Butanolide hydrogenation

SCHEME 65. BINAP-Ru-catalyzed hydrogenation of 2-acetyl-4-butanolide. 

Scheme 65 shows a reaction occurring with opposite 2,3-syn diaste-reoselection. Hydrogenation of racemic 2-acetyl-4-butanolide, in which the 2-substituent and directing group are linked, proceeds with 98 2 yn anti selectivity to form the diastereomeric hydroxyethyl lactones. 

Furfural (18) was oxidized to give the butenolide 19, which on Michael condensation with diethyl ethylmalonate afforded lactone 20. Hydrolysis of 20 yielded the dilactone 21. This was treated with ethanol in the presence of sulfuric acid. The isomer mixture obtained was separated by preparative thin-layer chromatography, yielding 4-ethoxy-3-ethoxycarbonylmethyl-2-ethyl-4-butanolide (22). Elimination of ethanol, with the aid of p-amino-benzenesulfonic acid, gave 2-(3//)-furanone 23 the 2-(5//)-furanone isomer 24 was obtained with the aid of orthophosphoric acid. Both isomeric esters gave the acid 25 after hydrolysis, which on catalytic hydrogenation afforded m-3-carboxymethyl-2-ethyl-4-butanolide 26, identical to homopilopic acid. This synthesis of homopilopic acid differs from earlier syntheses because the less stable cw-2,3-disubstituted butanolide (26) is formed in the last step. 

Diketene is quantitatively hydrogenated to 3-methyl-3-propanolide in 92% ee (eq 9). Certain 4-methylene- and 2-alkylidene-4-butanolides as well as 2-alkylidenecyclopentanone are also hydrogenated with high enantioselectivity. ... 

An original example of DKR was observed by Noyori and colleagues during the hydrogenation of 2-acetyl-4-butanolide, proceeding with 96% syn diastereo-selectivity and good enantiomeric excesses, as shown in Scheme 2.6. ... 

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