Über die Stereospezifität der α‐Alkylierung von β‐Hydroxycarbonsäureestern. Vorläufige Mitteilung

Abstract

About the Stereospecific α‐Alkylation of β‐Hydroxyesters It was found, that dianions derived from β‐hydroxyesters with lithium diisopropylamide (LDA) at −50 to −20° were alkylated stereospecifically ( Scheme 1 ). The stereospecificity was 95–98%, the threo ‐compound ( threo ‐2, ‐3 and ‐4) being the main product. This was proved for threo ‐2 and ‐3 by preparing the β‐lactones 7 and 8 , respectively, which were pyrolyzed to trans ‐1, 4‐hexadiene (9) and trans ‐1‐phenyl‐2‐butene (10) , respectively ( Scheme 2 ). Moreover, the acid threo ‐6 from threo ‐3 was converted by dimethylformamide‐dimethylacetal to cis ‐1‐phenyl‐2‐butene (11) (s. footnote 6). The alkylation of α‐monosubstituted β‐hydroxyesters also turned out to be stereospecific. Reduction of 16 and 18 with actively fermenting yeast furnished (+) ‐17 and (+) ‐2. respectively ( Scheme 4 ), which were each mixtures of the (2 R , 3 S )‐ and the (2 S , 3 S )‐isomers. Alkylation of (+) ‐17 with allyl bromide yielded after chromatography (2 S , 3 S ) ‐19 and of (+) ‐2 with methyl iodide (2 R , 3 S ) ‐19 , the oxidation of which finally gave ( S )‐(−) ‐20 and ( R )‐(+) ‐20 , respectively.