Mild and efficient method for preparation of tert-butyl esters

Article abstract of DOI:10.1055/s-2001-13356

Aliphatic and aromatic methyl esters undergo efficient transesterification at ambient temperature with potassium tert-butoxide in diethyl ether.

Full text of DOI:10.1055/s-2001-13356

658
LETTER
Mild and Efficient Method for Preparation of tert-Butyl Esters
V. A. Vasin,*a V. V. Razinb
a N.P. Ogarev Mordovian State University, 430000 Saransk, Russian Federation
b Department of Chemistry, St. Petersburg State University, 198904 St. Petersburg, Russian Federation
Fax +7 8342 327527; E-mail: vasin@mrsu.ru
Received 19 January 2001
during development of synthesis of methyl esters of bi-
cyc-lobutane-1-carboxylic acids by means of dehydroha-
logena-tion of corresponding esters of 3-halocyclobutane-
1-carbo-xylic acids by the action of potassium tert-butox-
ide.10–12 The conservation of configuration of the initial
methyl esters of geometrical isomers 13 and 1413 testifies
that base catalyzed epimerization at the -position to the
ester group does not occur to any appreciable extent under
the experimental conditions. Such epimerization usually
requires more harsh conditions.11 Exocyclic migration of
the double bond in cyclopropenes 11 and 12, which occurs
under basic conditions reported earlier by Vidal et al,14 is
not observed.
Abstract: Aliphatic and aromatic methyl esters undergo efficient
transesterification at ambient temperature with potassium tert-bu-
toxide in diethyl ether.
Key words: methyl esters, transesterification, potassium tert-bu-
toxide, tert-butyl esters
The tert-butyl ester enjoys a unique place among protect-
ing groups for carboxylic acids due to its relative resis-
tance to nucleophilic attack and its ready removal under
mild acidic conditions. Among the methods attractive for
preparation of tert-butyl esters, their preparation from
more readily or commercially available methyl or ethyl
esters by means of transesterification is represented.
Moreover, the ester-to-ester transformation is especially
useful when the parent carboxylic acids are labile or diffi-
cult to isolate.
However, despite of great number of variants of transes-
terification suitable for preparation of esters of primary
and secondary alcohols,1 the majority of them appear in-
efficient for synthesis of esters of tert-butyl and other ster-
ically hindered alcohols.2 For example, such modern
variants of transesterification using as catalyst titanium
alkoxide3 or DBU / LiBr4 are unsuitable for tert-butanol
acylation.
Scheme
It is necessary to draw to attention a number of features of
the procedure. For successful transesterification it is nec-
essary to use freshly prepared potassium tert-butoxide15
and absolute ether. Any tert-butanol in the t-BuOK or
traces of moisture in the ether can considerably lower
yields. There are also some restrictions on the structure of
the acid part of the ester. For example, we did not manage
to convert methyl esters of malonic, fumaric, and acrylic
acids into tert-butyl esters. Instead of potassium tert-bu-
toxide it is possible to use sodium tert-butoxide which is
less soluble in ether. However the necessity for using
more dilute solutions (the concentration t-BuONa < 0.16
M) creates certain inconveniences. For example, under
typical reaction conditions transformation of methyl and
ethyl benzoates into compound 3 was not complete (con-
version 60-70%) and the yields of pure product amounted
to only 38% and 27% accordingly.
Now it is possible to point out only three preparative
methods for transformation of methyl (ethyl) esters into
tert-butyl esters: 1. Action of a large surplus of tert-bu-
tanol in the presence of the catalytic additives of potassi-
um tert-butoxide and the utilization of molecular sieves;5
2. Action of 1–2 equivalents of tert-butanol and butyl-lith-
ium in THF;6 3. Action of surplus tert-butyl acetate and
catalysis by potassium tert-butoxide in THF.7
We herein suggest a very mild and efficient method for
preparation of tert-butyl esters by means of interaction of
methyl esters with 1–1.2 equivalents of potassium tert-bu-
toxide in diethyl ether. On mixing ether solutions of sub-
ο
stances at 0–20 C the reactions proceed rapidly and in
In summary, a mild and efficient method for converting
methyl (ethyl) esters into tert-butyl esters has been devel-
oped that uses available reagents and simple work up, and
thus compares favorably with known methods for prepa-
ration of tert-butyl esters.
good yield thanks to the high reactivity of t-BuOK8 and
the complete insolubility of potassium methoxide in di-
ethyl ether9 (Scheme). The method was tested on a num-
ber of examples submitted in the Table (compound 1–14).
The particular selection of examples, on which the proce-
dure of transesterification is fulfilled, is caused by our in-
terest in esters of bicyclobutane- and cyclopropene-
carboxylic acids and that the proposed method has arisen
Typical procedure for preparation of tert-buthyl esters. The
0.01 M solution of methyl ester in 10 mL of anhydrous ether at 20
οC under nitrogen and at stirring was added to 40 mL of 0.3 M so-
Synlett 2001, No. 5, 658–660 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
Mild and Efficient Method for Preparation of tert-Butyl Esters
659
Table Yields and Constants of tert-Butyl Esters 1–14
a) Yields represent isolated materials of > 98% purity (by GC) without optimisation. b) Microanalysis was in satisfactory agreement with
calc. values: C, H 0.25%. c) 2.2 Equivalents of potassium tert-butoxide.
lution of t-BuOK in anhydrous ether. The residue was formed im-
References and Notes
mediately. The stirring was continued for 20–30 min. Potassium
methoxide was filtered through the thin layer of Al2O3 and then the
tert-butyl ester was isolated from filtrate by distillation or crystalli-
zation. In the other variant of treatment of a reaction mixture for dis-
solution of potassium methoxide, cold water was added and the
organic layer was dried and evaporated. All products gave satisfac-
tory 1H NMR and were homogeneous by GC or TLC.
1H NMR data for new compounds (Bruker AC–200 spectrometer,
CDCl3, , ppm): 9 1.10 (s, 9H, t-Bu), 1.45 (s, 2H, exo-H2,4), 2.80 (s,
2H, endo-H2,4), 7.20 (m, 5H, Ph); 10 1.40 (s, 9H, t-Bu), 1.40 (m, 6H,
H3–5), 2.0 (t, 1H, H7, J 3.2 Hz), 3.02 (br. s, 2H, H2,6); 11 1.37 (s, 9H,
t-Bu), 1.97 (s, 1H, H1); 2.07 (s, 6H, Me); 12 1.0 (t, 6H, Me, J 7.2
Hz), 1.38 (s, 9H, t-Bu), 1.53 (m, 4H, -CH2); 1.85 (s, 1H, H1), 2.4
(t, 4H, -CH2, J 7.2 Hz).
(1) Otera, J. Chem. Rev. 1993, 93, 1449.
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(4) Seebach, D.; Thaler, A.; Blaser, D.; Soo, Y. Ko. Helv. Chim.
Acta 1991, 74, 1102.
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660
V. A. Vasin, V. V. Razin
LETTER
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Article Identifier:
1437-2096,E;2001,0,05,0658,0660,ftx,en;D01801ST.pdf
Synlett 2001, No. 5, 658–660 ISSN 0936-5214 © Thieme Stuttgart · New York