Carbon backbone numbering

Fig. 3. Chemical structure and carbon backbone numbering of p-menthane (left) and bornane (right)...

A classification based first on ion specificity, then on stmctural features has been suggested for the polyethers (7). Another method uses the presence of unsaturation or of aromatic groups in the molecular skeleton (8). In this review the compounds are classified based on the number of carbons in the backbone according to the numbering system proposed in reference 9. The carbon backbone or skeleton refers to the longest chain of contiguous carbons between the carboxyl group and the terminal carbon. 

As the number of carbon atoms in the alkane increases, so does the number of possible stractural isomers. Thousands of different alkanes exist, because there are no limits on the length of the carbon chain. Regardless of the number of the chain length, alkanes have tetrahedral geometry around all of their carbon atoms. The structure of decane, Cio H22, shown in Figure 9-15. illustrates this feature. Notice that the carbon backbone of decane has a zigzag pattern because of the 109.5° bond angles that characterize the tetrahedron. 

In our study of the simple hydrocarbons, there are only two functional groups. One is a carbon-to-carbon double bond. Hydrocarbons that contain a carbon-to-carbon double bond are called alkenes. Naming alkenes is very similar to naming alkanes. The major difference is that the carbon base has an -ene ending instead of the -ane ending. The carbon backbone of the base hydrocarbon is numbered so the position of the double bond has the lowest location number. 

The retrosynthetic analysis proceeds according to Scheme 13.4.1 and starts with the application of the heuristic principle number 8 (HP-8) i.e., performing the systematic disconnection of the nucleophilic heteroatom attached to the carbon backbone. Of the three bonds connecting the nitrogen atom, bond a is the most... 

The oxidation of either primary alcohols or aldehydes doesn t change the carbon backbone, so you end up with a carboxylic acid containing the same number of carbon atoms as the aldehyde or alcohol. Alcohols require considerably stronger oxidizing conditions than aldehydes do. 

A limited number of reactions can add or remove carbon atoms. For this reason, comparing the carbon backbones of the materials involved in the reaction is useful. These reactions require the presence or absence of certain functional groups. 

Apart from the all-carbon backbone, poly(vinyl ester)s also exhibit a unique 1,3-diol structure (see Fig. 1). This structure is a common motif in many natural materials, e.g. carbohydrates. A number of oxidative or reductive electron transfer processes catalysed by natural redox systems are imaginable for this motif. The 1,3-diol structure is unique for a synthetic polymer and cannot be found in any other synthetic polymer class of significance. This explains the unusual biodegradation properties discussed below. 

FIGURE 3-8 Uncommon amino acids, (a) Some uncommon amino acids found in proteins. All are derived from common amino acids. Extra functional groups added by modification reactions are shown in red. Desmosine is formed from four Lys residues (the four carbon backbones are shaded in yellow). Note the use of either numbers or Creek letters to identify the carbon atoms in these structures, (b) Ornithine and citrulline, which are not found in proteins, are intermediates in the biosynthesis of arginine and in the urea cycle. 

The beta pleated sheet, or simply beta sheet, structure is illustrated in Figure 12.53. The carbon backbone in the beta sheet is fully extended, and adjacent chains are held together by a large number of hydrogen bonds. 

Fig. 18. Stereodrawing of the antigen-binding site of the Fab fragment NEW. The a-carbon backbone and numbered aromatic side chains show the large number of aromatic-aromatic interactions present at the VL-VH interface. These interactions occur both within and between the two domains. Courtesy of J. Novotny.

After mercury(II)-assisted hydrolysis of the thioenol ether, aldehyde 3 was obtained. This was then subjected to the critical vinylogous aldol reaction needed to complete the carbon backbone of the natural product. The latter process furnished a 3.5 1 mixture of the y to ot addition products. The stereoselectivity observed in the installation of the C(5)-hydroxyl (natural product numbering) was only 2 1. Fortunately, the predominant isomer was the desired product 2. In retrospect, it can be seen that the level of selectivity attained conformed to the predictions of the Still model.4... 

Alkanes contain only single bonds. Alkanes have the maximum number of hydrogen atoms possible for their carbon backbone, so they are called saturated. Alkenes, alkynes, and aromatics are unsaturated because they have fewer hydrogen atoms.. 

---