Substituent‐Controlled Electrocyclization of 2,4‐Dienones: Synthesis of 2,3,6‐Trisubstituted 2H‐Pyran‐5‐carboxylates and Their Transformations

Abstract

A facile access to 2,3,6-trisubstituted 2H-pyran-5-carboxylates is developed by employing 2-alkyl-2-enals as reactants with acetoacetates. The reaction involves Knoevenagel condensation followed by a 6π-electrocyclization, in which the presence of the C2 alkyl substituent in the enals favors the

Self-Sensitized Photooxygenation of 2H-Pyrans: Characterization of Unexpected Products Assisted by Computed Structural Elucidation and Residual Dipolar Couplings

“…1 Generally, singlet oxygen is the reagent of choice in these processes, requiring the use of an external sensitizer for the energy transfer process to occur. [2][3][4][5] During our studies on the photochemical behavior of dienones in the presence of molecular oxygen, we established an easy one-pot approach for the preparation of bridged 1,2,4-trioxanes (Scheme 1). [6][7][8] The process involves a singlet 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 oxygen Diels-Alder type oxygenation of 2H-pyran in situ generated intermediates, revealing the ability of dienones bearing an ionone-type skeleton to act as singlet oxygen sensitizers.…”

Recent Advances in Oxa-6π Electrocyclization Reactivity for the Synthesis of Privileged Natural Product Scaffolds

“…Another steric clash occurs when R 2 ≠ H or for disubstituted R 2 /R 2′ ≠ H in terminal E-alkenes, thus forcing the s-cis,s-cis dienone π-system of 1″ out of conjugation which ultimately results in a favorable oxa-6π electrocyclic ringclosure towards the 2H-pyran isomer 2. Aside from these steric factors, the oxa-6π electrocyclic ring-closure is also favored towards 2H-(benzo)pyrans 2 by the presence of either electron-withdrawing groups at the C5 and/or C6 positions (reduced nucleophilicity of the enol ether moiety), or the stabilizing π-aromaticity of arenes (C5-C6) [18,19]. For most oxatriene substrates 1 which do not possess the stereoelectronic characteristics mentioned above, the oxa-6π electrocyclic ring-closure is often reversible unless being funneled through a cascade reaction [20,21].…”

Recent Advances in the Synthesis of 2H-Pyrans

“…Although the use of cyclic 1,3-dienones has been beneficial for the synthesis and stability of the resulting 2HPs, it is not a mandatory requirement, and acyclic active methylene compounds, such as methyl acetoacetate 27 , have been successfully condensed with 2-alkyl-2-enals 28 to deliver the corresponding stable 2,3,6-trisubstituted 2HPs 29 (Scheme 12) [41].…”