Retusaminic acid

Synthetic work by another Japanese group has resulted in the synthesis of a-and j -retusanecic acids, and retusaminic acid. The retusanecic acids are obtained by the alkaline hydrolysis of retusine (44), and have been shown to be epimeric 4-hydroxy-2,3,4-trimethylglutaric acid lactones. Since in acidic solution the equilibrium favours the j3-acid, it is formulated as (45), in which the C-2 methyl group is trans with respect to the C-3 methyl group a-retusanecic acid is... 

The synthesis of retusaminic acid (51), the hydrolysis product of retusamine (52), followed similar lines. Ethylation of the keto-ester (47) via its ethylene... 

This acid is derived from retusamine (Table3Pi), and has been synthesised by Kiyooka etal. 162) as shown in Scheme 18. The starting material (55) was the same as that used in the synthesis of a- and ji-retusanecic acids cf. Scheme 17), and a similar strategy was employed, except that ethylation of the starting material (55) preceded stereospecific cyanohydrin formation and cyclisation to give lactone (66). The absolute configuration of the resolved lactone acid, which was identical with retusaminic acid (67), was already known from the X-ray structure analysis of retusamine carried out by Wunderlich 319). 

Kiyooka, S., T. Hase, and J. D. Edwards Pyrrolizidine Alkaloids. The Total Synthesis of Retusaminic Acid. Chem. Lett. 1973, 963. 

The absolute configuration of monocrotalic acid (44), the necic acid component of monocrotaline (45), has been elucidated aside from the acid derived from retusamine, whose structure has been defined by X-ray crystallography, this is the first necic acid component of a pyrrolizidine diester alkaloid containing an eleven-membered ring to have its stereochemistry unambiguously established. 

The structure LXXXIX proposed previously (154a) would place this acid in this group as containing the same carbon skeleton with a carboxyl group in place of a methyl group, a relationship observed between lati-folic acid and viridifloric and trachelanthic acids (see Chart II). Wunderlich (64) has proposed an alternative structure (XCa) indicative of a relationship with the acid in retusamine (CXb) (64). 

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