Oxabicyclo[3.2.1]octenes in organic synthesis - Direct ring opening of oxabicyclo[3.2.1] systems employing silyl ketene acetals in concentrated solutions of lithium perchlorate-diethyl ether: Application to the synthesis of the C(19)-C(27) fragment of rifamycin S

[GRAPHICS] The direct opening at the bridgehead of oxabicyclo[3.2.1]octenes employing silyl ketene acetals in 4.0-5.0 M lithium perchlorate in diethyl ether has been realized, which gives rise to highly functionalized cycloheptadienes that can be further manipulated for use in natural product synthesis. The bridgehead opening reaction has been employed in the construction of the C(19)-C(27) fragment of Rifamycin S.