Reaction of 2,6-disubstituted phenols with vinyllead triacetates and alk-1-ynyllead triacetates: synthesis of 6-vinyl- and 6-alkynyl-cyclohexa-2,4-dienones and crystal structure of 1,3,5,7-tetramethyl-3,5-bis(phenylethynyl)-1,3,4,4a,5,8a-hexahydro-1,4-ethenonaphthalene-2,6-dione