Martina Vicente

Cyanogen Bromide as a Useful Brominating Agent, Synthesis of α-Bromo-β-Aminoenones

Abstract Cyanogen bromide reacts with β-aminoenones, in very mild conditions, leading to α-bromo-β-aminoenones with excellent yields.

A DIASTEREOSELECTIVE APPROACH TO ENANTIOPURE 3-SUBSTITUTED PYRROLIDINES FROM MASKED LITHIUM HOMOENOLATES DERIVED FROM NOREPHEDRINE

Abstract N-alkoxycarbonyl-2-(2′-bromoethyl)oxazolidines derived from norephedrines are transformed into lithium derivatives by reaction with 1 or 2 equiv of tert-butyllithium in Et2O. The lithium intermediates readily cyclize to bicyclic lactams that react with acetaldehyde and benzaldehyde leading to the addition products in excellent chemical yields and moderate diastereoselectivities. Removal of the chiral appendage provides enantiopure 3-substituted pyrrolidines.

The Reaction of β-Aminoenones with Cyanamide. A High Efficient Synthesis of 2-Aminopyrimidines

Abstract β-aminoenones react with cyanamide, molar ratio 1:2, to yield 2-aminopyrimidines in nearly quantitative yields.

Lithium derivatives from enantiomerically pure 2-(2′-haloethyl) dioxolanes: Formation and use as chiral propanal homoenolate synthons

Abstract Condensation of acetals of 3-chloro- and 3-bromopropanal with (−)-1,4-di-O-benzyl-L-threitol and (−)-cis-2,3-bornanediol leads to the corresponding 2-(2′-chloroethyl-)- and 2-(2′-bromoethyl)dioxolanes that were converted into the Li-derivatives by transhalometallation with t-butyllithium in excellent yields. These chiral propanal homoenolates react with benzaldehyde and acetophenone yielding carbinols in moderate chemical yields and low diastereomeric excesses.

Elimination versus diastereoselective alkylation in homochiral 2-(β-ethoxy carbonyl) acetals

Abstract The reaction of the homochiral 2-(β-ethoxycarbonyl) dioxolane derived from (−)- exo -camphanediol with methyl iodide in the presence of one equivalent of LDA lead to the alkylated product in very good chemical yield and poor diastereomeric excess. 2-(β-Ethoxycarbonyl)-1,3-dioxanes derived from 2,4-pentanediol behave in a different way, whereas the diequatorial form is alkylated in excellent chemical yield, the equatorial-axial diastereoisomer yields a mixture of alkylation and elimination products.

Differential reactivity of β-amino enones and 3-dimethylaminoacrylaldehyde towards α-amino derivatives

Unsubstituted β-amino enones react with α-amino derivatives by a well established route which implies a fast transamination process — 1,4-addition followed by elimination — and cyclodehydration of the intermediate to 3-functionalized pyrroles. In contrast, 3-dimethylaminoacrylaldehyde undergoes 1,2-addition followed by cyclization to give the final 2-substituted pyrroles. Isolation of the intermediates supports the proposed mechanism for each reaction.

Diastereoselective aldol condensation with 2-(β-ethoxycarbonyl)oxazolidine derived from norephedrine. A chiral masked synthon of ethyl formylacetate

Abstract Deprotonation of chiral masked synthetic equivalents of ethyl glyoxalate, derived from norephedrine, with a novel base formed by addition of tert-BuLi to tert-butyl formate, and subsequent reaction with aromatic aldehydes yields β-hydroxy esters in good yield and moderate to excellent diastereomeric excesses. Reductive transformation of the condensation products, and elimination of the chiral appendage leads to enantiomerically pure 2-aminomethyl-1,3-propanediol derivatives.

Reaction of 3-halogeno-2H-1-benzopyran-2-ones with organometallic compounds. Synthesis of 4-Alkyl-2H-1-benzopyran-2-ones. X-Ray molecular structure of 3-bromo-3,4-dihydro-4-isopropylcoumarin

3-Halogeno-2H-1-benzopyran-2-ones react with magnesium, lithium, aluminium and copper derivatives to give 3,4-dihydrocoumarins and 3-(o-hydroxyphenyl)propenols as major products. The nature and the ratio of the products in the final mixture depend on the solvent and on the organometallic reagent. Grignard derivatives yield 1,4-monoalkylation compounds in tetrahydrofuran (THF) or 1,2-dialkylation derivatives in toluene. In some cases the dehalogenation competes with the 1,2-alkylation process in the reactions with alkyllithiums. The presence of the halogen at C-3 increases the reductive ability of organoaluminiums. In general, the reaction with lithium dialkylcuprates leads to complex mixtures of products. The 4-alky-3-halogeno-3,4-dihydrocoumarins obtained undergo dehydrohalogenation easily, and lead to 4-alkylcoumarins in good yields. The tandem alkylation-dehydrohalogenation of 3-halogeno-2H-1-benzopyran-2-ones constitutes a versatile synthesis of 4-alkylcoumarins.

Substitution and elimination reactions of 3- and 4-alkyl-2H-1-benzopyrans with organoaluminium and organomagnesium reagents

2,3-Dialkyl- and 2,2,3-trialkyl-2H-1-benzopyrans react with triethylaluminium or ethylmagnesium bromide yielding substituted o-allylphenols in good yield. The reactions of 2,3-dialkyl-2H-1-benzopyrans are stereoselective. Either the E or Z isomers can be formed preferentially depending upon the experimental conditions; the stereoselectivity increases with the bulk of the alkyl substituent at C-3. However 2,4-dialkyl- and 2,2,4-trialkyl-2H-1-benzopyrans lead to o-allylphenols only with ethylmagnesium bromide; the mixture of E and Z isomers thus formed was found to be very difficult to separate.