Abstract
All stereoisomers (4a,b–7a,b) of 4-carboxy-4-ethyl-3-hydroxy-2-isopropyl-4-butanolide, the necic acid component of axillaridine, have been synthesized. Methyl(E)-(S)-(+)-2-ethyl-4-methoxycarbonyl-5-methyl-2- hexenoate (12a) was converted into γ-lactone acids, 4a (2S,3S,4S), 5a (2S,3R,4R), and 5b (2R,3S,4S), by a series of reactions: Epoxidation with m-chloroperbenzoic acid, treatment with acetone in the presence of tin(IV) chloride, acidic hydrolysis with formic acid, and alkaline hydrolysis with barium hydroxide. Similarly, (R)-(−)-enantiomer (12b) was also transformed into γ-lactone acids 4b (2R,3R,4R), 5b and 5a. Subsequently, (E)-(S)-(+)-4-carboxy-2-ethyl-5-methyl-2-hexenoic acid (13a) and its (R)-(−)-enantiomer (13b) were esterified with chloromethyl methyl ether in the presence of triethylamine to give the corresponding bis(methoxymethyl) esters (26a and 26b). Oxidation of 26a with potassium permanganate and subsequent acidic hydrolysis afforded γ-lactone acids, 6a (2S,3R,4S) and 7a (2S,3S,4R). Simi...
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