Germana Mazzanti

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.

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)

Spin adducts of group IVB organometallic radicals with Thioketones. an esr study

Abstract The radical adducts resulting from the reaction between group IVB organometallic radicals MR 3 (M = Si, Ge, Sn; R = alkyl or aryl) and a number of thioketones of the chromone and flavone series have been investigated by ESR spectroscopy. The spectral parameters obtained for these species have been compared with those of similar adducts of the corresponding ketones. Both classes of radicals adopt an orthogonal conformation, and the differences between the former and the latter paramagnetic species can be accounted for by the different electronegativity of oxygen and sulphur. the effects of substitution of the heterocyclic oxygen with a sulphur or selenium atom on the spin density distribution are likewise explained. INDO calculations have been carried out on the model systems CH 2 XSiH 3 (X = O, S).

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.

Thiophilic allylation of dithioesters and trithiocarbonates

Abstract Reaction of dithioesters and trithiocarbonates with silylated nucleophiles under fluoride ion conditions affords a novel example of silyl mediated regiochemical control in the thiophilic functionalization.

[4+2]-Cycloadditions of silylated and non silylated thiones with vinyl trimethylsilyl ketone: Novel synthesis of 4H-1,3-Oxathiins.

Abstract 4H-1,3-Oxathiins are obtained by an high yield heterocycloaddition of thiones with vinyltrimethylsilylketone, behaving as heterodiene. 4H-1,3 Oxathiins can be oxidized to the corresponding S-oxides, which are the formal cycloadducts of sulphines.

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.

1,2-diastereoselection induced by chiral silicon in the addition of Grignard reagents to acylsilanes

Abstract The reaction of acylsilanes 1 with Grignard reagents gives the diastereomeric α-hydroxysilanes 3 . The level of asymmetric induction depends on the nature of R 1 and R 2 . Good results were obtained for R 1 =t-Bu, R 2 =Me and R 1 =α-Napht, R 2 =Ph. Subsequent desilylation occurs with predominant inversion of configuration at carbon.