The Thiopyran Route to Polypropionates: Enantioselective Synthesis of Membrenone B from Racemic Fragments

Enantioselective Pd-Catalyzed Decarboxylative Allylic Alkylation of Thiopyranones. Access to Acyclic, Stereogenic α-Quaternary Ketones

A catalytic, enantioselective decarboxylative allylic alkylation of 4-thiopyranones is reported. The α-quaternary 4-thiopyranones produced are challenging to access by standard enolate alkylation owing to facile ring-opening β-sulfur elimination. In addition, reduction of the carbon-sulfur bonds provides access to elusive acyclic α-quaternary ketones. The alkylated products are obtained in up to 92% yield and 94% enantiomeric excess.

The thiopyran route to polypropionates

The strategic use of thiopyran templates to facilitate polypropionate synthesis was first demonstrated in Woodward's landmark total synthesis of erythromycin A in 1981 where the topology of a fused bicyclic system was exploited. In the ensuing three decades, various alternative strategic applications of thiopyran motifs to achieve key stereoselective transformations have emerged including, inter alia, unique substrates for chemoenzymatic syntheses, surrogates for 3-pentanone in enantioselective aldol reactions, and templates for enantiotopic group selective reactions. This review summarizes these developments.

Highly stereoselective and scalable anti-aldol reactions using N-(p-dodecylphenylsulfonyl)-2-pyrrolidinecarboxamide: scope and origins of stereoselectivities

A highly enantio- and diastereoselective anti-aldol process (up to >99% ee, >99:1 dr) catalyzed by a proline mimetic-N-(p-dodecylphenylsulfonyl)-2-pyrrolidinecarboxamide-has been developed. Catalyst loading as low as 2 mol % can be employed. Use of industry-friendly solvents for this transformation as well as neat reaction conditions have been demonstrated. The scope of this transformation on a range of aldehydes and ketones is explored. Density functional theory computations reveal that the origins of enhanced diastereoselectivity are due to the presence of nonclassical hydrogen bonds between the sulfonamide, the electrophile, and the catalyst enamine that favor the major anti-Re aldol TS in the Houk-List model.