2-Benzothiazolyl Propargyl Sulfides: Versatile Precursors for Enantiopure Allenes, E-α,β-Unsaturated Ketones and Z-Sulfonylalkenes

A novel, stereospecific route to allenes is described. The approach
proceeds via a retro-ene reaction, initiated by lithium aluminium
hydride mediated cleavage of benzothiazole from the respective (chiral)
2-benzothiazolyl propargyl sulfone precursors. As such, allenes
in up to 92% yield are formed under evolution of sulfur
dioxide at ambient temperature. Furthermore, a new and mild way
to generate E-α,β-unsaturated
ketones has been discovered. These substrates are formed in around
50% yield by treating 2-benzothiazolyl propargyl sulfides
with hydrogen peroxide and a catalytic amount of ammonium molybdate
at room temperature. In the course of this research it was also
found that treatment of 2-benzothiazolyl propargyl sulfones with
sodium borohydride led to reduction of the triple bond exclusively
to the Z-double bond.