A. Palma et al. / Tetrahedron Letters 56 (2015) 674–676
675
O
C
O
O
O
O
O
C
O
O
O
O
O
O
O
H
O
O
O
i
R
R
R
3
R1
R
O
O
HO
OH
Δ
O
O
R
R
3
O
O
O
O
O
O
R1
C
O
O
O
C
O
H O
O
O
2
8
n
2
R =
Scheme 2. General reaction scheme for the synthesis of poly b ketoesters.
a
b
Scheme 5. Synthesis of diketo-diesters 8a,b. Reagents and conditions: (i) Solvent in
a sealed tube (PhMe or 1,4-dioxane), 2,2-dimethylpropan-1-ol (2 equiv), 100 °C.
O
OtBu
iii
O
O
N
ii
O
O
N
N
O
O
O
O
COOH
i
O
C
O
O
O
O
COOH
O
H
O
O
N
HO R1 OH
O
6a
OtBu
O
2a
O
R
Δ
O
O
R
R
3
4a
5
O
R1
Scheme 3. Synthesis of 2a. Reagents and conditions: (i) CDI, THF, 100%; (ii) LDA,
tBuOAc, THF, À30 °C, 100%; (iii) Me2CO (20 equiv), Ac2O (30 equiv), H2SO4, 0–20 °C,
60% yield over 2 steps.
O
O
C
O
H
O
O
O
n
2
P1-16
O
OtBu
iii
O
O
Me
N
O
O
O
O
O
O
OMe
R1
=
COOH
R =
ii
i
a
b
OMe
Scheme 6. Synthesis of poly b ketoesters P1–16.
COOH
N
Me
O
OtBu
O
O
sets of conditions, the conversion yields of double dioxinones
2a,b into the double b ketoesters 8a,b were greater than 95%. Both
sets of conditions were subsequently applied to generate a variety
of different poly b ketoesters by employing a number of different
diols (Scheme 6).
4b
7
6b
2b
Scheme 4. Synthesis of 2b. Reagents and conditions: (i) a) ClCOCOCl, CH2Cl2, DMF
(catalyst); (b) MeNH2OMeÁCl, pyridine, CH2Cl2, 0–20 °C, 85% yield over 2 steps; (ii)
t
LDA, BuOAc, THF, À30 °C, 100%; (iii) Me2CO (20 equiv), Ac2O (30 equiv), H2SO4, 0–
20 °C, 65% yield over 2 steps.
Polymers P1–16 (Table 1) were characterized using 1H NMR and
IR spectroscopy, mass spectrometry and GPC techniques. The 1H
NMR spectra showed broad peaks in the expected regions charac-
teristic of polymeric material. 1H NMR peaks at 5.21 ppm and
1.65 ppm corresponding respectively to the proton on the sp2 car-
bon of the dioxinone unit and the methyl groups in the starting
materials 2a,b were not present in all products P1–16. GPC analyses
of the polymers P1–16 were performed in order to acquire struc-
tural information about the size of the polymers. This was carried
sulfuric acid gave the double dioxinone 2a in 60% yield over the
two steps (Scheme 3).
Double dioxinone 2b was readily prepared starting from tere-
phthalic acid (4b), which was converted into the diamide 7 in
85% yield over two steps. The lithium enolate prepared by deproto-
nation of tert-butyl acetate with LDA, was allowed to react with
diamide 7 yielding diketo-diester 6b. Subsequent reaction of
diketo-diester 6b with a mixture of acetic anhydride, acetone
and concentrated sulfuric acid gave the double dioxinone 2b in
65% yield over two steps (Scheme 4).
out with a Polymer Labs PL-GPC 50 with two PL-Gel 5 lm MIXED-
D columns at 40 °C using DMF with 1% v/v triethylamine and 1% v/
v acetic acid as the eluent at a flow rate of 0.5 ml/min. Solutions of
all samples were filtered using Whatman Puradisc 13 200 nm PTFE
syringe filters. These GPC analyses showed a clear relationship
between the size of the polymers and the reaction conditions
employed with Mw values significantly increasing when 1,4-diox-
ane was used as the solvent compared to toluene (Table 1 and
Fig. 1).16
In conclusion, syntheses of several prototype poly b ketoesters
from double dioxinones and diols are reported. The key polymeri-
zation process was the generation of acyl ketene intermediates via
retro-Diels Alder fragmentation of 6-substituted-2,2-dimethyl-4H-
1,3-dioxin-4-ones 2a,b and trapping with a range of diols. Further
Generation of acylketenes 3a,b via the thermolysis of double-
dioxinones 2a,b at 100 °C, and trapping with 2,2-dimethylpro-
pan-1-ol was next examined (Scheme 5).15
Since we considered that solvent polarity would have a major
impact on polymer Mw and MwD, both toluene (bp 110 °C) and
1,4-dioxane (bp 104 °C) were examined, as the reflux temperature
of each is appropriate for the generation of the acylketenes. These
test reactions were carried out in parallel where the other reaction
conditions were kept constant (concentration, temperature and
reaction time). After evaporation of the solvent, the crude residues
were analyzed by 1H NMR spectroscopy. Pleasingly, under both