Synthesis of π-Extended Carbazole Dimers Via Oxidative Cyclization Using DDQ and Sulfonic Acid and Elucidation of the Reaction Mechanism.

This paper describes the synthesis of various π-extended carbazole dimers via intramolecular oxidative cyclization using a chain precursor consisting of two carbazole units bridged by a nitrogen atom. A careful selection of the reaction conditions using 2,3-dichloro-5,6-dicyano-p-benzoquinone/sulfonic acid enabled the selective synthesis of a π-extended analogue with an angular (L-shape) structure in moderate yield. The preferential positions for the bonding of the two carbazole units in this π-extended derivative were determined by performing density functional theory calculations.

Asymmetric total synthesis of (-)-stenine and 9a-epi-stenine.

A route for the asymmetric synthesis of (-)-stenine, a member of the Stemona alkaloid family used as folk medicine in Asian countries, is described. The key features of the sequence employed include stereoselective transformations on a cyclohexane ring controlled by a chiral auxiliary unit and an intramolecular Mitsunobu reaction to construct the perhydroindole ring system. By using an intermediate in the route to (-)-stenine, an asymmetric synthesis of 9a-epi-stenine was also executed. The C(9a) stereocenter in 9a-epi-stenine was installed by using a Staudinger/aza-Wittig reaction of a keto-azide precursor followed by reduction of the resulting imine. The results of this effort demonstrate the applicability of the chiral auxiliary based strategy to the preparation of naturally occurring alkaloids that contain highly functionalized cyclohexane cores.

Stereoselective synthesis of cis-2,5-disubstituted THFs: application to adjacent bis-THF cores of Annonaceous acetogenins.

The iodocyclization of γ,δ-unsaturated alcohols in the presence of a silyl enol ether produced cis-2,5-disubstituted tetrahydrofurans in one pot via siloxy intermediates. N-Iodosuccinimide (NIS) effectively worked as an activator of the double bonds in the substrates and the silyl enol ether. Application to an expedient synthesis of the adjacent bis-tetrahydrofuran core of Annonaceous acetogenins with a cis/threo/cis relative stereochemistry is also described.

Intramolecular iodoetherification of ene or diene ketals: facile synthesis of spiroketals.

A novel and rapid approach to chiral mono- or di-substituted spiroketals based on remote asymmetric induction by intramolecular iodoetherification of ene or diene ketals has been developed. This strategy concisely offers 5,5- and 5,6-spiroketals including the natural insect pheromone of the wasp.