R -Umbelactone

The pyrazoline derivative 260 was also the precursor for the synthesis (252) of the naturally occurring umbelactone. Reaction of butenolide 159c with diazomethane gave the pyrazoline 260, which was subjected to pyrolysis to give (—)-(5 )-umbelactone (261). As the natural umbelactone was described as being dextrorotatory, the synthesis of (+)-(R)-umbelactone from 159c was also performed. 

The condensation of the dilithio derivative of (R)-(+)-3-(p-tolylsulfinyl)propionic acid with protected glycoaldehy-des (O-r-butyl and 0-benzyl) gives 5-alkoxy-4-hydroxy-3-(p-tolylsulfinyl)pentanoic acids, which spontaneously cyclize to the corresponding 3-sulfinyl-4-alkoxymethyl butanolides (eq 4). Pure diastereomers can be separated by flash chromatography and are obtained in comparable amounts. The corresponding optically pure butenolides are obtained by pyrolytic elimination of the sulfoxides and then transformed into natural (-i-)-(/ )-umbelactone (eq5). 

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