Cyclization of Malonate Derivatives with Iodine(III) Reagents

Article abstract of DOI:10.1002/ejoc.201601587

The cyclization of malonate derivatives by using hypervalent iodine(III) reagents is described. In this reaction, double bonds are dioxygenated to give five-membered lactones in up to 70 % yield with diastereomeric ratios of up to 11:1.

Full text of DOI:10.1002/ejoc.201601587

A Journal of
Accepted Article
Title: Cyclization of malonate derivatives with iodine(III) reagents
Authors: Florence Malmedy and Thomas Wirth
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201601587
Link to VoR: http://dx.doi.org/10.1002/ejoc.201601587
Supported by

European Journal of Organic Chemistry
10.1002/ejoc.201601587
COMMUNICATION
Cyclization of malonate derivatives with iodine(III) reagents
[
a]
[a]
Florence Malmedy and Thomas Wirth*
Abstract: The cyclization of malonate derivatives using hypervalent
iodine(III) reagents is described. In this reaction, double bonds are
dioxygenated and 5-membered lactones obtained with up to 70%
yield and in diastereomeric ratios of up to 11:1.
O
MeO
2
C
CO
2
Me
iodine reagent
additives
MeO
2
C
MeO
2
C
MeO C
2
O
+
CH Cl , 20 h
2
2
OAc
–20 ºC
R
O
1
2a: R = OAc
2b: R = OH
3
Hypervalent iodine reagents have become extremely versatile
reagents in organic chemistry. The mild reaction conditions
associated with the low toxicity and the environmentally friendly
behavior of these compounds render them attractive to use in
Table 1. Optimization of reaction conditions for lactonization.
Entry
Iodine reagent (Eq.)
Additive (Eq.)
Yield
d.r.
[
1 ]
[%]
(trans:cis)
organic synthesis.
Hypervalent iodine reagents are very
selective oxidants and several derivatives have been reported
[
2]
1
2
3
PhI(OAc)
2
(1)
-
-
0
-
[
3]
as enantiomerically pure reagents. Due to their electrophilic
nature and their excellent leaving group ability, they can react
with a broad range of nucleophiles in reactions such as the
I(OAc) (1)
3
0
-
[
4 ]
PhI(OAc)
PhI(OAc)
2
2
(1)
BF
BF
3
3
• OEt
• OEt
2
2
(2)
(2)
2a: 16
2a: 15
10:1
10:1
6:1
oxidation of sulfides to sulfoxides,
the dearomatization of
phenols, the α-arylation and the α-oxygenation of carbonyl
[5]
[6]
[
[
a]
b]
4
5
(1.2)
[7]
compounds but also in the functionalization of carbon-carbon
[
8
]
[
9
]
PhI(OAc) (1.2)
BF • OEt (2)
2a: 48
3: 45
2
3
2
double bonds through dioxygenation
,
diamination
,
[
10]
[11]
[12]
oxyamination,
iodoamination,
oxytrifluoromethylation
or
[
13
]
[
c]
aminofluorination.
The facile generation of cationic
6
7
8
PhI(OH)OTs (1.2)
BF
BF
BF
3
3
3
• OEt
• OEt
• OEt
2
2
2
(2)
(2)
(3)
2b: 20
0
n.d.
-
intermediates by hypervalent iodine reagents allows either the
direct reaction with a nucleophile or the formation of rearranged
3 2
PhI(OCOCF ) (1.5)
[
14]
[15]
[16]
products
with ring contraction,
ring expansion,
or aryl
PhI(OAc)
PhI(OAc)
PhI(OAc)
PhI(OAc)
2
2
2
2
(1.2)
(1.2)
(1.2)
(1.2)
2a:
5:1
[
17]
[
d]
migration.
the catalytic use of those hypervalent iodine reagents.
Finally, intensive efforts have been made towards
(
83)
[
3b,18]
9
1
1
BF
3
• OEt
2
(2)
2a: 24
3: 32
8:1
Herein we report the intramolecular synthesis of lactones from
malonate derivatives using hypervalent iodine reagents. Initially,
investigations were carried out using substrate 1 which was
easily synthesized from dimethyl malonate by two subsequent
alkylations. The reaction was initially performed using only the
AcOH (12.5)
0
1
BF • OEt (3)
3
2
2a: 58
3: 6
11:1
5:1
AcOH (12.5)
BF
3
• OEt
2
(2)
2a: 39
2 3
iodine(III) reagent at –20 °C. Both PhI(OAc) and I(OAc) did not
H
2
O (0.01)
mediate any cyclization even after 3 days of reaction and the
starting material was fully recovered (Table 1, Entries 1 and 2). It
was suspected that the iodine reagent alone was not reactive
enough and an activating Lewis acid was added. With the Lewis
[
a] Reaction time: 3 days. [b] Reaction performed at 0 ºC. A reaction performed
at rt led to almost complete decomposition. [c] Not determined. [d] Conversion,
yield not determined.
3 2
acid BF •OEt the cyclized product 2a was isolated in 16% yield
and with a 10:1 diastereomeric ratio (Table 1, Entry 3). The size
of the lactone ring was unambiguously established from its
[
19]
crystal structure (Figure 1). The crystal structure also revealed
the relative stereochemistry of the major diastereomer, with the
methyl ester and the CH
other.
2
-acetate substituent being trans to each
[a]
Dr. F. Malmedy, Prof. Dr. T. Wirth
School of Chemistry
Cardiff University
Park Place, Main Building, Cardiff CF10 3AT (UK)
E-Mail: wirth@cf.ac.uk
Figure 1. Crystal structure of lactone 2a.
Homepage: http://blogs.cardiff.ac.uk/wirth/
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

European Journal of Organic Chemistry
10.1002/ejoc.201601587
COMMUNICATION
A slight excess of the iodine reagent did not make any
difference and the diastereomeric ratio remained unchanged
[c]
2
3
PhI(OAc)
PhI(OAc)
2
2
(1.2)
(1.2)
TMSOTf (2)
5c: 28
n.d.
-
(
Table 1, Entry 4). The yield was not improved by prolonging the
TMSOTf (2)
TFE
0
reaction time to three days. Increasing the temperature to 0 °C
improved the conversion to 100% and the cyclized product 2a
was isolated in 48% yield. However, the formation of a major
side product was observed. A competing oxidation of the triple
bond occurred and the oxidized compound 3 was isolated in
[c]
4
5
6
PhIO (1)
BF
BF
BF
3
3
3
• OEt
• OEt
• OEt
2
2
2
(2)
(2)
(2)
5b: 37
5b: 21
0
n.d.
2:1
-
PhI(OH)OTs (1.5)
PhI(OCOCF
1.5)
3 2
)
4
5% yield in addition to 2 (Table 1, Entry 5). When the reaction
(
was warmed up to room temperature, considerable
decomposition was observed and products could no longer be
isolated.
[
a] Reaction time: 3 days. [b] Reaction performed at 0 ºC. A reaction
performed at rt led to almost complete decomposition. [c] Not determined.
Two other iodine reagents were also evaluated. Koser’s
The cyclization reaction was also performed with ethyl 2-
cyanoacetate derivative 4, which was easily prepared from ethyl
2-cyanoacetate. The expected cyclized product 5a with the
acetoxy group addition was isolated in 43% yield but the
diastereomeric ratio was only 3:1 (Table 2, Entry 1). Another
Lewis acid (TMSOTf) was also used here. The cyclization
occurred with lower yield (28%) while the triflate anion reacted
as the nucleophile to give product 5c (Table 2, Entry 2). The
addition of TFE resulted again in complete degradation of the
starting material (Table 2, Entry 3). Changing the iodine(III)
reagent did neither improve the yield of the cyclized product 5
nor the selectivity. Iodosylbenzene and Koser’s reagent both
mediated the addition of a hydroxyl group onto the double bond
in 37% and 21% yield, respectively (Table 2, Entries 4 and 5).
Finally, the use of [bis(trifluoroacetoxyiodo)]benzene resulted in
complete degradation into unknown products (Table 2, Entry 6).
reagent [PhI(OH)OTs] gave the cyclized product 2b (R = OH) in
1
only 20% yield. The selectivity could not be determined by
H
NMR as the peaks were overlapping (Table 1, Entry 6). Using
bis(trifluoroacetoxyiodo)]benzene together with BF • OEt led to
complete decomposition (Table 1, Entry 7).
[
3
2
Increasing the amount of BF
3 2
• OEt to 3 equivalents led to a
better conversion (83%), but the selectivity dropped to
a
diastereomeric ratio of 5:1 (Table 1, Entry 8). Additional acetic
acid was added to the reaction mixture and the cyclized product
was isolated in 58% yield with slightly improved diastereomeric
ratio of 11:1 (Table 1, Entry 10). As the increased amount of
Lewis acid was detrimental for the selectivity, the reaction was
3 2
also performed with only two equivalents of BF • OEt . The
starting material was fully converted but surprisingly, more side
product 3 was formed as well as one other uncharacterized side
product. Furthermore, the selectivity decreased to a ratio of 8:1
After the detailed investigation of the two substrates 1 and 4,
different homoallyl esters were cyclized under identical
conditions. The results are summarized in Table 3.
(
Table 1, Entry 9). Finally, it has to be noted than in the
presence of 1 mol% water the cyclization still proceeded but with
a lower conversion. Only product 2b was isolated in 39% yield
and with a diastereomeric ratio of 5:1 (Table 1, Entry 11).
[
a]
Table 3. Cyclization of homoallyl esters.
Entry
Substrate
Product
Yield
%]
d.r.
The influence of the solvent was also studied. When the
reaction was performed in acetonitrile, no product was isolated
due to complete degradation of the reaction mixture into
uncharacterized products. The addition of acetic acid did not
lead to a cleaner reaction. 2,2,2-Trifluorethanol (TFE) is known
to coordinate to the iodine reagents and has, in several cases of
[
(trans:cis)
O
MeO2C
MeO C
2
O
1
MeO2C
61
15
1:1.3
OAc
6
[
20]
7
iodine(III)-mediated reactions, led to increased selectivities.
However, when a mixture of dichloromethane and TFE (5:1) was
used as the solvent, complete degradation of the starting
material was observed.
OAc
O
MeO C
2
O
[
b]
MeO2C
O
[c]
2
n.d.
O
AcO
8
O
NC CO2Et
iodine reagent
additives
9
NC
O
O
CH2Cl2, 20 h
MeO C
2
–
20 ºC
R
HO C
2
O
[
c]
2
a: 28
n.d.
n.d.
4
5a: R = OAc
MeO2C
3
[c]
5
5
b: R = OH
c: R = OTf
R
2b: 13
10
2
2
a: R = OAc
b: R = OH
Table 2. Cyclization of ethyl 2-cyanoacetate derivative 4.
Entry
1
Iodine reagent
Eq.)
Additive (Eq.)
Yield [%]
d.r.
(
(trans:cis)
PhI(OAc)
2
(1.2)
BF
3
• OEt
2
(2)
5a: 43
3:1
This article is protected by copyright. All rights reserved.

European Journal of Organic Chemistry
10.1002/ejoc.201601587
COMMUNICATION
O
Acknowledgements
tBuO2C
tBuO2C
t
BuO C
2
O
We thank Dr. B. Kariuki, Cardiff University, for the X-ray analysis
of compound 2a. Support from the School of Chemistry, Cardiff
University, is gratefully acknowledged. We thank the EPSRC
National Mass Spectrometry Facility, Swansea, for mass
spectrometric data.
4
5
58
70
1:4
1:4
OAc
1
1
1
2
tBuO C
O
2
tBuO2C
tBuO2C
O
Keywords: cyclization • hypervalent iodine reagents •
1
3
OAc
1
4
lactonization • malonate derivatives
[
a] 1.5 eq. PhI(OAc)
2
, 2 eq. BF
3
• OEt
2
, 12.5 eq. AcOH, –20 ºC, 20 h. [b] 2.5
eq. PhI(OAc)
determined.
2
, 4 eq. BF OEt ,
3
•
2
25 eq. AcOH, –20 ºC, 20 h. [c] Not
[1]
a) Hypervalent Iodine Chemistry, Ed. T. Wirth, (Topics in Current
Chemistry), Springer, Berlin, 2016, vol. 373. b) V. V. Zhdankin,
Hypervalent Iodine Chemistry, Wiley, Chichester, 2014.
The mono-allylated dimethyl malonate 6 gave compound 7
in 61% yield and with a diastereomeric ratio of 1.3:1 (Table 3,
Entry 1). The cyclization also proceeded with the di-allylated
substrate 8. As two carbon-carbon double bonds and two esters
are present in compound 8 the amounts of reagents were
increased and the double cyclized spiro product 9 was isolated
in 15% yield as the major product (Table 3, Entry 2). Even if the
substrate contains one hydrolyzed ester group as in 10 there
seems to be no apparent advantage for the cyclization. Two
different cyclized products were formed during the reaction, one
with the addition of a hydroxyl group onto the double bond (2b,
[
2]
3]
T. Wirth, Angew. Chem. Int. Ed. 2005, 44, 3656-3665.
[
a) R. Kumar, T. Wirth, Top. Curr. Chem. 2016, 373, 243-261; b) F.
Berthiol, Synthesis 2015, 47, 587-603; c) F. V. Singh, T. Wirth, Chem.
Asian J. 2014, 9, 950-972; d) A. Parra, S. Reboredo, Chem. Eur. J.
2
013, 19, 17244-17260; e) U. Farid, T. Wirth, in Asymmetric Synthesis
– The Essentials, Eds.: S. Bräse, C. Christmann, Wiley VCH, 2012,
197-203; f) H. Liang, M. A. Ciufolini, Angew. Chem. Int. Ed. 2011, 50,
11849-11851; g) M. Ngatimin, D. W. Lupton, Aust. J. Chem. 2010, 63,
6
53-658.
a) S. M. Altermann, S. Schäfer, T. Wirth, Tetrahedron 2010, 66, 5902-
907; b) V. V. Zhdankin, J. T. Smart, P. Zhao, P. Kiprof, Tetrahedron
[
4]
5]
5
Lett. 2000, 41, 5299-5302; c) H. Tohma, S. Takizawa, H. Watanabe, Y.
Fukuoka, T. Maegawa, Y. Kita, J. Org. Chem. 1999, 64, 3519-3523; d)
D. G. Ray, G. F. Koser, J. Am. Chem. Soc. 1990, 112, 5672-5673; e) T.
Imamoto, H. Koto, Chem. Lett. 1986, 967-968.
1
3%) and the other one with an acetoxy moiety (2a, 28% yield)
(
Table 3, Entry 3).
Di-tert-butyl malonate derivatives can be used similarly well.
[
a) A. M. Harned, Tetrahedron Lett. 2014, 55, 4681-4689; b) K. A. Volp,
A. M. Harned, Chem. Commun. 2013, 49, 3001-3003; c) M. Uyanik, T.
Yasui, K. Ishihara, Angew. Chem. Int. Ed. 2013, 52, 9215-9218; d) T.
Dohi, N. Takenaga, T. Nakae, Y. Toyoda, M. Yamasaki, M. Shiro, H.
Fujioka, A. Maruyama, Y. Kita, J. Am. Chem. Soc. 2013, 135, 4558-
4566; e) S. Quideau, G. Lyvinec, M. Marguerit, K. Bathany, A. Ozanne-
Beaudenon, T. Buffeteau, D. Cavagnat, A. Chénedé, Angew. Chem. Int.
Ed. 2009, 48, 4605-4609.
Substrate 11 gave the desired cyclized product 12 in reasonable
yield (58%) and with moderate selectivity (d.r. = 4:1) (Table 3,
Entry 4). The mono-allylated di-tert-butyl malonate 13 led to the
cyclized product 14 in good yield (70%) but with only moderate
selectivity of 4:1 (Table 3, Entry 5). The stereochemistry of the
major isomer (cis) was determined by a NOESY experiment
(
see supporting information).
[6]
a) M. Brown, M. Delorme, F. Malmedy, J. Malmgren, B. Olofsson, T.
Wirth, Synlett 2015, 26, 1573-1577; b) V. K. Aggarwal, B. Olofsson,
Angew. Chem. Int. Ed. 2005, 44, 5516-5519; c) M. Ochiai, Y. Kitagawa,
N. Takayama, Y. Takaoka, M. Shiro, J. Am. Chem. Soc. 1999, 121,
The substitution of (diacetoxyiodo)benzene with a chiral
[
21]
derivative did not lead to any cyclization product, probably due
to reduced reactivity of the chiral reagent.
9
233-9234.
In conclusion, several malonate derivatives were
successfully cyclized to highly functionalized lactones with
moderate to good diastereoselectivities.
[7]
a) B. Basdevant, C. Y. Legault, J. Org. Chem. 2015, 80, 6897-6902; b)
B. Basdevant, C. Y. Legault, Org. Lett. 2015, 17, 4918-4921; c) P.
Mizar, T. Wirth, Angew. Chem. Int. Ed. 2014, 53, 5993-5997; d) U.
Farooq, S. Schäfer, A.-H. Shah, D. Freudendahl, T. Wirth, Synthesis
2
010, 2010, 1023-1029; e) S. M. Altermann, R. D. Richardson, T. K.
Page, R. K. Schmidt, E. Holland, U. Mohammed, S. M. Paradine, A. N.
French, C. Richter, A. M. Bahar, B. Witulski, T. Wirth, Eur. J. Org.
Chem. 2008, 5315-5328; f) R. Richardson, T. Page, S. Altermann, S.
Paradine, A. French, T. Wirth, Synlett 2007, 538-542; E. Hatzigrigoriou,
A. Varvoglis, M. Bakola-Christianopoulou, J. Org. Chem. 1990, 55, 315-
318.
Experimental Section
PhI(OAc)
2
(108 mg, 0.340 mmol) was dissolved in dry dichloromethane
(
2 mL) under argon and the reaction was cooled down to –20 °C. BF •
3
2
OEt (70 µL, 0.56 mmol) and AcOH (200 µL, 3.5 mmol) was then added
and the resulting mixture was stirred for 5 min. A solution of starting
material (0.28 mmol) in dry dichloromethane (2 mL) was then added and
the resulting mixture was stirred for 19 h at –20 °C. The reaction was
quenched with sat. aq. solution of sodium bicarbonate (0.5 mL) and the
organic phase was extracted with dichloromethane (3 x 5 mL). The
[8]
a) T. Takesue, M. Fujita, T. Sugimura, H. Akutsu, Org. Lett. 2014, 16,
4634-4637; b) M. Shimogaki, M. Fujita, T. Sugimura, Eur. J. Org. Chem.
2013, 7128-7138; c) M. Fujita, K. Mori, M. Shimogaki, T. Sugimura, Org.
Lett. 2012, 14, 1294-1297; d) M. Fujita, Y. Yoshida, K. Miyata, A.
Wakisaka, T. Sugimura, Angew. Chem. Int. Ed. 2010, 49, 7068-7071;
e) M. Fujita, Y. Ookubo, T. Sugimura, Tetrahedron Lett. 2009, 50,
1298-1300; f) M. Fujita, S. Okuno, H. J. Lee, T. Sugimura, T. Okuyama,
Tetrahedron Lett. 2007, 48, 8691-8694; g) U. H. Hirt, M. F. H. Schuster,
®
combined organic layers were dried through a Telos phase separator
and concentrated under vacuum. The pure product was obtained after
purification by column chromatography (0 to 30% EtOAc in hexane).
This article is protected by copyright. All rights reserved.

European Journal of Organic Chemistry
10.1002/ejoc.201601587
COMMUNICATION
A. N. French, O. G. Wiest, T. Wirth, Eur. J. Org. Chem. 2001, 1569-
2006, 11, 421-434; c) F. A. Siqueira, E. E. Ishikawa, A. Fogaça, A. T.
Faccio, V. M. T. Carneiro, R. R. S. Soares, A. Utaka, I. R. M. Tébéka,
M. Bielawski, B. Olofsson, L. F. Silva, J. Braz. Chem. Soc. 2011, 22,
1795-1807.
1
579; h) T. Wirth, U. H. Hirt, Tetrahedron: Asymmetry 1997, 8, 23-26.
[
9]
a) P. Mizar, A. Laverny, M. El-Sherbini, U. Farid, M. Brown, F. Malmedy,
T. Wirth, Chem. Eur. J. 2014, 20, 9910-9913; b) C. Röben, J. A. Souto,
E. C. Escudero-Adán, K. Muñiz, Org. Lett. 2013, 15, 1008-1011; c) C.
Röben, J. A. Souto, Y. González, A. Lishchynskyi, K. Muñiz, Angew.
Chem. Int. Ed. 2011, 50, 9478-9482.
[16] a) L. F. Silva, R. S. Vasconcelos, M. A. Nogueira, Org. Lett. 2008, 10,
1017-1020; b) M. Justik, G. Koser, Molecules 2005, 10, 217-225.
[17] a) M. Brown, R. Kumar, J. Rehbein, T. Wirth, Chem. Eur. J. 2016, 22,
4030-4035; b) S. M. Banik, J. W. Medley, E. N. Jacobsen, Science
2016, 353, 51-54; c) U. Farid, F. Malmedy, R. Claveau, L. Albers, T.
Wirth, Angew. Chem. Int. Ed. 2013, 52, 7018-7022; d) A. C. Boye, D.
Meyer, C. K. Ingison, A. N. French, T. Wirth, Org. Lett. 2003, 5, 2157-
2159.
[
10] U. Farid, T. Wirth, Angew. Chem. Int. Ed. 2012, 51, 3462-3465.
11] a) P. Mizar, T. Wirth, Synthesis 2016, 48, in press; b) P. Mizar, A.
Burrelli, E. Günther, M. Söftje, U. Farooq, T. Wirth, Chem. Eur. J. 2014,
[
2
0, 13113-13116.
[
12] R. Zhu, S. L. Buchwald, Angew. Chem. Int. Ed. 2013, 52, 12655-12658.
13] a) H. Chen, A. Kaga, S. Chiba, Org. Biomol. Chem. 2016, 16, 5481-
[
[18] R. D. Richardson, T. Wirth, Angew. Chem. Int. Ed. 2006, 45, 4402-4404.
[19] CCDC-1512668 (2a) contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
5
485; b) W. Kong, P. Feige, T. de Haro, C. Nevado, Angew. Chem. Int.
Ed. 2013, 52, 2469-2473.
[
14] a) G. Maertens, S. Canesi, Top. Curr. Chem. 2016, 373, 223-242; b) F.
V. Singh, T. Wirth, Synthesis 2013, 45, 2499-2511; c) G. Jacquemot, S.
Canesi, J. Org. Chem. 2012, 77, 7588-7594; d) F. V Singh, J. Rehbein,
T. Wirth, ChemistryOpen 2012, 1, 245-250; e) M.-A. Beaulieu, C. Sabot,
N. Achache, K. C. Guérard, S. Canesi, Chem. Eur. J. 2010, 16, 11224-
Cambridge
Crystallographic
Data
Centre
via
www.ccdc.cam.ac.uk/data_request/cif.
[20] T. Dohi, N. Yamaoka, Y. Kita, Tetrahedron 2010, 66, 5775–5785.
3
[21] The
chiral
reagent
dimethyl-2,2'-((2-(diacetoxy-λ -iodanyl)-1,3-
phenylene)bis(oxy))(2R,2'R)-dipropionate was investigated in this
reaction, see: M. Uyanik, T. Yasui, K. Ishihara, Angew. Chem. Int. Ed.
2010, 49, 2175-2177.
1
1228; f) A. A. Zagulyaeva, C. T. Banek, M. S. Yusubov, V. V Zhdankin,
Org. Lett. 2010, 12, 4644-4647; g) J.-M. Vatèle, Synlett 2008, 1785-
788.
1
[15] a) L. F. Silva, F. A. Siqueira, E. C. Pedrozo, F. Y. M. Vieira, A. C.
Doriguetto, Org. Lett. 2007, 9, 1433-1436; b) L. F. Silva, Molecules
This article is protected by copyright. All rights reserved.

European Journal of Organic Chemistry
10.1002/ejoc.201601587
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
F. Malmedy, T. Wirth*
Page No. – Page No.
Cyclization of malonate derivatives
with iodine(III) reagents
Double bonds are dioxygenated and 5-membered lactone products obtained in
good yields and diastereomeric ratios.
This article is protected by copyright. All rights reserved.