A Mild, Room-Temperature Protection of Ketones and Aldehydes as 1,3-­Dioxolanes under Basic Conditions

Abstract

Protection of ketones or aldehydes as 1,3-dioxolane derivatives proceeds within minutes at room temperature in the presence of N-hydroxybenzenesulfonamide, its O-benzyl derivative, or the tosyl analogue, in the absence of strong protonic acids, and in the presence of base (Et₃N). Acid-sensitive groups such as O-THP, O-TBS, or N-Boc are unaffected.

Key words

ketals - protecting groups - synthetic methods - ketones - aldehydes

• References and Notes

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• 19 Protection of 2 as 1,3-Dioxolane 3 (Table 1); General Procedure: A solution of ketone or aldehyde 2 (1 mmol), 1a (commercial, 121 mg, 0.7 mmol), and ethylene glycol (124 mg, 2 mmol) in CH₂Cl₂ (4 mL) was stirred at r.t. for 10 min. The solution was washed with water and brine, and dried over anhydrous MgSO₄ [washing with 2 M NaOH (2 × 10 mL) followed by acidification led to only partial recovery of 1a]. Concentration under vacuum afforded 3. Flash chromatography (basic alumina; EtOAc–hexane, 1:9) gave pure 3 in 80–90% yield (Table 1). The analytical data of 1,3-dioxolanes 3 were in good agreement with those of authentic compounds.