[4+2] Annulation – Convenient Synthesis of Substituted Dihydropyranones in Aqueous Media

Abstract

A facile and convenient synthesis of dihydropyranones has been developed by a formal [4+2] annulation of readily available α-acetyl ketene S,S-acetals with various aldehydes, involving a tandem aldol reaction and conjugate addition-elimination reaction, in the presence of NaOH in water.

Direct Synthesis of Polyfunctionalized Unsaturated δ-Lactones and δ-Lactams from α-Alkenoyl α-Carboxyl/Carbamoyl Ketene S,S-Acetals under Vilsmeier Conditions

“…On the basis of the above results and related studies, − a plausible mechanism for the formation of unsaturated δ-lactones 2 is presented in Scheme . The first step of the reaction may involve the formation of an unstable intramolecular Michael adduct, δ-lactone I .…”

“…In addition, tetronic acid, tetramic acid, and thiophene derivatives were synthesized with α-alkenoyl α-carboxyl/carbamoyl ketene S , S -acetals as starting materials through intramolecular oxa/aza/thia-anti-Michael addition reactions, respectively. However, in comparison, the direct intramolecular oxa/aza-Michael addition of either α-alkenoyl α-carboxyl or α-alkenoyl α-carbamoyl ketene S , S -acetals leading to the corresponding six-membered heterocycles has not been established , due to decarboxylation of the former under both basic and acidic conditions , and competition of the aza/thia-anti-Michael additions of the latter under basic conditions. , Very recently, as an alternative route, a one-pot synthesis of functionalized unsaturated δ-lactones via reduction−lactonizations of α-alkenoyl α-carboxyl ketene S , S -acetals has been reported by us . As part of a continuing interest in solving the problems in the synthesis of the corresponding six-membered heterocycles starting directly from the easily available α-alkenoyl α-carboxyl and α-alkenoyl α-carbamoyl ketene S , S -acetals, the Vilsmeier reagent (VR) mediated cyclization reactions of α-alkenoyl α-carboxyl/carbamoyl ketene S , S -acetals were studied .…”

Recent developments of ketene dithioacetal chemistry

“…183 On the other hand, dihydropyranones were prepared in moderate yields by a formal [4 + 2] annulation of a-acetyl ketene dimethylthioacetals with various aldehydes via a tandem aldol reaction and conjugate addition-elimination reaction, in the presence of NaOH in water. 184…”

Divergent Synthesis of α,α‐Dihaloamides through α,α‐Dihalogenation of β‐Oxo Amides by Using N‐Halosuccinimides

“…During the course of our studies on the chemistry and applications of β‐oxo amide derivatives,11 we successfully developed convenient syntheses of α‐oxo ketene S , S ‐acetals,12a dihydropyranones,12b cyclopropyl amides,12c and isoxazoles12d in aqueous media. We also noticed that β‐oxo amides are easily deacylated under appropriate conditions 13.…”