A convenient and practical method for the selective benzoylation of primary hydroxyl groups using microwave heating

Article abstract of DOI:10.1016/S0040-4020(01)00592-0

A convenient method for the selective protection of primary hydroxyl groups in 1,n diols is described. The use of microwave heating is shown to be advantageous.

Full text of DOI:10.1016/S0040-4020(01)00592-0

TETRAHEDRON
Pergamon
Tetrahedron 57 (2001) 6305±6310
A convenient and practical method for the selective benzoylation
of primary hydroxyl groups using microwave heating
Stephen Caddick,a,p Andrew J. McCarrolla and David A. Sandhamb
aThe Chemistry Laboratory, School of Chemistry, Physics and Environmental Sciences, University of Sussex, Falmer,
Brighton BN1 9QJ, UK
bNovartis Horsham Research Centre, Wimblehurst Road, Horsham, West Sussex RH12 5AB, UK
Received 20 February 2001; revised 3 May 2001; accepted 24 May 2001
AbstractÐA convenient method for the selective protection of primary hydroxyl groups in 1,n diols is described. The use of microwave
heating is shown to be advantageous. q 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction
example of a `microwave-speci®c reaction' had been
uncovered.9 Although the amount of tin was reduced, it
was not completely eliminated, and the reactions were not
always completely regioselective.
The selective protection of hydroxyl groups is a valuable
technique in organic synthesis.1,2 The benzoyl moiety (Bz)
is frequently employed as a protecting group owing to its
stability to hydrolysis, and selective benzoylation of polyols
has been described by many groups. Reagents such
as N-benzyl-2-(6-methyl)pyridinecarbamoyl chloride,3 3-
benzoylthiazolidine-2-thione,4 and 1-(benzoyloxy)benzo-
triazole5 have been designed speci®cally for this purpose,
however these can be expensive or awkward to prepare, and
selectivity is not always excellent. An alternative approach
has seen tin oxides and ethers used to preform stannylene
complexes from 1,2 or 1,3-diols.6 The nucleophilicity of one
of the oxygen atoms is enhanced, leading to regioselective
protection. The method has been widely used, especially in
carbohydrate chemistry,7 but there are still several draw-
backs: tin compounds are extremely toxic, the stannylene
intermediate has to be preformed, and again selectivity is
not always exceptional. In an interesting development, it
was reported that by using microwave conditions benzoyl-
ation could be effected using substoichiometric quantities
of the dibutyltin oxide.8 Notably, it appeared that a rare
We wish to report that primary hydroxyl groups can be
protected with complete selectivity in the presence of
secondary hydroxyl groups using inexpensive reagents in
a one-pot reaction. The use of microwave irradiation
enables precise heating time and thus better control, facili-
tating the interception of a reaction at an intermediate point.
2. Results and discussion
The starting point for our investigations was the afore-
mentioned literature report of benzoylation under micro-
wave conditions.8 We were intrigued by the possibility
that the regioselective stannane-mediated monoprotection
of diols could be selectively accelerated in the microwave
oven, and set about investigating the reaction further using
1,2-decanediol (Scheme 1). The microwave oven employed
was a commercially available machine that had been
Scheme 1.
Keywords: acylation; diols; microwave heating; protecting groups.
Corresponding author. Tel.: 144(0)-1273-606755; fax: 144(0)-1273-677196/678734; e-mail: s.caddick@sussex.ac.uk
p
0040±4020/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(01)00592-0

6306
S. Caddick et al. / Tetrahedron 57 (2001) 6305±6310
Table 1. Protection of 1,2-decanediol 1a with 1 equiv. of PhCOCl
Entry
Catalyst
Base
Reaction time (min)a
Solvent
Yieldsb
2a
3a
4a
S.M.
1
2
3
4
5
6
7
Bu2SnO
None
None
Et3N
Et3N
Et3N
Et3N
Collidine
K2CO3
Et3N
9 £ 1
9 £ 1
5 £ 5
1 £ 40
9 £ 1
9 £ 1
1 £ 9
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
77
29
24
32
77c
14
69
16
0
0
1
0
0
0
0
0
0
0
0
7
71
76
67
23
82
16
None
Bu2SnO
Bu2SnO
Bu2SnO
4
15
a
9£1 min denotes that the reaction mixture was irradiated 9 times for 1 min each time, punctuated by 30 s intervals.
b
c
Yields refer to NMR yields.
Yield increased to 92% by use of a 10% excess of collidine and benzoyl chloride.
adapted to allow a re¯ux condenser to be ®tted, which
(entry 3) and surprisingly, there was little bis-benzoylation,
leading to 4a. Thus the procedure could be optimised by
using a reaction time of 7 min leading to quantitative yield
of desired product (entry 5).
enabled prolonged irradiation periods. Crude yields and
ratios were determined by 1H NMR spectroscopic analysis,
by comparison of integration of the peaks due to the HnCO
protons occurring between 3.5 and 5.8 ppm.
The heating conditions provided by microwave ovens differ
greatly from those provided by conventional heating
sources. One consequence is that heating is almost instan-
taneous in the microwave oven, allowing a more accurate
measure of heating time. This phenomenon was simulated
using conventional conditions by immersing the reaction
mixture in an oil bath preheated to 1408C, and timing
from the moment the internal temperature reached 1098C
(method B). The technique led to almost identical results
(entry 6) as those obtained using microwave irradiation. The
slight discrepancy occurs because of the inertia present in
heat transfer from the oil bath to the reaction mixture;
heating is not instant, and heating time is therefore slightly
longer than indicated. Once the reactions were removed
from the heat source they were allowed to cool slowly to
room temperature after both microwave and oil bath
heating. It is worth noting that it was not necessary to
perform the reaction under nitrogen to obtain the excellent
yields shown.
The results obtained in the presence or absence of dibutyltin
oxide as a catalyst were generally in accord with the litera-
ture, although we only observed mono-benzoylated product
in the absence of a tin catalyst (entry 2). Prolonged micro-
wave heating gave only marginal advantage in terms of
product yields. We examined the use of 2,4,6-collidine as
base (entry 5), which has been used for the selective
acetylation of diols,10 which led to high selectivity in the
tin-catalysed procedure, but use of potassium carbonate as a
heterogeneous base (entry 6)11 gave poor results. It was also
established that the discontinuous sequences of radiation
were not necessary (entry 7, cf. entry 1).
We were intrigued by the apparent selectivity of the micro-
wave-mediated reactions using 1 equiv. of triethylamine
and benzoyl chloride (Table 1, entry 2), and undertook to
utilise this in a synthetically useful transformation. The
results of these further studies are presented in Table 2.
Increasing the conversion was a priority but as previously
noted, extended microwave irradiation did not lead to
improved yield. However improved conversion could be
achieved simply by increasing the concentration of the
triethylamine and benzoyl chloride by a factor of ®ve
In control experiments, using only 1 equiv. of reagents
under conventional conditions, selectivity was shown to
be poor, and extended reaction times were required (entry
7). Extended exposure to microwave conditions using an
Table 2. Optimising the benzoylation of 1,2-decanediol
Entry
Methoda
Solvent
Time (min)
NEt3, PhCOCl (equiv.)
Yieldsb
3a
2a
4a
S.M.
1
2
3
4
5
6
7
8
A
A
A
A
A
B
B
A
C
A
B
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
MeCNc
MeCN
9
9
9
5
7
1
3
5
5
5
5
1.1
5
5
29
76
95
80
0
,1
0
0
0
0
17
0
0
0
0
0
5
0
0
71
23
0
20
0
0
9
0
0
.99 (90)
94 (87)
61
7
6
8220
240
60
0.5
0.5
13
35
5
0
0
65
95
.99
.99
9
10
11
5
5
0
0
0
a
Conditions A: microwaved continuously at full power for time indicated. B: mixture lowered into oilbath at 1408C (1108C with acetonitrile as solvent).
Timing started from moment reaction temperature reached 1098C. C: mixture heated from rt.
Yields refer to NMR yields. Figures in parentheses refer to isolated yields.
b
c
Acetonitrile (MeCN) was lost rapidly on microwave irradiation.

S. Caddick et al. / Tetrahedron 57 (2001) 6305±6310
6307
Table 3. Protection and isolation of diols
Entry
Diol
Method
Time
(min)
NEt3,
PhCOCl (equiv.)
Yieldsa
Secondary
benzoate
Primary
benzoate
Bisbenzoate
S.M.
1
2
A
B
7
7
5
5
.99 (90)
94 (87)
0
0
0
6
0
0
3
4
A
B
7
7
5
5
.99 (90)
94 (82)
0
0
0
6
0
0
5
6
A
B
7
7
5
5
.99 (84)
91(90)
0
0
0
9
0
0
7
8
A
B
7
7
5
5
.99 (88)
95 (82)
0
0
0
5
0
0
9
A
A
A
9
9
9
5
5
5
0
±
±
±
0
.99 (96)
0
0
10
11
100
,20
Conditions A: microwaved continuously at full power for time indicated. B: mixture lowered into oil bath at ,1408C. Timing started from the moment reaction
temperature reached 1098C.
Yields refer to NMR yields. Figures in parentheses refer to isolated yields.
a
excess of reagents led to signi®cant formation of 1,2-
bis(benzoyloxy)decane 4a (entry 8), indicating that micro-
wave irradiation does not inhibit this reaction. Stuerga and
co-workers have demonstrated that improved selectivity in
the sulfonation of naphthalene can be induced under micro-
wave conditions owing to the heating rate in the microwave
oven.12 In the present study however, we were able to show
that the observed selectivity is not a result of the rate of
heating. When we allowed the reaction mixture to warm
slowly from room temperature to re¯ux under conventional
heating conditions we obtained similar results (cf. entries 3,
6 and 9).
this reaction. The results, summarised in Table 3 indicate
that very good selectivity can be achieved in simple diols
containing a primary and a secondary hydroxyl group
(entries 1±8). Crude yields were again determined from
NMR spectroscopic integration13±17 and isolated yields of
over 85% were obtained in most cases. The reactions
performed using the microwave oven (entries 1, 3, 5 and
7) gave generally higher yields than oil bath heating re¯ect-
ing the inherent inef®ciency of heating under conventional
conditions. Not unexpectedly, both of the primary hydroxyl
groups of 1,4-butanediol were benzoylated (entry 9).
However under the optimised conditions only bis-benzoyl-
ation was observed from reaction of dimethyl tartrate,
whereas 2,3-butanediol was almost unreactive.
The reaction was also performed in acetonitrile (entries 10
and 11). Even with the adapted apparatus, cooling was not
ef®cient enough to prevent immediate rapid solvent loss, a
serious disadvantage. However, good crude yields were
observed after only 30 s of irradiation. A second dis-
advantage was noted using this solvent; unlike in toluene,
the triethylamine hydrochloride by-product is soluble and
cannot simply be removed by ®ltration.
Attempts to selectively protect methyl a-d-glucopyranoside
10, were unsuccessful due to its insolubility in toluene. It is
also only very sparingly soluble in acetonitrile, and this does
not facilitate the selective protection, especially if the
1
product is more soluble. The H NMR spectrum of the
product mixture was complex, but was consistent with
tris-protected methyl 2,3,6-tribenzoyl a-d-glucopyranoside
11 as the main product.7a
Our investigations were extended to determine the scope of

6308
S. Caddick et al. / Tetrahedron 57 (2001) 6305±6310
Scheme 2.
product ratio (2±3±4±1ˆ2:1:4:0.2) showed that reaction
of the secondary hydroxyl group was more favourable in
the absence of triethylamine. An alternative rationale, in
which R3N acts as a Brùnsted base, has been proposed by
Yamamoto and co-workers to explain selectivity differences
for related transformations using a variety of bases.21
The excellent selectivity observed compares well with that
observed using tin-containing compounds, in which selec-
tivity is compromised by post-acylation equilibrium in
simple diols.22
The use of microwave ovens to perform organic transforma-
tions has aroused increasing interest since early reports
describing the signi®cant rate enhancements that are pos-
sible.18 The use of microwaves often results in a reduction in
reaction time and gives cleaner reactions than those
performed under standard conditions. Our results show
that microwave ovens can be also used to precisely control
heating time, to bene®cial effect.
4. Conclusions
In summary, we have shown that the benzoylation of 1,n
diols can be achieved with high regioselectivity at the
primary hydroxyl group by use of a ®vefold excess of
benzoyl chloride and triethylamine both of which are inex-
pensive, commercially available reagents. Careful control
of heating time prevents undesired further reaction, and
this is most readily achieved using microwave irradiation.
The stoichiometry and mode of heating appears to have
an important effect on the regiochemical outcome of the
reaction.
3. Mechanism
Given the myriad procedures reported for the selective
monoprotection of primary alcohols it is surprising that
this simple selective mono-protection protocol has not
previously been described as a general procedure. Recently,
Paquette noted that protection of an isotaxane tetraol was
completely selective at room temperature when performed
using 20 equiv. of benzoic anhydride in pyridine; only when
DMAP was added were secondary and tertiary hydroxyl
groups acylated.19 However, this selectivity was not further
investigated. Sharpless noted a concentration dependence
during the protection of phenylethylene glycol using
p-toluenesulfonyl chloride,20 and also observed that the
regioselectivity could be enhanced by using more sterically
encumbered arenesulfonyl chlorides.20
5. Experimental
Microwave irradiation was carried out in a domestic oven
(Sharp, 2450 MHz, 800 W), operated at full power that had
been modi®ed to incorporate a condenser. The diols, methyl
a-d-glucopyranoside and benzoyl chloride were available
commercially (from Aldrich or Lancaster) and were used as
received. For all other general experimental procedures see
previous work.23
We considered the different results obtained under conven-
tional conditions using 1 and 5 equiv. of reagents (Table 2,
entries 6 and 7) could only be accounted for if different
mechanisms were in operation for the protection of the
primary and secondary hydroxyl group. Under the condi-
tions employed, reaction of triethylamine with benzoyl
chloride could be accelerated potentially leading to the
formation of a more sterically hindered acylammonium
electrophile 12 which would undergo reaction selectively
with the primary hydroxyl group of the diol (Scheme 2).
We considered that any other benzoylation product, such
as 3 (Table 2, entry 7), was formed by the direct reaction
of the secondary hydroxyl group with benzoyl chloride.
That this suggestion was viable was con®rmed by perform-
ing the protection of 1,2-decanediol in the absence of
5.1. Typical procedure for the dibutyltin oxide catalysed
selective protection of diols under microwave conditions
To a mixture of 1,2-decanediol (0.174 g, 1.0 mmol) and
triethylamine (0.101 g, 1.0 mmol) in toluene (30 mL) was
added dibutyltin oxide (0.05 g, 0.25 mmol) and benzoyl
chloride (0.141 g, 5.0 mmol). The ¯ask was attached to a
re¯ux condenser in an adapted microwave oven, and
subjected to microwave irradiation (800 W) for 1 min.
The mixture was allowed to stand for 30 s, and then
subjected to a further minute of irradiation. The process
was repeated until the total irradiation period was 9 min,
then the mixture was allowed to cool, then ®ltered and
concentrated at reduced pressure. Column chromatography
(petroleum ether±ethyl acetate) yielded 1-benzoyloxy-2-
hydroxydecane 2a (0.214 g, 77%) as white crystals, and
²
triethylamine using 5 equiv. of benzoyl chloride. The
²
The reaction mixture was re¯uxed for 4 h, and analysed in the normal
way.

S. Caddick et al. / Tetrahedron 57 (2001) 6305±6310
6309
2-benzoyloxy-1-hydroxydecane 3a (0.44 g; 16%) as a
colourless oil.
5.3.1. 1,2-Bis(benzoyloxy)decane 4a. Puri®ed by column
chromatography to give a colourless oil, Rfˆ0.74 (4:1 petro-
leum ether±ethyl acetate) nmax (®lm) 1725 cm21 (CvO).
dH 0.87 (3H, t, Jˆ6.3 Hz, CH3), 1.00±1.55 (12H, m),
1.76±1.87 (2H, m, CH2), 4.43±4.59 (2H, m, OCH2),
5.48±5.53 (1H, m, OCH), 7.40±7.60 (6H, m, ArH), 8.00±
8.10 (4H, m, ArH). dC 14.2, 22.7, 25.2, 29.2, 29.4, 29.5,
31.0, 31.8, 65.8, 72.3, 128.4, 129.7, 129.9, 130.3, 133.1,
133.1, 166.2, 166.4. (Found MH1 383.2226. C24H30O41H
requires 383.2226.)
5.1.1. 1-Benzoyloxy-2-hydroxydecane. Rfˆ0.53 (4:1
petroleum ether±ethyl acetate). mp 39±408C. nmax (Nujol)
3507 (O±H), 1697 cm21 (CvO). dH 0.88 (3H, m, CH3),
1.0±1.5 (12H, m), 2.15 (1H, s, OH), 3.86±4.04 (1H, m,
CH), 4.23 (1H, dd, J1ˆ7.2 Hz, J2ˆ11.5 Hz, OHCH), 4.40
(1H, dd, J1ˆ7.2 Hz, J2ˆ11.5 Hz, OHCH), 7.40±7.60 (3H,
m, ArH), 8.02±8.10 (2H, m, ArH). dC 13.8, 22.4, 25.1, 29.0,
29.2, 29.3, 31.6, 33.2, 69.0 (all CH2), 69.8 (CH), 128.1,
129.4, 129.6, 132.9, 166.5. (Found MH1 279.1881.
C17H26O31H requires 279.1960.)
5.3.2. 1-Benzoyloxy-3-hydroxybutane.16 Puri®ed by
column chromatography to give a colourless oil, Rfˆ0.25
(4:1 petroleum ether±ethyl acetate). nmax (®lm) 3417 (O±
H), 1719 cm21 (CvO). dH 1.20 (3H, Jˆ6.2 Hz, CH3),
1.70±1.94 (2H, m, CCH2C), 2.10 (1H, bs, OH), 3.86±3.96
(1H, m), 4.26±4.36 (1H, m), 4.50±4.59 (1H, m), 7.34±7.51
(2H, m, ArH), 7.46±7.53 (1H, m), 7.94±8.00 (2H, m, ArH).
5.1.2. 2-Benzoyloxy-1-hydroxydecane. Rfˆ0.45 (4:1
petroleum ether±ethyl acetate). nmax (®lm) 3433 (O±H),
1719 cm21 (CvO). dH 0.88 (3H, t, Jˆ6.7 Hz, CH3), 1.1±
1.5 (12H, m), 1.64±1.80 (2H, m, CH2), 2.10 (1H, t, Jˆ
6.3 Hz, OH), 3.72±3.88 (2H, m, OCH2), 5.12±5.22 (1H,
m, CH), 7.42±7.62 (3H, m, ArH), 8.02±8.10 (2H, m,
ArH). dC 13.8, 22.3, 25.1, 28.9, 29.1, 29.2, 30.4, 31.5,
64.8, 128.1, 129.4, 129.9, 132.8, 166.7. (Found MH1
279.1927. C17H26O31H requires 279.1960.)
5.3.3. 1-Benzoyloxy-4-hydroxypentane.16 Puri®ed by
column chromatography to give a colourless oil, Rfˆ0.21
(4:1 petroleum ether±ethyl acetate). nmax (®lm) 3418 (O±
H), 1718 cm21 (CvO). dH 1.24 (3H, d, Jˆ6.2 Hz, CH3),
1.57±1.65 (2H, m, CH2CH(OH)), 1.78±1.98 (2H, m, CH2),
2.10 (1H, bs, OH), 3.85±3.95 (1H, m, CHOH), 4.36 (2H, t,
Jˆ6.6 Hz, CH2OCO), 7.38±7.47 (2H, m, ArH), 7.54±7.60
(1H, m, ArH), 8.03±8.06 (2H, m, ArH).
5.2. Typical procedure for the selective protection of
diols under microwave conditions
To a mixture of 1,2-decanediol (0.174 g, 1.0 mmol) and
triethylamine (0.505 g, 5.0 mmol) in toluene (30 mL) was
added benzoyl chloride (0.705 g, 5.0 mmol). The ¯ask was
attached to a re¯ux condenser in an adapted microwave
oven, and subjected to microwave irradiation (800 W) for
7 min. The mixture was allowed to cool, then ®ltered and
concentrated under reduced pressure with a minimum of
5.3.4. 1,4-Bis(benzoyloxy)butane.13 Puri®ed by column
chromatography (on neutral alumina) to give white crystals,
Rfˆ0.58 (4:1 petroleum ether±ethyl acetate on silica gel),
mp 79±80.58C (lit.13 798C) nmax (Nujol) 1721 cm21
(CvO). dH 1.93±1.99 (4H, m, 2£CH2), 4.36, (4H, m,
2£CH2O), 7.40±7.48 (4H, m, ArH), 7.52±7.60 (2H, m,
ArH), 8.01±8.08 (2H, m, ArH). dC 25.6, 64.5, 128.4,
129.5, 130.2, 132.9, 166.6.
³
heating (,408C). Following 1H NMR spectroscopic analy-
sis, the crude mixture was washed rapidly through a short
silica plug with copious petroleum ether, and the product
removed from the column using ethyl acetate. A further
column chromatography (petroleum ether±ethyl acetate)
yielded pure 1-benzoyloxy-2-hydroxydecane (0.250 g,
90%) as white crystals.
Acknowledgements
We would like to thank Novartis for funding this project and
Clive Aldcroft for performing mass spectral analysis. We
also gratefully acknowledge EPSRC Mass Spectrometry
Service at Swansea and Drs Avent and Abdul-Sada for
their contribution. Financial support of our research
programme from Glaxo Wellcome, AstraZeneca, Smith-
Kline Beecham, P®zer, BBSRC, EPSRC and AICR (St
Andrews) is also gratefully acknowledged.
5.3. Typical procedure for the selective protection of
diols under re¯ux conditions
To a mixture of 1,2-butanediol (0.090 g, 1.0 mmol) and
triethylamine (0.505 g, 5.0 mmol) in toluene (30 mL) was
added benzoyl chloride (0.705 g; 5.0 mmol). The ¯ask was
lowered into a hot oil bath at 1408C. When the internal
temperature reached 1098C, the mixture was re¯uxed for a
further 7 min, then removed from the heat and allowed to
cool. Puri®cation was performed as above to give pure
1-benzoyloxy-2-hydroxybutane15 2b (0.146 g; 82%) as a
colourless oil. Rf 0.33 (4:1 petroleum ether±ethyl acetate).
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