ABSTRACT
A visible-light photoredox catalyzed radical cascade cyclization of simple ethers with cyanamides is developed at room temperature. This strategy involves sequential inert Csp3-H/Csp2-H functionalizations through intermolecular addition reaction of oxyalkyl radicals to N-cyano groups followed by radical cyclization of iminyl radicals in situ generated with C-2 aryl rings. This method allows for efficient synthesis of tetracyclic benzo[4,5]imidazo[1,2-c]quinazolines. Importantly, this is the first example of an intermolecular-intramolecular radical cascade cyclization reaction of cyanamides.
ABSTRACT
Rehmannia glutinosa, a perennial herbaceous species, belongs to the family Scrophularia-ceae. As a staple medicinal material, its tuberous roots are highly valued in traditional Chinese medicine. However, R. glutinosa suffers from serious consecutive monoculture problems in production, which leads to a decline in both productivity and quality. Phyllosphere bacteria, the most abundant component of phyllosphere microorganisms, play crucial roles in plant growth and health. Characterization of phyllosphere bacteria could provide new insights into the mechanisms of consecutive monoculture problems and their control measures. Meanwhile, the varied taxa could be served as an important indicator of consecutive monoculture problems. The barcoded pyrosequencing of 16S rDNA genes combined with a culture-dependent approach was applied to characterize the shifts of bacterial community structure and diversity in the phyllosphere under consecutive monoculture of R. glutinosa. The results showed that consecutive monoculture clearly affected bacterial community structure in the phyllosphere. The phyllosphere bacterial communities of the two-year monocultured (TY) and the diseased plants (DP) were more similar, and different from the one-year monocultured (OY). The evenness, Shannon and Simpson diversity indices were significantly lower in TY and DP than in OY. Species annotation showed that bacterial community in R. glutinosa phyllosphere mainly consisted of Proteobacteria (91.2%), Firmicutes (5.1%) and Actinobacteria (3.7%). There was no significant difference in the number of detected bacterial taxa. However, Proteobacteria was significantly increased while Firmicutes and Actinobacteria were significantly decreased under consecutive monoculture. At the genus level, the relative abundances of genera Exiguobacterium, Bacillus and Arthrobacter, potentially beneficial microorganisms, were significantly higher in OY than that in TY and DP, but it was opposite for the genus Pseudomonas. The results from the culture-dependent approach and pathogenicity test showed that Pseudomonas plecoglossicida D9, widely isolated from the diseased leaves, was highly pathogenic to leaves. In conclusion, R. glutinosa monoculture resulted in distinct phyllosphere bacterial community variation with the accumulation of pathogen loads at the expense of beneficial microorganisms, which could contribute to the occurrence of leaf disease symptoms,and aggravate R. glutinosa replant disease in a monoculture regime.
