Asymmetric synthesis of (−)-(1R,7aS)-absouline

General Approach to Enantiopure 1-Aminopyrrolizidines: Application to the Asymmetric Synthesis of the Loline Alkaloids

The synthesis of a range of loline alkaloids is reported. The C(7) and C(7a) stereogenic centers for the targets were formed by the established conjugate addition of lithium (S)-N-benzyl-N-(α-methylbenzyl)amide to tert-butyl 5-benzyloxypent-2-enoate, ensuing enolate oxidation to give an α-hydroxy-β-amino ester, and then formal exchange of the resultant amino and hydroxyl functionalities (via the intermediacy of the corresponding aziridinium ion) to give an α-amino-β-hydroxy ester. Subsequent transformation gave a 3-hydroxyprolinal derivative which was converted to the corresponding N-tert-butylsulfinylimine. Mannich-type reaction with the enolate derived from O-Boc protected methyl glycolate then formed the remaining C(1) and C(2) stereogenic centers for the targets. The 2,7-ether bridge was formed by a displacement reaction, completing construction of the loline alkaloid core. Facile manipulations then gave a range of loline alkaloids, including loline itself.

Mutual kinetic resolution: probing enantiorecognition phenomena and screening for kinetic resolution with racemic reagents

Enantiorecognition between a racemic reagent and a racemic substrate can be a valuable process in organic synthesis. This review highlights representative examples of this phenomenon and the use of mutual kinetic resolution as a method for screening of kinetic and/or parallel kinetic resolutions.

New Advances in the Synthetic Application of Enantiomeric 1-Phenylethylamine (α-PEA): Privileged Chiral Inducer and Auxiliary

New developments in the synthesis, resolution, and synthetic applications of chiral 1-phenylethylamine (α-PEA) reported in the last decade have been reviewed. In particular, improvements in the synthesis of α-PEA and its derivatives and chiral resolution, as well as their applications in the resolution of other compounds, were discussed. α-PEA was used as a chiral auxiliary in the diastereoselective synthesis of medicinal substances and natural products. Chiral ligands with α-PEA moieties were applied in asymmetric reactions, and effective modular chiral organocatalysts were constructed with α-PEA fragments and used in important synthetic reactions.

Homoallylic amines as efficient chiral inducing frameworks in the conjugate addition of amides to α,β-unsaturated esters. An entry to enantio-enriched diversely substituted amines

The diastereoselective conjugate addition of secondary homoallylamines, obtained in the enantioenriched form via allylmetallation of imines, to α,β-unsaturated esters is reported. This method allows access to valuable building blocks as well as heterocyclic skeletons, providing tertiary amines bearing two chains integrating a stereogenic center adjacent to the nitrogen atom.

Enantiopure 4-oxazolin-2-ones and 4-methylene-2-oxazolidinones as chiral building blocks in a divergent asymmetric synthesis of heterocycles

Enantiopure 3-((R)- and 3-((S)-1-phenylethyl)-4-oxazoline-2-ones were evaluated as chiral building blocks for the divergent construction of heterocycles with stereogenic quaternary centers. The N-(R)- or N-(S)-1-phenylethyl group of these compounds proved to be an efficient chiral auxiliary for the asymmetric induction of the 4- and 5-positions of the 4-oxazolin-2-one ring through thermal and MW-promoted nucleophilic conjugated addition to Michael acceptors and alkyl halides. The resulting adducts were transformed via a cascade process into fused six-membered carbo- and heterocycles. The structure of the reaction products depended on the electrophiles and reaction conditions used. Alternative isomeric 4-methylene-2-oxazolidinones served as chiral precursors for a versatile and divergent approach to highly substituted cyclic carbamates. DFT quantum calculations showed that the formation of bicyclic pyranyl compounds was generated by a diastereoselective concerted hetero-Diels-Alder cycloaddition.

Pyrrolizidine alkaloids: occurrence, biology, and chemical synthesis

Covering: 2013 up to the end of 2015This review covers the isolation and structure of new pyrrolizidines; pyrrolizidine biosynthesis; biological activity, including the occurrence of pyrrolizidines as toxic components or contaminants in foods and beverages; and formal and total syntheses of naturally-occurring pyrrolizidine alkaloids and closely related non-natural analogues.

A versatile access to vicinal diamine motifs by highly anti-selective asymmetric vinylogous Mannich reactions: an efficient total synthesis of (+)-absouline

An anti-stereoselective vinylogous Mannich reaction between 2-TBSO-pyrrole and N-tert-butanesulfinyl imines and its application to the efficient synthesis of (+)-absouline were reported.

To be or not to be butterfly: New mechanistic insights in the Aza-Michael asymmetric addition of lithium (R)-N-benzyl-N-(α-methylbenzyl)amide

The asymmetric Aza-Michael addition of homochiral lithium benzylamides to α,β-unsaturated esters represents an extended protocol to obtain enantioenriched β-amino esters. An exhaustive mechanistic revision of the originally proposed mechanism is reported, developing a quantum mechanics/molecular mechanics protocol for the asymmetric Aza-Michael reaction of homochiral lithium benzylamides. Explicit and implicit solvent schemes were considered, together with a proper account of long-range dispersion forces, evaluated through a density functional theory benchmark of different functionals. Theoretical results showed that the diastereoselectivity is mainly controlled by the N-α-methylbenzyl moiety placing, deriving a Si/Re 99:1 diastereoselective ratio, in good agreement with reported experimental results. The main transition state geometries are two transition state conformers in a "V-stacked" orientation of the amide's phenyl rings, differing in the tetrahydrofuran molecule arrangement coordinated to the metal center. Extensive conformational sampling and quantum-level refinement give reasonable good speed/accuracy results, allowing this protocol to be extended to other similar Aza-Michael reaction systems.

Asymmetric synthesis of (-)-martinellic acid

A high-yielding total asymmetric synthesis of (-)-martinellic acid is reported. The conjugate addition of lithium (R)-N-allyl-N-(α-methyl-4-methoxybenzyl)amide to tert-butyl (E)-3-[2'-(N,N-diallylamino)-5'-bromophenyl]propenoate and alkylation of the resultant β-amino ester have been used as the key steps to install the C(9b) and C(3a) stereogenic centers, respectively, and a highly diastereoselective Wittig reaction/intramolecular Michael addition was then used to create the C(4) stereogenic center within this tricyclic molecular architecture.