Benzopyran derivatives, heterocyclic synthesis

The synthesis of a variety of other heterocycles has been achieved using similar methodology [108], For example, benzopyran derivatives are readily prepared from 2-allyl phenols [110] note that the Pd( 0) precatalyst is oxidized to Pd(ll) by air before the reaction commences. Nitrogen heterocycles are also accessible from Wacker-type transformations, as demonstrated by the Hegedus indole synthesis described below in Section 1.10. 

The synthesis of heterocycles by means of Lewis bases has been carried out using the reactions described above. For instance, the dipole resulting from the reaction of pyridine with DMAD was intercepted with phenylisocyanate giving a 1,4 dipole that ultimately led to pyrimidindione derivatives. Wang and coworkers have described the synthesis of benzoxazoles by reaction of ynals with N-protected-2-aminophenols using pyrrolidine as catalyst. Salicylaldehyde has been the starting material for the synthesis of many benzoheterocycles. When using DBU as catalyst, it was transformed into benzopyran derivatives by reaction with 2,2-disubstituted allene esters. When DABCO is used as catalyst, the reaction of AT-tosylimines with ethyl... 

Nitrile oxides have been used extensively in heterocyclic synthesis. They are readily generated from oximes, using sodium hypochlorite under phase-transfer conditions, in high yields. They are versatile, lending themselves to one-pot procedures and reaction in situ with dipolarophiles. Lee discusses a wide variety of their reactions, notably the intramolecular reaction in which the benzopyran derivatives (48) are formed in yields that are more than double those previously reported. 

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