Solvent‐Free Carbon–Oxygen Bond Formation Catalysed by CeCl₃·7 H₂O/NaI: Tetrahydropyranylation of Hydroxy Groups

Abstract

An efficient and highly chemoselective method fo the protection of free hydroxy compounds with 3,4‐dihydro‐2H‐pyran is reported. Since the deprotection of THP‐ethers occurs very readily at room temperature, the successful use of this type of protecting group depends only upon how readily it can be introduced. For this we have examined the tetrahydropyranylation of alcohols and phenols catalysed by the CeCl3·7 H2O/NaI system surface under solvent‐free conditions. The reaction presents the advantage of being performable under extremely mild conditions by use of catalytic amounts of an interesting Lewis acidic system consisting of the CeCl3·7 H2O/NaI catalyst combination, which can be easily separated from the reaction mixture. The advantages of this procedure, which utilizes cheap and “friendly” reagents, over the previously reported ones are discussed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

An Efficient Lewis Acid Catalyzed Povarov Reaction for the One-Pot Stereocontrolled Synthesis of Polyfunctionalized Tetrahydroquinolines­

“…On the other hand, by-products like 13 were already reported in the Povarov reaction. The coordination of Cerium with the oxygen atom in tetrahydroquinolines 4 and 5 favors the ether ring opening, 30 then the iminodiene acts as hydrogen acceptor in the oxidation to quinoline by an hydrogen transfer mechanism. 31 Furthermore tetrahydropyranylation of the free hydroxyl groups could occur, because of the known ability of CeCl 3 •7H 2 O/NaI to catalyze this kind of protection of alcohols.…”

Development of a New Heterogeneous Lewis Acid Catalyst for Chemoselective Tetrahydropyranylation of Different Hydroxyl Compounds

“…Amongst them, tetrahydropyranylation is the most frequently used method because of its low cost, ease of preparation, and ease of removing protecting groups as well as the remarkable stability of tetrahydropyranyl ethers under a variety of conditions such as Grignard reagents, alkyl lithiums, oxidative reagents, and acylating reagents [11,12]. A variety of catalysts that have been reported for this conversion include the use of LiOTf [13], Pyridinium p-toluenesulfonate [14], silicasulphuric acid [15], SiO 2 -AlCl 3 [16], polystyrene-AlCl 3 [17], CeCl 3 ⋅7H 2 O/NaI [18], copper nitrate/acetic acid [19], heteropolyacids [20], and 1-alkyl-3methylimidazolium tetrachloroindate under microwave irradiation [21]. Although these methods are suitable, many of them are associated with several drawbacks, which include long reaction time, high cost, harsh and acidic conditions, poor selectivity, formation of polymeric by-products of the dihydropyran, and isomerization.…”

Tetrahydropyranylation of Alcohols over Modified Zeolites

“…Tetrahydropyranylation of alcohols can be accompanied by using PTSA [3], BF 3 ÁOEt 2 [4], PPTS [5] and zeolites [6][7][8][9]. Some of the recently used reagents that can catalyze tetrahydropyranylation are AlCl 3 Á6H 2 O [10], [11], Fe 2 (SO 4 ) 3 ÁXH 2 O [12], ferric perchlorate [13], Ru(acac) 3 [14], La(NO 4 ) 3 Á6H 2 O [15], 2,4,6-Trichloro [1,3,5] triazine [16], TBATB [17], In(OTf) 3 [18], InCl 3 immobilized on ionic liquids [19], bromodimethylsulfonium bromide [20], tetrabutylammonium bromide [17], CeCl 3 Á7H 2 O-NaI [21], PdCl 2 (CH 3 CN) 2 [22]. On the other hand, many methods have also been developed using heterogeneous catalysts and these have been reviewed recently [23].…”