2,6-dialkyl-3,4-dihydropyrans

The reachon of y-(trimethylsilyl)allyltributylstannane with aliphatic aldehydes leads to the formahon of 2,6-dialkyl-3,4-dihydropyrans with cis diastereoselechvity (Scheme 8.120) [164]. Aromahc aldehydes did not lead to the cychzahon products. 

Applications of 3 and 4 during the past decade include azidonations of triiso-propylsilyl (TIPS) enol ethers, glycals and dihydropyrans, aryl Ny N-dialkyl-amines, cyclic amides, and cyclic sulfides. Because the relative concentrations of 3 and 4 in solvents employed for their use are uncertain, the azidoiodanes are usually represented collectively as PhIO/TMSN3. 

The interaction of an ethenyl ether and a trialkyl phosphite (trimethyl phosphite was actually used) under dry acid conditions (HCl gas in MeOH) leads to a phosphonic diester according to Scheme 2. Little dealkylation of the phosphite triester appears to occur, and the reaction appears therefore not to involve addition of dialkyl hydrogenphosphonate (the product of phosphite dealkylation), all the more so since triphenylphosphine also reacts under the same conditions to give related triphenylphosphonium salts. Ethenyl ethyl ether, 2,3-dihydrofuran and 2,3-dihydropyran were used as substratesThe ease of reaction under mild conditions, coupled with high yields, testifies to the importance of a cationic intermediate species. 

The cycloaddition reaction of substituted dialkyl a-keto-P,y-unsaturated phospho-nates with cyclopentadiene, cyclohexadiene, dihydrofuran, and dihydropyran was reported. The authors surveyed a series of Lewis acids, Sc(OTf)3, Yb(OTf)3, BiCl3, TiCU, and SnCU to assess their influence on the ratio of the hetero Diels-Alder product (3) and the Diels-Alder cycloadduct (4). BiCl3 was found to be not efficient concerning the selectivity between these two competitive reactions (Equation 22) [40a]. 

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