The direct and highly enantioselective synthesis of tetrahydroacridines was achieved through the phosphoric acid catalyzed addition of enamides to in situ generated ortho-quinone methide imines and subsequent elimination. This novel one-step process constitutes a very efficient, elegant, and selective synthetic approach to valuable N-heterocycles with a 1,4-dihydroquinoline motif. By subsequent highly diastereoselective hydrogenation and N-deprotection the reaction products were easily converted into free hexahydroacridines with a total of three new stereogenic centers.

A straightforward, catalytic, enantioselective approach toward the synthesis of a broad range of 1,4-dihydroquinoline-3-carboxylates is described. Under phosphoric acid catalysis in situ-generated ortho-quinone methide imines reacted with β-keto esters to form the nitrogen heterocycles with good yields and enantioselectivities in just one synthetic step under ambient reactions conditions.

Spirocyclic dihydroquinolones have been obtained with good yields and excellent diastereoselectivity (> 20:1 d.r.), and enantioselectivity (up to 99:1 e.r.) from in situ generated ortho-quinone methide imines and cyclic b-oxo esters. This one-step domino Michael addition-lactamization processf eatures mild reactionc onditions, easily accessible startingm aterials, and products with two adjacent chiral centers one of whichi sq uaternary.M echanistic studies revealedthe in situ generatedc hiralm agnesium phosphate salt rather than the free phosphoric acid to be the more reactive catalystfor this reaction.

Ruthenium-catalyzed cycloadditions to form five-, six-, and seven-membered rings are summarized, including applications in natural product total synthesis. Content is organized by ring size and reaction type. Coverage is limited to processes that involve formation of at least one C–C bond. Processes that are stoichiometric in ruthenium or exploit ruthenium as a Lewis acid (without intervention of organometallic intermediates), ring formations that occur through dehydrogenative condensation-reduction, σ-bond activation-initiated annulations that do not result in net reduction of bond multiplicity, and photochemically promoted ruthenium-catalyzed cycloadditions are not covered.