Studies toward the Total Synthesis of Nominine

Asymmetric Synthesis of the Functionalized A/E-Ring Fragment of C18-Diterpenoid Alkaloids

A new asymmetric synthesis of the A/E-ring fragment of C18-diterpenoid alkaloids is described. The crucial contiguous stereogenic centers at C4, C5, and C11 were established through an asymmetric Michael addition/allylation sequence. The unique azabicyclo[3.3.1]nonane motif (A/E rings) was assembled by employing ring-closing metathesis and Mitsunobu reaction as key strategies.

Total Syntheses of Aromatic Abietane Diterpenoids Utilizing Advances in the Pummerer Rearrangement

The first total syntheses of triptobenzene T, vitexifolin C, 4- epi-triptobenzene L, triptobenzene L, and nepetaefolin F have been accomplished through an enantioselective, common intermediate approach and have enabled the confirmation and/or establishment of the absolute stereochemistry of each natural product synthesized. Application of three new and/or underutilized Pummerer reaction pathways proved critical to the synthetic work. A proline sulfonamide-catalyzed Yamada-Otani reaction was used to access the highly functionalized cyclohexane A ring core, including the C₁₀ all-carbon quaternary stereocenter. Additionally, the importance of the A ring unsaturation for controlling the stereoselectivity during the C₄ alkylation is showcased.

Stereoselective Total Synthesis of Hetisine-type C20 -Diterpenoid Alkaloids: Spirasine IV and XI

The first total synthesis of the architecturally complex hetisine-type heptacyclic C₂₀ -diterpenoid alkaloids (±)-spirasine IV and XI is reported. The A/F/G/C tetracyclic skeleton with the challenging N-C6 and C14-C20 linkages was efficiently constructed by an intramolecular azomethine-ylide-based 1,3-dipolar cycloaddition with unusual regioselectivity. SmI₂ -mediated free-radical addition to the arene moiety without prior dearomatization and a stereoselective intramolecular aldol reaction further enabled rapid access to the hetisine core, providing a bicyclo[2.2.2]octane ring with a new oxygen substitution pattern.

(4+3) Cycloaddition Reactions of N-Alkyl Oxidopyridinium Ions

N-Methylation of methyl 5-hydroxynicotinate followed by reaction with a diene in the presence of triethylamine afforded (4+3) cycloadducts in good to excellent yields. High regioselectivity was observed with 1-substituted and 1,2-disubstituted butadienes. Density functional theory calculations indicate that the cycloaddition involves concerted addition of the diene onto the oxidopyridinium ion. The process provides rapid access to bicyclic nitrogenous structures resembling natural alkaloids.

Diterpenoid alkaloids

The lasting attention that researchers have devoted to diterpenoid alkaloids is due to their various bioactivities and toxicities, structural complexity, and intriguing chemistry. From 1998 to the end of 2008, more than 300 new diterpenoid alkaloids were isolated from Nature. This review focuses on their structural relationships, and investigations into their chemical reactions, synthesis, and biological activities. A table that lists the names, plant sources, and structural types is given along with 363 references.

Cyclic Metalated Nitriles: Stereoselective Cyclizations to cis- and trans-Hydrindanes, Decalins, and Bicyclo[4.3.0]undecanes

Metalated nitriles are nucleophilic chameleons whose precise identity is determined by the nature of the metal, the solvent, the temperature, and the structure of the nitrile. The review surveys the different structural types and their cyclization trajectories to show how to selectively tune the metalated nitrile geometry for stereoselective cyclizations to a variety of cis or trans hydrindanes, decalins, and bicyclo[4.3.0]undecanes.

Formation of the C6 - N - C20 Bridge for Entry into the Hetisane Skeleton

As a result of recent investigations into the total synthesis of the complex diterpene alkaloid nominine, the preparation of two new heterocyclic intermediates is now described. The cyclic ether 10 was derived from an intramolecular Mitsunobu reaction. The precursor hydroxylated compound 9 was the minor product of the hydroboration of alkene 7, which proceeded with an unexpected stereochemical outcome. The lactam 3 was derived from the hydrogenation of oxime 18 through presumed cyclization of the amino ester.