Vinyl ethers reactions with benzaldehyde

Benzaldehyde and ethyl vinyl ether also gave a 3 to 1 ratio of 3 and 4. 7 8> The reaction with triplet ketone was found to be less stereoselective than the reaction with singlet ketone. 

The relative reactivities of acetals and orthoesters in Lewis acid catalyzed reactions with enol ethers have been investigated. For BF3-OEt2-catalyzed reactions with methyl vinyl ether, the following order has been found saturated acetals < methyl orthoformate < benzaldehyde acetals < a,p-unsaturated acetals. ... 

Bidentate Lewis acid. This useful catalyst (1) with a high propensity for double coordination of the carbonyl group is prepared from the corresponding phenol and two equivalents of McjAI in CH Clj at room temperature. It catalyzes the reduction of 5-nonanone by BujSnH at -78° in 86% yield, whereas a reaction in the presence of the monodentate 0-dimethylaluminum 2,6-xylenoxide affords 5-nonanol in only 6%.. Accordingly, different catalytic efficiencies are also found in the Mukaiyama aldol reaction (e.g., 87% vs. 0% in the reaction between 1-trimethylsiloxy-l-cyclohexene and benzaldehyde) and the Claisen rearrangement of (fil-cinnamyl vinyl ether (96% vs. 0%). The contrasting ( >Zi-selectivity of the Michael adducts also reflects the different coordination states. 

The monomers were synthesized using a similar experimental procedure. Starting from 4-hydroxybenzaldehyde, the phenolic group was first blocked with the vinyl ethers to form the acetal-benzaldehydes and then Wittig reaction converted the aldehyde group into the vinyl functionality. To demonstrate the reaction sequences. Scheme 1 illustrates the preparation of 4-(l-phenoxyetho) )styrene and its free radical polymerization. 

To a flask was added 200 mg chromium(II) chloride (1.63 mmol, 2.5 eq.) then 2.5 mL THF was added under vigorous stirring. After a few minutes, 229 mg benzaldehyde (2.16 mmol, 3.0 eq.) and 156.3 mg 3,7-dimethyl-l-bromo-oct-2,6-diene (0.72 mmol, 1.0 eq.) were added. The resulting mixture was stirred for 3 h at 55" C, and the reaction was quenched with brine. The aqueous layer was extracted three times with diethyl ether, and the combined organic fractions were washed twice with brine, dried over MgS04, and concentrated in vacuo. The residue was purified by silica gel column chromatography using diethyl ether as the eluent, to afford 64% 2,6-dimethyl-l-phenyl-2-vinyl-hept-5-en-l-one as a colorless oil, in a yield of 64%, Rf = 0.64 (hexane/EtOAc, 98 2). 

The nudeophOic addition of the phenolic functionali to vin phenyl ether was found to proceed satisfactorify with PPTS as the catafyst. Excess 4-hydrcH -benzaldehyde was used in order to adiieve maximum conversion of expensive vinyl phen ether. The unreacted 4-hydn enzaldehyde could be easily removed from the reaction mixture 1 washing with diluted aqueous base solutions and in the meantime, the catalyst, PPTS, was also removed by aqueous washing. The protected 4-hydroi benzaldehyde was then taken to a high yield Wittig methylenation to afford the desired product. 

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