22 Molecular Recognition by Inclusion …
469
22.3.2 Enantioselective Inclusion of Alcohols Into Salts
Between N-Trityl (S)-Tyrosine and Tert-Butylamine
[33]
In 2012, we reported that N-tritylamino acid tert-butylamine salts enantioselectively
included aliphatic alcohol (Fig. 22.12) [34]. Salts between carboxylic acids and
primary amines have an inherent one-dimensional hydrogen bonding network. In
this work, we utilized trityl groups to break the hydrogen bonds and form inclusion
cavities, and alcohols were included to compensate for the loss of hydrogen bonds. In
tert-butylammonium salts of N-tritylphenylalanine, enantioselective inclusion of 1chloro-2-propanol was observed in 69% ee(S). This salt host included only alkanols
and benzyl alcohol, but 1-arylethanols such as 1-phenylethanol were not included at
all.
Here, we report additional developments of N-tritylamino acid tert-butylamine
salts. In our previous work, the enantioselective inclusion ability of phenylalanine
was superior to five other amino acids we examined (Ala, Val, Leu, Ile, Phe, and
Phg (an abbreviation for phenylglycine)). Among about twenty naturally occurring
amino acids, the structure of tyrosine is close to phenylalanine, whereas the hydroxy
group is in the para-position of the phenyl group. We theorize whether the hydroxyl
group incorporates the hydrogen bonding network and affects their inclusion ability
and enantioselectivity (Fig. 22.13).
To evaluate the ability of the host to include secondary alcohols with enantioselectivity, we chose several 1-arylethanols as racemic alcohols. The crystalline
host (tert-butylammonium salts of N-trityltyrosine: Tr-Tyr·tBuNH 2 ) and racemic 1arylethanols (4 equiv.) were dissolved in toluene and recrystallized by diffusion of
hexane vapor. The Tr-Tyr·tBuNH 2 included the corresponding alcohols to yield 1:1
inclusion crystals, which were collected by filtration and confirmed by a
1 H NMR
spectra. After inclusion crystals were dissolved in chloroform, a 1 M aqueous NaOH
Fig. 22.12 Crystallization of Tr-Phe·tBuNH 2 with alcohols to prepare inclusion crystals
469
22.3.2 Enantioselective Inclusion of Alcohols Into Salts
Between N-Trityl (S)-Tyrosine and Tert-Butylamine
[33]
In 2012, we reported that N-tritylamino acid tert-butylamine salts enantioselectively
included aliphatic alcohol (Fig. 22.12) [34]. Salts between carboxylic acids and
primary amines have an inherent one-dimensional hydrogen bonding network. In
this work, we utilized trityl groups to break the hydrogen bonds and form inclusion
cavities, and alcohols were included to compensate for the loss of hydrogen bonds. In
tert-butylammonium salts of N-tritylphenylalanine, enantioselective inclusion of 1chloro-2-propanol was observed in 69% ee(S). This salt host included only alkanols
and benzyl alcohol, but 1-arylethanols such as 1-phenylethanol were not included at
all.
Here, we report additional developments of N-tritylamino acid tert-butylamine
salts. In our previous work, the enantioselective inclusion ability of phenylalanine
was superior to five other amino acids we examined (Ala, Val, Leu, Ile, Phe, and
Phg (an abbreviation for phenylglycine)). Among about twenty naturally occurring
amino acids, the structure of tyrosine is close to phenylalanine, whereas the hydroxy
group is in the para-position of the phenyl group. We theorize whether the hydroxyl
group incorporates the hydrogen bonding network and affects their inclusion ability
and enantioselectivity (Fig. 22.13).
To evaluate the ability of the host to include secondary alcohols with enantioselectivity, we chose several 1-arylethanols as racemic alcohols. The crystalline
host (tert-butylammonium salts of N-trityltyrosine: Tr-Tyr·tBuNH 2 ) and racemic 1arylethanols (4 equiv.) were dissolved in toluene and recrystallized by diffusion of
hexane vapor. The Tr-Tyr·tBuNH 2 included the corresponding alcohols to yield 1:1
inclusion crystals, which were collected by filtration and confirmed by a
1 H NMR
spectra. After inclusion crystals were dissolved in chloroform, a 1 M aqueous NaOH
Fig. 22.12 Crystallization of Tr-Phe·tBuNH 2 with alcohols to prepare inclusion crystals