Synthesis of furo[3,4-c]furans using a rhodium(II)-catalyzed cyclization/Diels-Alder cycloaddition sequence.
A series of 2-alkynyl 2-diazo-3-oxobutanoates, when treated with a catalytic quantity of rhodium(II) acetate, afforded furo[3,4-c]furans in good yield. The reaction proceeds by addition of a rhodium-stabilized carbenoid onto the acetylenic pi-bond to give a vinyl carbenoid that subsequently cyclizes onto the neighboring carbonyl group to produce the furan ring. These furo[3,4-c]furans react with various dienophiles, furnishing anisole derivatives derived by loss of water from the initially formed Diels-Alder cycloadducts. The Rh(II)-catalyzed cyclization reaction was quite versatile with regard to the nature of the interacting carbonyl group. The methodology was applied to the synthesis of several oxa-polyheterocyclic systems by first generating a 2-alkoxy-substituted furan and then allowing it to undergo a subsequent intramolecular Diels-Alder cycloaddition. Ring opening of the resulting cycloadduct is followed by deprotonation to furnish a rearranged keto lactone. The potential use of this method for the synthesis of the alkaloid strychnine was probed using suitable model diazo compounds. To establish the viability of this approach, the Rh(II)-catalyzed cyclization/cycloaddition sequence of alpha-diazo amides 64 and 68 were studied. Both compounds underwent the sequential process in good overall yield, leading to novel pentacyclic products. The structural features of the resultant products present numerous opportunities for postcycloaddition manipulations that could be exploited to synthetic advantage.