Koji Iwamoto

Syntheses of all possible conformational isomers of O-alkyl-p-t-butylcalix[4]arenes

Abstract In p-t-butylcalix[4]arene (1H 4 ) and its mono-, di-, tri-, and tetra-O-alkyl derivatives (1HR 3 , 1H 2 R 2 , 1HR 3 , and 1R 4 respectively), 23 different homologs can exist (including 1H 4 ). We found that the OH group in the unmodified phenol unit is permeable through the calix[4]arene ring. Thus, several conformational isomers become equivalent after the oxygen-through-the-annulus rotation of the OH group and the number of the possible homologs is reduced to 13 (including 1H 4 ). We here report the syntheses of all of these possible conformational isomers by using a protection-deprotection method with a benzyl group and metal template effects. The results are very useful for understanding the conformer distribution in 1H 2 R 2 , 1HR 3 , and 1R 4 , which has confused calixarene chemists for a long time and is still a matter of discussion.

Synthetic approaches to regioisomerically pure porphyrins bearing four different meso-substituents

Abstract Regioisomerically pure porphyrins bearing four different meso -substituents have been synthesized via a 9-step route starting from pyrrole and carbonyl-containing compounds. This synthesis builds on a one-flask synthesis of 1,9-unsubstituted dipyrromethanes. An acyl group is introduced selectively in the 1-position of the dipyrromethane by use of an acid chloride and the dipyrromethane Grignard reagent, which resembles the pyrrole Grignard reagent. In contrast to the 2- and 5-positions of a monomeric pyrrole, the 1- and 9-positions of a dipyrromethane are relatively non-interacting and can be functionalized independently. A 2-aryl-1,3-benzoxathiolium tetrafluoroborate, available from carbonyl containing compounds, serves as a latent acyl equivalent and alkylates regiospecifically the 9-position of a 1-acyldipyrromethane. Alternatively the 1- and 9-positions of a dipyrromethane can be functionalized independently by successive alkylations with two different 2-aryl-1,3-benzoxathiolium tetrafluoroborates. Hydrolysis of the mono or di(benzoxathiolyl)dipyrromethane followed by reduction of the 1,9-diacyl-dipyrromethane affords the corresponding dipyrromethane-diol. An acid-catalyzed MacDonald-type 2 + 2 condensation of the dipyrromethane-diol and a 1,9-unsubstituted dipyrromethane at room temperature followed by oxidation with DDQ gives the porphyrin bearing four different meso -substituents. The reaction sequence resulted in a single porphyrin isomer without acidolytic scrambling of the four meso -substituents. The porphyrin structures were confirmed by laser desorption mass spectrometry and by high field high resolution proton NMR spectroscopy. An entire synthesis can be performed in about two weeks. The controlled stepwise synthesis of porphyrins bearing four different meso -substituents should enable preparation of multi-functionalized porphyrin building blocks for application in the synthesis of bioorganic model systems.

Cone - "Partial-cone" Isomerism in Tetramethoxycalix[4]arenes. Influence of Solvent Polarity and Metal Ions

Abstract It was found that the “cone” - “partial-cone” equilibrium in 25,26,27,28-tetramethoxycalix[4]arene derivatives is sensitively affected by the solvent polarity: the cone% increases with the increase in the solvent polarity. The novel finding was rationalized in terms of the difference in the dipole moment: cone conformers with four dipole moments orientating into the same direction are more polar than partial-cone conformers with one inverted phenyl unit and therefore are more stabilized in polar solvents. Alkali metal cations such as Li + and Na + strongly interact with four methoxy oxygens, resulting in the shift of the equilibrium to cone.

Synthesis and optical resolution of an asymmetrically substituted calix[4]arene

A cone-shaped, asymmetrically substituted calix[4]arene, 1, has been synthesized and successfully optically resolved for the first time.

Fluorogenic calix[4]arene

5-Benzothiazol-1-yl-28-hydroxy-25,26,27-trimethoxycalix[4]arene has been synthesized: this compound acted as a novel fluorogenic calix[4]arene showing ‘perfect’ Li+-selectivity.

How is the 1,2-alternate conformer formed in tetra-O-alkylation of p-tert-butylcalix[4]arene?

The reaction route for the formation of a new 1,2-alternate conformer from p-tert-butylcalix[4]arene has been elucidated.