2H-Pyran ring

Horley AG, Mahon EM, Muhammed M, Molloy CK, Haycock PW, Meyers CP. Antimony (1-diketonates and alkoxide/p-diketonates remarkahle formation of a 3,4-dihydro-2H-pyran ring by couphng of 1,1,1,5,5,5-hexafluoro- 2,4-pentanedione ligands. J Chem Soc Dalton Trans 2002 4416-21. 

H-pyran ring, 3,4-dihydro-, 2-amino- 43,702 Alder-Bong reaction... 

Spiro-2H-chromenes of type 15 show the phenomenon of photochromy on irradiation, the 2H-pyran ring is opened reversibly at the O-C-2-bond in a ring-chain valence tautomerization a colored zwitterionic merocyanine system 16 is formed, which can undergo spontaneous thermal re-cyclization to the spirocompound 15 [28] ... 

Intermediates. 3,4-Dihydro-2H-pyran [110-87-2] is prepared by a ring-expanding dehydration of tetrahydrofurfuryl alcohol. It is used as a protecting agent for hydroxyl compounds and as an intermediate. 2-Methylfuran is a chemical intermediate for 5-methylfurfural [620-02-0] (151) and... 

In connection with studies on the ring-opening polymerization of cyclic acetals, we have undertaken investigations on the polymerization of bicyclic acetals, bicyclic oxalactone, and bicyclic oxalactam, which yield polysaccharide analogs, macrocyclic oligoesters, and a hydrophilic polyamide, respectively, some of which can be expected to be useful as novel speciality polymers. The monomers employed in the studies were prepared via synthetic routes presented in Scheme 1, starting from 3,4-dihydro-2H-pyran-2-carbaldehyde (acrolein dimer) I. 

Studies of 6-aminopyranopyrazoles 11 (02CHE1180) show that they always exist in the crystal phase as a 2H (11a), instead of a IH-tautomer (11b), as considered earlier. Molecules are joined into dimers by H-bonds, NH2 Nl, which, in turn, form ribbons as the result of inter-molecular binding of the N(2)H and N=C groups. Pyrazole and pyran rings together (except for O and C4) are planar (dihedral angle is as low as 1.6°). 

As mentioned above, the main problem with the 2H-naphtho[l,2-fc]pyran ring system (1.20) is the very slow rate of fading back to the colourless state (1.20 R, ... 

A facile synthesis of cyclobutylmethanols has been devised by reacting 2-ethoxy-5-alkyl-3,4-dihydro-2H-pyrans with aluminum alkyls (Scheme 149) (80TL4525). When (648) is reacted with triisobutylaluminum the cyclobutylmethanol (649) is formed quantitatively. While several mechanisms have been proposed for this process, initial rupture of the carbon-oxygen bond of the pyran ring to form an aluminum enolate, which then undergoes ring closure and reduction, appears to be most likely. 

The reaction of pyrylium salts with sodium methoxide was shown to give 2H-pyrans in part by virtue of the lack of symmetry of the 13C spectra (79H(12)775). The ring carbon atoms of 2-amino-2/f-pyrans are shielded relative to the analogous 2-methoxy compounds. Phenyl groups attached at C-4 of 2H-pyrans are deshielded at C-T in keeping with the non-aromatic nature of these pyrans (81CS(18)256). 

An anomeric effect is observed in 3,4-dihydro-2H- pyrans. For example, a 2-alkoxy group preferentially occupies an axial position (71DOKd96)367>. Indeed, a study of the NMR spectra of some 2-alkoxy-3,4-dihydro-2//-pyrans and their 4-methyl derivatives established that the anomeric effect was more important in the unsaturated heterocycles than in the corresponding tetrahydropyrans (72BSF1077). The axial preference of an alkoxy group is even more accentuated when the double bond is associated with a fused benzenoid ring, as in the 2-alkoxychromans. It is also of interest to note that the role of the polarity of the solvent on the conformational equilibrium is less important than for the saturated analogues. 

The thermal ring opening of the cyclobutene (16) also leads to a 2H-pyran (17) and this too has been trapped as a Diels-Alder adduct (69TL4987). 

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