Dehydrohalogenation, ring-closure

Dehydrohalogenation has been utilised successfully for certain ring closure reactions. 

Ring closure to the cis-trans isomers of the thietanes can be achieved by intramolecular dehydrohalogenation of 1,3-substituted chlorothiols (Eq. 13). 

Ring closure by an intramolecular acylation reaction followed by a halogenation-dehydrohalogenation sequence (equation 70) may account for the isolation of the... 

Such ring closures have been effected by a variety of methods generally involving either the direct cyclodehydration of the acids or the cyclo-dehydrohalogenation of the acid chlorides by an intramolecular Friedel-... 

The a-methyl derivative is produced from the acetate of o-allylphenol, HOC4H6CHjCH= CHj, by the addition of hydrogen bromide in the absence of peroxides followed by dehydrohalogenation of the free phenol with potassium hydroxide. In the presence of air or peroxides the mode of addition of hydrogen bromide is reversed and cyclization gives benzo-pyran. /3-Keto derivatives ate prepared by a similar ring closure. ... 

When this halogenated compound is treated with ethanolic ammonium hydroxide for several days, as in Experiment [F3], the system undergoes several reactions (dehydrohalogenation, amination, and ring closure) to ultimately yield a trans-substituted aziridine product ... 

Ring-closures may result not only from dehydrohalogenation but also through loss of water from two ds-ligands. On heating the rhodium(III) complex, Na[Rh(OH)2(HL)2] H20 (HL = deprotonated bidentate iminodiacetic... 

---