Carbon skeleton diagrams

In these simplified representations, called bond-line formulas or carbon skeleton diagrams, the only atoms specifically written in are those that are neither carbon nor hydrogen bound to carbon. Flydrogens bound to these heteroatoms are shown, however. 

Bond lengths. See Bond distances Bond-line formulas, 21, 59, l. See also Carbon skeleton diagrams... 

Carbon skeleton diagram Synonymous with bond-line formula. 

Dash formula diagram all atoms, bonds as dashes Bond line formula hide H, show carbon skeleton as... 

The broken lines in the diagrams show the trace of the plane of the n orbitals. A reaction will occur readily if X lies on this plane and makes an obtuse angle with the C=Y bond. Molecular models show that the carbon backbone is long and flexible enough to satisfy both of these criteria for the exo reactions. The 6-endo reaction poses problems. If X lies in the n plane, the carbon skeleton has to adopt a boat conformation, leading to a perpendicular attack. However, if X moves slightly out of the n plane, an acceptable compromise can be achieved the attack trajectory becomes non-perpendicular, with a fair nucleophile-n overlap. However, neither condition can be satisfied for a 5-endo reaction. Note that a direct application of Baldwin s empirical rules would have masked these subtleties. 

The aromatic acid is available and we need consider only the pyrazole (core pyrazole ring in black in the diagram). The aromatic amino group can be put in by nitration and reduction and the amide can be made from the corresponding ester. This leaves a carbon skeleton, which must be made by ring synthesis. 

This skeleton is a little more difficult, but you will discover that in carbon compounds, the carbon is usually the central element. Therefore, the skeleton diagram will look like ... 

Estimate the length and width of the carbon skeleton in anthracene [see Problem 9-43(o) for a diagram]. Assume hexagonal rings with equal carbon-carbon distances of 140 pm. 

Figure 17. Diagram summarizing isotopic relationships between inorganic carbon pools and carbon in photosynthate and C4 carbon skeletons, is the equilibrium isotopic fractionation between dissolved CO2 and bicarbonate (Mook et al. 1974). is the isotope effect associated with carbon fixation. The estimate of d for internal CO2 is based on = -25%o and % = 27%o. is the isotope effect associated with phosphenolpymvate carboxylase (O Leary et al. 1982). The carbon pools represented in the right-hand colurrm are, from top to bottom, the carbon added by the carboxylation of phosphoenolpymvate, the total carbon in aspartic acid and in oxaloacetate, and the total carbon in pyravate.

Figure 27. Molecular structures showing the that junctions between isoprene units can be recognized by counting the number of CH2 groups between methyl branches. The branching diagram indicates biosynthetic relationships between various isoprenoid carbon skeletons. Names of typical products appear in parentheses (n. b., geraniol and phytol are merely examples and are far from the only Qo and C20 isoprenoids). The irregular-junction case is an example based on Rowland et al. (1995)...

Carbon-proton bonds are then assigned by means of the one-bond CH connectivities observed in the CH COSY diagram (Table 44.2). This completes partial structures A and B to the CH skeletons C and D. 

Triterpenoid compounds are derived from a precursor, squalene, which was first isolated from shark liver (Bruneton 1995). They have similar configurations to steroids (found in plants and animals) whose C27 skeletons are also derived from squalene. According to convention, the rings are labelled A-D (or E if present) and the carbons are numbered as shown on the diagram. 

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