Decumbenine synthesis

For synthesis decumbenine B, compound 2-14 was condensed with 2-7 in THF with LDA (lithium diisopropylamide) at -70°C followed by deprotonation with dilute hydrochloric acid. The desired intermediate 2-15 was obtained successfully in 49% yield. Lithium aluminum hydride reduction of 2-15 afforded the amine 2-16, which was converted to 2-17 by debenzylation with palladium on charcoal in acetic acid. The final step was dehydrogenation, after comparison with several reagents including palladium on charcoal in acetic acid and DDQ (2,3-dichloro-5,6-dicyano-l,4-benzoquinone)/l,4-dioxane, the best results were obtained by using DDQ/benzene. The yield of decumbenine B was 41% (Scheme 3). 

As in the case of quinolines, isoquinolines can be prepared by metal-mediated annotation processes. Reaction of t-butylimines of iodo-benzaldehydes with acetylenes in the presence of palladium catalyst affords 3,4-disubstituted isoquinolines. The power of this reaction relies on its ability to introduce different type of substituents on 4-position of the heterocycle by using the formed intermediate palladium complex in the cross-coupling reaction with aryl and alkyl halides or alkenes. This methodology was successfully applied to the total synthesis of decumbenine... 

Roesch, K. R., Larock, R. C. (2002). Synthesis of isoquinolines and pyridines by the paUadium/copper-catalyzed coupling and cyclization of terminal acetylenes and unsaturated imines. The total synthesis of decumbenine. J. Org Chem 67,86-94. 

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