Ring initiators

The two isomeric possibilities are 1,2- and 2,1-benzisoxazole. Both are preferentially halogenated by electrophilic halogen in the homocyclic ring, initially in the 5-position, although substituents can modify this behavior [67AHC(8)277]. Nucleophiles attack the heteroring (84MI26). 

Acyl radicals can be generated and they cyclize in the usual manner. A polyene-cyclization reaction generated four rings, initiating the sequence by treatment of a phenylseleno ester with Bu3SnH/AIBN to form the acyl radical, which added to the first alkene unit. The newly formed carbon radical added to the next alkene, and so on. Acyl radicals generated firom Ts(R)NCOSePh derivatives cyclize to form lactams. ... 

Hydroxylation of the aromatic ring initially produces dopa (3,4-dihydroxyphenyl-alanine). This reaction uses the unusual coenzyme tetrahydrobiopterin (THB). Dopa (cf p. 6) is also used in the treatment of Parkinson s disease. 

Fig. 42 Ring expansion polymerization of lactone with spiro-ring initiator...

The DDQ-mediated transformations of 180a-c are oxidative couplings that may actually proceed through nucleophilic capture of a benzylic cation (or an equivalent thereof) 193 by the adjacent aryl ring. Initial one-electron aryl oxidation (180 —> 191) followed by proton loss (191 —> 192) and subsequent one-electron removal may precede benzyl cation 193 formation, as shown in Scheme 46 [129]. 

The 2-alkylhydroanthraquinone (A) when in contact with the catalyst will undergo a small amount of catalytic reduction (B) on the ring, initially on the unsubstituted ring, yielding a tetrahydroalkylanthrahydroquinone. Unfortu-... 

The resulting cyclic product 197 contains an unexpected seven-membercd ring due to the insertion of an oxygen atom attributed to the opening of the intermediate six-membered ring, initially formed by aminomethylation, and elimination of the halogen atom. 

Acyl radicals can be generated and they cychze in the usual manner.poly-ene-cyclization reaction generated four rings, initiating the sequence by treatment... 

Conformational drawuigs reveal that the product has the larger group equatorial but it is not so easy this time to draw good conformations for the cyclization itself. We need to get the p orbitals of the alkenes pointing at each other. The best we can do is to form the new six-membered ring initially in a twist-boat conformation and flip it into a chair afterwards. You may well have done better. 

Photoadditions of this type frequently give rise to four-membered rings initially <80AHC(27)24i>. Photooxidations, which probably proceed by cycloaddition of singlet oxygen, have been discussed earlier (see Section 3.02.5.3.11). 

These authors postulate that even in the stronger acid solutions the initial product is the OH adduct. This then undergoes acid-catalyzed hetero-lytic bond fission in which the significant step is the loss of OH" from the ring, initiated by an electron switch from the side-chain, leading ultimately to a resonance-stabilized radical—e.g... 

The fully iniramolecular Pd-catalyzed cascade cyclization of 2-bromo-l-ene-n,m-diynes is a reliable access to tricyclic system in which the central benzene moiety is formed by a formal [2 + 2 + 2] assembly as long as both rings initially formed by intramolecular carbopalla-dation are flve-membered or at most six-membered (Scheme The yields range from... 

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