A new synthesis of exo-methylene butyrolactones from nitroalkanes

Article abstract of DOI:10.1055/s-2001-16098

A versatile route to exo-methylene butyrolactones was developed by employing a three step reaction sequence consisting of Michael addition of primary nitroalkanes 1 to ethyl (2-bromomethyl) acrylate (2), then Nef conversion of the nitro derivatives 3, and subsequent lactonization of the obtained keto esters 4. The method is chemoselective for important functionalities such as ester, C=C double bond and hydroxyl.

Full text of DOI:10.1055/s-2001-16098

PAPER
1519
A New Synthesis of exo-Methylene Butyrolactones from Nitroalkanes
A
N
ew Synthesis
o
of ex
o
-Meth
b
ylene Butyr
o
e
lactones fr
r
om
N
itro
t
alkane
o
s
Ballini,* Giovanna Bosica, Damiana Livi
Dipartimento di Scienze Chimiche dell’Università, Via S. Agostino 1, 62032 Camerino (MC), Italy
Fax +39(737)637345; E-mail: ballini@camserv.unicam.it
Received 17 May 2001
NO2
Br
Abstract: A versatile route to exo-methylene butyrolactones was
0.5 N NaOH
THF
NO2
OEt
developed by employing a three step reaction sequence consisting
of Michael addition of primary nitroalkanes 1 to ethyl (2-bromom-
ethyl)acrylate (2), then Nef conversion of the nitro derivatives 3,
and subsequent lactonization of the obtained keto esters 4. The
method is chemoselective for important functionalities such as es-
ter, C=C double bond and hydroxyl.
R
OEt
+
72–92%
R
O
3a–j
O
2
1a–j
1) MeONa/MeOH
2) conc. H2SO4,
–50 °C
1) NaBH4, MeOH
Na2HPO4•12H2O
–10 °C to r.t.
Key words: nitro compounds, Michael addition, lactones, Nef re-
action, elimination
O
O
2) 6 N HCl
OEt/Me
O
R
R
66–81% from 3
O
4a–j
The lactone functionality is present in a large variety of
natural products and biologically active compounds.1
They are also largely used as starting material for the
preparation of butenolides, furans, cyclopentenones, alka-
loids, macrocyclic antibiotics, pheromones, antileuke-
mics, flavor compounds, etc.2
5a–i
Scheme 1
Thus, conjugate addition of the nitro compounds 1 to 2,
performed using 0.5 N sodium hydroxide in THF, afford-
ed nitro adducts 3 (72–92%) through tandem nucleophilic
addition-elimination reaction. Successive conversion of
the nitro group to carbonyl (Nef reaction, 3 to 5) by for-
mation of the nitronate anion (MeONa, 1.2 equivalents),
followed by its addition, at –50 °C, to concentrated sul-
phuric acid10 allowed the formation of the keto derivatives
4 as a mixture of ethyl and methyl esters, due to the partial
transesterification. Thus, both the unsaturated keto esters
are prone to lactonization by reduction of the
exo-Methylene butyrolactones constitute a very important
part of this class of compounds,3 mainly due to their pres-
ence as the major structural feature in many biologically
active natural products.4 Their occurrence coupled with
their pharmacophoric activity have spurred the develop-
ment of numerous procedures for their preparation,5 but
these methods suffer from certain drawbacks such as re-
stricted generality, the need for tedious procedures, and
expensive chemicals. Much attention has been focused on
the synthesis of -methylene unit from -phosphono- -
butyrolactones,6 but their synthesis required a multi-step
sequence, high temperature, and -halo- -lactones as
starting material, some of which are not easily available.7
Table 1 Synthesis of Adduct 3a–j and Lactones 5a–j
Entry
R
Yield (%)a of 3 Yield (%)a of 5
from 3
Recently, we reported the synthesis of -(alkylmethyl-
ene)butyrolactone derivatives8 by conjugate addition of
nitroalkanes to methyl trans-4-oxo-2-pentenoate, fol-
lowed by in situ elimination of nitrous acid from the
Michael adduct, then lactonization by sodium borohy-
dride reduction or by Grignard reagents in the presence of
dry cerium(III) chloride.
a
b
c
d
e
f
CH3(CH2)3
76
75
77
72
78
88
86
88
92
75
66
77
70
71
80
75
81
80
78
75b,c
CH3(CH2)4
CH3(CH2)2
CH3OCO(CH2)4
HO(CH2)5
As part of our study on the synthetic potential of aliphatic
nitrocompounds,9 we wish to report here an efficient
method for the preparation of functionalised exo-methyl-
ene butyrolactones 5 starting from primary nitroalkanes 1
and, commercially available, ethyl (2-bromomethyl)acry-
late (2) (Scheme 1).
CH3(CH2)8
g
h
i
CH3OCO(CH2)10
CH2=CH(CH2)8
PhCH2
j
CH3CO(CH2)2
a Yield of isolated, purified product.
Synthesis 2001, No. 10, 30 07 2001. Article Identifier:
1437-210X,E;2001,0,10,1519,1522,ftx,en;Z05101SS.pdf.
© Georg Thieme Verlag Stuttgart · New York
ISSN 0039-7881
b The alkyl group (R) for 5j is CH3CH(OH)(CH2)2.
c As diastereomeric mixture (0.55:0.45).

1520
R. Ballini et al.
PAPER
crude compounds 4 at –10 °C to room temperature with
sodium borohydride in the presence of a catalytic
amount of sodium hydrogenophosphate dodecahydrate
(Na2HPO4•12H2O), so that, the title compounds 5 are ob-
tained in good yields (66–81%, Table 1), directly from 3.
The spectroscopic data for compounds 3 and 5 are given
in Tables 2 and 3, respectively.
NO2
OEt
R
O
3
1) MeONa (excess)/MeOH
2) concd. H2SO4,
–50 °C
OMe
O
O
The amount of MeONa (1.2 equivalents) employed in the
formation of the nitronate form is very important, since,
increasing the base equivalents favours the formation of
compound 6 as a by-product (Scheme 2).
OEt/Me
OEt/Me
R
+
R
O
O
4
6
Scheme 2
It should be emphasized that other important functional-
ities such as ester, C=C double bond and hydroxyl are pre-
served, however, when a third carbonyl moiety is
enclosed in the alkyl structure 4j, its conversion to a hy-
droxy group (4j to 5j, Scheme 3) is observed.
bromomethyl)acrylate (2) through a simple chemical pro-
cedure, and is further evidence of the great versatility of
the aliphatic nitro compounds in organic synthesis.
In conclusion, this method demonstrates that the title
compounds 5 can be conveniently prepared in two steps
starting from easily available nitroalkanes 1 and ethyl (2-
All reactions were monitored by TLC and gas chromatographic
analyses, performed on a Carlo Erba Fractovap 4160 using a capil-
Table 2 Spectroscopic Data for Adducts 3a–j
Product
IR (film)
(cm–1)
1H NMR (CDCl3) (ppm), J (Hz)
MS: m/z
3a
1713, 1630, 1557
1710, 1631, 1551
1714, 1632, 1551
0.90 (t, 3 H, J = 6.7), 1.32 (t, 3 H, J = 7.1), 1.30–1.50 (m, 4 H), 1.70–2.10 184, 153, 137, 109 (100),
(m, 2 H), 2.82 (d, 2 H, J = 9.0), 4.23 (q, 2 H, J = 7.1), 4.70–4.80 (m, 1
95, 81, 67, 55, 41, 29
H), 5.65 (s, 1 H), 6.25 (s, 1 H)
3b
3c
3d
3e
3f
0.90 (t, 3 H, J = 6.4), 1.32 (t, 3 H, J = 7.1), 1.20–1.40 (m, 6 H), 1.60–2.00 198, 197, 123 (100), 95,
(m, 2 H), 2.82 (t, 2 H, J = 7.6), 4.23 (q, 2 H, J = 7.1), 4.70–4.80 (m, 1 H), 81, 67, 55, 41, 29
5.64 (s, 1 H), 6.25 (s, 1 H)
0.94 (t, 3 H, J = 7.3), 1.30 (t, 3 H, J = 7.1), 1.20–1.40 (m, 2 H), 1.60–2.00 170, 169, 139, 123, 95
(m, 2 H), 2.81 (d, 2 H, J = 7.5), 4.22 (q, 2 H, J = 7.1), 4.70–4.90 (m, 1
(100), 67, 55, 41, 29
H), 5.63 (s, 1 H), 6.23 (s, 1 H)
1737, 1713, 1631,
1551
1.30 (t, 3 H, J = 7.1), 1.20–1.40 (m, 2 H), 1.60–1.70 (m, 2 H), 1.90–2.10 256, 241, 209, 179, 163
(m, 2 H), 2.32 (t, 2 H, J = 7.3), 2.80 (d, 2 H, J = 7.5), 3.67 (s, 3 H), 4.22 (100), 153, 107, 93, 79,
(q, 2 H, J = 7.1), 4.70–4.80 (m, 1 H), 5.64 (s, 1 H), 6.24 (s, 1 H)
55
3403, 1708, 1631,
1549
1.31 (t, 3 H, J = 7.1), 1.30–1.50 (m, 2 H), 1.50–1.70 (m, 2 H), 1.70–1.90 213, 167, 149, 121, 107,
(m, 2 H), 1.90–2.10 (m, 2 H), 2.80 (d, 2 H, J = 7.6), 3.64 (t, 2 H, J = 6.3), 93, 79 (100), 67, 55
4.23 (q, 2 H, J = 7.1), 4.70–4.80 (m, 1 H), 5.64 (s, 1 H), 6.24 (s, 1 H)
1719, 1630, 1551
0.98 (t, 3 H, J = 6.4), 1.30 (t, 3 H, J = 7.1), 1.30–2.10 (m, 16 H), 2.80 (d, 253, 179, 135, 123, 109,
2 H, J = 7.0), 4.23 (q, 2 H, J = 7.1), 4.70–4.80 (m, 1 H), 5.64 (s, 1 H),
95 (100), 81, 67, 55
6.24 (s, 1 H)
3g
3h
3i
1736, 1716, 1631,
1551
1.20–1.70 (m, 18 H), 1.30 (t, 3 H, J = 7.1), 2.30 (t, 2 H, J = 7.5), 2.80 (d, 280, 123, 109, 95, 81, 67,
2 H, J = 7.6), 3.65 (s, 3 H), 4.22 (q, 2 H, J = 7.1), 4.70–4.80 (m, 1 H),
55 (100)
5.64 (s, 1 H), 6.26 (s, 1 H)
1714, 1638, 1552
1718, 1629, 1550
1.30–1.40 (m, 11 H), 1.50–2.10 (m, 8 H), 2.81 (d, 2 H, J = 8.8), 4.22 (q, 265, 189, 109, 95, 81, 67,
2 H, J = 7.1), 4.70–4.80 (m, 1 H), 4.90–5.10 (m, 2 H), 5.64 (s, 1 H), 5.70– 55 (100)
5.90 (m, 1 H), 6.24 (s, 1 H)
1.29 (t, 3 H, J = 7.1), 2.10–2.20 (m, 1 H), 2.80–2.90 (m, 1 H), 3.10 (dd, 216, 171, 143, 128, 115,
1 H, J = 5.8, 14.2), 3.29 (dd, 1 H, J = 8.7, 14.2), 4.21 (q, 2 H, J = 7.1),
91 (100), 65
5.00–5.10 (m, 1 H), 5.64 (s, 1 H), 6.27 (m, 1 H), 7.20–7.40 (m, 5 H)
3j
1715, 1709, 1630,
1546
1.30 (t, 3 H, J = 7.1), 2.10–2.20 (m, 2 H), 2.16 (s, 3 H), 2.52 (t, 2 H, J = 197, 167, 151 (100), 123,
7.1), 2.82 (d, 2 H, J = 6.3), 4.25 (q, 2 H, J = 7.1), 4.70–4.90 (m, 1 H), 5.64 109, 97, 91, 79, 67
(s, 1 H), 6.27 (s, 1 H)
Synthesis 2001, No. 10, 1519–1522 ISSN 0039-7881 © Thieme Stuttgart · New York

PAPER
A New Synthesis of exo-Methylene Butyrolactones from Nitroalkanes
1521
Table 3 Spectroscopic Data for Methylene Lactones 5a–j
Product IR (film)
(cm–1)
1H NMR (200 MHz, CDCl3) ppm, J (Hz)
13C NMR (75 MHz,
CDCl3) (ppm)
MS: m/z
5a
5b
5c
5d
1763, 1685
0.90 (t, 3 H, J = 7.0), 1.20–1.40 (m, 4 H), 1.50–1.70 (m, 170.80, 135.26, 122.38,
2 H), 2.57 (ddt, 1 H, J = 3.0, 6.1, 17.1), 3.06 (ddt, 1 H, 77.50, 36.46, 34.05, 27.45, 108, 97 (100), 69,
J = 2.5, 7.6, 17.0), 4.40–4.60 (m, 1 H), 5.62 (t, 1 H, J = 22.89, 14.40
2.5), 6.22 (t, 1 H, J = 2.9)
154 (M+), 125,
55, 41
1765, 1666
0.90 (t, 3 H, J = 6.5), 1.30–1.80 (m, 8 H), 2.57 (ddt, 1 H, 170.89, 135.26, 122.38,
168 (M+), 139,
J = 3.0, 6.1, 17.1), 3.05 (ddt, 1 H, J = 2.5, 7.6, 17.1),
4.50–4.60 (m, 1 H), 5.62 (t, 1 H, J = 2.5), 6.22 (t, 1 H,
J = 2.9)
78.09, 36.73, 34.04, 31.94, 122, 97 (100), 69,
25.02, 22.97, 14.44
41
1762, 1667
0.95 (t, 3 H, J = 7.3), 1.20–1.70 (m, 4 H), 2.56 (ddt, 1 H, 170.34, 134.77, 121.83,
140 (M+), 111, 97
J = 3.0, 6.1, 17.0), 3.10 (ddt, 1 H, J = 2.5, 7.6, 17.0),
4.50–4.60 (m, 1 H), 5.62 (t, 1 H, J = 2.5), 6.22 (t, 1 H,
J = 3.0)
77.32, 38.35, 33.57, 18.20, (100), 69, 55, 41,
13.83
27
1760, 1735, 1667
1.50–1.70 (m, 6 H), 2.33 (t, 2 H, J = 7.2), 2.56 (ddt, 1 H, 173.79, 170.21, 134.55,
J = 3.0, 6.1, 17.0), 3.06 (ddt, 1 H, J = 2.5, 7.6, 17.0), 3.67 122.07, 77.19, 51.54,
(s, 3H), 4.40–4.60 (m, 1 H), 5.62 (t, 1 H, J = 2.5), 6.22 35.95, 33.77, 33.52, 24.53,
194, 162, 135, 97
(100), 69, 55, 41
(t, 1 H,
24.50
J = 3.0)
5e
5f
3401, 1759, 1667
1763, 1664
1.40–1.80 (m, 8 H), 2.57 (ddt, 1 H, J = 3.0, 6.1, 17.0),
3.06 (ddt, 1 H, J = 2.5, 7.6, 17.0), 3.65 (t, 2 H, J = 6.3), 77.04, 62.69, 36.27, 33.56, (100), 81, 69, 41
4.40–4.60 (m, 1 H), 5.62 (t, 1 H, J = 2.5), 6.22 (t, 1 H,
J = 2.8)
170.39, 134.63, 122.08,
166, 148, 111, 97
32.49, 25.48, 24.75
0.87 (t, 3 H, J = 6.9), 1.20–1.80 (m, 14 H), 1.50–1.60 (m, 170.87, 135.28, 122.37,
1 H), 1.70–1.80 (m, 1 H), 2.55 (ddt, 1 H, J = 3.0, 6.1, 77.50, 36.78, 34.06, 32.34, (100), 83, 69, 55,
16.9), 3.03 (ddt, 1 H, J = 2.5, 7.6, 16.9), 4.40–4.50 (m, 1 29.95, 29.76, 25.35, 23.15, 41
H), 5.60 (t, 1 H, J = 2.5), 6.20 (t, 1 H, J = 2.9) 14.59
224 (M+), 140, 97
5g
1764, 1736, 1667
1.20–1.70 (m, 18 H), 2.30 (t, 2 H, J = 7.4), 2.57 (ddt, 1 174.34, 170.39, 134.77,
H, J = 2.9, 6.1, 17.0), 3.06 (ddt, 1 H, J = 2.5, 7.6, 17.0), 121.90, 77.03, 51.46,
278, 264, 112, 97
(100), 81, 69, 55,
3.37 (s, 3 H), 4.40–4.60 (m, 1 H), 5.62 (t, 1 H, J = 2.5), 36.29, 34.11, 33.57, 29.41, 41
6.22 (t, 1 H, J = 2.9)
29.36, 29.27, 29.22, 29.12,
24.94, 24.86
5h
1765, 1664, 1639
1.20–1.70 (m, 14 H), 2.04 (q, 2 H, J = 6.9), 2.57 (ddt, 1 170.30, 139.16, 134.76,
H, J = 3.0, 6.1, 17.0), 3.06 (ddt, 1 H, J = 2.5, 7.6, 17.0), 121.91, 114.15, 77.02,
236 (M+), 151,
109, 97 (100), 81,
4.40–4.60 (m, 1 H), 4.90–5.10 (m, 2 H), 5.62 (t, 1 H,
36.29, 33.78, 33.57, 29.39, 68, 55, 41
J = 2.5), 5.70–5.90 (m, 1 H), 6.22 (t, 1 H, J = 2.8)
29.34, 29.28, 29.06, 28.89,
24.86
5i
5j
1759, 1666
3435, 1759
2.68 (ddt, 1 H, J = 3.0, 6.0, 17.1), 2.92 (dd, 1 H, J = 3.4, 170.12, 135.5, 134.34,
14.0), 2.98 (ddt, 1 H, J = 2.5, 7.6, 17.1), 3.13 (dd, 1 H, 129.52, 128.67, 127.04,
J = 3.0, 14.0), 4.70–4.80 (m, 1 H), 5.58 (t, 1 H, J = 2.5), 122.12, 77.45, 41.86,
188 (M+), 143, 97
(100), 91, 69, 41
6.20 (t, 1 H, J = 2.9), 7.20–7.40 (m, 5 H)
32.72
1.09 (d, 3 H, J = 6.0), 1.50–1.80 (m, 4 H), 2.56 (ddt, 1
(diastereomeric mixture,
155, 126, 111, 97
(100), 81, 69, 45,
39
H, J = 2.9, 9.1, 17.0), 3.05 (ddt, 1 H, J = 2.4, 7.6, 17.0), 0.55:0.45): 170.31 (2C),
3.80–3.90 (m, 1 H), 4.50–4.60 (m, 1 H), 5.61 (t, 1 H,
J = 2.6), 6.21 (t, 1 H, J = 2.9)
134.58, 134.54, 122.16
(2C), 77.03 (2C), 67.68,
67.32, 34.57, 34.05, 33.64,
33.50, 32.83, 32.32, 23.81,
23.74
lary column of duran glass (0.32 mm 25 m), stationary phase OV1
(film thickness 0.4–0.45 nm). 1H and 13C NMR spectra were record-
ed in CDCl3 at 200 and 75 MHz, respectively, on a Varian Gemini
200. Chemical shifts are expressed in ppm downfield from TMS.
Mass spectra were determined on a capillary GC/MS operating in
the split mode with He carrier gas and fitted with a mass-selective
detector (MDS). IR spectra were recorded with a Perkin–Elmer 257
spectrophotometer. All products were purified by flash chromatog-
raphy on Merck silica gel using EtOAc–petroleum ether as eluent.
Elemental analyses were performed using a C, H, N Analyzer Mod-
el 185 from Hewlett–Packard. Satisfactory microanalyses were ob-
tained for each compound; the data are available as supporting
information from the publisher.
Synthesis 2001, No. 10, 1519–1522 ISSN 0039-7881 © Thieme Stuttgart · New York

1522
R. Ballini et al.
PAPER
1) NaBH4, MeOH
Na2HPO4•12H2O
–10 °C to r.t.
O
O
O
2) 6 N HCl
OEt/Me
75%
O
O
5j
OH
4j
Scheme 3
2-Methylene-4-Nitro Esters (3); General Procedure
(3) (a) Carlson, R. M.; Yang, Q. Tetrahedron Lett. 1994, 35,
7919. (b) Lu, X.; Wang, Z.; Ji, J. Tetrahedron Lett. 1994, 35,
613. (c) Dulcere, J.-P.; Mihabi, M. N.; Rodriguez, J. J. Org.
Chem. 1993, 58, 5709. (d) Lu, X.; Zhu, G. Synlett 1993, 68.
(e) Petragnani, N.; Ferraz, H. M. C.; Silva, G. V. J. Synthesis
1986, 157. (f) Grieco, P. A. Synthesis 1975, 67. (g) Martin,
V. S.; Rodriguez, C. M.; Martin, T. Org. Prep. Proced. Int.
1998, 30, 291.
(4) (a) Kupchan, S. M.; Britto, R. W.; Ziegler, M. P.; Gilmore,
C. J.; Restivo, R. G.; Bryan, R. F. J. Am. Chem. Soc. 1973,
95, 1335. (b) Amos, R. A.; Katzenellenbogen, J. J. Org.
Chem. 1978, 43, 560. (c) Yamamoto, M. J. Chem. Soc.,
Perkin Trans. 1 1981, 582. (d) Jellal, A.; Grimaldi, J.;
Santelli, M. Tetrahedron Lett. 1984, 25, 3179. (e) Gollin, I.
J. Chem. Soc., Perkin Trans. 1 1998, 1869. (f) Hoffman, H.
M. R.; Rabe, J. Angew. Chem., Int. Ed. Engl. 1985, 24, 94.
(g) Mulzer, J. In Comprehensive Organic Synthesis, Vol. 6;
Fleming, I.; Trost, B. M., Eds.; Pergamon: Oxford, 1991,
323.
(5) (a) Andrews, R. C.; Marhall, J. A.; DeHoff, B. S. Synth.
Commun. 1986, 16, 1953. (b) Patterson, J. W.; Mc Murry, J.
J. Chem. Soc., Chem. Commun. 1971, 488. (c) Chan, D. M.
T.; Marder, T. B.; Milstein, D.; Taylor, N. J. J. Am. Chem.
Soc. 1987, 109, 6385. (d) Bryan, V. J.; Chan, T.-H.
Tetrahedron Lett. 1996, 37, 5341. (e) Tamaru, Y.;Hojo, M.;
Yoshida, Z. J. Org. Chem. 1991, 56, 1099. (f) Rosini, G.;
Laffi, F.; Marotta, E.; Pagani, I.; Righi, P. J. Org. Chem.
1998, 62, 2398. (g) Leroy, B.; Dumeunier, R.; Markò, I. E.
Tetrahedron Lett. 2000, 41, 10215.
To a solution of nitroalkane 1 (5 mmol) and ethyl (2-bromometh-
yl)acrylate (2, 0.97 g, 5 mmol) in THF (25 mL), NaOH (0.5 N, 16
mL, 8 mmol) was added dropwise at r.t. The resulting solution was
stirred for 12 h, then extracted with Et2O (3 25 mL), and the or-
ganic layer was dried (MgSO4) and evaporated. The crude product
was pure enough (>93% by GC), however, purification by flash
chromatography (EtOAc–petroleum ether) afforded the pure com-
pounds 3.
exo-Methylene Butyrolactones (5); General Procedure
The nitro ester 3 (2 mmol) was added at r.t. and under a N2 atm, to
a solution of MeONa/MeOH obtained by mixing Na (552 mg, 2.4
mmol) and anhyd MeOH (6 mL). After 30 min, the formed nitronate
was added dropwise to a cooled (–50 °C) solution of concd H2SO4
(1.2 mL) and MeOH (6 mL). After 1–6 h (see GC), H2O (15 mL)
was added and the mixture was concentrated in order to reduce the
amount of MeOH. The mixture was extracted with CH2Cl2 (3 20
mL), and the organic layer was dried (MgSO4) and evaporated. The
crude product 4 was dissolved in MeOH (15 mL) and the solution
was cooled to –10 °C. Na2HPO4•12H2O (83 mg, 0.23 mmol) and
NaBH4 (83 mg, 2.2 mmol) were added and the mixture was stirred
at the same temperature for 2 h, and then at r.t. for 13 h. The mixture
was acidified to pH 2 with 6 N HCl, stirred for a further 1 h, extract-
ed with CH2Cl2 (3 15 mL), dried (MgSO4), and evaporated to give
the lactone 5, which was then purified by flash chromatography
(EtOAc–petroleum ether).
Acknowledgement
(6) Lee, C.-W.; Gil, J. M.; Oh, D. Y. Heterocycles 1997, 45,
943.
(7) (a) Buechel, K. L.; Roechling, H.; Korte, F. Liebigs Ann.
Chem. 1965, 685, 10. (b) Jackson, J. A.; Hammond, G. B.;
Wiemer, D. F. J. Org. Chem. 1989, 54, 4759.
This work was carried out in the framework of the National Project
“Stereoselezione in Sintesi Organica. Metodologie ed Applicazio-
ni“ supported by Ministero dell’Università e della Ricerca Scienti-
fica e Tecnologica, Rome-Italy and by Fondazione della Cassa di
Risparmio della Provincia di Macerata-Italy.
(8) Ballini, R.; Marcantoni, E.; Perella, S. J. Org. Chem. 1999,
64, 2954.
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G.; Ballini, R.; Petrini, M.; Marotta, E.; Righi, P. Org. Prep.
Proced. Int. 1990, 22, 707. (c) Ballini, R. In Studies in
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