Applications of the 3-methyl-.gamma.-butyrolactone strategy to the synthesis of polypropionates: the Prelog-Djerassi lactonic ester, ent-invictolide, and the C19-C27 fragment of rifamycin S
3-Methyl-γ-butyrolactone as a source of 2-methyl-3-hydroxyketones and 2-methyl-1,3-diols: a synthesis of the C19C27 fragment of rifamycin S by linear iteration