A domino method for the rapid syntheses of 1-azabicyclo[x.y.0]alkane scaffolds, such as indolizidines, quinolizidines, decahydropyridoazepines, and their derivatives, has been developed. This strategy involved a rhodium-catalyzed hydroformylation of allyl-, 3-butenyl-, or homoallyl amides, followed by two spontaneous ring closures under mild conditions. The reaction scope and diastereoselectivity were fully explored by changing the substitution pattern on the amide and by altering the length of the carbon chain. This method was applied to the syntheses of natural alkaloids tashiromine and epilupinine. The clear differences between the reactivities of two isomeric amide substrates, 3-butenamide 1 j and homoallyl amide 1 i, could be attributed to better HOMO-LUMO overlap in the transition states that were derived from butenamides during cyclization. This explanation was supported by DFT calculations.
Keywords: alkaloids; domino reactions; hydroformylation; rhodium; synthetic methods.
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