A Domino Strategy for the Synthesis of 2H -Pyrans from Propargyl Vinyl Ethers

Ensembles of Metal Triflate Additives in Gold(I)-Catalyzed Formal [2 + 2 + 2] Cycloaddition of Carbonyls with Activated Terminal Alkynes: One-Pot Synthesis of 2H-Pyrans

Herein, we describe a gold(I)-catalyzed intermolecular formal [2 + 2 + 2] cycloaddition of a carbonyl compound and a propiolate. A series of highly substituted 2H-pyrans are accessed regioselectively from acyclic and cyclic ketones and aldehydes with various substituted propiolates using Ph3PAuCl (0.1-1 mol %) with two distinct metal triflate additives [0.75-7.5 mol %; In(OTf)3/Cu(OTf)2 for ketones, Bi(OTf)3/Hf(OTf)4 for aldehydes]. This method should provide an opportunity for further development of synthetic access to other heterocyclic compounds.

Oxa-[3+3] annulation of MBH-carbonates of propiolaldehydes with α-nitro/bromo ketones to access 2H-pyrans

A novel alkyne-assisted annulation reaction of MBH-carbonates of propiolaldehydes with α-nitro/bromo ketones is reported, providing a facile synthesis of substituted 2H-pyrans in good yields. This reaction divulges the inimitable reactivity of the MBH-carbonates of propiolaldehydes as C3-synthons wherein the alkyne functionality promoted the reaction without participating in the oxa-[3+3] annulation. The obtained products, having alkyne and ester functionalities, allowed further annulations to generate diverse pyrano[3,4-c]pyran-1-ones.

Recent Advances in the Synthesis of 2H-Pyrans

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.