10.1016/S0040-4039(03)00563-X
The study details the enantiospecific total synthesis of the indolizidine alkaloid (+)-ipalbidine, a nonaddictive analgesic with additional biological activities. The synthesis starts from (?)-pyroglutamic acid and involves several key steps. Initially, the alcohol derived from pyroglutamic acid methyl ester is converted to a tosylate, which is then reacted with a higher-order cuprate reagent to form an olefinic amide. This amide is condensed with a bromide, prepared from an ester through reduction and bromination steps, to yield a diene. The diene undergoes ozonolysis to form a diketone, which is then subjected to an intramolecular McMurry coupling reaction using low-valent titanium to construct the desired carbon-carbon double bond, yielding the core structure of ipalbidine. The final steps include reduction of the amide function and debenzylation to obtain the natural product. The study also explores an alternative synthetic path involving the elimination of a vic-diol function from a major byproduct of the McMurry coupling. The developed synthetic strategy is noted for its potential applicability to the synthesis of other biologically active alkaloids.