Reference molecule asymmetric similarity

Asymmetric similarity can provide some benefits in similarity searches not afforded by its symmetric competitors. For example, consider as in Subheading 2.2.1., the query and target molecules, Q and T, respectively, and the asymmetric similarity coefficients given in Eqs. 2.26 and 2.27. If Q is relatively small (N.B. small and large are used here refer to the size of the set and not to the size of the corresponding molecule), that is, if Q T, then target molecules for which Q is an approximate subset will be selected using Eq. 2.26, that is,... 

Fig. 13.3. Comparison of symmetric and asymmetric similarity scores. A virtual product from VRXN-2-00051 is used as a query molecule. The two corresponding Basis Products are VRXN-2-00051 A 1 and VRXN-2-00051 B 1. In reference to the query molecule, their corresponding similarity scores are listed under SS and AS (see equations [1] and...

UV radiation hypothetical, but so is the transport of molecules from outer space to Earth. Recent analyses of the Murchison meteorite by two scientists from the University of Arizona, Tucson (Cronin and Pizzarello, 1997 Cronin, 1998) have shown it to contain the four stereoisomeric amino acids DL-a-methylisoleucine and DL-a-methylalloisoleucine. In both cases, the L-enantiomer is present in a clear excess (7.0 and 9.1%). Similar results were obtained for two other a-methyl amino acids, isovaline and a-methylvaline. Contamination by terrestrial proteins can be ruled out, since these amino acids are either not found in nature or are present in only very small amounts. Since the carbonaceous chondrites are thought to have been formed around 4.5 billion years ago (see Sect. 3.3.2), the amino acids referred to above must have been subject to one or more asymmetric effects prior to biogenesis. 

This reaction was first reported by Marckwald in 1904. It is the synthesis of chiral L-valeric acid (a-methyl propanoic acid) from the pyrolysis of brucine salt of racemic o -methyl-o -ethylmalonic acid. Therefore, it is generally known as the Marckwald asymmetric synthesis. Occasionally, it is also referred to as the Marckwald method. In this reaction, the brucine salts of racemic a-methyl-a-ethylmalonic acid essentially exist as a pair of diastereomers that are separated by fractional crystallization the one with lower solubility is isolated. Upon pyrolysis of such crystalline salt at 170°C, the corresponding brucine salt of L-valeric acid forms upon decarboxylation, resulting in a 10% e.e. In addition, Marckwald defined the asymmetric synthesis as reactions that produce optically active molecules from symmetrically constituted compounds with the use of optically active materials and exclusion of any analytical processes, such as resolution. However, this work was challenged as not being a trae asymmetric synthesis because the procedure was similar to that of Pasteur. In fact, the If actional crystallization of the diastereomers is a resolution process. This process is used as base for many other preparations of chiral molecules, such as tartaric acid and under its influence, the kinetic resolution and tme asymmetric synthesis have been developed in modem organic synthesis. The asymmetric synthesis has been redefined by Morrison and Mosher as the reaction by which an achiral unit of the substrate is converted into a chiral unit in such a manner that the two resulting stereoisomers are produced in unequal amounts. ... 

Very similar results have also been reported with the use of sulfur ylides as donor molecules [179]. With regard to the asymmetric Biginelli reaction [180] and mulhcomponent coupling reachons [181] using 30 or its analog as a catalyst, only references are given here. 

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