Heterocycles dihydro benzopyrans

In contrast to the benzoannelated nitrogen heterocycles, only a few kinds of benzoannelated systems without nitrogen have been described (Fig. 3.15). Five-mem-bered rings such as benzo[b]furans (336) [184, 461] and hydrobenzo[b]furans (337) (Scheme 3.41) [337, 462, 463] and six-membered rings like 4H-l-benzopyran-4-ones (4-chromones) (338) and 2H-l-benzopyran-2-ones (coumarines) (339) [96, 464, 465] 2,3-dihydro-4H-l-benzopyran-4-ones (chroman-4-ones) (340) [466] and 3,4-dihydro-lH-2-benzopyrans (isochromans) (341) [444] have been reported. 

Morpholino-li/-2-benzopyran-l-one (208) adds aromatic aldehydes in hot acetic acid or acetonitrile to form dihydro-4-morpholinocarbonyl-l/7-2-benzopyran-l-ones 209 as mixtures of cis- and trans-isomers. The process is formally an ene reaction in which an acyl group is transferred (equation 88). Analogous reactions with imines, phenyl isocyanate, nitrosobenzene, arenediazonium salts and carbon disulphide lead to a variety of heterocycles (Scheme 7)110. 

Lu and Malinkova researched the regio- and diastereoselective insertion of allenes into stable oxapalladacycles giving 3,4-dihydro-2H-l-benzopyrans. The insertion of allenes into Pd-G bonds of cyclopalladated pyridine derivatives was studied giving novel cationic heterocycles (berberinium derivatives). ... 

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