Four-Step Synthesis of (-)-4-epi-Presilphiperfolan-8α-ol by Intramolecular Iron Hydride Atom Transfer-Mediated Ketone-Alkene Coupling and Studies to Access trans-Hydrindanols with a Botryane Scaffold

Theoretical Studies on the Competing Mechanism and Origin of Diastereoselectivity of NHC-Catalyzed Intramolecular [3 + 2] Annulations of Ynals

Chiral tricyclic 6,5,5-fused rings exhibit structural diversity and possess important biological activities in the synthesis of natural products. However, predicting the possible mechanism and origin of stereoselectivity in these reactions remains a challenge. In this article, we conducted a theoretical investigation into the NHC-catalyzed intramolecular [3 + 2] annulations of ynals to generate tricyclic 6,5,5-fused rings. Our calculations revealed that NHC could nucleophilically attack the carbonyl group of the ynal reactant, leading to the formation of a Breslow intermediate via a 1,2-proton transfer. Subsequently, an intramolecular Michael addition takes place, resulting in a 6-5 bicyclic intermediate. We then compared the competitive processes involving proton transfer and the Mannich reaction. The more energetically favorable process involves an HOAc-assisted proton transfer process, followed by the Mannich reaction. To ascertain the origin of the diastereoselectivity, we performed noncovalent interaction (NCI) and atom-in-molecule (AIM) analyses. This work is useful for understanding the general principles and detailed mechanisms of the synthesis of chiral 6,5,5-fused tricyclic scaffolds with unique diastereoselectivity.

Metal-Catalyzed Hydrogen Atom Transfer (MHAT) Hydroalkylation with Electron-Deficient Alkynes

We present a novel strategy for olefin construction via the reductive coupling of electron-neutral alkenes with electron-deficient alkynes under metal-catalyzed hydrogen atom transfer conditions. This methodology provides selective access to both trans and the more challenging-to-synthesize cis isomers and permits the olefin to be installed next to sterically hindered centers, key factors in the synthesis of biologically active compounds. The reaction exhibits broad functional group tolerance and proceeds under mild, nontoxic conditions with high atom efficiency.

N-Heterocyclic Carbene-Catalyzed Formal Intramolecular [3 + 2] Annulations of Cyclohexadienone-Tethered Ynals

A formal [3 + 2] annulation of cyclohexadienone-tethered ynals is enabled by an N-heterocyclic carbene (NHC) catalyst, affording a tricyclo[6.2.1.04,11]undecane framework. This study represents the first demonstration of using C═C double bonds as the reaction partner in the NHC-catalyzed annulation of ynals. This strategy is characterized by mild reaction conditions and 100% atom economy as well as high catalytic performance and efficiency.

Dual-Catalysed Intermolecular Reductive Coupling of Dienes and Ketones

We report a mild, catalytic method for the intermolecular reductive coupling of feedstock dienes and styrenes with ketones. Our conditions allow concomitant formation of a cobalt hydride species and single-electron reduction of ketones. Subsequent selective hydrogen-atom transfer from the cobalt hydride generates an allylic radical which can selectively couple with the persistent radical-anion of the ketone. This radical-radical coupling negates unfavourable steric interactions of ionic pathways and avoids the unstable alkoxy radical of previous radical olefin-carbonyl couplings, which were limited, as a result, to aldehydes. Applications of this novel and straightforward approach include the efficient synthesis of drug molecules, key intermediates in drug synthesis and site-selective late-stage functionalisation.

Isocyanides as Acceptor Groups in MHAT Reactions with Unactivated Alkenes

The use of isocyanides as acceptor groups in metal-hydride hydrogen atom transfer (MHAT) coupling reactions with nonactivated alkenes to form heterocycles is described. Monosubstituted alkenes couple and cyclize directly, whereas more substituted alkenes proceed via a two-step, one-pot procedure involving MHAT reductive cyclization followed by a MHAT Minisci coupling upon the addition of acid. To highlight the utility of the methodology, a diverse variety of substituted heterocycles such as phenanthridines, indoles, and isoquinolines were prepared.