Tin-Free Access to the ABC Core of the Calyciphylline A Alkaloids and Unexpected Formation of a D-Ring-Contracted Tetracyclic Core

A tin-free strategy for the successful cyclization of a variety of internal alkyne-containing N-chloroamine precursors to the ABC core via cyclization of a neutral aminyl radical is established. Deuterium labeling experiments confirm that the solvent is the primary source of the final H atom in the cyclization cascade. These conditions enabled a streamlined route to a β-ketoester intermediate poised for intramolecular Knoevenagel condensation to construct the seven-membered D-ring of calyciphylline A alkaloids. However, exposure to CsF in t-BuOH at elevated temperatures led to an unexpected decarboxylation to form a D-ring-contracted tetracyclic core.

Toward the ABCD Core of the Calyciphylline A-Type Daphniphyllum Alkaloids: Solvent non-Innocence in Neutral Aminyl Radical Cyclizations

The Daphniphyllum alkaloids remain an attractive target in the synthetic community because of their unique framework and promising biological activities. We have shown that the ABC core of the calyciphylline A-type alkaloids can be rapidly accessed via the tandem cyclization of a neutral aminyl radical with a polarized cyclic olefin. Deuterium labeling experiments and reactions omitting a tin hydride reagent suggest that the solvent is the major source of the terminating hydrogen atom in the cyclization cascade. Incorporation of an internal alkyne in the radical pathway was tolerated in the reaction, and it provided the necessary atoms to enable completion of the D ring of the calyciphylline A-type alkaloids.

Rapid access to the heterocyclic core of the calyciphylline A and daphnicyclidin A-type Daphniphyllum alkaloids via tandem cyclization of a neutral aminyl radical

A streamlined approach to the tertiary amine-containing core of the calyciphylline A and daphnicyclidin A-type Daphniphyllum alkaloids is presented. A known carvone derivative is converted into the core structure in only four synthetic operations, and it is well poised for further elaboration. The key enabling methodology is a radical cyclization cascade beginning with addition of a secondary, neutral aminyl radical to the β-position of an enone, followed by trapping with a pendant alkyne.

Pyrrolidine and piperidine formation via copper(II) carboxylate-promoted intramolecular carboamination of unactivated olefins: diastereoselectivity and mechanism

An expanded substrate scope and in-depth analysis of the reaction mechanism of the copper(II) carboxylate-promoted intramolecular carboamination of unactivated alkenes is described. This method provides access to N-functionalized pyrrolidines and piperidines. Both aromatic and aliphatic gamma- and delta-alkenyl N-arylsulfonamides undergo the oxidative cyclization reaction efficiently. N-Benzoyl-2-allylaniline also underwent the oxidative cyclization. The terminal olefin substrates examined were more reactive than those with internal olefins, and the latter terminated in elimination rather than carbon-carbon bond formation. The efficiency of the reaction was enhanced by the use of more organic soluble copper(II) carboxylate salts, copper(II) neodecanoate in particular. The reaction times were reduced by the use of microwave heating. High levels of diastereoselectivity were observed in the synthesis of 2,5-disubstituted pyrrolidines, wherein the cis substitution pattern predominates. The mechanism of the reaction is discussed in the context of the observed reactivity and in comparison to analogous reactions promoted by other reagents and conditions. Our evidence supports a mechanism wherein the N-C bond is formed via intramolecular syn aminocupration and the C-C bond is formed via intramolecular addition of a primary carbon radical to an aromatic ring.