Methyl elenolate

A total synthesis of ( )-methyl elenolate has been described (73JA7156). 

In continuation of his biomimetic syntheses of heteroyohimbine alkaloids Brown" has succeeded in converting secologanin tetra-acetate (117) into elenolic acid (118), and in completing the synthesis of ajmalicine (80) essentially by the route published" earlier (Scheme 14). Contrary to the previous workers, who apparently isolated only ajmalicine. Brown eta/." obtained ajmalicine (80), 19-epiajmalicine (119) and tetrahydroalstonine (120). Since one of the lactams (121) afforded only 19-epiajmalicine (119) and in the general reaction sequence the proportion of (80), (119), and (120) in the final product depended on the length of time allowed for Schiff base formation rather than on the ratio of isomers present in the methyl elenolate [ester of (118)], it is apparent that interconversion of the imines (122) via the equilibria shown in Scheme 14 is responsible for the non-stereospecificity of the synthesis. 

The ester of 856, ( —)-methyl elenolate (857), upon reductive amination with tryptamine followed by Bischler—Napieralski cyclization, furnishes (— )-ajmalicine (859). 

Hatakeyama S, Ochi N, Numata H, Takano S (1988) A new route to substituted 3-methoxy-carbonyldihydropyrans enantioselective synthesis of (-)-methyl elenolate. J Chem Soc Chem Commun 1988 1202-1204... 

Two other non-glycosidic secoiridoids (see Figure 11) have been isolated from O. europaea. Compound 27 is the elenolic acid, described by Panizzi et al. [2], whereas compound 28 is its methyl ester whose total synthesis was accomplished by MacKellar et al [29], so demonstrating the absolute configuration of these two compounds. 

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