Synthesis of N-Phtaloyl Amino Acid p-tert-butylcalix [ 4 ] arene Esters

The chemistry of calixarenes has received increasing attention in the field of Supramolecular Chemistry, as this class of molecules offers a suitable platform for the synthesis of receptors for anions, cations and neutral molecules. Their structures allow acylation and alkylaton reactions of the phenolic groups. The conformation and degree of substitution of acylated products are related to the reagent used to promote the reaction; the substituent at para position of the calixarene and the acid chloride. For example, Gutsche reported tetrasubstitution in the reaction of p-tert-butylcalix[4]arene with p-substituted benzoyl chlorides, using AlCl3 or NaH as catalyst. In this case the products had 1,3 alternate, partial cone and cone conformations4a while when pyridine was used as catalyst, only trisubstituted calix[4]arene in partial cone was obtained6. The complexation and transport of racemic mixtures with chiral receptors has been pointed out as a smooth method for enantiomeric resolution, because it requires low concentrations of the receptor, which can be recovered at the end of the process. Chiral receptors based on calix[4]arene structures are obtained by three approaches: reaction of calixarenes with chiral compounds7, use of four different phenols as building blocks for cyclization in the synthesis of calixarenes8, and from functionalization of achiral calixarenes on meta-positions of the aromatic rings, leading to an inherently chiral conformation9. In this work, we report the synthesis of calixarene amino acid esters and their structures have been elucidated by 1H NMR spectroscopy. Results and Discussion

The chemistry of calixarenes has received increasing attention in the field of Supramolecular Chemistry, as this class of molecules offers a suitable platform for the synthesis of receptors for anions, cations and neutral molecules.Their structures allow acylation and alkylaton reactions of the phenolic groups.
The conformation and degree of substitution of acylated products are related to the reagent used to promote the reaction; the substituent at para position of the calixarene and the acid chloride.For example, Gutsche reported tetrasubstitution in the reaction of p-tert-butylcalix [4]arene with p-substituted benzoyl chlorides, using AlCl 3 or NaH as catalyst.In this case the products had 1,3 alternate, partial cone and cone conformations 4a while when pyridine was used as catalyst, only trisubstituted calix [4]arene in partial cone was obtained 6 .
The complexation and transport of racemic mixtures with chiral receptors has been pointed out as a smooth method for enantiomeric resolution, because it requires low concentrations of the receptor, which can be recovered at the end of the process.Chiral receptors based on calix [4]arene structures are obtained by three approaches: reaction of calixarenes with chiral compounds 7 , use of four different phenols as building blocks for cyclization in the synthesis of calixarenes 8 , and from functionalization of achiral calixarenes on meta-positions of the aromatic rings, leading to an inherently chiral conformation 9 .
In this work, we report the synthesis of calixarene amino acid esters and their structures have been elucidated by 1 H NMR spectroscopy.

Results and Discussion
N-Phthaloyl amino acid calix [4]arene esters 1 -6 were synthesized from p-tert-butylcalix [4]arene and the corresponding N-Pht amino acid chlorides in the presence of Et 3 N, using a molar ratio of 10:10:1 of Et 3 N: amino acid chloride: p-tert-butylcalix [4]arene, respectively.In the case of the glycine derivative, the reaction was also performed using AlCl 3 .In this reaction, a molar ratio of 70:10:1 of AlCl 3 : amino acid chloride: p-tert-butyl-calix [4]arene was used and only non-reacted calixarene was recovered when a molar ratio of 20:10:1 was used.This lack of efficiency in lower AlCl 3 , amino acid chloride ratio is attributed to the Lewis acid characteristics of AlCl 3 , that would be inactivated by interaction with the lone pairs of N-phtaloyl amino acid derivative.
Both methods yield bis substituted products on distal phenolic oxygens, and the product of tetra-substitution was not isolated, although a large excess of all reagents in relation to calixarene was used.The products adopt 1,3-alternate conformation, as two semi-calixes, an unusual pattern for bis-substituted calix [4]arenes, by the lack of intramolecular hydrogen bonds as additional stabilization forces.The yields of the reactions were 70-80 %, and the pure products were obtained after recrystallization from chloroform-methanol, with exception of Gly derivative 1 which was recrystallized from chloroform.
In the IR spectra, the OH stretching (ν OH ) appears as a narrow band above 3500 cm -1 for all aminoacid ester derivatives.The frequency depends on the aminoacid residue, and is higher for bulkier residues (N-Pht-Phe 2, N-Pht-Leu 3; ν OH 3550 cm -1 ).This is consistent with a 1,3-alternate conformation, adopted by the calixarene, where the remaining phenolic hydrogens are placed at opposite sides in relation to the substituted oxygens.Consequently, intramolecular hydrogen bonds between O-H and substituted oxygens are not possible.In the case of bis-substituted cone conformers, whose four oxygen atoms are located in suitable positions for intramolecular hydrogen bonds, a broadening of the bands and a decreasing of the frequency (ν OH 3200-3300 cm -1 ) is observed 7b .
The 1 H NMR of the N-Pht-Gly derivative 1 shows two tert-butyl signals with equal integrals at δ 1.11 and 1.38 , attributed to the tert-butyl hydrogens of the phenolic and the ester residues, and two doublets that correspond to methylenic hydrogens at δ 3.65 and 3.81; the difference of ∆δ 0.16 between both signals is typical of a 1,3 alternate conformer.This ∆δ value can be used to assign the structure, where higher values (∆δ ~ 1.0) are obseved for the cone conformer (less symmetric conformer) and lower values for 1,3 alternate conformer (more symmetric conformer).shielded than in the corresponding N-Pht-Gly derivative, with doublets centered at δ 3.40 and 3.70.
For chiral amino acid esters, the spectra are more complex for two reasons: diastereotopism of aromatic and methylenic hydrogens and racemization, with consequent formation of diastereoisomers.The diastereoisomeric excess was estimated by the difference in the integrals of the hydrogen signals for both diastereoisomers, and was found to be 72% for calixarene N-Pht-Phe 2, 50% for N-Pht-Leu 3 and bis-N-Pht-Orn 5 derivatives and complete racemization for N-Pht-Ala ester 6.The degree of racemization seems to be related to the volume of the side chain, that makes difficult the removal of the α-hydrogen by the base.
The 1 H NMR of 6 shows three tert-butyl signals and eight doublets, corresponding to the methylene signals.Surprisingly, the phenolic hydrogen signals are affected by the chiral centers, and an integration ratio of 1:2:1 was observed.For the (R, R)/(S, S) pair, both phenolic hydrogens are magnetically equivalent, but the (R, S)/(S, R) pair presents two signals corresponding to phenolic hydrogens, which indicates that intra-or intermolecular hydrogen exchange does not occur in NMR time scale.Scheme 1: Synthesis of p-tert-butylcalix [4]arene-N-Pht-amino acid esters.Noteworthy are the chemical shifts of tert-butyl protons.The δ values for tert-butyl hydrogens of substituted phenol rings, located on the same side of non-reacted OHs remain unchanged in the amino acid series (δ ~ 1.35) , while for hydrogens of tert-butyl groups of unsubstituted rings, located in the same side of N-Pht amino acid, we observed a shift to higher field with the increase of the size of the side chain of the amino acid.The highest δ value corresponds to N-Pht-Gly derivative 1 (δ 1.11) and lowest for N-Pht-Leu 3 (δ 0.83).This indicates an increase in steric crowding between the side chain of the amino acid and the tert-butyl group of the unsubstituted phenolic unit.Consequently, there is a distortion of the structure that may also be detected by the difference in the values of NMR signals of the methylenic hydrogens (∆δ) .Here, the lowest δ value corresponds to N-Pht-Gly derivative and the highest corresponding to N-Pht-Phe derivative.This feature is attributed to differences in the chemical environment between these hydrogens for branched amino acids, related with the proximity with phenolic oxygens.
The analysis of these data allows to conclude that the geometrical parameters of the cavity can be controlled by the linked amino acid, and this result in a a fine tuning of opening of calixarene hollow.
In conclusion, bis-substituted calix [4]arenes amino acid esters with 1,3 alternate conformation were obtained, in contrast to the usual cone conformation of bis-substituted derivatives, which may be further functionalized by acylation or alkylation reactions of the remaining phenolic oxygens.As a sequence of this work, ditopic receptors will be synthesized by linkage of groups with metal complexational properties (e. g. crown ethers) .