Acid isomerization of diterpenic acids

Acetylphenylhydroquinone, 34, 1 5-Acetyl-k-valeric acid, 31, 3 Acid chlorides, synthesis of, 37, 69 Acid isomerization of diterpenic acids,... 

Isomerization of the diterpeue A B ring juncture. The steroidal type A/B ring juncture of diterpenes can be isomerized to the antipodal one by treatment with a 10% palladium-charcoal catalyst in refluxing triglyme. Thus methyl 5a,10 -podocarpa-8,1 l,13-triene-15-oale (1) can be isomerized in 83% yield to methyl 5j, IOa-podocarpa-8,ll,13-triene-l3-oate (2). The reaction was used in a synthesis of (- J-podocarpic acid from (+)-dehydroabietic acid. 

Other examples of the use of intramolecular Diels-Alder reactions in preparative chemistry have appeared the o-quinodimethane (186), produced as a transient intermediate in the pyrolysis of the analogous benzocyclobutene, cyclizes to give (187 65%), the desulphurization of which (Raney nickel) affords a potential intermediate for the preparation of tetracyclic diterpenes. Elevated temperatures result in the isomerization of (188) into the internal [4 -I- 2] adduct (189) in which it is clear that a cyano-group has acted as a dienophile (189) is the result of a subsequent double-bond shift, and the structure was validated by crystallographic structure analysis. Bridged y-lactams have been successfully prepared by the intramolecular Diels-Alder reactions of NN-diallyl- and iV-(l,l-dimethylprop-2-yn-l-yl)amides of various penta-2,4-diene carboxylic acids. Thus, pyrolysis of (190) gave an 80%... 

When CLXVIII was heated under reflux with methanol-d for 24 hours, conditions sufficient to isomerize atisine, no deuterium was incorporated in the product, which was identical with starting material. Refluxing with added NaOD for 3 hours resulted in less than 5% incorporation. Refluxing in a mixture of dioxane and deuterium oxide at 85° for 24 hours resulted in < 30% incorporation of one deuterium atom per molecule. Thus it is evident that without the steric driving force which is present in the alkaloid system, strenuous conditions are required to effect isomerization (54). In another model system of the arylaralkyl oxazolidine type (CLXXII and CLXXIII), in which there was no steric differentiation between the two a-W-carbons, but in which the protons on these particular carbons were relatively more acidic than in the diterpene... 

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