Carbon skeleton, 129 substitutions

The question of (p — d)-ir bonding and ring planarity also arises in compounds having an anthracene-like carbon skeleton substituted at either or both of the 9 and 10 positions by atoms bearing vacant d-orbitals (4). Heteroatom substitution in 9,10-bis(dimethylgermyl)-... 

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. 

Substitution of thiophene rings for the benzene rings and a light modification in the carbon skeleton leads to a marked... 

In this section, the emphasis is on carbocation reactions that modify the carbon skeleton, including carbon-carbon bond formation, rearrangements, and fragmentation reactions. The fundamental structural and reactivity characteristics of carbocations toward nucleophilic substitution were explored in Chapter 4 of Part A. 

One of the sex pheromone components of the housefly, Musca domestica, is Z9-21 H that is found on the cuticular surface of the fly. This compound is formed by the elongation of Z9-18 CoA using malonyl-CoA and NADPH to Z15-24 CoA which is decarboxylated to form Z9-21 Hc (Fig. 3) [78-80]. Other pheromone components include an epoxide and ketone that are produced from Z9-21 Hc by a cytochrome P450 [81,82] and methyl-branched alkanes that are produced by the substitution of methylmalonyl-CoA in place of malonyl-CoA at specific points during chain elongation [83,84]. A novel microsomal fatty acid synthase is involved in production of methyl-branched alkanes in most insects [85-87]. This fatty acid synthase is different from the ubiquitous soluble fatty acid synthase that produces saturated straight chain fatty acids in that it is found in the microsomes and prefers methylmalonyl-CoA. The amino acids valine and isoleucine can provide the carbon skeletons for methylmalonyl-CoA as well as propionate [83]. 

The intramolecular addition of alkynyl-substituted a-diazoketones is catalyzed by Rh2(OAc)4 to give transient cyclopropenes, which spontaneously rearrange to vinylogous a-keto carbene intermediates for further carbon-skeleton transformations [54]. 

The course of decomposition of confirmed or presumed metallocyclo-butane intermediates is important, but most results reported deal with stoichiometric rather than catalytic processes. Retention of the 3-carbon skeleton via pathways d or f in Eq. (26) occurs much more frequently than does cleavage to metathesis-related products. For example, thermolysis of phenyl-substituted platinocyclobutanes yields propenylben-zenes and phenyl-cyclopropane, but no styrene or ethylene (77). Similarly, the decomposition of tantalum carbene adducts (8) with olefins... 

Alternatively, it is possible to add a functional group (FGA), either to functionalise the carbon skeleton or to create new consonances (dissonances must be always avoided ) which can provide valid bond disconnection mechanisms (HP-4). Of some special interest is the introduction of a double bond (in the a,P-position if a carbonyl group is already present in the target molecule) since it is a typical ambivalent group, of type A, which provides different valid bond disconnection mechanisms, either directly or after substitution by an equivalent synthon, such as a... 

Finally, another important heuristic principle is to carry out the systematic disconnection of substituted nucleophilic heteroatoms (O, N, S) directly attached to the carbon skeleton, especially if they are attached to a primary sp carbon atom (HP-8) ... 

The clustering calculations also provide other kinds of Information about chemical classification of Infrared spectra. These Include the effect of single substituents, carbon skeleton and multiple substitution. These effects were noticeable even under the more difficult circumstance wherein two classes were clustered simultaneously. 

Prelog and co-workers (160, 161) chose 9,9 -spirobifluorene as a starting material for synthesizing chiral crown ethers since (i) it has a more rigid carbon skeleton than the 1,1 -binaphthyl unit, and (ii) it can be substituted easily in the 2 and 2 positions by electrophilic reagents. Thus, the 2,2 -diacetyl derivative (Figure 19) obtained after a Friedel-Crafts on 9,9 -spirobifluoiene can be con-... 

The development of schemes for the total synthesis of the carbon skeleton of morphine revealed that the fused furan ring was not necessary for biological activity. More recently it has been found that substitution of a pyran ring for the terminal ali cyclic is also consistent with... 

The Hantsch pyridine synthesis provides the final step in the preparation of all dihydrop-yridines. This reaction consists in essence in the condensation of an aromatic aldehyde with an excess of an acetoacetate ester and ammonia. Tlie need to produce unsymmetrically substituted dihydropyridines led to the development of modifications on the synthesis. (The chirality in unsymmetrical compounds leads to marked enhancement in potency.) Methyl acetoacetate fonns an aldol product (30) with aldehyde 29 conjugate addition of ethyl acetoacetate would complete assembly of the carbon skeleton. Ammonia would provide the heterocyclic atom. Thus, application of this modified reaction affords the mixed diester felodipine 31 [8]. 

Askani has observed that 3- and 7-substituted semibullvalenes, of which 369 is exemplary, undergo cycloaddition to N-phenyltriazolinedione with rearrangement of the carbon skeleton The products are dihydrodiazatriquinacenes (e.g. 370) which when converted to their azo counterparts and heated to 80 °C or above are transformed into isomeric semibullvalenes such as 371. 

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