Mariafrancesca Fochi

The Emergence of Quinone Methides in Asymmetric Organocatalysis

Quinone methides (QMs) are highly reactive compounds that have been defined as “elusive” intermediates, or even as a “synthetic enigma” in organic chemistry. Indeed, there were just a handful of examples of their utilization in catalytic asymmetric settings until some years ago. This review collects organocatalytic asymmetric reactions that employ QMs as substrates and intermediates, from the early examples, mostly based on stabilized QMs bearing specific substitution patterns, to more recent contributions, which have dramatically expanded the scope of QM chemistry. In fact, it was only very recently that the generation of QMs in situ through strategies compatible with organocatalytic methodologies has been realized. This tactic has finally opened the gate to the full exploitation of these unstable intermediates, leading to a series of remarkable disclosures. Several types of synthetically powerful asymmetric addition and cycloaddition reactions, applicable to a broad range of QMs, are now available.

Catalytic Asymmetric Addition of Meldrum’s Acid, Malononitrile, and 1,3‐Dicarbonyls to ortho‐Quinone Methides Generated In Situ Under Basic Conditions

A new approach to the utilization of highly reactive and unstable ortho-quinone methides (o-QMs) in catalytic asymmetric settings is presented. The enantioselective reactions are catalysed by bifunctional organocatalysts, and the o-QM intermediates are formed in situ from 2-sulfonylalkyl phenols through base-promoted elimination of sulfinic acid. The use of mild Brønsted basic conditions for transiently generating o-QMs in catalytic asymmetric processes is unprecedented, and allows engaging productively in the reactions nucleophiles such as Meldrums acid, malononitrile and 1,3-dicarbonyls. The catalytic transformations give new and general entries to 3,4-dihydrocoumarins, 4H-chromenes and xanthenones. These frameworks are recurring structures in natural product and medicinal chemistry, as testified by the formal syntheses of (R)-tolterodine and (S)-4-methoxydalbergione from the catalytic adducts.

NEWLY DESIGNED ACYLSILANES AS VERSATILE TOOLS IN ORGANIC SYNTHESIS

Abstract Structural variations in acylsilane molecules allow a number of new selective synthetic processes to be performed which lead to sulfur-containing heterocycles, highly functionalized and unsaturated polycarbonyl derivatives, polyenes, and β - and γ -aminoalcohols. The synthesis of these compounds, most of them not easily accessible through conventional routes, takes advantage of the site-selective reactions occurring at the C–Si bonds, of the increased stability of acylsilanes with respect to that of the corresponding aldehydes and of the high diastereofacial selectivity introduced by the SiR 3 group. Herein we report the different synthetic strategies leading to the synthesis of several functionalized acylsilanes and their synthetic applications. The use of new selective polymetallic reagents for the nucleophilic silylation will be presented as well.

Allylation reactions of acylsilanes as synthetic equivalents of aldehydes. Application to a stereocontrolled synthesis of (1S,2S,5S)-(10S)-and-(10R)-allyl myrtanol

Abstract The allylation of acylsilanes with tetraallyltin in the presence of catalytic amounts of Sc(OTf) 3 proceeded smoothly to afford the silylated homoallylic alcohols in good yields. Subsequent protiodesilylation gave the formal adducts of the corresponding aldehydes. The homochiral acylsilane 11 gave a reversal of asymmetric induction in the Sc(OTf) 3 catalyzed allylation and in the Sakurai reaction.

Catalytic Asymmetric Reactions of 4-Substituted Indoles with Nitroethene: A Direct Entry to Ergot Alkaloid Structures

Abstract A domino Friedel–Crafts/nitro‐Michael reaction between 4‐substituted indoles and nitroethene is presented. The reaction is catalyzed by BINOL‐derived phosphoric acid catalysts, and delivers the corresponding 3,4‐ring‐fused indoles with very good results in terms of yields and diastereo‐ and enantioselectivities. The tricyclic benzo[cd]indole products bear a nitro group at the right position to serve as precursors of ergot alkaloids, as demonstrated by the formal synthesis of 6,7‐secoagroclavine from one of the adducts. DFT calculations suggest that the outcome of the reaction stems from the preferential evolution of a key nitronic acid intermediate through a nucleophilic addition pathway, rather than to the expected “quenching” through protonation.

EXTREMELY FACILE FORMATION AND HIGH REACTIVITY OF NEW THIOACYLSILANES CONTAINING THE FERROCENE MOIETY

Abstract Thioacylsilanes containing the ferrocene moiety, easily prepared from the corresponding acylsilanes with Lawessons reagent at room temperature, can be transformed into vinyl silanes, sulfur heterocycles and sulfines.

Synthesis and Reactivities of Enantiomerically Pure β-Hydroxyalkyl and β-Aminoalkyl Ferrocenyl Sulfides

Enantiomerically pure β-hydroxyalkyl and β-aminoalkyl ferrocenyl sulfides have been synthesized in good yields from mercaptoferrocene and amino alcohol derivatives. Primary β-aminoalkyl sulfides allowed the synthesis of tetrahydro-1,4-thiazepines containing the ferrocene moiety with good diastereoselectivity and β-iminoalkyl sulfides. Some of these derivatives have successfully been employed as ligands for palladium-catalyzed allylic substitution, with asymmetric induction of up to 99% ee. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Synthesis of Enantiopure β- and γ-Amino Alcohols from Homochiral α- and β-Aminoacylsilanes as Stable Synthetic Equivalents of α- and β-Amino Aldehydes

A practical route is described for the synthesis of enantiopure β- and γ-amino alcohols with two stereocenters, starting from homochiral α- (1 and 5) and β- (13 and 16) -aminoacylsilanes, and involving stereoselective addition of allylmetal compounds and subsequent stereospecific protiodesilylation of the adducts. The degree of diastereoselectivity achieved in the nucleophilic addition step depends on both the nitrogen-protecting group and the reagents used. Diastereomeric excess (de) values equal to or higher than 98% were obtained in the TiCl4-promoted allylation of the N-Pht-aminoacylsilanes 1 and 13 and of the N-Ts-aminoacylsilane 5 with allyltrimethylsilane. Lower de values were obtained in the Sc(OTf)3-catalyzed allylation of 5 with tetraallyltin and in the additions of both allyltrimethylsilane and tetraallyltin to the N-Ts-β-aminoacylsilane 16. Protiodesilylation of the adducts, leading to the β- and γ-amino alcohols, was accomplished with TBAF, except in the case of the adducts obtained from 5. For these, a preliminary removal of the tosyl group was necessary, which was accomplished with simultaneous desilylation by treatment with Na in liquid ammonia.

Chemistry of silyl thioketones part 9. A new selective synthesis of 1-silyl-1-enethiols and of 2-silyl-thiacycloalk-2-enes of common to large ring size☆

Abstract ω-Haloacylsilanes 1 were selectively transformed via thioacylsilanes into Z-silyl-enethiols 4 with solid sodium hydrogen carbonate or into 2-silylthiacycloalk-2-enes 5 of common to large ring size with solid sodium hydroxide. Compounds 5 underwent protiodesilylation.

Chemistry of silyl thioketones. Part 10. Synthesis and reactivity of α-silyl vinyl sulfides

Aliphatic silyl thioketones containing an α-hydrogen atom undergo enethiolization to Z-α-silyl enethiols 2. Compounds 2 react with a variety of halides R3X to give open-chain α-silyl vinyl sulfides 3. Protiodesilylation of 3 was achieved upon treatment with fluoride ion to give vinyl sulfides 4. Reaction of 3 with Grignard reagents, in the presence of an appropriate nickel catalyst, results in a series of vinylsilanes 5 with a specific geometry.