Biomedical subjects

J A Dobado

Publications and source records attributed to J A Dobado.

3 recordsLinked to original sources

PubMed2000

On the aromaticity and Meisenheimer rearrangement of strained heterocyclic amine, phosphine, and arsine oxides.

A theoretical investigation (AIM and ELF analyses together with NMR chemical shifts) has been conducted for three-membered heterocycle (N, P, and As) oxides. An aromatic stabilization was found for the P and As rings. However, the N derivatives displayed a net negative hyperconjugation in the N-O bond formation, without ring aromaticity observed for their electronic properties. The calculated delta(C) and delta(H) shifts also supported the ring delocalization for the P and As unsaturated heterocycle oxides (delta(C) approximately 165 and delta(H) approximately 9 ppm). In addition, these values for 1H-azirine oxide resembled standard C=C double bond values (delta(C) approximately 130 and delta(H) approximately 7 ppm). The different behavior for the N oxides was also observed in their Meisenheimer rearrangement (MR). All the reaction paths, yielding the corresponding hydroxyl structures, were exothermic (G2 method). However, the N derivatives showed the lowest values for activation enthalpy, DeltaH(). The C=C bond influence in the MR was slight, with the same DeltaH values for the saturated and unsaturated paths. This rearrangement for the P and As oxides yielded TSs closer to the reactives; however, the corresponding TSs resembled the products for the N-derivatives. The different reaction paths have been examined by their corresponding AIM and ELF analyses at the B3LYP/6-311G level.

Journal Article↗

PubMed2000

Regioselective enzymatic acylations of polyhydroxylated eudesmanes: semisynthesis, theoretical calculations, and biotransformation of cyclic sulfites.

Different lipase enzymes have been tested in order to perform regioselective acetylations on the eudesmane tetrol from vulgarin. High yields (95%) of 1,12-diacetoxy derivative (4) were achieved in 1 h with Candida antarctica lipase (CAL). However, only the 12-acetyl derivative (6) was obtained in similar yield with Mucor miehei (MML) or Candida cylindracea (CCL) lipases. The enzymatic protection at C-1 and C-12 has been used to form eudesmane cyclic-sulfites between C-6 and C-4 atoms. The R/S-sulfur configuration has been assigned by means of the experimental and theoretical (13)C and (1)H NMR chemical shifts. The theoretical shifts were calculated using the GIAO method, with a MM+ geometry optimization followed by a single-point calculation at the B3LYP/6-31G(*) level (B3LYP/6-31G(*)//MM+). Moreover, B3LYP/6-31G(*) geometry optimizations were carried out to test the B3LYP/6-31G(*)//MM+ results, for the deacetylated sulfites (12 and 15). In addition to the delta(C) and delta(H) shifts, the (3)J(HH) coupling constants were also calculated and compared with the experimental values when available. Finally, different reactivities have been checked in both sulfites by biotransformation with Rhizopus nigricans. While the R-sulfite gave 2 alpha- and 11 beta-hydroxylated metabolites, the S-sulfite yielded only regioselective deacetylations. Furthermore, both sulfites showed different reactivities in redox processes.

Acylation↗

PubMed2000

Multiple bonding in four-coordinated titanium(IV) compounds.

Theoretical description (MP2/6-311G* and B3LYP/6-311G*) is presented for hypovalent titanium alkoxide model compounds showing linear angleTi-O-C angles. This feature is explained by the existence of a polarized triple Ti-O bond. In contrast, a series of 18 electron germanium derivatives displaying bent angleGe-O-C angles contain polarized single Ge-O bonds. The nature of the Ti-O and Ge-O bonds is established by means of natural bond order, atoms-in-molecules theory, and electron localization function analyses.

Journal Article↗