Selective Oxidations in the Synthesis of Complex Natural -Kauranes and -Beyeranes.

Syntheses of two natural products derived from the -kaurene kaurenoic acid are described for the first time using regio- and diastereoselective oxidations. Palladium- and manganese-mediated oxidations were used to accomplish the syntheses of two -beyerane metabolites. The use of the White-Gormisky-Zhao catalyst Mn(CF-PDP) enabled the first application of a nondirected metal-catalyzed oxidation in an unactivated C-H bond in a total synthesis.

Chemoselective methylene oxidation in aromatic molecules.

Despite significant progress in the development of site-selective aliphatic C-H oxidations over the past decade, the ability to oxidize strong methylene C-H bonds in the presence of more oxidatively labile aromatic functionalities remains a major unsolved problem. Such chemoselective reactivity is highly desirable for enabling late-stage oxidative derivatizations of pharmaceuticals and medicinally important natural products that often contain such functionality. Here, we report a simple manganese small-molecule catalyst Mn(CF-PDP) system that achieves such chemoselectivity via an unexpected synergy of catalyst design and acid additive.

Enantioselective Synthesis of Phthalides and Isochromanones via Heck-Matsuda Arylation of Dihydrofurans.

In this communication, the enantioselective synthesis of phthalides and isochromanones is described through a new palladium-catalyzed Heck-Matsuda arylation/NaBH -reduction/lactonization sequence of 2,3- and 2,5-dihydrofurans in good overall yields and excellent enantioselectivities (up to 98:2 er). This expeditious synthesis of chiral Heck lactol intermediates allowed the diversification of the strategy to obtain medicinally relevant chiral lactones, amines, and olefins. The natural product 3-butylphthalide was obtained in three steps with an overall yield of 33 % yield in 98:2 er.

Remote, Late-Stage Oxidation of Aliphatic C-H Bonds in Amide-Containing Molecules.

Amide-containing molecules are ubiquitous in natural products, pharmaceuticals, and materials science. Due to their intermediate electron-richness, they are not amenable to any of the previously developed N-protection strategies known to enable remote aliphatic C-H oxidations. Using information gleaned from a systematic study of the main features that makes remote oxidations of amides in peptide settings possible, we developed an imidate salt protecting strategy that employs methyl trifluoromethanesulfonate as a reversible alkylating agent.

Total Synthesis of (-)-Marinisporolide C.

The first total synthesis of (-)-marinisporolide C is described, which establishes unequivocally the relative and absolute configuration of this oxopolyene macrolide. Key features of this synthesis include a series of highly stereoselective aldol reactions followed by directed reductions to build the polyol domain, a Stille cross-coupling reaction to assemble the polyene, and an intramolecular Horner-Wadsworth-Emmons olefination to forge the macrocyclic ring. Despite the initial approach to marinisporolide A using a Yamaguchi macrolactonization reaction that was unsuccessful due to steric hindrance of the oxygen at the C33 position, we were able to prepare a known derivative of marinisporolide A and consequently confirm its stereochemical assignment.

Total Synthesis of the Oxopolyene Macrolide (-)-Marinisporolide C.

The first total synthesis of (-)-marinisporolide C was performed in 25 steps (longest linear sequence) and an overall yield of 1%. Due to the high degree of convergence and robustness, the C9-C35 fragment that corresponds to the polyol portion was obtained in gram quantity. Highlights of this synthesis include five highly stereoselective aldol reactions responsible for the construction of five C-C bonds and six stereogenic centers.

The role of β-bulky substituents in aldol reactions of boron enolates of methylketones with aldehydes: experimental and theoretical studies by DFT analysis.

In this work, we show the influence of the volume of the β-substituents on the levels of 1,5-stereoselectivities of aldol reactions of boron enolates generated from β-alkoxy methylketones with aldehydes. Excellent levels of 1,5-syn stereoinduction were obtained when the β-protecting group is a silicon ether. This remarkable selectivity is attributed to the volume of the β-bulky substituent of the corresponding boron enolate.

Influence of β-substituents in aldol reactions of boron enolates of β-alkoxy methylketones.

Moderate to good levels of substrate-based 1,5-syn-stereocontrol could be achieved in the boron-mediated aldol reactions of β-tert-butyl methylketones with achiral aldehydes, independent of the nature of the β-alkoxy protecting group (P = PMB or TBS). The analysis of the relative energies of the transition structures by theoretical calculations using the density functional B3LYP shows relative energies favoring the corresponding OUT-1,5-SYN transition structures, explaining the observed 1,5-syn stereoinduction.