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In organic chemistry, an ortho ester is a functional group containing three attached to one carbon atom, i.e. with the general formula . Orthoesters may be considered as products of exhaustive alkylation of unstable Ortho acid and it is from these that the name 'ortho ester' is derived. An example is ethyl orthoacetate, , more correctly known as 1,1,1-triethoxyethane. In total synthesis, bicyclic OBO ortho esters are used as for carboxylic acids and esters.
can also drive the reaction from the corresponding amide, e.g.:
Although a less common method, ortho esters were first produced by reaction of 1,1,1-trichloroalkanes with sodium alkoxide:
Carboxylic acids naturally form a trithio ortho ester when heated with a mercaptan of appropriate stoichiometry. The resulting compound undergoes transesterification to a traditional orthoester in the presence of zinc chloride.
Transesterification from a cheaper ortho ester is also possible; but performs best with unstabilized (electron-poorer) ortho esters. Stabilized ortho-esters tend to collapse to the corresponding non-ortho ester.
For example, trimethyl orthoformate CH(OCH3)3 may be hydrolyzed (under acidic conditions) to methyl formate and methanol; and may be further hydrolyzed (under alkaline conditions) to salts of formic acid and methanol.United States Patent Application 20070049501, Saini; Rajesh K.; and Savery; Karen, March 1, 2007
Adamantane-structured orthoesters, formally derived from all- cis-1,3,5-trihydroxycyclohexane, hydrolyze particularly slowly, for which reason they have been used as .
The threefold symmetry of the cyclohexanehexol isomer scyllo-inositol (scyllitol) yields the triply-bridged orthoformate esters scyllitol orthoformate with an adamantane-like skeleton, and scyllitol bis-orthoformate with a diamantane-like skeleton.
Compounds with an adjacent hydrogen atom on R tend to undergo elimination instead. Traditional can be converted to α,αdichloro ethers with phosphorus pentachloride. The resulting halogenated compounds undergo ether synthesis like the trichloroalkanes.
Reactions
Hydrolysis
Ortho esters are readily hydrolysis in mild aqueous acid to form :
Johnson–Claisen rearrangement
The Johnson–Claisen rearrangement is the reaction of an alcohol with an ortho ester containing a deprotonatable alpha carbon (e.g. triethyl orthoacetate) to give a ester.
Bodroux–Chichibabin aldehyde synthesis
In the Bodroux–Chichibabin aldehyde synthesis an ortho ester reacts with a Grignard reagent to form an aldehyde; this is an example of a formylation reaction.
Examples
Examples of orthoesters include the reagents trimethyl orthoformate and triethylorthoacetate. Another example is the bicyclic OBO protecting group (4-methyl-2,6,7-tri oxa- bicyclo2.2.2 octan-1-yl) which is formed by the action of on activated carboxylic acids in the presence of Lewis acids such as . The group is base stable and can be cleaved in two steps under mild conditions. Mildly acidic hydrolysis yields the ester of tris(hydroxymethyl)ethane which is then cleaved using e.g. an aqueous carbonate solution.
See also
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