Selectivity of Daucus carota roots and baker's yeast in the enantioselective reduction of γ-nitroketones

Article abstract of DOI:10.1016/j.tetasy.2005.03.002

The enantioselective reduction of a series of aromatic γ-nitroketones was achieved by using Daucus carota roots in water, which afforded the corresponding (S)-alcohols with ees ranging from 73% to 100%. A comparison of these results with the data obtained by reducing the same substrates with baker's yeast resulted in D. carota always being more enantioselective than baker's yeast, although a lower number of substrates were reduced. The possible influence of the aromatic ring substituents on the reaction outcome is also discussed.

Full text of DOI:10.1016/j.tetasy.2005.03.002

Tetrahedron:
Asymmetry
Tetrahedron: Asymmetry 16 (2005) 1479–1483
Selectivity of Daucus carota roots and bakerꢀs yeast
in the enantioselective reduction of c-nitroketones
Dina Scarpi,* Ernesto G. Occhiato and Antonio Guarna
Dipartimento di Chimica Organica ‘Ugo Schiff ’, Via della Lastruccia 13, 50019Sesto Fiorentino, Italy
Received 13 January 2005;accepted 1 March 2005
Available online 23 March 2005
Abstract—The enantioselective reduction of a series of aromatic c-nitroketones was achieved by using Daucus carota roots in water,
which afforded the corresponding (S)-alcohols with ees ranging from 73% to 100%. A comparison of these results with the data
obtained by reducing the same substrates with bakerꢀs yeast resulted in D. carota always being more enantioselective than bakerꢀs
yeast, although a lower number of substrates were reduced. The possible influence of the aromatic ring substituents on the reaction
outcome is also discussed.
ꢀ 2005 Elsevier Ltd. All rights reserved.
1. Introduction
Moreover, it should be highlighted that the use of D.
carota roots as reducing system applied to the organic
synthesis has not been widely explored and very recently
have a few articles been published,13–19 which disclosed
the potential for the enantioselective reductions of
carbonyl compounds.
The synthetic usefulness of enantiopure d-nitroalcohols
as precursors in the synthesis of natural substances has
been well established.1–4 Previously, we reported that
alkaloids such as pyrrolidines,5 indolizines6 and pyr-
rolizidines,7 pheromones, such as spiroketals8,9 and
lactones5 can all be easily prepared starting from d-
nitroalcohols and d-nitrodiols. The synthesis of enantio-
pure d-nitroalcohols has been based mainly on the
reduction of the corresponding c-nitroketones, in partic-
ular by reduction using bakerꢀs5,10 and other yeasts,11,12
and chemical reduction by diisopinocamphenylchloro-
borane (DIP-ClTM).9 The latter procedure allows the
enantioselective synthesis of both enantiomers but the
reagent is quite expensive. The reduction with bakerꢀs
yeast is much more convenient although some of the
c-nitroketones are not reduced at all and the product
selectivity with these substrates is often poor.5 Thus,
for finding an inexpensive alternative procedure for the
synthesis of enantiopure d-nitroalcohols, we decided to
evaluate the reduction of a series of c-nitroketones
by Daucus carota roots. Recently, D. carota has been
shown to reduce acetophenones13–17 and other ke-
tones,13,18,19 with excellent selectivity. It has some
advantages compared to bakerꢀs yeast;for instance the
very low cost, the availability of the material and the
ease of the reaction work-up and product recovery.13,18
2. Results and discussion
Herein we report on the bio-reduction of a series of aro-
matic c-nitroketones that have already been used as sub-
strates for bakerꢀs yeast reductions.5 Furthermore, three
new ketones have been synthesised to better explore the
effects of the substituents on the aromatic ring on the
enzyme selectivity.
All substrates 3a–f were synthesised (Scheme 1) as re-
ported starting from the corresponding acetophenones
1a–f,20 by treatment of the intermediate Mannich bases
with CH3NO2,5 with the exception of 3g (R = p-Cl)
which was prepared from the commercially available
b-chloropropiophenone 2 and CH3NO2 under basic
conditions.21
The reduction of these carbonyl compounds was carried
out according to the experimental procedures reported
in the literature: one13 employs cut roots in water, while
the other15 involves minced roots in 0.1 M phosphate
buffer at pH 6.5. The results are reported in Table 1 to-
gether with the data obtained by bakerꢀs yeast reduction
of the same substrates for comparison.
*
Corresponding author. Tel.: +39 055 4573502;fax: +39 055 4573531;
e-mail: dina.scarpi@unifi.it
0957-4166/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2005.03.002

1480
D. Scarpi et al. / Tetrahedron: Asymmetry 16 (2005) 1479–1483
O
Cl
2
Cl
a: R = H
e: R = o-Br
f: R = m-NO2
g: R = p-Cl
b: R = o-OMe
c: R = m-OMe
d: R = p-OMe
b
(64%)
O
O
OH
NO2
a
NO2
ee 73-100%
c or d
*
R
R
R
1a-f
3a-g
4a-g
Scheme 1. Reagents and conditions: (a) (i) (HCHO)n, Me2NHÆHCl, EtOH, H+, reflux, 2 h;(ii) NaOH aq;(iii) CH 3NO2, TRITON B, reflux, 2 h;
(b) CH3NO2, NaOH, MeOH–H2O, rt, 1.5 h;(c) Daucus carota root, H2O, rt, 10 days;(d) Daucus carota root, 0.1 M phosphate buffer, pH 6.5, rt,
10 days.
Table 1.
Entry
Substrate
Product
Conditionsa
Daucus carota
Bakerꢀs yeast5
Conv.b (%)
Ee (%)c
Conf.d
Conv.b % (time)
Ee (%)c
Conf.d
1
3a
3a
3b
3c
3c
3d
3e
3f
3g
6
4a
4a
4b
4c
4c
4d
4e
4f
A
67
97e
0
>99
53
S
89 (7d)
78
S
2
B
A or B
S
3
—
—
S
64 (7d)
67 (7d)
>99
76
S
S
4
A
100
82f
0
>99
90
5
B
A or B
S
6
—
—
—
—
R
82 (7d)
0
76
—
58
71
—
95
S
7
A or B
A
0
—
R
8
53
73
0
73
94
85 (9d)
50 (8d)
0
9
4g
A
A or B
S
S
10
11
12
—
—
—
—
S,S
—
S,S
7
8
8
A
B
0
76
58 (7d)
7
58
a Reductions were carried out at room temperature over 10 days either in water (A) or in 0.1 M phosphate buffer pH 6.5 (B).
b Determined by 1H NMR of the crude reaction mixture.
c Determined by 1H NMR analysis of the Mosher esters.5,27
d Determined by the sign of the specific rotation and by 1H NMR analysis of the Mosher esters.5,27
e 4a (68%) and 5a (29%).
f 4c (57%) and 5c (25%).
The reductions by D. carota roots were quite slow under
both conditions and were all stopped after 10 days. With
the exception of m-NO2 substituted ketone 3f (entry 8),
reduction by D. carota of the aromatic nitroketones
afforded the corresponding alcohols with (S) absolute
configuration. In all cases the roots proved more enan-
tioselective than bakerꢀs yeast. For example, alcohols
4a (R = H), 4c (R = m-OMe) and 4g (R = p-Cl) were
all obtained with ee >94% when the reduction was
carried out with D. carota in water (entries 1, 4 and 9),
whereas the yeast afforded the same products with ee
ranging from 71% to 78%. Conversely the range of sub-
strates accepted by D. carota seems more limited as the
reduction of 3b (R = o-OMe) and 3d (R = p-OMe) was
not achieved (entries 3 and 6). It should be noted that
in these two ketones, the substituent on the aromatic
ring is the electron donor methoxy group in ortho and
para positions, which could account for the decrease in
the reduction rate of the carbonyl group by D. carota.
Instead, as in the case of substrate 3c, the same elec-
tronic effect could not be exerted by the m-OMe substi-
tuent and thus the reduction to the corresponding
alcohol 4c is achieved. It must be pointed out, however,
that in the case of 3b, the steric hindrance due to the
o-OMe group, as a cause of the lack of reactivity, cannot
be excluded. In fact substrate 3e, bearing an o-Br group,
was not reduced at all either by D. carota or bakerꢀs
yeast (entry 7). Two more c-nitroketones, bearing elec-
tron withdrawing substituents on the ring, that is, m-
NO2 3f and p-Cl 3g were subjected to reduction. Since
in the reduction of 3a and c the use of the buffer
decreased the enantioselectivity, the reduction of keto-
nes 3f and g was performed only in water. Both ketones
were converted into the corresponding alcohols by D.
carota (entries 8 and 9), thus suggesting that electronic
effects do play a role in determining the reduction rate.
Interestingly, bakerꢀs yeast seems less sensitive to the
same electronic effects since all methoxy-substituted
substrates were reduced to 4b–d.
Reduction of ketone 3f (R = m-NO2) by D. carota afford-
ed alcohol 4f in good enantiomeric excess (73%, entry
8) and, surprisingly, with an (R)-configuration. Also
the reduction by bakerꢀs yeast of 3f afforded the

D. Scarpi et al. / Tetrahedron: Asymmetry 16 (2005) 1479–1483
1481
(R)-alcohol albeit, as in the other cases discussed so far,
with a lower enantiomeric excess (58%). We cannot
explain, at the moment, this change of facial selectivity
in the reduction of 3f by both systems.
was used, the corresponding (S,S)-diol 8 was obtained,
but in lower enantiomeric excess (58%) than with
bakerꢀs yeast.
It is interesting to note at this point the consistency of
our results to those obtained by other authors in the
reduction of simple acetophenones 1a–e and g,13,14
which suggests that the effects of the ring substituents
on the reduction of aromatic ketones by D. carota are
quite general. First, the corresponding (S)-alcohols were
obtained in all cases, with enantiomeric excesses ranging
from 78% to 100%. More significantly, methoxy-substi-
tuted substrates 1b and d and o-bromo-substituted com-
pound 1e were only slowly converted in buffer to the
corresponding alcohol with conversions never exceeding
12%.14,22 By contrast, as in the case of nitroketone 3g,
p-chloro-substituted ketone 1g was easily converted to
the corresponding alcohol in 76% yield and 95% ee.13
3. Conclusion
In conclusion, the enantioselective reduction of a series
of aromatic c-nitroketones was achieved by using D.
carota roots in water, which afforded the corresponding
(S)-alcohols with ees ranging from 73% to 100%. A com-
parison of these results to the data obtained by reducing
the same substrates with bakerꢀs yeast resulted in D. car-
ota always being more enantioselective than bakerꢀs
yeast, although a lower number of substrates were
reduced. The stereoselectivity of the reduction seems to
be influenced by the reaction conditions and, for the
substrates presented herein, reductions in plain water
gave the best results. The possibility of using D. carota
as an alternative to bakerꢀs yeast in the enantioselective
reduction of c-nitroketones increases the range of enan-
tiopure d-nitroalcohols which can be used as precursors
in the synthesis of natural products. The availability of
the enzymatic system, its low cost and the ease of the
work-up and product recovery all are advantages that
cannot be neglected.
Interestingly, in the reduction of 3a (entry 2) and 3c (en-
try 5) only, when carried out in buffer, the formation of
chiral c-lactones 5a and c (Fig. 1) as byproducts was ob-
served. These compounds could derive from a Nef reac-
tion23 underwent by the corresponding nitroalcohols24
4a and c. However, treating the isolated alcohols with
buffer did not yield even traces of the lactones after
10 days, while the same experiment carried out on 4c
in the presence of D. carota roots afforded a 2.5:1 mix-
ture of the starting alcohol and lactone 5c. Thus D. car-
ota must play a role in the Nef reaction undergone by
the alcohols.25
4. Experimental
Melting points are uncorrected. Chromatographic sepa-
rations were performed under pressure on silica gel by
flash-column techniques; Rf values refer to TLC carried
out on 25 mm silica gel plates (Merck F254), with the
same eluent as indicated for the column chromatography.
1H NMR (200 MHz) and 13C NMR (50.33 MHz) spectra
were recorded with a Varian XL 200 instrument in CDCl3
solution. Mass spectra were carried out by EI at 70 eV,
unless otherwise stated, on QMD 1000 Carlo Erba instru-
ments. Microanalyses were carried out with a Perkin-
Elmer 2400/2 elemental analyser. Optical rotations were
determined with a JASCO DIP-370 instrument.
Finally, since D. carota proved such an efficient reduc-
tion system, we tested its ability on diketone 6 (Fig. 1),
a precursor in the synthesis of chiral pyrrolizidines.7
However, like bakerꢀs yeast, D. carota failed in the
reduction of this substrate (entry 10), most probably
because of its very low solubility in aqueous medium.
When the analogous dialkyl compound 7 (Scheme 2)
O
4-Nitro-1-phenylbutan-1-one 3a,5 1-(20-methoxyphe-
nyl)-4-nitrobutan-1-one 3b,5 1-(30-methoxyphenyl)-4-
nitrobutan-1-one 3c,5 1-(40-methoxyphenyl)-4-nitro-
butan-1-one 3d,5 4-nitro-1,7-diphenylheptane-1,7-dione
6,9 and 5-nitrononane-2,8-dione 726 were synthesised
as reported. Bakerꢀs yeast (YSC2, Sigma) reductions
were performed under the already reported conditions5
on 100 mg of the substrate.
O
O
O
NO2
R
6
5a, 5c
a: R = H
c: R = m-OMe
Figure 1.
4.1. 1-(20-Bromophenyl)-4-nitrobutan-1-one 3e
The compound was prepared according to the already
reported procedure,5,20 starting from 20-bromoacetophen-
one 1e. Compound 3e: Rf = 0.16 (eluent: EtOAc–petro-
leum ether, 1:12). 1H NMR d (ppm): 7.61 (d, J = 8.8 Hz,
1H), 7.39–7.24 (m, 3H), 4.53 (t, J = 6.6 Hz, 2H), 3.08 (t,
J = 6.6 Hz, 2H), 2.43 (quint, J = 6.6 Hz, 2H). 13C NMR
d (ppm): 202.0 (s), 167.9 (s), 133.8 (d), 131.9 (d), 128.3
(d), 127.6 (d), 74.4 (t), 38.7 (t), 21.5 (t). MS m/z (%)
271–273 (M+, 1/1), 183–185 (95/87), 155–157 (35/35).
OH
OH
O
O
a or b
NO2
NO2
ee 58%
8
7
Scheme 2. Reagents and conditions: (a) Daucus carota root, H2O, rt,
10 days;(b) D. carota root, 0.1 M phosphate buffer, pH 6.5, rt,
10 days.

1482
D. Scarpi et al. / Tetrahedron: Asymmetry 16 (2005) 1479–1483
Anal. Calcd for C10H10BrNO3: C, 44.14;H, 3.70;N,
5.15. Found: C, 44.37;H, 3.87;N, 5.27.
(ppm): 8.24 (s, 1H), 8.16 (d, J = 8.2 Hz, 1H), 7.69 (d,
J = 8.2 Hz, 1H), 7.55 (t, J = 7.7 Hz, 1H), 4.91 (t,
J = 6.2 Hz, 1H), 4.46 (td, J = 7.0, 2.6 Hz, 2H), 2.26–
2.04 (m, 2H), 1.90–1.79 (m, 2H). 13C NMR d (ppm):
146.1 (s), 131.6 (d), 130.0 (s), 129.5 (d), 122.7 (d),
120.5 (d), 75.2 (t), 72.5 (d), 35.6 (t), 23.7 (t). MS m/z
(%) 240 (M+, 1), 239 (M+À1, 6), 150 (34), 104 (15).
Anal. Calcd for C10H12N2O5: C, 50.00;H, 5.04;N,
11.66. Found: C, 49.92;H, 5.23;N, 11.42.
4.2. 4-Nitro-1-(30-nitrophenyl)butan-1-one 3f
The compound was prepared accordingly to the already
reported procedure,5,20 starting from 30-nitroacetophen-
one 1f. Compound 3f: Rf = 0.21 (eluent: EtOAc–petro-
1
leum ether, 1:4). H NMR d (ppm): 8.77 (s, 1H), 8.45
(d, J = 8.0 Hz, 1H), 8.29 (d, J = 7.8 Hz, 1H), 7.71 (t,
J = 8.0 Hz, 1H), 4.57 (t, J = 6.6 Hz, 2H), 3.23 (t,
J = 6.6 Hz, 2H), 2.49 (quint, J = 6.6 Hz, 2H). 13C
NMR d (ppm): 195.8 (s), 148.4 (s), 137.5 (s), 133.5 (d),
130.1 (d), 127.7 (d), 122.8 (d), 74.3 (t), 34.9 (t), 21.2
(t). MS m/z (%) 192 (M+ÀNO2, 1), 150
(M+ÀCH2CH2CH2NO2, 100), 104 (31), 76 (30). Anal.
Calcd for C10H10N2O5: C, 50.42;H, 4.23;N, 11.76.
Found: C, 50.29;H, 4.21;N, 11.57.
4.6. (S)-1-(40-Chlorophenyl)-4-nitrobutan-1-ol 4g
Compound 4g: Rf = 0.20 (eluent: EtOAc–petroleum
24
D
ether, 1:3). ½aꢀ = À43.1 (c 0.95, CHCl3). 1H NMR
d (ppm): 7.36–7.24 (m, 4H), 4.74 (dd, J = 7.4, 5.4 Hz,
1H), 4.43 (td, J = 7.0, 2.6 Hz, 2H), 2.25–1.98 (m, 2H),
1.95–1.73 (m, 2H). 13C NMR d (ppm): 142.1 (s), 133.4
(s), 128.6 (d, 2C), 126.9 (d, 2C), 75.3 (t), 72.9 (d), 35.3
(t), 23.8 (t). MS m/z (%) 183 (M+ÀNO2, 2), 142–140
(7.3), 141–139 (84.27). Anal. Calcd for C10H12ClNO3:
C, 52.30;H, 5.27;N, 6.10. Found: C, 52.24;H, 5.36;
N, 6.07.
4.3. 1-(40-Chlorophenyl)-4-nitrobutan-1-one 3g
To a stirred solution of CH3NO2 (1.4 mL, 26.0 mmol) in
MeOH (26 mL), cooled at 0 ꢁC, was slowly added a
2.0 M NaOH aqueous solution (13 mL, 26.0 mmol),
keeping the temperature under 5 ꢁC during the addition.
After 20 min, 3,40-dichloropropiophenone 2 (1.0 g,
4.92 mmol) was slowly added and the resulting solution
stirred at first for 30 min at 0 ꢁC and then at room tem-
perature until TLC revealed that the reaction was com-
plete (1.5 h). After cooling at 0 ꢁC, glacial CH3COOH
(3 mL) and H2O (40 mL) were added, the product
extracted with DCM (3 · 30 mL) and the combined
organic phases dried over Na2SO4. After filtration and
evaporation of the solvent, crude 3g was obtained. Puri-
fication by chromatography (eluent: EtOAc–petroleum
ether, 1:5, Rf = 0.38) afforded pure 3g (721 mg, 64%) as
4.7. General procedure for D. carota root reduction in
buffer
Healthy and freshly cut roots were minced in an electric
mixer for 2 min and the vegetable pulp (70 mL) sus-
pended in 0.1 M phosphate buffer pH 6.5 (170 mL). A
solution of the substrate (100 mg) in acetone (1 mL)
was then added and the resulting mixture stirred at
room temperature. After 5 days, a second portion of
minced roots was added (35 mL) and after 10 days, the
reaction was stopped by filtration over absorbent
cotton. The product was extracted with EtOAc and,
after filtration and evaporation of the solvent, crude
alcohol was purified by chromatography.
1
a yellow oil. Compound 3g: H NMR d (ppm): 7.89 (d,
J = 8.8 Hz, 2H), 7.45 (d, J = 8.8 Hz, 2H), 4.54 (t,
J = 6.6 Hz, 2H), 3.12 (t, J = 6.6 Hz, 2H), 2.44 (quint,
J = 6.6 Hz, 2H). 13C NMR d (ppm): 196.7 (s), 140.0 (s),
134.6 (s), 129.4 (d, 2C), 129.1 (d, 2C), 74.6 (t), 34.5 (t),
21.4 (t). MS m/z (%) 227 (M+, 2), 139–141 (100/34),
111–113 (30/10). Anal. Calcd for C10H10ClNO3: C,
52.76;H, 4.43;N, 6.15. Found: C, 52.40;H, 4.46;N, 5.85.
Acknowledgements
`
We thank MIUR and Universita di Firenze for financial
support. Mr. Maurizio Passaponti and Mrs. Brunella
Innocenti are acknowledged for their technical support.
Dr. Daniela Stranges is acknowledged for carrying out
some experiments. Ente Cassa di Risparmio di Firenze
4.4. General procedure for D. carota root reduction in
water
is acknowledged for granting
spectrometer.
a 400 MHz NMR
To a suspension of healthy and freshly cut roots (10 g) in
water (70 mL), kept under vigorous stirring, a solution
of the substrate (100 mg) in abs. EtOH (0.5 mL) was
slowly added. The resulting mixture was stirred at room
temperature. After 5 days, a second portion of freshly
cut roots (5 g) was added and after 10 days the reaction
was stopped by filtration over adsorbent cotton. The
product was extracted with EtOAc and, after filtration
and evaporation of the solvent, crude alcohol was puri-
fied by chromatography.
References
1. Wade, P. A.;Giuliano, R. M. In Nitro Compounds Rec.
Adv. Synth. Chem.;Feuer, H., Nielsen, A. T., Eds.;VCH,
1990;pp 137–265.
2. Ono, N. In The Nitro Group in Organic Synthesis;Wiley-
VCH, 2001, pp 30–69.
´
3. Ballini, R.;Fiorni, D.;Gil, M. V.;Palmieri, A.;Roma
n,
E.;Serrano, J. A. Tetrahedron Lett. 2003, 44, 2795–
2797.
4. Ballini, R.;Barboni, L.;Giarlo, G. J. Org. Chem. 2003, 68,
9173–9176.
4.5. (R)-4-Nitro-1-(30-nitrophenyl)butan-1-ol 4f
Compound 4f: Rf = 0.07 (eluent: EtOAc–petroleum
ether, 1:5). ½aꢀ = +24.7 (c 0.64, CHCl3). H NMR d
5. Guarna, A.;Occhiato, E. G.;Spinetti, L. M.;Vallecchi,
M. E.;Scarpi, D. Tetrahedron 1995, 51, 1775–1788.
25
D
1

D. Scarpi et al. / Tetrahedron: Asymmetry 16 (2005) 1479–1483
1483
6. Occhiato, E. G.;Guarna, A.;Spinetti, L. M. Tetrahedron 17. Comasseto, J. V.;Omori, A. T.;Porto, A. L. M.;Andrade,
1993, 49, 10629–10642.
7. Scarpi, D.;Occhiato, E. G.;Guarna, A. J. Org. Chem.
1999, 64, 1727–1732.
L. H. Tetrahedron Lett. 2004, 45, 473–476.
18. Baldassarre, F.;Bertoni, G.;Chiappe, C.;Marioni, F.
J. Mol. Catal. B 2000, 11, 55–58.
8. Occhiato, E. G.;Guarna, A.;De Sarlo, F.;Scarpi, D.
Tetrahedron: Asymmetry 1995, 6, 2971–2976.
9. Occhiato, E. G.;Scarpi, D.;Menchi, G.;Guarna, A.
Tetrahedron: Asymmetry 1996, 7, 1929–1942.
19. Yadav, J. S.;Thirupathi Reddy, P.;Nanda, S.;Bhaskar
Rao, A. Tetrahedron: Asymmetry 2001, 12, 3381–3385.
20. Maxwell, C. E. Organic Synthesis;Wiley and Sons: New
York, 1955;Vol. 3, pp 305–306.
10. Forzato, C.;Nitti, P.;Pitacco, G.;Valentin, E.
dron: Asymmetry 1997, 8, 1811–1820.
Tetrahe-
21. Feuer, H.;Harmetz, R. J. Org. Chem. 1961, 26, 1061–1072.
22. The influence of the experimental conditions on the
reactivity and selectivity of D. carota is not well under-
stood;in fact, as in the case of 1d, the reduction in water
afforded the corresponding alcohol in 72% yield and
94% ee. See Ref. 13.
23. Ballini, R.;Petrini, M. Tetrahedron 2004, 60, 1017–1047.
24. Hafner, T.;Reissig, H. U. Liebigs Ann. Chem. 1989, 937–938.
25. The presence of an oxidant is required for the Nef
reaction. See Ref. 23.
11. Molinari, F.;Occhiato, E. G.;Aragozzini, F.;Guarna, A.
Tetrahedron: Asymmetry 1998, 9, 1389–1394.
12. Molinari, F.;Gandolfi, R.;Villa, R.;Occhiato, E. G.
Tetrahedron: Asymmetry 1999, 10, 3515–3520.
13. Yadav, J. S.;Nanda, S.;Thirupathi Reddy, P.;Bhaskar
Rao, A. J. Org. Chem. 2002, 67, 3900–3903.
14. Ma˛czka, W. K.;Mironowicz, A. Tetrahedron: Asymmetry
2004, 15, 1965–1967.
15. Ma˛czka, W. K.;Mironowicz, A. Tetrahedron: Asymmetry
2002, 13, 2299–2302.
26. Ballini, R.;Bosica, G.;Uselli, A. J. Heterocycl. Chem.
1994, 31, 259–260.
16. Bruni, R.;Fantin, G.;Medici, A.;Pedrini, P.;Sacchetti,
G. Tetrahedron Lett. 2002, 43, 3377–3379.
27. Dale, J. A.;Mosher, H. S. J. Am. Chem. Soc. 1973, 95,
512–519.