Classes of compounds

It may be regarded as the parent of a number of important classes of compounds derived from the y-pyrone skeleton (e.g. flavone, xan-thone) and the important chroman derivatives called the tocopherols (vitamin E). 

M.p. 296 C. Accepts an electron from suitable donors forming a radical anion. Used for colorimetric determination of free radical precursors, replacement of Mn02 in aluminium solid electrolytic capacitors, construction of heat-sensitive resistors and ion-specific electrodes and for inducing radical polymerizations. The charge transfer complexes it forms with certain donors behave electrically like metals with anisotropic conductivity. Like tetracyanoethylene it belongs to a class of compounds called rr-acids. tetracyclines An important group of antibiotics isolated from Streptomyces spp., having structures based on a naphthacene skeleton. Tetracycline, the parent compound, has the structure ... 

Olefin polymers alpha-olefin polymers (PAO), polybutenes and alkylaromatics, in particular the dialkylbenzenes (DAB). This class of compounds is the most widespread and accounted for 44% of the synthetic base market in France in 1992. 

The purpose of this chapter is to present a special class of compounds, additives, that plays an Important role in the formulation of fuels as well as lubricants. 

The Lindemaim mechanism for thennally activated imimolecular reactions is a simple example of a particular class of compound reaction mechanisms. They are mechanisms whose constituent reactions individually follow first-order rate laws [11, 20, 36, 48, 49, 50, 51, 52, 53, 54, 55 and 56] ... 

Two of the material systems shown in figure G2.16.3 are of particular importance. These are the ternary compounds fonned from group III elements such as A1 and Ga in combination with As and quaternary compounds fonned from Ga and In in combination with As and P [8,15,]. Ternary Al Ga s grown on GaAs is the best known of the general class of compounds Quaternary Ga In As grown on InP is... 

Empirical molecular formulas and molecular weights usual identify a whole class of compounds (chemical isomers) rather than a single structure. Further-... 

Substructure searches provide another method of searching for available starting materials. They arc used primarily for planning the synthesis of combinatorial libraries. After the target compound has been dissected into a set of suitable precursors, substructure searches can provide for each of them a series of representatives of a certain class of compounds, Siibsti ucturc searches enable the user to specify attributes such as open sites or atom lists at certain positions of the structure. Figure 10.3-38 shows the possible specification elements for the query in a substructure search. 

Glycine is the simplest member of a large and very important class of compounds, the a-amino-carboxylic acids. TTiere are many different methods available for the synthesis of amino-acids, but glycine can be readily prepared by the action of an excess of ammonia on chloroacetic acid ... 

It must be emphasised that the following substances are merely representative of widely different classes of compounds, and that the test when applied to an unknown compound often provides only a general indication of the probable class to which the compound belongs. Moreoi er, the behaviour of a compound when heated is often determined more by the nature of its substituent groups than by its general character. 

The following classes of compound yield carbon dioxide when treated in this way. 

Special methods are available for particular classes of compounds, Hinsberg s method of separating primary, secondary and tertiary amines (p. 249)-... 

The industrial process for preparing the reagent usually permits a little hydrolysis to occur, and the product may contain a little free calcium hydroxide or basic chloride. It cannot therefore be employed for drying acids or acidic liquids. Calcium chloride combines with alcohols, phenols, amines, amino-acids, amides, ketones, and some aldehydes and esters, and thus cannot be used with these classes of compounds. 

Table XI,5,1 shows this grouping of organic compounds, together with the commoner classes of compounds that fall into the respective solubility groups.

It is convenient to consider the indiflferent or neutral oxygen derivatives of the hydrocarbons—(a) aldehydes and kelones, (b) esters and anhydrides, (c) alcohols and ethers—together. All of these, with the exception of the water-soluble members of low molecular weight, are soluble only in concentrated sulphuric acid, i.e., fall into Solubility Group V. The above classes of compounds must be tested for in the order in which they are listed, otherwise erroneous conclusions may be drawn from the reactions for functional groups about to be described. 

Both aldehydes and ketones contain the carbonyl group, hence a general test for carbonyl compounds will Immediately characterise both classes of compounds. The preferred reagent is 2 4-dinilrophenylhydrazine, which gives sparingly soluble phenylhydrazones with carbonyl compounds ... 

Suggestions as to the methods for identifying the above classes of compounds will be found under Class Reactiona in Section XI,7. Some fimther remarks upon enolic compounds (see Table IV,1I4A) may be made here. Enols may be divided into (a) p-keto esters and (b) 1 3-diketones. With 5 per cent, sodium hydroxide solution, a p-keto ester yields the salt of the corresponding acid, which when heated with dilute hydrochloric acid is decarboxylated to a ketone ... 

Separations based upon differences in the chemical properties of the components. Thus a mixture of toluene and anihne may be separated by extraction with dilute hydrochloric acid the aniline passes into the aqueous layer in the form of the salt, anihne hydrochloride, and may be recovered by neutralisation. Similarly, a mixture of phenol and toluene may be separated by treatment with dilute sodium hydroxide. The above examples are, of comse, simple apphcations of the fact that the various components fah into different solubihty groups (compare Section XI,5). Another example is the separation of a mixture of di-n-butyl ether and chlorobenzene concentrated sulphuric acid dissolves only the w-butyl other and it may be recovered from solution by dilution with water. With some classes of compounds, e.g., unsaturated compounds, concentrated sulphuric acid leads to polymerisation, sulphona-tion, etc., so that the original component cannot be recovered unchanged this solvent, therefore, possesses hmited apphcation. Phenols may be separated from acids (for example, o-cresol from benzoic acid) by a dilute solution of sodium bicarbonate the weakly acidic phenols (and also enols) are not converted into salts by this reagent and may be removed by ether extraction or by other means the acids pass into solution as the sodium salts and may be recovered after acidification. Aldehydes, e.g., benzaldehyde, may be separated from liquid hydrocarbons and other neutral, water-insoluble hquid compounds by shaking with a solution of sodium bisulphite the aldehyde forms a sohd bisulphite compound, which may be filtered off and decomposed with dilute acid or with sodium bicarbonate solution in order to recover the aldehyde. 

Group additivity methods must be derived as a consistent set. It is not correct to combine fragments from different group additivity techniques, even for the same property. This additivity approximation essentially ignores effects due to the location of one functional group relative to another. Some of these methods have a series of corrections for various classes of compounds to correct for this. Other methods use some sort of topological description. 

Another technique for obtaining an ionization potential is to use the negative of the HOMO energy from a Hartree-Fock calculation. This is called Koopman s theorem it estimates vertical transitions. This does not apply to methods other than HF but gives a good prediction of the ionization potential for many classes of compounds. 

Most of the envisioned practical applications for nonlinear optical materials would require solid materials. Unfortunately, only gas-phase calculations have been developed to a reliable level. Most often, the relationship between gas-phase and condensed-phase behavior for a particular class of compounds is determined experimentally. Theoretical calculations for the gas phase are then scaled accordingly. 

Molecular mechanics methods have only been used to a limited extent for these classes of compounds. However, molecular mechanics methods do fairly well in describing the geometries and relative energies of compounds with these elements. It is perhaps only for historical and economic reasons that molecular mechanics has not been used more for modeling these elements. Subsequently, there are not as many force fields available. 

In the past, when molecular mechanics methods were used for transition metals, it was by having a set of parameters for the metal that were parameterized specifically for one class of compounds. There have been a number of full periodic table force fields created, with the most successful being the UFF force field. All the full periodic molecular mechanics methods still give completely unreasonable results for certain classes of compounds. 

Material properties can be further classified into fundamental properties and derived properties. Fundamental properties are a direct consequence of the molecular structure, such as van der Waals volume, cohesive energy, and heat capacity. Derived properties are not readily identified with a certain aspect of molecular structure. Glass transition temperature, density, solubility, and bulk modulus would be considered derived properties. The way in which fundamental properties are obtained from a simulation is often readily apparent. The way in which derived properties are computed is often an empirically determined combination of fundamental properties. Such empirical methods can give more erratic results, reliable for one class of compounds but not for another. 

Single-bond cleavage with molecular hydrogen is termed hydrogenolysis. Palladium is the best catalyst for this purpose, platinum is not useful. Desulfurizations are most efficiently per-formed with Raney nickel (with or without hydrogen G.R. Pettit, 1962 A or with alkali metals in liquid ammonia or amines. The scheme below summarizes some classes of compounds most susceptible to hydrogenolysis. 

In this chapter some important synthetic reactions specific to each class of compounds are described. Only small parts of certain total syntheses will be discussed. With the given references, however, the interested reader will easily locate the complete descriptions of the syntheses. I. Fleming s (1973) book is recommended as a guide through some ingenious classic total syntheses. 

In the last fifteen years macrolides have been the major target molecules for complex stereoselective total syntheses. This choice has been made independently by R.B. Woodward and E.J. Corey in Harvard, and has been followed by many famous fellow Americans, e.g., G. Stork, K.C. Nicolaou, S. Masamune, C.H. Heathcock, and S.L. Schreiber, to name only a few. There is also no other class of compounds which is so suitable for retrosynthetic analysis and for the application of modem synthetic reactions, such as Sharpless epoxidation, Noyori hydrogenation, and stereoselective alkylation and aldol reactions. We have chosen a classical synthesis by E.J. Corey and two recent syntheses by A.R. Chamberlin and S.L. Schreiber as examples. 

The principal reactions of this class of compounds are summarized in Scheme 172. In most of these reactions the reactive nucleophilic center is the terminal NHj group, although the other exocyclic nitrogen may also be involved, as shown by acetylation, which yields 284 and 285. However, the structure of compound 281 is not the one proposed in a recent report (1582) that attributes the attack to the other exocyclic nitrogen. The formation of osazones (287) from sugars, 2-hydrazinothiazoles, and hydrazine has been reported (525, 531). 

Alkylidenehydrazinothiazoles (297) can be prepared either from 2-hydrazinothiazoles (549) or by direct heterocyclization (527). Their characteristic infrared bands have been reported (550). The main mass spectrometric peaks of (4-coumarinyl-2-thiazolyl)hydrazone (302) (Scheme 179) (134, 551) are situated at mle = 361. 244, 243, 118, 216, 202, 174, 117 the proposed interpretation of the fragmentation pattern should, however, be reconsidered. Scheme l80 summarizes some representative reactions of this class of compounds. 

Not many examples of this class of compounds have been reported. They are prepared by the action of sodium azide on 2-diazonium salts of 2-aminothiazole (590, 597, 598). 

Aniino4-methyl-5-phenylthiazole-3-oxide (353) was synthesized from the a-chlorooxime and Ba(SCN)2 (Scheme 202) (682). This class of compounds is usually prepared by such heterocyclizations (683). 

Imino-4-thiazolines are far more basic than their isomeric 2-aminothiazoles (see Table VI-1). They react with most electrophDic centers through the exocyclic nitrogen and are easily acylated (37, 477, 706) and sulfonated (652). The reaction of 2-imino-3-methyi-4-thiazoline (378) with a-chloracetic anhydride yields 379 (Scheme 217) (707). This exclusive reactivity of the exocyclic nitrogen precludes the direct synthesis of endocyclic quaternary salts of 2-imino-4-thiazolines. although this class of compounds was prepared recently according to Scheme 218 (493). 

Reaction of 2-imino-3-alkyl-4-thiazolines with alkyl isocyanates gives the ureas (382), which when nitrated on C-5 give the schistosomicide class of compounds (383) (Scheme 219), When nitration takes place on the ring... 

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