Nucleophilic benzoylation using a mandelic acid dioxolanone as a synthetic equivalent of the benzoyl carbanion. Oxidative decarboxylation of α-hydroxyacids

Article abstract of DOI:10.3390/90500365

The synthesis of alkyl aryl ketones using a mandelic acid dioxolanone as a synthetic equivalent (Umpolung) of the benzoyl carbanion is reported. The methodology involves alkylation of the mandelic acid dioxolanone, hydrolysis of the dioxolanone moiety in the alkylated products and oxidative decarboxylation of the resulting α-hydroxyacids. The last step is carried out in a catalytic aerobic way using a Co (III) complex in the presence of pivalaldehyde under very mild conditions.

Full text of DOI:10.3390/90500365

Molecules 2004, 9, 365-372
molecules
ISSN 1420-3049
http://www.mdpi.org
Nucleophilic Benzoylation Using a Mandelic Acid Dioxolanone
as a Synthetic Equivalent of the Benzoyl Carbanion. Oxidative
Decarboxylation of α-Hydroxyacids
Gonzalo Blay, Isabel Fernández, Belén Monje and José R. Pedro*
Departament de Química Orgànica, Facultat de Química, Universitat de València, E-46100
Burjassot(València), Spain. Tel. +34-963544329, Fax +34-963544328.
* Author to whom correspondence should be addressed; e-mail: Jose.R.Pedro@uv.es
Received: 5 March 2004 / Accepted: 18 March 2004 / Published: 30 April 2004
Abstract: The synthesis of alkyl aryl ketones using a mandelic acid dioxolanone as a
synthetic equivalent (Umpolung) of the benzoyl carbanion is reported. The methodology
involves alkylation of the mandelic acid dioxolanone, hydrolysis of the dioxolanone moiety
in the alkylated products and oxidative decarboxylation of the resulting α-hydroxyacids.
The last step is carried out in a catalytic aerobic way using a Co (III) complex in the
presence of pivalaldehyde under very mild conditions.
Keywords: Alkylation, dioxolanone, decarboxylation, catalysts, cobalt, Umpolung.
Introduction
We have recently reported the use of methyl mandelate as a masked d1-synthon for nucleophilic
benzoylation ("Umpolung") of the carbonyl group [1] in a synthesis of aryl alkyl ketones, which
involves alkylation of methyl mandelate, hydrolysis of the ester group in the alkylated products and
oxidative decarboxylation of the resulting α-hydroxyacids (Scheme 1). Because of the importance of
the carbonyl group in organic chemistry, it is not surprising that several “Umpolung” methods for this
group have been described, including the use of dithianes and dithiolanes [2], TosMIC [3], enol ethers
[4], vinyl sulfides [5], α-aminonitriles [6], cyanohydrins [7], SAMP-hydrazones [8], or heterocycles

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366
[9] among other. In this paper, we wish to report a related procedure which uses a different mandelic
acid derivative, namely 2,2-dimethyl-5-phenyl-1,3-dioxolan-4-one (3) as a new umpoled synthon of
the benzoyl carbanion leading to alkyl aryl ketones.
Results and Discussion
In our earlier work [1] we carried out the alkylation of the dianion of methyl mandelate (2) using 2
equiv. of LDA and 1 equiv. of an alkyl halide as it had been described in the literature [10]. Now, we
have carried out the alkylation of a mandelic acid dioxolanone, namely 2,2-dimethyl-5-phenyl-1,3-
dioxolan-4-one (3), which has the advantage of requiring only one equivalent of strong base (Scheme
1).
Dioxolanone 3 was readily prepared by acid catalysed condensation of mandelic acid (1) with
acetone. The lithium enolate derived from 3 was alkylated with several organic halides (Table 1). The
use of alkyl, allyl or benzyl chlorides as alkylating reagents was unsuccessful as the starting materials
were recovered unreacted in all of the cases. Nevertheless the introduction of benzylic or allylic groups
could be successfully achieved using the corresponding bromides as alkylating reagents [11], while the
introduction of an alkyl group required the use of the corresponding iodide and the addition of 20%
HMPA as cosolvent. Following this methodology we were able to prepare in good yields several α-
alkylated dioxolanones 5 bearing primary, allyl or benzyl groups, and other additional functional
groups such as a bromide or a carboxyester.
Scheme 1
O
O
MeO
HO
MeO
HO
Ph
Ph
R
2
4
O
HO
HO
O
O
HO
HO
1
6
Ph
R
Ph
Ph
R
7
O
O
O
O
O
O
Ph
Ph
R
3
5

Molecules 2004, 9
Table 1. Alkylation of 2,2-dimethyl-5-phenyl-1,3-dioxolan-4-one (3) and oxidative
367
decarboxylation of α-alkylated mandelic acids (6).
Hydrolysis
time (h)
Yield of
Yield of
5 (%)
Entry
a
7 (%)c
Compound 5 a
Compound 6b
Ketone 7
O
O
O
HO
CO2H
CH3
6.0
2.0
76
87
Ph
Ph
O
CH3
O
Ph
O
O
HO
CO2H
(CH2)11CH3
b
c
65
62
95
75
Ph
Ph
(CH2)11)CH3
O
9
Ph
O
O
HO
CO2H
O
2.5
Ph
Ph
O
Ph
O
O
HO
Ph
CO2H
Ph
O
3.0
3.0
d
e
65
75
93
94
Ph
O
Ph
Ph
O
O
O
Br
Br
O
HOOC
OH
Ph
Ph
Ph
Br
O
O
O
O
HO
CO2H
CO2H
7.0
52
f
97
Ph
O
Ph
Ph
Ph
HO
Ph
O
O
Br
g
5.5
9.5
55
77
81
50
O
Ph
Br
Br
O
O
O
HO
CO2H
CO2H
h
COOCH3
CO2H
O
Ph
Ph
Ph
a Entries a-c required RI as alkylating reagent and the use of 20% of HMPA as cosolvent.
Entries d-h were carried out with RBr as alkylating reagent.
b Nearly quantitative yields of the hydrolysis products 6 were obtained in all cases.
c The reaction was carried out at room temperature; entries f-g at 0 °C and entry h at -47 °C.

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368
Upon basic hydrolysis, the products 5 were transformed into the corresponding α-hydroxyacids 6 in
almost quantitative yields. Finally, oxidative decarboxylation of 6 gave the corresponding alkyl aryl
ketones 7. This transformation was achieved using a catalytic procedure developed in our laboratory
which employs molecular oxygen as terminal oxidant in the presence of pivalaldehyde as co-reductant
and a catalytic amount of the Co(III) ortho-phenylene-bis(N’-methyloxamidate) complex [Co(III)–
Me2opba] (Figure 1) [12].
Figure 1
O
O
O
O
N
N
N
N
Co
NMe4
Me Me
Conclusions
In summary, we have presented in this paper a procedure for the use of a mandelic acid derivative,
namely 2,2-dimethyl-5-phenyl-1,3-dioxolan-4-one (3) as a new masked d1-synthon for the benzoyl
group (Umpolung), in which the key step is the aerobic oxidative decarboxylation with oxygen in the
presence of pivalaldehyde as co-reductant and the Co(III)-Me2opba complex as catalysts. This reaction
is carried out under very mild conditions and it is compatible with the presence in the molecule of
other oxidisable groups.
Acknowledgements
This work was financially supported by the Spanish Government (MCYT, project BQU 2001-
3017) and in part by Generalitat Valenciana. (AVCYT, Grupos 03/168). BM thanks MCYT for a grant.
Experimental
General
All melting points are uncorrected. Column chromatography was performed on silica gel 60
(Merck, 0.040-0.063 mm). Unless specified otherwise NMR experiments were run for CDCl3 solutions
1
13
at 300 MHz for H- and at 75 MHz for C-NMR, and referenced to the solvent as internal standard.
The carbon type was determined by DEPT experiments. Mass spectra were run by electron impact (70
eV) or by chemical ionisation using methane as ionising gas. Co(III)-Me2opba complex was prepared
according to the previously reported procedure [12].

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Preparation of 2,2-dimethyl-5-phenyl-1,3-dioxolan-4-one (3)
Conc. H2SO4 (1.5 mL, 28.36 mmol) was added very slowly to a solution of mandelic acid (2 g,
13.16 mmol) in toluene (160 mL) and acetone (40 mL). The resulting solution was stirred at room
temperature until the reaction was complete (24 hours). The reaction mixture was then poured into
water (80 mL), extracted with ether (3 x 50 mL), washed with saturated solution of NaHCO3 and brine,
and dried over MgSO4. After removal of the solvent under reduced pressure compound 3 (1.64 g, 65%)
was obtained as an oil: 1H-NMR δ 7.45-7.35 (5H, m), 5.38 (1H, m), 1.71 (3H, s), 1.66 (3H, s);
13C-NMR δ 171.7 (s), 134.7 (s), 129.2 (d), 129.0 (d), 126.7 (d), 111.2 (s), 76.1 (d), 27.5 (q), 26.4 (q);
MS (EI) m/z 192 (M+, 9.5), 177 (19.6), 148 (35.9), 107 (100), 90 (25.2), 79 (46.8); HRMS (EI) m/z
required for C11H12O3 192.0786, found 192.0796.
General procedure for alkylation of 2,2-dimethyl-5-phenyl-1,3-dioxolan-4-one (3)
A 1.6 M solution of n-BuLi in hexane (0.78 mL, 1.25 mmol) was added dropwise to a solution of
diisopropylamine (0.175 mL, 1.25 mmol) in dry THF (0.8 mL) at 0 oC under argon. After 30 min, the
o
solution was cooled to –78 C and a solution of dioxolanone 3 (192 mg, 1 mmol) in THF (0.65 mL)
o
was added. The resulting solution was stirred at –78 C for 30 min and then the corresponding alkyl
halide (1.25 mmol) in THF (0.39 mL) was added. The reaction mixture was stirred at this temperature
for 2 hours and then at room temperature until the reaction was complete (2 hours). After this time, the
reaction mixture was poured into aq. NH4Cl (30 mL), extracted with ether (3 x 30 mL), washed with
brine and dried over MgSO4. After filtration and removal of the solvent under reduced pressure, the
products 5 were obtained by column chromatography (elution with hexane-diethyl ether).
1
2,2,5-Trimethyl-5-phenyl-1,3-dioxolan-4-one (5a). An oil; H-NMR δ 7.54 (2H, dd, J = 8.1, 1.2 Hz),
13
7.4-7-2 (3H, m), 1.67 (3H, s), 1.61 (3H, s), 1.37 (3H, s); C-NMR δ 172.7 (s), 139.8 (s), 127.4 (d),
127.0 (d), 123.5 (d), 109.1 (s), 79.5 (s), 28.2 (q), 27.4 (q), 26.6 (q); MS (EI) m/z 206 (M+, 6.7), 191
(7.8), 162 (83.5), 121 (36.1), 104 (100), 77 (31.8); HRMS (EI) m/z required for C12H14O3 206.0943,
found 206.0949.
1
5-Dodecyl-2,2-dimethyl-5-phenyl-1,3-dioxolan-4-one (5b). An oil; H-NMR δ 7.57 (2H, dd, J = 8.1,
1.2 Hz), 7.4-7.2 (3H, m), 1.89 (2H, m), 1.64 (3H, s), 1.36 (3H, s), 1.20 (20H, m), 0.83 (3H, t, J = 7.0
13
Hz); C-NMR δ 173.3 (s), 140.2 (s), 128.3 (d), 127.8 (d), 124.8 (d), 110.0 (s), 83.6 (s), 41.9 (t), 31.9
(t), 29.59 (t, two overlapped signals), 29.56 (t), 29.48 (t), 29.31 (t), 29.29 (t, two overlapped signals),
27.8 (q), 24.0 (t), 22.7 (t), 14.1 (q); MS (EI) m/z 360 (M+, 24.6), 316 (47.9), 275 (28.4), 191 (61.9),
163 (98.7), 105 (100), 77 (33.5); HRMS (EI) m/z required for C23H36O3 360.2664, found 360.2650.
1
2,2-Dimethyl-5-(3,7-dimethyl-6-octenyl)-5-phenyl-1,3-dioxolan-4-one (5c). An oil; H-NMR δ 7.56
(2H, dd, J = 8.1, 1.2 Hz), 7.4.7.2 (3H, m), 5.00 (1H, m), 2.0-1.8 (4H, m), 1.64 (3H, s), 1.61 (3H, s),

Molecules 2004, 9
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1.52 (3H, s), 1.36 (3H, s), 1.35-1.10 (5H, m), 0.79 (3H, t, J = 6.4 Hz); 13C-NMR δ 173.3 (s), 140.2 (s),
131.2 (s), 128.3 (d), 127.8 (d), 124.8 (d), 124.7 (d), 110.0 (s), 83.6 (s), 39.4 (t), 36.7 (t), 32.1 (d),
30.8 (t), 27.8 (q), 25.7 (q), 25.3 (t), 19.4 (q), 19.3 (q), 17.6 (q); MS (CI) m/z 331 (M++1, 0.5), 301
(8.1), 273 (100), 245 (15.9), 227 (32.0), 189 (17.7), 137 (13.1), 107 (8.7); HRMS (CI) m/z required for
C21H31O3 331.2273, found 331.2263(M++1).
o
1
5-Benzyl-2,2-dimethyl-5-phenyl-1,3-dioxolan-4-one (5d). M.p. 47-49 C (from CH2Cl2); H-NMR δ
7.63 (2H, dd, J = 8.1, 1.2 Hz), 7.35-7.22 (3H, m), 7.20-7.10 (5H, m), 3.31 (1H, d, J = 13.9 Hz), 2.98
13
(1H, d, J = 13.9 Hz), 1.27 (3H, s), 1.01 (3H, s); C-NMR δ 172.5 (s), 140.0 (s), 135.0 (s), 131.0 (d),
128.4 (d), 128.0 (d), 127.1 (d), 124.8 (d), 110.4 (s), 84.3 (s), 47.7 (t), 27.9 (q), 27.0 (q); MS (EI) m/z
282 (M+, 6.4), 191 (96.8), 178 (16.6), 163 (23.1), 105 (100), 77 (45.9); HRMS (EI) m/z required for
C18H18O3 282.1256, found 282.1243.
1
5-(p-Bromobenzyl)-2,2-dimethyl-5-phenyl-1,3-dioxolan-4-one (5e). M.p. 65-66 oC (from CH2Cl2); H-
NMR δ 7.65 (2H, dd, J = 8.1, 1.2 Hz), 7.40-7.30 (5H, m), 7.05 (2H, d, J = 8.4 Hz), 3.29 (1H, d, J =
13.9 Hz), 3.02 (1H, d, J = 13.9 Hz), 1.34 (3H, s), 1.19 (3H, s); 13C-NMR δ 172.2 (s), 139.6 (s), 133.9
(s), 132.7 (d), 131.1 (d), 128.5 (d), 128.2 (d), 124.8 (d), 121.3 (s), 110.5 (s), 83.9 (s), 47.1 (t), 27.8 (q),
27.2 (q); MS (EI) m/z 362/360 (M+, 2.0/1.8), 277/275 (7.0/7.4), 191 (100), 178 (25.9), 163 (46.1), 105
(47.8), 77 (99.0); HRMS (EI) m/z required for C18H17O3Br 362.0341 / 360.0361, found 362.0382 /
360.0360.
1
5-Allyl-2,2-dimethyl-5-phenyl-1,3-dioxolan-4-one (5f). An oil; H-NMR δ 7.62 (2H, dd, J = 8.1, 1.2
Hz), 7.40-7.25 (3H, m), 5.72 (1H, m), 5.15 (1H, brd, J = 9.9 Hz), 5.13 (1H, brd, J = 17.4 Hz), 2.70 (2H,
13
m), 1.66 (3H, s), 1.40 (3H, s); C-NMR δ 172.5 (s), 139.5 (s), 131.3 (d), 128.4 (d), 128.0 (d), 124.8
(d), 120.3 (t), 110.2 (s), 83.3 (s), 45.9 (t), 27.8 (q), 27.7 (q); MS (CI) m/z 233 (M++1, 0.8), 215 (2.3),
203 (5.0), 191 (4.5), 175 (100), 157 (7.9), 147 (12.0), 131(30.6) 129 (26.2), 105 (27.5); HRMS (CI)
m/z required for C14H17O3 233.1178, found 233.1171 (M++1).
5-(2-Bromoallyl)-2,2-dimethyl-5-phenyl-1,3-dioxolan-4-one (5g). An oil; 1H-NMR δ 7.58 (2H, dd, J =
8.1, 1.2 Hz), 7.30-7.24 (3H, m), 5.61 (1H, s), 5.55 (1H, s), 3.14 (1H d, J = 15.1 Hz), 2.93 (1H, d, J =
13
15.1 Hz), 1.67 (3H, s), 1.36 (3H, s); C-NMR δ 172.0 (s), 139.0 (s), 128.5 (d), 128.4 (d), 125.2 (s),
124.9 (d), 122.9 (t), 110.9 (s), 83.1 (s), 51.6 (t), 27.9 (q), 27.6 (q); MS (CI) m/z 283/281 (M+- C2H5,
3.5/3.9), 255/253 (65.8/70.1), 237/235 (26.7/27.8), 227/225 (22.8/24.0), 191 (64.3), 173 (75.7), 129
(63.8), 105 (100); HRMS (CI) m/z required for C12H10O3Br 2872.9793 / 280.9813, found 282.9757 /
280.9824,.
o
2,2-Dimethyl-5-(methoxycarbonylmethyl)-5-phenyl-1,3-dioxolan-4-one (5h). M.p. 105-107 C (from
1
CH2Cl2); H-NMR δ 7.62 (2H, dd, J = 8.1, 1.2 Hz), 7.39-7.27 (3H, m), 3.69 (3H, s), 3.10 (1H, d, J =
16.8 Hz), 2.89 (1H, d, J = 16.8 Hz), 1.66 (3H, s), 1.35 (3H, s); 13C-NMR δ 171.9 (s), 169.0 (s), 138.9

Molecules 2004, 9
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(s), 128.6 (d), 128.4 (d), 124.7 (d), 110.8 (s), 80.6 (s), 51.9 (q), 45.5 (t), 27.8 (q), 26.7 (q); MS (EI) m/z
264 (M+, 4.4), 220 (31.8), 205 (8.3), 179 (25.6), 133 (21.5), 105 (100), 77 (49.5); HRMS (EI) m/z
required for C14H16O5 264.0998, found 264.1005.
General procedure for hydrolysis of α-alkylated dioxolanones 5
The α-alkylated dioxolanones 5 (0.3 mmol) were treated with 5% ethanolic KOH (0.75 mL, 0.6
mmol) at room temperature until complete reaction of the starting material (as indicated by TLC). The
solution was poured into ice and acidified with 1M HCl to pH ~ 2. The aqueous mixture was extracted
with EtOAc (3 x 20 mL), the organic layers were washed with brine until neutral, dried, filtered and
concentrated under reduced pressure to give the α–alkylated mandelic acids 6 in almost quantitative
yield. For characterisation of compounds 6a, 6b, 6d and 6f-6h see ref [1].
o
1
2-Hydroxy-5,9-dimethyl-2-phenyl-8-decenoic acid (6c). M.p. 78-80 C (from ethyl acetate); H-NMR
δ 7.61 (2H, d, J = 7.0 Hz), 7.40-7.20 (3H, m), 5.06 (1H, m), 2.3-1.8 (4H, m), 1.67 (3H, s), 1.57 (3H, s),
1.45-1.10 (5H, m), 0.87 (3H, t, J = 6.2 Hz); 13C-NMR δ 180.6 (s), 141.0 (s), 131.1 (s), 128.3 (d), 128.0
(d), 125.5 (d), 124.7 (d), 78.4 (s), 37.1 (t), 36.7 (t), 32.3 (d), 30.3 (t), 25.7 (q), 25.4 (t), 19.4 (q),
17.6 (q); MS (EI) m/z 290 (M+, 0.8), 272 (49.4), 245 (22.2), 133 (22.3); 110 (77.5), 105 (100), 69
(64.3); HRMS (EI) m/z required for C18H26O3 290.1882, found 290.1872.
o
1
3-(p-Bromophenyl)-2-hydroxy-2-phenylpropanoic acid (6e). M.p. 210-212 C (from CH2Cl2); H-
NMR (DMSO-d6) δ 7.62 (2H, d, J = 7.2 Hz), 7.40-7.25 (5H, m), 7.16 (2H, d, J = 7.2 Hz) 3.44 (1H, d,
J = 13.6 Hz), 3.19 (1H, d, J = 13.6 Hz); 13C-NMR (DMSO-d6) δ 175.0 (s), 142.4 (s), 136.2 (s), 132.7
(d), 130.1 (d), 127.7 (d), 127.1 (d), 125.6 (d), 119.4 (s), 77.8 (s), 44.2 (t); MS (EI) m/z 304/302 (M+-
H2O, 4.0/3.8), 178 (9.3), 171 (12.3), 169 (13.0), 149 (16.5), 105 (100), 77 (28.8); HRMS (EI) m/z
required for C15H11BrO2 303.9922 / 301.9942, found 303.9919 / 301.9937.
General procedure for catalytic aerobic decarboxylation of α–hydroxyacids 6
Co (III)-Me2opba complex (3.3 mg, 7.7 x 10-3 mmol) and pivalaldehyde (46 µL, 0.39 mmol) were
added to a stirred solution of alkylated α-hydroxyacids 6 (0.13 mmol) in acetonitrile (0.5 mL) under a
dioxygen atmosphere. The mixture was stirred at the indicated temperature until consumption of the
starting α-hydroxyacid, as indicated by TLC. The reaction products 7 were purified by flash
chromatography. For characterisation of compounds 7a, 7b, 7d and 7f-7h see ref [1].
3,7-Dimethyl-6-octenyl phenyl ketone (7c). An oil; 1H-NMR δ 7.94 (2H, dd, J = 8.1 and 1.2 Hz), 7.53
(1H, tt, J = 8.1 and 1.2 Hz) 7.43 (2H, td, J = 8.1 and 1.2 Hz), 5.08 (1H, m), 2.94 (2H, m), 1.98 (2H, m),
1.75 (2H, m), 1.66 (3H, s), 1.58 (3H, s), 1.50 (2H, m), 1.35 (1H, m), 0.92 (3H, d, J = 6.2 Hz);
13C-NMR δ 200.8 (s), 137.0 (s), 132.8 (d), 131.2 (s), 128.5 (d), 128.0 (d), 124.7 (d), 36.9 (t), 36.3 (t),

Molecules 2004, 9
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32.2 (d), 31.3 (t), 25.7 (q), 25.5 (t), 19.4 (q), 17.6 (q); MS (EI) m/z 244 (M+, 58.8), 201 (6.9), 173
(18.5), 133 (78.9); 122 (77.5), 120 (71.4); 105 (100), 77 (46.6); HRMS (EI) m/z required for C17H24O
244.1827, found 244.1829.
p-Bromobenzyl phenyl ketone (7e). M.p. 136-138 oC (from CH2Cl2); 1H-NMR δ 8.00 (2H, dd, J = 8.1,
13
1.2 Hz), 7.65-7.40 (5H, m), 7.15 (2H, d, J = 8.4 Hz), 4.24 (2H, s); C-NMR δ 197.2 (s), 136.6 (s),
135.3 (s), 133.6 (d), 132.0 (d), 131.5 (d), 129.0 (d), 128.8 (d), 121.2 (s), 45.0 (t); MS (EI) m/z 276/274
(M+, 9.3/9.8), 185/183 (15.8/15.0), 171 (9.7), 165 (5.0), 157 (5.8), 105 (73.8), 90 (22.4), 77 (100);
HRMS (EI) m/z required for C14H11OBr 275.9973 / 273.9993, found 275.9963 / 273.9982.
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Samples Availability: Available from the authors.
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