Activating Imides with Triflic Acid: A General Intramolecular Aldol Condensation Strategy Toward Indolizidine, Quinolizidine, and Valmerin Alkaloids

Synthesis of fungicidal morpholines and isochromenopyridinones via acid-catalyzed intramolecular reactions of isoindolinones

Acid-catalyzed intramolecular cyclization or rearrangement of isoindolinone derivatives is described. 3-Hydroxy/ethoxy-3,4-dihydro-6H-[1,4]-oxazino-[3,4-a]-isoindol-6-ones are obtained in moderate to good yields. Further acid-catalyzed intramolecular rearrangement reactions give 6H-isochromeno-[4,3-b]-pyridin-6-ones. The mild reaction conditions with convenient starting materials show broad substrate scope and provide the target compounds as novel pesticide leads with good fungicidal or systemical acquired resistance activities.

An aza-Robinson Annulation Strategy for the Synthesis of Fused Bicyclic Amides: Synthesis of (±)-Coniceine and Quinolizidine

An aza-Robinson annulation strategy is described using a NaOEt-catalyzed conjugate addition of cyclic imides onto vinyl ketones, followed by a TfOH-mediated intramolecular aldol condensation to afford densely functionalized fused bicyclic amides. The potential use of these amides in the synthesis of alkaloids is demonstrated by the sequential conversion of appropriate precursors to (±)-coniceine and quinolizidine in two additional steps, thus allowing their preparation in overall 40 and 44% yields, respectively.

Alkylation of N,N-Dibenzylaminoacetonitrile: From Five- to Seven-Membered Nitrogen-Containing Heterocyclic Systems

The syntheses of several alkaloids and nitrogen-containing compounds including N-Boc-coniine (14b), pyrrolizidine (1), δ-coniceine (2), and pyrrolo[1,2a]azepine (3) are described. New C-C bonds in the α position relative to the nitrogen atom were formed by the alkylation of metalated α-aminonitriles 4 and 6a-c with alkyl iodides possessing the requisite size and functionality. In all of the reported cases, the pyrrolidine ring was formed in the aqueous medium through a favorable 5-exo-tet process involving a primary or a secondary amino group and a terminal δ-leaving group. Conversely, the azepane ring was efficiently formed in N,N-dimethylformamide (DMF), as the preferred aprotic solvent, through an unreported 7-exo-tet cyclization process involving a more nucleophilic sodium amide and a terminal mesylate borne by a saturated six carbon chain unit. In this way, we successfully synthesized pyrrolo[1,2a]azepane 3 and 2-propyl-azepane 14c in good yields from inexpensive and readily available materials without tedious separation methods.

Synthesis of Saturated N-Heterocycles via a Catalytic Hydrogenation Cascade

Saturated N-heterocycles are prominent motifs found in various natural products and pharmaceuticals. Despite the increasing interest in this class of compounds, the synthesis of saturated bicyclic azacycles requires tedious multi-step syntheses. Herein, we present a one-pot protocol for the synthesis of octahydroindoles, decahydroquinolines, and octahydroindolizines through a cascade reaction.

Skeletal remodeling of chalcone-based pyridinium salts to access isoindoline polycycles and their bridged derivatives

Simultaneous deconstructive ring-opening and skeletal reconstruction of an inert, aromatic pyridinium ring is of great importance in synthetic communities. However, research in this area is still in its infancy. Here, a skeletal re-modeling strategy was developed to transform chalcone-based pyridinium salts into structurally intriguing polycyclic isoindolines through a dearomative ring-opening/ring-closing sequence. Two distinct driving forces for the deconstruction of the pyridinium core were involved in these transformations. One was the unprecedented harnessing of the instability of in situ generated cyclic β-aminoketones, and the other was the instability of the resultant N,N-ketals. The desired isoindoline polycycles could undergo the Wittig reaction with various phosphorus ylides to achieve structural diversity and complexity. Notably, by tuning the Wittig conditions by addition of one equivalent of base, an additional bridged ring was introduced. A plausible mechanism was proposed on the basis of control experiments and theoretical calculations.

5-Aryloxazolidines as Reagents for Double Alkylation of Arenes: A Novel Synthesis of 4-Aryltetrahydroisoquinolines

5-Aryloxazolidines react with arenes under Lewis or Brønsted acid conditions via the Friedel-Crafts/Pictet-Spengler double alkylation sequence to give alkaloid-like 4-aryltetrahydroisoquinolines in 12-94% yields. Three approaches for the controlled insertion of substituents into the target molecules and application of oxazolidine derivatives such as 1-arylethanol-2-amines or 4-hydroxytetrahydroisoquinolines in the alkylation of arenes are also described. An unprecedented two-step easily scalable synthesis of the 4-aryltetrahydroisoquinoline core from aromatic aldehyde was achieved applying oxazolidine methodology.

Catalyst-free construction of spiro [benzoquinolizidine-chromanones] via a tandem condensation/1,5-hydride transfer/cyclization process

A catalyst-free tandem 1,5-hydride shift/cyclization process to form spiro [benzoquinolizidine-chromanones] is developed, which features high atom- and step-economy, high levels of stereocontrol, mild conditions, and a simple workup process. A series of new polycyclic spiro [benzoquinolizidine-chromanones] were obtained in high yields with excellent diastereoselectivities (up to 91% yield, >20 : 1 dr).

Biologically active indolizidine alkaloids

Indolizidine alkaloids are chemical constituents isolated from various marine and terrestrial plants and animals, including but not limited to trees, fungi, ants, and frogs, with a myriad of important biological activities. In this review, we discuss the biological activity and pharmacological effects of indolizidine alkaloids and offer new avenues toward the discovery of new and better drugs based on these naturally occurring compounds.

C-H/C-C Functionalization Approach to N-Fused Heterocycles from Saturated Azacycles

Herein we report the synthesis of substituted indolizidines and related N-fused bicycles from simple saturated cyclic amines through sequential C-H and C-C bond functionalizations. Inspired by the Norrish-Yang Type II reaction, C-H functionalization of azacycles is achieved by forming α-hydroxy-β-lactams from precursor α-ketoamide derivatives under mild, visible light conditions. Selective cleavage of the distal C(sp2)-C(sp3) bond in α-hydroxy-β-lactams using a Rh-complex leads to α-acyl intermediates which undergo sequential Rh-catalyzed decarbonylation, 1,4-addition to an electrophile, and aldol cyclization, to afford N-fused bicycles including indolizidines. Computational studies provide mechanistic insight into the observed positional selectivity of C-C cleavage, which depends strongly on the groups bound to Rh trans to the phosphine ligand.

Enzymatic synthesis of 2-hydroxy-4H-quinolizin-4-one scaffolds by integrating coenzyme a ligases and a type III PKS from Huperzia serrata

2-Hydroxy-4H-quinolizin-4-one scaffolds were enzymatically synthesized by integrating three enzymes including phenylacetate-CoA ligase (PcPCL) from an endophytic fungus Penicillium chrysogenum MT-12, malonyl-CoA synthase (AtMatB) from Arabidopsis thaliana, and a type III polyketide synthase (HsPKS3) from Chinese club moss Huperzia serrata. The findings paved the way to produce these kinds of structurally interesting alkaloids by engineered microorganisms.

One-pot enzymatic synthesis of 2-hydroxy-4H-quinolizin-4-one scaffolds was developed by integrating three enzymes PcPCL, AtMatB, and HsPKS3.