Coumaran ring

Further degradation by side chain oxidation and opening of phenyl coumaran ring... 

The phenyl coumaran, dihydro-dehydro-di- oeugenol (XXVIIa) and its methyl ether (XVIIb) reacted via cleavage of the B-ring, forming a dilactone (Figure... 

The ether containing the five-membered ring is more stable entropically and therefore it is the a-bromide that underwent the Williamson reaction. The fact that the a-bromide used its phenol group in the reaction is verified by the product s insolubility in base. Had this not been true, the hydroxyl group would lose its proton to form the anion and therefore dissolve. The product, coumarane, is therefore ... 

Scope and Limitations. Carbonyl Component. For practice purposes this reaction is limited to aromatic aldehydes, of which a wide variety has been studied, and to a,/3-unsaturated aliphatic aldehydes. The sub-stitutents on the ring of the aromatic aldehydes include alkyl, fluoro, chloro, bromo, iodo, hydroxyl, alkoxy, acyloxy, carbethoxy, nitro, and various combinations of two or more of these groups. Aldehydes of the naphthalene, pyrene, biphenyl, thiophene, furmi, pyrrole, indole, chro-mane, coumarane, and thiazole series also have been employed. 

Nucleophiles other than amines or hydroxyl can cause ring openings analogous to the Zincke reaction. With acetone and sodium carbonate, (139) gives i (141), and in related reactions l-(2,4-dinitrophenyl)pyridinium chloride and active methylene compounds yield cyanines, while pyridine, coumaran-2-one and an aroyl chloride give (142). 

Figure 6. One-dimensional potential energy function for the ring-puckering of coumaran.

---