Chloroformate ester-induced reaction

Product equilibration is not likely under room temperature reaction conditions, but there are some cases where it has been deliberately used to enhance the ( )-enoate selectivity of reactions that otherwise afford mixtures. Entries 47-49 in Table 19 demonstrate that catalytic amounts of p-dimethyl-aminopyridine (DMAP) will induce the Z E equilibration of a, -unsaturated thiol esters at room temperature (chloroform solution), resulting in enhanced ( )-enoate selectivity (101b). This procedure is not effective with the corresponding oxygen esters, however. 

In a similar way, chloroform may be added to isoprenol (which derives from acetone and acetylene). Acid-induced dehydration leads to 5,5,5-trichloro-2-meth-ylpent-2-ene, which can be used directly for the reaction with the diazoacetate ester, or it may optionally be first converted with base to the desired diene. 

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