Ethers, propargylic thermolysis

Formation of the requisite triene 41 was readily achieved by alkylation of hydroxy-diene 24 with l-bromobut-2-yne (Scheme 10-13). The stereospecificity of the concerted Diels-Alder reaction required a cis-configured dienophile to react through the. sterically less demanding exo T. S. to afford the desired product 42. A chemoselective hydrobora-tion/protonolysis sequence on propargylic ether 43 allowed access to the desired cis olefin 41 in good yield. Subsequent thermolysis at 165 °C in toluene afforded a 4 1 mixture of stereoisomers with the major product being the desired exo isomer 42. Unactivated primary alkyl ethers are rarely used as tethers, owing to the relatively harsh reaction con-... 

Pyrolysis of vinyl ethers in a retro-ene fashion [equation (15)] has been investigated and shown to occur at temperatures 40—50 °C lower than for the corresponding acetates. Another retro-ene thermolysis, that of propargyl ethers, has... 

Thermal rearrangements of tropolone ethers have been described. Thermolyses of the 7-substituted tropolone propargyl ethers (135 R = Cl, Ph R = H) give lactones (136 R = Cl, Ph) via [3, 3] shifts and cyclization. However, lactones (136 R = Ph, Cl) are also formed on thermolysis of the 3-substituted tropolone propargyl ethers (135 R = H R = Ph or Cl). The formation of these lactones from the 3-substituted tropolone ethers was interpreted in terms of a [3,7]shift to give the... 

The removal of an allylic functionality is always accompanied by an allylic rearrangement when it proceeds by a retro-ene reaction. In most cases such a reaction requires high temperatures as in the thermolysis of allylic or, preparatively more useful, propargylic ethers (Scheme 84), but allylic sulfinic acids undergo fragmentation slightly above room temperature with the potential of high stereocontrol, as the example demonstrates (equation 46). ... 

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