Diastereoselective aldol reactions of cyclohexanone lithium enolate

Origin of High Diastereoselectivity in Reactions of Seven-Membered-Ring Enolates

Unlike many reactions of their six-membered-ring counterparts, the reactions of chiral seven-membered-ring enolates are highly diastereoselective. Diastereoselectivity was observed for a range of substrates, including lactam, lactone, and cyclic ketone derivatives. The stereoselectivity arises from torsional and steric interactions that develop when electrophiles approach the diastereotopic π-faces of the enolates, which are distinguished by subtle differences in the orientation of nearby atoms of the ring.

Biomimetic Aerobic C-H Olefination of Cyclic Enaminones at Room Temperature: Development toward the Synthesis of 1,3,5-Trisubstituted Benzenes

A green and mild protocol for the dehydrogenative olefination of cyclic enaminones was devised via palladium catalysis at room temperature using oxygen as the terminal oxidant. The synthetic utility of the olefinated cyclic enaminones afforded a series of unique 1,3,5-trisubstituted benzenes via an unanticipated Diels-Alder tandem reaction. The broad substrate scope and good yields achieved with this new protocol provide an alternative pathway for arene functionalization.

The thiopyran route to polypropionates

The strategic use of thiopyran templates to facilitate polypropionate synthesis was first demonstrated in Woodward's landmark total synthesis of erythromycin A in 1981 where the topology of a fused bicyclic system was exploited. In the ensuing three decades, various alternative strategic applications of thiopyran motifs to achieve key stereoselective transformations have emerged including, inter alia, unique substrates for chemoenzymatic syntheses, surrogates for 3-pentanone in enantioselective aldol reactions, and templates for enantiotopic group selective reactions. This review summarizes these developments.

Triethylgallium as a Nonnucleophilic Base to Generate Enolates from Ketones

Triethylgallium deprotonated cyclic and acyclic ketones at 125-175 degrees C without forming carbonyl addition products, and the resulting gallium enolates underwent facile C-benzoylation and an aldol reaction. Unsymmetrical ketones were preferentially enolized at the methylene moiety, which was under kinetic control. [reaction: see text]

Aldol addition of lithium and boron enolates of 1,3-dioxan-5-ones to aldehydes. A new entry into monosaccharide derivatives

Methods allowing control of stereoselectivity in aldol reactions of enolates derived from 1,3-dioxan-5-ones (4) are described. Boron enolates, generated in situ, react with benzaldehyde to give the corresponding anti aldol selectively (the anti:syn ratio of up to 96:4) and in high yield. Lithium enolates give high anti selectivity only with aldehydes branched at the alpha-position. Enantioselective deprotonation of C(S) symmetrical dioxanones (e.g., 4b) can be accomplished efficiently, with enantiomeric excess of up to 90%, with chiral lithium amide bases of general structure PhCH(Me)N(Li)R (9, 10) if the R group is sufficiently bulky (e.g, R = adamantyl) or is fluorinated (e.g., R = CH2CF3). Dioxanone boron and lithium enolates react readily with glyceraldehyde derivatives (19), yielding protected ketohexoses (20 and 21).

Stereoselective Reactions of N-(9-Phenylfluoren-9-yl)-4-oxoproline Enolates. An Expedient Route for the Preparation of Conformationally Restricted Amino Acid Analogues

Methodology for the stereoselective preparation of 3-alkylprolines from N-(9-phenylfluoren-9-yl)-4-oxoproline is presented. The enolate of a N-(9-phenylfluoren-9-yl)-4-oxoproline ester was shown to give stereoselective aldol condensations with aromatic and aliphatic aldehydes. The enolate was preferentially approached by the electrophile through the Re face, and high threo selectivity was also observed and provided a route to trans 3-substituted prolines. The reactions are kinetically controlled except for the case of electron-rich or sterically hindered aromatic aldehydes. The ease of the equilibration between the erythro- and threo-aldolates is dictated by the electronic nature of the group at C-4 in substituted benzaldehydes, and the steric compression of the aldehyde group increases the rate of erythro/threo equilibration. Very high stereoselection was also observed in the reductions of the keto group in 3-substituted 4-oxoproline esters. Alkylation or Michael additions of the same enolate were poorly stereoselective, probably due to equilibration of the initially formed products. Kinetic protonation of enolates of 3-alkyl-4-oxoprolines proceeded with high diastereoselection to provide the corresponding cis-3-alkylprolines.

Samarium Ion-Promoted Cross-Aldol Reactions and Tandem Aldol/Evans-Tishchenko Reactions

Cross-aldol reactions of carbonyl compounds were achieved by the catalysis of SmI(2) or SmI(3), together with molecular sieves, at ambient temperature. 1,3-Dichloroacetone and 1-chloroacetone can be used as acceptor substrates in the cross-aldol reactions with donor substrates such as acetone, cyclopentanone, and cyclohexanone. The cross-aldol reactions with (R)-glyceraldehyde acetonide gave optically pure compounds 25-32, the stereochemistry of which was in agreement with a chairlike chelate transition state of dipolar mode. SmI(2)-molecular sieves or SmI(3)-molecular sieves also functioned as effective Lewis acids to catalyze tandem aldol/Evans-Tishchenko reactions. The aldol/Evans-Tishchenko reactions of methyl ketones with aldehydes occurred at 0 degrees C to give alpha,gamma-anti diol monoesters 53a-59a. When the reactions were conducted at room temperature, a certain degree of transesterification took place. The aldol/Evans-Tishchenko reactions of ethyl or benzyl ketones with aldehydes yielded alpha,beta-anti-alpha,gamma-anti diol monoesters 60a-65a. However, the aldol/Evans-Tishchenko reactions of cyclic ketones with benzaldehyde occurred with a different stereoselectivity to give alpha,beta-syn-alpha,gamma-anti diol monoesters 66a-76a. The structures of products were determined by chemical and spectroscopic methods including an X-ray diffraction analysis of 72a derived from the reaction of 4-tert-butylcyclohexanone and benzaldehyde. A reaction mechanism involving dissociation-recombination of aldols followed by intramolecular stereoselective hydride shift is proposed, based on some experimental evidence, to explain the dichotomous stereoselectivity using acyclic or cyclic ketones as the reaction substrates.

Synthesis of the Forssman pentasaccharide and terminal tetra-, tri-, and di-saccharide fragments

The 2-(trimethylsilyl)ethyl (TMSEt) beta-glycosides of the Forssman pentasaccharide [alpha-D-GalNAc-(1-->3)-beta-D-GalNAc-(1-->3)-alpha-D-Gal- (1-->4)-beta-D-Gal-(1-->4)-D-Glc] and the terminal tetrasaccharide, as well as the methyl glycosides 1 and 2 of the terminal di- and tri-saccharides, were synthesised by silver trifluoromethanesulfonate-promoted alpha-glycosylation of suitably protected mono-, di-, tri-, and tetrasaccharide alcohols with 3,4,6-tri-O-acetyl-2-azido-2-deoxy-alpha-D-galactopyranosyl bromide, followed by removal of protecting groups. The anomeric TMSEt group of the Forssman pentasaccharide and terminal tetrasaccharide was removed with trifluoroacetic acid-dichloromethane, to give the corresponding hemiacetal sugars 4 and 6.