Anion traps 3 + 2 Annulations

The anion derived from furobenzopyrandione 28 reacts with Michael acceptors regiospecifically to yield xanthones and various annulated derivatives <03OL3753> and intramolecular trapping of the anion by a carbamate side-chain leads to spiro- pyrrolidine and piperidine 9-xanthene derivatives <03TL9291>. 

The capnellane (109) has been synthesized, using two dififerent annulation procedures (Scheme 11). The a/b ring junction was formed through consecutive Michael and ene-type reactions by addition of a lithium cuprate to 3-methyl-cyclopent-2-enone and trapping of the enolate anion that is thus formed as the enol acetate (110). Lewis-acid-induced cyclization then formed the ketone (111). 

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