Elisa Bandini

Diastereofacial selectivity in the reaction of chiral N-trimethylsilyl imines with ester enolates: Preparation of trans-azetidin-2-ones in high stereocontrolled fashion.

Abstract Highly efficient chiral ester enolate-imine condensation, giving 3,4-disubstituted β-lactams with excellent trans-syn or trans-anti diastereoselectivity, is reported. The chiral information is included in the electrophilic partner of the condensation, nominally chiral α-, β-silyloxy- or alkoxy-N-trimethylsilyl-imines. The high diastereofacial selectivity is determined by the correct choice of the metal cations present in the reaction medium and the very nature of the hydroxy protecting group. By this procedure a number of azetidinones, intermediates in the synthesis of commercially interesting β-lactam antibiotics have been obtained. Fully assigned 1H and 13C NMR spectra of the so obtained azetidinones are reported.

Metallo-imines: useful reagents in organic synthesis

Ihe preparation of aluminum, silicon and tin imines from nitriles, aldehydes and ketones, enolizable as well as nonenolizable ones, is reported. The reactivity of these relatively new organometallic compounds has also been explored.

β-Lactams from ester enolates and silylimines: An enantiospecific synthesis of monocyclic β-lactams.

Abstract Optically active 3,4-disubsutituted azetidin-2-ones have been prepared by annelation of chiral silylimines derived from (S) or (R)-lactaldehyde with the ester enolate of the ethyl 2,2,5,5,-tetramethyl-1,2,5-azadisilolidin-1-acetate (STABASE). Oxidation of the hydroxyethyl side chain on the C-4 position of the β-lactam ring, followed by Baeyer-Villiger oxidation led to the optically active (3S, 4S ) 3-amino-4-acetoxy-β-laclam. The absolute configuration of this compound was determined by elaboration of this substrate to a key intermediate in the synthesis of the antibiotic “Aztreonam”. Nucleophilic displacement of the acetoxy group led to optically active 3-amino-4-alkyl (aryl)-azetidin-2-ones.

Trans -Diastereoselective synthesis of 3-phthalimido β-lactams via a two step-staudinger reaction

Abstract New conditions for the Staudinger reaction provide N -unsubstituted-3-phthalimido-β-lactams in satisfactory yields with complete trans-selectivity.

Silicon directed asymmetric synthesis of (1R, 2S)-(−)-(1,2-Epoxypropyl)phosphonic acid (fosfomycin) from (S)-lactaldehyde

Abstract The title compound is obtained in high diastereomeric purity based on the stereoselective addition of trimethylsilyldibenzylphosphite (TMSDBP) to O-triisopropylsilyloxy(S)-lactaldehyde.

Stereochemical Aspects of a Two-Step Staudinger Reaction − Asymmetric Synthesis of Chiral Azetidine-2-ones

Reaction of N-trialkylsilylimines with a variety of glycine-derived ketenes produced 1,3-azadienes, which in some cases have been isolated and characterised. A conrotatory ring closure of these compounds gave rise to the formation of the azetidine-2-ones, thus allowing a formal two-step Staudinger reaction. Exclusive trans diastereoselectivity was observed. A less stringent diastereofacial selectivity was obtained. A set of experiments has been performed in order to evaluate the influence of the structural parameters as well as reaction conditions. Ab initio studies at MP2/6−31G* and QCISD(T)/6−311G** levels on model compounds provide a rationalization of the experimental results obtained. From the experimental as well as theoretical data it is clear that the presence of the silyl enol group in the intermediate azadiene is crucial in its stabilisation and plays a fundamental role in the conrotatory ring closure and, therefore, in the formation of the azetidine-2-one ring.

Synthesis of perhydroxazin-4-ones. Competitive Mukaiyama versus hetero Diels–Alder reaction in the cycloaddition of 2-aza-3-trimethylsilyloxy-1,3-butadiene and aldehydes

Abstract The reactions of 2-aza-3-trimethylsilyloxy-1,3-butadiene with carbonyl dienophiles are described. 2-Aza-1,3-butadienes participate as dienes in the [4+2] cycloaddition with aldehydes to afford perhydroxazin-4-ones in good yields. Experimental results, however, show that a Mukaiyama type two-step reaction must be taken into account. The cycloadducts obtained have proved to be useful intermediates in the synthesis of α-amino-β-hydroxy acids.

Stereochemical aspects of the asymmetric synthesis of chiral α,β-dihydroxy phosphonates. Synthesis of α,β-dihydroxy phosphonic acids

Abstract Stereocontrol in the asymmetric phosphonylation of aldehydes via organophosphorous esters has been obtained starting from chiral aldehydes. The nature of the O-protecting group is crucial to obtain, in terms of diastereoselectivity and chemical yields, the best results. An ab initio molecular orbital study on 2-silyloxy propanal and MM2 studies on 2-alkoxy propanal show the existence of stable cyclic and acyclic conformers, which are presumably responsible for the high syn diastereoselectivity observed in the addition of non-metal carrying phosphites.

Stereoselective synthesis of chiral 3,4,5-trisubstituted 1,5-dihydropyrrol-2-ones from azadienes

Abstract A new access to enantiopure 3-phthalimido-4-amino-5-(1-hydroxyalkyl) 1,5-dihydropyrrol-2-ones has been developed from 1,3-azadienes. Stereoselective Lewis acid catalyzed addition of a cyano group to an azadiene, followed by intramolecular ring closure, in a four-step one-pot synthesis, results in the formation of γ-lactams in satisfactory yields.

Synthesis of carbapenems via metalloimines-ester enolates condensation: a new synthesis of (+)-1β-methyl PS-5

Abstract The synthesis of the new β-lactam antibiotic, namely the (+)-1β-methyl PS-5, is described. The key step involves the ester enolate-metallo imine cycloaddition reaction.