An Enantioselective Organocatalytic Approach to Both Enantiomers of Lasubine II

Enantiodivergent Approach to the Synthesis of Cis-2,6-Disubstituted Piperidin-4-ones

Enantiopure β-amino ketone derivatives were synthesized by decarboxylative Mannich reaction of chiral N- tert-butanesulfinyl imines with β-keto acids and were subsequently transformed into cis-2,6-disubstituted piperidin-4-ones through an organocatalyzed condensation with aldehydes. Both enantiomers were accessible from the same precursors by inverting the order in the reaction sequence of the aldehydes involved in the imine formation and the intramolecular Mannich condensation. The synthesis of the piperidine alkaloids (+)-241D, (-)-epimyrtine, and (-)-lasubine II demonstrated the utility of this methodology.

Direct and highly stereoselective synthesis of quinolizidine iminosugars promoted by l-proline-Et3N

A mild and effective method for the synthesis of polyhydroxylated quinolizidine iminosugars is described. The Mannich-type reaction of iminosugar C-glycosides with aldehyde in the presence of l-proline-Et3N provides polyhydroxylated quinolizidine iminosugars, and desired products as the potential glucosidase inhibitors were obtained in good to excellent yields with excellent stereoselectivity.

Privileged heterocycles: bioactivity and synthesis of 1,9-diazaspiro[5.5]undecane-containing compounds

This review discusses the biological activity and synthesis of 1,9-diazaspiro[5.5]undecanes, including those ring-fused with arenes and heteroarenes and/or containing a carbonyl group at position 2. These compounds could be used for the treatment of obesity, pain, as well as various immune system, cell signaling, cardiovascular, and psychotic disorders.

Synthesis of β-Amino-Substituted Enones by Addition of Substituted Methyl Enones to Sulfinimines: Application to the Total Synthesis of Alkaloids (+)-Lasubine II and (+)-241D and the Formal Total Synthesis of (-)-Lasubine I

Addition of silyl enol ethers obtained from substituted methyl enones to chiral sulfinimines afforded the β-amino-substituted enones with excellent selectivity. Utility of the obtained N-sulfinyl β-amino ketones possessing α,β-unsaturation is exemplified in the total synthesis of the quinolizidine alkaloid natural products (-)-lasubine I, (+)-lasubine II, and substituted piperidine alkaloid (+)-241-D.

Synthesis of (±)-Lasubine II Using N-Methoxyamines

The synthesis of (±)-lasubine II has been achieved through a three-component allylation capitalizing on the unique properties of N-methoxyamines. This reaction enabled the installation of all the carbon atoms of lasubine II in a single operation. The N-methoxy group was efficiently used for the subsequent nitrone formation. A single-step cyclization of isoxazolidines or N-methoxyamines to form functionalized piperidine rings was also developed.

Formal Synthesis of (+)-Lasubine II and (-)-Subcosine II via Organocatalytic Michael Addition of a Ketone to an α-Nitrostyrene

The first examples of an organocatalytic Michael addition of a ketone to in situ generated α-nitrostyrenes are reported. A suitably functionalized γ-nitroketone obtained from the organocatalyzed Michael addition was converted into (+)-2-epi-lasubine II, the immediate synthetic precursor of (+)-lasubine II and (-)-subcosine II (enantiomers of the natural quinolizidine alkaloids). Two of the three stereocenters in (+)-2-epi-lasubine II are set by the Michael reaction.

Simple Indolizidine and Quinolizidine Alkaloids

This review of simple indolizidine and quinolizidine alkaloids (i.e., those in which the parent bicyclic systems are in general not embedded in polycyclic arrays) is an update of the previous coverage in Volume 55 of this series (2001). The present survey covers the literature from mid-1999 to the end of 2013; and in addition to aspects of the isolation, characterization, and biological activity of the alkaloids, much emphasis is placed on their total synthesis. A brief introduction to the topic is followed by an overview of relevant alkaloids from fungal and microbial sources, among them slaframine, cyclizidine, Steptomyces metabolites, and the pantocins. The important iminosugar alkaloids lentiginosine, steviamine, swainsonine, castanospermine, and related hydroxyindolizidines are dealt with in the subsequent section. The fourth and fifth sections cover metabolites from terrestrial plants. Pertinent plant alkaloids bearing alkyl, functionalized alkyl or alkenyl substituents include dendroprimine, anibamine, simple alkaloids belonging to the genera Prosopis, Elaeocarpus, Lycopodium, and Poranthera, and bicyclic alkaloids of the lupin family. Plant alkaloids bearing aryl or heteroaryl substituents include ipalbidine and analogs, secophenanthroindolizidine and secophenanthroquinolizidine alkaloids (among them septicine, julandine, and analogs), ficuseptine, lasubines, and other simple quinolizidines of the Lythraceae, the simple furyl-substituted Nuphar alkaloids, and a mixed quinolizidine-quinazoline alkaloid. The penultimate section of the review deals with the sizable group of simple indolizidine and quinolizidine alkaloids isolated from, or detected in, ants, mites, and terrestrial amphibians, and includes an overview of the "dietary hypothesis" for the origin of the amphibian metabolites. The final section surveys relevant alkaloids from marine sources, and includes clathryimines and analogs, stellettamides, the clavepictines and pictamine, and bis(quinolizidine) alkaloids.

Emergence of single-molecular chirality from achiral reactants

The synthesis of enantiopure molecules from achiral precursors without the need for pre-existing chirality is a major challenge associated with the origin of life. We here show that an enantiopure product can be obtained from achiral starting materials in a single organic reaction. An essential characteristic of this reaction is that the chiral product precipitates from the solution, introducing a crystal–solution interface which functions as an asymmetric autocatalytic system that provides sufficient chiral amplification to reach an enantiopure end state. This approach not only provides more insight into the origin of life but also offers a pathway to acquire enantiopure compounds for industrial applications.

Conversion of achiral starting materials into enantiopure products without additional chiral additives is a challenging task. Here, the authors show a reaction where the precipitation of chiral product induces autocatalysis, ultimately leading to an enantiopure compound.

Enantiodivergency and enantioconvergency in the synthesis of the dendrobate alkaloid 241D

A diastereodivergent preparation of two N-alkenylnitrones (9 and 11) from easily available (R)-2,3-O-cyclohexylideneglyceraldehyde (5) led to an enantiodivergent synthesis of both enantiomers of the dendrobate alkaloid 241D in a sequential two-directional approach involving intramolecular nitrone cycloaddition as the key step. Either of these two nitrones could, in principle, be utilized for the preparation of the title compounds in an enantioconvergent fashion as well. The methodology was extended to prepare an analogue (33) of (-)-241D.