Formal total synthesis of (±)-estrone via the furano diene approach.

We present in this report the development and realization of a novel formal total synthesis of estrone (1) via the Torgov diene (24) by the furano diene approach, first attempted by Woodward in 1937. The core ring structure 16 was established by an acid-mediated regioselective and stereospecific cyclization of the endo-oxabicyclo[2.2.1]heptene derivative 14, which is readily available from the AlCl(3)-catalyzed Diels-Alder cycloaddition of 2-(3-methoxyphenethyl)furan (4) and dimethyl maleate. The mechanistic pathway of this S(N)' type cyclization is discussed, and the earlier perspectives in our preliminary report (Org. Lett. 2004, 6, 1333) are corrected.