Traceless Stereoinduction in the One-Pot Assembly of All Three Rings of Hexahydrodibenzopyrans

2008-03-12T00:00:00Z (GMT) by Keith A. Korthals William D. Wulff
A one-pot method has been developed for the synthesis of all three rings of the hexahydrodibenzopyran ring system. This process involves a tandem sequence consisting of a benzannulation reaction of an alkenyl Fischer carbene complex and an alkyne followed by an elimination reaction that generates an <i>o</i>-quinone methide chromium tricarbonyl complex that is finally followed by an intramolecular heteroatom Diels−Alder cycloaddition. The stereoinduction for the overall process begins with the transfer of stereochemical information from the chiral center at the propargylic ether in the alkyne to the planar center of chirality in the in situ generated arene chromium tricarbonyl complexed intermediate and finally, after base-induced elimination to generate an <i>o</i>-quinone methide chromium tricarbonyl complexed intermediate, a transfer of chirality from the planar center of chirality in the <i>o</i>-quinone methide complex to the two new centers of chirality at positions 6a and 10a in the hexahydrodibenzopyran products upon intramolecular Diels−Alder reaction. The reactions of diastereomeric chiral propargyl ethers that contain an additional center of chirality can lead to high diastereoselectivity in the matched case and to low selectivity in the mismatched case.