Directed Aldol Condensation

Abstract

Crossed aldol condensation has traditionally been carried out by mixing two different carbonyl units under protic conditions with general acids or bases as the triggers of the reaction. [1] Development in this area has significantly expanded the scope of directed aldol condensation. Its synthetic potential has been particularly well-featured by those reactions that occurr in an intramolecular manner, such as Robinson annulation [2] and Dickemann condensation; [3] their analogous processes [4] have also been found quite attractive. Unfortunately, because of difficulties in directing the coupling intermolecularly, the conventional aldol condensatiton has serious disadvantages:1) It gives low selectivities including stereo-, regio-and chemoselectivity involved in both enolization and aldolization processes. 2) It cannot necessarily suppress dehydration; this leads to a,b-unsaturated carbonyl compounds that can undergo further side reactions, such as Michael addition with enolates, to contaminate the reaction. This sometimes leads to further oligomerization and/or polymerization.3) The method often suffers from proton transfer from a carbonyl compound to a reactive enolate; this results in multiple functionalization. As a consequence, a desirable cross-coupling is significantly retarded or completely suppressed.[a] Prof.

Co–Fe catalyst supported on acidified bentonite for selective hydrogenation of cinnamaldehyde

“…ACBT exhibited high activity for aldolization because dissolved ACBT could generate H + as a catalyst for the reaction to form acetal. 35,36 The same result can be obtained for 10Co-10Fe/BT with acid. When small amounts of nitric acid were involved in the reaction, 10Co-10Fe/BT exhibited high selectivity for acetals.…”

An Aldolase-Based New Pathway for Bioconversion of Formaldehyde and Ethanol into 1,3-Propanediol in Escherichia coli

“…Chemical aldol condensation of HCHO and ACALD was reported in literature. , However, the reaction temperature was very high (above 250 °C), which can lead to serious side reactions such as dehydration, oligomerization, and polymerization. Biocatalysis can principally take place at environmental temperature and with high selectivity, and therefore overcomes these problems.…”

Towards peptide isostere libraries: aqueous aldol reactions on hydrophilic solid supports

“…By far, most of the common catalysts for this reaction are Lewis acids, and chiral induction has been successfully demonstrated in the aldol reaction by use of chiral Lewis acids. [23][24][25][26] However, as most Lewis acids are very sensitive to water, the reactions normally require strictly anhydrous conditions as even small amounts of water significantly lower the yields due to decomposition of the catalyst and hydrolysis of the silyl enol ethers. Thus, the discovery of lanthanide trifluoromethanesulfonates (triflates) as water tolerant Lewis acids 27-29 has greatly expanded the scope of the reaction, especially from an industrial point of view.…”