Site-Selective trans-Hydrostannation of 1,3- and 1,n-Diynes: Application to the Total Synthesis of Typhonosides E and F, and a Fluorinated Cerebroside Analogue.

Propargyl alcohols are privileged substrates for stereochemically unorthodox trans-hydrostannation reactions catalyzed by [Cp*RuCl]4 (Cp*=pentamethylcyclopentadienyl), because an incipient hydrogen bond between the -OH group and the polarized [Ru-Cl] unit assists substrate binding. For this very reason, it is also possible to subject diyne derivatives carrying one -OH group to site-selective stannylation, even if the acetylene units are conjugated and hence, electronically coupled. An unusual temperature dependence was observed in that heating tends to improve site-selectivity, whereas per-stannylation is favored when the reaction is carried out in the cold. This counterintuitive trend can be rationalized based on spectroscopic data; additional support comes from the isolation of the unusual bimetallic complex 11. The bridging fulvene and enynyl ligands in 11 are thought to reflect an interligand redox isomerization process likely triggered by synchronous activation of the 1,3-diyne substrate by two metal centers. The preparative relevance of site-selective trans-hydrostannation is illustrated by the total synthesis of two members of the typhonoside series of glycolipids, which are endowed with neuroprotective properties. Moreover, the preparation of a fluoroalkene sphingosine analogue shows that the tin residue also serves as a versatile handle for late-stage modification of a bioactive target compound.