Head-to-tail link

Also deoxycholic acid (6) crystallizes in an inclusion lattice with channel-shaped cavities 13). Figure 3 shows that they are formed by facing molecules of deoxycholic acid, 4). This characteristic structural unit is a double layer of head-to-tail linked deoxycholic acid molecules at which specific H-bridges between hydroxy and carboxy groups are the decisive fact. The channels as such (e.g. in case of the orthorhombic crystal, see Fig. 3) are lined with lipophilic groups. Thus only van der Waals contacts are kept between the included guest molecules (also for polar molecules like acetone, Fig. 3) and the molecules of the channel wall. 

The cyclization to the desired head-to-tail linked bis-benzimidazoles could also be performed utilizing aryl or alkyl isothiocyanates with N, N -dicyclohexylcarbodiimidc (DCC) [82]. Upon completion, the insoluble N,N -dicyclohexylthiourea formed had to be removed by filtration and the desired PEG-bound products were precipitated by the addition of diethyl ether. The results were essentially the same as those of the cyclizations with the above mentioned aldehydes. 

In addition to head-to-tail links, -CH2-CHCI-CH2-CHCI-, other links such as head-to-head or tail-to-tail can occur, as in -CH2-CHCI-CHCI-CH2-CH2-CHCI-. 

A nickel-based Ziegler-Natta catalyst, Ni(Acac)2—Al(/-Bu)3, yields a regular allene polymer consisting almost exclusively of head-to-tail linked (1,2-inserted) monomeric units [441],... 

The cyclopolymerisation of unsymmetrical a, co-diolefins such as 2-methyl-1,5-hexadiene in the presence of catalysts such as Cp2 ZrMc2 M(Me)0 x, [Cp2 , ZrMe]+ [B(C6F5)4] or [Cp2 ZrMe]4 [McB(C6 F5)2] yields highly regiore-gular cyclopolymers [501]. The perfectly head-to-tail linked monomeric units in the formed poly[methylene-l,3-(l-methylcyclopentane)] arises from the chemo-selective insertion of the less hindered terminus of 2-methyl-1,5-hexadiene into the active Mt—P bond, followed by cyclisation involving the insertion of the disubstituted olefinic bond (Figure 3.50) [497]. The insertion of the disubsti-tuted olefinic bond is made easier by its intramolecular nature. 

The crystalline fraction was found to be formed via a cleavage of the C1g-0(CH2—O) bond in the monomer molecule and proved many years ago to be an isotactic polymer (with regular head-to-tail linkages) (Figure 9.1) [42]. The structure of the amorphous fraction, on the other hand, varies depending on the kind of catalyst. Some amorphous polypropylene oxide)s prepared with catalysts such as diethylzinc-methanol [43] or aluminium isopropoxide-zinc chloride [44] consist of regular head-to-tail linked units, but they are atactic (the mole fraction of isotactic diads is less than 0.6) [43]. Some other amorphous polypropylene oxide)s obtained with catalysts derived from reactions in the diethylzinc-water [44,45], and triethylaluminium-water [46] systems, and with aluminium isopropoxide [44], have been found to be irregular, i.e. to contain head-to-head and tail-to-tail enchained monomer units. 

Tetrabromocyclohexadienone has been shown to be an efficient alternative to A -bromosuccinimide for the preparation of terminal bromohydrins of polyenes like squalene/° A stereoselective synthesis of mokupalide (4), an unusual head-to-tail linked hexaprenoid from a marine sponge, is summarized in the Scheme. ... 

These are head-to-tail linked bases that tend to occur most commonly in the several genera of the Menispermaceae. 

Sutherland have confirmed an earlier report about the dimerization of iso-prene over a nickel-based catalyst, and have additionally shown that the proportions of the products [(10), (11), and (12)] are 78 8 14, the cyclo-octadiene (10) being more than 98% head-to-tail linked. Some interesting reactions of the cyclo-octadiene (10) are described. Dimerization of isoprene with nickel-ligand... 

A simple example should illustrate the concept. Consider the degradation of a polystyrene chain. We have chosen a segment with head-to-tail links including one head-to-head link. We consider now three mechanisms of degradation that can result in the formation of styrene. These mechanisms are shown in Figs. 18.5, 18.6, and 18.7. 

Isoprene rule Terpenes are composed of repeating head-to-tail-linked isoprene units. 

Rubber hydrocarbon is the principle component of raw rubber. The subject is discussed in greater detail in Chapter 7. Natural rubber is 97% cw-l,4-polyisoprene. It is obtained by tapping the bark of rubber trees (Hevea brasiliensis) and collecting the exudate, a latex consisting of about 32-35% rubber. A similar material can also be found in the sap of many other plants and shrubs. The structure of natural rubber has been investigated over 100 years, but it was only after 1920, however, that the chemical structure was elucidated. It was shown to be a linear polymer consisting of head-to-tail links of isoprene units, 98% bonded 1,4. 

During the course of chain polymerisation, head-to-tail linking is generally predominant, as illustrated in Figure 1.9. 

Figure 1.9 Usual head-to-tail linking (a), irregularities in poly(vinyl acetate) (b) and in poly(vinyl...

When poly(l-amido-1-R-ethylene) is dissolved in solutions with a pH less than 2, it starts to link adjacent amide units into 6-member rings. The reaction, imidization, for head-to-tail linked, repeat units is... 

Head-to-tail links, -CH2-CHCI-CH2-CHCI-, can be observed as well as head-to-head or tail-to-tail links, -CH2-CHCI-CHCI-CH2-CH2CHCI-. In copolymers, the situation becomes even more complex because the individual monomers can combine in many different ways resulting in statistical, random, alternating, periodic, block, and graft copolymers. As mentioned in Section 1.18.1, this situation required the development of a structure-based nomenclature and a source-based nomenclature system. 

Neolignans are head-to-tail linked dimeric phenylpropanoids, i.e., a Cj-side chain of one monolignol (coniferyl alcohol) is linked to the aromatic nucleus of a second one. This may proceed via C-O-C linkages, e.g., 8.0.4 -type. A comprehensive review on neolignans was published by Gottlieb (1978). 

The Michael-induced Ramberg-Backlund reaction has been used as the key step in a sequence for the homologation of head-to-tail linked conjugated isoprenoids (Scheme 84). Alternatively, homologation can be performed using the Cs-synthon (206) (Scheme 85), but yields are only moderate. ... 

The highest conversions of isoprene are obtained with palladium catalysts such as [Pd(PPh3)4], but the highest amount of terpenols was found with platinum catalysts as [Pt(PPh3)4] or [Pt(PPh3)2(CH2=CH2)j. The head-to-tail linked terpenoid a-linalool could be synthesized in selectivities up to 20%. 

When an unsymmetrical monomer such as styrene or vinyl chloride is polymerized, two propagating species are possible. A combination of steric and electronic factors will favor one of the two possibilities and will determine the stereochemical course of the reaction (see above, section on Polymer Structure and Nomenclature ). This results in head-to-tail linking. In coupling-termination reactions, however, head-to-head linking occurs. 

The fatty acids are primary metabolites, but are also the source of many secondary metabolites. They exist chiefly in nature as triglycerides (Figure 3.1), in which three molecules of fatty acids (usually a mixture of acids) are esterified to one molecule of glycerol. J. N. Collie Journal of the Chemical Society, 1907, 91, 1806) noticed that the acids of natural fats always contained even numbers of carbon atoms, and suggested that they are made by head-to-tail linking of acetic acid units. Later when radioactive isotopes became available, his suggestion was shown to be correct. 

---