H Activation by Non-Transition-Metal Systems: Hydrogenation of Aldimines and Ketimines with LiN(SiMe ).

In recent years, H activation at non-transition-metal centers has met with increasing attention. Here, a system in which H is activated and transferred to aldimines and ketimines using substoichiometric amounts of lithium bis(trimethylsilyl)amide is reported. Notably, the reaction tolerates the presence of acidic protons in the α-position.

Meroterpenoid Synthesis via Sequential Polyketide Aromatization and Cationic Polyene Cyclization: Total Syntheses of (+)-Hongoquercin A and B and Related Meroterpenoids.

(+)-Hongoquercin A and B were synthesized from commercially available trans, trans-farnesol in six and eleven steps, respectively, using dual biomimetic strategies with polyketide aromatization and subsequent polyene functionalization from a common farnesyl-resorcylate intermediate. Key steps involve Pd(0)-catalyzed decarboxylative allylic rearrangement of a dioxinone β,δ-diketo ester to a β,δ-diketo dioxinone, which was readily aromatized into the corresponding resorcylate, and subsequent polyene cyclization via enantioselective protonation or regioselective terminal alkene oxidation and cationic cyclization of enantiomerically enriched epoxide to furnish the tetracyclic natural product cores. Analogues of the hongoquercin were synthesized via halonium-induced polyene cyclizations, and the meroterpenoid could be further functionalized via saponification, hydrolytic decarboxylation, reduction, and amidation reactions.

Importance of a 4-Alkyl Substituent for Activity in the Englerin Series.

The ring closing metathesis/transannular etherification approach to the englerin nucleus was adapted to provide two key intermediates for analogue synthesis: the 4-desmethyl Δ tricycle and the 4-oxo Δ tricycle. The former was elaborated to 4-desmethyl englerin A and the latter served as a common precursor for englerin A, 4-ethyl englerin A, and 4-isopropyl englerin A. 4-Desmethyl englerin A was less active than the natural product by an order of magnitude, but the 4-ethyl and 4-isopropyl analogues were comparable in activity to englerin A.

Sequential Ketene Generation from Dioxane-4,6-dione-keto-dioxinones for the Synthesis of Terpenoid Resorcylates.

Trapping of the ketene generated from the thermolysis of 2-methyl-2-phenyl-1,3-dioxane-4,6-dione-keto-dioxinone at 50 °C with primary, secondary, or tertiary alcohols gave the corresponding dioxinone β-keto-esters in good yield under neutral conditions. These intermediates were converted by palladium(0)-catalyzed decarboxylative allyl migration and aromatization into the corresponding β-resorcylates. These transformations were applied to the syntheses of the natural products (±)-cannabiorcichromenic and (±)-daurichromenic acid.