Typical reactions catalyzed by proton transfer are esterfications and aldol reactions. These imines can be used to direct subsequent reactions like an aldol condensation. In the usual aldol addition, a carbonyl compound is deprotonated to form the enolate. Carbonyls also may be alkylated by enolates as in aldol reactions. Enones are typically produced using an aldol condensation or Knoevenagel condensation. Chiral oxazolidinones have been employed most widely in stereoselective aldol reactions. Often, a single diastereomer may be obtained by one crystallization of the aldol adduct. One approach, demonstrated by Evans, is to silylate the aldol adduct. These products arise via an initial aldol condensation to give diacetone alcohol. The product is the alkoxide salt of the aldol product. It is a modification of the aldol condensation. The combination of these two steps is known as the aldol condensation. Since the aldol addition reaction creates two new stereocenters, up to four stereoisomers may result. This is called "aldol condensation". Schmidt reaction is an example of a crossed aldol process.