Academic literature on the topic 'Viento estelar'

En el presente libro se exponen diversas experiencias con sistemas agroforestales en el país, como la utilización del tamarugo y algarrobo para obtención de frutos y follajes para alimentación animal, el uso de especies forrajeras como Atriplex spp. y de uso múltiple como Acacia saligna, de las estepas de Acacia caven con fines forrajeros y producción de leña, en la zona norte; se muestran resultados de la utilización de pino radiata y otras especies como nogal, castaño, pino piñonero y otras, en sistemas silvopastorales en la zona central y centro sur; y sistemas combinados con pino ponderosa

La región asiste a una constante sucesión de reformas procesales, producto de la irrefutable insatisfacción de la sociedad con sus sistemas de enjuiciamiento. De entre las múltiples preocupaciones que vienen aparejadas con esos movimientos reformistas destaca aquella por la independencia judicial, un tópico de indagación multidisciplinar: desde la ciencia política a la ciencia jurídica, desde la teoría del Estado a la teoría constitucional y, por cierto, a la teoría procesal. La propia conceptualización de la independencia judicial no es unívoca y, así, los acentos aparecen en distintos aspect

It has generally been observed that prospective intramolecular Diels-Alder cycloadditions that would form a γ-lactone are reluctant to proceed. In the course of a synthesis of (±)-neovibsanin B 4, Hiroshi Imagawa and Mugio Nishizawa of Tokushima Bunri University reversed (Organic Lett. 2009, 11, 1253) the usual connectivity and found that the dienyl ester 1 could be induced to cyclize to 3. The solvent 2 improved both the yield and the diastereoselectivity of the cycloaddition. It was not surprising that Johann Mulzer of the University of Vienna could see (Organic Lett. 2009, 11, 1151) no evidence of cyclization on heating the acrylate 5. In contrast, following the lead of Barriault, they found that MgBr2 -tethered cycloaddition of methyl acrylate 7 with the alcohol 6 proceeded smoothly, to give 8, which they carried on to valerenic acid 9. Intramolecular Diels-Alder reactions to form 6,6-systems are often facile. Mohammad Movassaghi of MIT, en route to (-)-himandrine 12, showed (J. Am. Chem. Soc. 2009, 131, 9648) that the tetraene 10 cyclized to 11 at 95°C with 5:1 dr. In this case the solvent was the more typical acetonitrile with 10% diethyl aniline. The intramolecular Diels-Alder reaction is concerted but nonsynchronous (Tetrahedron Lett. 1981, 22, 5141), with β-bond formation preceding α-bond formation. This contributes to the reluctance of 8 to cyclize. In contrast, Henry N. C. Wong of the Chinese University of Hong Kong observed (Angewandte Chem. Int. Ed. 2009, 48, 2351) that the tetraene 13, which is polarity matched, cyclized to 14 spontaneously as soon as it was formed. Functional group conversion completed the synthesis of (±)-pallavicinolide A 15. The transannular Diels-Alder (TADA) reaction can proceed with high diastereocontrol, but the factors directing these cyclizations are not completely understood. In the course of a synthesis of (+)-phomopsidin 18, Masahisa Nakada of Waseda University found (Tetrahedron 2009, 65, 888) that 16 led to 17 with 16:1 dr. In contrast, the triene epimeric to 16 at the silyloxy group cyclized with only 2:1 dr.

It is thought that the pseudopterane class of diterpenoid natural products, of which 11-gorgiacerol is a member, arises biosynthetically by a photo-ring contraction of the related furanocembranes. Johann Mulzer at the University of Vienna has applied (Org. Lett. 2012, 14, 2834) this logic to realize the total synthesis of 11-gorgiacerol. Ringclosing metathesis of the butenolide 1 using the Grubbs second generation catalyst produced the tricycle 2. When irradiated, 2 undergoes a 1,3-rearrangement to furnish the natural product in good yield. Whether this rearrangement is concerted, or occurs stepwise via a diradical intermediate, is not known. Although ring-closing metathesis has become a reliable method for macrocycle construction, its use here to set what then becomes an extracyclic olefin is notable. Berkelic acid is produced by an extremophile bacterium penicillium species that lives in the toxic waters of an abandoned copper mine, and this natural product has been found to possess some very intriguing biological activities. Not surprisingly, berkelic acid has attracted significant attention from synthetic chemists, including Francisco J. Fañanás of Universidad de Oviedo in Spain, who has developed (Angew. Chem. Int. Ed. 2012, 51, 4930) a scalable, protecting-group free total synthesis. The key step in this route is the remarkable silver(I)-catalyzed coupling of alkyne 3 and aldehyde 4 to produce, after hydrogenation, the structural core 5 of (–)-berkelic acid on a gram scale. Some tools from the field of organocatalysis have been brought to bear (Angew. Chem. Int. Ed. 2012, 51, 5735) on a new total synthesis of the macrolide (+)-dactylolide by Hyoungsu Kim of Ajou University in Korea and Jiyong Hong of Duke University. The bridging tetrahydropyranyl ring is fashioned by way of an intramolecular 1,6-oxa conjugate addition of dienal 6 to produce 8 under catalysis by the secondary amine 7. Following some synthetic manipulations, the macrocyclic ring 12 is subsequently forged by an NHC-catalyzed oxidative macrolactonization using the carbene catalyst 10 and diphenoquinone 11 as the oxidant. A new approach to the nanomolar antimitotic agent spirastrellolide F methyl ester has been reported (Angew. Chem. Int. Ed. 2012, 51, 8739) by Alois Fürstner of the Max-Planck-Institut, Mülheim. Two elegant metal-catalyzed processes form the key basis of this strategy.