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Abstract
In this review, we discuss the nature of the different physicochemical factors affecting the valence isomerism between 2H-pyrans (2HPs) and 1-oxatrienes, and we describe the most versatile synthetic methods reported in recent literature to access to 2HPs, with the only exception of 2HPs fused to aromatic rings (i.e., 2H-chromenes), which are not included in this review.
Highlights
In we discuss discuss the the nature physicochemical factors factors affecting the Abstract: In this this review, review, we nature of of the the different different physicochemical affecting the valence isomerism between
Other structural factors, such asasannulation, the ItIthas well established annulation favors the closed form by restricting conformational freedom (entropic established that annulation favors the closed form by conformational freedom (entropic established that annulation favors closed form byinrestricting conformational freedom,6 trap), and it has been used as a the design element therestricting synthesis of stable 2HPs
The most most common for synthesizing these heterocycles relies on the oxa-6πThe most common route for synthesizing these heterocycles relies on section, the oxa-6πof dienones, the so-called
Summary
Equilibrium prone undergospontaneous spontaneous valence valence isomerization [16]. 1are are prone to to undergo spontaneous valence isomerization the corresponding. Was that hand, simpler dienones 8–12, which could adopt a stable planar conformation, existed in the openedsteric steric destabilization of the dienones shifted the equilibria toward the. 6) by a substituent observed that the, substitution of a δ-alkyl Is that the existence of the form depends non-stable planar conformations, preferred their closed forms Along with these results, the authors authors observed substituent (R5(Ror. 6) by observed that thatthe thesubstitution substitutionofofa aδ-alkyl δ-alkyl substituent a substituent to extend conjugation of the π-system (e.g, vinyl group) favored the dienone form. The design of stable 2HPs must include, among other structural/electronic criteria, enough steric destabilization on the dienone to penalize the valence isomerization.
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