Catalytic asymmetric synthesis of R207910

Article abstract of DOI:10.1021/ja103183r

The first asymmetric synthesis of a very promising antituberculosis drug candidate, R207910, was achieved by developing two novel catalytic transformations; a catalytic enantioselective proton migration and a catalytic diastereoselective allylation of an intermediate α-chiral ketone. Using 2.5 mol % of a Y-catalyst derived from Y(HMDS)₃ and the new chiral ligand 9, 1.25 mol % of p-methoxypyridine N-oxide (MEPO), and 0.5 mol % of Bu₄NCl, α-chiral ketone 3 was produced from enone 4 with 88% ee. This reaction proceeded through a catalytic chiral Y-dienolate generation via deprotonation at the γ-position of 4, followed by regio- and enantioselective protonation at the α-position of the resulting dienolate. Preliminary mechanistic studies suggested that a Y: 9: MEPO = 2: 3: 1 ternary complex was the active catalyst. Bu₄NCl markedly accelerated the reaction without affecting enantioselectivity. Enantiomerically pure 3 was obtained through a single recrystallization. The second key catalytic allylation of ketone 3 was promoted by CuF·3PPh₃·2EtOH (10 mol %) in the presence of KO^tBu (15 mol %), ZnCl₂ (1 equiv), and Bu₄PBF₄ (1 equiv), giving the desired diastereomer 2 in quantitative yield with a 14: 1 ratio without any epimerization at the α-stereocenter. It is noteworthy that conventional organometallic addition reactions did not produce the desired products due to the high steric demand and a fairly acidic α-proton in substrate ketone 3. This first catalytic asymmetric synthesis of R207910 includes 12 longest linear steps from commercially available compounds with an overall yield of 5%.