DBU-CH3I, a potential substitute for CH2N2 in the preparation of methyl esters and methyl aryl ethers: Studies with assorted acids

Article abstract of DOI:10.1080/00397910802238809

DBU-CH3I has been poised to be a substitute for diazomethane in the preparation of methyl esters from carboxylic acids. The reactions can be carried out in commercial untreated acetone and acetonitrile, which have been exemplified with several methyl esters, otherwise it is difficult to prepare. Bis-esterification using diiodomethane can also be achieved in a similar fashion. Sufficiently acidic phenols are also conveniently O-methylated by the method. Copyright Taylor & Francis Group, LLC.

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DBU-CH3I, a Potential
Substitute for CH2N2 in the
Preparation of Methyl Esters
and Methyl Aryl Ethers: Studies
with Assorted Acids
Dipakranjan Mal a , Amit Jana a , Sutapa Ray a
,
Sourav Bhattacharya a , Asit Patra a & Saroj R. De a
a Department of Chemistry, Indian Institute of
Technology, Kharagpur, India
Published online: 20 Oct 2008.
To cite this article: Dipakranjan Mal , Amit Jana , Sutapa Ray , Sourav Bhattacharya ,
Asit Patra & Saroj R. De (2008): DBU-CH3I, a Potential Substitute for CH2N2 in the
Preparation of Methyl Esters and Methyl Aryl Ethers: Studies with Assorted Acids,
Synthetic Communications: An International Journal for Rapid Communication of
Synthetic Organic Chemistry, 38:22, 3937-3946
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Synthetic Communications1, 38: 3937–3946, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910802238809
DBU-CH3I, a Potential Substitute for CH2N2 in
the Preparation of Methyl Esters and Methyl
Aryl Ethers: Studies with Assorted Acids
Dipakranjan Mal, Amit Jana, Sutapa Ray, Sourav Bhattacharya,
Asit Patra, and Saroj R. De
Department of Chemistry, Indian Institute of Technology,
Kharagpur, India
Abstract: DBU-CH3I has been poised to be a substitute for diazomethane in the
preparation of methyl esters from carboxylic acids. The reactions can be carried
out in commercial untreated acetone and acetonitrile, which have been exemplified
with several methyl esters, otherwise it is difficult to prepare. Bis-esterification
using diiodomethane can also be achieved in a similar fashion. Sufficiently acidic
phenols are also conveniently O-methylated by the method.
Keywords: DBU, diiodomethane, iodomethane, phenols, phthalaldehydic acids,
pyridine carboxylic acids
Methyl esterification of carboxylic acids is an important process in the
purification, characterization, and selective transformation of
carboxylic acids.[1a] It is generally achieved by the use of preprepared
ethereal solution of diazomethane. Despite the toxicity and direct
inaccessibility, it enjoys wide acceptance because of its selectivity and
mildness of reaction conditions. Shioiri’s method, based on commercially
available trimethylsilyldiazomethane, appears to be the best alternative
to diazomethane.[1b,1c]
Received April 21, 2008.
Address correspondence to Dipakranjan Mal, Department of Chemistry,
Indian Institute of Technology, Kharagpur 721302, India. E-mail: dmal@chem.
iitkgp.ernet.in
3937

3938
D. Mal et al.
Quite some time ago, we reported a facile chemoselective O-methylation
of carboxylic acids with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)-CH3I,
following the original report[2a] of Ono et al., and introduced the notion that
such a methylation is faster for the stronger OH acids.[2b] The method has
since found a good number of applications in organic chemistry.[3,4] To widen
the scope of the method to solving some of the commonly encountered litera-
ture problems, we studied O-methylation of assorted carboxylic acids and
phenols using readily available iodomethane. It is now amply demon-
strated that DBU-CH3I could be a good substitute for toxic diazomethane,
at least for O-methylation of carboxylic acids and sufficiently acidic
phenols. The method can be conveniently conducted on a large scale in
commercial acetone as a solvent. It can also be extended to methylenation
with diiodomethane.
This work was initiated to solve the commonly encountered
problems of methyl esterification of ortho formylbenzoic acids 1, which
exit in the form of phthalaldehydic acids 2. The common method of
methyl esterification involving methanol and a catalytic amount of
mineral acids almost always yields 3-methoxyphthalides 3 as the sole
products (Scheme 1).[5a] Attempted methyl esterification by treatment
with SOCl2, followed by methanol, resulted in a 57:43 mixture of methyl
2-formylbenzoate and 3-methoxyphthalide.[5b] With K2CO3 and a three-
fold excess of iodomethane in acetonitrile, phthalaldehydic acid (4) was
methylated in 70% yield after 24 h of reflux.[5c,d] Methyl opianate, a
dimethoxyphthalaldehydic acid methyl ester, was recently prepared from
the acid by treatment with ethereal diazomethane.[5e]
In the present work, a number of phthalaldehydic acids were
O-methylated more conveniently at room temperature with DBU-
CH3I. When commercially available phthalaldehydic acid (4) was treated
with 1 equivalent of DBU in untreated acetonitrile followed by
iodomethane (3 equiv) (Table 1), the reaction provided methyl o-
formylbenzoate 5[6] in 87% yield. The corresponding methoxyphthalide
was not formed. Likewise, substituted phthalaldehydic acids 6, 8, and
10 underwent smooth O-methylation with DBU-CH3I to give the
Scheme 1. Reaction of phthalaldehydic acids with methanol.

DBU-CH3I, a Potential Substitute for CH2N2
3939
Table 1. Esterification of carboxylic acids with DBU and an alkyl halide
Acid
no.
Solvent
(DBU:CH3I)
Product
no.
Yielda
(%)
Acid
Product
4
6
Acetonitrile (1:3)
5
7
87
82
Acetone (2:5)
8
Acetone (1:3)
Acetone (1:3)
9
94
94
10
11
12
Acetone (1:1.2)
13
62
14
16
Acetonitrile (1:3)
Acetonitrile (2:5)
15
17
72
93
(Continued )

3940
D. Mal et al.
Table 1. Continued
Acid
no.
Solvent
(DBU:CH3I)
Product
no.
Yielda
(%)
Acid
Product
18
Acetonitrile (2:1)b
19
69
20
22
Acetonitrile (2:1)b
Acetonitrile (2:5)
21
23
66
85
92
24
26
Acetonitrile (1.1:3) 25
Acetone (2:5)
Acetone (1:3)
27
29
28c
91
30
Acetone (1:3)
31
92
(Continued )

DBU-CH3I, a Potential Substitute for CH2N2
3941
Table 1. Continued
Acid
no.
Solvent
(DBU:CH3I)
Product
no.
Yielda
(%)
Acid
Product
32
Acetone (1:3)
33
80
34
Acetonitrile (1:3)
35
84
36
Acetone (1:3)
37
98
89
38
40
Acetone (1:3)
Acetone (1:0)
39
41
96
68
42
Acetone (2:1.2)d
43
aYields refer to the isolated yields.
bDiiodomethane.
cMixture of cis and trans isomers.
d3,3-Dimethyl-3-chloropropyne.

3942
D. Mal et al.
corresponding esters 7[6], 9, and 11 in excellent yields. No side products were
found. It is worthy of note that untreated commercial acetone works well
as the reaction medium. The products were conveniently purified by
quick silica-
gel filtration. Most often, they were obtained in sufficiently pure (>90%)
form for further use without resorting to chromatographic purification.
For pyridine carboxylic acids 12, 14, and 16, the acid-catalyzed
methyl esterification with methanol is conventionally followed, which
often requires long reaction time ranging between 1 and 2 days at reflux
temperature.[7] Occasionally, the method based on dimethyl acetal of
acetone in the presence of conc. HCl is used.[7c] The present method,
however, worked well with pyridine carboxylic acids and gave esters
13, 15, and 17, respectively, with the best yield for pyridine-2,6-
dicarboxylic acid 16. Analysis of NMR spectra of the crude reaction mix-
tures revealed that the monocarboxylic acids 13 and 15 formed some
amount of N-methylated esters.
Considering the importance of the methylene esters in the develop-
ment of organic materials,[8] we briefly examined the present method of
preparing the esters using DBU. Indeed, the reactions of benzoic acid
18 and p-toluic acid 20 with DBU-CH2I2 at room temperature gave the
corresponding esters 19 and 21 in good yields.
Mildness of the reaction conditions of the present method was
demonstrated by the acids 22, 24, 26, and 28. Dimethyl ester 23 obtained
from itaconic acid (22) was free from contamination with its double-
bond-isomerized product.[9] However, the attempted monomethylation
of the acid 22 by limiting the amount of iodomethane resulted in a mix-
ture of monoesters and diester. For the cis cinnamic acid 24, the reaction
was clean. There was no trace of the more stable trans isomer. For homo-
phthalic acid (26), which could be considered a vinylogous malonic acid,
no complication arose during the methylation, although there was a pos-
sibility of C-alkylation at the benzylic position. With the furan acid 28,
the dichloromethyl group did not suffer any loss of HCl, and the desired
product 29 was obtained in good yield.
In our earlier publication,[2b] we reported that O-methylation of the
carboxyl groups can be performed neatly while phenolic hydroxyl groups
remain intact. To test the viability of the reaction for large-scale pre-
paration, we chose methyl 1,4-dihydroxy-2-naphthoate (31), which is
prepared from the acid by diazomethane treatment[10a] or via the reaction
of preformed sodium salt of the acid with methyl iodide in hexamethyl-
phosphoramide (HMPA).[10b] In view of the toxicity of diazomethane
and HMPA, we submitted the acid 30 to the present method of using
DBU-CH3I in acetone on a 5-g scale. The reaction provided the ester
31 in 92% yield.

DBU-CH3I, a Potential Substitute for CH2N2
3943
During the course of our work[3c] on angucycline, we noted that with
longer time and an excess of iodomethane, phenolic OH groups are
converted to the methyl derivative to an appreciable extent. To find the
scope of the reactivity, we submitted salicylaldehyde (32) to DBU-
CH3I. The ether 33 was formed in good yield. Similarly, vanillin (34),
nitrophenol (36), and thiophenol (38) were methylated to give 35,[11]
37, and 39 respectively. For the phenols, the reactions were carried out
with 1 equiv. of DBU and 1 equiv. of MeI in a stoppered flask. Because
they were incomplete (by TLC) after 2 h, an additional molar equivalent
of DBU and MeI was added. The reactions soon became complete.
Workup by removal of acetone and dilution with ethyl acetate followed
by washing with brine yielded the products in practically pure form.
Intramolecular cyclization[12] of o-bromomethylbenzoic acids is a use-
ful methodology for the preparation of the phthalides. To improve upon
the process, the acid 38 was treated with 1 equivalent of DBU in acetone.
Phthalide (41) was formed in excellent yield at room temperature.
In conjunction with the present work, we came across the problem of
preparing[13] dimethylallyl esters via the corresponding propargyl esters.
To overcome the problem, we applied the present method to benzoic acid
and 3,3-dimethyl-3-chloropropyne. The ester 43 was obtained in good yield.
In conclusion, the DBU-CH3I method is ideally suited for room
temperature O-methylation of phthalaldehydic acids, pyridine carbo-
xylic acids, and phenols, and thus the use of hazardous diazomethane
can be avoided. Short reaction time is also a characteristic of the method.
EXPERIMENTAL
Typical Experimental Procedure
DBU was added to a stirred solution of an acid or a phenol (1 mmol) in
acetonitrile or acetone (5 mL) (see Table 1 for the molar ratio), and the
reaction was stirred for 10 min. Iodomethane (see Table 1 for the molar
ratios) was added to the mixture over a period of 10–15 min, and stirring
continued for 4 h at room temperature. The reaction mixture was then
concentrated and diluted with ether=ethyl acetate (50 mL). The resulting
solution was washed successively with hydrochloric acid (5 mL, 2 M),
water (10 mL), saturated aqueous solution of sodium thiosulfate (5 mL),
and brine (10 mL). The organic layer was dried (anhydrous Na2SO4) and
concentrated. The residue was purified by silica-gel filtration or column=
preparative thin-layer chromatography (TLC) to afford pure methyl ester
or methyl ether. In most cases during workup, washing the organic layer
with brine only leads to a NMR-pure product.

3944
D. Mal et al.
Physical Data
The characterization data that are not reported or readily accessible are
provided.
Ester 9: mp: 40–42 ꢀC; nmax (KBr): cmꢁ1: 1729, 1567, 1454, 1270,
1110, 950, 811; 1H NMR (200 MHz, CDCl3): d 9.92 (s, 1H), 7.60
(ABq, 2H, J ¼ 8.4 Hz), 4.02 (s, 3H), 3.94 (s, 3H); 13C NMR (50 MHz,
CDCl3): d 188.7, 165.8, 153.5, 134.7, 132.8, 131.8, 129.9, 127.3, 62.2, 52.8.
Ester 11: mp: 64–66 ꢀC; nmax (KBr): cmꢁ1: 1727, 1533, 1444, 1286,
1
1135, 977, 815; H NMR (200 MHz, CDCl3): d 10.7 (s, 1H), 8.85 (d,
1H, J ¼ 1.8 Hz), 8.48 (dd, 1H, J ¼ 1.8, 8.4 Hz), 8.09 (d, 1H, J ¼ 8.4 Hz),
4.06 (s, 3H); 13C NMR (50 MHz, CDCl3): d 190.3, 164.5, 149.8, 141.3,
132.8, 129.9, 127.0, 125.6, 53.4.
Ester 15: 1H NMR (400MHz, CDCl3): d 9.22 (t, 1H, J ¼ 1.6 Hz), 8.77
(dd, 1H, J¼ 3.2, 9.7 Hz), 8.35–8.20 (m, 1H), 7.45–7.30 (m, 1H), 3.95 (s, 3H).
Ester 21: 1H NMR (400 MHz, CDCl3): d 7.98 (d, 4H, J ¼ 8 Hz), 7.24
(d, 4H, J ¼ 8.4 Hz), 6.22 (s, 2H), 2.41(s, 6H).
1
Ester 23: H NMR (200 MHz, CDCl3): d 6.31 (s, 1H), 5.70 (s, 1H),
3.75 (s, 3H), 3.68 (s, 3H), 3.33 (s, 2H); 13C NMR (400 MHz, CDCl3): d
171.1, 166.5, 133.5, 128.5, 52.09, 52.01, 37.4.
Ester 25: mp: 61–62 ꢀC; 7.70–7.62 (m, 2H), 7.57 (dt, 1H, J ¼ 7.7,
1.4 Hz), 7.41 (dt, 1H, J ¼ 7.7, 1.4 Hz), 7.18 (d, 1H, J ¼ 12.3 Hz), 6.21 (d,
1H, J ¼ 12.3 Hz), 3.68 (s, 3H).
Ester 27: 1H NMR (400 MHz, CDCl3): d 8.02 (d, 1H, J ¼ 8 Hz), 7.49
(t, 1H, J ¼ 8 Hz), 7.37 (t, 1H, J ¼ 7.6 Hz), 7.27 (d, 1H, J ¼ 7.4 Hz), 4.01 (s,
2H), 3.87 (s, 3H), 3.70 (s, 3H); 13C NMR (400 MHz, CDCl3): d 171.9,
167.4, 135.8, 132.3, 132.2, 130.9, 129.5, 127.3, 51.9, 51.8, 40.4.
1
Ester 29: H NMR (400 MHz, CDCl3), mixture of isomers: d 5.99
(d, 1H, J ¼ 8 Hz), 5.83 (d, 1H, J ¼ 3.6 Hz), 4.53–4.7 (m, 1H), 4.34–4.42
(m, 1H), 4.10–4.25 (m, 1H), 3.82–4.12 (m, 4H), 3.75 (s, 3H), 3.73
(s, 3H), 3.15–3.35 (m, 2H), 2.16–2.41 (m, 3H).
1
Ether 33: H NMR (400 MHz, CDCl3): d 10.39 (s, 1H), 7.73 (d, 1H,
J ¼ 7.6 Hz), 7.43–7.36 (m, 1H), 6.92–6.80 (m, 2H), 3.82 (s, 3H).
1
Ether 35: H NMR (400 MHz, CDCl3): d 9.84 (s, 1H), 7.46 (dd, 1H,
J ¼ 8.2, 1.6 Hz), 7.41 (d, 1H, J ¼ 1.6 Hz), 6.98 (d, 1H, J ¼ 8.2 Hz), 3.96 (s,
3H), 3.94 (s, 3H).
ACKNOWLEDGMENTS
The authors are grateful to the department of Science and Technology,
New Delhi, for financial support. A. J. thanks University Grants
Commission for the research fellowship. S. R., A. P., and S. R. D. thank

DBU-CH3I, a Potential Substitute for CH2N2
3945
Council of Scientific and Industrial Research for their research
fellowships.
REFERENCES
1. (a) Otera, J. Esterification: Methods, Reactions, and Applications; Wiley-VCH
Verlag GmbH & Co.: Weinheun, 2003; pp. 1–326; (b) Hashimoto, N.;
Aoyama, T.; Shioiri, T. New methods and reagents in organic synthesis,
14: A simple efficient preparation of methyl esters with trimethylsilyldiazo-
methane (TMSCHN2) and its application to gas chromatographic analysis
of fatty acids. Chem. Pharm. Bull. 1981, 29, 1475–1478; (c) Kuhnel, E.;
Laffan, D. D. P.; Lloyd-Jones, G. C.; Martinez del Campo, T.; Shepperson,
I. R.; Slaughter, J. L. Mechanism of methyl esterification of carboxylic acids
by trimethylsilyldiazomethane. Angew. Chem. Int. Ed. 2007, 46, 7075–7078.
2. (a) Ono, N.; Yamada, T.; Saito, T.; Tanaka, K.; Kaji, A. A convenient pro-
cedure for esterification of carboxylic acids. Bull. Chem. Soc. Jpn. 1978, 51,
2401–2404; (b) Mal, D. Chemoselective methylation of carboxylic acids using
DBU and iodomethane. Synth. Commun. 1986, 16, 331–335.
3. (a) Ray, S.; Patra, A.; Mal, D. Tandem annulation strategy for the conver-
gent synthesis of benzonaphthopyranones: Total synthesis of chartarin and
O-methylhayumicinone. Tetrahedron 2008, 64, 3253–3267; (b) Li, M. S.;
O’Doherty, G. A. De novo formal synthesis of (ꢁ)-apicularen A via an itera-
tive asymmetric hydration sequence. Org. Lett. 2006, 8, 6087–6090; (c) Mal,
D.; Dey, S. Synthesis of chlorine-containing angucycline BE-23254 and its
analogs. Tetrahedron 2006, 62, 9589–9602; (d) Gauthier, C.; Legault, J.;
Lebrun, M. Glycosidation of lupane-type triterpenoids as potent in vitro
cytotoxic agents. Biorg. Med. Chem. 2006, 14, 6713–6725; (e) Herb, C.;
Maier, M. E. A formal total synthesis of the salicylihalamides. J. Org. Chem.
2003, 68, 8129–8135; (f) Kuhnert, S. M.; Maier, M. E. Synthesis of the core
structure of apicularen A by transannular cyclization. Org. Lett. 2002, 4,
643–646; (g) Scheufler, F.; Maier, M. E. Synthesis of a model system for
the macrocyclic subunit of the oximidines. Synlett 2001, 1221–1224; (h)
Wang, X. S.; Jackson, R. A.; Armes, S. P. Facile synthesis of acidic copolymers
via atom transfer radical polymerization in aqueous media at ambient tempera-
ture. Macromolecules 2000, 33, 255–257; (i) Berteina, S.; De Mesmaeker, A.
Radical and palladium-mediated cyclizations of ortho-iodo benzyl enamines:
A scope and limitation study. Synlett 1998, 1227–1230; (j) Tyman, J. H. P.;
Visani, N. Synthesis of saturated anacardic acids, and alkenyl and alkynyl ana-
logs. J. Chem. Res. 1997, 181–181; (h) Morin, C.; Nedjar, N. Oxidative degra-
dation of larixol and larixyl acetate. Tetrahedron Lett. 1996, 37, 4705–4706.
4. Reviews: (a) Hermecz, I. Chemistry of diazabicycloundecene (DBU) and
other pyrimidoazepines. Adv. Heterocycl. Chem. 1987, 42, 83–202; (b)
Ghosh, N. DBU (1,8-diazabicyclo[5.4.0]undec-7-ene)—A nucleophillic base.
Synlett, 2004, 574–575.
5. (a) Wheeler, D. D.; Young, D. C.; Erley, D. S. Reactions of phthalaldehydic
acids. J. Org. Chem. 1957, 22, 547–555; (b) Morgan, D. O.; Ollis, W. D.;

3946
D. Mal et al.
Stanforth, S. P. Preparation and cycloaddition reactions of novel heterocyclic
mesomeric betaines. Tetrahedron 2000, 56, 5523–5534; (c) Khartulyari, A. S.;
Kapur, M.; Maier, M. E. Concise strategy to the core structure of the
macrolide queenslandon. Org. Lett. 2006, 8, 5833–5836; (d) Ye, B.-H.; Nar-
uta, Y. A novel method for the synthesis of regiospecifically sulfonated por-
phyrin monomers and dimers. Tetrahedron 2003, 59, 3593–3601; (e) Marcos,
C. F.; Marcaccini, S.; Menchi, G; Pepinob, R.; Torroba, T. Studies on isocya-
nides: Synthesis of tetrazolyl-isoindolinones via tandem Ugi four-component
condensation=intramolecular amidation. Tetrahedron Lett. 2008, 49, 149–152.
6. Che, C.; Xiang, J.; Wang, G.-X.; Fathi, R.; Quan, J.-M.; Yang, Z. One-pot
synthesis of quinoline-based tetracycles by a tandem three-component reac-
tion. J. Comb. Chem. 2007, 9, 982–989.
7. (a) Lee, J. C., Jr.; Yao, W.; Crabtree, R. H. Fluxionality in
[ReH5(PPh3)2(pyridine)]. Inorg. Chem. 1996, 35, 695–699; (b) Schmuck, C.;
Machon, U. Amino acid binding by 2-(guanidiniocarbonyl)pyridines in
aqueous solvents: A comparative binding study correlating complex stabil-
ity with stereoelectronic factors. Chem. Eur. J. 2005, 11, 1109–1118; (c)
Alcock, N. W.; Clarkson, G.; Glover, P. B.; Lawrance, G. A.; Moore, P.;
Napitupulu, M. Complexes of 2,6-bis[N-(20-pyridylmethyl)carbamyl]-
pyridine: Formation of mononuclear complexes, and self-assembly of double
helical dinuclear and tetranuclear copper(II) and trinuclear nickel(II) com-
plexes. Dalton Trans. 2005, 518–527.
8. (a) Al-Kaysi, R. O.; Dillon, R. J.; Kaiser, M. J.; Mueller, L. J.; Guiradoand,
G.; Bardeen, C. J. Photopolymerization of organic molecular crystal nano-
rods. Macromolecules 2007, 40, 9040–9044; (b) Kavitake, B. P.; Salunkhe,
M. M.; Wadgaonkar, P. P. Efficient method for synthesis of methylene
diesters using polyethylene glycol as a phase transfer catalyst. Synth. Com-
mun. 1997, 27, 1703–1710.
9. Murata, T.; Koinuma, Y.; Anami, K. Preparation of mesaconic acid diesters
by isomerization of itaconates. Jpn. Kokai Tokkyo Koho 1989, Application:
JP 01104030 19890421.
10. (a) Cho, H.; Park, S.-J.; Lee, K.-I. Naphthofuroquinone derivatives: DNA
topoisomerase-I inhibition and cytotoxicity. Bull. Korean Chem. Soc. 2005,
26, 1829–1830; (b) Mamaghani, M.; Pourali, A. Synthesis of methyl 3,6-
dioxo-endo-tricyclo[6.2.1.02,7]undeca-4,9-diene-2-carboxylate as synthetic
intermediate for conduritol derivatives. Russ. J. Org. Chem. 2002, 38, 369–371.
11. Kao, C.-L.; Chern, J.-W. A novel strategy for the synthesis of benzofuran
skeleton neolignans: Application to ailanthoidol, XH-14, and obovaten.
J. Org. Chem. 2002, 67, 6772–6787.
12. Khan, K.; Hayat, S.; Choudhary, M. I.; Maharvi, G. M.; Bayer, E. An
alternative method for the synthesis of c-lactones by using cesium fluoride-
celite=acetonitrile combination. Synth. Commun. 2003, 33, 3435–3453.
13. Sedighi, M.; Calimsiz, S.; Lipton, M. A. A convenient, general synthesis of
1,1-dimethylallyl esters as protecting groups for carboxylic acids. Org. Lett.
2005, 7, 1473–1475.