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| - Synthesis of Furo[3,4-c]furans Using a Rhodium(II)-CatalyzedCyclization/Diels−Alder Cycloaddition Sequence
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| - 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-stablilized carbenoid onto the acetylenic π-bond to give a vinyl carbenoid that subsequently cyclizesonto the neighboring carbonyl group to produce the furan ring. These furo[3,4-c]furans react withvarious dienophiles, furnishing anisole derivatives derived by loss of water from the initially formedDiels−Alder cycloadducts. The Rh(II)-catalyzed cyclization reaction was quite versatile with regardto the nature of the interacting carbonyl group. The methodology was applied to the synthesis ofseveral oxa-polyheterocyclic systems by first generating a 2-alkoxy-substituted furan and thenallowing it to undergo a subsequent intramolecular Diels−Alder cycloaddition. Ring opening ofthe resulting cycloadduct is followed by deprotonation to furnish a rearranged keto lactone. Thepotential use of this method for the synthesis of the alkaloid strychnine was probed using suitablemodel diazo compounds. To establish the viability of this approach, the Rh(II)-catalyzed cyclization/cycloaddition sequence of α-diazo amides 64 and 68 were studied. Both compounds underwent thesequential process in good overall yield, leading to novel pentacyclic products. The structural featuresof the resultant products present numerous opportunities for postcycloaddition manipulations thatcould be exploited to synthetic advantage.
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