The aldol reaction, usually carried out in protic solvents with base or acid as the catalyst, is one of the most versatile methods in organic synthesis. By application of this reaction a great number of aldols and related compounds have been prepared from various carbonyl compounds. However, because of difficulty in directing the coupling, the conventional method has serious synthetic limitations. This is particularly notable when two different carbonyl compounds are used in a cross-coupling; the reaction is often accompanied by undesirable side reactions such as self-condensation and polycondensation. The synthetic limitation arises because the reaction is reversible and cannot be driven to completion if the aldol is less stable than the parent carbonyl compounds. In addition, the reverse reaction, in the presence of acid or base, generates regioisomeric enols or enolates, which in turn attack the carbonyl compounds to yield a mixture of aldols. Furthermore, the aldols are often dehydrated and the resulting unsaturated carbonyl compounds may undergo a Michael addition between enolate anions to give a complex reaction mixture.
During the last decade new methods have been developed for the directed coupling of two different carbonyl compounds (or carbonyl equivalents) to give specific carboncarbon bond formation between the α-carbon atom of one carbonyl compound and the other carbonyl component to produce a desired crossed aldol. These methods provide regiospecific reactions for forming carboncarbon bonds and allow the synthesis of a wide variety of aldols by directed self- or cross-coupling.
The success of the reaction depends on effective interception of the aldol-type adduct by formation of a stable six-membered chelate. This interception can usually be achieved by reaction of a carbonyl compound with a suitably reactive metal enolate or enol ether that is derived regioselectively from the other carbonyl compound. Reaction of a carbonyl compound with lithium enolates or preformed lithio derivatives of imines is a typical method used for preparation of crossed aldols, even though the reaction is carried out under strongly basic conditions. Use of magnesium, aluminum, or zinc enolates permits rather milder reaction conditions. A clean aldol reaction has been achieved using vinyloxyboranes in a process that is carried out under neutral conditions to produce various crossed aldols in excellent yields. The titanium tetrachloride–promoted coupling of silyl enol ethers, enol ethers, or enol esters with carbonyl compounds, acetals, or ketals is another particularly useful method for the preparation of aldols. The powerful electrophilic activation of the carbonyl acceptor by titanium tetrachloride provides the driving force for this reaction. The acidic reaction medium makes the method useful for compounds that have base-sensitive functional groups.
These directed aldol reactions provide efficient methods for the regiospecific formation of new carboncarbon bonds and can be used for the preparation of key intermediates in the synthesis of important natural products.
This chapter includes examples where the coupling reactions are of aldehydes, ketones, or their equivalents.