Abstract:The stereoselectivities obtained in Lewis acid-promoted Mukaiyama aldol additions and Sakurai allylations of mono-, and syn-and anti-disubstituted aldehydes possessing various polar a-and bsubstituents under non-chelating conditions are presented. The stereochemical outcome in the nucleophilic addition to a-substituted aldehydes containing an a-benzyloxy, a-fluoro or a-sulfonamide substituent are accurately predicted by current stereoinduction models.In contrast, the selectivitites obtained from addition of sterically demanding nucleophiles to a-chloro-substituted aldehydes cannot be rationalized by the same models and an alternative is discussed. The stereochemichal outcome in the additions to a,b-disubstituted aldehydes is more complex and cannot be predicted using current models.

A short formal synthesis of ent-Cephalotaxine is achieved. The approach features a new Lewis acid-mediated [2,3]-Stevens rearrangement of N-allylated prolineamide to generate a key quaternary stereogenic center. Additionally, a one-pot Parham-aldol sequence was developed to rapidly assemble two of the four rings in the cephalotaxine core.

The addition of sterically demanding enolsilanes to alpha-chloro aldehydes results unexpectedly in preferential formation of the anti-PFA product (1,2-syn), while the addition of the corresponding boron enolate furnishes the expected polar Felkin-Anh product (1,2-anti). A stereoinduction model explaining these observations is proposed.

A 1,3-dipolar cycloaddition reaction of azomethine ylides to aldehydes is described. The azomethine ylides, generated by thermal electrocyclic ring opening of aziridines, adds to aldehydes in good yields with moderate to good selectivities to

A Diels-Alder reaction is used as a key step in a synthetic study toward communesin F, in order to simultaneously introduce both of the all-carbon quaternary stereocenters with complete control of relative stereochemistry. Further manipulations of the cycloadduct, toward the hexacyclic core-structure of communesin F, are also disclosed.