The chiro-Inositols and Related Cyclitols as Chiral Monomers for Polymerization: Expansion of a Family of Chiral Polymers

Abstract

Three different synthetic approaches to three different classes of oligomeric and polymeric inositol and inositol-like derivatives are presented: Grubbs cross-metathesis polymerization, Lewis acid catalyzed epoxide opening, and head-to-tail Diels-Alder polymerization. All three approaches lead to the formation of oligomers and polymers. The oligomers/polymers formed range from three (low-molecular-weight fraction of Diels-Alder polymerization) to 60 (Lewis acid catalyzed polymerization) repeats of inositol or inositol-like units. The molecular weights of the polymers were characterized by gel-permeation chromatography (GPC), MALDI, and NMR end-group analysis.Chiral polymers are of great interest for diverse applications including their use as stationary phases for the separation of enantiomers 1 or as heterogeneous and easily recoverable ligands or catalysts. 2 The complex, varied, chiral, and highly hydroxylated structure of carbohydrates makes oligosaccharides one of the most useful naturally occurring polymers for these purposes, but the lability of the glycosidic linkage somewhat limits their use in the presence of harsh conditions. An isosteric, but more stable, alternative to the monosaccharides would be the inositols; however, it is more difficult to generate polymers of this class because of the lack of a clear chemoselective differentiation of the hydroxyl groups. The de novo preparation of functionalized polyhydroxycyclohexanes is available by the whole cell fermentation of arenes, such as bromobenzene, with E. coli JM109 (pDTG601A) strain that overexpresses the enzyme toluene dioxygenase. This protocol allows for subsequent selective introduction of the remaining functionalities in the homochiral monomers prior to their polymerization. We would like to report a series of recent efforts in our laboratory towards the preparation of chiral polymers based on D-or L-chiro-inositol and simple tetrahydroxylated derivatives. We have previously reported the synthesis and preparation of several well-defined inositol oligomers, 3 and a bisbutenyl ether of polymer of D-chiro-inositol. 4 Three different approaches and polymeric structures are presented to demonstrate the chemical and structural versatility of these building blocks: a Grubbs metathesis approach providing chiral inositols connected through a carbon-chain linker; an aluminum-catalyzed epoxide opening approach leading to polymers of oxygen substituted cyclohexene oxides lacking spacers between the cyclitols; and a head-to-tail Diels-Alder polymerization of functionalized inositols generating cyclohexene spacers between the repeating monomers. A general method of synthesis for conduritols 5 and inositols, 6 including their fluorinated 7 and isotopically 8 labeled derivatives, has been developed previously in our laboratory and have been extensively reviewed. 9 Mono-or disubstituted benzene rings are enantioselectively cis-dihydroxylated by toluene dioxygenase, providing the required functionality for elaboration into conduritols, inositols, pseudosugars...