l-Proline·H₂SO₄: a sustainable ionic liquid as a catalyst for the tetrahydropyranylation of alcohols in water

A highly environmentally adaptable and green method is developed for the tetrahydropyranylation of alcohols in water, catalyzed by an ionic liquid, l-proline·H₂SO₄, derived from nature's chiral pool. The tetrahydropyranylation of primary, secondary, and tertiary alcohols was accomplished within an hour at room temperature. The catalyst was equally efficient for the deprotection of tetrahydropyranyl ethers in the presence of methanol to give the corresponding alcohols in quantitative yields. The method invites further attention due to the reusability of the l-proline·H₂SO₄ ionic liquid in water (as the reaction medium); it could be reused up to six times without any processing after each batch. The efficacy of the method was also established by carrying out the reaction at the multigram level, achieving excellent transformation without any loss of catalytic activity. Significantly low process mass intensity (2.99) and E-factor (1.97) in the synthesis of tertrahydropyranyl ether demonstrate the remarkable green compliance of the protocol.