Studies in Syntheses of Non-Isoprenoid Sesquiterpenes

The purpose of this study was to find the shortest route to prepare the 1,9-dimethyl decalone system I, a key intermediate in the synthesis of non-isoprenoid sesquiterpenes. The decalone system I was prepared in good yield by dehydrating the hydroxyketone II with 50% H2SO4. The dehydration of hydroxyketone II was also affected by heating in the presence of iodine crystals to yield 80% of the decalone system I. The basic kinds of approaches under investigation were the Diels-Alder reaction and the Robinson annellation. The Diels-Alder reaction between the conjugated diene, 1-acetoxybutadiene (III) and 2,3-dimethylcyclohexenone (IV) under typical conditions as well as in the presence of catalysts such as anhydrous AlCl3, CCl3COOH and SnCl4 was studied. The mixture of 1-acetoxybutadiene (III) and 2,3-dimethylcyclohexenone (IV) was heated for 15 hours but the formation of the compounds VII or VIII was not accomplished. The same mixture was dissolved in dry benzene and refluxed for 15 hours but found no evidence by vpc the formation of the compounds VII or VIII. Further attempts were made in the presence of the catalysts, such as anhydrous AlCl3, CCl3COOH and SnCl4 but the results were not satisfactory. Similarly, the reactions between 1-acetoxybutadiene and Hagemann\u27s ester (V) as well as methylated Hagemann\u27s ester (VI) were found unsatisfactory. Tho condensation of methyl vinyl ketone (XIV) or 1-diethylamino-3-butanone (XIII) and methylated Hagemann\u27s ester (VI) gave compound XVII in unsatisfactory yield. Similar condensations with 2-methyl-2-cyclohexenone (XV) and 2,3-dimethyl-2-cyclohexenone (IV) were not accomplished. Other attempts were made to condense 1,3-dichloro-2-butene (XVI) with 2,3-dimethyl-2-cyclohexenone (IV) in presence of NaH in dry benzene as an alternate route to decalone system I through the chloroketone XIX. But the chloroketone was obtained in too poor of a yield to proceed further. The hydroxymethylene derivative XX was prepared from the hydroxyketone II by the treatment with HCOOEt and NaH in dry benzene. Further reaction of the hydroxymethylene derivative XX with ethyl α-bromopropionate was not accomplished to yield the compound XXI