The 1,6-conjugate addition of nucleophiles to dienyl diketones produces either cyclopentenone or 2H-pyran products with high selectivity through either Nazarov (4π) or 6π electrocyclization, respectively. The outcome of the reaction is dependent upon the nature of the nucleophile used. Nucleophiles that are anionic or easily deprotonated exclusively produce cyclopentenones via Nazarov cyclizati...

The mol-ecular structure of the title compound, C(19)H(20)O(2), reveals a slightly distorted chair conformation for the tetra-hydro-pyran ring with the two methyl and two phenyl substituents in equatorial positions.

In the title compound, C(17)H(14)O(3), the pyran ring adopts a boat conformation and the dihedral angle between the aromatic ring planes is 59.1 (1)°. In the crystal structure inter-molecular C-H⋯O hydrogen bonds and C-H⋯π inter-actions link the mol-ecules.

In the title compound, C(19)H(19)NO(2), commonly called koenimbine, the pyran ring adopts a sofa conformation. The carbazole ring system is planar [r.m.s. deviation = 0.063 (1) Å]. A C(10) zigzag chain running along the b axis is formed through inter-molecular C-H⋯O hydrogen bonds. The chains are linked via weak C-H⋯π and N-H⋯π inter-actions.

Asymmetric allylic alkylation is a powerful reaction that allows the enantioselective formation of C-C bonds. Here we describe the asymmetric alkylation of alkylzirconium species to racemic 3,6-dihydro-2H-pyrans. Two systems were examined: 3-chloro-3,6-dihydro-2H-pyran using linear optimization (45-93% ee, up to 33% yield, 5 examples) and 3,6-dihydro-2H-pyran-3-yl diethyl phosphate with the ass...

In the title compound, C(22)H(21)NOS, the thio-pyran-one ring adopts a chair-like conformation with the substituent in the axial position. The relative configuration of the racemic compound is 3R,7S according to the numbering scheme used in this publication. In the crystal packing, centrosymmetric dimers are built up via N-H⋯O hydrogen bonds, with graph set R(2) (2)(8).

The title compound, C(14)H(19)NO(6), was synthesized by the condensation reaction between hecilid (4-formyl-phenl-β-d-allopyran-oside) and methyl-amine in methanol. In the crystal structure, the pyran ring adopts a chair conformation and adjacent mol-ecules are linked by inter-molecular O-H⋯O and O-H⋯N hydrogen bonds, forming a three-dimensional network.

The regiodivergent ring contraction of diastereomeric 2-silyl-5,6-dihydro-6-aryl-(2H)-pyrans via [1,2]- and [1,4]-Wittig rearrangements to the corresponding α-silylcyclopentenols or (α-cyclopropyl)acylsilanes favor the [1,4]-pathway by ortho and para directing groups in the aromatic appendage and/or by sterically demanding silyl groups. The [1,2]-pathway is dominant with meta directing or elect...

Hantzsch condensation of two equivalents of methyl-3-aminocrotonate with (m- and p)-methoxybenzaldehyde afforded the expected products 2,6-dimethyl-3,5-dimethoxycarbonyl-4-(m-methoxyphenyl)-1,4-dihydropyridine and 2,6-dimethyl-3,5-dimethoxycarbonyl-4-(p-methoxyphenyl)-1,4-dihydropyridine, whereas o-methoxy-benzaldehyde produced mainly 1-amino-2-methoxycarbonyl-3,5-bis(o-methoxy-phenyl)-4-oxa-cy...

The pyran ring of the title compound, C(13)H(18)N(2)O(3), is almost planar (r.m.s. deviation = 0.059 Å). The crystal packing is stabilized by N-H⋯O and N-H⋯N hydrogen bonds.