Positional prefixes

Do not capitalize chemical names or nonproprietary drug names unless they are at the beginning of a sentence or are in a title or heading. In such cases, capitalize the first letter of the English word, not the locant, stereoisomer descriptor, or positional prefix. (See Chapter 12, Names and Numbers for Chemical Compounds .)... 

Many simple common compounds are known by both a trivial and a systematic name. The systematic names are helpful in learning the structures of organic compounds, but the trivial names are often simpler and can reflect the source or dominant reactivity of the compound concerned. The systematic name for a compound has a stem that describes the carbon skeleton, prefixes and suffixes that indicate the functional groups, and numbers (locants) that define their position. Prefixes may also be added to indicate modifications to the carbon skeleton and to define the stereochemistry. A list of the more common prefixes, suffixes and stems is given in Table 1.2. 

FIGURE 39.2 Schematic structure of microcystins and nodularins. For each position in the peptide ring (see Figure 39.1), all amino acid moieties are listed that have been found in that particular position. Prefixes indicate N-methylation (M), O- or C-methylation (m), acetylation (Ac). For abbreviations of amino acids see text. 

OH groups are in the para or 1,4 position to each other. This use of the prefix is confined to disubstituted benzene derivatives in such cases as para-hydrogen and paraldehyde the prefix has no uniform structural significance and is always written in full. 

Double bonds m the mam chain are signaled by the ending enow acid and their position IS designated by a numerical prefix Entries 6 and 7 are representative carboxylic acids that contain double bonds Double bond stereochemistry is specified by using either the cis-trans or the E-Z notation... 

If the same alkyl group occurs more than once as a side chain, this is indicated by the prefixes di-, tri-, tetra-, etc. Side chains are cited in alphabetical order (before insertion of any multiplying prefix). The name of a complex radical (side chain) is considered to begin with the first letter of its complete name. Where names of complex radicals are composed of identical words, priority for citation is given to that radical which contains the lowest-numbered locant at the first cited point of difference in the radical. If two or more side chains are in equivalent positions, the one to be assigned the lowest-numbered locant is that cited first in the name. The complete expression for the side chain may be enclosed in parentheses for clarity or the carbon atoms in side chains may be indicated by primed locants. 

Isomers are distinguished by lettering the peripheral sides of the parent beginning with a for the side 1,2, and so on, lettering every side around the periphery. If necessary for clarity, the numbers of the attached position (1,2, for example) of the substituent ring are also denoted. The prefixes are cited in alphabetical order. The numbers and letters are enclosed in square brackets and placed immediately after the designation of the attached component. Examples are... 

Azoxy Compounds. Where the position of the azoxy oxygen atom is unknown or immaterial, the compound is named in accordance with azo rules, with the affix azo replaced by azoxy. When the position of the azoxy oxygen atom in an unsymmetrical compound is designated, a prefix NNO- or ONN- is used. When both the groups attached to the azoxy radical are cited in the name of the compound, the prefix NNO- specifies that the second of these two groups is attached directly... 

Table 1 lists some of the common binucleophiles utilized in heterocyclic synthesis, the numerical prefixes referring to the relative positions of the nucleophilic centers to each other. Higher order binucleophiles, e.g. 1,5-systems, come readily to mind and the above illustrative examples rapidly increase in scope when the incorporation of these structural elements into heterocyclic systems is considered. This last group offers many opportunities for ring annulations. 

In theory, three isoxazolines are capable of existence 2-isoxazoline (2), 3-isoxazoline and 4-isoxazoline. The position of the double bond may also be designated by the use of the prefix A with an appropriate numerical superscript. Of these only the 2-isoxazolines have been investigated in any detail. The preparation of the first isoxazoline, 3,5-diphenyl-2-isoxazoline, from the reaction of )3-chloro-)3-phenylpropiophenone with hydroxylamine was reported in 1895 (1895CB957). Two major syntheses of 2-isoxazolines are the cycloaddition of nitrile A-oxides to alkenes and the reaction of a,/3-unsaturated ketones with hydroxylamine. Since 2-isoxazolines are readily oxidized to isoxazoles and possess some of the unique properties of isoxazoles, they also serve as key intermediates for the synthesis of other heterocycles and natural products. 

As described in Section 1.02.2.3, any heterocycle can be named by indicating (with appropriate prefixes) the positions of heteroatoms in the corresponding carbocycle. The carbocycle can be named systematically, as described in later Sections however, the replacement procedure can also be applied to naming heterocyclic analogues of trivially named carbocycles. A list of polycarbocycles with their trivial names is given in Table 5 (taken from Rule A-21.2 in the lUPAC Blue Book (B-79MI10200)) replacement names may be derived as shown in examples (27) and (28). As demonstrated by example (28), it is unnecessary to add hydro prefixes if the heterocycle cannot accommodate as many double bonds as the original carbocycle. 

The extension to polycycles with three or more rings is straightforward prefixes tricyclo-, tetracyclo-, etc. are used and the positions of the additional bridges are specified by superscripts (examples 59-61). 

In substitutive nomenclature the use of the prefix oxo- or the suffix -one means the insertion of =0 in place of two hydrogen atoms. This is straightforward when the parent carries a CHz group at the appropriate position (example 206), but often the insertion of =0 requires prior reduction of a double bond. When the prefix oxo- is employed, this is achieved by using the appropriate hydro- prefix terms (example 207). However, the suffix -one can be used without hydro , the required reduction being implied rather than overtly stated (example 208) if desired the position of the hydrogen atom introduced to accommodate the carbonyl can be indicated in parentheses (see also examples 123 and 124). 

As you can see, cycloalkanes are naned, under the lUPAC system, by adding the prefix cyclo- to the nane of the unbranched alkane with the same number of carbons as the ring. Substituent groups are identified in the usual way. Their- positions are specified by numbering the carbon atoms of the ring in the direction that gives the lowest number to the substituents at the first point of difference. 

If several groups are attached to the benzene ring, their names as well as their relative positions should be indicated. For example, dimethylbenzene or xylene, CgH (CH,)2, has three geometric isomers, with prefixes ortho-, meta-, and para-, indicating the relative positions of the two methyl groups. 

When two hydrogens in the ring are substituted by the same reagent, three isomers are possible. The prefixes ortho, meta, and para are used to indicate the location of the substituents in 1,2- 1,3- or 1,4-positions. For... 

The sugar component in RNA is ribose, and the sugar in DNA is 2 -deoxy-ribose. (The prefix 2 -deoxv indicates that oxygen is missing from the 2 position of ribose.) DNA contains four different amine bases, two substituted purines (adenine and guanine) and two substituted pyrimidines (cytosine and thymine). Adenine, guanine, and cytosine also occur in RNA, but thymine is replaced in RNA by a closely related pyrimidine base called uracil. 

The systematic name of a diketose is formed by replacing the terminal -se of the stem name by -diulose . The locants of the (potential) carbonyl groups must be the lowest possible and appear before the ending. The stem name is preceded by the appropriate configurational prefix. If there is a choice of names, a decision is made on the basis of 2-Carb-2.2.2. In cyclic forms, locants may be needed for the positions of ring closure that of the (potential) carbonyl group is cited first. 

If there is a carbonyl group in the branch (or a terminal COOH or its equivalent), its position (assigned lowest number when stereochemistry is being considered) is used to define the configurational prefix (see examples 1 and 3 in Chart V). Use of the R,S system is generally preferred, as less open to misinterpretation. 

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