Etherification pathways

In an effort to identify the origin of the formation of the minor diastereomer 19 and understand whether its formation was a function of a breakdown in the SN2 pathway leading to an SN1 pathway, the activation of the chiral imidate 67 was next investigated. In the etherification reaction between 10 and 67, the acid catalyst increases the electrophilicity of imidate 67 through coordination between the acid... 

When the concentration of BF3 increased, competitive deactivation leading to the formation of 97 resulted in unreacted starting material at the end of the etherification reaction. Efforts to break up this coordination and increase the conversion by the addition of certain additives, such as water, NaPlv, KPF6, l.iPF(l, or NasSiF6, either led to no improvement in conversion or to a complete shut down in the SN2 pathway and significant erosion in the diastereoselectivity resulted. 

Several methods and reaction pathways have been reported for the conversion of glycerol in the literature, such as etherification, esterification [1], and oxidation [2], Via ionic dehydration acetol [3] and acrolein can be produced. The radical steps result in aldehydes, allyl alcohol, etc. [4], If the dehydration is followed by a hydrogenation step, propanediols (1,2- or 1,3-) can be obtained [5-6]. 

Here we report an overview of the different heterogeneously-catalyzed pathways designed for the selective conversion of carbohydrates. On the basis of these results, we shall try to determine the key parameters allowing a better control of the reaction selectivity. Water being commonly used as solvent in carbohydrate chemistry, we will also discuss the stability of solid catalysts in the aqueous phase. In this review, heterogeneously-catalyzed hydrolysis, dehydration, oxidation, esterification, and etherification of monosaccharides and polysaccharides are reported. 

An interesting alternative in using diols, but starting from 3,4-dihydroxythiophene-2,5-dicarboxylic acid diethyl ester (the same intermediate as used in the Gogte pathway above), was developed independently by the groups of Reynolds [25] and Bauerle [26], who utilized the Mitsunobu reaction with azodicarboxylic acid ester-phosphane as the etherification agent. 

Scheme 8.20 Reaction Pathway for the Etherification of Glycols and Fatty Alcohols. With...

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