Studies in terpenns dyhydrogenation studies of abietic acid woth tetrachloro-o-Benzo quinone and a novel route

Abstract

The thesis entitled "Studies in Terpenes - Dehydrogenation studies of abietic acid with tetrachloro-o-benzoquinone and a novel route to the synthesis of (I) methyl dehydroabietate and (II) methyl 13-methoxydeisopropyldehydroabietate" is divided into four chapters. Chapter I is a brief review of the chemistry of abietic acid (Ia) bringing out its stereochemical identity at the asymmetric centres, C^8 and C^13 with the naturally occurring steroids and terpenes. Chapter II describes the available methods of preparation of methyl dehydroabietate (IIb) and methyl 13-methoxydeisopropyldehydroabietate (IV) from abietic acid as well as by total syntheses. The importance of methyl 13-methoxydeisopropyldehydroabietate (IV), already utilized for the synthesis of abietic acid (Ia), for the synthesis of modified steroids has been emphasized. Chapter III deals with dehydrogenation studies of abietic acid with tetrachloro-o-benzoquinone. Initially, we reinvestigated the reported chloranil dehydrogenation of abietic acid. As we failed to obtain pure dehydroabietic acid (IIa) by this method, we resorted to tetrachloro-o-benzoquinone dehydrogenation, as it is known that tetrachloro-o-benzoquinone reacts very much faster than chloranil in the dehydrogenation reactions. When methyl abietate was treated with one mole of this quinone in xylene, methyl dehydroabietate (IIb) was not obtained; instead, a solid adduct, UV ? 236 m? and 302 m? was obtained. Catalytic hydrogenation of the adduct gave a tetrahydro derivative. On the basis of the following data, viz., 1) UV, 2) presence of chlorine, 3) molecular weight of 546 for the adduct (sum of molecular weights, 302 and 246, of abietic acid and tetrachloro-o-benzoquinone respectively, minus two hydrogens) and 4) IR evidence for modification of the isopropyl group, the structures (V) and (VI) were suggested for the adduct and its tetrahydro derivative. The proposed structures were further corroborated by the NMR spectra of the acid (Va) and its tetrahydro derivative (VIa). The mass spectral fragmentation of the adduct (Va) and its tetrahydro derivative (VIa) lent additional support to the proposed structures. A mechanism of this adduct formation involving a dehydrogenation-addition sequence has been proposed. According to this mechanism two moles of tetrachloro-o-benzoquinone are required for one mole of methyl abietate (Ib). Actually, when two moles of the quinone were used for one mole of methyl abietate, as expected, the yield was almost doubled (79.5%). The generality of this reaction with simple systems containing either isopropenyl group or activated isopropyl group has been established. Thus, both limonene and p-cymene gave rise to the same adduct (VII), the structure of which has been established from the NMR spectrum. In Chapter IV a novel route to the synthesis of methyl 13-methoxydeisopropyldehydroabietate (IV) from the adduct (Vb) is described. Incidentally, a new method of preparing dehydroabietic acid (IIa) from the adduct (Vb), in good yield, resulted. Dehydrogenation of (Vb) using either chloranil (1 mole) or tetrachloro-o-benzoquinone (1 mole) gave the dehydroabietic acid derivative (VIII). Several unsuccessful methods to cleave the benzo-1,4-dioxan moiety in (Vb) and (VIIIb) have been discussed. With a view to converting the compound (VIIIb) to the styrene derivative (IX), which on two successive oxidations was expected to furnish the phenol ester (III), the former was treated with HI/HIO?. But dehydroabietic acid (IIa) was obtained in poor yield, as the only isolable product, presumably by reduction of the intermediate styrene (IX) with hydroiodic acid. By heating the adduct (Vb) at 200°/1 mm there was obtained a mixture of methyl ?^18-dehydrodehydroabietate (IX), methyl dehydroabietate (IIb) and tetrachlorocatechol, which after removal of the alkali soluble phenol on hydrogenation and subsequent saponification gave dehydroabietic acid (IIa). The neutral pyrolytic mixture gave pure methyl 18-dehydrodehydroabietate (IX) after careful chromatography. A mechanism for this pyrolysis has been suggested. Ozonolysis of (IX) furnished the methylketone (X), oxidation of which using perbenzoic acid afforded the phenolic ester. Methylation of (III) gave methyl 13-methoxydeisopropyldehydroabietate (IV) in good yield. Thus a convenient and short route for the synthesis of methyl 13-methoxydeisopropyldehydroabietate (IV) and methyl dehydroabietate (IIb) was accomplished. The appendix contains the following two papers:

"Dehydrogenation studies with tetrachloro-o-benzoquinone Structure of a novel dehydrogenation-addition product of abietic acid", T.R. Kasturi, E. Raghavan, Sukh Dev and D.K. Banerjee, Tetrahedron, 22, 745 (1966). "Synthesis of methyl 13-methoxydeisopropyldehydroabietate", D.K. Banerjee, Sukh Dev, T.R. Kasturi and E. Raghavan, Indian J. Chem. (in press).