Activity function for DCT coded images

By combining a measure of activity in transform-coded image blocks with frequency sensitivity data, suitable psychovisual coefficient quantisation step sizes can be calculated. A number of possible definitions for an activity function A/sub F/ are proposed. The most suitable of these is determined by comparing results on a subjective basis.<>     

Limitations of some esterification techniques in structural studies of kerogen-derived acids

Oxidative degradation techniques for characterizing kerogens in oil shales often lead to major yields of precipitated acids (PA), which are less amenable than soluble acid products to analysis by gas chromatography/mass spectrometry (g.c./m.s.). A PA complex produced by oxidation of a Julia Creek (Queensland) kerogen was studied by applying four different esterification techniques before g.c./m.s. Methods based on reaction with diazomethane or boron trifluoride/methanol were of limited effectiveness. Reaction with dimethyl sulphate in acetone with potassium carbonate catalyst led to the detection of by far the widest range of components in the PA complex. This method is recommended for esterifying acid products derived from kerogen, especially in comparative characterization studies of kerogens.     

Asymmetric aziridine synthesis by aza-Darzens reaction of N-diphenylphosphinylimines with chiral enolates. Part 2: Inversion of diastereoselectivity

The aza-Darzens (‘ADZ’) reactions of N-diphenylphosphinyl (‘N-Dpp’) imines with chiral enolates derived from N-bromoacetyl 2S-2,10-camphorsultam proceed in generally good yield to give N-diphenylphosphinyl aziridinoyl sultams. However, the stereoselectivity of the reaction is dependent upon the structure of the imine substituent: when the chiral enolate was reacted with arylimines substituted in the ortho-position, mixtures of cis- and trans-2′R,3′R-aziridines were obtained, often with a complete selectivity in favour of the trans-isomer.