Phosphine Oxide-Catalyzed Enantioselective Intramolecular Aldol Reaction via Regioselective Enolization of Unsymmetrical Diketones with Tetrachlorosilane

Enantioselective and Chemoselective Phosphine Oxide-catalyzed Aldol Reactions of N-Unprotected Cyclic Carboxyimides

Asymmetric catalytic transformations of N-unprotected cyclic carboxyimides such as succinimides, hydantoins, oxazolidinediones, and glitazones, is a powerful way of directly accessing variety of biologically valuable chiral compounds. Herein, a bis(trichlorosilyl) nucleophilic intermediate formed from cyclic carboxyimides was reacted with aldehydes via (S)-SEGPHOS dioxide (SEGPHOSO), proceeding the aldol reaction in highly enantioselective fashion through a cyclic transition state. Furthermore, N-unprotected carboxyimides were chemoselectively activated, even in the presence of N-alkylated carboxyimides, to undergo stereoselective and chemoselective aldol reactions via in situ silicon tetrachloride activation. The functionalized cyclic carboxyimides is readily derived to the several synthetic units derivatization to various chiral building blocks without unnecessary protection/deprotection steps.

Cascade cyclization of 1,2,7,8-tetraones and total synthesis of (±)-nesteretal A

For the short synthesis of highly oxidized complex molecules, regioselectivity and stereoselectivity of intramolecular domino aldol cyclization/acetalizations of 1,2,7,8-tetraones were investigated by using catalytic amounts of Brønsted bases, Brønsted acids,...

TetraPh-Tol-BITIOPO: a new atropisomeric 3,3'-bithiophene based phosphine oxide as an organocatalyst in Lewis base-catalyzed Lewis acid mediated reactions

A new chiral phosphine oxide based on a 3,3'-bithiophene scaffold (TetraPh-Tol-BITIOPO) was synthesized, fully characterized and separated into antipodes through chiral HPLC. This new compound was successfully employed as an organocatalyst in Lewis base-catalyzed Lewis acid mediated reactions involving trichlorosilyl compounds. The new atropisomeric catalyst was able to promote the allylation of aldehydes with allyltrichlorosilane in up to 98% yield and up to 96% enantiomeric excess (ee), and the direct aldol reaction to afford β-hydroxy ketones and β-hydroxy thioesters, with good chemical yields and modest stereochemical efficiency. Computational studies helped to elucidate and to rationalize the stereochemical outcome of the reactions catalyzed by TetraPh-Tol-BITIOPO that was found to favour the formation of the isomer with an opposite absolute configuration in comparison with the products obtained with the previously reported 3,3'-bithiophene-based catalyst.

Phosphine Oxide-Catalyzed Asymmetric Aldol Reactions and Double Aldol Reactions

Chiral phosphine oxides successfully catalyze asymmetric cross-aldol reactions of various carbonyl compounds in a highly enantioselective manner. The phosphine oxide catalysts coordinate to chlorosilanes to form chiral hypervalent silicon complexes in situ, which activate both aldol donors and acceptors, thus realizing cross-aldol reactions between a ketone and an aldehyde, between two aldehydes, between two ketones, and of 2,6-diketones. The use of phosphine oxide catalysis can be further extended to achieve the first catalytic enantioselective double aldol reactions, realizing one-pot stereoselective construction of up to four stereogenic centers.

Brønsted Acid Catalyzed Synthesis of Functionalized 1,4- and 1,6-Dicarbonyl Monosilyl Enol Ethers under Operationally Practical Conditions

Herein, we report an improved protocol for the concise synthesis of functionalized 1,4- and 1,6-dicarbonyl-derived monosilyl enol ethers via ionization of α'-hydroxy silyl enol ethers to generate unsymmetrical silyloxyallyl cations that were subsequently captured by TBS silyl enolates. These transformations were efficiently performed in acetonitrile at room temperature by employing pyridinium triflate as a catalyst. Our new reaction conditions are operationally more practical and broaden the accessibility of various 1,4- and 1,6-dicarbonyl groups, which include diketone, ketoester, and ketothioester functionalities.

Synthesis of Renewable Triketones, Diketones, and Jet-Fuel Range Cycloalkanes with 5-Hydroxymethylfurfural and Ketones

A series of renewable C₉ -C₁₂ triketones with repeating [COCH₂ CH₂ ] units were synthesized in high carbon yields (ca. 90 %) by the aqueous-phase hydrogenation of the aldol-condensation products of 5-hydroxylmethylfurfural (HMF) and ketones over an Au/TiO₂ catalyst. Compared with the reported routes, this new route has many advantages such as being environmentally friendly, having fewer steps, using a cheaper and reusable catalyst, etc. The triketones as obtained can be used as feedstocks in the production of conducting or semi-conducting polymers. Through a solvent-free intramolecular aldol condensation over solid-base catalysts, the triketones were selectively converted to diketones, which can be used as intermediates in the synthesis of useful chemicals or polymers. As another application, the tri- and diketones can also be utilized as precursors for the synthesis of jet-fuel range branched cycloalkanes with low freezing points (224-248 K) and high densities (ca. 0.81 g mL^-1 ).

Concise Asymmetric Construction of C2 -symmetric 1,9-Diarylnonanoids Using a Hypervalent Silicon Complex: Total Synthesis of (-)-Ericanone

By using a phosphine oxide-catalyzed enantioselective double aldol reaction, we achieved the concise construction of C2 -symmetric 1,9-diarylnonanoids, enabling the synthesis of (-)-ericanone from p-hydroxybenzaldehyde in 6 steps with 65 % overall yield. The enantioselective double aldol reaction is useful for establishing C2 -symmetric 1,9-diaryl-3,7-dihydroxy-5-nonanones with a single operation. Furthermore, the use of o-nosyl-protected p-hydroxybenzaldehyde and a 4,4'-disubstituted BINAP dioxide catalyst dramatically improved the reactivity and selectivity in the double aldol reaction, enabling the total synthesis of (-)-ericanone with high yield and with excellent enantiopurity.