Topology and Group Theory-Tools for Determinating the Stereochemistry of Molecules

Modern Chemistry requires to handle complex stereochemical problems with a computer. Therefore, it is necessary to find a suitable way of modelling ensembles of molecules and their reactions. This kind of modelling must also consider dynamic structural properties of molecules. This cannot be described sufficiently by rigid models. Developing concepts and computer programs for describing stereochemical and constitutional regards simultanously requires an exact chemical nomenclature. Thus, a nomenclature must be found which is easily applicable and produces onlyasmall amount of data. Especially concerning stereochemistry this nomenclature must be applicable to atoms of any validity. No rules for special cases should be defined. For a definite description of ensembles of molecules our algorithms profit by topological and graph theoretical methods. Based on uncoloured molecular graphs, first the chemical nature of the vertices and finally by group theoretical aspects stereochemistry is introduced. This hierachical ordering allows to compare structural properties of chemical different molecules. Thus, for example reaction patterns of molecules can be transferred between chemical different but structural identical ensembles

Multicomponent Reactions and Their Libraries - a New Approach to Preparative Organic Chemistry

Classical chemical syntheses from n starting materials usually require sequences of at least n-I preparation steps, including separation and purification of the intermediates. A perfect alternative for the rapid syntheses of a large variety of agrichemically and pharmaceutically relevant products are one-pot syntheses by multicomponent reactions (MCR)on the basis of isocyanides. Four to seven different types of components mixed in a reaction vessel undergo the transformation to one molecule. Due to the last irreversible step that involves the isocyanides, a stable product results in quantitative yields. Using more than one representative of each type of starting materials (i.e. different isocyanides, amines, etc.) in the same vessel, all possible combinations will lead to a molecular library with hundreds and thousands of products formed according to a given reaction scheme. The design of such syntheses and the handling of the results require adequate mathematics and computer tools

Further flavonol glycosides of Embelia schimperi leaves

Fractionation of the methanolic extract of Embelia schimperi leaves has led to the isolation of two novel flavonol glycosides. The compounds were characterized as isorhamnetin 3-O-b-galactoysyl (1® 4)-b-galactoside and quercetin 3-O-[a-rhamnosyl (1®2)] [a-rhamnosyl (1® 4)]-a-rhamnoside. Also reported from the same extracts were known compounds quercetin, myricetin, quercetin 3-O-a-rhamnoside, quercetin 3-O-b-glucoside, quercetin 3-O-rutinoside, myricetin 3-O-b-xyloside, isorhamentin 3-O-b-glucoside and myricetin 3-O-b-glucoside. Their structural elucidation was accomplished using spectral measurements and chemical methods. KEY WORDS: Embelia schimperi, Flavonol glycosides, Isorhamnetin 3-O-b-galactoysyl (1® 4)-bgalactoside, Quercetin 3-O-[a-rhamnosyl (1®2)] [a-rhamnosyl (1® 4)]-a-rhamnoside  Bull. Chem. Soc. Ethiop. 2004, 18(1), 51-57

MCR XVII. Three Types of MCRs and the Libraries – Their Chemistry of Natural Events and Preparative Chemistry

The one-pot Multicomponent Reactions (MCRs)1 convert more than two different components into their products with at least two new chemical bonds, and the products contain all educts or at least some parts of them. Many chemical reactions have several, but not all, aspects of the MCRs. Three different basic types (I–III) and two subclasses (A and B) of MCRs can take place. Chemistry had started in the nature of our world roughly 4.6 billion years ago, including MCRs of the types I and II, forming libraries of many different products. A little later, the living cells came into existence, and their biochemical MCRs of all three types started. In their various local parts their biochemical products are selectively formed by their enzyme-assisted procedures, but many of their MCRs belong to type III. The preparative chemistry of MCRs started in the middle of the last century, when the first equilibrating but isolateable 3CR products of type IB were formed. The pre-final reactions of type I form compounds, which react further and form their final products irreversibly by MCRs of type II. The type IIA products are usually heterocycles, whereas those of type IIB are generally products of isocyanides. The U-4CR of type IIB was introduced and this led to a new preparative MCR chemistry. Their educts and intermediate products equilibrate (type IA) and undergo irreversible CII → CIV &alpha,-additions of the isocyanides, followed by a variety of rearrangements into their final products (type IIB). In recent years, unions of higher numbers of components were introduced, forming even more diverse types of products. The MCR libraries were proposed in 1961, and since 1995 this chemistry has become an essential part of the chemical research in industrial search for new desirable products. This methodology requires much less work than all previous methods and proceeds many orders of magnitude faster

MCR XVII. Three Types of MCRs and the Libraries – Their Chemistry of Natural Events and Preparative Chemistry

The one-pot Multicomponent Reactions (MCRs)1 convert more than two different components into their products with at least two new chemical bonds, and the products contain all educts or at least some parts of them. Many chemical reactions have several, but not all, aspects of the MCRs. Three different basic types (I–III) and two subclasses (A and B) of MCRs can take place. Chemistry had started in the nature of our world roughly 4.6 billion years ago, including MCRs of the types I and II, forming libraries of many different products. A little later, the living cells came into existence, and their biochemical MCRs of all three types started. In their various local parts their biochemical products are selectively formed by their enzyme-assisted procedures, but many of their MCRs belong to type III. The preparative chemistry of MCRs started in the middle of the last century, when the first equilibrating but isolateable 3CR products of type IB were formed. The pre-final reactions of type I form compounds, which react further and form their final products irreversibly by MCRs of type II. The type IIA products are usually heterocycles, whereas those of type IIB are generally products of isocyanides. The U-4CR of type IIB was introduced and this led to a new preparative MCR chemistry. Their educts and intermediate products equilibrate (type IA) and undergo irreversible CII → CIV &alpha,-additions of the isocyanides, followed by a variety of rearrangements into their final products (type IIB). In recent years, unions of higher numbers of components were introduced, forming even more diverse types of products. The MCR libraries were proposed in 1961, and since 1995 this chemistry has become an essential part of the chemical research in industrial search for new desirable products. This methodology requires much less work than all previous methods and proceeds many orders of magnitude faster

<b>Further flavonol glycosides of <i>Embelia schimperi</i> leaves</b>

Fractionation of the methanolic extract of <i>Embelia schimperi</i> leaves has led to the isolation of two novel flavonol glycosides. The compounds were characterized as isorhamnetin 3-<i>O</i>-&beta;-galactoysyl (1 &rarr; 4)- &beta;-galactoside and quercetin 3-<i>O</i>-[&alpha;-rhamnosyl (1&rarr;2)] [&alpha;-rhamnosyl (1&rarr; 4)]- &alpha;-rhamnoside. Also reported from the same extracts were known compounds quercetin, myricetin, quercetin 3-<i>O</i>-&alpha;-rhamnoside, quercetin 3-<i>O</i>-&beta;-glucoside, quercetin 3-<i>O</i>-rutinoside, myricetin 3-<i>O</i>-&beta;-xyloside, isorhamentin 3-<i>O</i>-&beta;-glucoside and myricetin 3-<i>O</i>-&beta;-glucoside. Their structural elucidation was accomplished using spectral measurements and chemical methods

Employment of a steroidal aldehyde in a new synthesis of β-lactam derivatives

In this article a β-lactam-steroid product 4 containing a steroid and a β-lactam unit is described. Product 4 is easily produced in one step by the means of Ugi reactions. A medium scale high yield procedure for the synthesis of dehydrocholic aldehyde is described

The formation of β-lactam derivatives and a C3-symmetrical heterocycle from 5,6-dihydro-2H-1,3-oxazines

In this article a short approach towards highly functionalized β-lactam derivatives is described. Diastereoselective addition of malonic acid to 5,6-dihydro-2H-1,3-oxazines leads to the corresponding saturated carboxymethyl derivatives. After hydrolysis of the O,N-acetal to the β-amino acids, these are transformed to β-lactam derivatives via the Ugi-reaction. Depending on the bulkyness of the rest in 2-position of the 5,6-dihydro-2H-1,3-oxazines, a β-amino acid or a tricyclic C3-symmetrical heterocycle is formed

Heuristik, Genetischer Algorithmus und andere Zufälligkeiten in der Computerchemie: Computer Chemistry

Heuristic procedures have been used increasingly in recent years in synthesis planning, chemometries and combinatorial chemistry. Applications of heuristic procedures have been presented in the chemical literature, but their use has been discussed only poorly. However, it has become evident that heuristic methods are not equally applicable to all problems. Indeed, the usefulness of the genetic algorithm can be deduced from the problem to be tackled: the most important prerequisite is that the quality does not playa significant role, i.e. a second best or even worse solution would be adequate and that the best solution is not necessary to solve the problem. Beyond that, the use of heuristics must be justified by the absence or high degree of complexity of non-heuristic methods