Benzopyrans

About: Benzopyrans is a research topic. Over the lifetime, 694 publications have been published within this topic receiving 8830 citations.

Radical-Promoted Cyclizations of Aromatic Carbonyl Compounds to Benzopyrans Using Titanocene(III) Chloride.

Abstract: Aromatic carbonyl compounds undergo smooth intramolecular radical cyclization with alkenes or alkynes using titanocene(III) chloride to furnish the corresponding benzopyrans. The radical initiator, Cp2TiCl, was prepared in situ from commercially available titanocene dichloride (Cp2TiCl2) and zinc dust in THF under argon.

Regio‐ and Diastereoselective Insertion of Allenes into Stable Oxapalladacycles with a Metal‐Bonded Stereogenic Carbon. Preparation of Contiguously Substituted 3,4‐Dihydro‐2H‐1‐Benzopyrans.

Abstract: Insertion of monosubstituted allenes into stable oxapalladacycle I was studied. The aim of this work was to define steric and electronic parameters of allenes that would allow for a regio- and diastereoselective synthesis of 2,3-disubstituted 3,4-dihydro-2H-1-benzopyrans, which could not be prepared via related catalytic protocols. Allenes with electron-donating alkyl substituents R sterically unencumbered at the C-3 and C-4 carbons reacted with palladacycles I to afford benzopyrans IV in good yields (45-81%), exclusively as cis diastereomers. Less than 10% of the regioisomeric benzopyrans V was detected in the crude reaction mixtures. Methoxy 1,2-propadiene afforded benzopyran IV in 98% yield as the trans diastereomer in 92% de. In contrast, allenes with electron-withdrawing substituents yielded benzopyrans V with an E double bond exclusively. Nonracemic palladacycles featuring a palladium-bonded stereogenic carbon as the only element of asymmetry underwent the allene insertion with 63-93% retention of the stereochemical information, providing benzopyrans IV or V in 40-47% ee. These results demonstrated that O-bonded palladium enolates did not operate as predominant intermediates in the insertion process. The study highlights the configurational stability of carbon-bonded palladium ester enolates, especially notable in systems lacking chiral nonracemic auxiliary ligands.

Adventures in Search of New Dyes Absorbing in the Red or Near Infrared Region

Abstract: Unexpected results were obtained when trying to synthesize new long wavelength absorbing methine dyes starting from bridged benzopyrans or bridged aniline derivatives. Thus, by reaction of mineral acids with bridged 2-amino-substituted benzopyrans 21 instead of corresponding bridged benzopyrylium salts 15 dimers of the intermediate bridged 2-alkylidene benzopyrans 22 are formed. Analogously, reaction of the bridged aniline and 1-naphthylamine derivatives 62, 65, 68, and 72 with nitrous acid yields 4-aminophenyl substituted quinolines or quinolinium salts 64, 70, or 74, respectively. Unexpected results also have been obtained in course of the synthesis of 2,4-diamino-thiazoles 36 and 2-amino-selanzoles 41 which, by analogy to the 2-aminosubstituted heterocyclic compound of the general structure 48, are versatile educts for the synthesis of the new methine dyes 50-53 and the azamethine dyes 57 and 58. Their spectral properties are recorded for some representative examples.

Synthesis of Heterocyclic-Fused Benzopyrans via the Pd(II)-Catalyzed C—H Alkenylation/C—O Cyclization of Flavones and Coumarins.

Abstract: An efficient and practical method for effecting a tandem C-H alkenylation/C-O cyclization has been achieved via the C-H functionalization of flavone derivatives. The synthetic utility of the one-pot sequence was demonstrated by obtaining convenient access to coumarin-annelated benzopyrans. The reaction scope for the transformation was found to be fairly broad, affording good yields of a wide range of flavone- or coumarin-fused benzopyran motifs, which are privileged structures in many biologically active compounds.