Microbial metabolites. Part XI. Total synthesis and absolute configuration of (S)-carlosic acid (4-butyryl-2,5-dihydro-3-hydroxy-5-oxo-furan-2-acetic acid) and conversion of (R)-5-methyltetronic acid into (R)-carolic acid {3,4-dihydro-8-methylfuro[3,4-b]oxepin-5,6(2H,8H)-di-one}