Epoxides, reaction with alkyne anions

The A1 atom is thought to coordinate with the oxygen of the epoxide in the first step of these reactions. The alkyne anion then reacts intramolecularly giving the product. 

As with cyanide, Sn2 reactions of alkyne anions can be done with substrates other than halides or sulfonate esters. Epoxides are opened by acetylides at the less sterically hindered carbon to give an alkynyl alcohol. A synthetic example is the reaction of epoxide 38 with the indicated lithium alkyne anion gave an 85% yield of 39, an intermediate in the Sinha et al. synthesis of squamotacin.49... 

Now, let s draw out the forward scheme. This multi-step synthesis uses three equivalents of ethylene (labeled A, B, C in the scheme below) and one equivalent of acetic acid (labeled D). Ethylene (A) is converted to 1,2-dibromoethane upon treatment with bromine. Subsequent reaction with excess sodium amide produces an acetylide anion which is then treated with bromoethane [made tfom ethylene (B) and HBr] to produce 1-butyne. Deprotonation with sodium amide, followed by reaction with an epoxide [prepared by epoxidation of ethylene (C)] and water workup, produces a compound with an alkyne group and an alcohol group. Reduction of the alkyne to the cis alkene is accomplished with H2 and Lindlar s catalyst, after which the alcohol is converted to a tosylate with tosyl chloride. Reaction with the conjugate base of acetic acid [produced by treating acetic acid (D) with NaOH] allows for an Sn2 reaction, thus yielding the desired product, Z-hexenyl acetate. 

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