MODELO DE DATOS

The synthesis and characterisation of amino acid-based BTA precursors used is shown below in Scheme 4.2. The amino acids chosen were phenylalanine methyl ester and the ethyl ester derivative, along with the alkyl ester 60 and its carboxylic acid derivative 63 described in Chapter 2. These were chosen due to their aggregation properties and previous reports of gelation behaviour.92,166,169 The synthesis was achieved using the established conditions for the tri-substitution of benzene-1,3,5-tricarbonyl trichloride, using the ester protected amino acids in the presence of base. In this case, the reactions were carried out in dry DCM under an argon atmosphere in the presence of Et3N and left to stir at room temperature for 24 h. Once complete

Scheme 4.2 Synthetic scheme for the BTA derivatives where (i) rt, dry DCM, Et3N, argon, 24 h and (ii) MeCN or

Chapter 4. Network polymers derived from BTA-based precursors

and following an aqueous workup, the ester derivatives were recovered as white solids in good to excellent yields. 60 was hydrolysed to form the carboxylic acid derivative 63 by base hydrolysis as described in Chapter 2. The compounds were then fully characterised by 1H and

13_C{1_{H} NMR spectroscopy, IR spectroscopy, mass spectrometry and melting point and were}

consistent with the tri-substituted products. The 1H NMR spectra of the BTA phenylalanine derivatives 74a and 74b, as with the derivatives discussed in Chapter 2, showed the aromatic protons to appear as a sharp singlet around 8.3 ppm, indicative of C3 symmetry of the molecule,

Figure 4.1.

After the synthesis of the BTA derivatives, the attachment of the alkoxy amine chains was attempted using a variety of reaction conditions. Firstly, the phenylalanine derivative (3 equivalents) was dissolved in (i) MeCN or (ii) MeOH, followed by the addition of amine 75 (1 equivalent) to the respective reaction mixtures. The reactions were then refluxed for 24 h. The resulting reaction mixtures were analysed by TLC and indicated the presence of the free amine chain under the UV light after staining with ninhydrin, along with the presence of starting material, thus indicating that the reaction did not work or was very inefficient. This reaction was also carried out in neat, excess amine, however, this too proved unsuccessful.

Figure 4.11_{H NMR (600 MHz and 400 MHz respectively, DMSO-d}

6) of the three BTA derivatives used, 63

Chapter 4. Network polymers derived from BTA-based precursors

Changing the approach slightly, it was decided to use a carboxylic acid derivative, 63, and first convert it to its acyl chloride derivative using either oxalyl chloride or thionyl chloride, followed by reaction with the amine, as shown in Scheme 4.3. 63 was dissolved in SOCl2 and

with DMF as catalyst and left to stir at 50 °C for 24 h. The SOCl2 and DMF were then removed

by vacuum distillation to reveal a yellow-ish oil. Due to the reactivity of acid chlorides, this product was used without further purification. Six equivalents of the amine (assuming quantitative yield), was dissolved in dry MeCN and Et3N, with the acid chloride (in dry MeCN)

was added to this mixture over a one-hour period using a syringe pump, resembling a pseudo high dilution method in order to minimise di/trimerization and polymerisation.170 This reaction mixture was then left to stir at room temperature for 48 h, after a yellow oil was recovered and analysed by 1H NMR spectroscopy. The 1H NMR spectrum did not show the presence of as many alkyl protons as would be expected and looked to show the presence of only one amine peak. One of the potential reasons for these reactions being unsuccessful could be due to the length of the chains and the increased steric bulk on each sequential amide formation making

Scheme 4.3 Reaction of 63 with (i) thionyl chloride or oxalyl chloride to result in the acid chloride, followed by (ii) reaction with the amine.

Chapter 4. Network polymers derived from BTA-based precursors

the tri-substitution inefficient. Successful tri-substitution of a BTA core with bulky groups also proved difficult with some of the derivatives discussed in Chapters 2 and 3. Taking this into consideration, the approach was again modified and is reflected in Scheme 4.4. The amine was mono-protect one of the amine with a tert-butyloxycarbonyl (Boc) group as per a literature procedure, thus allowing the use of more forcing and varied conditions while preventing unselective or uncontrolled polymerisation.162 This involved dissolving the amine in dry DCM, followed by addition of the Boc anhydride under an argon atmosphere at room temperature for 24 h. After 24 h, the organic layer was subject to aqueous workup, affording a pale-yellow oil. The 1H NMR spectrum of the product was consistent with what was expected and with that reported in the literature.162

The next step in the reaction was to react the boc-protected amine with benzene-1,3,5- tricarbonyl trichloride, Scheme 4.4. The benzene-1,3,5-tricarbonyl trichloride was treated with 3.3 equivalents of Et3N and 75 in dry DCM. This was then left to stir at room temperature for

72 h. The reaction mixture was then analysed by TLC which revealed the presence of a number Scheme 4.4 Proposed reaction scheme for the generation of a cross-linked polymer system, where (i) di-tert-butyl dicarbonate, dry DCM, rt, 24 h., (ii) dry DCM, Et3N, rt, 72 h.

Chapter 4. Network polymers derived from BTA-based precursors

of species which were separated by column chromatography on silica. One of the fractions was found to contain benzene-1,3,5-tricarboxylic acid, indicating the inefficiency of the reaction. Another component was isolated as a yellow oil, potentially the desired product and was analysed by NMR spectroscopy, Figure 4.2. Despite attempted purification by column chromatography, the NMR spectrum showed a number of impurities. For the successful tri- substitution of the core, it would be expected that the C3 symmetry of the molecule would result

in CH2 protons that are symmetric and result in quite a simple NMR spectrum with one set of

chemical shifts for the arms. In this case, however, there are more resonances in the aliphatic region that expected, indicating that the tri-substitution was not successful, and a mixture of asymmetric products was obtained. The aromatic region contained three prominent shifts which would be expected for the aromatic core protons and the two amides, however, it too contained a number of other smaller resonances, indicating the presence of other species. A similar observation was made for the 13C{1H} NMR, in which there was an excess of signals and can be seen in the appendix.

Figure 4.2 1_{H NMR (400 MHz, DMSO-d}

6) of 79 showing the excess peaks in the aliphatic region and the inset

Chapter 4. Network polymers derived from BTA-based precursors

Taking these results into account, the approach was modified further and a shorter chain amine, 2,2(ethlenedioxy)bis(ethylamine), was used for the tri-substitution of a leucine functionalised BTA core, shown in Scheme 4.5. This was inspired by a report by Chen and co- workers in which they reported the synthesis of lysine-based, hyperbranched polymers that mimic the behaviour of endosomolytic cell-penetrating peptides for the purpose of drug delivery.167 These polymers contained an aromatic core, decorated with lysine side chains and cross-linked with aromatic groups; the chains described here were more hydrophilic in nature. The first step in this approach was to mono-protect one of the amines of the chain, using the same procedure as for the 75, Figure 4.4.162 Once the reaction was complete, the product was analysed by NMR spectroscopy and mass spectrometry, with the 1H NMR spectrum showing the appearance of the CH3 of the Boc protecting group. The mono-protection was also

confirmed by the mass spectroscopy, as detailed in 6.2.3. Following this, it was again attempted to attach this group to the leucine functionalised BTA core, using a coupling reaction, using the

Scheme 4.5 Synthetic scheme for the synthesis of a leucine-based BTA polymer, 86, where (i) di-tert-butyl dicarbonate, dry DCM, rt, 24 h, (ii) EDC.HCl, DMAP, dry DCM, rt, argon, 72 h, with (iii) and (iv) being the planned further steps.

Chapter 4. Network polymers derived from BTA-based precursors

same conditions as used previously in Chapters 2 and 3 for the coupling of an amine and carboxylic acid. Unfortunately, this reaction was unsuccessful, with the 1_{H NMR of the material}

isolated after the reaction found to be the BTA-leucine starting material.

The last attempt made to generate an extended BTA polymer was to first couple a simple amino acid to the amine chain and then attempt to react this with benzene-1,3-5-tricarbonyl trichloride, as demonstrated in Scheme 4.6 to generate an extended polymer by reaction of both amines with the BTA-Cl. The first step was to protect the amine of the leucine which was carried out using a procedure reported by Adolfsson and co-workers.171 This was then to be coupled to the mono-protected amine chain previously synthesised, using the standard coupling reaction conditions. This resulted in a white solid which was then subject to column chromatography. One of the fractions obtained appeared to contain the product by TLC analysis, thus this was evaporated to give a small amount of yellow oil. NMR studies, along with mass spectrometry indicated that the reaction was a success, however, the poor yield, the presence of impurities even after purification attempts and general inefficiency of the reaction,

Scheme 4.6 Synthetic scheme for the polymer with the shorter chain where (i) di-tert-butyl dicarbonate, dry DCM, rt, 24 h, (ii) dry DCM, EDC.HCl, DMAP, argon, rt, 72 h, with (iii) and (iv) representing planned further steps.

Chapter 4. Network polymers derived from BTA-based precursors

prompted a change in focus to generating more dynamic polymers which will be discussed in the next section.