Asymmetric induction in Manganese(III)-based oxidative free-radical cyclizations of chiral esters and amides

Asymmetric syntheses of ent-pimarane diterpenoids

Aromatic ent-pimaranes are a group of aromatized tricyclic diterpenoids that exhibit diverse bioactivities. In this work, the first total syntheses of two aromatic ent-pimaranes were achieved via a C-ABC construction sequence enabled by chiral auxiliary controlled asymmetric radical polyene cyclization, and the subsequent substrate-controlled stereo-/regio-specific hydroboration of alkene allowed for access to both natural products with C19 oxidation modifications.

Asymmetric Total Synthesis of (+)-Phainanoid A and Biological Evaluation of the Natural Product and Its Synthetic Analogues

Here, we report our detailed efforts toward the synthesis of phainanoids, a novel class of dammarane-type triterpenoids with potent immunosuppressive activities and unique structural features. Systematic model studies have been carried out, and efficient approaches have been established to construct the benzofuranone-based 4,5-spirocycle, the D/E/F tricyclic core, the [4.3.1] propellane, and the 5,5-oxaspirolactone moieties. The asymmetric synthesis of (+)-phainanoid A has been achieved through kinetic resolution of the tricyclic core followed by diastereoselective installation of the A/B/C and G/H rings and fragment coupling with the enantioenriched I/J rings. In addition, novel estrone-derived phainanoid analogues have been prepared. The immunosuppressive and cell survival assays revealed that (+)-phainanoid A and some of its synthetic analogues can specifically inhibit stimulation-induced lymphocyte proliferation but not cell survival at their effective concentrations. Preliminary structure-activity relationship information has been obtained, which could inspire future design of immunosuppressive phainanoid analogues.

Enantioselective Radical-Polar Crossover Reactions of Indanonecarboxamides with Alkenes

Highly efficient asymmetric intermolecular radical-polar crossover reactions were realized by combining a chiral N,N'-dioxide/Ni^II complex catalyst with Ag₂ O under mild reaction conditions. Various terminal alkenes and indanonecarboxamides/esters underwent radical addition/cyclization reactions to afford spiro-iminolactones and spirolactones with good to excellent yields (up to 99 %) and enantioselectivities (up to 97 % ee). Furthermore, a range of different radical-mediated oxidation/elimination or epoxide ring-opening products were obtained under mild reaction conditions. The Lewis acid catalysts exhibited excellent performance and precluded the strong background reaction.

Chiral auxiliaries for asymmetric radical cyclization reactions: application to the enantioselective synthesis of (+)-triptocallol

[figure: see text] A series of epimeric 8-aryl menthyl derivatives 5a-d and 6a-l, prepared from the same chiral source (R)-pulegone, were employed as chiral auxiliaries in the asymmetric radical cyclization reactions of beta-keto esters mediated by Mn(OAc)3. Chiral precursors 8c and 8d provided the cyclization products 10c and 10d, respectively, as single isomers (dr > 99:1), whereas the cyclization of precursor 9k gave 13k with good stereoselectivity (dr = 24:1). Diastereomer 13e was employed as the key intermediate in the enantioselective synthesis of (+)-triptocallol in 90% ee.

Metallic salt promoted radical cyclization of beta-keto carboxamides and their corresponding beta-enamino carboxamides

Substituted lactams and spirolactams were obtained by Mn(III)-induced radical cyclization of unsaturated beta-keto carboxamides. Treatment of the corresponding tertiary enamines under similar reaction conditions and in the presence of K(2)CO(3) afforded the same cyclized products but with inversion of diastereoselectivity. The oxidation of optically pure secondary enamines leads to diastereomeric spirolactams in an approximately 3:1 ratio.

Diastereoselectivity in Mn(III)-promoted 4-exo-trig cyclization of enamides to beta-lactams

[reaction: see text] The effect of chiral substituents on the enamide nitrogen atom upon the diastereoselection of the Mn(III)-mediated 4-exo-trig cyclization to beta-lactams was studied. A significant level of diastereoselectivity was achieved when an amino acid ester moiety was included into the enamidic skeleton. The structure of the major diastereoisomer was suggested by semiempirical calculations.