Toward Asymmetric Aldol-Tishchenko Reactions with Enolizable Aldehydes: Access to Defined Configured Stereotriads, Tetrads, and Stereopentads

Enantioselective Synthesis of Acyclic Stereotriads Featuring Fluorinated Tetrasubstituted Stereocenters

Elaboration of enantioenriched complex acyclic stereotriads represents a challenge for modern synthesis even more when fluorinated tetrasubstituted stereocenters are targeted. We have been able to develop a simple strategy in a sequence of two unprecedented steps combining a diastereoselective aldol-Tishchenko reaction and an enantioselective organocatalyzed kinetic resolution. The aldol-Tishchenko reaction directly generates a large panel of acyclic 1,3-diols possessing a fluorinated tetrasubstituted stereocenter by condensation of fluorinated ketones with aldehydes under very mild basic conditions. The anti 1,3-diols featuring three contiguous stereogenic centers are generated with excellent diastereocontrol (typically >99 : 1 dr). Depending upon the precursors both diastereomers of stereotriads are accessible through this flexible reaction. Furthermore, from the obtained racemic scaffolds, development of an organocatalyzed kinetic resolution enabled to generate the desired enantioenriched stereotriads with excellent selectivity (typically er >95 : 5).

Potassium Base-Promoted Diastereoselective Synthesis of 1,3-Diols from Allylic Alcohols and Aldehydes through a Tandem Allylic-Isomerization/Aldol-Tishchenko Reaction

This study reports the first base-promoted aldol-Tishchenko reactions of allylic alcohols with aldehydes initiated by allylic isomerization. The reaction enables the diastereoselective synthesis of a variety of 1,3-diols with three contiguous stereogenic centers. Unlike commonly reported systems, our method allows the use of readily available allylic alcohols as nucleophiles instead of enolizable aldehydes and ketones.

Stereoelectronic power of oxygen in control of chemical reactivity: the anomeric effect is not alone

Although carbon is the central element of organic chemistry, oxygen is the central element of stereoelectronic control in organic chemistry. Generally, a molecule with a C-O bond has both a strong donor (a lone pair) and a strong acceptor (e.g., a σ*_C-O orbital), a combination that provides opportunities to influence chemical transformations at both ends of the electron demand spectrum. Oxygen is a stereoelectronic chameleon that adapts to the varying situations in radical, cationic, anionic, and metal-mediated transformations. Arguably, the most historically important stereoelectronic effect is the anomeric effect (AE), i.e., the axial preference of acceptor groups at the anomeric position of sugars. Although AE is generally attributed to hyperconjugative interactions of σ-acceptors with a lone pair at oxygen (negative hyperconjugation), recent literature reports suggested alternative explanations. In this context, it is timely to evaluate the fundamental connections between the AE and a broad variety of O-functional groups. Such connections illustrate the general role of hyperconjugation with oxygen lone pairs in reactivity. Lessons from the AE can be used as the conceptual framework for organizing disjointed observations into a logical body of knowledge. In contrast, neglect of hyperconjugation can be deeply misleading as it removes the stereoelectronic cornerstone on which, as we show in this review, the chemistry of organic oxygen functionalities is largely based. As negative hyperconjugation releases the "underutilized" stereoelectronic power of unshared electrons (the lone pairs) for the stabilization of a developing positive charge, the role of orbital interactions increases when the electronic demand is high and molecules distort from their equilibrium geometries. From this perspective, hyperconjugative anomeric interactions play a unique role in guiding reaction design. In this manuscript, we discuss the reactivity of organic O-functionalities, outline variations in the possible hyperconjugative patterns, and showcase the vast implications of AE for the structure and reactivity. On our journey through a variety of O-containing organic functional groups, from textbook to exotic, we will illustrate how this knowledge can predict chemical reactivity and unlock new useful synthetic transformations.

Stereoselective Synthesis of 2-Fluoro-1,3-Diols via Lithium Binaphtholate-Catalyzed Aldol-Tishchenko Reaction

Lithium binaphtholate, readily prepared from ( R)-3,3'-I₂-BINOL and lithium tert-butoxide, efficaciously catalyzed the enantioselective aldol-Tishchenko tandem reaction of α-fluoroketones with aldehydes, achieving the enantioselective synthesis of 2-fluoro-1,3-diols with three contiguous stereogenic centers. Kinetic studies revealed that the aldol reaction and the subsequent hemiacetal formation are in equilibrium under the reaction conditions and that the lithium binaphtholate catalyst selectively promotes hydride shift of one of the eight stereoisomers to produce 2-fluoro-1,3-diols containing a tetrasubstituted fluorinated carbon center.

Sequential Catalysis of Phosphine Oxide for Stereoselective Synthesis of Stereopentads

An efficient method for accessing enantiomerically pure stereopentads via a catalytic asymmetric sequential aldol reaction has been developed for the first time. The enantioselective sequential aldol reaction produces a wide range of chiral stereopentad precursors in good yields with excellent enantioselectivities. The key to success is the use of the sequential catalytic system involving a chiral phosphine oxide catalyst and trichlorosilyl triflate.

2-Cyclo-hexyl-idene-N-methyl-hydrazine-carbothio-amide

The title compound C₈H₁₅N₃S has two mol­ecules in the asymmetric unit in which cis–trans isomerism is exhibited around the N(NH)C=S bonds. The cyclo­hexyl rings in both mol­ecules adopt a chair conformation. In the crystal, N—H⋯S hydrogen bonding produces dimers, which are inter­connected through further N—H⋯S hydrogen bonds, forming chains along the b-axis direction.

Direct enantioselective aldol-Tishchenko reaction catalyzed by chiral lithium diphenylbinaphtholate

Chiral lithium diphenylbinaphtholate is an effective catalyst for the enantioselective aldol-Tishchenko reaction, affording 1,3-diol derivatives with three contiguous chiral centers and high stereoselectivities. Successive aldol-aldol-Tishchenko reactions gave a triol derivative with five consecutive chiral centers. The present reaction was applicable to highly enantioselective Evans-Tishchenko reduction.

Evans-Tishchenko coupling of heteroaryl aldehydes

The low-temperature Evans-Tishchenko coupling of a range of functionalized heteroaryl aldehydes with β-hydroxy ketones in the presence of a Sm(III) catalyst has been achieved with high yields (90-99%) and good to excellent diastereoselectivity (90:10 → 95:5 dr). However, at room temperature a retro-aldol aldol-Tishchenko reaction was found to compete with the desired Evans-Tishchenko reaction. Identification of these byproducts has allowed the corresponding aldol-Tishchenko reaction to be optimized for several heteroaryl aldehydes.