Facile synthesis of the tricyclic core of sarain A. 3-oxidopyridinium betaine cycloaddition approach

[GRAPHICS] A new approach to a suitably functionalized tricyclic core of sarains has b een developed by means of Katritzky's cycloaddition using 3-oxidopyridinium betaines. A key step was the regioselective differentiation of the two nea rly identical hydroxy groups derived from oxidative cleavage of the double bond in 8 to afford 14. A stereocontrolled construction of the tricyclic co re 20 of sarains containing the requisite side chain at C-3' was achieved b y an intramolecular conjugate addition.