Preparation of geminal diacylates (acylals) of aldehydes - Scope and reactivity of aldehydes with acid anhydrides

Article abstract of DOI:10.1002/ejoc.200600737

The geminal diacylates of a variety of aldehydes were prepared in good yields without any catalysts or solvent by simply refluxing the aldehydes with aliphatic acid anhydrides. The scope of the reactions and relative reactivities of aldehydes and acid anhydrides were examined. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejoc.200600737

FULL PAPER
DOI: 10.1002/ejoc.200600737
Preparation of Geminal Diacylates (Acylals) of Aldehydes – Scope and
Reactivity of Aldehydes with Acid Anhydrides
Motiur A. F. M. Rahman[a] and Yurngdong Jahng*[a]
Keywords: Geminal diacylates / Aldehydes / Acylals / Green chemistry
The geminal diacylates of a variety of aldehydes were pre-
pared in good yields without any catalysts or solvent by sim-
ply refluxing the aldehydes with aliphatic acid anhydrides.
The scope of the reactions and relative reactivities of alde-
hydes and acid anhydrides were examined.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
tic acids[7] were employed as catalysts, but reactions suffered
from poor yields and long reaction times. Lewis acids and
various metal salts[8] replaced protic acids in order to im-
prove yields and reduce reaction times substantially. Ad-
ditionally, P2O5,[9] I2,[10] and NBS[11] as well as solid acidic
materials such as Nafion-H,[12] tungstosilicic acid and
HZSM-5,[13] Y-zeolite,[14] β-zeolite,[15] graphite,[16] Am-
berlyst-15,[17] montmorillonites,[18] PVC-FeCl3 complex,[19]
Introduction
The merits of geminal diacylates (acylals) as useful
protecting groups for aldehydes stem from not only their
stability in neutral and basic media[1] as well as towards
aqueous acids[2] but also the ease with which they can be
deprotected.[3] The geminal diacylates have an additional
advantage over acetals in that water, generated during the
formation of acetals, must be removed either by physical
or chemical methods with a water scavenger, which is not
necessary during the diacylation of aldehydes.
Additionally, geminal diacylates are stable towards a
variety of reactions. Thus, geminal diacylates have been
used as important protecting groups for aldehydes and
played important roles in organic synthesis.[4] Geminal di-
acylates derived from α,β-unsaturated aldehydes are not
only important starting materials for the synthesis of ace-
toxy dienes and vinyl acetates[4a] but are also good sub-
strates for the SN2Ј coupling reaction with organocuprates
to afford β-substituted aldehydes after hydrolysis.[4c]
[20]
and ZrCl4 were recently introduced as catalysts, most of
which employed solvent-free reaction conditions. Neverthe-
less, up to this point the development of procedures for the
preparation of geminal diacylates has relied on the use of
rather strong acids or toxic and/or expensive reagents,
which has led to a continuing interest in the development
of a more efficient procedure. A practical and more efficient
alternative with an inexpensive reagent under solvent-free
condition is of considerable interest. We herein describe a
simple and efficient procedure for the preparation of gemi-
nal diacylates of aldehydes and examine the scope of such
a reaction.[21]
Recently, Kavala and Patel extensively studied the reac-
tion mechanism for the formation of geminal diacylates
with tetrabutylammonium tribromide (TBATB).[5] The sug-
gested mechanism included reaction initiation with acid and
proceeded by a nucleophilic attack of an anhydride on an
aldehyde carbonyl group, followed by nucleophilic attack of
the hemiacylate intermediate on a second molecule of the
anhydride, which was followed by an intermolecular attack
of a second acylate to generate the anhydride.
Results and Discussion
The present method describes the preparation of geminal
acylals by simply refluxing aldehydes and acid anhydrides
without any additional catalyst or solvent. Although it was
recently claimed that no considerable amount of the corre-
sponding diacetate was formed upon the reactions of alde-
hydes with Ac2O in the absence of catalyst,[8m,8r] we found
that the reactions proceeded smoothly. Although the reac-
tion of benzaldehyde (1a) with Ac2O in refluxing acetoni-
trile failed to provide the corresponding geminal diace-
tate,[8t] we successfully isolated benzylidene diacetate in
85% yield. Although a low yield (4%) was reported for 4-
nitrobenzaldehyde in the presence of a catalyst,[8e] all three
isomers (compounds 1e, i, and m) yielded the corresponding
The reaction of aldehydes with simple acid anhydrides
was originally pursued in the presence of acidic catalysts
and afforded geminal diacylates.[6] A variety of strong pro-
[a] College of Pharmacy, Yeungnam University,
Gyeongsan 712-749, Korea
Fax: +82-53-810-4654
E-mail: ydjahng@yumail.ac.kr
Eur. J. Org. Chem. 2007, 379–383
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
379

M. A. F. M. Rahman, Y. Jahng
FULL PAPER
geminal diacetates in 82–93% yield. Methyl ether (2o) and hydes, was additionally examined and found to give the cor-
methylthio ether (2p) moieties were not disturbed under the responding diacetate (4f) in 57% yield along with 3-(fur-2-
reaction condition.
yl)crotonic acid (5) in 10% yield.
Although a variety of aldehydes were converted into the
corresponding geminal diacetates in good yields (Table 1),
we failed to convert 2-chlorobenzaldehyde (1c) and 4-(di-
methylamino)benzaldehyde (1q) to their geminal diacetates.
The lack of reactivity of the latter could be explained by the
electron-donating effect of the N,N–dimethylamino moiety,
which reduces the electron deficiency of the carbonyl car-
bon and induces the quinonoid resonance structure, as had
been studied.[7c,8r] The lack of reactivity of 2-chlorobenzal-
dehyde was somewhat surprising and remained to be ex-
plored since the reactions in the presence of a catalyst[6,8r]
were reported to produce the corresponding diacetate in
good yields.
Table 1. Preparation of geminal diacetates from aromatic al-
dehdyes.
The chemoselectivity of this reaction was studied with a
variety of aromatic substituents of varying electron-with-
drawing ability. The reaction of a mixture of equimolar 4-
methoxybenzaldehyde or 4-nitrobenzaldehyde with 1 equiv.
of Ac2O gave the corresponding diacetates in 20% or 57%
isolated yield, respectively. Such a result may reflect the
electrophilicity of the carbonyl carbon. This result is incon-
sistent with a previous report, in which 4-methoxybenzalde-
hyde showed more reactivity than 4-nitrobenzaldehyde by a
factor of 93.[5] Although one can claim that the reaction
temperature and presence of a catalyst (TBATB) may affect
the reaction, it is clear that the more electron-deficient car-
bonyl carbon on 4-nitrobenzaldehyde is more reactive
towards nucleophilic attack than is the less electron-de-
ficient carbonyl of 4-methoxybenzaldehyde.
R
Time [h][a] Yield [%][a,b]
M.p. [°C]
2a
2b
2c
2d
2e
2f
2g
2h
2i
H
2-F
1.5
3.5
24
4
6
2
3
4
5.5
2
3
4
5.5
1
1.5
3
24
85
82
43–44 (45.8[6]
)
27
–
2-Cl
2-Br
2-NO2
3-F
3-Cl
3-Br
3-NO2
4-F
no reaction
73
85
78
75
79
93
74
65
82
82
70
71
83
80
92 (91–92[8e]
30–31
)
)
65 (65–66[8e]
thick liquid
Additionally, the chemoselectivity of aldehyde versus
ketone was examined by reacting a mixture of benzaldehyde
and acetophenone with 1 equiv. of Ac2O to afford only
benzylidene diacetate in 80% yield. The reaction of aceto-
phenone alone with Ac2O was also pursued, but failed to
give any evidence of the corresponding diacetate.
63 (64–66[2]
)
2j
2k
2l
39
4-Cl
4-Br
84–85 (80–81[22]
)
85
2m 4-NO2
121 (125–126[4e]
)
2n
2o
2p
2q
4-SCH3
4-OCH3
3,5-(OMe)2, 4-OAc
4-(dimethylamino)
50
65 (64–65[4e]
125–127
–
)
Only
a very limited number of acid anhydrides
(propionic, butyric, isobutyric, pivalic, and benzoic anhy-
dride)[5,17,20] were previously employed in the pursuit of ge-
minal diacylates. To examine the generality of the reaction,
the present reaction conditions were applied to propionic
anhydride, benzoic anhydride, and phthalic anhydride with
benzaldehyde or substituted benzaldehydes. Although all
the reactions with propionic anhydride proceeded smoothly
no reaction
[a] Reaction times and yields are not optimized. In most cases, the
value given is from a single experiment. [b] All yields refer to pure
isolated products.
The scope of the reaction was examined with selected to afford the corresponding geminal dipropionates (6a–d)
α,β-unsaturated aldehydes and aliphatic aldehydes. The re- in 72–82% yield, the reactions with benzoic anhydride and
actions of (E)-crotonaldehyde (3a) and (E)-cinnamaldehyde phthalic anhydride were too sluggish to afford the corre-
(3b) provided the corresponding geminal diacetates 4a and sponding acylates. A reaction with benzoic anhydride in the
4b in 84% and 83% yield, respectively. Similarly, the reac- presence of H2SO4, however, provided the corresponding
tions of selected aliphatic aldehydes such as butyraldehyde dibenzoate in 77% yield (not described herein in detail). On
(3c), isobutyraldehyde (3d), and cyclohexanecarbaldehyde the other hand, a reaction with phthalic anhydride did not
(3e) also proceeded smoothly to afford the corresponding proceed, even in the presence of H2SO4. Such results may
diacetates (4c, d, and e) in 82–92% yields. The reactivity of imply that the nucleophilicity of the acid anhydride is im-
furan-2-carbaldehyde (3f), one of the most fragile alde- portant for the reaction.
380
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© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2007, 379–383

Preparation of Geminal Diacylates
FULL PAPER
(3ϫ30 mL). The combined organic layers were washed with water
successively and dried with anhydrous MgSO4. The solvent was
evaporated under reduced pressure to provide crude product, which
was chromatographed on silica gel with CH2Cl2/n-hexane (1:1).
The early fractions afforded the desired geminal diacetates pure
enough for the analysis. Spectral data of unreported compounds
and selected diacylates follows.
Crotonylidene Diacetate (4a): Colorless liquid (84% yield), b.p. 45–
48 °C (14 Torr), ref.[10] b.p. 44–46 °C (14 Torr). IR (thin film) ν =
˜
1760, 1390, 1250, 1215 cm–1. 1H NMR (CDCl3, 250 MHz): δ =
7.08 (d, J = 6.6 Hz, 1 H, 1-H), 6.03 (dq, J = 15.5, 6.6 Hz, 1 H, 3-
H), 5.57 (ddt, J = 15.5, 6.6, 1.2 Hz, 1 H, 2-H), 2.08 (s, 6 H), 1.76
(dd, J = 6.6, 1.5 Hz, 3 H) ppm. 13C NMR (CDCl3, 62.5 MHz): δ
= 168.5, 133.0, 124.4, 89.5, 20.7, 17.4 ppm.
Conclusions
In summary, a simple and efficient procedure for the
preparation of geminal diacylates of a variety of aldehydes
such as aliphatic, aromatic, and α,β-unsaturated aldehydes
was established by simply refluxing aldehydes with acid an-
hydrides. Although the application of the present method
seems to be limited to aliphatic acid anhydrides, the method
has advantages over previously described methods in yield,
simplicity of operation, and economy as well as the environ-
mental aspect of running the reaction without any haloge-
nated solvents, additional additives, and/or catalyst.
Cinnamylidene Diacetate (4b): White solid (83% yield), m.p. 84–85
°C (ref.[8d] m.p. 84–87 °C), R = 0.3 (CH Cl ). IR (thin film) ν =
˜
f
2
2
1760, 1390, 1250, 1215 cm–1. 1H NMR (CDCl3, 250 MHz): δ =
7.42–7.28 (m, 6 H), 6.85 (d, J = 15.6 Hz, 1 H, 1-H), 6.19 (dd, J =
15.5, 6.6 Hz, 1 H, 3-H), 2.10 (s, 6 H) ppm. 13C NMR (CDCl3,
62.5 MHz): δ = 168.7, 135.6, 135.1, 128.8, 128.6, 127.0, 121.6, 89.7,
20.9 ppm.
Butylidene Diacetate (4c): Colorless liquid (92% yield). IR (thin
film) ν = 2960, 1760, 1375, 1250, 1220 cm–1. 1H NMR (CDCl3,
˜
250 MHz): δ = 6.74 (t, J = 5.6 Hz, 1 H, 1-H), 2.05 (s, 6 H), 1.75–
1.68 (m, 2 H, H-2), 1.44–1.34 (m, 2 H, H-3), 0.93 (t, J = 6.1 Hz, 3
H, H-4) ppm. 13C NMR (CDCl3, 62.5 MHz): δ = 168.6, 90.7,
27.12, 31.12, 20.70, 14.02 ppm. MS (ESI): calcd. for C8H14O4 175
[M + H]+; found 175.
Experimental Section
Melting points were determined with a Fisher–Jones melting point
apparatus and are uncorrected. Infrared spectra were recorded with
KBr pellets for solids and neat liquids with a Perkin–Elmer 1330
grating spectrometer. NMR spectra were obtained with a Bruker-
250 spectrometer (250 MHz for 1H NMR and 62.5 MHz for 13C
NMR) and reported as parts per million (ppm) from the internal
standard, tetramethylsilane (TMS). Column chromatography was
carried out with 60–120 mesh silica gel. Chemicals and solvents
were commercial reagent grade and used without further purifica-
tion. Electrospray ionization (ESI) mass spectrometry (MS) experi-
ments were performed with a LCQ advantage-trap mass spectrome-
ter (Thermo Finnigan, San Jose, CA, USA).
Isobutylidene Diacetate (4d): Pale yellow liquid (82% yield). IR
(thin film) ν = 2965, 1765, 1370, 1248, 1210 cm–1. 1H NMR
˜
(CDCl3, 250 MHz): δ = 6.58 (d, J = 5.3 Hz, 1 H, 1-H), 2.08–2.03
(m, 1 H, H-2), 2.05 (s, 6 H), 1.15 (d, J = 6.8 Hz, 6 H) ppm. MS
(ESI): calcd. for C8H14O4 175 [M + H]+; found 175.
Cyclohexylmethylene Diacetate (4e): Pale yellow liquid (86% yield).
1H NMR (CDCl3, 250 MHz): δ = 6.54 (d, J = 5.3 Hz, 1 H, 1-H),
2.00 (s, 6 H), 1.67–1.44 (m, 6 H), 1.19–0.98 (m, 5 H) ppm. 13C
NMR (CDCl3, 62.5 MHz): δ = 169.0, 98.2, 70.7, 26.6, 26.0, 25.3,
20.6 ppm. MS (ESI): calcd. for C11H18O4 215 [M + H]+; found
215.
Preparation of Diacetates of Aldehydes (General Method): A mix-
ture of aldehyde (1.00 mmol) and Ac2O (10 mL) was refluxed for
1–6 h. The reaction was monitored by TLC. After the given reac-
tion time, the reaction mixture was poured into 5% aqueous
NaOH. The resulting mixture was extracted with CH2Cl2
(Furan-2-yl)methylene Diacetate (4f): A mixture of furan-2-carbal-
dehyde (0.96 g, 10.0 mmol) and Ac2O (10 mL) was refluxed for 4 h.
The reaction mixture was poured into 5% aqueous NaOH and was
extracted with CH2Cl2 (3ϫ30 mL). The combined organic layers
Eur. J. Org. Chem. 2007, 379–383
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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381

M. A. F. M. Rahman, Y. Jahng
FULL PAPER
were washed with water successively and dried with anhydrous
MgSO4. The solvent was evaporated under reduced pressure to pro-
vide the crude product, which was chromatographed on silica gel
with CH2Cl2. The early fractions afforded the desired geminal di-
acetate as colorless liquid [57% yield, Rf = 0.6 (CH2Cl2)], which
could be crystallized in the refrigerator. M.p. 54–55 °C (ref.[23] m.p.
52–53 °C). 1H NMR (CDCl3, 250 MHz): δ = 7.70 (s, 1 H), 7.44 (d,
J = 0.8 Hz, 1 H), 6.52 (d, J = 2.2 Hz, 1 H), 6.37 (dd, J = 3.2,
1.8 Hz, 1 H), 2.12 (s, 6 H) ppm. 13C NMR (CDCl3, 62.5 MHz): δ
= 186.4, 147.8, 143.6, 110.3, 109.7, 83.4, 20.7 ppm. The latter frac-
tions afforded 3-(fur-2-yl)crotonic acid (5) as colorless liquid [10%
Acknowledgments
Support from a Korean Research Foundation Grant (KRF-2005-
041-E00496) is gratefully acknowledged.
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Benzylidene Dipropionate (6a): Colorless liquid [78% yield, Rf = 0.2
(CH2Cl2)]. H NMR (CDCl3, 250 MHz): δ = 7.71 (s, 1 H), 7.52–
7.49 (m, 2 H), 7.40–7.37 (m, 3 H), 2.40 (overlapped q, J = 6.7 Hz,
4 H), 1.14 (t, J = 7.5 Hz, 6 H) ppm. 13C NMR (CDCl3, 62.5 MHz):
δ = 172.2, 135.6, 129.6, 128.5, 126.5, 89.5, 27.3, 8.6 ppm. MS (ESI):
calcd. for C13H16O4 237 [M + H]+; found 237.
1
3-Bromobenzylidene Dipropionate (6b): Colorless liquid [82% yield,
1
Rf = 0.27 (CH2Cl2)]. H NMR (CDCl3, 250 MHz): δ = 7.64 (s, 1
H), 7.63 (s, 1 H), 7.50 (dt, J = 8.1, 1.3 Hz, 1 H), 7.40 (dt, J = 7.8,
1.5 Hz, 1 H), 7.24 (t, J = 8.0 Hz, 1 H), 2.40 (q, J = 7.7 Hz, 2 H),
2.39 (q, J = 7.7 Hz, 2 H), 2.13 (t, J = 7.7 Hz, 6 H) ppm. 13C NMR
(CDCl3, 62.5 MHz): δ = 172.1, 137.8, 132.7, 130.1, 129.7, 125.4,
122.5, 88.6, 27.3, 8.7 ppm. MS (ESI): calcd. for C13H15BrO4 316
[M + H]+; found 316.
4-Methoxybenzylidene Dipropionate (6c): Colorless liquid [72%
1
yield, Rf = 0.4 (CH2Cl2)]. H NMR (CDCl3, 250 MHz): δ = 7.65
(s, 1 H), 7.42 (dm, J = 8.5 Hz, 2 H), 6.87 (dm, J = 8.5 Hz, 2 H),
3.81 (s, 3 H), 2.37 (q, J = 7.7 Hz, 2 H), 2.35 (q, J = 7.7 Hz, 2 H),
2.13 (t, J = 7.7 Hz, 6 H) ppm. 13C NMR (CDCl3, 62.5 MHz): δ =
172.2, 160.6, 128.2, 128.1, 114.2, 89.8, 55.6, 27.8, 9.2 ppm. MS
(ESI): calcd. for C14H18O4 267 [M + H]+; found 267.
4-Nitrobenzylidene Dipropionate (6d): Colorless liquid [82% yield,
1
Rf = 0.4 (CH2Cl2)]. H NMR (CDCl3, 250 MHz): δ = 8.24 (dm, J
= 8.5 Hz, 2 H), 7.74 (s, 1 H), 7.67 (dm, J = 8.5 Hz, 2 H), 2.43 (q,
J = 7.5 Hz, 2 H), 2.42 (q, J = 7.6 Hz, 2 H), 1.15 (t, J = 7.7 Hz, 6
H) ppm. 13C NMR (CDCl3, 62.5 MHz): δ = 172.1, 142.2, 127.8,
123.8, 123.7, 88.2, 27.3, 8.6 ppm. MS (ESI): calcd. for C13H15NO6
282 [M + H]+; found 282.
Study of Relative Reactivity: An equimolar mixture of 4-nitrobenz-
aldehyde (1.51 g, 10 mmol) and 4-methoxybenzaldehyde (1.36 g,
10 mmol) was refluxed with 1 equiv. of Ac2O (1.02 g, 10 mmol) for
12 h. Work up as described above in the general method afforded a
mixture of 4-nitrobenzylidene diacetate and 4-methoxybenzylidene
diacetate in 77% yield. Each compound was separated by column
chromatography with CH2Cl2/n-hexane (1:1) to give 1.46 g (57%
yield) of 4-nitrobenzylidene diacetate and 0.48 g (20% yield) of 4-
methoxybenzylidene diacetate.
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fluxed with 1 equiv. of Ac2O (1.02 g, 10 mmol) for 12 h. Work up
as described above in the general method afforded benzylidene di-
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recovered.
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Preparation of Geminal Diacylates
FULL PAPER
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Received: August 20, 2006
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Eur. J. Org. Chem. 2007, 379–383
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