Enyne [4+4] Cycloaddition/Oxidation: Ring Contraction via Cyclopropanones and Their Anionic Ring‐Opening Reactions

Irradiation of a 1,3‐enyne tethered to a 2‐pyridone, in the presence of oxygen, leads to formation of a seven‐membered ring product, an overall [4+4−1] reaction. This transformation involves two unstable intermediates and a sequence of unusual reactions. An initial [4+4] photocycloaddition of the enyne with the pyridone yields a 1,2,5‐cyclooctatriene. Photooxidation of this triene forms a cyclopropanone and subsequent photoextrusion of carbon monoxide gives the observed 1,4‐cycloheptadiene product. The first‐formed cyclooctatriene and the cyclopropanone could be observed and characterized spectroscopically. The cyclopropanone underwent CO extrusion both photochemically and thermally to give the cycloheptadiene product. Addition of fluoride or acetylide to the most stable cyclopropanone occurred chemoselectively at the two different silicon groups rather than the carbonyl group. The resulting cyclopropanone ring openings gave unsaturated aldehydes.     

Total Syntheses of Sarcandrolide J and Shizukaol D: Lindenane Sesquiterpenoid [4+2] Dimers

The syntheses of members of a family of lindenane sesquiterpenoid [4+2] dimers led to the total syntheses of sarcandrolide J and shizukaol D. Inspired by a modified biosynthetic pathway, a cascade featuring furan formation/alkene isomerization/Diels–Alder cycloaddition was devised to construct the congested polycyclic architecture of the target molecules with the correct stereochemistry. This study presents a pioneering synthetic entry to this family of natural products and paves the way for fully exploring their biological functions.     

Stereoselective Synthesis of Piperidines by Iridium‐Catalyzed Cyclocondensation

An iridium‐catalyzed cyclocondensation of amino alcohols and aldehydes is reported. Intramolecular allylic substitution by an enamine intermediate and subsequent in situ reduction furnishes 3,4‐disubstituted piperidines with high enantiospecificity and good diastereoselectivity. The modular approach and the broad functional group tolerance provide access to diverse piperidine derivatives, which were further functionalized to give a versatile set of products.