Deracemization of silyl enol ethers

Stereopermutation on the Putative Structure of the Marine Natural Product Mucosin

A stereodivergent total synthesis has been executed based on the plausibly misassigned structure of the unusual marine hydrindane mucosin (1). The topological connectivity of the four contiguous all-carbon stereocenters has been examined by selective permutation on the highlighted core. Thus, capitalizing on an unprecedented stereofacial preference of the cis-fused bicycle[4.3.0]non-3-ene system when a Michael acceptor motif is incorporated, copper-mediated conjugate addition furnished a single diastereomer. Cued by the relative relationship reported for the appendices in the natural product, the resulting anti-adduct was elaborated into a probative target structure 1*.

Catalytic asymmetric protonation of alpha-amino acid-derived ketene disilyl acetals using P-spiro diaminodioxaphosphonium barfates as chiral proton

Chiral diaminodioxaphosphonium salts have been developed and their unique abilities as a chiral proton have been revealed through the establishment of a highly enantioselective protonation of alpha-amino acid-derived ketene disilyl acetals.

Asymmetric protonation of ketone enolates using chiral beta-hydroxyethers: acidity-tuned enantioselectivity

New chiral hydroxyethers 1a-f were prepared for asymmetric protonation of achiral enolates prepared from prochiral ketones. The enantioselectivity of protonation was highly dependent upon the acidity of the chiral alcohols, the highest enantioselectivity (90% ee) being achieved with 3,5-dichloro-substituted beta-hydroxyether 1c. A salt-free enolate generated from trimethylsilyl enol ether 4 provided product of the highest ee. Unlike other reagents, chloro-substituted alcohols provided almost consistent enantioselections throughout the reaction temperatures examined (-25 to -98 degrees C). Protonation of other aromatic ketones showed selectivity similar to that of 2-methyl-1-tetralone.

Chiral proton donor reagents: tin tetrachloride--coordinated optically active binaphthol derivatives

The Lewis acid-assisted chiral Brønsted acids (chiral LBAs), which are prepared from tin tetrachloride and optically active binaphthol derivatives, are highly effective chiral proton donor reagents for enantioselective protonation and biomimetic polyene cyclization. These chiral LBAs can directly protonate various silyl enol ethers and ketene disilyl acetals to give the corresponding alpha-aryl or alpha-halo ketones and alpha-arylcarboxylic acids, respectively, with high enantiomeric excess (up to 98% ee). A catalytic version of enantioselective protonation was also achieved using stoichiometric amounts of 2,6-dimethylphenol and catalytic amounts of monomethyl ether of optically active binaphthol in the presence of tin tetrachloride. The biomimetic cyclization of simple isoprenoids to polycyclic isoprenoids using chiral LBA is also described. This is the first example of a chiral Brønsted acid-induced enantioselective ene cyclization in synthetic chemistry. Geranyl phenyl ethers, o-geranylphenols, and homogeranylphenol derivatives were directly cyclized in the presence of (R)-binaphthol derivatives and tin tetrachloride (up to 90% ee). Compounds bearing a farnesyl group could also be cyclized under the same conditions to give the natural products (-)-ambrox((R)) and (-)-chromazonarol, and (-)-tetracyclic polyprenoids of sedimentary origin. These chiral LBAs recognize the prochiral face of a trisubstituted terminal olefin and site selectively generate carbocations on the substrates.