2-phenyl-4-substituted derivatives, ring opening

Unlike other diene photopericyclic reactions, cyclohexadiene ring opening proceeds with reasonable efficiency in phenyl-substituted derivatives, although the quantum yields are generally significantly lower than those of non-arylated systems. Two examples of the conversion of 93 to 94 are shown in equation 36178,179... 

The reaction is highly regioselective for substituted cyclic ortho esters, with ring opening mainly at the primary carbon center. However, reversed reactivity is observed for the phenyl-substituted derivative. Corresponding 1,4- and 1,5-diol derivatives are obtained in reaction of six- and seven-membered cyclic ortho esters. 

Numerous papers have been published describing the photochemical production of enones from carbonyl compounds and alkynes. The first detection of an oxetene intermediate involved low-temperature (-78 C) photolysis of 2-butyne and benzaldehyde to form the photoproduct (61), which was observed by NMR. The oxetene undergoes further photoreaction with benzaldehyde to form the novel fused oxetane (62). Recently, Friedrich has reported Either studies on the reactivity of oxetenes and developed alternative syntheses of the parent compound and 3-phenyloxete (64). The parent oxetene is found to have a thermal half-life of t proximately 8 h in solution at room temperature. The phenyl-substituted derivative (64) underwent slow ring-opening under acidic conditions to form 2-phenylprq)enal and air oxidation to yield a formate derivative, probably via a radical process. 

There are very few homolytic reactions on triazolopyridines. A suggestion that the ring opening reactions of compound 1 involved free radical intermediates is not substantiated (98T9785). The involvement of radical intermediates in additions to ylides is discussed in Section IV.I. The reaction of radicals with compound 5 and its 1-substituted derivatives gives 4-substituted compounds such as 234 (96ZOK1085). A more detailed study of the reaction of the 1-methyl and 1-phenyl derivatives with r-butanol and ammonium persulfate produced 4-methyl substitution with a silver nitrate catalyst, and the side chain alcohol 235 without the catalyst (96ZOK1412). 

Substituted 1,2,4-triazoles have received less attention than their isomeric counterparts, but those results that are available indicate that their lithio derivatives are less stable than their 1-substituted isomers. Thus, in addition to undergoing the expected a-lithiation and reaction at the 5-position, 3-phenyl-4-benzyl-1.2.4-triazole also gave products resulting from ring-opening, even at -78°C (Scheme 65) (86JHC1257). 

The asymmetric alcoholytic ring opening of 4-substituted-2-phenyl-4,5-dihydro-l,3-oxazin-6-ones proved to be a efficient method for the preparation of enatiomerically pure /3-amino acid derivatives <2005AGE7466>. Treatment of 2,4-diphenyl-4,5-dihydro-l,3-oxazin-6-one 208 in the presence of the bifunctional chiral thiourea catalyst 211 resulted in formation of an enantiomerically enriched mixture of the unchanged oxazinone (iJ)-208 and allyl (4)-3-benzoyl-amino-3-phenylpropanoate 209. The resolved material (iJ)-208 and the product 209 could easily be separated by a selective hydrolytic procedure that converted oxazinone (iJ)-208 quantitatively into the insoluble iV-benzoyl /3-amino acid 210 (Scheme 37). 

The oxidation of AT-substituted 5//-dibenz[6,/] azepines with MCPBA is complex and depends upon the nature of the N-substituent. AT-Acyl derivatives do not form the N-oxide but suffer epoxidation of the 10,11-bond. AT-Aryl derivatives undergo hydroxylation of the phenyl ring, whereas N-alkyl congeners, with the exception of the AT-methyl compound, yield mixtures of diphenylamines and acridones. The N-oxide is obtained from the A/-methyl derivative along with ring-opened and ring-contracted products (81CPB1221). 

Carbon-13 shift values of parent heterocycloalkanes [408] collected in Table 4.61 are essentally determined by the heteroatom electronegativity, in analogy to the behavior of open-chain ethers, acetals, thioethers, thioacetals, secondary and tertiary amines. Similarly to cyclopropanes, three-membered heterocycloalkanes (oxirane, thiirane, and azirane derivatives) display outstandingly small carbon-13 shift values due to their particular bonding state. Empirical increment systems based on eq. (4.1) permit shift predictions of alkyl- and phenyl-substituted oxiranes [409] and of methyl-substituted tetrahydropyrans, tetrahydrothiapyrans, piperidines, 1,3-dithianes, and 1,3-oxathianes [408], respectively. Methyl increments of these heterocycloalkanes are closely related to those derived for cyclohexane (Table 4.7) due to common structural features of six-membered rings. 

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