Enrica Narisano

Enantiospecific and diastereoselective synthesis of anti α-hydrazino- and α-amino-β-hydroxyacids through “electrophilic amination” of β-hydroxyesters

Abstract β-Hydroxyesters 1a–1d were transformed into corresponding dianions and condensed with di-t-butylazodicarboxylate to give anti protected α-hydrazino-β-hydroxyesters 2a–2d with good diastereoselectivities (up to 94:6). Cleavage of protecting groups followed by ester hydrolysis gave the previously unknown anti α-hydrazino-β-hydroxyacids 4a–4d, which were in turn converted by hydrogenolysis into anti α-amino-β-hydroxyacids 5a–5d. Starting from (S) 1a, enantiomerically pure (2S, 3S) allo-threonine 5a was obtained in good overall yields. On the contrary, reaction of silyl ketene acetal 10, derived from 1a, with a diazonium salt furnished predominantly the syn isomer, but in unsatisfactory yield.

Diastereoselection in trimethylsilyl trifluoromethanesulphonatecatalyzed reaction of silyl ketene acetals with imines

Abstract The trimethylsilyl trifluoromethanesulphonate catalyzed condensation of silyl ketene acetals with imines afforded β-amino esters with prevalent anti relative diastereoselectivity (up to 100%). Some anti β-amino esters have been then cyclized to trans β-lactams.

Enzymes in asymmetric synthesis: Effect of reaction media on the PLE catalysed hydrolysis of diesters

Abstract Organic cosolvents and other addenda aan influence fhe rate and the enantioselectivity of pig liver esterase catalysed hydrolysis of meso diacetates and diesters, the magnitude of the effect being strongly dependent on the nature of substrate.

Enzyme catalyzed monohydrolysis of 2-aryl-1 3-propanediol diacetates. A study of structural effects of the aryl moiety on the enantioselectivity

Abstract PPL catalyzed monohydrolysis of 2-aryl substituted 13-propanediol diacetates afforded the corresponding monoacetates in acceptable to fair chemical and optical yields. Electronic effects in the aromatic ring were examined. Elaboration of some 2-arylpropanediol monoacetates to optically active 2-arylpropanols and propanoic acids was performed.

Microbiological synthesis of variously protected L-glyceraldehydes in high optical purity

Abstract Variously protected L-glyceraldehydes have been enantioselectively synthesized through a sequence involving acylation of formylanion equivalents with glycolic acid derivatives followed by bakers yeast mediated reduction of the resulting ketones.

Stereoselective preparation of synthetic equivalents of 2-deoxy-2-amino- and 3-deoxy-3-aminotetroses from malic acid. Application to the synthesis of C18-D-ribo-phytosphingosine

Abstract “Electrophilic amination” of β-hydroxyester 5, derived from (S) malic acid, with di- t -butylazodicarboxylate allowed preparation of new synthetic equivalents of 2-deoxy-2-amino- and 3-deoxy-3-amino-tetroses. As an application, D- ribo -C 18 -phytosphingosine was stereoselectively synthesized.

Enzymes as selective reagents in organic synthesis: Enantioselective preparation of “asymmetrized tris (hydroxymethyl)methane”

Abstract A new C-4 chiral building block, that is a tris (hydroxymethyl)methane derivative where the three equivalent hydroxymethyl groups have been differentiated [(THYM)*], was prepared with excellent enantioselection, through PPL catalysed monohydrolysis of prochiral diacetate 3.

Dibenzylaminoacetates as useful synthetic equivalents of glycine in the synthesis of α-amino-β-hydroxyacids1

Abstract The stereochemical course of three new simple methodologies for the preparation of α-amino-β-hydroxyacids starting from dibenzylaminoacetates as synthetic equivalents of glycine is described. While the aldol-type condensation via lithium enolates gave results highly dependent on the aldehyde employed, producing in some cases diastereoselectivities up to 5:1 for the a nti isomers, the acid-catalysed aldol condensation of silyl ketene acetals yielded predominantly syn adducts with selectivities from 5:1 to 32:1. Finally the acylation-reduction procedure gave the best results in terms of yields and stereoselectivities, affording syn isomers with excellent induction (⩾ 13 : 1).

Enzymes in organic synthesis: remarkable influence of a π system on the enantioselectivity in PPL catalysed monohydrolysis of 2-substituted 1,3-diacetoxypropanes.

Abstract The results of PPL catalysed monohydrolysis of a series of 2-substituted-1,3-diacetoxypropanes showed that alkinyl and ( E ) alkenyl substituents led to improvement of yield and enantioselectivity, compared to their saturated analogues. On the contrary, ( Z ) alkeny substituents provoked a net reversal of asymmetric induction.

A stereocontrolled synthesis of 3-(1′-hydroxyethyl)-2-azetidinones through trimethylsilyl trifluoromethanesulphonate catalyzed condensation of silyl ketene acetal derived from ethyl 3-hydroxybutyrate and imine

Abstract The silyl ketene acetal 2, prepared from ethyl 3-hydroxybutyrate, reacted with benzylidene aniline in the presence of trimethylsilyl trifluoromethanesulphonate to furnish in satisfactory yield the β-amino ester 3 as a single diastereomer, which was subsequentially cyclized to the corresponding β-lactam.