Catalytic enantioselective conjugate addition of dialkyl zinc reagents to α,β-unsaturated ketones mediated by new phosphite ligands containing binaphthalene/1,2-diphenylethane moieties: A practical synthesis of (R)-(-)-muscone

Article abstract of DOI:10.1016/j.tetasy.2003.09.017

New diastereoisomeric phosphites based on either (R)- or (S)-2,2′-dihydroxy-1,1′-binaphthyl (BINOL) and having the chiral alcoholic moiety derived from the monobenzyl ether of (R,R)-1,2-diphenylethane- 1,2-diol have been prepared and used as chiral ligands in the enantioselective copper-catalyzed 1,4-addition of diethylzinc to chalcone and 2-cyclohexen-1-one (enantiomeric excesses up to 48%). With the (aR,R,R) ligand dimethylzinc adds enantioselectively to (E)-cyclopentadecen-2-en-1-one to give (R)-(-)-muscone (68% yield, 78% ee). This provides an efficient access to a valuable ingredient of the perfume industry. However, with the (aS,R,R) ligand, (S)-(+)-muscone is obtained with longer reaction times (37% yield and 10% ee) with a very high double diastereoselection effect being observed.

Full text of DOI:10.1016/j.tetasy.2003.09.017

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 14 (2003) 3873–3877
Catalytic enantioselective conjugate addition of dialkyl zinc
reagents to a,b-unsaturated ketones mediated by new phosphite
ligands containing binaphthalene/1,2-diphenylethane moieties: a
practical synthesis of (R)-(−)-muscone
Patrizia Scafato, Stefania Labano, Giovanni Cunsolo and Carlo Rosini*
Dipartimento di Chimica, Universita` della Basilicata, via Nazario Sauro 85, 85100 Potenza, Italy
Received 21 August 2003; accepted 1 September 2003
Abstract—New diastereoisomeric phosphites based on either (R)- or (S)-2,2%-dihydroxy-1,1%-binaphthyl (BINOL) and having the
chiral alcoholic moiety derived from the monobenzyl ether of (R,R)-1,2-diphenylethane-1,2-diol have been prepared and used as
chiral ligands in the enantioselective copper-catalyzed 1,4-addition of diethylzinc to chalcone and 2-cyclohexen-1-one (enan-
tiomeric excesses up to 48%). With the (aR,R,R) ligand dimethylzinc adds enantioselectively to (E)-cyclopentadecen-2-en-1-one to
give (R)-(−)-muscone (68% yield, 78% ee). This provides an efficient access to a valuable ingredient of the perfume industry.
However, with the (aS,R,R) ligand, (S)-(+)-muscone is obtained with longer reaction times (37% yield and 10% ee) with a very
high double diastereoselection effect being observed.
© 2003 Elsevier Ltd. All rights reserved.
1. Introduction
could be prepared by coupling the diol framework of
BINOL with a second enantiopure alcoholic moiety
obtained by a simple monoether of 1,2-diphenylethane-
1,2-diol 1 (Fig. 1), i.e. a type of compound already
employed as a chiral controller.6
Over the last few years, significant progress in the field
of the catalytic asymmetric conjugated addition of
organozinc compounds to a,b-unsaturated ketones has
been made.1 This process allows the construction of
CꢀC bonds in high chemical yields with efficient stereo-
control, thus providing synthetic organic chemistry with
a very useful tool to assemble large and polyfunctional
molecules.2 The success obtained in such a process
relies mainly in the development of efficient catalytic
precursors which are constituted by Cu(II) or Cu(I)
salts coordinated by enantiopure phosphorus ligands,
i.e. phosphites3 and phosphoroamidites.4 In these lig-
ands the chiral source is often derived from enantio-
pure 2,2%-dihydroxy-1,1%-binaphthyl, BINOL, or
modified binaphthols, coupled to another chiral
residue, so the phenomenon of double asymmetric
induction results.5
Herein we report the synthesis of the diastereomeric
phosphites 3a and 3b (Fig. 1) and their application as
chiral controllers in the asymmetric conjugated addition
of diethylzinc to chalcone and 2-cyclohexen-1-one,
assumed as model compounds of acyclic and cyclic
ketones. In addition, the asymmetric conjugate addition
of dimethylzinc to (E)-cyclopentadec-2-en-1-one is also
reported in order to provide a new, practical catalytic
process2a,7 for the synthesis of (R)-(−)-muscone, a rare
and valuable perfumery ingredient isolated from the
male musk deer Moschus moschiferus.
2. Results and discussion
Stimulated by recent literature reports, we reasoned
that a new family of enantiopure phosphite compounds
2.1. Synthesis of phosphites
The two novel enantiomerically pure phosphites 3a and
3b were synthesized by treating chlorophosphite 2, pre-
pared by starting from monobenzyl ether of (R,R)-1,2-
* Corresponding author. Tel.: +390971202241; fax: +390971202223;
e-mail: rosini@unibas.it
0957-4166/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2003.09.017

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P. Scafato et al. / Tetrahedron: Asymmetry 14 (2003) 3873–3877
ratio 1:2 between Cu(OTf)2 and the chiral ligand 3a or
3b. The results obtained are shown in Table 1 for
chalcone and in Table 2 for 2-cyclohexen-1-one.
As far as the addition to chalcone was concerned, we
saw that both 3a and 3b acted as efficient catalytic
precursors of the conjugate addition. In fact, the prod-
ucts were obtained in short reaction times with satisfac-
tory yields (in both cases the presence of a secondary
product obtained by the Michael addition of the result-
ing zinc enolate to the starting enone was observed
while the 1,2-adduct was not detected). However, the
diastereoisomeric relationship between the two ligands
had a clear effect on stereoselection. In fact, the
(aR,R,R) ligand gave the (S)-stereoisomer with 48%
e.e. (entry 1) and the (aS,R,R) ligand gave (R) with
11% e.e. (entry 2). The stereochemical outcome was
clearly determined by the BINOL moiety: 3a derived
from (R)-BINOL induced the formation of the (S)
product, whilst 3b, derived from (S)-BINOL, induced
the formation of the (R)-product. This is a well-known
behavior concerning these types of ligands.1 As far as
the results obtained in the catalytic 1,4-addition of
diethylzinc to cyclohexenone (Table 2) it is interesting
to note that both 3a and 3b acted as very efficient
catalytic precursors of this conjugate addition even if
the stereoselectivity values obtained were unsatisfactory
in both cases, i.e. no double diastereoselection was
observed in this case.
Figure 1. Structure of alcohol (R,R)-1 and phosphite deriva-
tives.
diphenylethane-1,2-diol 1 and PCl3, with (R)- or (S)-
BINOL 4, in the presence of triethylamine and DMAP
(Scheme 1). The monobenzylether of (R,R)-1,2-
diphenylethane-1,2-diol 1 can easily be prepared, as
previously reported,6a,b from enantiopure (R,R)-1,2-
diphenylethane-1,2-diol.8
2.2. Asymmetric conjugate reactions
The effectiveness of this novel catalytic system was first
tested by studying the conjugate addition of diethylzinc
on chalcone and 2-cyclohexen-1-one. In a typical proce-
dure the catalytic system was generated in situ by
stirring a toluene solution containing the ligand, 3a or
3b, and the copper salt, Cu(OTf)2, for 1 h at room
temperature. The additions of diethylzinc to chalcone
and to 2-cyclohexen-1-one were carried out in toluene
at −40°C, using 3 mol% of the catalyst with a molar
The shortest route to synthesize the (R)-(−)-muscone is
the enantioselective addition of a methyl group to
Scheme 1.
Table 1. Catalytic conjugate addition of diethylzinc to chalcone
Entry
Ligand
Cu(OTf)2 (mol%)
Cu(OTf)2/L*
T (°C)
Time (h)
Yielda (%)
E.e.b,c (%)
1
2
aR,R,R
aS,R,R
3
3
1:2
1:2
−40
−40
1.30
1.30
66
61
48 (S)
11 (R)
a Isolated product.
b Determined by HPLC analyses: Daicel Chiralcel OJ, hexane/2-propanol 99.5:0.5, 1 mL/min., u=254 nm, tr=18.0 (S), tr=25.0 (R).
c Absolute configuration assigned by the sign of the optical rotation.

P. Scafato et al. / Tetrahedron: Asymmetry 14 (2003) 3873–3877
Table 2. Catalytic conjugate addition of diethylzinc to 2-cyclohexen-1-one
3875
Entry
Ligand
Cu(OTf)2 (mol%)
Cu(OTf)2/L*
T (°C)
Time (h)
Yielda (%)
E.e.b,c (%)
1
2
aR,R,R
aS,R,R
3
3
1:2
1:2
−40
−40
2
2
98
98
19 (R)
12 (S)
a Isolated product.
b Determined by HPLC analyses after derivatization with (R,R)-1,2-diphenylethane-1,2-diol: Daicel Chiralcel OD, hexane/2-propanol 99.7:0.3, 0.5
mL/min., u=254 nm, tr=8.0 (R,R,R), tr=10.0 (S,R,R).
c Absolute configuration assigned by the sign of the specific rotation.
3a is clearly a more easily prepared (and thus more
useful from a practical point of view) than the ligands
described in the references.2a,7 In addition, a remark-
able difference in behavior between 3a and 3b is seen: in
practice, 3b is not even capable in promoting the reac-
tion; in fact (+)-muscone is obtained in low chemical
yield with a very unsatisfactory enantiomeric excess. Of
course, such difference in double stereoselection
requires further, detailed studies.
(E)-cyclopentadec-2-en-1-one, since this unsaturated
15-membered ketone can be readily prepared from
commercially available cyclopentadecanone.9 As
a
result, we have focused our attention on the catalytic
conjugate addition of dimethylzinc to (E)-cyclopenta-
dec-2-en-1-one 9, using the new phosphites 3a and 3b.
The results are shown in Table 3.
The data collected in Tables 1–3 deserves some com-
ments: first of all it is interesting to observe that
substrates 5 and 9 give rise to lower chemical yields of
products 6 and 10, respectively, than substrate 7. Addi-
tionally, in the crude reaction mixture, by-products
derived from the addition of the zinc enolate to the
starting ketone (in the cases of 5 and 9) as well as
1,2-addition product (only for 9) can be detected. The
similar behavior of 5 and 9 can be related to the fact
that for both of them s-cis and s-trans conformers are
possible whilst 7 is fixed in an s-trans-structure (see, for
instance, Ref. 1c). However, the most important practi-
cal result is that of 3a (entry 1 of Table 3), which works
as an efficient catalytic precursor in the conjugate addi-
tion of dimethylzinc to 9, providing a new route to
(−)-muscone. It is noteworthy that only few methods of
this type are known2a,7 for the synthesis of (−)-muscone:
3. Conclusions
Herein the asymmetric 1,4-addition of dialkylzinc
reagents to a,b-unsaturated ketones using, as chiral
promoters, new diastereoisomeric phosphites derived
from (R)- or (S)-2,2%-dihydroxy-1,1%-binaphthalene and
(R,R)-1,2-diphenylethane-2-benzyloxyethanol, has been
described. The most significant results are as follows:
with the (aR,R,R) ligand, (R)-(−)-muscone, the key
flavor component of musk, is obtained in 68% yield and
78% ee, whilst the (aS,R,R) ligand did not even pro-
mote the reaction. It should be noted that (+)-muscone
was obtained after much longer reaction times at higher
temperatures, but with a poor yield and ee. It is also
Table 3. Catalytic conjugate addition of dimethylzinc to (E)-cyclopentadec-2-en-1-one
Entry
Ligand
Cu(OTf)2 (mol%)
Cu(OTf)2/L*
T (°C)
Time (h)
Yielda (%)
E.e.b (%)
1
2
aR,R,R
aS,R,R
3
3
1:2
1:2
−10
2
68
37
78 (R)
10 (S)
0
20c
a Isolated product.
b Determined by specific rotation measurements [lit.10: for (R)-(−)-muscone [h]D=−11.7 (c 0.8, MeOH)].
c The reaction does not work at −10°C, significant (10–15%) amounts of the addition product of the resulting zinc enolate to 9 and of the
1,2-addition product are found.

3876
P. Scafato et al. / Tetrahedron: Asymmetry 14 (2003) 3873–3877
important to see that the results present interesting
implications for both the practical and theoretical
points of view. In practice a new, convenient access
(which uses simply-to-be-prepared, cheap ligands) has
been provided to this important ingredient of the per-
fume industry: the achievement of higher yields and ees
can be related to a suitably designed monoether of
(R,R)-1,2-diphenylethane-1,2-diol. From the theoretical
point of view, the huge dependence of the chemical and
stereochemical outcome of the reaction on the absolute
configuration of the ligand, constitutes an aspect
deserving detailed experimental and computational
studies. Hopefully, its understanding will provide a
fundamental contribution to the comprehension of the
mechanism of this important reaction.
Hz), 4.53 (part B of AB system, 1H, J=11.7 Hz), 5.42
(dd, 1H, J1=8.8 Hz, J2=8.1 Hz), 6.81 (d, 1H, J=8.7
Hz), 6.93 (d, 2H, J=7.2 Hz), 7.06 (d, 2H, J=6.4 Hz),
7.1–7.4 (m, 18H), 7.72 (d, 1H, J=9.0 Hz), 7.8–7.9 (m,
3H); 13C NMR (75 MHz, CDCl3): l 70,97, 81.46, 85.91,
121.95, 122.57, 124.68, 124.91, 125.69, 126.10, 127.03,
127.09, 127.39, 127.53, 127.67, 127.72, 127.95, 128.08,
129.24, 128.28, 129.20, 130.06, 131.19, 131,49, 132.63,
132.86, 137.30, 137.55, 138.46, 147.70, 148.23.
(aS,R,R)-3b: (50%); [h]D=+221 (c 0.53, CHCl3). Mp:
1
98–100°C; H NMR (300 MHz, CDCl3): l 4.50 (part A
of AB system, 1H, J=11.7 Hz), 4.56 (part B of AB
system, 1H, J=11.7 Hz), 4.64 (d, 1H, J=7.5 Hz), 5.43
(dd, 1H, J1=7.8 Hz, J2=7.5 Hz), 6.90 (d, 1H, J=7.5
Hz), 7.06 (d, 2H, J=8.0 Hz), 7.1–7.5 (m, 20H), 7.69 (d,
1H, J=9.0 Hz), 7.9–8.0 (m, 3H); 13C NMR (75 MHz,
CDCl3): l 70.98, 81.18, 85.42, 122.30, 122.41, 124.93,
125.19, 126.16, 126.38, 127.30, 127.71, 127.80, 127.90,
128.04, 128.22, 128.34, 128.53, 129.72, 130.40, 131.72,
133.07, 137.48, 138.03, 138.31, 147.60.
4. Experimental
4.1. General
1H and 13C NMR spectra were recorded in CDCl3 on a
Bruker Aspect 300 MHz NMR spectrometer, using
TMS as the external standard. TLC analyses were
performed on silica gel 60 Macherey–Nagel sheets; flash
chromatography separations were carried out on ade-
quate dimension columns using silica gel 60 (230–400
mesh). HPLC analyses were performed on a JASCO
PU-1580 intelligent HPLC pump equipped with a
Varian 2550 UV detector. Optical rotations were mea-
sured with a JASCO DIP-370 digital polarimeter. Melt-
ing points were taken using a Kofler Reichert–Jung
Thermovar apparatus and are uncorrected. Toluene
and dichloromethane were refluxed over sodium–ben-
zophenone and calcium hydride, respectively, and dis-
tilled before the use. Unless otherwise specified the
reagents were used without any purification. (1R,2R)-
1,2-Diphenyl-2-benzyloxyethanol 16a,b and cyclopen-
tadec-2-en-1-one9 were obtained according to literature
procedures.
4.3. Enantioselective conjugate addition of diethylzinc
to chalcone
A solution of Cu(OTf)2 (5 mg, 0.014 mmol, 3.0 mol%)
and the chiral ligand (17 mg, 0.028 mmol, 6.0 mol%) in
toluene (3 mL) was stirred for 1 h at rt under nitrogen
atmosphere. To this catalyst solution chalcone (96 mg,
0.46 mmol) was added and, after cooling to −40°C,
diethylzinc (1 mL, 2 equiv.) was added dropwise. The
reaction was monitored by TLC. After stirring for 90
min at −40°C the reaction mixture was poured in 10
mL of 1 M HCl solution and extracted three times with
diethyl ether. The combined organic phases were
washed with brine, dried with anhydrous Na2SO4,
filtered and concentrated under reduced pressure. The
crude product was purified by column chromatography
(SiO2, petroleum ether/diethylether 8:2), affording pure
1,3-diphenyl-pentanone.
4.2. Synthesis of phosphites (aR,R,R)-3a and (aS,R,R)-
3b
4.4. Enantioselective conjugate addition of diethylzinc
to 2-cyclohexen-1-one
A solution of (1R,2R)-1,2-diphenyl-2-benzyloxyethanol
(552 mg, 1.8 mmol) in toluene (5 mL) was added in 15
min to a cooled solution (−60°C) of freshly distilled
phosphorus trichloride (157 mL, 1.8 mmol) and triethyl-
amine (1.3 mL, 9 mmol) in toluene (3 mL). The reac-
tion mixture was stirred at −60°C for 2 h. Then, to the
reaction mixture was added DMAP (240 mg, 1.96
mmol) and, dropwise, a solution of (R) or (S)-BINOL
(515 mg, 1.8 mmol) in toluene (14 mL). The reaction
mixture was stirred at −60°C for 2 h and then at rt for
20 h. The reaction was monitored by TLC. The reac-
tion mixture was filtered and concentrated under
reduced pressure. The crude product was purified by
flash column chromatography (SiO2, CH2Cl2), afford-
ing the pure phosphite as white solid.
A solution of Cu(OTf)2 (5 mg, 0.014 mmol, 3.0 mol%)
and the chiral ligand (17 mg, 0.028 mmol, 6.0 mol%) in
toluene (3 mL) was stirred for 1 h at rt under nitrogen
atmosphere. The solution was cooled to −40°C and
2-cyclohexen-1-one (44 mg, 0.46 mmol) followed by
Et2Zn (1 mL, 2 equiv.) were added slowly. The reaction
was monitored by GC–MS. After stirring for 2 h at
−40°C, the reaction mixture was poured into 10 mL of
1 M HCl solution and extracted three times with
diethyl ether. The combined organic phases were
washed with brine, dried with anhydrous Na2SO4 and
filtered. Removal of the diethyl ether under reduced
pressure, 700–350 mbar, at rt yielded the crude product
in toluene, which was purified by flash column chro-
matography (SiO2, pentane/diethylether 5:1) to afford
3-ethylcyclohexan-1-one (98%) as a colorless liquid.
The e.e. was determined by HPLC analysis after deriva-
tization with (R,R)-1,2-diphenyletan-1,2-diol. To a
solution of 3-ethylcyclohexan-1-one (67 mg, 0.53 mmol)
(aR,R,R)-3a: (38%); [h]D=−246 (c 0.55, CHCl3). Mp:
1
107–110°C; H NMR (300 MHz, CDCl3): l 4.48 (part
A of AB system, 1H, J=11.7 Hz), 4.49 (d, 1H, J=8.1

P. Scafato et al. / Tetrahedron: Asymmetry 14 (2003) 3873–3877
3877
,
in CH2Cl2 (13 mL), activated 4 A molecular sieves
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to 2-cyclopentadecen-1-one
A solution of Cu(OTf)2 (5.4 mg, 0.015 mmol, 3.0
mol%) and the chiral ligand (18.5 mg, 0.03 mmol, 6.0
mol%) in toluene (5 mL) was stirred for 1 h at rt
under nitrogen atmosphere. After cooling to −10°C,
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solution of 2-cyclopentadecen-1-one (111 mg, 0.5
mmol) in toluene (1 mL). The reaction was moni-
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−10°C to the reaction mixture was added 10 mL of 1
M HCl solution and 5 mL of diethyl ether, which
was left to stir for a few minutes. The solution was
then extracted three times with diethyl ether. The
combined organic phases were washed with brine,
dried with anhydrous Na2SO4, filtered and concen-
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purified by flash column chromatography (SiO2,
petroleum ether/diethylether 95:5), affording (−)-mus-
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