A mechanistic investigation of the carbohydrate/DBU cocatalyzed enantioselective diboration of alkenes is presented. These studies provide an understanding of the origin of stereoselectivity and also reveal a strategy for enhancing reactivity and broadening the substrate scope.
Densely
substituted and highly oxygenated carbocycles are challenging
targets for synthesis. In particular, those possessing numerous contiguous,
fully substituted carbon atoms (i.e., tertiary alcohols and quaternary
centers) are often not accessible in a direct fashion, necessitating
the strategic decoupling of ring-formation from the establishment
of functionality about the system. Here, we describe an approach to
the construction of highly oxygenated mono-, di-, and polycyclic carbocycles
from the reaction of disubstituted alkynes with β- or γ-dicarbonyl
systems. These processes embrace a variant of metallacycle-mediated
annulation chemistry where initial alkyne–carbonyl coupling
is followed by a second, now intramolecular, stereoselective
C–C bond-forming event. In addition to revealing the basic
reactivity pattern in intermolecular settings, we demonstrate that
this class of reactivity is quite powerful in a fully intramolecular
context and, when terminated by a stereoselective oxidation
process, can be used to generate polycyclic systems containing a fully
substituted and highly oxygenated five-membered ring.
An asymmetric synthesis of C14-desmethylene corialactone D is described on the basis of strategic application of a metallacycle-mediated annulative cross-coupling reaction, a Still [2,3]-Wittig rearrangement, and Morken's hydroxyl-directed diboration reaction. While representing a convenient approach to access novel compositions of matter inspired by the sesquiterpenoid natural product class (including classic natural product synthesis targets including the picrotaxanes and dendrobine), these studies have led to the discovery of natural product-inspired agents that inhibit nerve growth factor (NGF)-mediated neurite outgrowth in PC-12 cells. Letter pubs.acs.org/OrgLett
A complementary process to the Pauson-Khand annulation is described that is well suited to forging densely substituted/oxygenated cyclopentenone products (including fully substituted variants). The reaction is thought to proceed through a sequence of metallacycle-mediated bondforming events that engages an internal alkyne and a β-keto ester in an annulation process that forges two C-C bonds. A variant of this annulation process has also been established that delivers deoxygenated cyclopentenones that lack the allylic tertiary alcohol.
INFORMACIÓN Los compuestos fenólicos en la autodefensa de los vegetalesPor R. Maestro-Durán, R. León y V. Ruiz-Gutiérrez Instituto de la Grasa y sus Derivados (CSIC). Avda. Padre García Tejero, 4.41012-SEVILLA RESUMEN Los compuestos fenólicos en la autodefensa de los vegetales.En este trabajo se recogen los estudios sobre la actividad como fitoalexinas (compuestos post-infeccionales cuya formación implica o desrrepresión de genes o activación de un sistema enzimático que no operaba antes de la infección), tanto frente a hongos como frente a bacterias, de compuestos de tipo fenólico (ácidos cinámicos, flavonoides, isoflavonoides, deoxiantocianinas, estilbenos, cumarinas, cromonas). Se señala la relación entre el balance hidrófilo-lipófilo y la actividad como fitoalexinas.Finalmente se recogen trabajos sobre la obtención de fitoalexinas sinté-ticas de estructuras similares a las naturales (estilbenos, derivados de epicatequinas) de interés comercial como pesticidas, insecticidas o fungicidas.
PALABRA-CLAVE: Autodefensa -Compuestos fenólicos -Información (artículo) -Vegetal.
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