1.2- dibromoethane, dehydrobromination

The a-methylene-) -butyrolactone grouping is incorporated in a large number of sesquiterpenoids, many of which have significant biological activity. A number of routes have been devised for the synthesis of this moiety but recently an important contribution from Ourisson s laboratory has demonstrated that the more accessible a-methyl-y-lactones can be converted in two steps into the a-methylene analogues.This is achieved by reaction of the lactone (263 R = H) with triphenylmethyl-lithium and quenching the resultant enolate with 1,2-dibromoethane. Dehydrobromination of the derived bromide (263 R = Br) with diazabicyclononene gives exclusively the exocyclic methylene lactone (264). This method is only suitable for c/s-y-lactones since the trans-lactone yields exclusively the endocyclic isomer. To illustrate this method Ourisson et at. have converted dihydro-6-epi-santonin (265) into (- )-frullanolide (266). More recently the same authors have developed a method which is applicable to... 

Phenylthioacetylene has been prepared by elimination of thiophenol and dehydrobromination of cis-1,2-bis(phenylthio)ethylene5 and cis-1-bromo-2-phenylthioethylene,2 7 respectively. The latter was obtained by addition of thiophenol to propiolic acid in ethanol and subsequent one-pot bromine addition, decarboxylative dehalogenation, and careful distillation to remove the trans isomer.2.7 on the other hand, cis-1,2-bis(phenylthio)ethylene was prepared by double addition of thiophenol to cis-1,2-dichloroethylene.5a d Although these procedures can provide useful amounts of phenylthioacetylene, they were found to be somewhat less satisfactory in our hands as far as operation and/or overall yields are concerned. Furthermore, we have encountered problems with regard to the reproducibility of one-pot dehydrobrominations of phenylthio-1,2-dibromoethane.6 However, the stepwise execution of the double dehydrobromination, as described in the modified procedure reported here, provides preparatively useful quantities of phenylthioacetylene in a practical manner. 

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