Aldol Reactions between L-Erythrulose Derivatives and Chiral α-Amino and α-Fluoro Aldehydes: Competition between Felkin-Anh and Cornforth Transition States

Stereodivergent approach in the protected glycal synthesis of L-vancosamine, L-saccharosamine, L-daunosamine and L-ristosamine involving a ring-closing metathesis step

In this paper, a new access to several chiral 3-aminoglycals as potential precursors for glycosylated natural products is reported from a common starting material, (-)-methyl-L-lactate. The stereodivergent strategy is based on the implementation of a ring-closing metathesis of vinyl ethers as key step of reaction sequences developed.

Stereocontrol by quaternary centres: a stereoselective synthesis of (-)-luminacin D

Very high diastereoselectivity can be achieved by 1,3-chelation-controlled allylation of aldehydes that possess a non-chelating α-ether substituent, even if the α-position is a quaternary centre and/or a spiro-epoxide. This reaction was used as a key step in an enantioselective synthesis of the angiogenesis inhibitor luminacin D.

Hydrogen-bonding network of N,N'-bis[2-(tert-butyldimethylsiloxy)ethyl]ethylenediammonium dichloride

The title salt, C18H46N2O2Si2(2+)·2Cl(-), has been synthesized by reaction of N,N'-bis(2-hydroxyethyl)ethylenediamine with tert-butyldimethylsilyl chloride. The zigzag backbone dication is located across an inversion centre and the two chloride anions are related by inversion symmetry. The ionic components form a supramolecular two-dimensional network via N-H···Cl hydrogen bonding, which is responsible for the high melting point compared with the oily compound N,N'-bis[2-(tert-butyldimethylsiloxy)ethyl]ethylenediamine.

α-Haloaldehydes: versatile building blocks for natural product synthesis

The diastereoselective addition of organometallic reagents to α-chloroaldehydes was first reported in 1959 and occupies a historically significant role as the prototypical reaction for Cornforth's model of stereoinduction. Despite clear synthetic potential for these reagents, difficulties associated with producing enantiomerically enriched α-haloaldehydes limited their use in natural product synthesis through the latter half of the 20th century. In recent years, however, a variety of robust, organocatalytic processes have been reported that now provide direct access to optically enriched α-haloaldehydes and have motivated renewed interest in their use as building blocks for natural product synthesis. This Highlight summarizes the methods available for the enantioselective preparation of α-haloaldehydes and their stereoselective conversion into natural products.

β-Siloxy-α-haloketones through highly diastereoselective single and double mukaiyama aldol reactions

Double-action haloketones: A super silyl group enabled the first highly diastereoselective Mukaiyama aldol reactions of α-chloro- and α-fluoroketones with a wide range of aldehydes, providing anti-β-siloxy-α-haloketones. This process is compatible with one-pot double-aldol methodology and allows for rapid access to new halogen-modified polyketide fragments bearing up to four contiguous stereocenters.

Rapid, one-pot synthesis of β-siloxy-α-haloaldehydes

The Mukaiyama cross-aldol reaction of α-fluoro-, α-chloro-, and α-bromoacetaldehyde-derived (Z)-tris(trimethylsilyl)silyl enol ethers is described, furnishing anti-β-siloxy-α-haloaldehydes. A highly diastereoselective, one-pot, sequential double-aldol process is developed, affording novel β,δ-bissiloxy-α,γ-bishaloaldehydes. Reactions are catalyzed by C(6)F(5)CHTf(2) and C(6)F(5)CTf(2)AlMe(2) (0.5-1.5 mol %) and provide access to halogenated polyketide fragments.

Role of pseudoephedrine as chiral auxiliary in the "acetate-type" aldol reaction with chiral aldehydes; asymmetric synthesis of highly functionalized chiral building blocks

We have studied in depth the aldol reaction between acetamide enolates and chiral α-heterosubstituted aldehydes using pseudoephedrine as chiral auxiliary under double stereodifferentiation conditions, showing that high diastereoselectivities can only be achieved under the matched combination of reagents and provided that the α-heteroatom-containing substituent of the chiral aldehyde is conveniently protected. Moreover, the obtained highly functionalized aldols have been employed as very useful starting materials for the stereocontrolled preparation of other interesting compounds and chiral building blocks such as pyrrolidines, indolizidines, and densely functionalized β-hydroxy and β-amino ketones using simple and high-yielding methodologies.

Mukaiyama aldol addition to alpha-chloro-substituted aldehydes. Origin of the unexpected syn selectivity

The addition of sterically demanding enolsilanes to alpha-chloro aldehydes results unexpectedly in preferential formation of the anti-PFA product (1,2-syn), while the addition of the corresponding boron enolate furnishes the expected polar Felkin-Anh product (1,2-anti). A stereoinduction model explaining these observations is proposed.