Annulations Using Phosphonium Salts and Enolate Anions - A Direct Route to Naphthalenes

Abstract

Ketone enolates react with an aldehyde phosphonium salt to produce naphthalenes in a one-pot reaction

Key words

annulations - phosphonium salts - ketone enolates

References

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14

To a freshly prepared solution of LDA (1 mmol in 3 mL of THF) was added the ketone (1 mmol) in 1 mL of THF dropwise at -78 °C and the reaction was stirred at -78 °C for 1 h. The cold solution was added via a cannula to the phosphonium salt (1 mmol) in 5 mL of THF at -78 °C. The solution was allowed to slowly warm to 0 °C. One-Pot Reaction: To the solution was then added 0.5 mL of concentrated HCl and was stirred at r.t. for 0.5-3 h (except for the adduct from cyclopentanone which required 10 h). The aqueous layer was extracted twice with Et₂O. The combined organic layers were dried over MgSO₄, filtered, concentrated in vacuo and purified by flash chromatography on silica gel using EtOAc:hexane.
Two-Step Reaction: To the solution was then added glacial acetic acid followed by water. The aqueous layer was extracted three times with EtOAc. The combined organic layers were dried over Na₂SO₄, filtered and concentrated in vacuo. To the alcohol in 4 mL of THF was added PTSA (0.25-0.50 equiv.) The reaction was stirred for 2-18 h. After aqueous workup, the crude product was purified by chromatography on silica gel using EtOAc:hexane.
Adduct from methyl hepten-2-one: ¹H NMR (CDCl₃): δ = 1.58 (s, 3 H), 1.69 (s, 3 H), 2.34-2.42 (m, 2 H), 2.79 (t, J = 8.1 Hz, 2 H), 5.18-5.23 (m, 1 H), 7.32-7.47 (m, 3 H), 7.62 (s, 1 H), 7.74-7.81 (m, 3 H). ¹³C NMR (CDCl₃): δ = 17.9, 25.9, 30.1, 36.5, 123.9, 125.2, 126.0, 126.5, 127.6, 127.7, 127.8, 127.9, 132.1, 132.4, 133.8, 140.1. HMRS calcd for C₁₆H₁₈: 210.1408. Found: 210.1413.