Francisco Sánchez-Ferrando

Stereoselective reactions of lithium enolates derived from N-BOC protected pyroglutamic esters

Abstract The lithium enolates of N-Boc protected pyroglutamic ethyl or tert -butyl esters react with electrophiles in good yield without epimerization of the chiral centre. With benzyl bromides the process is stereospecific, yielding exclusively the trans isomer. However, with other reactive electrophiles a 2:1 trans/cis diastereomeric mixture was obtained, regardless of the steric bulk of the ester group.

Studies on structurally simple α,β-butenolides—I: New syntheses of racemic γ-hydroxymethyl-α,β-butenolide and derivatives

Abstract Exhaustive approaches to the synthesis of racemic γ-heterosubstituted γ-methyl-α,β-butenolides are presented, starting mainly from C 3 synthons (glyceraldehyde, glycidaldehyde, acrolein and 2,3-epoxypropyl ethers). Good general methods for the preparation of γ-hydroxymethyl-α,β-butenolide 2 , several of its ether derivatives, as well as of γ-bromomethyl-α,β-butenolide 5 , are given. The reactivities of these structurally simple but highly functionalized compounds, convenient synthons for more complex molecules, are preliminarily explored.

β-Cyclodextrin inclusion complex with adamantane intermolecular 1H{1H} NOE determinations and molecular mechanics calculations

Abstract NOE experiments on the β-cyclodextrin (CD) /adamantane complex unequivocally demonstrate that the guest is inside the host cavity. Molecular mechanics calculations indicate the absence of preference by the guest for one single orientation. The host/guest supermolecule has been fully optimized and results show that adamantane is in the narrower half of the β-CD torus.

Diels-alder cycloadditions of chiral butenolides with butadiene and isoprene:diastereofacial selectivity and regioselectivity

Abstract Chiral butenolides react with butadiene at 210° giving optically active bicyclic compounds with excellent diastereoselectivity, in good yields. Optical purity of the adducts has been verified either by chemical correlation and/or by the use of Eu(hfc) 3 . Reactions with isoprene at 125–220° afford 1:1 mixtures of regioisomers, selectivity being increased by the use of AlCl 3 as catalyst, at lower temperatures.

The influence of cross correlation on multiplet patterns in nuclear Overhauser effect spectra

Abstract In the presence of cross correlation between the chemical-shift anisotropy and dipole-dipole relaxation mechanisms the individual lines of a weakly coupled multiplet can receive different NOE enhancements. The resulting multiplet asymmetry is due to the presence of antiphase terms which arise solely as a result of relaxation, rather than their more usual origin through selective population transfer effects. The enhancements measured from the overall integrated multiplet intensity are not substantially affected by cross correlation. Experimentally, the multiplet asymmetry is especially pronounced in three spin {H}19FH systems, when observed at high-field strengths. Similar effects are predicted for cross correlation between separate dipole-dipole relaxation pathways. The generation of these antiphase terms is related to the appearance of “forbidden” cross peaks in multiple-quantum-filtered COSY spectra.

Asteriscunolides A, B, C and D, the first humulanolides; Two pairs of conformationally stable stereoisomers

Abstract Following a preliminary comunication on Asteriscunolide A (the first reported natural humulanolide), three more configurationally and/or conformationally isomeric Asteriscunolides ( B, C and D ) from the same plant ( Asteriscus aquaticus L.) are described. The absolute configurations and the stable conformations of all four Asteriscunolides are established by an array of spectroscopic methods (CD, 1H and 13C NMR) without resorting to X ray analysis.

Diels-alder cycloadditions of chiral butenolides with cyclopentadiene: endo/exo selectivity

Abstract Chiral butenolides are revealed to be good dienophiles in Diels-Alder reactions with cyclopentadiene, affording endo and exo adducts, whose fully characterization is provided. Selectivity of these cycloaddition reactions has been shown to be temperature dependent not as a result of a thermodynamic equilibrium between the isomers, but due to a kinetically controlled process.

Chiral butenolides in diels-alder cycloadditions with isoprene and cyclopentadiene

Abstract Chiral α,β-butenolides react with isoprene at 125–220° giving a 1:1 mixture of Diels-Alder regioisomers, selectivity being dramatically increased by the use of AlCl 3 as catalyst. While these butenolides give no reaction with furan even in the presence of catalysts, they react smoothly with cyclopentadiene, endo / exo selectivity being temperature dependent.

Asteriscunolide A: Humulanolide from asteriscus aquaticus

A number of humulanolides and other sesquiterpene lactones have been isolated from Asteriscus aquaticus (L). The structure, absolute stereochemistry and conformation of the major component, asteriscunolide A, have been determined.

A general building block to introduce carbon multiplicity information into multi-dimensional HSQC-type experiments

A general building block to achieve editing according to the multiplicity of the X nuclei in sensitivity‐improved multidimensional 1H–X HSQC experiments is proposed. Its simple implementation on standard experiments and the useful additional information obtained by such an approach are the two main reasons for its use even in routine NMR work. Illustrative examples on sensitivity‐improved 2D 1H–13C HSQC, HSQC‐TOCSY and 1,1‐ADEQUATE experiments are given.