Cumambrins A and B fromTanacetum santolina
To isolate the lactones, this plant (1350 g), collected in the flowering stage in June, 1975, on the Ustyurt plateau (Karakalpakia) was extracted five times with chloroform, and the solvent was distilled off to dryness. The concentrated residue was treated with aqueous ethanol (i:I), and the precipitate that deposited was filtered off. The neutral substances were extracted from the filtrate with hexane and chloroform. In the chloroform-extracted material two substances were detected with Rf 0.38 and 0.24 [TLC, benzene--methanol (9:1), Silufol UV-254]. This resin (22 g) was chromatographed on silica gel. When the column was eluted with henzene--hexane (7:3) and with benzene, two substances were isolated.
The guaianolides of ambrosia cumanensis HBK : The structures of cumambrins A and B☆
Abstract Cumambrins A and B isolated from Ambrosia cumanensis HBK were shown to be guaianolides with structures Ia and Ib.
Modelling of Mooney viscosity relaxation in natural rubber
Abstract Better assessment of natural rubber (NR) quality and processability, through the identification of new attributes, is still a major challenge. With this scope, 12 model samples of natural polyisoprene and two of synthetic polyisoprene were characterized in terms of Mooney torque relaxation, using the standardized ISO289 method (ML(1+4)100, explained later in the Section 1 ). The main purpose of this study was to identify a model that gave a better fit for the experimental decay of Mooney torque over time (ML(t)=f(t)) than the usual power law model used before (Y=at−b, [G.M. Bristow, A.G. Sears, The use of novel parameters in the assessment of natural rubber processability, NR Technol. 18 (1987); H.G. Burhin, W. Spreutels, J.M. Campbell, MV2000 Mooney viscometer: Mooney relaxation measurements on raw polymers and compounded rubber stocks, Kautsch. Gummi Kunstst. 43 (1990) 431]). The results showed that a generalized Maxwell model, and the Wu–Abbott empirical model ( Y = 1 − a Ln ( t ) − ( b t / ( c + t ) ) ) [T. Wu, J.A. Abbott, Firmness and force relaxation characteristics of tomatoes stored intact or as slices, Postharvest Biol. Technol. 24 (2002) 59–68], recently used for describing stress relaxation of tomato flesh, allowed a better description of the relaxation curve for the samples studied than the power law or stretched exponential traditionally used. Moreover, the total relaxation time (τm) given by the generalized Maxwell model showed good correlation with the total gel content of the samples studied.