A Facile, Selective Preparation of Monoketals from Pentaerythritol and Ketones

Article abstract of DOI:10.1055/s-2001-14569

The selective preparation of monoketals 3a-f from pentaerythritol 1 and cyclic, acyclic, aromatic, and aliphatic ketones 2a-f was achieved by a facile method. The extreme polarity and low solubility of pentaerythritol in almost all organic solvents were the main difficulties to be overcome for the preparation of monoketals in good yields and high selectivity. A benzene-dimethylformamide (40:60) mixture proved to be excellent for the ketalization. The one-step procedure developed allowed the preparation of monoketals in good yields and good to excellent selectivity (higher than 90 percent).

Full text of DOI:10.1055/s-2001-14569

PAPER
1093
A Facile, Selective Preparation of Monoketals from Pentaerythritol and
Ketones
Selective Synthesis of
a
M
onoketals
r
from
P
e
c
ntaerythrito
e
l
and
K
eto
l
nes o C. Murguía,1a Santiago E. Vaillard,1b Ricardo J. Grau*1c
Instituto de Desarrollo Tecnológico para la Industria Química (INTEC), Universidad Nacional del Litoral and CONICET Güemes 3450,
(3000) Santa Fe, Argentina
Fax +54(342)4551918; E-mail: cqfina@ceride.gov.ar
Received 14 November 2000; revised 27 February 2001
dealing with the studies concerning the preparation of
Abstract: The selective preparation of monoketals 3a–f from pen-
monoketals. If the ketalization is carried out under low
solubility of pentaerythritol conditions, the diketal is
mainly formed due to the presence of ketone in excess. All
taerythritol 1 and cyclic, acyclic, aromatic, and aliphatic ketones
2a–f was achieved by a facile method. The extreme polarity and low
solubility of pentaerythritol in almost all organic solvents were the
main difficulties to be overcome for the preparation of monoketals available methods give low yields of monoketals. Conrad
et al.10 have reported the preparation of monoketals from
diketals by partial hydrolysis obtaining the monoketal of
in good yields and high selectivity. A benzene–dimethylformamide
(40:60) mixture proved to be excellent for the ketalization. The one-
step procedure developed allowed the preparation of monoketals in
pentaerythritol and cyclohexanone in 12% yield.
good yields and good to excellent selectivity (higher than 90%).
Schneider et al.11 have described methods for preparation
Key words: pentaerythritol, 1,3-diols, monoketals, monoprotec-
of the mono and diketals of pentaerythritol in benzene–di-
oxane (1:1) as solvent, but neither yields nor selectivities
tion, 1,3-dioxanes
towards both ketalization products have been reported.
Bonner et al.15 have prepared the monoketal derived from
Ketals find widespread applications in the pharmaceuti-
acetone and pentaerythritol in 28% yield by a tedious pro-
cedure. Schaeffer and Stevens16 have reported the prepa-
ration of the monoketal derived from the cyclohexanone
in 3.5% yield by the Issidroides and Gulen procedure.
cal, phytopharmaceutical, fragrance, and lacquer indus-
tries.2,3 They are of interest in synthetic steroid and
carbohydrate chemistry.4,5 In addition, ketals are useful in
strategies for protecting groups of carbonyl, hydroxy and
diol functions in organic synthetic sequences.6,7 Particu-
larly, protection for the hydroxyl groups of 1,2- and 1,3-
diols can be accomplished by ketalization in acidic medi-
um. On the other hand, the development of new amphip-
athic compounds derived from polyhydric alcohols has
recently grown in importance because different surface
and micellar properties can be expected for a molecule
with distinct content of lipophilic and hydrophilic
groups.8,9 Pentaerythritol is one of the most common tet-
raols and it would be a useful symmetric 1,3-diol to syn-
thesize novel amphipatic compounds bearing four
lipophilic alkyl chains and four hydrophilic groups. This
possibility prompted us to protect, in selective form, a pair
of hydroxyl groups of pentaerythritol with ketones as the
key step in the synthetic sequence to prepare well-defined
amphipathic compounds.
Our exploratory studies started with ketones since they
might be less reactive than the analogous aldehydes. We
report herein a one-step procedure to prepare monoketals
3a–f, in good yields and good to excellent selectivities,
from pentaerythritol 1 and cyclic, acyclic, aromatic, and
aliphatic ketones 2a–f in the presence of a catalytic
amount of p-toluenesulfonic acid monohydrate (Table 1,
Scheme).
A solvent capable of dissolving both 1 and 2 was neces-
sary for the success of the selective ketalization toward
the monoketal 3 rather than the diketal 5. Another key fac-
tor was to establish reaction conditions for removing the
water formed to shift the equilibrium toward ketals, with-
out losing the volatile ketones. Therefore, ketalization re-
actions were tried with benzene–DMF mixtures.
Dimethylformamide was chosen as solvent because it dis-
solves both reagents and it allows, due to its low relative
volatility, temperatures high enough to remove the water
formed by reactive distillation. Benzene was used as recy-
cling co-solvent to enhance the recovery of the volatile
ketones, which might be lost together with water from the
distillate. Upon varying the benzene–DMF molar ratio
from 9:1 to 1:9, the selective preparation of the monoket-
als was best accomplished in a benzene–DMF (4:6) mix-
ture. This composition was finally chosen since it makes
it possible to carry out the ketalization reaction at temper-
atures in the 95 115 °C range, with high removal rates of
the water formed and with high reflux rates of co-solvent,
enhancing the recovery and recycling of the ketones evap-
orated. Three physical methods for the simultaneous re-
In connection with the ketalization of pentaerythritol, nu-
merous diketals have been reported.10,11 However, an ex-
amination of the literature for the preparation of
monoketals revealed that only few efforts have been made
to obtain these synthetic intermediates, since Böeseken,12
Orthner and Freyss,13 and Orthner14 published pioneer
works regarding it. The extreme polarity of pentaerythri-
tol and consequently its insolubility in almost all organic
solvents appear to be the main difficulties found when
Synthesis 2001, No. 7, 01 06 2001. Article Identifier:
1437-210X,E;2001,0,07,1093,1097,ftx,en;M03400SS.pdf.
© Georg Thieme Verlag Stuttgart · New York
ISSN 0039-7881

1094
M. C. Murguía et al.
PAPER
Scheme
Table 1 Monoketalization Reactions of Pentaerythritol 1 with Diverse Ketones 2a
Entry
1
Ketones 2
Reaction Conditions
Temp.(°C) Time (h)
Products 3
Conv.
(%)
Select.
(%)
Yields
(%)
Methods
A
115
48
99
92
90b (91)c
3a
2a
2
115
120
80
95
68.4b (76)c
A
2b
2c
3b
3c
3d
3
4
90
95
48
48
64
56
95
93
56.7b (61)c A, C
47b (52)c
41b (45)c
B, C
2d
2e
5
6
110
115
96
48
37
94
90
B
3e
3f
120
31b (33)c
B
2f
a All acetalization reactions were carried out in benzene–DMF (40:60) in the presence of a catalytic amount of p-toluenesulfonic acid monohy-
drate.
b Product quantified by isolation.
c Product quantified by GC analysis using the method of internal standard.
moval of the water formed were used (see experimental, When cyclohexanone 2a was reacted with a solution of
preparation of ketals).
pentaerythritol 1 and benzene–DMF (40:60), in the pres-
ence of a catalytic amount of p-toluenesulfonic acid
monohydrate (0.1 equivalent of TsOHH2O for 1 equiva-
Synthesis 2001, No. 7, 1093–1097 ISSN 0039-7881 © Thieme Stuttgart · New York

PAPER
Selective Synthesis of Monoketals from Pentaerythritol and Ketones
1095
lent of ketone) for 48 hours at 115°C, 3a was obtained in and if the ketalization rate of pentaerythritol is much
90% yield, with 99% conversion and 92% selectivity (en- greater than the monoketal one k1[1][2]>>k3[2][3], then
try 1, Table 1). Under the same conditions and long d{[5]/[3]}/dt 0.
enough reaction times (greater than 120 hours), acetophe-
none 2b gave the corresponding monoketal 3b in 68.4%
yield, with 80% conversion and 95% selectivity (entry 2,
Table 1). Despite the lower yield, in comparison with 3a,
the selectivity towards the monoketal was successfully
higher (95%). As expected, 2b is less reactive than 2a. In
Therefore, no change in the [5]/[3] ratio would be expect-
ed, as observed experimentally. This result may be ex-
plained by considering that the ketalization rate is
critically dependent on the relative positions of the pair of
hydroxyls and hence the stereochemistry and the confor-
mational stability of the ketal produced. Preliminary Mo-
both cases, the monoketal to diketal ratio attained was
lecular Modeling based on semi empirical calculations by
high (92:8 and 95:5, respectively), and nearly constant
the AM1 method showed that one of the hydroxyl groups
in the optimized structure of 3 would be highly hindered
by the six-membered ring, consequently 3 has a much
lesser ability than 1 to undergo ketalization reaction.
from the beginning of the reaction (Table 2). This unex-
pected result suggested that the ability of a pair of hydrox-
yls groups of 1 to undergo ketal formation is much greater
than those of 3, and that the hydrolysis rates of 3 and 5 are
The above procedure (Method A) failed when applied to
negligible with respect to the ketalization rates of 1 and 3,
the preparation of aliphatic ketals. In this case, the water
respectively. If both conditions are fulfilled, the monoket-
formed during the ketalization reaction was removed inef-
al to diketal molar ratio must be nearly constant during the
ficiently by means of a Dean-Stark trap. It seemed that the
ketalization reaction as it can be seen by inspection of the
greater content of volatile ketones in the recycling current
kinetic equations of reaction rates.
dissolved some water, which was consequently recycled
Table 2 Ketalization Reaction of Pentaerythritol with 2ba
from the trap to the reaction medium. Therefore, poor
yields and worse selectivities of monoketals were ob-
tained; accordingly, these compounds are unstable in the
presence of water.10 Instead, the use of a jacket containing
anhydrous magnesium sulfate as dehydrating agent
proved successful (Method B). In this way, monoketals 3c
to 3f were selectively prepared from acetone (2c), 2-bu-
tanone (2d), 3-methyl-2-butanone (2e), and 3,3-dimethyl-
2-butanone (2f), respectively (entries 3–6, Table 1). After
little experimentation, we were able to obtain acetone, cy-
clic 2,2-bis(hydroxymethyl)trimethylene ketal 3c in
56.7% yield and high mono-/diketal product ratio
(95:5) (entry 3, Table 1). 2-Butanone, cyclic 2,2-bis(hy-
droxymethyl)trimethylene ketalt 3d was obtained from
2d in 47% yield and high selectivity (93:7) (entry 4).
Likewise, 2-methyl-2-(1-methylethyl)-1,3-dioxane-5,5-
dimethanol (3e) (41%, 94:6 selectivity), and 2-methyl-2-
Entry 3b, Conver- 3b, Yield 5b, Yield 3b, Selec- Reaction
sion (%)
(%)
26
39
63
76
76
(%)
0.6
2.1
4.7
4.0
5.7
tivity (%) time (h)
1
2
3
4
5
27
97.8
94.9
93.1
95.0
93.0
24
41
48
67
94
80
120
164
81
a Reaction carried out in benzene DMF (4:6) in the presence of 0.1
equiv TsOHH2O for 1 equiv of 2b, at 115°C. All values were calcu-
lated by GC using the method of internal standard.
The reaction rates of monoketal and diketal are given by
(1,1-dimethylethyl)-1,3-dioxane-5,5-dimethanol
(3f)
(31%, 90:10 selectivity), were prepared (entries 5 and 6).
The conversion of the aliphatic ketones decreased in the
order: acetone > 2-butanone > 3-methyl-2-butanone >
3,3-dimethyl-2-butanone. A reaction time of 48 hours (at
90°C) was required for 64% conversion of 3c; 48 hours
(at 95°C) for 56% conversion of 3d; 96 hours (at 110°C)
for 48% conversion of 3e; and 120 hours (at 115°C) 37%
conversion of 3f. The sterical hindrances might account
for this observed sequence. Some improvement of reac-
tion time was corroborated by carrying out the ketaliza-
tion reaction in the lower container of a Soxhlet apparatus
(Method C).
Equation 1
Thus, if the hydrolysis reaction rates are negligible with
respect to the ketalization ones, the rate of the change of
the diketal to monoketal ratio reduces to
In conclusion, several monoketals from pentaerythritol
and cyclic, acyclic, aromatic and aliphatic ketones were
easily obtained in good yields and good to excellent selec-
tivities. A benzene–DMF (40:60) mixture proved excel-
lent for the selective ketalization reaction. Currently,
further ketalization of more complex ketones and diverse
aldehydes are in progress to extend this simple protocol to
Equation 2
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1096
M. C. Murguía et al.
PAPER
Then the reaction mixture was treated as described above. Com-
pounds 3c and 3d were obtained as white solids. Conversions, se-
lectivities and yields are summarized in Table 1 (entries 3 and 4).
the selective preparation of monoketals as intermediates
to be used in strategies of the synthesis of well-defined
amphiphatic compounds.
1,5-Dioxaspiro[5.5]undecande-3,3-dimethanol (3a)
Mp 123 124°C (Lit.11 mp 123°C).
Mps (uncorrected) were determined on a Büchi 510 micro melting
point apparatus. IR spectra were recorded on a Shimadzu 8201 PC
IR (KBr): = 920.0, 1039.6, 1062.7, 1107.1, 1369.4, 2856.4,
2922.0, 3273.0 cm–1.
1
spectrophotometer. H and 13C NMR spectra were recorded on a
Bruker FT-200 spectrometer, using (CD3)2SO as solvent. Chemical
shifts ( ) are reported in ppm relative to internal TMS. Mass spectra
were performed on a Shimadzu GCMS-QP 5000 spectrometer. Ele-
mental analyses were performed at the Galbraith Laboratories, Inc.,
Knoxville, U.S.A. GLC analyses were performed on a Shimadzu
GC-17AATF chromatograph equipped with a methyl silicone cap-
illary column (30 m 0.32 mm, 0.25 m film thickness) and flame
ionization detector. Isolated and authenticated compounds were
used as internal standard to perform quantitative GC analyses. Col-
umn chromatography was performed on silica gel (70 230 mesh
ASTM).
1H NMR [200 MHz, (CD3)2SO]: = 1.31–1.57 (m, 6H), 1.61–1.78
(m, 4H), 3.27–3.48 (m, 4H), 3.61 (s, 4H), 4.47 (t, 2H).
13C NMR [50 MHz, (CD3)2SO]: = 22.11, 25.19, 32.25, 60.63,
60.79, 61.19, 96.88.
MS: m/z (%) = 216 (M+, 6), 187 (13), 173 (96), 160 (4), 125 (5), 101
(6), 99 (14), 83 (34), 71 (40), 55 (100), 41 (72).
Anal. Calcd for C11H20O4 (216.3): C, 61.08; H, 9.32; O, 29.59.
Found: C, 61.01; H, 9.61; O, 29.38.
2-Methyl-2-phenyl-1,3-dioxane-5,5-dimethanol (3b)
Mp 126–127°C.
All reactants and solvents were of analytical grade. Benzene and
DMF were double distilled and stored on molecular sieves (4 Å).
High purity p-toluenesulfonic acid monohydrate was dried at 100
° C under vacuum prior to use. Pentaerythritol was purified by sub-
limation according to a reported procedure.17 Cyclohexanone, ace-
tophenone, acetone, 2-butanone, 3-methyl-2-butanone and 3,3-
dimethyl-2-butanone were distilled and dried as previously report-
ed.17,18
IR (KBr): = 675.0, 707.8, 871.8, 1001.0, 1019.8, 1040.0, 1407.9,
2871.8, 2954.7, 3323.1 cm–1.
1H NMR [200 MHz, (CD3)2SO]: = 1.41 (s, 3H), 3.07–3.67 (m,
6H), 4.10–4.23 (m, 2H), 4.27 (t, 1H, J = 6.0 Hz), 4.55 (t, 1H, J = 6.0
Hz), 7.33–7.48 (m, 5H).
13C NMR [50 MHz, (CD3)2SO]: = 31.31, 45.46, 59.77, 61.06,
61.20, 63.11, 99.47, 126.20, 127.55, 128.60, 141.06.
MS: m/z (%) = 223 (M+ – 15, 11), 161 (14), 121 (11), 105 (38), 71
Preparation of Ketals; General Procedure
Method A: Synthesis of ketals attended by reactive distillation.
The reaction was carried out in a 100-mL three-neck round-bot-
tomed flask equipped with a magnetic stirring bar and a Dean-Stark
trap topped with a condenser. A catalytic amount of p-toluene-
sulfonic acid monohydrate (5% w/w relative to the ketone) was add-
ed to a dispersion of pentaerythritol (22 mmol) and benzene–DMF
(40:60, 50 mL) at r.t. The well-stirred dispersion was warmed to 80
°C until complete dissolution, and the ketone (15 mmol) was added
dropwise while the reaction mixture was heated to 115 °C. The H2O
formed during the reaction was removed by reactive distillation and
collected in the Dean-Stark trap. The reaction was stopped when
there was no further increase in the collected H2O. After cooling,
the reaction mixture was poured into H2O (30 mL), neutralized with
K2CO3 and extracted with CH2Cl2 (3 30 mL). The organic layer
was dried (MgSO4) and evaporated in vacuo. Compounds 3a to 3c
were isolated as white solids by Kugelrohr distillation and purified
by column chromatography on silica gel (Et2O–light petroleum,
80:20). Conversions, selectivities and yields are summarized in
Table 1 (entries 1 to 3).
(10), 44 (64), 32 (100).
Anal. Calcd for C13H18O4 (238.3): C, 65.52; H, 7.61; O, 26.86.
Found: C, 65.31; H, 7.84; O, 26.85.
2,2-Dimethyl-1,3-dioxane-5,5-dimethanol (3c)
Mp 125–126°C (Lit.15 mp 126–127°C).
IR (KBr): = 673.1, 827.4, 1002.9, 1020.1, 1041.2, 1369.4, 2877.6,
2941.2, 2995.2, 3323.1 cm–1.
1H NMR [200 MHz, (CD3)2SO]: = 1.31 (s, 6H), 3.34–3.39 (m,
6H), 3.60 (s, 2H), 4.23 (t, 1H, J = 5.9 Hz), 4.50 (t, 1H, J = 5.9 Hz).
13C NMR [50 MHz, (CD3)2SO]: = 23.78, 45.54, 60.47, 61.03,
61.65, 97.04.
MS: m/z (%) = 161 (M+ – 15, 11), 113 (3), 83 (5), 71 (21), 59 (60),
43 (100).
Anal. Calcd for C8H16O4 (176.2): C, 54.52; H, 9.15; O, 36.32.
Found: C, 54.47; H, 9.51; O, 36.02.
2-Ethyl-2-methyl-1,3-dioxane-5,5-dimethanol (3d)
Mp 73–74°C.
Method B: Synthesis of ketals attended by a dehydrating agent.
The upper part of the apparatus described above was provided with
a jacket containing anhyd MgSO4 to remove the H2O formed during
the reaction by means of a dehydrating agent. The reaction was car-
ried out as above, starting from the same amount of reagents and
solvents. Compounds 3d to 3f were obtained as white solids. Con-
versions, selectivities and yields are summarized in Table 1 (entries
4 to 6).
IR (KBr): = 721.3, 866.0, 898.8, 1042.8, 1043.4, 1365.5, 2881.5,
2925.8, 2974.0, 3259.5 cm–1.
1H NMR [200 MHz, (CD3)2SO]: = 0.85 (t, 3H, J = 7.0 Hz), 1.27
(s, 3H), 1.57–1.69 (m, 2H), 3.33–3.43 (m, 4H), 3.53–3.69 (m, 4H),
4.38–4.52 (m, 2H).
13C NMR [50 MHz, (CD3)2SO]: = 7.72, 19.80, 30.12, 60.33,
60.79, 61.11, 61.38, 98.55.
MS: m/z (%) = 175 (M+ – 15, 22), 161 (51), 127 (2), 101 (9), 83 (34),
Method C: Synthesis of ketals using a Soxhlet apparatus.
Pentaerythritol (22 mmol), benzene–DMF (40:60, 50 mL) and a cat-
alytic amount of p-toluenesulfonic acid monohydrate (5% w/w rel-
ative to the ketone) were placed into the lower container of a
Soxhlet apparatus. The upper part of the apparatus was provided
with a jacket containing anhyd MgSO4. The mixture was heated to
80 °C until complete dissolution, then the ketone (15 mmol) was
added dropwise, and finally the reaction mixture was kept at 115 °C.
73 (61), 71 (55), 57 (68), 55 (43), 43 (100).
Anal. Calcd for C9H18O4 (190.2): C, 56.82; H, 9.54; O, 33.64.
Found: C, 56.52; H, 9.79; O, 33.69.
2-Methyl-2-(1-methylethyl)-1,3-dioxane-5,5-dimethanol (3e)
Mp 76–77°C.
Synthesis 2001, No. 7, 1093–1097 ISSN 0039-7881 © Thieme Stuttgart · New York

PAPER
Selective Synthesis of Monoketals from Pentaerythritol and Ketones
1097
IR (KBr): = 549.7, 771.5, 852.5, 896.8, 1038.2, 1062.5, 1379.0,
1440.7, 2877.6, 2960.0, 3182.3 cm–1.
1H NMR [200 MHz, (CD3)2SO]: = 0.86 (s, 3H), 0.90 (s, 3H), 1.21
(s, 3H), 1.99 (dd, 1H), 3.28–3.72 (m, 8H), 4.43–4.50 (m, 2H).
13C NMR [50 MHz, (CD3)2SO]: = 15.46, 16.67, 34.38, 38.66,
60.25, 61.03, 61.22, 100.06.
References
(1) (a) Research Assistant from CONICET (b) Research
Assistant from U. N. L (c) Professor at U. N. L. and
Member of CONICET’s Research Staff
(2) Meskens, F. A. J. Synthesis 1981, 501.
(3) Schelhaas, M.; Waldmann, H. Angew. Chem., Int. Ed. Engl.
1996, 35, 2056.
MS: m/z (%) = 189 (M+ – 15, 5), 161 (17), 141 (2), 119 (2), 101 (3),
87 (9), 83 (10), 71 (15), 55 (10), 43 (100).
(4) Brown, J. J.; Lenhard, R. H.; Berstein, S. J. Am. Chem. Soc.
1964, 86, 2183.
Anal. Calcd for C10H20O4 (204.3): C, 58.79; H, 9.87; O, 31.33.
Found: C, 58.68; H, 10.15; O, 31.17.
(5) Coe, J. W.; Roush, W. R. J. Org. Chem. 1989, 54, 915.
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Organic Synthesis, 2nd ed.; Wiley: New York, 1991, 68–86.
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Organic Synthesis, 2nd ed.; Wiley: New York, 1991, 178–
198.
(7) Tanaka, N.; Masaki, Y. Synlett 1999, 12, 1960.
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1964, 10, 95.
2-Methyl-2-(1,1-dimethylethyl)-1,3-dioxane-5,5-dimethanol (3f)
Mp 145–146°C.
IR (KBr): = 669.3, 873.7, 1016.4, 1041.5, 1407.9, 2885.3, 2943.2,
3327.0 cm–1.
1H NMR [200 MHz, (CD3)2SO]: = 0.93 (s, 9H), 1.31 (s, 3H), 3.39
(s, 8H), 4.09–4.31 (m, 2H).
13C NMR [50 MHz, (CD3)2SO]: = 24.65, 45.46, 59.98, 61.17,
61.55.
MS: m/z (%) = 203 (M+ – 15, 13), 161 (39), 119 (5), 101 (20), 88 (6),
83 (27), 71 (31), 57 (71), 43 (100).
Anal. Calcd for C11H22O4 (218.3): C, 60.52; H, 10.16; O, 29.32.
Found: C, 60.28; H, 10.41; O, 29.31.
(12) Böeseken, J. Recl. Trav. Chim. Pays-Bas 1922, 41, 722.
(13) von Orthner, L.; Freyss, G. Justus Liebigs Ann. Chem. 1930,
484, 131.
(14) von Orthner, L. Chem. Ber. 1928, 61, 116.
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(16) Schaeffer, J. R.; Stevens, R. E. J. Org. Chem. 1973, 38,
1241.
Acknowledgement
We are grateful to the Consejo Nacional de Investigaciones Cientí-
ficas y Tecnológicas (CONICET) and the Universidad Nacional del
Litoral (U.N.L.) of Argentina for financial support.
(17) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification
of Laboratory Chemicals; Pergamon Press: London, 1980.
(18) Vogel, A. I. Vogel’s Textbook of Practical Organic
Chemistry, 5th ed.; Longman: London, 1989.
Synthesis 2001, No. 7, 1093–1097 ISSN 0039-7881 © Thieme Stuttgart · New York