Transition Metal-Catalyzed Reactions of Allenes in Diversity-Oriented Synthesis

Transition metal-catalyzed cyclization reactions involving allenes were demonstrated as useful methods for the synthesis of novel heterocyclic compounds. Multiple scaffolds were accessed from pivotal allenic amino-ester intermediates by varying the reaction conditions. Rhodium(I)-catalyzed allenic Alder-ene reaction of allenynes afforded cross-conjugated trienes in high yields. Double bond selectivity in the allenic cyclocarbonylation reaction of allenynes was accomplished by utilizing either rhodium(I)-catalyzed or molybdenum-mediated conditions, thus gaining access to 4-alkylidene or α-alkylidene cyclopentenones, respectively. In addition, a detailed study of the molybdenum-mediated reaction uncovered an unexpected diastereocontrol element based on the amino acid side chain. Furthermore, replacement of the alkyne with an alkene led to discovery of a novel rhodium(I)-catalyzed cycloisomerization of ene-allenes affording tetrahydroazepines. The novel molecular scaffolds obtained in this manner were studied in complexity generating reactions that were employed in the synthesis of libraries of compounds for biological evaluation. For example, stereoselective sequential Diels-Alder reactions of the cross-conjugated trienes allowed rapid access to polycyclic molecular skeletons. Additionally, a Stetter/Paal-Knorr reaction sequence was developed to gain access to novel tricyclic pyrroles