Aromatic sequence

R1 = (het)aryl, R2 = (het)aryl, C02Et Scheme 6.243 Biomimetic hetero-Diels-Alder-aromatization sequences. 

The aroyl-substituted heterocyclic ketene aminals 482 react with 4-chlorophenyl azide to give polysubstituted 1,2,3-triazoles 483 and imidazo[ 1. Z-r 1.2,4]triazoles 39 (Equation 112) <2000HAC387>. Polysubstituted 1,2,4-triazoles are formed by the nucleophilic attack of the ct-carbon of the azide. Then, through the cyclocondensation and aromatization sequences, the fused heterocycles resulted by a 1,3-dipolar addition at first, and then through a Dimroth rearrangement and deamination of chloroaniline <1992JOC184>. 

Advantage has been taken of the propensity for equilibrium lithiation demonstrated for diethyl nicotinamide (447a) which has been induced under a double self-condensation to form the 2,7-diazaanthraquinone (452) (Scheme 136) (88S388). The quinone 452 was transformed by a reduction-aromatization sequence into the pyrido[3,4-g]isoquinoline (453) in high overall yield. 

Another theoretical criterion applied to estimation of aromaticity of homo- and heteroaromatic ring system is aromatic stabilization energy (ASE). Based on this approach, the aromatic sequence of five-membered ring systems (ASE in kcal mol-1) is pyrrole (20.6) > thiophene (18.6) > selenophene (16.7) > phosphole (3.2) [29], According to geometric criterion HOMA, based on the harmonic oscillator model [30-33], thiophene is more aromatic than pyrrole and the decreasing order of aromaticity is thiophene (0.891) > pyrrole (0.879) > selenophene (0.877) > furan (0.298) > phosphole (0.236) [29],... 

Additions of (478) to furans which contain an appropriately placed aryl or vinyl bromide unit permit the construction of polycyclic systems by a Diels-Alder/radical cyclization/aromatization sequence (Scheme 116). ... 

Three separate laboratories have attached an acetylenic group to the C-19 methyl that is normally hydroxylated by aromatase in the demethylation-aromatization sequence and have shown that the resulting sterols Inactivate the placental enzyme.It has also been reported that norethisterone, a 17-ethinyl sterol shown earlier to alkylate the prosthetic heme of hepatic cytochrome P-450,inactivates placental aromatase.Aromatase is furthermore irreversibly inactivated by the analogue of androst-4-ene-3,17-dione with an allene moiety instead of a methyl group at C-10. Other catalysis-dependent irreversible inhibitors of aromatase act by mechanisms which probably do not Involve the prosthetic heme,10 H0 li3... 

Structure of polyphthalamides PPA-1 and PPA-2 represents alternation of aliphatic and aromatic sequences. 

To check the influence of the length of the aliphatic and aromatic sequences in the chains, we synthesised copolyesters of different structure, such as (blends), block copolymers, random copolymers and alternating BTA copolyester, all with the same fraction of terephthalic acid (50 mol % of the total acid components) (Fig 5). 

Figure 5. Aliphatic and aromatic sequences in copolyesters with different structure...

MBPE-9. Macromolecules that contain aliphatic and aromatic sequences in their backbone structure have special possibiUties to exist as liquid crystals or as condis crystals (see Sect. 2.5). The thermal analysis of a polyether based on the semi-flexible mesogen l-(4-hydroxyphenyl)-2-(2-methyl-4-hydroxyphenyl)ethane is copied in Fig. 2.68. The computed vibration-only heat capacity is reached somewhat... 

The isotropization enthalpy of the homologous series of MBPEs is compared in Fig. 5.150 with DBA with flexible spacers. One notes immediately that the aromatic sequence of MBPE, in contrast to DBA, does not seem to contribute to the ordering... 

The HPX polymer of Fig. 5.149 has a rather flexible aromatic sequence, and, indeed, the polymer behaves like a typical semicrystalline polymer, just that the aromatic and aliphatic sequences cannot interdigitate in a satisfactory crystal structure... 

On the other hand, it has been established that aliphatic-aromatic copolyesters are indeed biodegradable and that the biodegradability of these copolyesters is related to the length of the aromatic sequence. Block copolyesters with relatively long aromatic sequences are not rapidly degraded by microorganisms. 

Synthesis of 2,3-disubstituted ffirans can be achieved from the readily available BayUs—Hillman adducts 28a—d (05EJOC1969). Mixed acetals 29a—d can be prepared by the previously described conditions. Following the RCM/aromatization sequence, ffirans 30a—d were obtained in moderate to good yields (Scheme 6). Additionally, exploiting the inherent reactivity of ftuans, the C-5 position can be ffinctionalized by several approaches such as (1) electrophilic substitution reactions and (2) a-metalation followed by (a) electrophilic quenches or (b) palladium-catalyzed couplings. 

Formal a-diarylmethylation of malonates was achieved through the development of a 1,6-selective conjugate addition-aromatization sequence with para-quinone methides as vinylogous electrophiles (Scheme 4) [8]. The driving force toward aromatization would make nucleophilic addition to the 8-position more thermodynamically favorable than addition to the p-position. Under solid-liquid biphasic conditions, the A-spiro chiral ammonium salt 5 appeared to be the most active and selective promoter. The electronic character of the aryl ester substituents on the malonate was linked to the stereochemical outcome, and introduction of a para-halophenyl group led to a significant decrease in the enantioselectivity. Based... 

Chamizo, Morgado, and Sosa put forward the absolute hardness, as calculated by the HOMO— LUMO gap, as a measure of the aromaticity of organometallic compounds. They calculated the hardnesses for a series of metallacycles containing W, Co, Ti, Fe, and Ir and a series of 15 heterobenzenes and heterocyclopentadienes. For the latter, they found that the dividing line between aromatic and nonaromatic species approximately corresponds to 1.28 eV. They, moreover, found the aromaticity sequence... 

Synthesis of C3 functionalized 2-unsubstituted benzo[l ]furans has been developed by a cyclocarbopalladation/cross-coupHng/aromatization sequence (13JOC4490). Substituted benzo[f>]furans were formed via the hydroxylation of2-haloalkynylarenes (130L5032).Palladium(0)-catalyzed insertion of C—C bonds into benzylic C(sp )-H bonds provided access to 2-methylene-2,3-dihydrobenzo[l ]furans, which transformed into... 

Ellman, Bergman, and coworker reported a rhodium-catalyzed procedure for the synthesis of pyridines from alkynes and a,/ -unsaturated N-benzyl aldimines and ketimines in 2008 [107]. The reaction proceeded via C-H alkenylation/electrocyclization/aromatization sequence through dihydropyridine intermediates. The C-H activated complex was isolated and determination by X-ray analysis. Good yields of highly substituted pyridines were produced in one-pot manner (Scheme 3.50). 

This phenomenon is attributed to the melting point of the material - a correlation already demonstrated by Tokiwa and co-workers for different aliphatic polyesters [79]. In aliphatic-aromatic copolyesters, the melting behaviour is mainly determined by the length of the aromatic sequences in the polymer chains, which depends both on the composition and structure [86, 87]. For many aliphatic polyesters, a correlation of the degradability with the melting point was observed [2]. Marten [86] interpreted it as a decrease in the mobility of the polyester chains at lower temperatures in this situation the polymer chains are highly fixed in the polymer crystals and cannot adjust easily into the active sites of the extracellular enzymes. A random insertion of some aromatic monomers in aliphatic polymer chains disturbs the formation of crystals. 

A major point of criticism for aliphatic-aromatic copolyesters is the final degradability of the aromatic sequences in the polymers. In such statistical copolyesters there are domains in the polymer chains, where several aromatic dicarboxylic acids are linked with the alcohol component, without being interrupted by an aliphatic dicarboxylic acid. The distribution of sequence lengths depends on the ratio of aliphatic and aromatic dicarboxylic acids, and can be calculated for an ideal random copolymerisation using (Equation 10.1) ... 

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