Stereospecific Synthesis of 4,8-Dimethylspiro(4,5)-deca-7-ene-l,6-dione A Synthetic Route to the Acoranes

The synthesis of the spiro(4,5)decane system is of interest because it constitutes the basic skeleton of the acoranic sesquiterpenes [1]. In this paper we wish to report a stereospecific synthesis of a promising precursor (1) of some of these natural products. According to literature [2] cyclohexanones asubstituted with a co-carboxyalkyl chain yield diketo-spiro systems on treatment with polyphosphoric acid. Thus, a spiro system like 1 can be related to 2 which in turn, is related to the aromatic system 3. The latter is a modified homolog of an easily accessible compound, 4,7-dimethylcoumarine (4) [3]. The transformation of 4 into 3 was accomplished through a series of standard reactions. The catalytic hydrogenation of the 3,4-double bond of 4 over 10% palladium-charcoal [4], followed by the etherification of the resulting lactone with dimethjdsulfate in basic medium, yielded 3-(o-methoxy-^)-tolyl)-butanoic acid. The desired 4-(o-methoxy-^)-tolyl)-pentanoic acid (3) [5] was obtained by homologation of the latter through the following secuence: lithium aluminum hydride reduction, treatment of the resultant alcohol with triphenylphosphine in carbon tetrachloride, cyanide displacement of the halide and basic hydrolysis of the nitrile. The transformation of the anisole 3 into the alicyclic acid 2 was accomplished by reduction under Birch condition followed by aqueous acid treatment of the dihydrobenzene product. The overall yield of 2 from 4 was 60%. Since a new asymmetric center is introduced in this step, 2 should be a mixture of