Mannaric Acid and Mannaric Acid Polyamides: Synthesis and Characterization
D-Mannose, an aldohexose and a C-2 epimer of the common monosaccharide D-glucose, occurs in a pyranose ring form as a component of a variety of plant polysaccharides and is the third most abundant naturally occurring aldohexose after D-glucose and D-galactose, respectively. Historically, D-mannaric acid has been prepared by the nitric acid oxidation of D-mannose. The first published report comes from Haworth et al. in 1944 with D-mannaric acid isolated as crystalline D-mannaro-1,4:6,3-dilactone. Nitric acid has been employed for the conversion of aldoses to aldaric acid for decades because it is such a potent oxidant. A new method for the nitric acid oxidation of D-mannose has been developed using a Mettler Toledo RC-1 Labmax reactor. The reaction is monitored with GC-MS and IC to ensure complete oxidation. D-mannaric acid which has been isolated as a new derivative, N,N'-dimethyl-D-mannaramide, in a yield of 52%. Characterization of N,N'-dimethyl-D-mannaramide was accomplished with GC-MS, 1H and 13C NMR, elemental analysis, and X-ray crystallography. D-mannaric acid has four chiral centers and a C-2 axis of symmetry but no plane of symmetry and consequently is a chiral molecule. Because of the C-2 axis of symmetry and chirality, polyamides derived from D-mannaric acid are chiral and stereoregular. D-Mannaric acid has been used here as an aldaric acid monomer for a new method of synthesizing polyhydroxypolyamides, PHPAs, based on a stoichiometrically balanced 1:1 ratio of aldaric acid: diamine. Alkylenediammonium salts of D-mannaric acid with varying alkyl chain lengths were synthesized and used as the starting material to give polyamides whose number average molecular weights were determined by 1H NMR end group analysis. All polyamides synthesized were subjected to a post-polymerization in hopes of increasing their size. As with the pre-polymers, the number average molecular weights of polyamides formed from post-polymerizations were determined by 1H NMR end group analysis. X-ray crystal analysis was carried out on octamethylenediammonium D-mannarate to better understand the conformation of the D-mannaryl unit of poly(alkylene D-mannaramides). Computational analysis has been carried out for D-glucaramides and its derivatives, but has never been attempted for D-mannaric acid or any derivatives thereof. In this work, MM3(96) computational analysis was performed on D-mannaric acid, its dimethyl ester, and three amide derivatives to learn more about the shapes and conformations of these molecules in solution with the hope of better understanding the polymerization reaction and gaining some insight into the three dimensional structures of the resultant polymers.
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