Lewis Acid-Catalyzed Tandem Diels−Alder Reaction/Retro-Claisen Rearrangement as an Equivalent of the Inverse Electron Demand Hetero Diels−Alder Reaction

Abstract

A highly stereoselective formal inverse electron demand hetero Diels-Alder reaction (HDA) occurs on reaction of 2-aryl-alpha,beta-unsaturated aldehydes with cyclopentadiene. The major pathway for this transformation is shown to be a Lewis acid-catalyzed tandem Diels-Alder reaction/retro-Claisen rearrangement.

Retro‐Diels–Alder Reactions of Masked Cyclopentadienones Catalyzed by Heterogeneous Brønsted Acids

“…FAU, *BEA, MOR and MWW was applied in the model reaction (entries [3][4][5][6][7][8][9][10]. The ultrastable Y zeolite ZF 510 (entry 3) and mordenite (entry 4), both with relatively low SiO 2 /Al 2 O 3 ratios, exhibit some initial activity but with no increase in yield at 18 h of reaction.…”

“…Dry CuI (60 mg, 0.32 mmol) was added to the turbid solution and the resultant mixture stirred at 0 8C for 30 min, then cooled to À78 8C and a solution of 8 (300 mg, 2 mmol) in dry THF (5 mL) was added over 5 min. The mixture was stirred for 1.5 h, then the reaction quenched by addition of saturated NH 4 Cl and the product recovered by extraction with EtOAc (3 ). The combined organic phases were dried over MgSO 4 and the solvent was removed under reduced pressure.…”

“…The resulting mixture was stirred at À5 8C for 15 min, after which the flask was cooled to À78 8C and a solution of 8 (300 mg, 2 mmol) in dry THF (5 mL) was added slowly and the solution stirred for 2 h at À78 8C. The reaction was quenched by addition of saturated aqueous NH 4 Cl, and the product recovered by extraction with ether (3 ), drying of the combined ether extracts over MgSO 4 , and removal of solvent under reduced pressure. The crude product was purified by column chromatography on silica gel using n-hexane:EtOAc (9:1) as eluent to obtain 9b as a light yellow oil; yield: 357 mg (85%).…”

“…4 h as a solution of ca. 3 g powder, 15 g NH 4 Cl and 500 mL water. The mixture was centrifuged, the powder dissolved in 500 mL water, 15 g NH 4 Cl were added and the mixture was refluxed again.…”

“…The use of MeAlCl 2 is associated with mild reaction conditions, typically involving temperatures around 55 8C and atmospheric pressure, although there are limitations in terms of the range of substrates amenable to this approach. [4] Microwave conditions have been employed to achieve shorter reaction times. [5] Other methods to effect cycloreversions under rather moderate conditions include the use of solid supports, [6] the adsorption of substrates on graphite under microwaves, [7] and photochemical retro-Diels-Alder reactions, [8] all with varying results.…”