Transesterification promoted by alkoxides

Description: Addition of alkoxide to an ester results in a new ester. This is called “transesterification”.
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Notes: An “alkoxide” is the conjugate base of the alcohol. This is an equilibrium reaction – the desired product is formed by using a large excess of the requisite alkoxide.

Examples:

Notes: Note that the third example is opening of a ring (often gives students trouble!)

Mechanism: Addition of alkoxide to the ester (Step 1, arrows A and B) results in a tetrahedral intermediate, which undergoes elimination of the original alkoxide (Step 2, arrows C and D) to give the new ester.

Notes: Note that this reaction is an equilibrium. It is driven to completion by the use of excess alkoxide.

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