Total Synthesis of the Labdane Diterpenes Galanal A and B from Geraniol

The
first total synthesis of galanal A and B has been achieved
from naturally occurring geraniol. Key steps in this synthesis are
the use of a Lewis acid assisted chiral Brønsted acid (chiral
LBA) mediated cationic polyene cyclization and a titanocene-mediated
radical cyclization for the asymmetric assembly of the “AB”
ring and the construction of the all-carbon quaternary center at the
junction of the “BC” ring, respectively.