Rh(I)-catalyzed bimolecular cyclization between two different 1,5-bisallenes: a combinatorial one-step approach to heterosteroids and mechanistic implications.

In this paper, a bimolecular-cyclization reaction between two different bis(allene)s with at least one heteroatom as the tether under the catalysis of trans-[RhCl(CO)(PPh3)2] is described. This protocol provides an efficient entry to different heterocyclic 18,19-norsteroid-like scaffolds. The tricyclic product was formed highly selectively from the cyclization reaction of bis(2,3-butadienyl)sulfide with dimethyl 2-bis(2',3'-butadienyl)malonate, which sheds light on the mechanism involving the metalla-[4.3.0]-bicyclic intermediate formed by the cyclometallation of the terminal and the internal C=C bonds of each of the two allene moieties in 2-bis(2',3'-butadienyl)malonate.