The Indium-Mediated Acyloxyallyation – A Tool for the Elongation of Aldoses Towards Higher-Carbon Sugars and Non-Natural Sugar Alcohols

Abstract

In carbohydrate chemistry, the synthesis of higher-carbon sugars and derivatives has been a challenge over decades. The addition of carbon atoms to the anomeric center of reducing sugars is often plagued by poor diastereoselectivity and most transformations require protection of the sugar. With the Indium-mediated acyloxyallylation (IMA) a method was developed and investigated in the last years, that allows direct elongation of unprotected aldoses by three carbons with high control of diastereoselectivity.[1,2]

In general, polyhydroxylated organic compounds are of high interest since they remain valuable for applications in various disciplines such as medicinal chemistry, material science or synthetic chemistry. Sugar alcohols, for example, can act as sugar mimics and chiral synthons and are therefore gaining popularity in the design of drugs and functional materials. Focusing on the latter, natural sugar alcohols like, e.g., erythritol or D-mannitol have been investigated intensively as potential phase change materials (PCMs) for thermal energy storage applications. With thermal storage densities of up to 350 kJ/kg, this substance class is considered as a promising candidate for thermal energy storage among organic materials. Extraordinarily high thermal storage densities of up to 450-500 kJ/kg have been predicted for experimentally unknown non-natural sugar alcohols in a recent computational study.[3] These outstanding values were found for sugar alcohols that fulfil three structural criteria: a linear carbon backbone, an even number of carbon atoms and 1,3-anti configuration of all hydroxyl groups. Based on the IMA as a tool for the elongation of reducing sugars in the required stereochemical fashion we developed an efficient strategy for the synthesis of non-natural sugar alcohols with more than six carbon atoms in the backbone that fulfil the stated rules. Our focus was on the synthesis of higher sugar alcohols with a syn-relationship between the two hydroxyl groups at the terminal stereocenters and a 1,3-anti-relationship of all hydroxyl groups, which we refer to as the "galacto-series". Furthermore, some of the investigated sugar alcohols with manno-configuration presented in the computational study [3] were accessible via IMA and the calculated values could be confirmed with experimental data.

[1] Palmelund A.; Madsen.R., Eur. J. Org. Chem., 2005, 70(20), 8248-8251. [2] Draskovits M.; Stanetty C., Baxendale I.R., Mihovilovic M.D., J. Org. Chem., 2018, 83(5), 2647-2659. [3] Inagaki T.; Ishida T., J. Am. Chem. Soc., 2016, 138, 11810-11819.