Aldol Reactions

Abstract

IntroductionThe aldol reaction and the related Claisen reaction are some of the most powerful methods for carbon-carbon bond formation [1]. Particularly since the mid-1980s, intensive research in this field has generated several new routes to stereo-and regiocontrol [2] as well as new catalytic reaction types. Although, overall, many variations of these reactions exist, only a few of them are generally applicable pathways for the creation of quaternary carbon centers.Formally, there exist two main reaction types to create a quaternary stereocenter via aldol additions (and related reactions). The first pathway is the addition of a carbonyl compound 1 (or a correspondding activated species) with two different -substituents (R 2 R 3 ) to another carbonyl compound 2. This method generates a quaternary carbon atom next to the carbonyl group of the aldols 3 and 4 (Scheme 3.1).The second pathway (Scheme 3.2) is the aldol addition of an enolizable carbonyl compound 5 to a carbonyl function 6 with two different substituents (R 2 R 3 ), which creates an aldol 7 with an asymmetric tertiary alcohol. Scheme 3.1 Scheme 3.2 Quaternary Stereocenters: Challenges and Solutions for Organic Synthesis. Edited

Ethnoscapes, national territorialisation and the Afghan war / Conrad Schetter.

“…An ethnic census, such as was envisioned at least for a time by the United Nations, is therefore doomed to failure and may even conceal the danger of an escalation of the conflict, as hardly any of the warring factions would accept its findings. 62 I would like here to concentrate in detail on the second strategy, that of the interpretation of ethnic origins, and in particular to look at the ethnoscapes that go along with such interpretations. As examples I intend to describe the ethnoscapes that have become widely shared among Pashtuns, Hazaras and Tajiks respectively, and to examine the dynamics these ethnoscapes have undergone.…”

Polyaldol Synthesis by Direct Organocatalyzed Crossed Polymerization of Bis(ketones) and Bis(aldehydes)

“…The aldol reaction is an extremely popular C–C bond forming addition reaction that is generally catalyzed by a Brønsted or Lewis acid or base, yielding a β-ketohydroxy adduct. − The term aldolization has been generalized to the addition reaction of any enolizable carbonyl-containing nucleophilic reagent (in general a ketone) onto an electrophilic nonenolizable substrate (mostly aldehydes) . Formation of a nucleophilic enolate, prior to its reaction with the aldehyde, is also common and refers to as the indirect aldolization.…”

“…Formation of a nucleophilic enolate, prior to its reaction with the aldehyde, is also common and refers to as the indirect aldolization. To this end, silylenol ethers have been extensively employed in the so-called Mukaiyama–aldol reaction. − The aldol adduct can yet undergo a dehydration (called crotonization) and be converted into the corresponding α,β-unsaturated carbonyl derivative, the driving force being the formation of a conjugated system. − …”

Enantioselective Equilibration−Access to Chiral Aldol Adducts of Mandelic Acid Esters

“…Cycloadditions, alkylations of ketones, Michael reactions and conjugate additions, allylic alkylation, and cross-coupling reactions have been suitable tools to achieve this goal . Several methods of aldol additions have also been employed successfully for special substrates, but they have failed to be generalized . During our ongoing studies of enantioselective aldol additions in the presence of chiral mandelic acid derivates, chiral 1,2-diols, aldol adducts of aldehydes, and mandelic acid derivatives needed to be synthesized.…”

“…2 Several methods of aldol additions have also been employed successfully for special substrates, but they have failed to be generalized. 3 During our ongoing studies of enantioselective aldol additions in the presence of chiral mandelic acid derivates, 4 chiral 1,2-diols, aldol adducts of aldehydes, and mandelic acid derivatives needed to be synthesized. Little is known about chiral aldol adducts of mandelic acid derivatives, 5 and there are only a few examples in the lactic acid series.…”