolidinone
5a (0.2 mmol, 0.10 g). Both solids were dissolved in anhydrous THF (25 mL)
and the reaction mixture was stirred for 15 min at ꢀ60 °C and then was added
NaHMDS (0.41 mmol, 0.41 mL) and the resulting solution was stirred
vigorously during 30 min. After, was added CH3I (1.2 mmol, 0.17 g), the
temperature was increased at ꢀ45 °C and the reaction mixture was stirred for
7 h. After the reaction was quenched by the addition of a saturated solution of
NH4Cl (3 mL), THF was removed under reduced pressure. The organic layer was
extracted with CH2Cl2 (3 ꢁ 30 mL), dried over Na2SO4, filtered and
concentrated under reduced pressure. The oily residue was purified by flash
column chromatography eluting with hexane- ethyl acetate (95:5) to give a
diastereomeric mixture of the compounds 7a and 8a. Their isolation was
carried out by preparative thin layer chromatography eluting with hexane
strong base. The products were found to be alkylated on the c-po-
sition with respect to the OBn group and possess an electron-rich
cis-alkene. The alkylation reaction was highly regioselective but
moderately diastereoselective for the compound 5a. However, for
the compound 5b this reaction was highly regioselective and dia-
stereoselective. The cis-alkene formation was highly stereospecific.
Our group is currently working to functionalize the cis-alkene as
well as to carry out this reaction with other electrophiles.
CH2Cl2
9:1.
(20R,4R,Z)-N-(40-benzyloxyl-2-methylbut-30-enoyl)-4,5,5-
triphenyloxazolidin-2-one 7a; ½a D25
ꢂ
+83 (c 2.0, CHCl3); 1H NMR (400 MHz,
CDCl3) d: 7.60 (2H, dd, J = 7.0, 1.6 Hz, Ph), 7.38–7.22 (8H, m, Ph), 7.12 –7.00
(10H, m, Ph), 6.20 (1H, s, H-4), 5.90 (1H, dd, J = 6.0, 1.2 Hz, H-40), 4.73 (1H, dq,
J = 8.4, 7.0 Hz, H-20), 4.60 (1H, d, J = 12.8 Hz, CHaHbPh), 4.56 (1H, d, J = 12.8 Hz,
CHaHbPh), 4.53 (1H, dd, J = 8.4, 6.0 Hz, H-30), 1.21 (3H, d, J = 7.0 Hz, CH3); 13C
NMR (100 MHz, CDCl3) d: 174.7 (C-10), 152.2 (C-2), 145.1 (H-40), 141.8, 138.1,
137.3, 136.0, 128.7, 128.6, 128.3, 128.2, 128.0, 127.7, 127.6, 127.3, 127.1, 126.2,
126.0 (Ph), 105.7 (H-30), 88.6 (C-5), 73.4 (CH2Ph), 66.2 (C-4), 34.5 (C-20), 17.8
Acknowledgments
The authors thank VIEP and CONACyT (project 80915).
References and notes
(CH3); IRt
max: 2922, 1782, 1703, 1450, 1329, 989, 696 cmꢀ1; FAB-HRMS:
calculated for (C33H29NO4), ([M+H])+ 504.2175; found, 504.2154.
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ꢀ13.4 (c 1.0, CHCl3), ee = 0.80; Our result ½a D25
ꢀ11.0 (c 1.9, CHCl3).
ꢂ
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1678; b R-4-(benzyloxyl)-2-methylbutan-1-ol 12, Lit. ½a D25
ꢂ
+10.1 (c 1.0, CHCl3),
Our result ½a 2D5
ꢂ
+10.5 (c 0.35, CHCl3).
16. Evans, D. A.; Britton, T. C.; Ellman, J. A. Tetrahedron Lett. 1987, 28, 6141–6144.
17. The compound ent-14 was obtained from removal of the chiral auxiliary on 9a
by Evans’ hydrolysis modified procedure.
18. (a) Jaime, C.; Segura, C.; Dinarés, I.; Font, J. J. Org. Chem. 1993, 58, 154–158; (b)
Jaime, C.; Ortuco, R. M.; Font, J. J. Org. Chem. 1986, 51, 3946–3951.
19. Geometry optimization of the four conformers c-lactone 13a–b in gas phase
was carried out using DFT calculations with the B3LYP hybrid functional and
the 6-31 + G(d) basis set as implemented in the quantum chemical program
gaussian09. Harmonic vibration frequency analyses were completed and all
vibration frequencies are positive. The computation of the coupling constants,
3JH–H, was done employing the NMR (Spin–Spin) keyword at B3LYP/6-31 + G(d)
level in gaussian09 package. (a) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988,
37, 785–789; (b) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54,
724–728; (C) Gaussian09, revision A.01, Gaussian, Inc. Exploring Chemistry
with Electronic Structure Methods Sec Ed., J. B. Foresman and Aleen Frisch.
20. Cerda-García-Rojas, C. M.; Zepeda, L. G.; Joseph-Nathan, P. Tetrahedron Comput.
Methodol. 1990, 3, 113–118.
9. The compounds 5a–b were achieved in low yield, however the starting
materials were recovered and used again.
10. Procedure for the isomerization (double bond migration). To an oven-dried
100 mL round-bottom equipped with a magnetic stir bar was added anhydrous
LiCl (0.36 mmol, 15.4 mg) under an argon atmosphere followed of
oxazolidinone 5a (0.3 mmol, 0.15 g). Both solids were dissolved in anhydrous
THF (25 mL) and the reaction mixture was stirred for 15 min at ꢀ60 oC and
then was added NaHMDS (0.61 mmol, 0.61 mL) and the resulting solution was
stirred vigorously. After 30 min the temperature was increased at ꢀ45 oC and