Facile and highly selective monoacylation of symmetric diols adsorbed on silica gel with acetyl chloride

Article abstract of DOI:10.1039/a707753j

Monoacetylated alcohols of symmetric 1,n-diols are synthesized quantitatively by refluxing a suspension of the diols adsorbed on silica gel with acetylchloride.

Full text of DOI:10.1039/a707753j

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Facile and highly selective monoacylation of symmetric diols adsorbed on silica
gel with acetyl chloride
Haruo Ogawa,*a† Misa Amanoa and Teiji Chiharab
a Department of Chemsitry, Tokyo Gakugei University, Koganei, Tokyo 184, Japan
b The Institute of Physical and Chemical Research (RIKEN), Wako, Saitama 351-01, Japan
Monoacetylated alcohols of symmetric 1,n-diols are synthe-
sized quantitatively by refluxing a suspension of the diols
adsorbed on silica gel with acetylchloride.
AcCl were introduced to 50–100 ml of cyclohexane and
refluxed for 2 h. The reaction period of 2 h was sufficient for the
reaction to reach completion. After the reaction the mixture was
filtered and the solid was washed with distilled water and DMF.
The washings plus the filtrate were concentrated, and the
products were analyzed by GLC with higher hydrocarbons such
as dodecane and heptadecane used as an internal standard. The
products were spectrometrically identified via comparison with
the authentic samples.
The application of solid adsorbents such as alumina and silica
gel as solid supports in organic synthesis affords a new
procedure for selective reactions1 involving oxidation,2 alkyla-
tion,3 condensation,4 acetylation5 and monomethyl esterifica-
tion.6 The significant potential of adsorbents is due to the milder
reactions conditions and simpler work-up required and the
selective organic transformations that they allow. It is important
for organic chemists to develop methods that permit the
selective protection or functionalization of one functional group
of a bi- or multi-functional molecule. Monoprotection of
polyols is achieved in some cases by carefully controlled
reaction conditions,7 continuous extraction,8 the use of silica
gel,9 alumina,10 phase-transfer catalysts11 and insoluble poly-
mer supports,12 or via the formation of cyclic compounds.13
Protection of hydroxy groups by acylation is common in
organic syntheses and here we report the facile and highly
selective monoacetylation of symmetric diols with acetyl
chloride (AcCl) by use of silica gel.
The results of acetylation of symmetric diols are listed in
Table 1. According to the adsorption method the monoacety-
lated products of 1,n-diols were quantitatively obtained, while
in the case of cyclohexane-1,4-diol the products were obtained
in lower yield. Even with a larger amount of decane-1,10-diol in
the adsorption sample (3.2 3 1023 mol g21 SiO2), the same
result (quantitative monoacetylation) was observed. This ad-
sorption method is insensitive to the amount of AcCl added, e.g.
2.5–20 equiv. of AcCl were effective for the quantitative
formation of the monoacetylated alcohol of decane-1,10-diol.
The dependence of selectivity on reaction temperature was
investigated in order to improve the selectivity in the reaction of
cyclohexane-1,4-diol, which showed the lowest selectivity.
Higher selectivity was observed with a decrease in the
temperature. Selectivity was improved to 73% at 0 °C, and to
76% at 0 °C in hexane suspension (Table 2). Silica gel is
considered to play a role as a protecting reagent for the
counterpart hydroxy group of the symmetric diol, which is
presumably adsorbed as a monomolecular layer on the surface
of silica gel as in the case of the methylation of decan-1-ol on
silica gel with CH2N2.14 This explains why a large excess of
AcCl yields only the monoacetylated alcohols of 1,n-diols. No
further evidence has been obtained at the present time, however,
this adsorption method should be applicable to the selective
monoprotection or monofunctionalization of polyols.
Acetylation of symmetric diols adsorbed on silica gel was
performed via the following method (adsorption method)
(Scheme 1). Alcohols were adsorbed on silica gel (C-200, Wako
Chemicals) as follows: 1 g of silica gel was added to a Et2O
solution of the alcohol; solvent was then eliminated under
reduced pressure. The obtained solid (adsorption sample) and
AcCl
HO(CH2)nOH / SiO2
AcO(CH2)nOH
cyclohexane
reflux, 2 h
Scheme 1
Table 1 Monoacetylation of symmetric diols adsorbed on silica gela
Amount of
AcClb (in
Maximum
homogeneous
reaction)
(equiv.)
Yield (%)
monoacetate diacetate
yield in
Selectivityc homogeneous
(%)
Substrate
reactiond (%)
Butane-1,4-diol
Hexane-1,6-diol
Octane-1,8-diol
Decane-1,10-diol
Decane-1,10-diole
Decane-1,10-diolf
Dodecane-1,12-diol
Hexadecane-1,16-diol
Cyclohexane-1,4-diol
trans-Cyclohexane-1,4-diol
Benzene-1,4-dimethanol
9.0 (2.0)
5.0 (7.0)
1.2 (7.0)
2.5 (20.0)
2.5 (20.0)
3.0 (20.0)
3.0 (3.0)
10.0 (5.0)
4.0 (4.2)
4.0 (7.0)
3.2 (3.0)
99.5
99.5
99.8
99.8
98.3
99.0
99.9
99.9
29.9
39.9
36.0
0.0
0.0
0.0
0.0
1.7
1.0
0.0
0.0
100
100
100
100
98.3
99.0
100
100
42.7
52.3
58.6
54.3
56.5
56.5
56.5
66.5
30.2
32.6
43.1
52.5
40.8
30.1
47.5
50.1
74.2
a Each experiment was carried out under reflux in cyclohexane for 2 h. Adsorption sample contains 3.2 3 1024 mol g21 SiO2. b Minimum amount of AcCl.
The same results were observed by the use of AcCl in the amount from the minimum to 20 equiv. c The value of [yield of the monoacetylated alcohol/(100
2 intract diol)] 3 100. d 2.5 3 1022 m solution of diols in DMF in the presence of pyridine (1 equiv.). e Gas–solid phase reaction at room temperature in
the bsence of cyclohexane solvent. f An adsorption sample containing 3.2 3 1023 mol g21 SiO2 of the alcohol was used.
Chem. Commun., 1998
495

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17.0
0.0
0.0c
29.9
54.6
52.4
61.4
40.8
41.6
19.3
19.4
42.7
56.8
73.1
76.0
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Adsorption sample of cyclohexane-1,4-diol (3.2 3 1024 mol g21 SiO2)
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Notes and References
† E-mail: ogawah@u-gakugei.ac.jp
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Received in Cambridge, UK, 28th October 1997; 7/07753J
496
Chem. Commun., 1998