Skeleton, carbon

Farnesol pyrophosphate is an immediate precursor of squalene, the key intermediate in steroid and triterpenoid biogenesis, which arises from the coupling of two farnesol pyrophosphate molecules or of C,s units derived therefrom. The numerous types of sesquiter-penoid carbon skeletons represent various modes of cyclization of farnesol (sometimes with rearrangement) and it is probable that farnesol pyrophosphate is also the source of these compounds. 

The octane numbers for olefins, as for paraffins, depends on the length and ramification of their chains. Olefin RONs are generally higher than those for paraffins having the same carbon skeleton. The displacement of the... 

Another scheme for estimating thermocheraical data, introduced by Allen [12], accumulated the deviations from simple bond additivity in the carbon skeleton. To achieve this, he introduced, over and beyond a contribution from a C-C and a C-H bond, a contribution G(CCC) every time a consecutive arrangement of three carbon atoms was met, and a contribution D(CCC) whenever three carbon atoms were bonded to a central carbon atom. Table 7-3 shows the substructures, the symbols, and the contributions to the heats of formation and to the heats of atomization. 

Figure 8-2. Edge graph of the carbon skeleton of 2-methylbutane (1).

Do the same thing with the unstable conliguration resulting from the carbon skeleton... 

PCMOIIEL. Draw the carbon skeleton (a single horizontal line for ethylene)... 

The mechanism of the reaction la not known with certainty. It is known from studies utilising as tracer that no change in the carbon skeleton occurs during the reaction, and also that unsaturated hydrocarbons can undergo reactions very similar to those of ketones thus both styiene and phenyl-acetylene can react with sulphur and morpholine to produce phenylaceto-thiomorphoUde, hydrolysis of which yields phenylacetic acid ... 

This reference work differs from Beilstein in that it is baaed upon structural formulae and compounds are grouped according to the carbon skeleton rather than the functional group the latter system has the advantage that closely related compounds are grouped together. The volumes are not published in numerical order but rather on the basis of fields of current interest. They are a valuable supplement to Beilstein. The volumes which have been published to date (1955) are ... 

Have you noticed that the disconnections involving H" are simply redox reactions and do not alter the carbon skeleton of the molecule They are not then reaUy discoimections at all but Functional Group Interconversions or FGI for short. 

The proton chemical shifts of the protons directly attached to the basic three carbon skeleton are found between 5.0 and 6.8 ppm. The J(H,H) between these protons is about -5 Hz. The shift region is similar to the region for similarly substituted alkenes, although the spread in shifts is smaller and the allene proton resonances are slightly upfield from the alkene resonances. We could not establish a reliable additivity rule for the allene proton shifts as we could for the shifts (vide infra) and therefore we found the proton shifts much less valuable for the structural analysis of the allene moiety than the NMR data on the basic three-carbon system. 

Benzylic anions, ArCHj, are of little importance in the construction of carbon skeletons, and allylic anions, R C—CR—CR", are discussed in the d -synthons section below. 

We shall only be concerned here with those reactions, which have been used in constructions of the carbon skeletons of complex compounds with concomitant regioselective incorporation of the double bond. 1,2-Eliminations are discussed on p. 137ff. 

In antithetical analyses of carbon skeletons the synthon approach described in chapter I is used in the reverse order, e.g. 1,3-difunctional target molecules are "transformed" by imaginary retro-aldol type reactions, cyclohexene derivatives by imaginary relro-Diels-Alder reactions. 

Open-chain carbon skeletons (including cyclic acetals, lactones, lactams, cyclic anhydrides, etc.)... 

Porphyrins and chlorophylls are the most widespread natural pigments. They are associated with the energy-converting processes of respiration and photosynthesis in living organisms, and the synthesis of specific porphyrin derivatives is often motivated by the desire to perform similar processes in the test tube. The structurally and biosynthetically related corrins (e.g. vitamin B,j) catalyze alkylations and rearrangements of carbon skeletons via organocobalt intermediates. The biosyntheses of these chromophores are also of topical interest. 

In these simplified representations called bond line formulas or carbon skeleton dia grams, the only atoms specifically written m are those that are neither carbon nor hydro gen bound to carbon Hydrogens bound to these heteroatoms are shown however... 

In this case the longest continuous chain has five carbon atoms the compound is named as a derivative of pentane The key word here is continuous It does not matter whether the carbon skeleton is drawn m an extended straight chain form or m one with many bends and turns All that matters is the number of carbons linked together m an uninterrupted sequence... 

The C4H9 alkyl groups may be derived either from the unbranched carbon skele ton of butane or from the branched carbon skeleton of isobutane Those derived from butane are the butyl (n butyl) group and the 1 methylpropyl (sec butyl) group... 

Consider the alkyl group having the same carbon skeleton as (CH3)4C All the hydrogens are equivalent replacing any one of them by a potential point of attachment is the same as replacing any of the others... 

All the parts of this problem refer to the alkane having the carbon skeleton shown... 

Place a double bond in the carbon skeleton shown so as to... 

A mixture of three alkenes was obtained m 80% yield having the composition shown The alkene having the same carbon skeleton as the starting alcohol 3 3 dimethyl 1 butene constituted only 3% of the alkene mixture The two alkenes present in greatest amount 2 3 dimethyl 2 butene and 2 3 dimethyl 1 butene both have carbon skeletons different from that of the starting alcohol... 

A certain species of grasshopper secretes an allenic substance of molecular formula C13H20O3 that acts as an ant repellent The carbon skeleton and location of various substituents in this substance are indicated in the partial structure shown Complete the structure adding double bonds where appropriate... 

Benzene is planar and its carbon skeleton has the shape of a regular hexagon There IS no evidence that it has alternating single and double bonds As shown m Figure 111 all the carbon-carbon bonds are the same length (140 pm) and the 120° bond angles... 

The two step acylation-reduction sequence is required Acylation of benzene puts the side chain on the ring with the correct carbon skeleton Clemmensen reduc tion converts the carbonyl group to a methylene group... 

We pointed out in Section 13 3 that both H and are nuclei that can provide useful structural information when studied by NMR Although a H NMR spectrum helps us infer much about the carbon skeleton of a molecule a NMR spectrum has the obvious advantage of probing the carbon skeleton directly NMR spectroscopy is analogous to H NMR in that the number of signals informs us about the number of different kinds of carbons and their chemical shifts are related to particular chemical environments... 

Commercial mesityl oxide (CH3)2C=CHCCH3 is often contam mated with about 10% of an isomer having the same carbon skeleton What is a Jikely structure for this compound ... 

Overall the carboxylation of Grignard reagents transforms an alkyl or aryl halide to a carboxylic acid in which the carbon skeleton has been extended by one carbon atom... 

The structural feature that distinguishes terpenes from other natural products is the iso prene unit The carbon skeleton of myrcene (exclusive of its double bonds) corresponds to the head to tail union of two isoprene units... 

The product of this reaction 3 hydroxy 3 methylglutaryl coenzyme A (HMG CoA) has the carbon skeleton of mevalonic acid and is converted to it by enzymatic reduction... 

Steric Factors. Initially, most of the coUisions of fluorine molecules with saturated or aromatic hydrocarbons occur at a hydrogen site or at a TT-bond (unsaturated) site. When coUision occurs at the TT-bond, the double bond disappears but the single bond remains because the energy released in initiation (eq. 4) is insufficient to fracture the carbon—carbon single bond. Once carbon—fluorine bonds have begun to form on the carbon skeleton of either an unsaturated or alkane system, the carbon skeleton is somewhat stericaUy protected by the sheath of fluorine atoms. Figure 2, which shows the crowded hehcal arrangement of fluorine around the carbon backbone of polytetrafluoroethylene (PTFE), is an example of an extreme case of steric protection of carbon—carbon bonds (29). 

---