An enantioselective synthesis of an intermediate in the Tanino total synthesis of solanoeclepin A has been developed. The key step was an intramolecular [2+2] photocycloaddition, which led to the tricyclo[5.2.1.0^1, 6]decane core in six steps. The first photosubstrate, prepared through an indium-mediated Barbier-type reaction, gave an excellent [2+2] cycloaddition, but it could not be obtained in sufficient enantiopurity. The second photosubstrate, prepared through an asymmetric allene diborylation in high enantiomeric excess, gave the [2+2] cycloaddition product in high yield on irradiation at 365 nm on 20 g scale in a flow system. Other important steps were the replacement of a boronate group at the quaternary carbon by a vinyl group and diastereoselective cyclopropanation of an allylic alcohol.

A new method has been developed for the synthesis of 2-alkylidenequinuclidines based on a silver triflate catalysed intramolecular hydroamination of 4-(prop-2-ynyl)piperidines. Monosubstituted piperidines reacted less efficiently than cis-disubstituted piperidines, and the reaction was selective for an alkyne moiety, even in the presence of a vinyl group at the 3-position. The hydroamination occurred readily with a terminal alkyne, as well as with an internal alkyne bearing an aliphatic or aromatic group at the terminal carbon atom. Using this silver-catalysed cyclization, a short procedure was developed for the relay synthesis of the cinchona alkaloids dihydroquinidine and dihydroquinine.

Racemic total syntheses of four spirocyclic oxindole alkaloids are reported. The general starting material was an N-2-butenylated 2-hydroxytryptamine, which underwent a base-mediated Mannich spirocyclisation with a functionalised aldehyde to generate the C-ring. The second key step was a Pd-catalysed cyclisation of an α-keto ester enolate (in the original aldehyde) onto an allylic carbonate (in the N-substituent) to form the D-ring. The stereoselectivities of the key steps were moderate, but the isomers were readily purified, so that the natural products could be conveniently prepared, three of them for the first time.

This report describes intramolecular [2+2] photocycloadditions of several butenolides with an allenylmethyl substituent at the 5-position. These photosubstrates were prepared from 2-(silyloxy)furans through silver-mediated reactions with allenylmethyl bromides. Two cases were studied where the lactone was replaced by an N-Boc-lactam. Irradiations of the photosubstrates were carried out with 300 nm UV-light in acetonitrile/acetone mixtures. Crossed cycloadducts were produced in good yields via cycloaddition of the heterocyclic double bond with the allene internal double bond. The products were γ-lactones (γ-lactams) containing the strained 5-methylenebicyclo[2.1.1]hexane skeleton. In some cases the products were unstable due to remarkably facile retro-ene reactions (1,5-hydrogen shifts) leading to unique α,β-unsaturated γ-lactone derivatives. These reactions occurred only with well-oriented C–H bonds so that strain-releasing four-membered ring opening could take place. This retro-ene reaction was studied in detail for more highly substituted derivatives in order to establish its relevance in connection with the instability of the natural terpenoid solanoeclepin A, the hatching agent of potato cyst nematodes. To this end, the methylenecyclobutane moiety in one of the cycloadducts was converted into a bicyclo[2.1.1]hexan-5-one (cyclobutanone) substructure.