From conventional to greener catalytic approaches for carbohydrates etherification - Carbohydrate Chemistry: Chemical and Biological Approaches - page 73

From conventional to greener catalytic
approaches for carbohydrates
etherification
Mathieu Sauthier,* Andr ´ Mortreux and Isabelle Suisse
DOI: 10.1039/9781849739986-00073
The synthesis of ethers derived carbohydrates/polyols used as intermediates in multistep
synthesis or large scale commodity products such as detergents is since a long time an
issue of concern for organic chemists. The conventional Williamson reaction is the most
used synthetic method to build an ether function from a free hydroxyl group. Very
selective transformations that use this procedure have been achieved and progresses
based on this reaction are always of actuality. Alternatively to the Williamson reaction,
catalytic approaches aiming at opening salt free accesses to ethers have more recently
emerged. This evolution is pushed by the growing interest dedicated to greener synthetic
approaches that find a particular echo in the field of carbohydrates chemistry. This chapter
sets up a panorama of the methods used to access ethers derived from the free hydroxyl
groups of carbohydrates/polyols. This overview of course includes the stoichiometric
Williamson reaction, largely and even, in some cases, routinely employed at the laboratory
and industrial scales. A particular attention is given to the recent developments in pro-
tecting groups for multistep synthesis and the large scale production of alkylcelluloses.
New approaches based on the use of homogeneous or heterogeneous catalysis have
more recently emerged and now offer a new panel of reactions to build ether functions.
Several examples of salt free acid or metal catalyzed reactions transformations are
depicted, thus showing progresses that have been and remain to be done in this field.
1 Introduction
Carbohydrates and derived polyols issuing from biomass are promising
alternative carbon sources to oil. They consequently represent an
important class of building blocks towards platform molecules,
surfactants, plasticizers or even materials for construction. Although the
chemistry of carbohydrates is being studied since centuries, this new
international context has pushed a renewed interest to this chemistry in
the last decades. The sustainability of a commercial product reaches a
very high level if both, clean chemical transformations with a limited
environmental impact and agro-based starting materials are used, and
carbohydrates fulfil this latter requirement. In order to obtain new
products with higher added values, carbohydrates can be either com-
pletely disrupted according to complex dehydration or oxidative reactions
or just chemically modified through the functionalization of one or more
hydroxyl groups. The first approach leads to the production of so called
platform molecules that are well identified as promising building blocks
(furfural, hydroxymethylfurfural, levulinic acid, isosorbide, glucaric
acid ...). 1 The non-destructive second approach is more generally based