Synthesis of the 5-6-7 Tricyclic Core of Daphnicyclidin-Type Alkaloids via a Tiffeneau-Demjanov Ring Enlargement Strategy.

An expedient construction of the 5-6-7 tricyclic core of daphnicyclidin-type alkaloids is described. The synthetically challenging cycloheptanone C ring was constructed via a Tiffeneau-Demjanov ring enlargement reaction from a 5-6-6 tricyclic precursor commonly found in calyciphylline A-type alkaloids. Other key transformations included Davis oxidation, 1,2-addition, oxidation, and dehydration to elaborate the essential cyclcohept-2-enone motif.

Total Syntheses of Phleghenrines A and C: A [4 + 2] Cycloaddition and Ring-Expansion Approach.

The first total syntheses of Lycopodium alkaloids phleghenrines A and C have been accomplished in 19 and 18 steps, respectively, relying on three (hetero)-Diels-Alder ([4 + 2]) cycloaddition reactions to forge the cyclic molecular backbone and two ring-expansion reactions to manipulate the ring size. A chiral precursor is synthesized through an auxiliary controlled Diels-Alder reaction, rendering the asymmetric synthesis accessible. The established strategy provides a general approach to the relevant novel Lycopodium alkaloids.