Structural isomers Applied

An older term, structural isomers, is recommended by the International Union of Pure and Applied Chemistry (IUPAC) to be abandoned. 

Isomers are substances that have the same number and kinds of atoms, but arranged differently. Constitutional (structural) isomers, as applied to coordination compounds, are isomers whose differences involve having more than a single coordination sphere or different donor atoms on the same ligand. 

Compounds that have the same molecular formulas but different structural formulas are called isomers. With hydrocarbons, this applies to a different arrangement of the carbon atoms. Isomers such as these are called structural isomers. Figure 18.3 shows the structural isomers of C5H12. Note that there are the same number of carbons and hydrogens in each structure. Only the way the carbons are bonded is different. 

Another area to which the MM method can be advantageously applied is the prediction of product distribution under thermodynamic control, where the errors in energy calculations tend to cancel if structurally related products are compared (120). A remarkable example is the dodecahydrogenation of phenanthrene, in which 25 structural isomer products are possible, each having one to four stable... 

The concept of "selectivity" must be clearly distinguished from the term "specificity" [4][5], Specific, applied to a reaction, means that two (or more) isomeric starting materials give -under the same reaction conditions- different reaction products which are also isomers. Depending upon the isomers we may be considering, we may refer to "regiospecificity" (structural isomers) or to "stereospecificity" (either diastereospecificity or enantiospecificity). For instance, the formation of we5o-2,3-dibromobutane by addition of bromine to ( )-2-butene, in contrast with the formation of the d,l - 2,3-dibromobutene from the (Z)-2-butene, is a case of diastereospecificity. 

In the following, experience on semiempirical hypersurface calculations - mostly applying the MNDO approximation(9) - is reported. As will be exemplified, correlation with experimental data on structures, isomer stabilities, ionization patterns or ESR coupling constants ranged between satisfactory and acceptable. Altogether, they have been of great help in tracking some short-lived molecules. 

Bis-allyl-metal complexes exist in three basic types of structural isomers bis-TT-ally 1-, tr, o-bis-allyl- and bis-o-allyl-moieties. Taking this into consideration and applying out principle results in the analogies shown in Scheme 2.5-1. These analogies are only complete if one con-... 

However, for the same structural transformations applied to linear peptides (e.g., 1A and 8 Scheme 1), the end groups cause inherent difficulties, which are absent in the end-to-end cyclic peptides. Since syntheses of linear enantiomers, their re/ro-isomers, and retrain verso-isomers (e.g., 1A, 6A, IB, 6B Scheme 1) and their end-group-modified peptides (e.g., 8,12,13 Scheme 1) follow the conventional protocols of peptide synthesis, they are not covered in this section. 

Another remarkable reaction is the methoxymercuration which leads, when applied to derivatives of lysergic acid, to the unique 10-methoxy-A8,9-ergolene derivatives (50) after treatment of the addition product with base and NaBH4 (51). In another recent paper (127) the synthesis of a new structural isomer of lysergic acid methyl ester (28) has been reported in which a novel modification of the Polonovski reaction has been used to introduce the 7,8 double bond. Thus, the A-oxide 51 on treatment with acetic anhydride and excess base, yielded the enamino ester (52) in about 50% yield. 

Structural (constitutional) isomers (Applied to coordination compounds.) Isomers whose differences involve more than a single coordination sphere or else different donor atoms they include ionization isomers, hydrate isomers, coordination isomers, and linkage isomers. 

By applying this methodology a series of para- mkeA structural isomers (17-20) has also been synthesized via coupling of the ortho-para" dimer. ... 

These general methods may also be applied to unsaturated Cn compounds and other structural isomers. 

Although in the first example the systematic name is 1,3-thiazole, the locants are generally omitted because, except for isothiazole (1,2-thiazole), no other structural isomers exist. Similar rules apply to oxazole (1,3-oxazole) and isoxazole (1,2-oxazole). 

A similar sequence of reactions was applied to 6-0-(3-amino-3-deoxy-a-D-glucopyranosyl)-2-deoxystreptamine (obtained by partial hydrolysis of the kanamydns) to yield a structural isomer of neamine, namely, 6-0- (3,6-diamino-3,6-dideoxy-a-D-glucopyranosyl )-2-deoxystrept-amine (84), which was devoid of activity. 

Another method which has been applied to obtain further structural information is GC coupled to a Fourier transform infrared (FT-IR) spectrophotometer. This technique allows the distinction of structural isomers which are otherwise difficult to distinguish by MS (Bicchi et al. 1989b). Since the diastereoisomers can be separated by GC, identification with this method can be carried out on a routine basis once the retention times for the pure stereoisomers have been established (Hendriks et al. 1987). 

The first elements of the series are A(x) = - -x- -x + x + 2x + 3x + 5x + 9x" + 18x + 35x + 7Sx -b. .., and to answer the section s question, 75 structural isomers exist for decane. Applying Eq. [19] the number of alkane isomers up to 25 carbon atoms is given in Table 4 in the Chemical Information subsection appearing later in this chapter. Note that a(x) can be evaluated computationally, with the product, sum, and scalar multipHcation operator on the polynomial A(x) representing the alkyl group coimting series. Considering the computational cost to evaluate A(x), alkanes up to n carbon atoms can be counted with no more than elementary arithmetic operations. 

Treatment of soybean tissues with Pms or the elicitor derived from it induced the accumulation of large amounts of the phytoalexin,glyceollin, and two of its structural isomers (Fig. 1) (Ref. 6). Soybean tissues are sensitive to very small amounts of Pms elicitor. When concentrations of a crude elicitor fraction in the range from 10 to 10 ug/ml were applied to wounded soybean cotyledons, the minimum concentration of elicitor giving detectable glyceollin accumulation was about 1 ug/ml, while concentrations of the order of 1 mg/ml resulted in msucimtua glyceollin accumulation (Fig. 2). 

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