RESUMEN
Vegetation 'greenness' characterized by spectral vegetation indices (VIs) is an integrative measure of vegetation leaf abundance, biochemical properties and pigment composition. Surprisingly, satellite observations reveal that several major VIs over the US Corn Belt are higher than those over the Amazon rainforest, despite the forests having a greater leaf area. This contradicting pattern underscores the pressing need to understand the underlying drivers and their impacts to prevent misinterpretations. Here we show that macroscale shadows cast by complex forest structures result in lower greenness measures compared with those cast by structurally simple and homogeneous crops. The shadow-induced contradictory pattern of VIs is inevitable because most Earth-observing satellites do not view the Earth in the solar direction and thus view shadows due to the sun-sensor geometry. The shadow impacts have important implications for the interpretation of VIs and solar-induced chlorophyll fluorescence as measures of global vegetation changes. For instance, a land-conversion process from forests to crops over the Amazon shows notable increases in VIs despite a decrease in leaf area. Our findings highlight the importance of considering shadow impacts to accurately interpret remotely sensed VIs and solar-induced chlorophyll fluorescence for assessing global vegetation and its changes.
Asunto(s)
Bosques , Bosque Lluvioso , Estaciones del Año , Sesgo , ClorofilaRESUMEN
The C-O bond cleavage for activation of alcohols is synthetically useful and practically challenging. This work describes carbazate as a new type of electrochemically activated alkylating agent derived from ubiquitous alcohols for direct functionalization of heteroarenes under mild electrolytic conditions. The simple undivided cell at low oxidative potentials with carbon/platinum electrode set-ups offers excellent substrate tolerance, affording a variety of primary, secondary and tertiary alkyl-decorated heterocycles in good chemical yields. Furthermore, the mechanism for this electrochemical deoxyalkylation reaction has been investigated.
RESUMEN
An efficient electrochemical 1,2-sulfonylation/alkynylation of alkenes via radical 1,4-alkynyl migration of alkynyl-substituted tertiary alcohols is described, which used sodium sulfinates as sulfonyl sources affording the corresponding α-sulfonyl-ß-alkynylated products in moderate to excellent yields. This electrochemical reaction proceeds smoothly without the use of any metal catalyst, additive and oxidant and thus represents a new and eco-friendly strategy for the difunctionalization of unactive olefins, and also the first example of the electrochemical distal radical migration reaction.
RESUMEN
A visible light-induced regioselective sulfonamidation of imidazo[1,2-a]pyridines was developed using sulfamides as the nitrogen sources and aqueous NaClO solution as the oxidant under mild conditions. With the imidazo[1,2-a]pyridines bearing various substituents, the reaction proceeded smoothly to furnish the C3-sulfonamidation products in moderate to good yields. The method was also suitable for the sulfonamidation of some other imidazoheterocycles.
RESUMEN
A regioselective alkoxycarbonylation of imidazoheterocycles using carbazates as ester group sources in DMSO was developed, in which an inexpensive FeCl2·4H2O was used as the catalyst and (NH4)2S2O8 was the oxidant. The reaction proceeded smoothly under an air atmosphere to give the 3-alkoxycarbonylated products in moderate to good yields.
RESUMEN
A regioselective synthesis of 3-fluorinated imidazo[1,2-a]pyridines using 1-chloromethyl-4-fluoro-1,4-diazoniabi cyclo[2.2.2]octane bis(tetrafluoroborate) (Selectfluor) as the fluorinating reagent in aqueous condition is described. In the presence of DMAP, the reaction mainly gave monofluorinated product via electrophilic fluorinated process in moderate to good yields.
