Characteristic activity of phosphorous acid in the dehydration condensation of a chitin-derived nitrogen-containing sugar alcohol

Chitin is a potential resource for the production of renewable organonitrogen compounds, as it contains biologically fixed nitrogen. Hydrolysis of chitin followed by hydrogenation gives the sugar alcohol named 2-acetamido-2-deoxysorbitol (ADS). The dehydration condensation of ADS produces 2-acetamido-2-deoxyisosorbide (ADI), which is a prospective precursor of nitrogen-containing polymers. However, the conversion of ADS to ADI in previous works needed a large amount of CF3SO3H as a superstrong acid catalyst, which is high cost and requires special corrosion-resistant equipment. In this work, we have found that H3PO3, a weak acid, can convert ADS to ADI. Possibly, H3PO3 alters the reaction mechanism from a simple acid-catalysed dehydration cyclisation to a cyclisation via phosphite esters. Density functional theory calculations suggest that the modified pathway via the phosphite esters significantly reduces the activation energy. In addition, the reducing ability of the acid likely decreases the formation of condensation by-products.