Journal of the American Chemical Society, Vol.132, No.23, 7905-7905, 2010
Catalytic Asymmetric Synthesis of R207910
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 alpha-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, a-chiral ketone 3 was produced from enone 4 with 88% ee. This reaction proceeded through a catalytic chiral Y-dienotate generation via deprotonation at the gamma-position of 4, followed by regio- and enantioselective protonation at the a-position of the resulting dienolate. Preliminary mechanistic studies suggested that a Y: 9: MEPO = 2: 3: 1 ternary complex was the active catalyst. Bu4NC1 markedly accelerated the reaction without affecting enantioselectivity. Enantiomerically pure 3 was obtained through a single recrystallization. The second key catalytic altylation of ketone 3 was promoted by CuF center dot 3PPh(3)center dot 2EtOH (10 mot %) in the presence of (KOBu)-Bu-t (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 a-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 a-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%.