Asymmetric synthesis of the C(6–18) bis(tetrahydropyran)spiroacetal fragment of the lituarines

We describe efforts to achieve a multigram synthesis of the tricyclic spiroacetal core of the lituarines based on the addition of acyl anion equivalent to 4-(2-furyl)butan-2-one (18). We report the first cases of chemoselective Achmatowicz reaction in the presence of a second furan ring that lacks an α-hydroxyl group. The use of lithiated methoxyallene provides an efficient one-step conversion of ketone 18 into a tricyclic Diels–Alder adduct (27). In the final route, asymmetric cyanosilylation of ketone 29 achieved the construction of the stereogenic C(12) 3°-alcohol centre. Subsequent butenylation, diastereoselective reduction of keto-alcohol (+)-33 and alkene cross metathesis set up an oxy-Michael reaction to close the C(8–12) tetrahydropyran ring. The second ring-closure, which completed the route, was achieved by oxidative spirocyclisation following our earlier work.