Acylative dimerization of tetrahydrofuran catalyzed by rare-earth triflates

Article abstract of DOI:10.1081/SCC-120034170

Ytterbium, scandium, and lanthanum triflates catalyze the cleavage reactions of cyclic ethers to give various products. Most notable was the acylative cleavage of tetrahydrofuran with acetic anhydride catalyzed by Yb(triflate)₃ to give the dime

Full text of DOI:10.1081/SCC-120034170

SYNTHETIC COMMUNICATIONSw
Vol. 34, No. 10, pp. 1871–1880, 2004
Acylative Dimerization of Tetrahydrofuran
Catalyzed by Rare-Earth Triflates
*
William H. Miles, Daniel A. Ruddy, Samira Tinorgah,
and Rebecca L. Geisler
Department of Chemistry, Lafayette College, Easton,
Pennsylvania, USA
ABSTRACT
Ytterbium, scandium, and lanthanum triflates catalyze the cleavage
reactions of cyclic ethers to give various products. Most notable was
the acylative cleavage of tetrahydrofuran with acetic anhydride catalyzed
by Yb(triflate)3 to give the dimeric compound CH3CO2(CH2)4O(CH2)4
OCOCH3 in 69% yield.
Key Words: Lewis acids; Tetrahydrofuran; Rare-earth triflates.
*
Correspondence: William H. Miles, Department of Chemistry, Lafayette College,
Easton, PA 18042, USA; E-mail: milesw@lafayette.edu.
1871
DOI: 10.1081/SCC-120034170
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

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Miles et al.
INTRODUCTION
Various Lewis acids and transition metals catalyze the cleavage of tetra-
hydrofuran into monomeric difunctional products such as 4-halobutyl alkano-
ates,[1–5] 4-halobutyl alcohols,[5] and TMS ethers of 4-phenylseleno
alcohols.[6] A handful of reactions give difunctional products incorporating
two tetrahydrofurans. For example, the cleavage of tetrahydrofuran with phos-
phorus oxychloride catalyzed by sulfuric acid gave the dimeric product
1-chloro-4-(4-chlorobutoxy)butane in 70% yield.[7] Acetic anhydride clea-
vage of tetrahydrofuran catalyzed by HClO4,[8] heteropolytungstic acids,[9]
or montmorillonite-type earth[10] gave good yields of the dimeric product
4-(4-acetoxybutoxy)butyl acetate (1a) using excess acetic anhydride. Stoi-
chiometric TiCl4 and excess THF gave 1-chloro-4-(4-chlorobutoxy)butane
in 55% yield.[11] Conditions leading to the polymerization of tetrahydrofuran
to give poly(tetramethylene oxide)[12] also has been intensely investigated,
since polymers and block copolymers of tetrahydrofuran are important
commercial products (e.g., spandex fibers). Polymerization catalysts such as
oxonium ions[13] and triflate esters[14] are typically used to initiate the cationic
polymerization.
Given the importance of the cleavage reactions of cyclic ethers, it is not
surprising that rare-earth metal triflates,[15] a newly emerging class of Lewis
acids, have been investigated as catalysts for ether cleavage reactions. The
rare-earth metal triflates are remarkably active Lewis acids, and are useful
water-tolerant, potentially recyclable catalysts in such diverse reactions as
the Diels–Alder, Michael, aldol, Friedel–Crafts, and Baeyer–Villiger
reactions.[15] The studies employing rare-earth metal triflates and related
complexes in ether cleavage reactions have, for the most part, looked at clea-
vage reactions of acyclic ethers used as protecting groups. Several highly
chemoselective methods for the cleavage of trityl,[16] TBS,[17] prenyl,[18]
MOM,[19] and benzylic ethers[20,21] using rare-earth metal catalysts have
been developed. Scandium triflate is an effective catalyst for the direct conver-
sion of O-trimethylsilyl cyanohydrins into the respective O-acetyl cyano-
hydrins.[22] The rare-earth metal triflates also are effective catalysts for the
ring-opening reactions of highly strained epoxides and oxetanes giving mono-
meric products,[15] but the only reports in the literature concerning the clea-
vage reactions of tetrahydrofurans has been in their role as catalysts leading
to polymeric materials.[23,24] Since these studies indicated the facile cleavage
of cyclic ethers using rare-earth triflates, we decided to examine the acylative
cleavage reactions of tetrahydrofuran catalyzed by ytterbium, scandium, or
lanthanum triflate under conditions favoring low molecular weight products.
When a 2.3 : 1 molar mixture of tetrahydrofuran and acetic anhydride was
heated (80–858C, 0.5 hr) with ytterbium triflate (4 mol%), the crude product

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Dimerization of Tetrahydrofuran
1873
contained dimeric AcO(CH2)4O(CH2)4OAc (1a) as the major product (ꢀ60%),
along with AcO(CH2)4OAc (2a; 5%), AcO(CH2)4O(CH2)4O(CH2)4OAc
(3a; 25%), and higher oligomers (Sch. 1). With a routine extractive work-
up procedure, we obtained 40–50% isolated yields of 1a. The isolated yield
of 1a improved markedly with two variations: direct distillation of the reaction
mixture after addition of Na4EDTA and/or careful addition of THF to the
acetic anhydride/catalyst mixture. Distillation directly from the reaction
flask takes advantage of the conversion of 3a (and higher oligomers) to 1a
(69% isolated yield; Table 1). The addition of Na4EDTA moderates the
activity of Yb(OTf)3 so that the formation of monomeric 2a is minimized.
Optimal crude yield of 1a (1a : 2a : 3a; 75 : 10 : 15) requires the careful balance
of catalyst activation, catalyst concentration, addition rate of THF, and temp-
erature (see Experimental) so that the THF concentration is not too low, which
promotes the formation of monomeric 2a, nor too high, which favors 3a and
higher oligomers. Various additives or solvents were tried, but they either did
not influence selectivity and yield (CH3NO2) or they deactivated the catalyst
(NaHCO3 and CH3CN). The wetness of the THF was only a minor determi-
nant of yield or selectivity, since the addition of unpurified THF containing
5 mol% water gave only slightly diminished yields (54%) of isolated 1a.
Scandium triflate gave comparable but more variable (55–70%) yields of
1a, and lanthanum triflate gave lower (55%) yields of 1a. Although water-
soluble rare-earth triflates are potentially recoverable, we were unable to
develop efficient procedures for separating the catalyst from the products.
When triflic acid (4 mol%) was used under similar conditions, 1a was isolated
in 52% yield. Perchloric acid-catalysis (4 mol%) of the reaction of THF
with acetic anhydride[8] required much longer reaction times and gave
lower yields of 1a.
The reaction of tetrahydrofuran with several other anhydrides–propionic,
chloroacetyl, benzoic, and crotonoic–gave fair to good yields of the respective
dimeric products 1 (Table 1) using conditions similar to the conditions
Scheme 1.

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Miles et al.
Table 1. Reaction of tetrahydrofuran with anhydrides.
Anhydride
Catalyst
Product (%yield)
(CH3CO)2O
Yb(OTf)3
CH3CO2(CH2)4O(CH2)4OCOCH3, 1a(65–69)
CH3CO2(CH2)4OCOCH3, 2a (10–15)
(CH3CO)2O
(CH3CO)2O
(CH3CO)2O
(CH3CO)2O
(CH3CH2CO)2O
Sc(OTf)3
La(OTf)3
HOTf
1a (55–70)
1a (55)
1a (52)
1a (21)
HClO4
Yb(OTf)3
CH3CH2CO2(CH2)4O(CH2)4OCOCH2CH3,
1b (59)
(ClCH2CO)2O Yb(OTf)3
(C6H5CO)2O Yb(OTf)3
(CH3CH55CHCO)2O Yb(OTf)3
ClCH2CO2(CH2)4O(CH2)4OCOCH2Cl, 1c (42)
C6H5CO2(CH2)4O(CH2)4OCOC6H5 1d (58)
CH3CH55CHCO2(CH2)4O(CH2)4OCOCH55
CHCH3, 1e (56)
developed for the Yb(OTf)3-catalyzed reaction of tetrahydrofuran with acetic
anhydride. Yb(OTf)3 catalyzed the polymerization of methacrylic anhydride
under these conditions, and we were unable to develop alternate conditions
leading to appreciable amounts of 1. We briefly looked at other cyclic ethers
under the reaction conditions described for the acylative dimerization of tetra-
hydrofuran, but these reactions were not as interesting or useful as the clea-
vage reactions of tetrahydrofuran. The Yb(OTf)3-catalyzed reaction of
2-methyltetrahydrofuran with acetic anhydride gave monomeric AcO(CH2)3
CH(CH3)OAc in 47% yield, whereas the analogous reaction employing
Sc(OTf)3 catalysis gave a mixture of 4-penten-1-yl acetate, (E)-3-penten-1-yl
acetate, and (Z)-3-penten-1-yl acetate with low yields of the dimeric diester.
The slow reaction of tetrahydrofuryl acetate and acetic anhydride using
Sc(OTf)3 catalysis gave 1,2,5-triacetoxypentane, similar to the reaction
employing HClO4 catalysis. Tetrahydropyran and dioxane reacted slowly
with acetic anhydride (4 mol% Yb(OTf)3) to give low yields of complex
mixtures. The acylative ring-opening reactions of oxetane catalyzed by
Yb(OTf)3 gave oligomeric compounds.
In order to probe the origin for the selectivity of the dimeric diester 1, we
performed several control and crossover experiments. When the diester 1a
was heated with Yb(OTf)3 (4 mol%) at 858C for 2 hr, it was recovered
1
unchanged. H NMR studies indicated that an equimolar mixture of diester
1a and acetic anhydride with Yb(OTf)3 (4 mol%) heated for 2 hr at 858C
also was unchanged. When 1a is heated to 1408C in the presence of
Yb(OTf)3 (4 mol%), however, we do see the formation of THF and
monomeric 2a, consistent with our observations of the distillation of the

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1875
crude reaction mixtures and with the observations by Drysdale in the depoly-
merization of polytetrahydrofuran.[23] When aliquots are removed during the
addition of THF to acetic anhydride using Yb(OTf)3 (4 mol%), the ratio of
1a : 2a : higher oligomers does not change substantially. These control exper-
iments rule out an equilibrium process favoring 1a, as has been proposed for
the related reaction of THF with phosphorous oxychloride,[7] although there is
significant equilibration of the products in the presence of the rare-earth
catalysts at the temperatures necessary for distillation. The control exper-
iments employing a rare-earth catalyst were in stark contrast to the results
we obtained employing triflic acid (4 mol%) as the catalyst: the ratio of
1a : 2a : higher oligomers changed significantly during the course of the reac-
tion, with greater than 60% yield of higher oligomers at 50% conversion of
THF (15 min) to a final product ratio of 25 : 60 : 15 (2 hr). We also observed
significant conversion of dimer 1a to monomer 2a under the standard reaction
conditions employing triflic acid (4 mol%, 808C), which we feel is strong
evidence for equilibrium control for the triflic acid-catalyzed reaction. In
a crossover experiment, the addition of THF to an equimolar mixture of acetic
and propionic anhydride with Yb(OTf)3 or Sc(OTf)3 catalysis gave nearly a
statistical mixture of the three possible diesters (1a : AcO(CH2)4O(CH2)4
OCOCH2CH3 : 1b; 1 : 2 : 1). This crossover experiment rules out a mechanism
in which both THF molecules and the anhydride form 1 entirely within the
coordination sphere of only one rare-earth metal.
We propose the following mechanism for the selectivity of the dimer in the
rare-earth catalyzed reactions of tetrahydrofuran with anhydrides (Sch. 2). The
activation of anhydrides for Friedel–Crafts reactions and esterification reactions
is well documented, so the rare-earth catalyst plays a pivotal role in the first step
of the mechanism, the acylation of tetrahydrofuran to give acyl cation 4. Nucleo-
philic attack by a carboxylate anion on intermediate 4 leads to monomeric pro-
duct 2, whereas nucleophilic attack by another tetrahydrofuran within the
coordination sphere of the Lewis acidic rare-earth metal leads to oxonium ion
5. As in the case of intermediate 4, oxonium ion 5 can undergo nucleophilic
attack by a carboxylate anion to give dimeric product 1 or by another tetrahydro-
furan to give oxonium ion 6 that ultimately leads to trimeric product 3 or higher
oligomers. The proposed mechanism, in which the rare-earth metal acts as a tem-
plate for the addition of the second THF (coordination number is unknown and
shown for illustration purposes only), is based on the proposed mechanisms for
the polymerization of THF,[12] the reaction of TiCl4 with THF to give 1-chloro-
4-(4-chlorobutoxy)butane,[11] and the TiCl4 oligomerization of THF in the
presence of acetyl chloride or epichlorohydrin.[25] Further studies will provide
a firmer basis for defining the mechanism for this reaction.
The rare-earth metal triflate-catalyzed acylative dimerization of tetrahy-
drofuran is an attractive method for the synthesis of dimeric diesters 1. The

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Miles et al.
Scheme 2.
traditional two-step synthesis of 1a (57% overall yield) based on the cleavage
of tetrahydrofuran with phosphorus oxychloride generates obnoxious bypro-
ducts. The synthesis of 1 presented in this paper is “green”: no byproducts.
The previous report using heteropoly acids such as H3PW12O40 to give
dimeric diester 1a requires a high Ac2O : THF ratio at incomplete conver-
sions,[9] whereas the rare-earth catalyzed reaction described here can be
conducted at nearly the desired stoichiometric ratio. We are continuing to
investigate the interesting mechanistic questions posed by this study and
are exploring conditions that will allow for the recovery of the rare-earth
catalyst. These studies and the synthesis of unique polymers will be reported
in due course.
EXPERIMENTAL
All reactions were carried out under Ar. IR spectra were obtained on a
Perkin Elmer 1600 Series FT-IR spectrometer. The 1H and 13C NMR spectra

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Dimerization of Tetrahydrofuran
1877
were recorded at 400 and 100 MHz, respectively, on a JEOL Eclipseþ400
spectrometer; all chemical shifts are reported in ppm relative to TMS
(d ¼ 0.00 ppm) in the proton NMR and relative to CDCl3 (d ¼ 77.0 ppm) in
the carbon NMR. Flash chromatography was performed on Merck silica gel
60 (230–400 mesh). Elemental analyses were performed by Galbraith Labora-
tories, Inc. Ytterbium trifluoromethanesulfonate hydrate (Yb(OTf)3),
scandium trifluoromethanesulfonate (Sc(OTf)3), and lanthanum trifluoro-
methanesulfonate (La(OTf)3) were purchased from Aldrich. THF (anhydrous,
100 ppm BHT; Aldrich) was used without further purification or was distilled
from sodium–benzophenone.
4-(4-Acetoxybutoxy)butyl acetate (1a).[9] Procedure A: In a 50 mL
3-neck RB flask equipped with two stoppers and take-off, ytterbium triflate
hydrate (1.98 g, 3.2 mmol) was charged in the flask and the reaction vessel
was heated in an oil bath (125–1308C) under vacuum (0.1 mm) for 25 min.
The oil bath and reaction vessel were allowed to cool to 758C. One stopper
on the reaction vessel was replaced with a thermometer adapter and thermo-
meter, and the take-off was replaced with a Claisen head fitted with a septum
and take-off. Acetic anhydride (7.5 mL, 8.1 g, 79 mmol) was added to the cata-
lyst and stirred for 5 min. Tetrahydrofuran (15.0 mL, 13.3 g, 184 mmol) was
added by syringe pump at the following rate: 5 mL at 0.8 mL/min, 5 mL at
0.6 mL/min, and 5 mL at 0.4 mL/min. At this rate of addition, the temperature
of the reaction mixture remained at 77–808C. The reaction mixture was stirred
for 10 min more after the completion of the THF addition. Na4EDTA hydrate
(1.23 g, 3.2 mmol) was added to the reaction mixture at 75–808C, and, after
replacement of the Claisen head with a short path distillation apparatus and
the thermometer with a stopper, the reaction mixture was distilled under
vacuum (2.0 mm) by heating the flask with an oil bath (100 ! 2008C). The
crude distillate (b.p. 75–1508C) was redistilled under vacuum (2.0 mm) to
give a forerunner containing a mixture of 1a and 2a (b.p. 85–1308C) and a
second fraction of .98% pure 1a (b.p. 138–1508C; 12.8 g, 65% yield) as
1
an oil. IR (film): 1739 cm21; H NMR (CDCl3): d 4.05 (t, 4H, J ¼ 6.4 Hz),
3.40 (t, 4H, J ¼ 6.2 Hz), 2.02 (s, 6H), 1.66 (m, 4H), 1.61 (m, 4H); 13C
NMR (CDCl3) d 171.1, 70.2, 64.2, 26.1, 25.4, 20.9.
Procedure B: In a 25-mL Schlenk flask, ytterbium triflate hydrate (1.98 g,
3.2 mmol) was heated in an oil bath (125–1308C) under vacuum (0.1 mm) for
25 min. The oil bath and reaction vessel were allowed to cool to 758C. Acetic
anhydride (7.5 mL, 8.1 g, 79 mmol) and then tetrahydrofuran (15.0 mL, 16.9 g,
224 mmol) were added and heated in an oil bath to 75–808C for 1 hr.
Na4EDTA (1.23 g, 3.2 mmol) was added to the reaction mixture and the reac-
tion mixture was distilled under vacuum (2.0 mm) by heating the flask with an
oil bath (100 ! 2008C). The crude distillate (b.p. 75–1508C) was redistilled
under vacuum (1.0 mm) to give .98% pure 1a (69% yield).

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Miles et al.
4-(4-Propanoylbutoxy)butyl propanoate (1b). Using the procedure
described in Procedure B for the synthesis of 1a, 1b (59% yield) was isolated
as an oil. B.p. (1.5 mm): 155–1658C; IR (film): 1737 cm21 1H NMR
;
(CDCl3): d 4.05 (t, 4H, J ¼ 6.4 Hz), 3.38 (t, 4H, J ¼ 6.2 Hz), 2.28 (q, 4H,
J ¼ 7.6 Hz), 1.65 (m, 4H), 1.61 (m, 4H), 1.09 (t, 6H, J ¼ 7.7 Hz); 13C NMR
(CDCl3) d 174.5, 70.2, 64.0, 27.5, 26.2, 25.5, 9.1.
4-[4-(2-Chloroacetyloxy)butoxy]butyl 2-chloroacetate (1c). Procedure
A for the synthesis of 1a was followed up to the distillation step of reaction
mixture. The crude product was transferred to a separatory funnel with
ethyl acetate (100 mL), washed with water (2 ꢀ 100 mL), 5% NaOH
(100 mL), and brine (100 mL), and then dried over Na2SO4. (The formation
of emulsions was a problem in the aqueous work-up of all reactions, so
additional brine and ethyl acetate were added as needed.) The crude product
was dissolved in cyclohexane/ethyl acetate (3 : 2), filtered through a plug of
silica gel, and distilled under vacuum to give 1c (42% yield) as an oil. B.p.
(0.1 mm): 175–1858C; IR (film): 1755 cm21 1H NMR (CDCl3): d 4.19
;
(t, 4H, J ¼ 6.4 Hz), 4.04, (s, 4H), 3.40 (t, 4H, J ¼ 6.2 Hz), 1.71 (m, 4H),
1.61 (m, 4H); 13C NMR (CDCl3) d 167.1, 69.9, 65.9, 40.7, 25.8, 25.2. Anal.
Calcd for C12H20Cl2O5: C, 45.69; H, 6.40. Found: C, 45.46; H, 6.43.
4-(4-Phenylcarbonyloxybutoxy)butyl benzoate (1d).[26] The procedure
described for the synthesis of 1c was followed until the silica gel filtration
step. Column chromatography (silica gel; 95 : 5, cyclohexane : ethyl acetate)
gave 1d contaminated with monomeric 2d. Recrystallization from hexane
gave pure 1d (58% yield) as a white solid. M.p.: 45–478C; IR (film):
1709 cm21
;
1H NMR (CDCl3): d 8.03 (d, 4H, J ¼ 7.3 Hz), 7.54 (t, 2H,
J ¼ 7.5 Hz), 7.42 (t, 4H, J ¼ 7.7 Hz), 4.34 (t, 4H, J ¼ 6.4 Hz), 3.48 (t, 4H,
J ¼ 6.4 Hz), 1.85 (m, 4H), 1.74 (m, 4H); 13C NMR (CDCl3) d 166.6, 132.8,
130.4, 129.5, 128.3, 70.3, 64.7, 26.3, 25.6.
4-[4-(2-But-2-enoyloxy)butoxy)butyl]butyl but-2-enoate (1e). Using
similar conditions as described for the synthesis of 1c, 1e (56% yield; E : Z,
96 : 4) was isolated by distillation as an oil. B.p. (0.1 mm): 158–1658C; IR
(film): 1722, 1659 cm21
;
1H NMR (CDCl3): d 6.91 (dq, 2H, J ¼ 15.4,
7.0 Hz), 5.79 (d, 2H, J ¼ 15.8 Hz), 4.09 (t, 4H, J ¼ 6.4 Hz), 3.38 (t, 4H,
J ¼ 6.2 Hz), 1.82 (dd, 6H, J ¼ 6.8, 1.8 Hz), 1.67 (m, 4H), 1.60 (m, 4H); 13C
NMR (CDCl3) d 166.5, 144.4, 122.6, 70.2, 63.8, 26.2, 25.5, 17.8. Anal.
Calcd for C16H26O5: C, 64.41; H, 8.78. Found: C, 64.70; H, 9.15.
ACKNOWLEDGMENTS
We gratefully acknowledge the generous financial support of Lafayette’s
Committee on Advanced Study and Research in the form of stipends under the

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EXCEL Scholar program to DAR, ST, and RLG. We gratefully acknowledge
a grant from the Kresge Foundation for the purchase of a 400 MHz NMR
spectrometer.
REFERENCES
1. Pri-Bar, I.; Stille, J.K. Acylative cleavage of ethers catalyzed by triorgan-
otin halides and palladium(II) complexes. J. Org. Chem. 1982, 47,
1215–1220.
2. Guo, Q.; Miyaji, T.; Hara, R.; Shen, B.; Takahashi, T. Group 5 and group
6 metal halide as very efficient catalysts for acylative cleavage of ethers.
Tetrahedron 2002, 58, 7327–7334.
3. Kwon, D.W.; Kim, Y.H.; Lee, K. Highly regioselective cleavages and iodi-
nations of cyclic ethers utilizing SmI2. J. Org. Chem. 2002, 67, 9488–9491.
4. Malladi, R.R.; Kabalka, G.W. One-pot synthesis of v-chloroesters via the
reaction of acid chlorides with tetrahydrofuran in the presence of trichloro-
borane. Synth. Commun. 2002, 32, 1997–2001.
5. Yadav, V.K.; Fallis, A.G. Regioselective cleavage of 2-methyltetrahydro-
furan: a versatile synthesis of 1-halo-4-pentanols and 4-halo-1-pentanols.
J. Org. Chem. 1986, 51, 3372–3374.
6. Miyoshi, N.; Hatayama, Y.; Ryu, I.; Kambe, N.; Murai, T.; Muria, S.;
Sonoda, N. Ring opening of tetrahydrofurans with phenyl trimethylsilyl
selenide catalyzed by moist zinc iodide. Synthesis 1988, 175–178.
7. Alexander, K.; Schniepp, L.E. 4,40-Dichlorodibutyl ether and its deriva-
tives from tetrahydrofuran. J. Am. Chem. Soc. 1948, 70, 1839–1842.
8. Rozenberg, B.A.; Babin, E.P. Telomerization of tetrahydrofuran. Zh.
Organ. Khim. 1965, 1, 1102–1104, Chem. Abstr. 1964, 63, 62845.
9. Izumi, Y.; Matsuo, K.; Urabe, K. Efficient homogeneous acid catalysis of
heteropoly acid and its characterization through ether cleavage reactions.
J. Mol. Catal. 1983, 18, 299–314.
¨
10. Dorfelt, C. US Patent 3,433,829, March 18, 1969.
11. Delaney, P.A.; Johnstone, R.A.W.; Entwistle, I.D. Metal-assisted reac-
tions. Part 17. Ring-opening and dimerization of cyclic ethers by titanium
halides. J. Chem. Soc. Perkin Trans. I 1986, 1855–1860.
12. Dreyfuss, P.; Dreyfuss, M.P.; Pruckmayr, G. Tetrahydrofuran polymers.
In Encyclopedia of Polymer Science and Engineering, 2nd Ed.; John
Wiley & Sons, Inc.: New York, 1989; Vol. 16, 649–681.
13. Penczek, S.; Kubisa, P.; Matyjaszewski, K. Cationic ring-opening poly-
merizations of heterocyclic monomers I. Mechanisms. Adv. Polym. Sci.
1980, 37, 1.

ORDER
REPRINTS
1880
Miles et al.
14. Hrkach, J.S.; Matyjaszewski, K. Trimethylsilyl triflate as an initiator for
cationic polymerization: improved initiation through the use of promo-
ters. J. Polym. Sci. Part A, Polym. Chem. 1995, 33, 285–298.
15. Kobayashi, S.; Sugiura, M.; Kitagawa, H.; Lam, W.W. Rare-earth metal
triflates in organic synthesis. Chem. Rev. 2002, 102, 2227–2302.
16. Lu, R.J.; Liu, D.; Giese, R.W. Detritylation with ytterbium triflate. Tetra-
hedron Lett. 2000, 41, 2817–2819.
17. Oriyama, T.; Kobayashi, Y.; Noda, K. Chemoselective and practical
deprotection of alkyl trialkylsilyl ethers in the presence of aryl trialkyl-
silyl ethers by a catalytic amount of Sc(OTf)3. Synlett. 1998, 1047–1048.
18. Sharma, G.V.M.; Ilangovan, A.; Mahalingam, A.K. A mild and highly
selective deprotective method of prenyl ethers using ytterbium triflate.
J. Org. Chem. 1998, 63, 9103–9104.
19. Oriyama, T.; Watahiki, T.; Kobayashi, Y.; Hirano, H.; Suzuki, T. A mild
and feasible deprotection of alcohol tetrahydropyranyl or methoxymethyl
ethers catalyzed by Sc(OTf)3. Synth. Commun. 2001, 31, 2305–2311.
20. Ishihara, K.; Hiraiwa, Y.; Yamamoto, H. Homogeneous debenzylation using
extremely active catalysts: tris(triflyl)methane, scandium(III) tris(triflyl)-
methide, and copper(II) tris(triflyl)methide. Synlett. 2000, 80–82.
21. Mehta, S.; Whitfield, D. Polymer-supported synthesis of a branched tri-
saccharide of the type IA group B Streptococcus capsular polysaccharide:
3-iodo-4-methoxybenzyl as a new O-protecting group. Tetrahedron 2000,
56, 6415–6425.
22. Norsikian, S.; Holmes, I.; Lagasse, F.; Kagan, H.B. A one-pot esterifica-
tion of chiral O-trimethysilyl-cyanohydrins with retention of stereochem-
istry. Tetrahedron Lett. 2002, 43, 5715–5717.
23. Drysdale, N.E.; Bockrath, R.E.; Herron, N.; Citron, J.D. US Patent
5,770,678, June 23, 1998.
24. Drysdale, N.E. US Patent 5,990,264, November 23, 1999.
´
´
25. Kucera, M.; Zahradnickova, A.; Majerova, K. Coordinate polymerization
of heterocycles: 2. Oligomerization of tetrahydrofuran by TiCl4 in the pre-
sence of acetyl chloride or epichloroydrin. Polymer 1976, 17, 528–535.
´
26. Botelho, G.; Queiros, A.; Gijsman, P. Thermo-oxidative studies of poly
(ether-esters) 2. Copolymer of poly(butylene terephthalate) and poly
buylene oxide. Polym. Degrad. Stab. 2000, 68, 35–42.
Received in the USA January 6, 2004

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