Synthesis and reactivity of pelletierine-derived building blocks and pelletierine analogs

Synthesis of 3-Alkyl Oxazolidines, Derived from 2-Hydroxymethyl Piperidine, as Analytical Standards for the Analysis of Volatile Aldehydes in the Workplace

Hexahydro-3-alkyl-3H-oxazolo[3,4-a]pyridines 4-15 for the quantitative analysis of various aldehydes were obtained in good yield via the condensation reaction of 2-hydroxymethylpiperidine (2-HMP) with aldehydes under mild conditions. When acrolein was used, the bicyclic 17 was obtained. This novel compound has suitable physical characteristics for an analytical standard. The hexahydro-3-vinyl-3H-oxazolo[3,4-a]pyridine 16 can be obtained at higher temperatures using an excess of acrolein (3 equiv). Following the same procedure as for 16, but with an excess of 2-HMP (2 equiv), a diastereomeric mixture of 18/19, which are both bisadducts of 2-HMP with acrolein, was obtained. The latter mixture can be easily converted into pure 18. Mechanistically, a thorough 1H-NMR study did not show any evidence that the condensation reaction proceeded via an enamine. The reaction probably proceeded through an elusive hemiaminal and fleeting iminium ion, which underwent subsequent cyclization to give hexahydro-3-alkyl-3H-oxazolo[3,4-a]pyridines 4-16. The reaction pathways for the preparation of 4-18 are described.

Stereoselective synthesis of analogues of deoxyfebrifugine

The preparation of six new optically active analogues of the natural product febrifugine (1) is reported. These analogues, lacking the hydroxy group from the natural product, were prepared from optically active N-protected S-pelletierine (7) and differ in terms of the specific quinazolinone portion included. The required S-7 (80% enantiomeric excess) was prepared from an asymmetric Mannich reaction between piperideine (8) and acetone in the presence of l-proline. The differently substituted quinazolinone used in this study (10a–10g) was either commercially available or was prepared from the corresponding substituted anthranilic acid and were installed via a bromination–alkylation sequence. N-Deprotection of the subsequent adducts (12a–12g) gave target compounds 13a–13f and completed the synthetic sequence.

The scaffold-forming steps of plant alkaloid biosynthesis

Alkaloids from plants are characterised by structural diversity and bioactivity, and maintain a privileged position in both modern and traditional medicines. In recent years, there have been significant advances in elucidating the biosynthetic origins of plant alkaloids. In this review, I will describe the progress made in determining the metabolic origins of the so-called true alkaloids, specialised metabolites derived from amino acids containing a nitrogen heterocycle. By identifying key biosynthetic steps that feature in the majority of pathways, I highlight the key roles played by modifications to primary metabolism, iminium reactivity and spontaneous reactions in the molecular and evolutionary origins of these pathways.

Simultaneous Construction of Planar and Central Chiralities as Well as Unprecedented Axial Chirality on and around a Ferrocene Backbone

Simultaneous generation of planar, central and axial chiralities on and around a ferrocene backbone via a d-phenylglycinol-induced intramolecular iminium cyclization was disclosed, which is rare and differs from known methods. A series of chiral spiro[cyclopentadienyl-1,2,3,3a-tetrahydropentalenyliron(II)-1,2'-pyrrolidine] derivatives were prepared acording to the new method, and their structures were characterized unambiguously. The axial chirality caused by the ferrocene backbone and the rigid spiral structure was verified by NOESY and variable-temperature NMR experiments and single-crystal XRD analyses. Mechanism for the stereoselective iminium cyclization reaction was suggested, which was influenced by steric hindrance and hydrogen bonding.

Biomimetic synthesis of 2-substituted N-heterocycle alkaloids by one-pot hydrolysis, transamination and decarboxylative Mannich reaction

A novel multi-enzymatic approach enabled the facile synthesis of a broad range of biologically active 2-substituted piperidine and pyrrolidine alkaloids.

Total Synthesis of (±)-Cermizine B

A practical synthesis of (±)-cermizine B was achieved. The nine-step synthesis mainly comprised two uninterrupted Michael additions including a highly diastereoselective 1,4-addition of 2-picoline to methyl 4-methyl-6-oxocyclohex-1-ene-1-carboxylate, Krapcho decarboxylation, a double reductive amination that resulted in ring closure and dearomatization of pyridine in 24% overall yield.

Trapping of carbolithiation-derived tertiary benzylic α-lithio piperidines with carbon electrophiles: Controlling the formation of α-amino quaternary and vicinal stereocenters

The interception of carbolithiation-derived tertiary benzylic α-lithio piperidines with carbon electrophiles has led to the diastereoselective synthesis of vicinally functionalized piperidines bearing α-amino quaternary stereocenters.

Expedient access to α,β-difunctionalized azepenes using α-halo eneformamides: application to the one-pot synthesis of 2-benzazepanes

The regioselective synthesis of α,β-difunctionalized (alkenyl, aryl, sulfonyl, allyl, or alkynyl) azepenes has been accomplished through α-halo eneformamides.

Direct access to functionalized benzotropones, azepanes, and piperidines by reductive cross-coupling of α-bromo enones with α-bromo enamides

High-yielding syntheses of functionalized azepenes and piperidines, bearing an α-benzotropone derivative, have been achieved through cobalt-catalysed reductive cross-coupling of α-bromo enamides with α-bromo enones.

Thermodynamic epimeric equilibration and crystallisation-induced dynamic resolution of lobelanine, norlobelanine and related analogues

The step-economical synthesis of lobelanine involving a ring closing double aza-Michael (RCDAM) reaction is revisited and successfully extended to the synthesis of various configurationally more stable analogues.