Dithioallyl cation (3 + 2) cycloadditions under aprotic reaction conditions: rapid access to spiro-fused cyclopentane scaffolds

We report a general method to effect all-carbon (3 + 2) cycloadditions that can elaborate cyclopentenes from a range of olefins. The required dithioallyl cation reagents can be generated in a newly developed mild protocol starting from 2-allyloxypyridine precursors, thus avoiding the use of strong Brønsted acids. The novel method significantly expands the substrate scope, which now also includes acid-sensitive olefins, and thus enables the preparation of previously inaccessible spiro-fused scaffold types from simple and readily available starting materials. Acid-sensitive conjugated alkenes can be transformed into attractive spirocarbocycles, using a newly developed mild generation mode of (thio)allyl cations giving a regio- and stereoselective carbocationic cascade reaction.