Regio- and Diastereoselective Synthesis of 2-Arylazetidines: Quantum Chemical Explanation of Baldwin's Rules for the Ring-Formation Reactions of Oxiranes†.

A general, scalable two-step regio- and diastereoselective method has been described for the synthesis of versatile alkaloid-type azetidines from simple building blocks with excellent overall yields. In the kinetically controlled reaction, only the formation of the strained four-membered ring can be achieved instead of the thermodynamically favorable five-membered rings under appropriate conditions. Remarkable functional group tolerance has also been demonstrated. In this paper, we give a new scope of Baldwin's rules by density functional theory (DFT) calculations with an explicit solvent model, confirming the proposed reaction mechanisms and the role of kinetic controls in the stereochemical outcome of the reported transition-metal-free carbon-carbon bond formation reactions.

Chemoselective Strategy for the Direct Formation of Tetrahydro-2,5-methanobenzo[ c]azepines or Azetotetrahydroisoquinolines via Regio- and Stereoselective Reactions.

The present study reports regio- and highly diastereoselective preparative methods for the synthesis of versatile alkaloid-type compounds from oxiranylmethyl tetrahydroisoquinolines. 2,5-Methanobenzo[ c]azepines or azetidine-fused heterocycles were synthesized in tandem reactions depending on the absence or presence of a BF3 co-reagent. A high functional group tolerance has also been demonstrated. DFT calculations with an explicit solvent model confirmed the proposed reaction mechanisms and the role of kinetic controls on the stereochemical outcome of the reported new methods.

Access to Fluorazones by Intramolecular Dehydrative Cyclization of Aromatic Tertiary Amides: A Synthetic and Mechanistic Study.

An efficient synthesis has been developed for the preparation of 9H-pyrrolo[1,2-a]indol-9-ones (fluorazones) from readily available anthranilic acid derivatives via a one-pot amide- and pyrrole-formation step, followed by an intramolecular cyclodehydration. The cyclodehydration process is mediated by the activation of aromatic tertiary amides by triflic anhydride (Tf2O). Comparison of various benzo-substituents is shown to demonstrate the high functional group tolerance of this transformation. In addition, study of the reaction mechanism is also presented to unfold the exact role of the applied base additive. Herein, as a first example, we report our findings that Tf2O-mediated amide activation is obstructed by the easy protonation of amides by the formed triflic acid during the activation step. Additionally, it has been also proven that the base additive is not involved in the transformation of O-triflyliminium triflates into reactive species (e.g., nitrilium triflates) and is only responsible to neutralize the superacid to avoid the protonation of both the secondary or tertiary amides.

Continuous Synthesis and Purification by Coupling a Multistep Flow Reaction with Centrifugal Partition Chromatography.

Continuous-flow multistep synthesis is combined with quasi-continuous final-product purification to produce pure products from crude reaction mixtures. In the nucleophilic aromatic substitution of 2,4-difluoronitrobenzene with morpholine followed by a heterogeneous catalytic hydrogenation, the desired monosubstituted product can be continuously separated from the co- and by-products in a purity of over 99 % by coupling a flow reactor sequence to a multiple dual-mode (MDM) centrifugal partition chromatography (CPC) device. This purification technique has many advantages over HPLC, such as higher resolution and no need for column replacement or silica recycling, and it does not suffer from irreversible adsorption.

The route from problem to solution in multistep continuous flow synthesis of pharmaceutical compounds.

Recent advances in the field of continuous flow chemistry allow the multistep preparation of complex molecules such as APIs (Active Pharmaceutical Ingredients) in a telescoped manner. Numerous examples of laboratory-scale applications are described, which are pointing towards novel manufacturing processes of pharmaceutical compounds, in accordance with recent regulatory, economical and quality guidances. The chemical and technical knowledge gained during these studies is considerable; nevertheless, connecting several individual chemical transformations and the attached analytics and purification holds hidden traps. In this review, we summarize innovative solutions for these challenges, in order to benefit chemists aiming to exploit flow chemistry systems for the synthesis of biologically active molecules.

Selecting Resolving Agents with Respect to Their Eutectic Compositions.

In order to develop a resolution procedure for a given racemic compound, the first and the most important step is finding the most suitable resolving agent. We studied 18 individual resolutions that were carried out with resolving agents having high eutectic composition. We found that very high enantiomeric excess values were obtained in all cases. We assume that the eutectic composition of a given resolving agent is one of the most important properties that should always be considered during the search for the most efficient resolving agent.

Synthesis of atropisomeric 1-phenylpyrrole-derived amino alcohols: new chiral ligands.

An efficient synthetic method has been developed for the preparation of a new family of atropisomeric amino alcohols with 1-phenylpyrrole backbone. The synthesis is based on the different reactivities of the two carboxylic groups in optically active 1-[2-carboxy-6-(trifluoromethyl)phenyl]-1H-pyrrole-2-carboxylic acid (1). The chemical structures of the key intermediates were confirmed by spectroscopic methods and single crystal X-ray diffraction measurements. The very first application of a new optically active amino alcohol as catalyst for the enantioselective addition of diethylzinc to benzaldehyde demonstrated the practical usefulness of atropisomeric compounds in which there are six-atom chains between the two functional groups.

Ring transformation of unsaturated N-bridgehead fused pyrimidin-4(3H)-ones: role of repulsive electrostatic nonbonded interaction.

Thermal ring transformation ability of unsaturated N-bridgehead fused pyrimidin-4(3H)-ones A is governed by both the steric and the electrostatic interactions between the oxygen of the carbonyl group and the substituent in the peri position.

Separation of non-racemic mixtures of enantiomers: an essential part of optical resolution.

Non-racemic enantiomeric mixtures form homochiral and heterochiral aggregates in melt or suspension, during adsorption or recrystallization, and these diastereomeric associations determine the distribution of the enantiomers between the solid and other (liquid or vapour) phases. That distribution depends on the stability order of the homo- and heterochiral aggregates (conglomerate or racemate formation). Therefore, there is a correlation between the binary melting point phase diagrams and the experimental ee(I)vs. ee(0) curves (ee(I) refers to the crystallized enantiomeric mixtures, ee(0) is the composition of the starting ones). Accordingly, distribution of the enantiomeric mixtures between two phases is characteristic and usually significant enrichment can be achieved. There are two exceptions: no enrichment could be observed under thermodynamically controlled conditions when the starting enantiomer composition corresponded to the eutectic composition, or when the method used was unsuitable for separation. In several cases, when kinetic control governed the crystallization, the character of the ee(0)-ee(I) curve did not correlate with the melting point binary phase diagram.

Synthesis and enantioselective rearrangement of (Z)-4-triphenylmethoxy-2,3-epoxybutan-1-ol enantiomers.

Efficient enzyme catalyzed kinetic resolutions of a synthetically useful chiral building block, (Z)-4-triphenylmethoxy-2,3-epoxybutan-1-ol, are reported. The highest selectivities were achieved by Lipozyme TL IM and Amano Lipase PS enzymes in the presence of vinyl acetate. Enantiomeric enrichment of the optically active acetate isomer was accomplished by selective crystallization of the racemic part of the enantiomeric mixture. Enzyme catalyzed hydrolysis of the acetate also provided an optically pure epoxybutanol derivative. O-Benzylation of (+)-(Z)-1-hydroxy-4-triphenylmethoxy-2,3-epoxybutane followed by super base promoted diastereo- and enantio-selective rearrangement resulted in (+)-(2R,3R,1'R)-3-[1-hydroxy-2-(triphenylmethoxy)ethyl]-2-phenyloxetane in >98% ee and de. Configurations of the new optically active products were determined by chemical correlation.

Advantages of structural similaritiesof the reactants in optical resolution processes.

Advantages and limitations of the use of structurally similar compounds in racemate resolution viadiastereoisomeric salt formation are discussed in this review. An effective conception on "derivativeresolving agents" (use of the optically active derivatives of a racemate as resolving agents)is presented by examples and the method is extended to the homologous series of the derivative resolvingagents and/or achiral additives. A recently developed distillation version of the family approachto optical resolution and the novel, solvent-free resolution methods using a half equivalent amountof resolving agent are also discussed.