Sulfones and sulfoxides in the total synthesis of biologically active natural compounds

Abstract

A graphical treatment of the tone reproduction problem in television systems satisfactorily answered by extensive experimental work, using the techniques of subjective appraisal which have been increasingly developed in recent years. Direct comparison of a reproduction and an original, with all the parameters of the system accurately known, calls, of course, for a considerable experimental effort. Such limited observations as have been made, however, suggest that the shape of the curves at least gives a good impression of the relative effects of viewing cathode-ray tubes with different values of A, and operating under, different conditions. It is not intended to draw any conclusions or express any as to what form of reproduction characteristic is most desirable. There can, however, be little doubt that there are instances where faithful reproduction is not desirable -where, in fact, the television system can improve on the eye and convey the maximum of information or enhance the asthetic value of a scene by emphasizing certain luminosity differences at the expense of others. To a large extent the value of such distortions must be a matter of personal preference.What constitutes the most desirable form of characteristic is, again, a question which calls for large-scale experiment on a subjective basis; factors such as signal-to-noise ratio and the effect of the characteristic on apparent definition have to be taken into account.Reference may be made in passing to the results of Jones's investigation into the preferences of a large number of observers in the matter of photographic prints.Q) This investigation tends to show that preferred prints are those in which the gradient of the logL,/logL, curve is approximately unity for the medium tones, and that a greater fall in the gradient is acceptable towards black. It would be unwise to assume, however, that the results obtained are necessarily applicable to television practice, where the form of the transfer characteristics and the viewing conditions are different. One general principle which does not seem to be in dispute is that, whereas a considerable tolerance exists when the reproduction is in monochrome, colour calls for a much closer approach to fidelity in luminosity reproduction.Large-area luminance range in modern aluminized cathoderay tubes, in the absence of external light, appears to offer little scope for worth-while improvement. It is worth noting, however, that the importance of good detail luminance range in determining the subjective quality of television pictures has been stressed by Law@) and other writers.Assuming that an optimum characteristic, or range of characteristics, can be defined, practical realization of the optimum raises a number of issues, e.g. the relative contributions of the transmitting and receiving apparatus to the overall result, and whether the desired objective can best be arrived at by selection of the camera or cathode-ray tube characteristic or by suitable circuit design.It is hoped that the present communication may help...

Selective and mild oxidation of sulfides to sulfoxides by H2O2 using DBUH-Br3 as catalyst

“…The reaction was not completed within 24 h (Table 1, entry 6). Consequently, it was performed in different molar ratios of the catalyst in the presence of oxidant in order to obtain the optimum reaction conditions (Table 1, entries [1][2][3][4][5].…”

“…Sulfoxides are important intermediates in the synthesis of various natural products [1,2]. Omeprazole and Figpronil pesticides are two typical examples of extensive applications of these important intermediates in pharmaceutical and fine chemical industries [3].…”

Recent Advances in Copper-Catalyzed Asymmetric Conjugate Addition of Grignard Reagents

“…中间体, 可用来制备醛、酮、炔烃、烯烃、烷烃、卤代 烷等化合物 [49] . 诸多有机金属试剂对 α,β-不饱和砜的不 对称加成可用来合成这类砜 [50,51] [60,61] (Scheme 2).…”

Catalytic Conjugate Additions of Geminal Bis(sulfone)s: Expanding the Chemistry of Sulfones as Simple Alkyl Anion Equivalents

“…Accordingly, alkanes bearing two geminal sulfonyl groups (gem-bisA C H T U N G T R E N N U N G (sulfonyl)s) may be regarded as synthetic equivalents of nucleophilic alkyl units with multiple applications in the field of natural product synthesis. [2] For years, realization of this concept has mainly relied on 1) irreversible generation of the sulfonyl a-carbanion by treatment with stoichiometric base in a separate step, 2) subsequent reaction with the corresponding electrophile; and 3) final reductive elimination of the sulfonyl group by action of metallic sodium or magnesium. [1][2][3] Although more advanced, direct methods capable of merging steps (1) and (2) by means of catalytic activation of the substrates still remain challenging.…”

“…[2] For years, realization of this concept has mainly relied on 1) irreversible generation of the sulfonyl a-carbanion by treatment with stoichiometric base in a separate step, 2) subsequent reaction with the corresponding electrophile; and 3) final reductive elimination of the sulfonyl group by action of metallic sodium or magnesium. [1][2][3] Although more advanced, direct methods capable of merging steps (1) and (2) by means of catalytic activation of the substrates still remain challenging. To date, sulfone-based direct catalytic C-C bond-forming methods are confined to either a-fluoro bis(phenylsulfonyl)methane [4] or monosulfones bearing an additional carbonyl, ester, or nitrile group at the a-position (Scheme 1).…”