Boron trifluoride dihydropyran

Tamura etal.2 described only briefly in their paper on dihydropyran derivatives that 6,8-dioxabicyclo[3.2.1]octan-7-one 53 and its methyl derivative underwent polymerization in the presence of a large amount of boron trifluoride etherate to give polymers with molecular weights of several hundreds. 

A more useful way of reducing esters to ethers is a two-step procedure applied to the reduction of lactones to cyclic ethers. First the lactone is treated with diisobutylaluminum hydride in toluene at —78°, and the product - a lactol - is subjected to the action of triethylsilane and boron trifluoride etherate at —20° to —70°. y-Phenyl-y-butyrolactone was thus transformed to 2-phenyltetrahydrofuran in 75% yield, and 5-lactone of 3-methyl-5-phenyl-5-hydroxy-2-pentenoic acid to 4-methyl-2-phenyl-2,3-dihydropyran in 72% yield [1034]. 

The groups of Trost and coworkers and Caserio and coworkers independently developed procedures for azasulfenylation. Caserio reported that when alkenes are treated with a sulfenamide such as MeSN-Me2 in the presence of the Meerwein reagent trimethyloxonium tetrafluoroborate, azasulfenylation of the alkene is observed, presumably via the thioammonium ion MeSNMe3+ (equation 4).20 Boron trifluoride etherate can also serve as a catalyst for addition of sulfenamides to alkenes. The adduct stereochemistry is strictly anti as established by treatment of the dihydropyran methanesulfenyl chloride adduct (see Scheme 2) with AgBF4 followed by trimethylamine. 

The stereoselective formation of cw-fused dihydropyrans and the steric requirements for the reaction of the norbornenyl examples support a concerted mechanism. However, boron trifluoride catalyzed rearrangements of mixtures of endo- and exo-compounds to bicyclic pyrans, i.e., stepwise reactions through cationic intermediates, are also possible38. 

Steroidal O-tetrahydropyran-2-yl derivatives, Ott et al. prepared the 0-tetra-hydropyran-2-yl derivative of testosterone by the reaction of testosterone with 2,3-dihydropyrane catalyzed by p-toluenesulfonic acid monohydrate. The reaction required three weeks and the yield was 59 %. Use of boron trifluoride etherate as catalyst raises the yield to 67% and lowers the reaction time to 10 hr. ... 

For the second approach, (37 )-3-hydroxy-p-cyclocitral (77) was protected with dihydropyran and the aldehyde function reduced to give 181. The hydroxy group was further reduced to give the unsaturated alcohol 182 which was transformed to the acetate 183. The double bond was epoxidized with MCPBA to form a mixture of the cis- and rra/i5-epoxides (184) in a ratio of 2 1. After separation, trans-184 underwent a pinacolic rearrangement and ring contraction with boron trifluoride as a catalyst saponification of the acetate then gave the desired building block 775 [57] (Scheme 41). 

Other Acid-catalyzed Cycloadditions. Isobutene has been used to form pyrylium salts (eqs 30 and 31), oxetanes by Boron Trifluoride-catalyzed [2 + 2] cycloaddition (eq 32), and 3,4-dihydropyrans by Diels-Alder reactions (eq 33). ... 

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