Synthesis and thermal properties of asymmetrical azo-peresters

Article abstract of DOI:10.1007/s00706-011-0459-9

New asymmetrical azo-perester derivatives of tert-butyl and tert-amyl hydroperoxides were obtained by reacting azo acids with hydroperoxides in the presence of N,N0-carbonyldiimidazole. The obtained azo-peresters possess two labile functional groups: the azo group and the perester group. Data from dynamic differential scanning calorimetry (DSC) experiments indicate that the azo groups decompose at somewhat lower temperatures than the perester groups.

Full text of DOI:10.1007/s00706-011-0459-9

Monatsh Chem (2011) 142:271–276
DOI 10.1007/s00706-011-0459-9
ORIGINAL PAPER
Synthesis and thermal properties of asymmetrical azo-peresters
•
Barbara Hefczyc Jan Zawadiak Henryk Janeczek
•
•
•
Marek Kowalczuk Beata Orlinska
´
Received: 9 March 2010 / Accepted: 4 February 2011 / Published online: 3 March 2011
ꢀ Springer-Verlag 2011
Abstract New asymmetrical azo-perester derivatives of
tert-butyl and tert-amyl hydroperoxides were obtained
by reacting azo acids with hydroperoxides in the presence
of N,N0-carbonyldiimidazole. The obtained azo-peresters
possess two labile functional groups: the azo group and the
perester group. Data from dynamic differential scanning
calorimetry (DSC) experiments indicate that the azo groups
decompose at somewhat lower temperatures than the pe-
rester groups.
techniques, and can be modified with different additives. To
prepare such copolymers, bi- and polyfunctional initiators
are frequently used. These initiators contain at least two
reactive groups in their structure and can initialise poly-
merisation processes according to specific mechanisms
(ionic, free radical etc.). Azo-peroxy compounds constitute
one important class of such bifunctional initiators. These
molecules contain labile azo and peroxy groups that pos-
sess different thermal and/or photochemical stabilities.
Consequently, these compounds undergo discrete thermal
decomposition processes at two different temperatures. This
allows them to initialise free radical processes in a one-pot,
two-stage fashion.
Keywords Differential scanning calorimetry ꢀ
Bifunctional initiators ꢀ NMR spectroscopy ꢀ Peroxides ꢀ
Diazo compounds
Most research about azo-peroxy compounds has focused
on their role in polymerisation processes, but there is sur-
prisingly little data regarding their thermal stability. This
work expands upon our previous research concerning
symmetrical azo-peresters [1].
Introduction
Polymeric materials are extensively employed in nearly all
branches of industry. Polymers with unique physical and
chemical properties have particular importance. Unfortu-
nately, most homopolymers contain a limited range of
molecular functionality and consequently possess a limited
set of macroscopic properties. Therefore, it is often neces-
sary to obtain required features through specially designed
tailor-made copolymers with more diverse structures. Those
copolymers can be obtained with different polymerisation
Results and discussion
In this work, we report the preparation and characterisation
of new asymmetrical compounds, as described in
Scheme 1.
Asymmetrical azo-peresters 3 and 4 were obtained by
reacting appropriate azo-functionalised carboxylic acids 2
with tert-alkyl hydroperoxides in the presence of N,N0-
carbonyldiimidazole.
´
B. Hefczyc (&) ꢀ J. Zawadiak ꢀ B. Orlinska
Department of Organic Chemical Technology
and Petrochemistry, Silesian University of Technology,
Gliwice, Poland
Asymmetrical carboxylic acids containing azo groups
were first obtained according to the procedure described by
Shaikh [2] from suitable ketoacids 1, sodium cyanide, and
tert-butylhydrazine hydrochloride. Among this set of
products, only 4-(tert-butylazo)-4-cyanopentanoic acid 2a
has been previously described (in a patent) [3], and in this
e-mail: Barbara.Hefczyc@polsl.pl
H. Janeczek ꢀ M. Kowalczuk
Center of Polymer and Carbon Materials,
Polish Academy of Sciences, Zabrze, Poland
123

272
B. Hefczyc et al.
protects the peroxy product from decomposition. Unfortu-
nately, this method is limited due to the sensitivity of N,N0-
carbonyldiimidazole and the intermediate acylating agent
to traces of water, which causes their decomposition.
The asymmetrical azo-peresters 3 and 4 were prepared
in moderate yields. There was no obvious correlation
between substituent or carbon chain length and yield. The
structure of each compound was confirmed by elemental
analysis and spectroscopic analysis by 1H NMR, 13C NMR,
and IR.
CH3
CN
O
H3C C N N C (CH2)n C O O R
CH3
CH3
a
b
c
n = 2
n = 3
n = 4
3
R = -C(CH3)3
4 R = -C(CH3)2CH2CH3
Scheme 1
Thermal decomposition of asymmetrical azo-peresters
case, relatively little information about this compound was
reported (Scheme 2).
The thermal decomposition of the obtained asymmetrical
azo-peresters 3 and 4 was evaluated by differential scan-
ning calorimetry (DSC). Results for two selected examples
are shown below—namely, for tert-butyl (Fig. 1) and tert-
amyl hydroperoxide derivatives (Fig. 2) of 4-(tert-butyl-
azo)-4-cyanopentanoic acid (2a).
Asymmetrical carboxylic acids 2 that contain azo groups
were obtained in high purities and with satisfactory yields.
Acids like 4-(tert-butylazo)-4-cyanopentanoic acid (2a),
5-(tert-butylazo)-5-cyanohexanoic acid (2b), and 6-(tert-
butylazo)-6-cyanoheptanoic acid (2c) were used to obtain a
series of asymmetrical azo-peresters with tert-amyl and
tert-butyl substituents. To obtain these compounds, we
employed a method described by Staab [4] that was pre-
viously used to obtain esters, peresters, and azo-peresters
directly from carboxylic acids [5, 6]. Staab’s method was
modified by:
In these figures, there are two peaks indicating the
exothermic decomposition of the azo and perester groups.
In all cases, the first peak (at lower temperature) corre-
sponds to the decomposition of the azo group, and the
second peak (at higher temperature) corresponds to
the decomposition of the perester group. Because some of
the azo-perester products were obtained in the form of an
oil, peaks corresponding to an endothermic melting process
were observed for a limited subset of products.
•
•
•
Usage of a greater excess of hydroxide to carboxylic
acid, which increased yields
Increasing the reaction time, to compensate for a higher
reaction mixture viscosity
To confirm the sequence of decomposition in asym-
metrical azo-peresters, an analogical test was conducted
using symmetrical compounds [1]. The experiments con-
sisted of heating two selected azo-peresters (3a and 4c)
with and without 1% cobalt stearate. The results are given
in Table 1.
Usage of a more concentrated NaOH solution for
washing the crude product mixture, in order to better
remove excess hydroperoxide.
This method generates an acylating agent in situ by
reacting N,N0-carbonyldiimidazole with a carboxylic acid
containing an azo group (Scheme 3). There is no need to
separate and purify the acylating agent, because it reacts
immediately with the hydroperoxide reagent. The reaction
is carried out using tetrahydrofuran (THF) as the solvent.
This prevents the decomposition of the azo-perester prod-
ucts, as was observed during preliminary studies in
chlorinated solvents. In spite of using a carboxylic acid
reagent, the reaction mixture has a slightly basic pH, which
The obtained results show that the addition of a cobalt
salt has no influence on the temperature of azo group
decomposition (Tp) and changes the corresponding peak
shape only slightly. However, the same salt appears to have
a greater influence on perester group decomposition. Tp
values for the peaks corresponding to perester group
decomposition shift to lower temperatures, which in the
case of 3a causes the peak from the perester group
1. HCl
CH3
H3C C N N C (CH2)n
CH3 CH3
CN
O
C
2. Br2
CH3CO(CH)nCOONa
+
(CH)3CN2H3*HCl
NaCN
OH
+
a
b
c
n = 2
n = 3
n = 4
1
2
Scheme 2
123

Synthesis and thermal properties of asymmetrical azo-peresters
273
CH3
CN
O
H3C C N N C (CH2)n C OH
CH3
CH3
2
O
1.
N
N
N
N
2. ROOH
CH3
CN
O
H3C C N N C (CH2)n C O O R
CH3
CH3
Fig. 2 DSC curves for samples of 1,1-dimethylpropyl 4-(tert-buty-
lazo)-4-cyanoperoxypentanoate (4a) at different heating rates: solid
line 20 ꢁC/min, large-dashed line 10 ꢁC/min, medium-dashed line
5 ꢁC/min, small-dashed line 2.5 ꢁC/min
a
b
c
n = 2
n = 3
n = 4
3
R = -C(CH3)3
4 R = -C(CH3)2CH2CH3
Table 1 Maximum exothermic temperatures (Tp) of asymmetrical
azo-peresters decomposed in the presence of 1% by mass of cobalt
stearate (constant heating rate 10 ꢁC/min)
Scheme 3
Compound
Temperature (ꢁC)
With addition of salt
Without addition of salt
–N=N–
–O–O–
–N=N–
–O–O–
a
3a
4c
a
129.7
135.9
–
131.0
137.3
162.2
155.8
136.6b
Peak invisible on DSC (whole peak of perester group decomposi-
tion is buried by the azo group decomposition peak)
b
Peak seen on DSC under main peak
Table 2 Maximum exothermic temperatures (Tp) of asymmetrical
azo-peresters with tert-butyl substituents under various heating rates
b (ꢁC/min) Temperature (ꢁC)
3a
3b
3ca
Fig. 1 DSC curves for samples of tert-butyl 4-(tert-butylazo)-4-
cyanoperoxypentanoate (3a) at different heating rates: solid line
20 ꢁC/min, large-dashed line 10 ꢁC/min, medium-dashed line 5 ꢁC/
min, small-dashed line 2.5 ꢁC/min
–N=N– –O–O– –N=N– –O–O– –N=N– –O–O–
1
112.5
119.0
125.3
131.0
139.3
140.1
148.4
155.4
162.2
169.8
112.9
122.5
128.2
133.6
142.1
135.5
144.9
151.9
158.5
166.6
112.6
123.5
128.9
135.4
142.9
136.2
144.3
151.0
158.6
166.7
2.5
5
decomposition to overlap with the peak from the azo group
decomposition. In the case of 4c, a similar effect can be
observed, but the coverage is not total.
10
20
a
Data regarding the Tp values for both labile groups in
the asymmetrical azo-peresters are shown in Tables 2 and
3 for tert-butyl and tert-amyl hydroperoxide derivatives,
respectively.
Compound obtained as an oil
Additionally, the maximum exothermic temperatures cor-
responding to azo group decomposition within asymmetri-
cal compounds show analogous behaviour to symmetrical
derivatives. The Tp values for azo group decomposition
within derivatives bearing a tert-amyl group are slightly
higher than those bearing a tert-butyl group.
The experimental data gathered in Tables 2 and 3 show
that in asymmetrical derivatives of tert-amyl hydroperox-
ide, perester groups decompose at lower temperatures
than in analogues that bear a tert-butyl group. A similar
situation was observed for symmetrical derivatives [1].
123

274
B. Hefczyc et al.
(0.16 mol) in 20 cm3 water was added slowly under very
intensive stirring. The mixture was stirred for an addi-
Table 3 Maximum exothermic temperatures (Tp) of asymmetrical
azo-peresters with tert-amyl substituents under various heating rates
tional 20 min.
A solution of 4-oxopentanoic (1a),
b (ꢁC/min) Temperature (ꢁC)
5-oxohexanoic (1b), or 6-oxoheptanoic acid (1c) neutra-
lised with sodium carbonate in 25 cm3 water (0.16 mol
of acid) was then slowly added. The final pH of the
reaction mixture was around 7. The mixture was subse-
quently allowed to stir for 8 h and then left for a further
36 h. Then, the reaction mixture was cooled to 5 ꢁC and
acidified with concentrated HCl. The acidified reaction
mixture was intensively stirred for 30 min. Bromine was
then added dropwise until the colour of the mixture
indicated the presence of excess reagent. The resulting
suspension was stirred for 1 h, and then the reaction
mixture was poured onto ice (30 g) and stirred until the
ice melted. The reaction mixture was transferred to a
freezer to crystallise the product. The obtained precipitate
was filtered and recrystallised from CH2Cl2.
4aa
4ba
4c
–N=N– –O–O– –N=N– –O–O– –N=N– –O–O–
1
111.7
120.3
126.3
131.9
139.0
136.7
144.5
151.4
158.8
166.5
114.7
122.2
129.1
135.3
142.2
134.6
142.8
149.4
156.1
163.2
115.8
124.0
130.7
137.3
144.1
134.2
142.5
149.1
155.8
162.3
2.5
5
10
20
a
Compounds obtained as an oil
Experimental
tert-Butyl hydroperoxide (Merck) was extracted with
hexane prior to use. tert-Amyl hydroperoxide (Pergan),
sodium cyanide (Fluka), tert-butylhydrazine hydrochloride,
N,N0-carbonyldiimidazole (Alfa Aesar GmbH & Co KG),
and 4-oxopentanoic acid (1a, Fluka) were sourced from the
indicated vendors and used without additional purification.
Diethyl ether was dried over metallic Na (P.P.H. POCH
S.A. Gliwice) prior to use. THF (P.P.H. POCH S.A.
Gliwice) was dried over metallic Na in the presence of
benzophenone and freshly distilled (65–66 ꢁC) prior to use.
5-Oxohexanoic acid (1b) was synthesised as described by
Bates [7] and 6-oxoheptanoic acid (1c) was synthesised as
described by Schaeffer [8].
1H and 13C NMR spectra were recorded in CDCl3 on a
Varian Unity Inova-300 spectrometer using TMS as an
internal standard. IR spectra were recorded on a Zeiss
Specord M 80 spectrometer. The results of elemental
analyses agreed favourably with the calculated values. The
thermal characteristics of the investigated materials were
obtained using a DSC 2010 TA instrument at heating rates
of 1, 2.5, 5, 10, or 20 ꢁC/min within a temperature range of
-80 to ?200 ꢁC. The temperature was calibrated with
indium and gallium standards over the whole explored
temperature range.
General procedure for preparation of asymmetrical
azo-peresters
A three-necked flask was equipped with a mechanical
stirrer, nitrogen supply, and dropping funnel, and a
solution of each carboxylic acid with an azo functional
group 2 (7.13 mmol) was introduced in 5 cm3 dry THF.
To this mixture was slowly added a solution of N,N0-
carbonyldiimidazole (9.38 mmol) in 15 cm3 dry THF.
The resulting suspension was intensively stirred for
approximately 0.5 h in an ice bath. The mixture was then
cooled to less than 5 ꢁC, and 89% tert-butyl or tert-amyl
hydroperoxide (11.05 mmol) was added. The progress of
the reaction was monitored by means of TLC (mobile
phase CH2Cl2/CH3COCH3 9:1; a solution of sodium
iodide in acetic acid was used for visualisation of the
separated peroxy substances). The reaction was allowed
to proceed for 6 h, after which 18 cm3 Et2O was added
and stirring was continued for an additional 20 min. The
mixture was then washed with 25 cm3 10% NaOH and
with 25 cm3 water. The organic layer was dried over
anhydrous MgSO4. Solutions of each asymmetrical
azo-perester were separated from the drying agent and
concentrated by rotary evaporation at room temperature.
The crude dried products had an oily consistency. Further
purification was conducted by column chromatography
(silica gel as stationary phase and CH2Cl2 as eluent).
The products tert-butyl 4-(tert-butylazo)-4-cyanoperoxy-
pentanoate (3a), tert-butyl 5-(tert-butylazo)-5-cyano-
peroxyhexanoate (3b), and 1,1-dimethylpropyl 6-(tert-
butylazo)-6-cyanoperoxyheptanoate (4c) were obtained in
crystalline form. All other asymmetrical compounds were
obtained as oils.
The obtained asymmetrical azo-peresters were kept
frozen in glass, tightly closed containers. Azo-peresters can
easily decompose at high temperature (over 80 ꢁC) or in
the presence traces of amines, transition metal salts, or
acidic substances. Because of a lack of literature data about
handling such compounds, extreme caution should be
undertaken, similar to work with explosive compounds.
General procedure for preparation of asymmetrical
carboxylic acids with azo functionality
To
a solution of tert-butylhydrazine hydrochloride
(0.16 mol) in 65 cm3 water, a solution of sodium cyanide
123

Synthesis and thermal properties of asymmetrical azo-peresters
275
4-(tert-Butylazo)-4-cyanopentanoic acid (2a)
Yield 66%; m.p.: 86–87 ꢁC (Ref. [3]: m.p.: 79–81 ꢁC).
ppm; 13C NMR (CDCl3, 75 MHz): d = 170.33 (COO),
119.26 (CN), 83.09 (C(CH3)3), 71.43 (C(CN)), 37.58
(CH2), 30.65 (CH2), 26.38 ((CH3)3C), 24.37 (CH3(C)CN),
5-(tert-Butylazo)-5-cyanohexanoic acid
(2b, C11H19N3O2)
23.67 (CH2), 23.56 ((CH3)C) ppm; IR (CH2Cl2):
-1
ꢀ
m = 2,990, 2,250, 1,780, 1,480, 1,375 cm
.
1
Yield 68%; m.p.: 54–56 ꢁC; H NMR (CDCl3, 300 MHz):
d = 11.03 (s, 1H, OH), 2.41–2.36 (t, J = 7.2 Hz, 2H,
CH2CO), 2.12–1.90 (m, 2H, CH2), 1.82–1.72 (m, 2H,
CH2), 1.67–1.58 (m, 2H, CH2), 1.55 (s, 3H, CH3), 1.20
(s, 9H, (CH3)3C) ppm; 13C NMR (CDCl3, 75 MHz):
d = 179.01 (CO), 119.33 (CN), 71.44 (C(CN)), 68.24
((CH3)C), 37.34 (CH2), 33.36 (CH3C(CN)), 26.55
1,1-Dimethylpropyl 4-(tert-butylazo)-4-cyanoperoxy-
pentanoate (4a, C15H27N3O3)
1
Yield 85%; H NMR (CDCl3, 300 MHz): d = 2.46–2.40
(m, 2H, CH2), 2.28–2.22 (m, 2H, CH2), 1.63–1.49 (m, 2H,
CH2), 1.55 (s, 3H, CH3CCN), 1.20 (s, 6H, (CH3)2C), 1.18
(s, 9H, (CH3)3C), 0.92–0.84 (m, 3H, CH3CH2) ppm; 13C
NMR (CDCl3, 75 MHz): d = 169.47 (COO), 118.73 (CN),
85.99 (C(CH3)2C2H5), 70.68 (C(CN)), 68.58 ((CH3)C),
32.93 (CH2CH3), 31.32 (CH2), 26.49 (CH2), 26.20 (CH2),
23.73 ((CH3)CCN), 23.63 (C(CH3)2C2H5), 8.15 (CH2CH3)
((CH3)3C), 23.76 (CH2), 19.28 (CH2) ppm; IR (CH2Cl2):
-1
ꢀ
m = 3,220–2,810, 2,260, 1,715, 1,460, 1,365 cm
.
6-(tert-Butylazo)-6-cyanoheptanoic acid
(2c, C12H21N3O2)
ꢀ
ppm; IR (CH2Cl2): m = 2,970, 2,250, 1,775, 1,445,
1
Yield 62%; m.p.: 52–55 ꢁC; H NMR (CDCl3, 300 MHz):
d = 10.02 (s, 1H, OH), 2.33–2.28 (t, J = 7.5 Hz, 2H,
CH2CO), 2.04–1.81 (m, 2H, CH2), 1.66–1.09 (m, 4H,
CH2), 1.51 (s, 3H, CH3), 1.16 (s, 9H, (CH3)3C) ppm; 13C
NMR (CDCl3, 75 MHz): d = 179.50 (CO), 119.47 (CN),
71.58 (C(CN)), 68.07 ((CH3)C), 37.74 (CH2), 33.55
(CH3C(CN)), 26.51 (CH2), 24.18 ((CH3)3C), 23.80
1,360 cm-1
.
1,1-Dimethylpropyl 5-(tert-butylazo)-5-cyanoperoxy-
hexanoate (4b, C16H29N3O3)
1
Yield 66%; H NMR (CDCl3, 300 MHz): d = 2.35–2.30
(t, J = 7.2 Hz, 2H, CH2CH3), 2.11–1.87 (m, 2H, CH2),
1.82–1.49 (m, 2H, CH2), 1.53 (s, 3H, CH3), 1.20 (s, 6H,
(CH3)2C), 1.18 (s, 9H, (CH3)3C), 0.89–0.84 (t, J = 7.5 Hz,
3H, CH3CH2) ppm; 13C NMR (CDCl3, 75 MHz):
d = 169.93 (COO), 119.22 (CN), 85.67 (C(CH3)2C2H5),
71.29 (C(CN)), 68.22 ((CH3)C), 37.27 (CH2CH3), 31.31
(CH2), 30.66 (CH2), 26.49 (CH2), 23.74 (CH3C(CN)),
ꢀ
(CH2), 23.56 (CH2) ppm; IR (CH2Cl2): m = 3,450–3,160,
2,240, 1,740, 1,350 cm-1
.
tert-Butyl 4-(tert-butylazo)-4-cyanoperoxypentanoate (3a)
Yield 89%; m.p.: 59–61 ꢁC (Ref. [9]: oil); 1H NMR
(CDCl3, 300 MHz): d = 2.52–2.31 (m, 2H, CH2CO),
2.31–2.26 (m, 2H, CH2), 1.54 (s, 3H, CH3), 1.25 (s, 9H,
(CH3)3C), 1.17 (s, 9H, (CH3)3C) ppm; 13C NMR (CDCl3,
75 MHz): d = 169.52 (CO), 118.79 (CN), 83.69
(C(CH3)3), 70.72 (C(CN)), 68.64 ((CH3)C), 33.01 (CH2),
26.63 (CH2), 26.57 ((CH3)3C), 26.08 (CH2), 23.79
23.62 (C(CH3)2C2H5), 19.61 (CH2), 8.12 (CH2CH3) ppm;
-1
ꢀ
IR (CH2Cl2): m = 2,975, 2,245, 1,765, 1,470, 1,375 cm
.
1,1-Dimethylpropyl 6-(tert-butylazo)-6-cyanoperoxy-
heptanoate (4c, C17H31N3O3)
1
Yield 85%; m.p.: 15–19 ꢁC; H NMR (CDCl3, 300 MHz):
d = 2.26–2.21 (t, J = 7.5 Hz, 2H, CH2CH3), 1.97–1.83
(m, 2H, CH2), 1.78–1.74 (m, 2H, CH2), 1.64–1.40 (m, 4H,
CH2), 1.47 (s, 3H, CH3), 1.15 (s, 6H, (CH3)2C), 1.13 (s,
9H, (CH3)3C), 0.84–0.79 (t, J = 7.5 Hz, 3H, CH3CH2)
ppm; 13C NMR (CDCl3, 75 MHz): d = 170.36 (COO),
119.25 (CN), 85.44 (C(CH3)2C2H5), 71.45 (C(CN)), 67.95
((CH3)C), 37.57 (CH2CH3), 31.19 (CH2), 30.70 (CH2),
26.36 (CH2), 24.37 (CH3C(CN)), 23.65 (C(CH3)2C2H5),
ꢀ
(CH3C(CN)) ppm; IR (CH2Cl2): m = 2,980, 2,300, 1,785,
1,465, 1,365 cm-1
.
tert-Butyl 5-(tert-butylazo)-5-cyanoperoxyhexanoate
(3b, C15H27N3O3)
1
Yield 61%; m.p.: 39–43 ꢁC; H NMR (CDCl3, 300 MHz):
d = 2.33–2.28 (t, J = 7.2 Hz, 2H, CH2), 2.07–1.85 (m,
4H, CH2), 1.50 (s, 3H, CH3), 1.23 (s, 9H, (CH3)3C), 1.14
(s, 9H, (CH3)3C) ppm; 13C NMR (CDCl3, 75 MHz):
d = 169.94 (COO), 119.12 (CN), 83.26 (C(CH3)3), 71.21
(C(CN)), 37.19 (CH2), 30.50 (CH2), 26.39 ((CH3)3C),
23.50 (CH2), 23.22 (CH2), 8.07 (CH2CH3) ppm; IR
-1
ꢀ
(CH2Cl2): m = 2,970, 2,245, 1,775, 1,470, 1,375 cm
.
25.39 ((CH3)C), 23.65 (CH3(C)CN), 19.52 (CH2) ppm; IR
-1
ꢀ
(CH2Cl2): m = 2,975, 2,240, 1,770, 1,465, 1,385 cm
.
References
tert-Butyl 6-(tert-butylazo)-6-cyanoperoxyheptanoate
(3c, C16H29N3O3)
1. Zawadiak J, Hefczyc B, Janeczek H, Kowalczuk M (2009)
Monatsh Chem 140:303
2. Shaikh AS, Comanita E, Dumitriu S, Simionescu CI (1981) Angew
Makromol Chem 100:147
3. Sheppard CS, MacLeay RE (1977) US Patent 4,055,714
4. Staab HA, Rohr W, Graf F (1965) Chem Ber 98:1122
1
Yield 80%; H NMR (CDCl3, 300 MHz): d = 2.2–62.21
(t, J = 7.5 Hz, 2H, CH2), 2.17–1.75 (m, 2H, CH2),
1.64–1.57 (m, 2H, CH2), 1.49–1.17 (m, 2H, CH2), 1.47
(s, 3H, CH3), 1.21 (s, 9H, C(CH3)3), 1.13 (s, 9H, C(CH3)3)
123

276
B. Hefczyc et al.
5. Engel PS, He SL, Smith WB (1997) J Am Chem Soc 119:6059
6. Engel PS, Wu A (1994) J Org Chem 59:3969
7. Bates HA, Deng PN (1983) J Org Chem 48:4479
8. Schaeffer JR, Snoddy AO (1963) Org Synth Coll 4:19
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4,088,642
123