Propargylic sulfmates

Two different types of substrates are used here, propargyl sulfenates and propargyl sulfmates. In both cases often the propargylic esters are prepared and then converted to the allenes in situ. 

The corresponding propargyl sulfmates behave similarly. Compound 145 [425] (Scheme 1.65), demonstrating the principle with an interesting alkynyl bromide as the substrate, is just one of numerous examples [322, 380, 403, 423, 426-443]. 

If propargyl sulfmates 422 are thermolyzed, the same carbocycles 424 are formed via [2,3]-sigmatropic rearrangement followed by intramolecular ene reaction. 

Table 9.28 Synthesis of propargylic silanes by SN2 displacement of propargylic sulfmates.

A diastereoselective synthesis of the racemic vinylallene 96 was described by Gordon and Tabacchi [96] in 1992 (Scheme 18.31). Here, the exocyclic allene was formed via anti-SN2 -substitution of the propargylic sulfmate 98 with lithium diisopropenyl-cyanocuprate, which, however, was also found to undergo side reactions such as reduction and direct attack of the nucleophile at the sulfmate. Deprotection of the substitution product 99 (which was obtained with only 35% yield) finally provided the desired target molecule 96. 

---