Monobenzylethers of (R,R)-1,2-diphenylethane-1,2-diol as a possible alternative to cyclohexyl-based chiral auxiliaries in the stereoselective reduction of α-ketoesters

Article abstract of DOI:10.1016/S0040-4020(01)01117-6

The monobenzylethers of (R,R)-1,2-diphenylethane-1,2-diol have been used as chiral auxiliaries in the stereoselective reduction of the corresponding phenylglyoxylates. The diastereoselectivity of the reduction with either L-selectride or DIBAL was found to be strictly dependent on the nature of the substitution on the aromatic ring of the auxiliary and a maximum d.r. of 87:13 was obtained with the p-CF₃ substituted derivative. This investigation also shows that with open chain chiral auxiliaries diastereoselectivities of preparative interest can be achieved, suggesting that such auxiliaries can constitute a promising alternative to the commonest cyclohexyl-based ones.

Full text of DOI:10.1016/S0040-4020(01)01117-6

TETRAHEDRON
Pergamon
Tetrahedron 58 +2002) 153±159
Monobenzylethers of ꢀR,R)-1,2-diphenylethane-1,2-diol as a
possible alternative to cyclohexyl-based chiral auxiliaries in the
stereoselective reduction of a-ketoesters
Patrizia Scafato, Laura Leo, Stefano Superchi and Carlo Rosinip
Á
Dipartimento di Chimica, Universita della Basilicata, Via N. Sauro 85, 85100 Potenza, Italy
Dedicated to Professor L. Lardicci on the occasion of his 75th birthday
Received 31 July 2001; revised 16 October 2001; accepted 8 November 2001
AbstractÐThe monobenzylethers of +R,R)-1,2-diphenylethane-1,2-diol have been used as chiral auxiliaries in the stereoselective reduction
of the corresponding phenylglyoxylates. The diastereoselectivity of the reduction with either l-selectride or DIBAL was found to be strictly
dependent on the nature of the substitution on the aromatic ring of the auxiliary and a maximum d.r. of 87:13 was obtained with the p-CF3
substituted derivative. This investigation also shows that with open chain chiral auxiliaries diastereoselectivities of preparative interest can
be achieved, suggesting that such auxiliaries can constitute a promising alternative to the commonest cyclohexyl-based ones. q 2002
Elsevier Science Ltd. All rights reserved.
1. Introduction
between these moieties. The main limitation of these
auxiliaries is however their high cost and, for +2)-8-phenyl-
menthol, the commercial availability of only one antipode.
For these reasons several efforts5 have been devoted to ®nd
simpler and economic alternatives to them.
Cyclohexyl-based chiral auxiliaries such as +2)-8-phenyl-
menthol1 and trans-2-phenyl-1-cyclohexanol2 are able to
induce high levels of stereoselectivity in several asymmetric
processes which involve the transformation of unsaturated
moieties +usually dicarbonyls or activated double bonds).
The high ef®ciency of these compounds results from a selec-
tive facial shielding of the functional group to be trans-
formed,2,3 due to a suitable face to face arrangement of
the phenyl group of such auxiliaries and the unsaturated
moiety. In these auxiliaries this arrangement is favored by
the presence of a ring, which restricts the conformational
mobility, and by the occurrence of p±p interactions3,4
In order to achieve this task we also recently proposed6 the
use of the acyclic auxiliary 2a, the monobenzylether of
+R,R)-1,2-diphenylethane-1,2-diol, as chiral auxiliary for
the diastereoselective reduction of a-keto esters. Compound
2a shows the advantage that both enantiomers can be easily
prepared from enantiopure 1,2-diphenylethane-1,2-diol,7 as
depicted in the Scheme 1. Signi®cant diastereoselectivity
+up to 60%) was obtained in the reduction of the
Scheme 1. +a) ArCHO, TsOH, benzene, re¯ux, Dean±Stark; +b) DIBAL +2.5 equiv.), toluene, 08C or LiAlH4/AlCl3 1:2, Et2O, 08C; +c) PhCOCOOH
+2.0 equiv.), DCC +2.0 equiv.), DMAP +0.5 equiv.), CH2Cl2, 258C.
Keywords: chiral auxiliaries; a symmetric synthesis; a-hydroxyesters.
Corresponding author. Tel.: 139-971-202241; fax: 139-971-202223; e-mail: rosini@unibas.it
p
0040±4020/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020+01)01117-6

154
P. Scafato et al. / Tetrahedron 58 )2002) 153±159
corresponding ketoester 3a,6 thus demonstrating that in
acyclic compounds like 3a a substantial facial shielding of
the dicarbonyl moiety can be achieved. Such shielding can,
in principle, be improved either by increasing the strength of
the p±p stacking interactions4 between the carbonyl and the
aryl groups, thus forcing their facial disposition, or by
increasing the size of the aryl moiety. In auxiliaries like
2a the nature of the shielding aryl group can be easily varied
simply by changing the aldehyde used in the starting acetal
+Scheme 1), and then an improvement of the ef®ciency of
such an auxiliary seemed feasible. We therefore decided to
carry out a systematic investigation on the effect of the
nature of the shielding group upon the stereoselectivity of
the reduction reaction with the aim to understand how the
face to face disposition of the two unsaturated residues
could be forced in order to improve the stereoselectivity.
+R,R)-1,2-diphenylethane-1,2-diol and did not involve the
stereogenic centers, all the intermediate compounds can
be safely considered enantiopure as well.
The reductions of a-ketoesters 3b±g +Fig. 1) were
performed in THF at 2788C with both l-selectride and
DIBAL, the reducing agents which afforded the best results
in our previous studies on 3a.6 The diastereoisomeric ratio
of the a-hydroxy esters 4b±g obtained +Table 1) was
1
directly determined by H NMR spectroscopic analysis of
the crude reaction mixture by measuring the areas of the
benzylic proton Ha +Fig. 2) in the two diastereoisomers
+Table 2). The absolute con®guration of the major diaster-
eoisomer was determined by hydrolyzing the hydroxy ester
mixture with K2CO3 in methanol and comparing the optical
rotation of the recovered mandelic acid with literature
values.13 This assignment allowed us to establish that the
lowest ®eld doublet is related to the +R,R,R) stereoisomer.
The basic hydrolysis also allowed quantitative recovery of
the starting chiral auxiliary without loss of enantiomeric
purity.
We report herein the synthesis and characterization of
compounds 2b±g, their transformation into the correspond-
ing phenylglyoxylates 3b±g, and the outcome of their
stereoselective reduction into the a-hydroxyesters 4b±g.
The aryl groups of compounds 2b±g were chosen in order
to test the effects on the stereoselectivity of both the elec-
tronic properties and the size of the aryl moieties. As a mater
of fact, in the auxiliaries 2b and 2c we have a strong electron
withdrawing group and electron donating group, respec-
tively, which should affect the electron density on the
benzene ring, and therefore the p-stacking. The auxiliary
2d, by comparison with 2b and 2c, should allow us to under-
stand the importance of simple steric effects, the iso-propyl
group having almost the same size of a CF3 moiety.8
Compound 2e was chosen since an extended aromatic
system could increase the p-stacking and/or the shielding
effect. The highly ¯uorinated benzene ring of 2f displays9 a
reversed electrostatic charge distribution with respect to
benzene, therefore should provide particularly ef®cient
facial p±p interactions10 with the phenylglyoxylate moiety.
Finally, in 2g the highly substituted and electron-rich
aromatic residue should constitute a wide and ef®cient facial
shield to the carbonyl group.
The results of the stereoselective reductions are collected in
the Table 1. For clarity of comparison results for the a-
ketoester 3a, coming from our previous work,6 are also
Figure 1.
2. Results and discussion
Table 1. Diastereoisomeric ratio of 4a±g from reduction of a-ketoesters
3a±g
Acetals 1b±g were prepared in 80±90% yield by reacting
+R,R)-1,2-diphenylethane-1,2-diol, a suitable arylaldehyde,
and a catalytic amount of p-TsOH, in re¯uxing benzene
under water removal using a Dean±Stark condenser. The
monobenzylethers 2b±e were obtained in 70±90% yield
by reduction of the acetals 1b±e with a three-fold excess
of DIBAL in dry toluene at 08C.11 Unexpectedly, this pro-
cedure gave only poor yields in the case of 1f and 1g, even
using a larger excess of DIBAL and employing either higher
reaction temperatures or longer reaction times. We could
not afford a satisfactorily explanation of this behavior,
however we succeeded in this transformation by reducing
the acetals 1f and 1g with a LiAlH4/AlCl3 +1:2) mixture in
THF at 08C.12 This procedure afforded 2f and 2g in 68%
yield. The auxiliaries 2b±g were then esteri®ed with
phenylglyoxylic acid in the presence of DCC +2.0 equiv.)
and DMAP +0.5 equiv.) in CH2Cl2 at 258C, affording the
required a-ketoesters 3b±g in 95±99% yield +Scheme 1).
Since all the reactions performed started from enantiopure
Entry Ester
X
Reducing agent Ratioa +R,R,S)/+R,R,R)
1b
2b
3
4
5
6
7
8
9
10
11
12
13
3a
3a
3b
3b
3c
3c
3d
3d
3e
3f
H
H
4-CF3
4-CF3
4-OCH3
4-OCH3
4-iPr
4-iPr
4-Ph
2,3,4,5,6-F
2,3,4,5,6-F
3,4,5-OCH3 l-Selectride
3,4,5-OCH3 DIBAL
l-Selectride
DIBAL
l-Selectride
DIBAL
l-Selectride
DIBAL
l-Selectride
DIBAL
78:22
73:27
87:13
57:43
70:30
68:32
83:17
70:30
80:20
75:25
75:25
80:20
30:70
l-Selectride
l-Selectride
DIBAL
3f
3g
3g
Reductions performed in dry toluene, at 2788C under N2 atmosphere +see
Section 3).
a
Directly determined on crude by 1H NMR spectroscopic analysis.
Data from Ref. 6.
b

P. Scafato et al. / Tetrahedron 58 )2002) 153±159
155
Figure 3.
Figure 2.
Table 2. 1H NMR +CDCl3, 300 MHz) chemical shift and coupling constant
of doublet due to proton Ha in a-hydroxyesters 4a±g
and usually the same +R,R,S) major diastereoisomer as
l-selectride. From this point of view the results obtained
in the case of the a-ketoester 3g appear however quite
unexpected. With such an ester the use of l-selectride
+entry 12) gave a 80:20 d.r. +R,R,S/R,R,R), whilst with
DIBAL +entry 13) we observed an inversion of selectivity
and a lower 30:70 d.r. Then in this case, starting from the
same a-ketoester it is possible to obtain either the R,R,R or
the R,R,S a-hydroxyester simply changing the reducing
agent.14 This result is quite important both from a prepara-
tive and mechanistic point of view. In fact two different
stereoisomers can be obtained starting from the same
a-ketoester, and the interpretation of this unexpected
stereochemical outcome could be very useful in understand-
ing the mechanism of the stereochemical induction of
this reaction.
Product
X
d +ppm)
J +Hz)
+R,R,R)-4a
+R,R,S)-4a
+R,R,R)-4b
+R,R,S)-4b
+R,R,R)-4c
+R,R,S)-4c
+R,R,R)-4d
+R,R,S)-4d
+R,R,R)-4e
+R,R,S)-4e
+R,R,R)-4f
+R,R,S)-4f
+R,R,R)-4g
+R,R,S)-4g
H
H
4-CF3
4-CF3
4-OCH3
4-OCH3
4-iPr
4-iPr
4-Ph
4-Ph
6.06
5.92
6.09
5.95
6.02
5.89
6.04
5.88
6.09
5.95
5.92
5.83
6.08
5.97
7.0
7.5
7.1
7.3
7.0
7.5
6.5
6.9
7.0
7.5
6.5
7.5
6.4
7.0
2,3,4,5,6-F
2,3,4,5,6-F
3,4,5-OCH3
3,4,5-OCH3
reported. This ester, which derives from the simplest alcohol
2a, afforded a diastereoisomeric ratio of ca 4:1, with both
l-selectride and DIBAL +entries 1 and 2). With all the
auxiliaries the reduction of the carbonyl group was quite
fast, affording the corresponding a-hydroxy esters 4b±g in
15±20 min, pointing out that the nature of the chiral
auxiliary does not affect the rate of the reaction. As
expected, however, a clear effect of the nature of the chiral
auxiliary on the diastereoisomeric ratio was observed. In
general, l-selectride provided higher stereoselectivity in
the reduction, inducing the preferential formation of the
+R,R,S) stereoisomer. With such a reducing agent, a signi®-
cant improvement of the stereoselectivity was observed in
the case of ester 3b +entry 3), where the benzene ring of the
shielding moiety is substituted with a strong electron with-
drawing group like CF3. On the contrary, introduction of the
electron donating OCH3 group on the benzyl moiety
reduced the diastereoselectivity +entry 5). These results
can be due to a major shield which occurs, thanks to a
more signi®cant p-stacking, in the ketoester 3b with respect
to 3c. p-Stacking is in fact mainly due to electrostatic inter-
actions and these should be more effective between a phenyl
and an electron poor arene.4 In order to determine if the
higher diastereoselectivity in 3b was due either to electronic
or simple steric effects, a comparison with the sterically
comparable ester 3d was done. The result obtained +entry
7), displaying a slightly lower selectivity in 3d with respect
to 3b, seems to point out that in the latter other effects than
simple steric interactions are operative. The use of the
extended aromatic ring in 3e +entry 9) did not afford an
increase of selectivity if compared with the previous ones.
Interestingly, in the case of 3f, where the p-stacking should
be enhanced by favorable electrostatic interactions,9 the d.r.
is almost the same as in the simple benzyl derivative 3a.
Reductions with DIBAL afforded lower stereoselectivity
The results reported in Table 1 can be qualitatively inter-
preted by the model we proposed6 in the case of the parent
a-ketoester 3a, derived from 2a, the simplest chiral
auxiliary of the series. In this model the a-ketoesters
3b±g assume the prevailing conformation depicted in
Fig. 3. In this conformation the reducing agent can attack
the carbonyl group only on the Re face, inducing the
preferred formation of an +S) con®gured stereocenter. At
present, we are not able to afford a clear interpretation of
the unexpected result obtained with 3g +entry 13), where an
inversion of diastereoselectivity was observed in the reduc-
tion with DIBAL. This outcome could be determined by two
different mechanisms: +a) a hydride attack from the inner Si
face of the carbonyl group of the conformer depicted in Fig.
3, promoted by coordination of DIBAL to the oxygens of
the trimethoxyphenyl group; +b) a trans to cis isomerization
of the dicarbonyl moiety due to chelation of the Al atom6,14
followed by hydride attack on the +in this conformation)
outer Si face. We do not have neither experimental nor
theoretical evidences to prefer one of these two possibilities
over the other. This analysis of the stereochemical outcome
of the reaction should prove instructive. Although we have
tested several aryl moieties promoting, in principle, intra-
molecular p-stacking interactions, the diastereoselectivity
was not improved so much with respect to the simple benzyl
auxiliary 2a. This result can be explained by arguing that the
conformational freedom of such an auxiliary, related to the
rotation around Cp±Cp, Cp±O and OCH2Ar bonds cannot be
completely overcome by the relatively weak stabilizing
effect of the intramolecular p-stacking. Working towards
a reduction of molecular ¯exibility using, for instance, an
aryl ether shielding moiety instead of a benzyl ether one
could afford success in the diastereoselective induction.
In conclusion, the present investigation has clearly shown

156
P. Scafato et al. / Tetrahedron 58 )2002) 153±159
that an open chain chiral auxiliary can be satisfactorily used
in the diastereoselective reduction of a-ketoesters, leading
in some cases to stereoselectivities of preparative value. It
has therefore pointed out that the alcohol 2b could provide a
practicable alternative to the cyclohexyl-based chiral
auxiliaries. The mechanistic rationale formulated to analyze
the stereochemical outcome has also indicated a possible
way to further improve the stereoselectivity. Work is now
in progress towards this end. As an important by-product,
this investigation could provide a contribution to the under-
standing of weak bonding interactions, such as the p±p
interactions, which play a fundamental role in chemistry,
biology15 and material science.10,16
chromatography +petroleum ether/Et2O 3:1), obtaining 1c as
a viscous oil +57% yield). [Found: C, 79.2; H, 6.1. C22H20O3
20
requires C, 79.50; H, 6.06]; [a]D ˆ17.2 +c 1.1, CHCl3); IR
+KBr): 3060, 3020, 2900, 1610, 1510, 1250, 1070, 1030,
1
760, 700 cm21; H NMR +300 MHz, CDCl3) d 3.85 +s,
3H), 4.95 +s, 2H), 6.36 +s, 1H), 6.98 +d, Jˆ8.6 Hz, 2H),
7.27±7.35 +m, 10H), 7.61 +d, Jˆ8.6 Hz, 2H); 13C NMR
+75 MHz, CDCl3) d 55.3, 85.2, 87.1, 104.7, 113.8, 126.4,
126.9, 128.1, 128.4, 128.5, 128.6, 136.7.
3.2.3. ꢀ4R,5R)-2-ꢀ4-Isopropylphenyl)-4,5-diphenyl-1,3-
dioxolane ꢀ1d). The residue was puri®ed by column chro-
matography +petroleum ether/Et2O 8:1), affording 1d as a
colorless oil +91% yield). [Found: C, 83.8; H, 7.5. C24H24O2
20
requires C, 83.69; H, 7.02]; [a]D ˆ110.1 +c 0.7, CHCl3);
3. Experimental
IR +KBr): 3030, 2960, 1450, 1390, 1090, 1020, 760,
700 cm21
;
1H NMR +300 MHz, CDCl3) d 1.28 +d,
3.1. General procedures
Jˆ7.0 Hz, 6H), 2.96 +sept, Jˆ7.0 Hz, 1H), 4.94 +d,
Jˆ9.3 Hz, 1H), 4.98 +d, Jˆ9.3 Hz, 1H), 6.38 +s, 1H),
7.25±7.38 +m, 12H), 7.60 +d, Jˆ8.1 Hz, 2H); 13C NMR
+75 MHz, CDCl3) d 23.9, 34.1, 85.2, 87.1, 104.7, 126.4,
126.6, 126.7, 126.9, 128.1, 128.5, 128.6, 135.7, 136.7,
138.4, 150.2.
Toluene, diethyl ether and THF were freshly distilled prior
to their use on sodium benzophenone ketyl under nitrogen
atmosphere. CH2Cl2 was distilled over P2O5 and stored over
CaCl2. Liquid aldehydes +Aldrich) were freshly distilled
prior to their use under nitrogen atmosphere. Reductions
were performed using syringe-septum cap techniques
under nitrogen atmosphere. DIBAL +Aldrich) was a 1.5 M
solution in toluene and l-selectridew +Aldrich) was a 1.0 M
solution in THF. Analytical TLC was performed on 0.2 mm
silica gel plate Merck 60 F-254 and column chromatography
was carried out with silica gel Merck 60 +80±230 mesh).
Melting points were determined with a Ko¯er hot-stage
3.2.4. ꢀ4R,5R)-2-ꢀ4-Biphenylyl)-4,5-diphenyl-1,3-dioxo-
lane ꢀ1e). The residue was puri®ed by column chromato-
graphy +petroleum ether/CH2Cl2 3:1) and then recrystallized
by heptane, affording 3e as white crystals +78% yield).
Mpˆ84±868C; [Found: C, 85.3; H, 6.2. C27H22O2 requires
20
C, 85.69; H, 5.86]; [a]D ˆ213.0 +c 1.0, CHCl3); IR +KBr):
3080, 3040, 2900, 1500, 1460, 1100, 1080, 1010, 760,
700 cm21 1H NMR +300 MHz, CDCl3) d 4.96 +d,
;
1
apparatus and are uncorrected. H NMR +300 MHz) and
13C NMR +75 MHz) spectra were recorded in CDCl3 on a
Bruker Aspect 300 spectrometer with TMS as internal
standard. Infrared spectra were recorded on a Perkin±
Elmer 883 instrument, using KBr disks.
Jˆ10.0 Hz, 1H), 4.99 +d, Jˆ10.0 Hz, 1H), 6.46 +s, 1H),
7.30±7.40 +m, 11H), 7.46 +t, Jˆ7.6 Hz, 2H), 7.62 +d,
Jˆ7.2 Hz, 2H), 7.65 +d, Jˆ8.3 Hz, 2H), 7.75 +d,
Jˆ8.3 Hz, 2H); 13C NMR +75 MHz, CDCl3) d 85.3, 87.2,
104.5, 126.4, 126.9, 127.1, 127.2, 127.3, 127.5, 128.2,
128.6, 128.8, 136.5, 140.8, 142.3.
3.2. Synthesis of ꢀ4R,5R)-2-aryl-4,5-diphenyl-1,3-
dioxolanes 1b±g
3.2.5.
ꢀ4R,5R)-2-ꢀ2,3,4,5,6-Penta¯uorophenyl)-4,5-di-
phenyl-1,3-dioxolane ꢀ1f). The crude residue was puri®ed
by column chromatography +petroleum ether/Et2O 4:1),
recovering 1f as a pale-yellow viscous oil +78% yield).
[Found: C, 64.5; H, 3.7. C21H13F5O2 requires C, 64.29; H,
A
solution of arylaldehyde +11.2 mmol), +R,R)-1,2-
diphenylethane-1,2-diol +9.33 mmol) and traces of p-toluen-
sulfonic acid in benzene +150 mL) was heated at re¯ux in a
Dean±Stark condenser. After removal of solvent the residue
was puri®ed by either recrystallization or column chromato-
graphy on silica gel.
20
3.34]; [a]D ˆ147.5 +c 1.0, CHCl3); IR +KBr): 3040, 2920,
1510, 1450, 1410, 1380, 1310, 1155, 1135, 1010, 935, 760,
700 cm21
;
1H NMR +300 MHz, CDCl3) d 4.86 +d,
Jˆ8.5 Hz, 1H), 5.02 +d, Jˆ8.5 Hz, 1H), 6.76 +s, 1H),
7.23±7.38 +m, 10H); 13C NMR +75 MHz, CDCl3) d 85.4,
87.9, 97.0, 112.2 +m), 126.4, 127.0, 128.5, 128.6, 128.9,
134.8, 136.0 +m), 136.9, 139.4 +m), 140.42 +m), 144.2
+m), 147.6 +m).
3.2.1. ꢀ4R,5R)-2-ꢀ4-Tri¯uoromethylphenyl)-4,5-diphenyl-
1,3-dioxolane ꢀ1b). The residue was puri®ed by column
chromatography +petroleum ether/Et2O 3:1), recovering 1b
as a viscous colorless oil +77% yield). [Found: C, 71.5; H,
20
4.3. C22H17F3O2 requires C, 71.35; H, 4.63]; [a]D ˆ16.3
+c 1.0, CHCl3); IR +KBr): 3040, 2900, 1330, 1170, 1120,
1
025, 1040, 760, 700 cm21; H NMR +300 MHz, CDCl3) d
3.2.6. ꢀ4R,5R)-2-ꢀ3,4,5-Trimethoxyphenyl)-4,5-diphenyl-
1,3-dioxolane ꢀ1g). The residue was puri®ed by column
chromatography +petroleum ether/Et2O 1:1), affording 1g
as viscous colorless oil +82% yield). [Found: C, 73.2; H,
4.90 +d, Jˆ8.0 Hz, 1H), 4.99 +d, Jˆ8.0 Hz, 1H), 6.45 +s,
1H), 7.27±7.40 +m, 10H), 7.71 +d, Jˆ8.4 Hz, 2H), 7.80 +d,
Jˆ8.4 Hz, 2H); 13C NMR +75 MHz, CDCl3) d 85.3, 87.3,
103.6, 125.4, 125.5, 126.4, 126.9, 127.0, 127.1 +q,
Jˆ215.9 Hz), 128.4, 128.6, 131.4 +q, Jˆ31.5 Hz), 136.1,
137.5, 142.3.
20
6.3. C24H24O5 requires C, 73.45; H, 6.16]; [a]D ˆ10.5 +c
1.0, CHCl3); IR +KBr): 3020, 2950, 1600, 1510, 1470, 1240,
1130, 1010, 765, 700 cm21; 1H NMR +300 MHz, CDCl3) d
3.88 +s, 3H), 3.91 +s, 6H), 4.94 +d, Jˆ7.9 Hz, 1H), 4.97 +d,
Jˆ7.9 Hz, 1H), 6.36 +s, 1H), 6.91 +s, 2H), 7.32±7.34 +m,
10H); 13C NMR +75 MHz, CDCl3) d 58.1, 60.7, 85.1, 87.0,
3.2.2. ꢀ4R,5R)-2-ꢀ4-Methoxyphenyl)-4,5-diphenyl-1,3-
dioxolane ꢀ1c). The crude residue was puri®ed by column

P. Scafato et al. / Tetrahedron 58 )2002) 153±159
157
103.5, 104.4, 126.4, 126.8, 128.2, 128.5, 133.7, 136.4,
137.9.
1.0, CHCl3); IR +KBr): 3370 +OH) 3070, 3040, 2910, 2870,
1
1460, 1395, 1090, 1010, 770, 735, 700 cm21; H NMR
+300 MHz, CDCl3) d 3.4 +bs, 1H), 4.41 +d, Jˆ11.4 Hz,
1H), 4.42 +d, Jˆ8.1 Hz, 1H), 4.59 +d, Jˆ11.4 Hz, 1H),
4.78 +d, Jˆ8.1 Hz, 1H), 7.1±7.7 +m, 19H); 13C NMR
+75 MHz, CDCl3) d 70.6, 78.6, 87.0, 127.1, 127.2, 127.3,
127.7, 127.8, 127.9, 128.1, 128.2, 128.4, 128.7, 136.7,
137.6, 139.2, 140.8.
3.3. Synthesis of monobenzylethers 2b±e
To a solution of the dioxolane 1 +4.96 mmol) in toluene
+25 mL), under nitrogen, was slowly added at 08C a solution
of DIBAL +1.6 M, in toluene, 12.42 mmol) and the reaction
was monitored by TLC. The mixture was quenched with
methanol +1 mL), diluted with Et2O, washed with 10%
aqueous NaOH, brine and dried over Na2SO4. After
evaporation of solvent the recovered residue was puri®ed
by column chromatography on silica gel.
3.4. Synthesis of monobenzylethers 2f and 2g
To a solution of AlCl3 +4.5 mmol) in 6 mL of Et2O LiAlH4
+2.25 mmol) was added at 08C, under N2. After 30 min of
stirring, to the resulting mixture was slowly added a solution
of the appropriate dioxolane 1 +1.5 mmol) in Et2O +8 mL)
and the reaction was monitored by TLC. After 24 h, the
mixture was quenched with 10% aqueous H2SO4, and
extracted with Et2O. The combined ether layers were
washed with brine and dried over Na2SO4. After removal
of solvent, the recovered residue was puri®ed by column
chromatography on silica gel.
3.3.1. ꢀ1R,2R)-1,2-Diphenyl-2-ꢀ4-tri¯uoromethylbenzyl-
oxy)ethanol ꢀ2b). The residue was puri®ed by column chro-
matography +petroleum ether/CH2Cl2 4:1), affording 2b as a
colorless viscous oil +84% yield). [Found: C, 71.4; H, 5.7.
20
C22H19F3O2 requires C, 70.96; H, 5.14]; [a]D ˆ25.9 +c
1.09, CHCl3); IR +KBr): 3400 +OH), 3020, 2920, 1450,
1330, 1160, 1125, 1070, 760, 700 cm21
;
1H NMR
+300 MHz, CDCl3) d 3.41 +s, 1H), 4.37 +d, Jˆ7.9 Hz,
1H), 4.41 +d, Jˆ12.1 Hz, 1H), 4.57 +d, Jˆ12.1 Hz, 1H),
4.77 +d, Jˆ7.9 Hz, 1H), 7.04±7.26 +m, 10H), 7.41 +d,
Jˆ8.0 Hz, 2H), 7.60 +d, Jˆ8.0 Hz, 2H); 13C NMR
+75 MHz, CDCl3) d 70.0, 78.5, 87.2, 124.1 +q, Jˆ
269.6 Hz), 125.4, 127.1, 127.2, 127.8, 127.9, 128.3, 130.5
+q, Jˆ31.4 Hz), 137.2, 139.2, 141.8.
3.4.1. ꢀ1R,2R)-1,2-Diphenyl-2-ꢀ2,3,4,5,6-penta¯uoroben-
zyloxy)ethanol ꢀ2f). The residue was puri®ed by column
chromatography +petroleum ether/Et2O 4:1), recovering 2f
as white crystals +68% yield). Mpˆ96±978C; [Found: C,
64.2; H, 3.5. C21H15F5O2 requires C, 63.96; H, 3.83];
20
[a]D ˆ28.0 +c 1.0, CHCl3); IR +KBr): 3620 +OH), 3040,
2880, 1540, 1510, 1460, 1140, 1060, 980, 940, 780, 750,
700 cm21; 1H NMR +300 MHz, CDCl3) d 3.29 +s, 1H), 4.32
+d, Jˆ8.0 Hz, 1H), 4.49 +d, Jˆ11.0 Hz, 1H), 4.60 +d,
Jˆ11.0 Hz, 1H), 4.67 +d, Jˆ8.0 Hz, 1H), 6.99±7.26 +m,
10H); 13C NMR +75 MHz, CDCl3) d 58.1, 78.4, 88.0,
111.2 +m), 127.1, 127.6, 127.8, 127.8, 128.2, 128.4, 135.8
+m), 136.7, 138.7 +m), 138.8, 143.1 +m), 144.0 +m), 147.3
+m).
3.3.2.
ꢀ1R,2R)-1,2-Diphenyl-2-ꢀ4-methoxybenzyloxy)-
ethanol ꢀ2c). The residue was puri®ed by column chromato-
graphy +petroleum ether/Et2O 4:1), affording 2c as an oil
+68% yield). [Found: C, 79.2; H, 6.3. C22H22O3 requires C,
20
79.02; H, 6.63]; [a]D ˆ233.7 +c1.1, CHCl3); IR +KBr):
3400 +OH), 3030, 2860, 1620, 1510, 1380, 1240, 1060,
1
1020, 750, 700 cm21; H NMR +300 MHz, CDCl3) d 3.53
+s, 1H), 3.81 +s, 3H), 4.26 +d, Jˆ11.0 Hz, 1H), 4.32 +d,
Jˆ8.0 Hz, 1H), 4.45 +d, Jˆ11.0 Hz, 1H), 4.70 +d,
Jˆ8.0 Hz, 1H), 6.90 +d, Jˆ8.7 Hz, 2H), 7.02±7.24 +m,
12H); 13C NMR +75 MHz, CDCl3) d 55.3, 70.5, 78.6,
86.6, 113.3, 127.3, 127.6, 127.8, 127.9, 128.0, 128.1,
129.6, 129.8, 137.7.
3.4.2. ꢀ1R,2R)-1,2-Diphenyl-2-ꢀ3,4,5-trimethoxybenzyl-
oxy) ethanol ꢀ2g). The residue was puri®ed by column
chromatography +petroleum ether/EtOAc 1:1), affording
2g as colorless viscous liquid +68% yield). [Found: C,
73.5; H, 6.7. C24H26O5 requires C, 73.08; H, 6.64];
20
[a]D ˆ220.8 +c 1.0, CHCl3); IR+KBr): 3500 +OH), 3010,
2940, 1590, 1505, 1465, 1455, 1420, 1235, 1130, 755,
3.3.3. ꢀ1R,2R)-1,2-Diphenyl-2-ꢀ4-isopropylbenzyloxy)-
ethanol ꢀ2d). The residue was puri®ed by column chroma-
tography +petroleum ether/Et2O 3:1), recovering 2d as white
crystals +91% yield). Mpˆ758C; [Found: C, 83.6; H, 7.3.
1
705 cm21; H NMR +300 MHz, CDCl3) d 3.60 +bs, 1H),
3.80 +s, 6H), 3.83 +s, 3H), 4.27 +d, Jˆ11.0 Hz, 1H), 4.37
+d, Jˆ7.8 Hz, 1H), 4.46 +d, Jˆ11.0 Hz, 1H), 4.74 +d,
Jˆ7.8 Hz, 1H), 6.49 +s, 2H), 7.03±7.23 +m, 10H); 13C
NMR +75 MHz, CDCl3) d 55.8, 60.5, 70.8, 78.2, 86.4,
104.8, 127.0, 127.4, 127.6, 127.7, 127.9, 133.2, 137.5,
139.2.
20
C24H26O2 requires C, 83.20; H, 7.56]; [a]D ˆ215.3 +c 1.0,
CHCl3); IR +KBr): 3400 +OH), 3070, 3040, 2970, 2870,
1
1460, 1390, 1200, 1090, 1045, 1020, 820, 700 cm21; H
NMR +300 MHz, CDCl3) d 1.26 +d, Jˆ7.0 Hz, 6H), 2.92
+sept, Jˆ7.0 Hz, 1H), 3.57 +s, 1H), 4.30 +d, Jˆ11.0 Hz, 1H),
4.36 +d, Jˆ8.3 Hz, 1H), 4.48 +d, Jˆ11.0 Hz, 1H), 4.71 +d,
Jˆ8.3 Hz, 1H), 7.2±7.0 +m, 14H); 13C NMR +75 MHz,
CDCl3) d 23.9, 33.9, 70.81, 78.6, 87.0, 126.5, 127.3,
127.6, 127.7, 127.8, 128.0, 128.1, 135.1, 137.7, 139.3,
148.6.
3.5. Synthesis of phenylglyoxylates 3b±g
To a mixture of the appropriate monobenzylether 2
+3.29 mmol), benzoylformic acid +6.58 mmol) and N,N-
dimethylaminopyridine +1.15 mmol) in CH2Cl2 +15 mL) a
solution of DCC +6.58 mmol) in CH2Cl2 +5 mL) was slowly
added at 08C. The resulting mixture was stirred overnight
then quenched by addition of EtOH +1 mL). The mixture
was then ®ltered under vacuum and the recovered solid
carefully washed with petroleum ether. The collected ®ltrate
was treated with 10% HCl, saturated aqueous NaHCO3,
3.3.4. ꢀ1R,2R)-1,2-Diphenyl-2-ꢀ4-phenylbenzyloxy)etha-
nol ꢀ2e). The residue was puri®ed by column chromato-
graphy +petroleum ether/Et2O 4:1), recovering 2e as white
needles +81% yield). Mpˆ109±1108C; [Found: C, 85.7; H,
20
6.8. C27H24O2 requires C, 85.23; H, 6.36]; [a]D ˆ225.5 +c

158
P. Scafato et al. / Tetrahedron 58 )2002) 153±159
brine and dried over Na2SO4. After evaporation of solvent
the residue was puri®ed by column chromatography on
silica gel.
+300 MHz, CDCl3) d 4.37 +d, Jˆ11.9 Hz, 1H), 4.59 +d,
Jˆ11.9 Hz, 1H), 4.74 +d, Jˆ8.0 Hz, 1H), 6.32 +d,
Jˆ8.0 Hz, 1H), 7.16±7.59 +m, 22H), 7.94 +d, Jˆ7.7 Hz,
2H); 13C NMR +75 MHz, CDCl3) d 70.4, 80.4, 83.7,
127.1, 127.2, 127.7, 127.9, 128.0, 128.2, 128.3, 128.4,
128.7, 130.1, 132.4, 134.7, 135.8, 136.9, 140.4, 140.9,
163.3, 186.5.
3.5.1. ꢀ1R,2R)-1,2-Diphenyl-2-ꢀ4-tri¯uoromethylbenzyl-
oxy)-ethylphenylglyoxylate ꢀ3b). The residue was puri®ed
by column chromatography +petroleum ether/Et2O 4:1),
obtaining 3b as a yellow viscous liquid, in 99% yield.
[Found: C, 71.7; H, 4.3. C30H23F3O4 requires C, 71.42; H,
3.5.5. ꢀ1R,2R)-1,2-Diphenyl-2-ꢀ2,3,4,5,6-penta¯uoroben-
zyloxy)-ethylphenylglyoxylate ꢀ3f). The residue was puri-
®ed by column chromatography +petroleum ether/CHCl3
1:2), affording 3f as a white solid +95% yield). Mpˆ107±
1088C; [Found: C, 66.5; H, 3.9. C29H19F5O4 requires C,
20
4.60]; [a]D ˆ231.1 +c 1.1, CHCl3); IR +KBr): 2960, 2920,
2860, 1790 +CO), 1690 +CO), 1450, 1330, 1120, 1070, 760,
;
700 cm21 1H NMR +300 MHz, CDCl3) d 4.40 +d,
Jˆ12.6 Hz, 1H), 4.58 +d, Jˆ12.6 Hz, 1H), 4.70 +d,
Jˆ7.9 Hz, 1H), 6.30 +d, Jˆ7.9 Hz, 1H), 7.12±7.38 +m,
14H), 7.52 +d, Jˆ8.1 Hz, 2H), 7.60 +t, Jˆ7.5 Hz, 1H),
7.92 +d, Jˆ8.1 Hz, 2H); 13C NMR +75 MHz, CDCl3) d
70.0, 80.3, 84.1, 125.2, 126.6 +q, Jˆ211.2 Hz), 127.4,
127.6, 127.9, 128.2, 128.4, 128.4, 128.5, 128.6, 128.7,
128.8, 130.0, 132.3, 134.8, 135.4, 136.8 +q, Jˆ57.7 Hz),
141.9, 163.1, 186.3.
20
66.16; H, 3.64]; [a]D ˆ243.0 +c 1.0, CHCl3); IR +KBr):
3060, 3040, 2960, 2920, 2880, 1740 +CO), 1690 +CO), 1505,
1200, 1180, 990, 940, 700 cm21 1H NMR +300 MHz,
;
CDCl3), 4.44 +d, Jˆ9.9 Hz, 1H), 4.57 +d, Jˆ9.9 Hz, 1H),
4.67 +d, Jˆ7.6 Hz, 1H), 6.16 +d, Jˆ7.6 Hz, 1H), 7.10±7.26
+m, 10H), 7.43 +t, Jˆ7.2 Hz, 2H), 7.64 +t, Jˆ7.2 Hz, 1H),
1
7.89 +d, Jˆ7.9 Hz, 2H); C NMR +75 MHz, CDCl3) 58.3,
80.6, 84.8, 111.0 +m), 127.5, 127.8, 128.2, 128.4, 128.5,
128.6, 128.7, 130.0, 132.4, 134.7, 135.4, 139.0 +m), 139.7
+m), 143.0 +m), 143.9 +m), 147.3 +m), 162.8, 185.9.
3.5.2.
ꢀ1R,2R)-1,2-Diphenyl-2-ꢀ4-methoxybenzyloxy)-
ethylphenylglyoxylate ꢀ3c). The residue was puri®ed by
column chromatography +petroleum ether/Et2O 1:1), obtain-
ing 3c as a white solid +65% yield). Mpˆ88±908C; [Found:
C, 77.5; H, 6.5. C30H26O5 requires C, 77.24; H, 6.62];
3.5.6. ꢀ1R,2R)-1,2-Diphenyl-2-ꢀ3,4,5-trimethoxybenzyl-
oxy)-ethylphenylglyoxylate ꢀ3g). The crude residue was
puri®ed by column chromatography +petroleum ether/
AcOEt 3:1), affording 3g as a colorless viscous liquid
+79% yield). [Found: C, 73.3; H, 6.1. C32H30O7 requires
20
[a]D ˆ256.0 +c 1.0, CHCl3); IR +KBr): 3060, 3020,
2940, 1740 +CO), 1690 +CO), 1510, 1250, 1200, 1180,
1
990, 700 cm21; H NMR +300 MHz, CDCl3) d 3.77 +s,
20
3H), 4.26 +d, Jˆ11.5 Hz, 1H), 4.47 +d, Jˆ11.5 Hz, 1H),
4.67 +d, Jˆ7.8 Hz, 1H), 6.28 +d, Jˆ7.8 Hz, 1H), 6.78 +d,
Jˆ12.7 Hz, 2H), 7.11±7.23 +m, 12H), 7.34 +t, Jˆ7.5 Hz,
2H), 7.58 +t, Jˆ7.5 Hz, 1H), 7.92 +d, Jˆ7.3 Hz, 2H); 13C
NMR +75 MHz, CDCl3) d 55.2, 70.3, 80.4, 83.2, 113.7,
127.2, 127.6, 127.8, 128.0, 128.1, 128.2, 128.3, 128.7,
129.1, 129.6, 129.8, 129.9, 130.1, 132.4, 134.7, 135.7,
136.9, 163.2, 186.5.
C, 72.99; H, 5.74]; [a]D ˆ252.4 +c 1.2, CHCl3); IR
+KBr): 3060, 3010, 2940, 1740 +CO), 1690 +CO), 1595,
1
1455, 1330, 1240, 1200, 1130, 990, 760, 700 cm21; H
NMR +300 MHz, CDCl3) d 3.71 +s, 6H), 3.83 +s, 3H),
4.28 +d, Jˆ12.0 Hz, 1H), 4.52 +d, Jˆ12.0 Hz, 1H), 4.71
+d, Jˆ8.1 Hz, 1H), 6.31 +d, Jˆ8.1 Hz, 1H), 6.48 +s, 2H),
7.13±7.26 +m, 10H), 7.38 +t, Jˆ7.9 Hz, 2H), 7.61 +t
Jˆ7.9 Hz, 1H), 7.94 +d, Jˆ8.2 Hz, 2H); 13C NMR
+75 MHz, CDCl3) d 56.0, 60.7, 70.7, 80.4, 83.3, 104.5,
127.8, 128.0, 128.1, 128.3, 128.4, 128.5, 128.8, 130.1,
132.4, 133.4, 134.8, 135.7, 136.7, 153.2, 186.4.
3.5.3. ꢀ1R,2R)-1,2-Diphenyl-2-ꢀ4-isopropylbenzyloxy)-
ethylphenylglyoxylate ꢀ3d). The residue was puri®ed by
column chromatography +petroleum ether/CHCl3 1:1),
affording 3d as a colorless viscous liquid +98% yield).
[Found: C, 80.5; H, 6.2. C32H30O4 requires C, 80.31; H,
3.6. General procedure of reduction of phenylglyoxylates
3b±g
20
6.32]; [a]D ˆ255.8 +c 1.0, CHCl3); IR +KBr): 3060,
3030, 2960, 1740 +CO), 1690 +CO), 1600, 1450, 1380,
1
1200, 1175, 990, 700 cm21; H NMR +300 MHz, CDCl3)
A solution of DIBAL +1.5 M in toluene, 0.23 mmol) was
slowly added by syringe to a solution of the phenyl-
glyoxylate 3 +0.23 mmol) in THF +5 mL) kept at 2788C
under N2 atmosphere. The reaction, monitored by TLC,
was quenched by addition of methanol +0.1 mL), diluted
with Et2O, extracted with NaOH 10%, brine and dried
over Na2SO4. After evaporation of solvent, the crude
mixture of a-hydroxyesters 4b±g was analyzed by 1H
NMR spectroscopy in order to measure the diastereo-
isomeric ratio.
d 1.24 +d, Jˆ6.9 Hz, 6H), 2.90 +sept, Jˆ6.9 Hz, 1H), 4.30
+d, Jˆ11.6 Hz, 1H), 4.50 +d, Jˆ11.6 Hz, 1H), 4.71 +d,
Jˆ7.9 Hz, 1H), 6.30 +d, Jˆ7.9 Hz, 1H), 7.12±7.23 +m,
14H), 7.31 +t, Jˆ8.2 Hz, 2H), 7.46 +t, Jˆ7.5 Hz, 1H), 7.93
+d, Jˆ8.2, 2H); 13C NMR +75 MHz, CDCl3) d 24.00, 33.9,
70.7, 80.4, 83.7, 126.4, 127.6, 127.7, 128.0, 128.3, 128.4,
128.7, 128.8, 130.2, 132.5, 134.6, 135.3, 135.7, 137.0, 140.2
164.1, 186.5.
3.5.4. ꢀ1R,2R)-1,2-Diphenyl-2-ꢀ4-phenylbenzyloxy)-ethyl-
phenylglyoxylate ꢀ3e). The crude residue was puri®ed by
column chromatography +petroleum ether/Et2O 8:1), reco-
vering 3e as a pale-yellow viscous liquid +74% yield).
[Found: C, 82.3; H, 5.4. C35H28O4 requires C, 82.01; H,
Acknowledgements
20
5.51]; [a]D ˆ258.0 +c 1.0, CHCl3); IR +KBr): 3060,
Financial support from MURST +COFIN2000) and
Á
Universita della Basilicata +Potenza) is gratefully
acknowledged.
3040, 2920, 2880, 1740 +CO), 1690 +CO), 1600, 1490,
1450, 1200, 1180, 990, 760, 700 cm21
;
1H NMR

P. Scafato et al. / Tetrahedron 58 )2002) 153±159
159
Tetrahedron Lett. 1985, 26, 429. +h) Nair, V.; Prabhakaran,
J. J. Chem. Soc., Perkin Trans. 1 1996, 593. +i) Chu, Y.-Y.;
Yu, C.-S.; Chen, C.-J.; Yang, K.-S.; Lain, J.-C.; Lin, C.-H.;
Chen, K. J. Org. Chem. 1999, 64, 6993. +j) Fukuzawa, S.-I.;
Miura, M.; Matsuzawa, H. Tetrahedron Lett. 2001, 42, 4167.
6. Superchi, S.; Contursi, M.; Rosini, C. Tetrahedron 1998, 54,
11247.
References
1. +a) Corey, E. J.; Ensley, H. E. J. Am. Chem. Soc. 1975, 97,
6908. +b) Whitesell, J. K.; Deyo, D.; Bhattacharya, A.
Â
J. Chem. Soc., Chem. Commun. 1983, 802. +c) Solladie-
Cavallo, A.; Bencheqroun, M. Tetrahedron: Asymmetry
1991, 2, 1165.
2. Whitesell, J. K. Chem. Rev. 1992, 92, 953.
7. Kolb, H. C.; Van Nieuwenhze, M. S.; Sharpless, K. B. Chem.
Rev. 1994, 94, 2483.
Á
3. +a) Solladie-Cavallo, A.; Kian, N.; Fisher, J.; Delian, A.
8. O'Hagan, D.; Rzepa, H. S. J. Chem. Soc., Chem. Commun.
1997, 645.
Tetrahedron 1991, 47, 249. +b) Whitesell, J. K.; Lawrence,
R. M.; Chien, H. J. Org. chem. 1985, 51, 4779. +c) Whitesell,
J. K.; Younathan, J. N.; Hurst, J. R.; Fox, M. A. J. Org. Chem.
1985, 50, 5499. +d) Maddaluno, J. F.; Gresh, N.; Giessner-
Prettre, C. J. Org. Chem. 1994, 59, 793. +e) Dumas, F.;
Mesrhab, B.; D'Angelo, J.; Riche, C.; Chiaroni, A. J. Org.
Chem. 1996, 61, 2293.
9. Sakai, T.; Tsuboi, M.; Takeuchi, H.; Ema, T.; Uneyama, K.;
Utaka, M. J. Org. Chem. 2001, 65, 2740 and references
therein.
10. Feast, W. J.; Lovenich, P. W.; Puschmann, H.; Taliani, C.
Chem. Commun. 2001, 505.
Â
11. Solladie, G.; Lohse, O. J. Org. Chem. 1993, 58, 4555.
4. The importance of p±p interaction in asymmetric synthesis
has been recently pointed out: Jones, G. B.; Chapmann, B. J.
Synthesis 1995, 475. For discussions about the arene±arene
interactions see: Hunter, C. A. Angew. Chem., Int. Ed. Engl.
1993, 32, 1585. +c) Hunter, C. A. Chem. Soc. Rev. 1994, 101.
+d) Cozzi, F.; Siegel, J. S. Pure Appl. Chem. 1995, 67, 1995.
5. +a) Hammon, D. P. G.; Holmann, J. W.; Massy-Westropp,
R. A. Tetrahedron 1993, 49, 9593. +b) Basavaiah, D.;
Pandiarajus, S.; Bakthadoss, M.; Muthukumaran, K.
Tetrahedron: Asymmetry 1996, 7, 997. +c) Xiang, Y. B.;
Snow, K.; Bellen, M. J. Org. Chem. 1993, 58, 993. +d) Maitra,
U.; Mathivanan, P. Tetrahedron: Asymmetry 1994, 5, 1171.
+e) Ghosh, A. K.; Chen, Y. Tetrahedron Lett. 1995, 36, 6811.
+f) Akiyama, T.; Nishimoto, H.; Ozaki, S. Tetrahedron Lett.
12. Eliel, E. L.; Badding, V. G.; Rerick, M. N. J. Am. Chem. Soc.
1962, 84, 237.
13. CRC Handbook of Organic Compounds; Weast, R. C., Ed.;
70th ed, CRC: Boca Raton, FL, 1989.
14. In our previous work +see Ref. 6) a similar inversion of dia-
stereoselectivity, even if in a minor extent, was observed
performing DIBAL reduction of 3a either in toluene or in
the presence of added ZnCl2. These results suggest that with
the present ¯exible auxiliaries, coordination factors deeply
affect the conformational equilibrium and therefore the stereo-
chemical outcome of the reduction.
15. +a) Dougherty, D. A. Science 1996, 271, 163. +b) Ngola, S. M.;
Dougherty, D. A. J. Org. Chem. 1998, 63, 4566.
16. Tamaki, R.; Samura, K.; Chujo, Y. J. Chem. Soc., Chem.
Commun. 1998, 1131.
Â
1991, 32, 1135. +g) Solladie-Cavallo, A.; Suffert, J.