L. Gerardo Zepeda

A PC program for calculation of dihedral angles from 1H NMR data

Abstract The program ALTONA, written in BASIC, calculates plots of H-C-C-H dihedral angles from proton-proton NMR vicinal coupling constants using an empirically generalized Karplus-type equation, which takes into account the electronegativity and the orientation of the substituents attached to the considered H-C-C-H fragment. Generation of a plot for the fragment results after introduction of the atomic symbols of the substituents (C, H, O, N, S, P, F, Cl, Br, and/or I) and their orientation (+ or −). ALTONA is compiled in Turbo BASIC for PC compatible computers and is included in the present paper as ALTONA.EXE on disk.

Absolute configuration of ()-myrtenal by vibrational circular dichroism

The VCD spectrum of the monoterpene (-)-myrtenal (1) was compared with theoretical spectra using ab initio density functional theory (DFT) calculations at the B3LYP/6-31G(d,p), B3LYP/6-31G+(d,p), B3LYP/6-311G+(d,p), B3LYP/DGDZVP, and B3PW91/DGTZVP levels of theory. Conformational analysis of 1 indicated that the lowest energy conformer was s-trans-C2-C10, which contributes more than 98.5% to the total conformational population regardless of the employed level of theory. The use of a recently developed confidence level algorithm demonstrated that VCD spectra calculated for the main conformer, using the indicated hybrid functionals and basis set, gave no significant changes, from where it follows that B3LYP/DGDZVP calculations provide a superior balance between computer cost and VCD spectral accuracy. The DGDZVP basis set demanded around a quarter the time than the 6-311G+(d,p) basis set while providing similar results. The spectral comparison also provided evidence that the levorotatory enantiomer of myrtenal has the 1R absolute configuration.

Highly diastereoselective nucleophilic additions using a novel myrtenal-derived oxathiane as a chiral auxiliary

Abstract The synthesis of novel oxathiane 3 and its acetyl derivative 12, from commercially available (−)-myrtenal 4, is described. The addition of several nucleophilic reagents to 12 furnished the corresponding tertiary carbinols in highly diastereomeric excess. The hydrolysis of 11a, b yielded the expected α-hydroxycarbonyl compounds in excellent enantiomeric excess.

Diastereoselective Preparation of (R)- and (S)-2-Methoxy-2- phenylpent-3-ynoic Acids and Their Use as Reliable Chiral Derivatizing Agents

Benzoyl-S,O-acetals 1a and 1b were used as chiral auxiliaries to achieve the diastereoselective preparation of both enantiomers of 2-methoxy-2-phenylpent-3-ynoic acids (MPPAs). The latter were condensed with several chiral secondary alcohols and some primary amines to evaluate their potential as chiral derivatizing agents (CDAs). The (1)H NMR spectra of the corresponding esters and amides showed strong consistency with the absolute configuration of the carbinol and amine moieties, whose observed ΔδL(1) and ΔδL(2) values were in the ranges of 0.1-0.4 and 0.02-0.12 ppm, respectively.

Parallel and automated library synthesis of 2-long alkyl chain benzoazoles and azole[4,5-b]pyridines under microwave irradiation

SummaryA versatile route to 40-membered library of 2-long alkyl chain substituted benzoazoles (1 and 2) and azole[4,5-b]pyridines (3 and 4) via microwave-assisted combinatorial synthesis was developed. The reactions were carried out in both monomode and multimode microwave oven. With the latter, all reactions were performed in high-throughput experimental settings consisting of an 8×5 combinatorial library designed to synthesize 40 compounds. Each step, from the addition of reagents to the recovery of final products, was automated. The microwave-assisted N-long chain alkylation reactions of 2-alkyl-1H-benzimidazole (1) and 2-alkyl-1H-benzimidazole[4,5-b] pyridines (3) were also studied.

Generation of the new quirogane skeleton by a vinylogous retro-Michael type rearrangement of longipinene derivatives

Abstract A new tricyclic hydrocarbon skeleton, named quirogane, was prepared by a vinylogous retro-Michael type molecular rearrangement of (4R,5S,7R,8R,9S,10R,11R)-7,8-diacetyloxy-9-mesyloxy-1-oxolongipin-2-ene (5). A remarkable difference in chemical behavior as compared to the corresponding 2,3-dihydroderivative of 5 is explained in terms of the stability of anionic intermediates, which were evaluated by AM1 calculations. The structure of the quirogane skeleton was confirmed by single crystal X-ray diffraction analysis of quirogadiene 6. A [2+2] photochemical cyclization of 6 afforded the highly strained pentacyclic sesquiterpenoid (10).

Synthese directe d'indoles anti-hypertensifs

Abstract The direct syntheses of two potencial anti-hypertensive drug candidates having the indole skeleton ( 2 and 3 ), using the β-cyanoaldehyde 6 or a γ-amide-aldehyde in Fischer condesations with ϱ-methoxyphenylhydrazine are described. As by-products from these syntheses we isolated the chloropyrrole ( 11 ) and two diasteroisomeric molecules ( 12 -E and 12 -Z), one of them leading selectivity to the N-acetylpyrrole 13 . The structure and stereochemistry of three of these minor products ( 10 , 12 -E and 14 ) were confirmed by single crystal X-ray diffraction studies.

NMR-based conformational analysis of perezone and analogues.

Complete assignment of the 1H NMR chemical shift and coupling constant values of perezone (1), O‐methylperezone (2) and 6‐hydroxyperezone (3) was carried out by total‐line‐shape‐fitting calculations using the PERCH iterative spectra analysis software (PERCH Solutions Ltd., Kuopio, Finland). The resulting simulated spectra for the three compounds showed strong similarity to their corresponding experimental spectra. Particularly, all vicinal, allylic and homoallylic coupling constant values for the side chain of the three compounds were very similar, thus revealing that the conformation of these three molecules in solution is indeed almost identical. This fact is in agreement with extended side chain conformations over folded chain conformations because 1, 2 and 3 undergo completely different intramolecular cycloaddition reactions. In addition, results of double pulsed field gradient spin echo NOESY 1D experiments performed on perezone (1) were unable to provide evidence for folded conformers. Copyright

A New Synthetic Route of 2-Aroyl- and 2-Benzyl-Benzofurans and their Application in the Total Synthesis of a Metabolite Isolated from Dorstenia gigas

The Lewis acid-catalyzed cyclization of the (Z)-3-(dimethylamino)-2-aryloxy-1-arylprop-2-en-1-ones 4a–h leads to a regioselective and short synthesis of 2-aroylbenzofurans 2a–h. The Wolff–Kishner reduction of the latter yielded a series of substituted 2-benzylbenzofurans 3a–h. This methodology was applied in the first total synthesis of the metabolite 2-(4-hydroxybenzyl)-6-methoxybenzofuran 1, which was isolated from the tropical plant Dorstenia gigas, and obtained through a six-step route and in a 24% overall yield.

The Chemical Constituents of Rhamnus serrata var. serrata

Abstract The leaves of Rhamnus serrata Schultez var. serrata yielded nonacosane, β-sitosterol and the anthraquinone emodine, while the roots afforded 7-hydroxy-5-methoxyphtalide, the anthraquinones emodine, physcion and chrysophanol, the known chromone eugenine and the new 3,5-dihydroxy-7-methoxy-2-methylchromone.