Lewis acid-mediated condensation of alkenols and aldehydes. A selective synthesis of tetrahydropyrans and oxepanes

Abstract

A stereoselective synthesis of different oxacycles is reported starting from aldehydes and unsaturated alcohols in the presence of A1C13 or AlBr3.Sir: Acetal-initiated cyclization reactions have been recently reported for the stereoselective preparation of oxygenated heterocycles.1 Vinylsilanes gave different unsaturated oxacycles with a good control of the double bond stereochemistry11" (route 1 in Scheme I), and (2-methoxyethoxy)methyl (MEM) ethers were also employed to give 4-halotetraydropyrans with a complete control of the stereochemistry of the substituents13 (route 2 in Scheme I).We and others recently reported2 that the coupling between allylsilanes and aldehydes could be conveniently used to prepare 2,6-disubstituted 4-halotetrahydropyrans (Scheme II).

Rapid and Efficient Protocol for the Synthesis of 4-Chlorotetrahydropyrans Using Niobium(V) Chloride

“…4 Generally, tetrahydropyran derivatives are prepared through Prins-cyclization using acid catalysis. [5][6][7][8] Recently indium halides are also found to be useful for this transformation. 9,10 However, many of these classical methods often involve the use of expensive reagents, extended reaction times and also generate a mixture of products.…”

A practical way to synthesize chiral fluoro-containing polyhydro-2H-chromenes from monoterpenoids

“…To create halogenated tetrahydropyranyl rings the Prins reactions between homoallylic alcohols and aldehydes catalyzed by appropriate halogen-containing Lewis acids or ionic liquids are usually used (Scheme 2) [10–16]. However, only a few examples of reactions for introducing a fluorine atom via the halo-Prins cyclization have been reported to date [15–21].…”

Stereoselective Synthesis of Tertiary Ethers through Geometric Control of Highly Substituted Oxocarbenium Ions

“…The condensation of acetophenone (34) with homoallylic alcohol 35 in the presence of TMSCl and NaI [12c] provided, after radical-mediated removal of iodide, tetrahydropyran 36 as a single isomer. Aldehyde 38 condensed with tertiary alcohol 37 and AlCl 3 [19] and yielded chlorotetrahydropyran 39, also as a single stereoisomer. The yields of these reactions were rather low because of the competitive ionization and of the oxonia-Cope-type reactions, [20] but the procedures were not optimized because our main interest was focused on determining the stereochemical outcomes of these transformations.…”