Beckmann and cyclization reactions of δ-oxo-α,β-unsaturated ketoximes obtained from pyrylium salts and hydroxylamine. formation of 2-aryl(or alkyl)amino-4,6-disubstituted pyrylium salts

Correction to Withdrawn Article[3,4] Cyclization Products of δ‐Oxo‐α,β‐unsaturated Ketoxime During Reaction with Hydrochloric Acid in Anhydrous Diethyl Ether

“…[2] Recyclization of keto-oxime 2 was performed under the conditions shown in Scheme 1. The products were separated by column chromatography and their yields are displayed.…”

“…A reasonable explanation is that 2-isoxazoline 3 originated from (1E)-oxime 2 as a Michael-addition product, whereas formation of pyridine 1-oxide 4 required prior isomerization to the (1Z)-oxime and then cyclization and dehydration, promoted under acidic conditions. [2] Should this mechanism be the case, a powerful protonating acid might accelerate both the oxime isomerization and dehydration steps, and raise the yield of pyridine 1-oxide accordingly. With this purpose in mind, cyclization of 2 was performed at room temperature in anhydrous diethyl ether that had previously been saturated with dry gaseous hydrogen chloride.…”

“…[2] Compound 2 should not be conserved since it cyclizes spontaneously on standing at room temperature giving 3 and 4. Compound 2 may be purified by chromatography on silica gel eluting with PE/EE (4:1, v/v).…”

“…The mechanism for the formation of these products involves the addition of hydroxylamine as an N-nucleophile to the α-position of the pyrylium cation, followed by (thermally allowed) electrocyclic ring opening and recyclization of the opened-chain oxime intermediate. [1] We have isolated, in a few favorable cases, the openedchain oxime intermediates (such as 2) [2] and have undertaken a study of their recyclization to explain the origin of the final products. We have claimed the formation of a stable 3-isoxazolidinol in the reaction with hydrochloric acid in anhydrous diethyl ether and we explained its formation by hydration of the CϭN double bond in situ.…”

N,N,N′,N′‐Tetramethylchloroformamidinium Chloride‐Mediated Cyclizations of β‐Oxo Amides: Facile and Divergent One‐Pot Synthesis of Substituted 2H‐Pyrans, 4H‐Pyrans and Pyridin‐2(1H)‐ones

“…The intramolecular cyclization of B leads the formation of 2H-pyran 2 via the intermediate C. In the presence of NaOH/EtOH, a ring-opening reaction of 2H-pyran 2 occurs to give intermediates D and D', as a pair of Z/E isomers, at different temperatures, which then undergo intramolecular cyclizations to afford 4H-pyrans 3 and pyridin-2(1H)-ones 4, respectively. [15] Conclusions In summary, the intermolecular cyclization of b-oxo amides 1 mediated by TMC/Et 3 N in DMF is disclosed for the first time, and thus a novel and divergent onepot synthesis of substituted 2H-pyrans 2, 4H-pyrans 3 and pyridin-2(1H)-ones 4 from b-oxo amides 1 has been developed based on a selection of the reaction conditions. The one-pot protocol features commercially available starting materials, mild conditions, high yields and rich functionality of the products.…”