Ynol ethers, addition

The enol precursor 151 was prepared in three steps by addition of Steri-col 150 onto trichloroethylene, which was followed by the formation of a sensitive ynol through addition of n-BuLi and excess allyl iodide (Scheme 43). The ynol was then partially hydrogenated with Pd on barium sulfate, furnishing the required enol ether 152 contaminated with 8% of the corresponding dihydro enol ether. [2+ 2]-Cycloaddition between 152 and dichloroketene, generated in situ from trichloroacetyl chloride, led to the cyclobutanone 153 as the major diastereomer with a 93 7 diastereocontrol. 

SCHEME 2.95 Silver-catalyzed addition of carboxylic acids to ynol ethers [140],... 

The synthesis of ynol ethers from sulfonamides using potassium alkoxides proceeded in moderate yields under mild conditions (Scheme 2.124) [195]. The chemistry entailed initial conversion of alcohols into potassium alkoxides through the addition of potassium to solutions of the alcohol, followed by the sulfonamide. An amine was needed for a successful conversion and dimethyl amine was a convenient choice. There was a level of operational complexity to this approach in that elemental potassium needed to be handled. Provided that the commercially available solutions were active in the chemistry, it would be more practical to obtain solutions of the alkoxides. 

In addition to these methods, ynol ethers and metal ynolates can be generated from dibromoearboxylic acids using strong bases [196-200]. While this methodology generated the desired ynol species in good to excellent yield, the process needs to start from a dihro-minated earboxyUc acid. 

The addition of lithiated ynol ethers to chiral A-sulfinyl imines proceeds in high yield and diastereoselectivity dr > 95 5)." The selectivity was completely reversed by the addition of boron trifluoride (Scheme 2). 

To a stirred solution of pyrido[2,3-small amounts over a period of 15 min. The addition caused slight warming of the mixture after which it was heated at reflux for 3 h. After cooling and removal of dioxane in vacuo, the syrupy mixture was subjected to successive extraction with toluene/petroleum ether (40 - 60 °C 1 1) such that a major part of the white crystalline Ph sP was left behind. Column chromatography (MeOH/CH2Cl2 1 99) was used for separation, recrystallization (CHCI,) gave colorless plates yield 0.179 g (25%) mp 160—162°C. 

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