Aldehydes aldol reaction

The aldehyde-aldehyde aldol reactions were first nsed in a natural product synthesis setting by Pihko and Erkkila, who prepared prelactone B in only three operations starting from isobutyraldehyde and propionaldehyde (Scheme 40). Crossed aldol reaction under proline catalysis, followed by TBS protection, afforded protected aldehyde 244 in >99% ee. A highly diastereoselective Mukaiyama aldol reaction and ring closure with aqueous HE completed the synthesis [112]. 

Many enzymes are both active and stable in carbon dioxide and have been used to conduct a number of reactions. Several different types of reactions have been examined, including hydrolysis (Lee et al., 1993 Randolph et al., 1985 Zheng and Tsao, 1996), oxidation (Hammond et al., 1985 Randolph et al., 1988), and esterification/transesterification (Kamihira et al., 1987 Nakamura et al., 1986 Rantakyla and Aaltonen, 1994), but there are other types of reactions that would make worthwhile investigations in carbon dioxide. These include preparation of amides, reduction of ketones, preparation of cyanohydrins from aldehydes, aldol reactions, hydroxylation reactions, and Baeyer-Villiger oxidation. 

Coupling of silyl enol ethers or boron enolates with Co2(CO)6-stabilized carbocations, generated via Lewis acid treatment of the appropriate propargyl ethers or aldehydes (aldol reaction), via the Nicholas reaction has been used to obtain large, highly strained, ring ketones. 

The reactions of enolates with aldehydes (aldol reactions) or with imines have been widely developed since the 1970s. Asymmetric aldol-type reactions are very important in the multistep synthesis of complex molecules such as ionophores or p-lactam antibiotics. Chirality has been introduced either on the substituents of boron, on the metal ligands or on the carbon skeleton of the enolate. Aldol reactions are usually run at low temperatures, and when metal enolates are used, the reactions are sometimes easily reversible [160, 209],... 

Scheme 4.15 Synthesis of polyketide stractuies by aldehyde-aldehyde aldol reaction...

General. Chiral 2-bromo-l,3-bis(4-methylphenyl sulfonyl)-4,5-diphenyl- 1,3,2-diazaborolidine (1) is used to control the stereochemistry of enantioselective aromatic Claisen rearrangements, allylations of aldehydes, aldol reactions, and formation of chiral propa-l,2-dienyl and propargyl alcohols. Included is the discussion of both the R,R) and the (5,5) chiral controllers. 

Nyberg, A.I., Usano, A. and Pihko, P.M., Proline-catalyzed ketone-aldehyde aldol reactions are accelerated in water, Synlett, 2004,1891 1896. 

A similar pattern is observed in the reactions of silyl enol ethers derived from aldehydes. Aldehyde-aldehyde aldol reactions had already been reduced to practice in the context of the Lewis base-catalyzed aldol reaction of trichlorosilyl enol ethers, thanks to the stabilizing effect of the trichlorosilyl chlorohydrin (Scheme 9) [25-27]. Because the product of the aldehyde-aldehyde aldol reaction is an aliphatic aldehyde, it can be quickly transformed into an umeactive chlorohydrin such... 

Tab. 2.20 Aminoind anol-based syn 2.4 Addition of Enolates with oc-Substituents to Aldehydes aldol reactions. ... 

Highly hindered bases such as (20) deprotonate the methyl group of methyl ketones specifically. In the presence of an aldehyde, aldol reaction occurs before enolate equilibration can take place. The same base promotes the addition of ester and lactone enolates to aldehydes and ketones. However, it has been noted that at low temperature (—70 °C), the more accessible lithium diethylamide deproto-nates only the methyl group of methyl ketones. Addition of a second aldehyde or ketone species at this temperature leads to the exclusive formation of jS-ketoIs derived from this enolate. ... 

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