Showing papers by "Carlos Cativiela published in 1994"

Asymmetric Synthesis of .beta.-Lactams. Highly Diastereoselective Alkylation of Chiral 2-Cyano Esters

Abstract: Enolates derived from 10-(dicyclohexylsulfamoyl)isobornyl 2-substituted-2-cyanoacetates were alkylated with very good yield and high diastereoselectivity. The reduction of the resulting reaction products and subsequent cyclization of the β-amino acids led to the corresponding β-lactam in high yields. This result paves the way for the development of a versatile and efficient asymmetric synthesis of enantiomerically pure (R)- and (S)-C(3)-disubstituted β-lactams

Solvent effects on endo/exo- and regio-selectivities of Diels–Alder reactions of carbonyl-containing dienophiles

Abstract: The selectivity of Diels–Alder reactions of methyl vinyl ketone and methyl acrylate, with cyclopentadiene and isoprene, have been studied in 18 pure solvents and aqueous mixtures. The results obtained have been analysed by means of regression models, using empirical solvent parameters. The endo/exo selectivity mainly depends on the solvophobic (Sp) and hydrogen bond donor (α) properties of the solvent. The influence of the dipolarity (π*) is only observed in the reactions with methyl acrylate. Regioselectivity almost exclusively depends on the hydrogen bond donor ability of the solvent. The double coordination observed with highly hydrogen bond donating solvents in the case of methyl vinyl ketone greatly increases the para/meta isomer ratio.

On the role of hexafluoroisopropanol in Diels-Alder reactions of acid-sensitive reagents

Abstract: 1,1,1,3,3,3-Hexafluoro-2-propanol (HFIP) is described as an excellent solvent for those Diels–Alder reactions in which Lewis-acid-sensitive reagents are used, leading to high yields with good regio...

Asymmetric Diels-Alder Reactions of Chiral (E)-2-Cyanocinnamates. 2. Synthesis of the Four 1-Amino-2-phenyl-1-cyclohexanecarboxylic Acids in Enantiomerically Pure Form

Abstract: High and complementary diastereoselectivities were obtained in the asymmetric Diels-Alder reactions of chiral (E)-2-cyanocinnamates with butadiene when (S)-ethyl lactate and (R)-pantolactone were used as chiral auxiliaries in the presence of TiCl4. The most selective reactions allowed the synthesis of the cycloadducts 2a and 3b, whose absolute configurations were assigned by an X-ray diffraction study of diastereoisomer 3b. The hydrolysis and subsequent hydrogenation of the stereoisomers gave the corresponding enantiomeric cyanocarboxylic acids. From these products the four 1-amino-2-phenyl-1-cyclohexanecarboxylic acids were synthesized in enantiomerically pure form following a protocol with stereocontrolled and stereodivergent transformations.

Stereoselective amination of chiral enolates : synthesis of chiral key intermediates for β-lactam antibiotics

Abstract: Stereoselective enolate trapping of lithium ( 1S, 2R, 4R )- 10-dicyclohexylsulfamoylisobornyl-2-cyano-3-phenylpropanoate with O-(diphenylphosphinyl) hydroxylamine followed by appropriate reduction, hydrolysis, and cyclisation processes allows the asymmetric synthesis of ( S )-3-amino-3-benzyl-2-azetidinone.

On the Role of Hexafluoroisopropanol in Diels-Alder Reactions of Acid- Sensitive Reagents.

Abstract: 1,1,1,3,3,3-Hexafluoro-2-propanol (HFIP) is described as an excellent solvent for those Diels–Alder reactions in which Lewis-acid-sensitive reagents are used, leading to high yields with good regio...

Synthesis of New Conformationally Rigid Phenylalanine Analogues.

Abstract: The reactivity of (Z)-2-phenyl-4-benzylidene-5(4H)-oxazolone 1 as dienophile in the Diels-Alder reaction with 1,3-butadiene and 2,3-dimethyl-1,3-butadiene is studied. The adducts obtained starting from the cycloaddition of 1 with both dienes are converted, through simple reactions into the conformationally restricted cyclic amino acids cis-1-amino-2-phenylcyclohexanecarboxylic acid 6 and 1-amino-c-4,c-5-dimethyl-t-2-phenyl-r-1-cyclohexanecarboxylic acid 10 , analogues of phenylalanine.