Preparation of Chiral Oxazolidin-2-ones and Vicinal Amino Alcohols

Copper-Catalyzed Enantioselective Reductive Aldol Reaction of α,β-Unsaturated Carboxylic Acids to Ketones: Silanes as Activator and Transient Protecting Group

We have developed the first enantioselective reductive aldol reaction of unprotected α,β-unsaturated carboxylic acids by employing a copper/bisphosphine catalyst. The reaction features in situ protection and activation of an α,β-unsaturated carboxylic acid by a hydrosilane. The copper enolate formed in situ reacts with a ketone to afford the β-hydroxy carboxylic acid with excellent enantioselectivity (up to 99% ee). The corresponding gram-scale reaction with a low catalyst loading and the derivatization of the β-hydroxy carboxylic acids highlight the practicality of this transformation.

Enantioselective synthesis of 2-oxazolidinones by ruthenium(ii)-NHC-catalysed asymmetric hydrogenation of 2-oxazolones

A highly enantioselective synthesis of optically active 4-substituted 2-oxazolidinones was described via ruthenium(ii)–NHC-catalysed asymmetric hydrogenation of 2-oxazolones.

An efficient synthesis of optically active 4-substituted 2-oxazolidinones by the ruthenium(ii)–NHC-catalysed asymmetric hydrogenation of 2-oxazolones was developed. Excellent enantioselectivities (up to 96% ee) and yields (up to 99%) were obtained for a variety of substrates, bearing a range of functional groups and useful motifs. The hydrogenation reaction was successfully scaled up to gram scale using low catalyst loading. Moreover, the utility of this methodology was demonstrated by the transformation of the enantioenriched product into the corresponding chiral β-amino alcohol, a bisoxazoline ligand, and the formal synthesis of (–)-aurantioclavine.

Synthesis of oxazolidinones: rhodium-catalyzed C-H amination of N-mesyloxycarbamates

N-Mesyloxycarbamates undergo intramolecular C-H amination reactions to afford oxazolidinones in good to excellent yields in the presence of rhodium(ii) carboxylate catalysts. The reaction is performed under green conditions and potassium carbonate is used, forming biodegradable potassium mesylate as a reaction by-product. This method enables the production of electron-rich, electron-deficient, aromatic and heteroaromatic oxazolidinones in good to excellent yields. Conformationally restricted cyclic secondary N-mesyloxycarbamates furnish cis-oxazolidinones in high yields and selectivity; DFT calculations are provided to account for the observed selectivity. trans-Oxazolidinones were prepared from acyclic secondary N-mesyloxycarbamates using Rh₂(oct)₄. The selectivity was reverted with a cytoxazone N-mesyloxycarbamate precursor using large chiral rhodium(ii) carboxylate complexes, affording the corresponding cis-oxazolidinone. This orthogonal selectivity was used to achieve the formal synthesis of (-)-cytoxazone.

Oxazolidinones and 2,5-Dihydrofurans via Zinc-Catalyzed Regioselective Allenylation Reactions of l-α-Amino Aldehydes

The simultaneous control of diastereoselectivity and regioselectivity in Zn-catalyzed allenylation reactions of N-protected l-α-amino aldehydes is reported. A reversal in diastereoselectivity could be realized by variation of the α-amino aldehyde protecting groups. A range of 1-allenyl-2-amino alcohols were obtained with excellent regioselectivity and converted to oxazolidinones and dihydrofurans. Many of which could be isolated as single diastereoisomers and without significant erosion of ee, making this a practical catalytic synthesis of highly functionalized heterocycles.

Buscopan labeled with carbon-14 and deuterium

Hyosine butyl bromide, the active ingredient in Buscopan, is an anticholinergic and antimuscarinic drug used to treat pain and discomfort caused by abdominal cramps. A straightforward synthesis of carbon-14- and deuterium-labeled Buscopan was developed using scopolamine, n-butyl-1-¹⁴ C bromide, and n-butyl-² H₉ bromide, respectively. In a second carbon-14 synthesis, the radioactive carbon was incorporated in the tropic acid moiety to follow its metabolism. Herein, we describe the detailed preparations of carbon-14- and deuterium-labeled Buscopan.

Synthesis of a smoothened cholesterol: 18,19-di-nor-cholesterol

Herein, we report the first synthesis of a demethylated form of cholesterol (18,19-di-nor-cholesterol), in which the C18 and C19 methyl groups of the β-face were eliminated. Recent molecular simulations modeling 18,19-di-nor-cholesterol have suggested that cholesterol's opposing rough β-face and smooth α-face play necessary roles in cholesterol's membrane condensing abilities and, additionally, that specific facial preferences are displayed as cholesterol interacts with different neighboring lipids and transmembrane proteins. Inspired by these poorly characterized biochemical interactions, an extensive 18-step synthesis was completed as part of a collaborative effort, wherein synthesizing a "smoothened" cholesterol analogue would provide a direct way to experimentally measure the significance of the β-face methyl groups. Starting from known perhydrochrysenone A, the synthesis of 18,19-di-nor-cholesterol was accomplished with an excellent overall yield of 3.5%. The use of the highly stereoselective Dieckmann condensation and the employment of Evans' chiral auxiliary were both key to ensuring the success of this synthesis.

Synthesis of novel adamantyl and homoadamantyl-substituted β-hydroxybutyric acids

Several new adamantyl and homoadamantyl-substituted [Formula: see text]-hydroxybutyric acids, 2-[2-(1-adamantyl)ethyl]-3-hydroxybutyric acid (2), 2-(3-homoadamantyl)-3-hydroxybutyric acid (3), and 2-(1-homoadamantyl)-3-hydroxybutyric acid (4), analogues of the 2-(1-adamantyl)-3-hydroxybutyric acid (1), have been prepared as mixtures of diastereoisomers using selective reduction of corresponding [Formula: see text]-keto esters or aldol condensation of the corresponding carboxylic acid and acetaldehyde. The rearrangement of adamantylmethyl and 3-homoadamantyl groups provided entry to both 3-homoadamantyl and 1-homoadamantyl-substituted hydroxy acids 3 and 4, respectively. The relative configurations of diastereoisomers 3 and 4 have been determined by NMR spectroscopy comparing the values of coupling constants. Adamantyl-substituted [Formula: see text]-hydroxybutyric acid 2 has also been prepared in enantiomerically pure form by Evan's asymmetric synthesis and the absolute configuration has been determined by X-ray crystallography. Contrary to the long-chain acid 2, the attempt to prepare short-chain hydroxy acids 1 and 4 by the same method failed indicating pronounced sensitivity of the used method to the vicinity of the bulky cage group.

Direct enantioselective organocatalytic hydroxymethylation of aldehydes catalyzed by alpha,alpha-diphenylprolinol trimethylsilyl ether

The direct enantioselective hydroxymethylation of aldehydes utilizing alpha,alpha-diphenylprolinol trimethylsilyl ether as an organocatalyst is described. The intermediate alpha-substituted beta-hydroxyaldehydes were not isolated but converted to the more readily isolable derivatives. For example, the derived hydroxy acids were isolated in up to 94% yield with excellent enantioselectivity.

First highly stereoselective synthesis of anti-α-trifluoromethyl-β-amino acid derivatives

The Reformatsky-type reaction of 2-bromo-3,3,3-trifluoropropanoic imide with various types of imines, in the presence of ZnBr2 as a Lewis acid in THF at 0 degrees C for 3 h, gave the corresponding alpha-trifluoromethyl-beta-amino acid derivatives in a highly anti-selective manner.

Palladium-Catalyzed Arylation of Trimethylsilyl Enolates of Esters and Imides. High Functional Group Tolerance and Stereoselective Synthesis of α-Aryl Carboxylic Acid Derivatives

A general procedure for the palladium-catalyzed arylation of trimethylsilyl enolates of esters and imides is reported. In the presence of ZnF2 or Zn(O-t-Bu)2 as an additive, the trimethylsilyl enolates of esters, including those bearing alpha-alkoxy derivatives, underwent arylation in high yield with high functional group tolerance. This arylation chemistry was extended to ester derivatives bearing chiral auxiliaries to form new tertiary stereocenters. The arylation of imides bearing the Evans auxiliary proceeded with selectivities up to 90% de. Further, the arylation of the ketal developed by Ley provided alpha-aryl glycolates with excellent diastereoselectivities (90 to >98% de). This reaction provides a convenient route to the synthesis of enantiopure alpha-aryl-alpha-hydroxy esters. Reactions conducted with Zn(O-t-Bu)2 as an additive occurred at room temperature to give enhanced diastereoselectivities with both chiral reagents. Mechanistic studies showed that the reaction conditions are neutral enough that the observed diastereomeric ratios reflect kinetic selectivities.

Fluorescence sensing of ammonium and organoammonium ions with tripodal oxazoline receptors

A new class of fluorescence sensors for ammonium and organoammonium ions has been disclosed. One of the sensors, an alaninol-derived tripodal oxazoline (1a) shows significant fluorescence enhancement upon binding NH(4)(+) but little response toward K(+), Na(+), and Mg(2+) ions. Owing to its chiral environment, a phenylglycinol-derived tripodal oxazoline (1b) shows chiral discrimination in fluorescence upon binding enantiomeric guests. [reaction: see text]

A new synthesis of cytoxazone and its diastereomers provides key initial SAR information

A short, enantioselective, and diastereoselective synthesis of cytoxazone, a Th2-selective immunomodulator from Streptomyces, is described. The route was readily adapted to the synthesis of the three other stereoisomers of natural cytoxazone. Evaluation of these compounds revealed that the stereochemical configuration of the oxazolidinone ring did not influence their biological activity.

Structure-activity relationships of the peptide deformylase inhibitor BB-3497: modification of the metal binding group

A series of analogues of the potent peptide deformylase (PDF) inhibitor BB-3497 containing alternative metal binding groups was synthesised. Enzyme inhibition and antibacterial activity data for these compounds revealed that the bidentate hydroxamic acid and N-formyl hydroxylamine structural motifs represent the optimum chelating groups on the pseudopeptidic BB-3497 backbone.

Determination of the relative and absolute stereochemistry of a potent and alpha1A-selective adrenoceptor antagonist

The binding affinities and selectivities of antagonists 1-4 for the alpha1A-adrenoceptor are dependent on the stereochemical orientation of the groups at the C-4 and C-5 positions of the oxazolidinone ring. The unambiguous assignment of the relative and absolute configurations of the diastereomers of SNAP 7915 (1) is reported.

Synthesis of aminocyclopentitols from chromium carbene complex derived aminocyclobutanones

Functionalized cyclopentenones were synthesized by the diazomethane ring expansion of cyclobutanones, produced by the photochemical reaction of vinyl oxazolidinones with chromium carbene complexes.