Electrophilic and Oxidative Reactions

With regard to the acid-catalyzed isomerization, 9,10-bridged anthra-cenophanes are capable of isomerizing to methylenedihydroanthracene (isotoluene) type isomers, since the energies of the tautomers would become more favorable as the size of the bridge decreases. Indeed, our attempt to synthesize 

Addition of bromine to [6]paracyclophane (la) took place in the 1,4-fashion to give unstable adduct 93a (Structures 16) [5b]. Similarly 1,4-adducts 93b-c (Structures 16) were obtained by reaction of Ij (Structures 2) with bromine and 

Addition of 2 mole of dichlorocarbene to the [6]paracyclophane derivative Ig (Structures 2) gave the cyclooctatetraenophane 105 as the major product together with the [8]paracycloph-l-ene 106 (Structures 20) [72]. Sequential addition of the carbene, first to Ig to give a cycloheptatriene followed by the second one to this intermediate, accounts for the formation of the products. 

The central ring of bridged anthracene should be even more reactive indeed, air oxidation of dithia[8]anthracenophane 42b (Structures 4), whose aromatic ring is only slightly deformed from planarity, was observed giving an-thraquinone presumably through an endoperoxide of 42b [37]. 

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Reactivity (General Topics, Reactions with Electrophiles and Oxidants, Reactions with Nucleophiles and Reducing Agents, Reactions toward Free Radicals, Carbenes, etc., Reactions with Cyclic Transition State, Reactivity of Substituents, Heterocycles as Intermediates in Organic Synthesis). 

A few typical examples of electrophilic and oxidative reactions are summarized in Scheme 8.5.6 bromination usually gives substitution rather than addition products, whereas in nitration reactions one also observes electrophilic substitution and the ribose must be protected. Hydroxy methylation with formaldehyde occurs preferrably at C5 of uridine. 

The high stability of porphyrins and metalloporphyrins is based on their aromaticity, so that porphyrins are not only most widespread in biological systems but also are found as geoporphyrins in sediments and have even been detected in interstellar space. The stability of the porphyrin ring system can be demonstrated by treatment with strong acids, which leave the macrocycle untouched. The instability of porphyrins occurs in reduction and oxidation reactions especially in the presence of light. The most common chemical reactivity of the porphyrin nucleus is electrophilic substitution which is typical for aromatic compounds. 

The reactivity of 464 was also used to prepare perhydrofuropyrans with the same methodology as shown in Scheme 129, but with an epoxide as second electrophile. Diols 467 were obtained in 35-68% yield which, after hydroboration and oxidation reactions (as shown in Scheme 128), gave perhydrofuropyrans 468 in 57-64% yield. This heterocyclic unit is present in many natural products, azadirachtins being probably the most important ones. ... 

Substituted 5-aminotetrazoles Reactions with electrophiles and oxidation... 

REACTIONS WITH ELECTROPHILES AND OXIDIZING AGENTS 2.07.3.1 Piperidines... 

The reaction of piperidine derivatives with electrophilic and oxidizing agents has been an effective method for functionalizing the positions adjacent to the nitrogen atom. This can be a useful synthetic process and a number of these methods are presented below. 

The vast majority of the reactions of the 5-5 fused systems have been the alkylation of nitrogen atoms by electrophiles and oxidants. Most of the examples have been in the alkylation of glycoluril derivatives and the general methods have been extensively covered in CHEC(1984) and CHEC-II(1996) and as such do not generally require comment here. Some of the alternative and less common methods however are outlined below. 

In 1,3-benzazoIes (benzoxazoles, benzothiazoies, benzoselenazoles) the heteroatom that is not nitrogen shows very low nucleophilicity and does not serve as a reaction center in electrophilic addition and oxidation-addition reactions. By contrast the tellurium atom in benzotellurazoles is quite susceptible to attack by electrophiles and oxidants. 

Electrophile Oxidation and Oxidative Reactions of Sulfur and Selenium... 

The ring carbon atoms of all possible l,4-(oxa/thia)-2-azoles are not expected to be attacked by electrophiles and such reactions have not been reported. A few known examples concerning alkylations or protonations at the ring nitrogen of certain types of dithiazoline derivatives, as well as oxidation of ring sulfur of certain 1,4,2-oxathiazolines to give sulfoxy compounds, have been discussed in CHEC-II(1996) <1996CHEC-II(4)519>. 

L.J. Mamett et al., Endogenous generation of reactive oxidants and electrophiles and their reactions with DNA and protein. J. Clin. Invest. Ill, 583-593 (2003)... 

B. D. Dombeck, and R. J. Angelici, Electrophilic and Oxidative Addition Reactions of Tungsten Thiocarbonyl Complexes, Inorg. Chem. 15, 2397-2402 (1976). 

The anionic tetrahedral clusters [Fe3(CO)9(/u,3-E)]2 (E = S (52), Se (53), Te (48)) can be obtained from the thermolytic reaction of K2E with [Fe(CO)5] in DMF. Although a lone pair of electrons is present on E, investigations of the reactivity of electrophiles and oxidants toward the lone pair of electrons on the apical Te atom in 48 suggest strongly that the apical Te atom is unreactive toward electrophiles and that reaction occurs at metal centers (Scheme 3).50... 

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