Aromatization thiophene ring

Duloxetine hydrochloride is an example of an aryl ether that is particularly unstable to hydrolysis under acidic conditions (84). The acid instability led to the development of an enteric-coated formulation to protect the compound from the acidic environment of the stomach. The reason for the susceptibility to hydrolysis is the stability of the cationic intermediate (Fig. 70), which is stabilized by delocalization into the aromatic thiophene ring. See Chapter 2 for additional discussion of the chemistry of this compound. 

Monomers such as 109a and 109b with two aromatic thiophene rings, which were prepared by Knoevenagel-... 

Jonsson et al. used synchrotron radiation to study the kinetic energy of photoelectrons from the S(2p) core level in PEDOTPSS films in electron spectroscopy for chemical analysis (ESCA). Photoelectrons from the S(2p) core level can be used as a quantitative measure of sulfur atoms in the aromatic thiophene ring of PEDOT and sulfur atoms in the sulfonic acid unit of PSS in the top layer of the film. By variation of the energy of the photons, the escape depth of photoelectrons from the polymer film can be tuned and a depth profile of the film can be obtained. Using this technique the authors demonstrated that pristine PEDOTPSS films have a PSS rich surface. With a photon energy of 270 eV, 95% of the signal stems from the outmost 15 A and a clear dominance of PSS is observed (see Table 9.4). When NMP and sorbitol are used as additives and the film is dried at room temperature, no PSS rich... 

Acetylenic dienophiles have also been used in conjunction with a different type of diene (150) for the combinatorial synthesis of isoindolones 151 fused with furan, pyrrole or thiophene rings (Fig. 30) [129]. In this case, the diene involved in IMDA is not the 5-membered heterocycle itself, but the one including the exocyclic double bond. The resulting dihydrobenzene is easily aromatized by oxidation with DDQ or C/O2. The synthetic protocol is accomplished in a one-pot process. 

Destroying the aromatic character of the thiophene ring has been accomplished by heating the 5-ammothieno[2,3-tf -pyrimidine 66 with orthophosphoric acid (Equation 27) the 5-oxo product 67 is obtained <2005CHJ211>. 

Thiophene is a sulphur-containing five-membered unsaturated heterocycle. The lone pair electrons of the sulphur are in the 35 orbital, and are less able to interact with the tt electrons of the double bonds. Therefore, thiophene is considered weakly aromatic. Acetylenic thiophene is found in some higher plant species. However, the thiophene ring is present in many important pharmaceutical products. 

It appears that the inhibition by both H2S and aromatic hydrocarbons involves competition between the inhibitor and the reactive sulfur compound for adsorption on the active site. However, inhibition could also be the result of occupancy of one or more of the bonding orbitals of the Co(Ni) by some nonreacting molecule, such as H2S or naphthalene. This would prevent the oxidative addition of the thiophene ring to the Co(Ni) in a mechanistic sequence such as that described in Figs. 

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