It allows you to transform an ester into the corresponding β - dicarbonyl compound .
It involves the use of monoethyl malonic acid and magnesium ethoxide.
Primarily the monoethyl malonic acid is treated with magnesium ethoxide (since Mg is a divalent metal it binds two alkoxide ions). According pretty complicated and only recently well-defined mechanism, the base generates two negative charges on monoethyl malonic acid ;a proton is torn off at the carboxyl portion, while the other is torn to the carbon placed between the two carbonyls. This proton is more acidic because carbon to which it is tied reflects the attraction of the two adjacent carbonyls ( δ + ). It follows that also this carbon has a partial positive charge ( δ + ) and easily yields the proton to rebalance its electronic situation.
Of the two species negatively charged (C and O), that is certainly carbanion the more nucleophilic. Now the ester on which you want reacting the carbanion is inserted. The carbanion will add to the electrophilic centre of the ester.
Eventually leading to the compound in an acid medium (slightly heat) it has decarboxylation with formation of the β- keto ester .Remember that when a carboxyl group is located in relation 1 - 3 with a carbonyl group (then it is a β- keto acid ), tends to spontaneously decarboxylate (see previous article ).
The Masamune reaction applications are the same as the Claisen condensation. Both systems allow you to get 1-3 carbonyl (or β- dicarbonyl) compounds and these are primarily used in the construction of new C - C links.
The simplest thing you can do is to generate the carbanion in position between the two carbonyl of β- ketoester to let him make a nucleophilic substitution on an halogen derivative.