Homochiral Diels-Alder adduct of 2,4-dimethylfuran to 1-cyanovinyl (1'R)-or (1's)-camphanate are transformed readily into polypropionate fragments containing four or five contiguous stereogenic centres. They can be condensed via cross-aldolizations to lithium enolates of 7-oxabicyclo[2.2.l]heptan-2-ones with high diastereoselectivity generating long--chain systems containing up to eleven stereogenic centres and tertiary alcoholic moieties.A large variety of natural products of biological interest contain polypropionate fragments (chain with alternating hydroxy and methyl substituents).l Among the many macrolide antibiotics, erythronolides (aglycons of erythromycins) have become the yardstick for measuring progress in the efficiency of stereoselective syntheses.3~4 The shortest synthesis of (9S)dihydroerythronolide A (1) is the linear approach of Stiirmer et aZ.3a Other more convergent approache@ imply a retrosynthetic disconnection at the C(6)--C(7) bond as illustrated with the synthesis of Stork et aZ.4 which couples the fragments 3 and 4 to generate 2 with high diastereoselectivity. If intermediate 3 could be derived from 5 which is the 2,3diepher of 4, one can envision these two polypropionate fragments and analogues to derive from the uronolactones 7 and 6, respectively, that w i l l be generated from the same homochiral(1R,4R)-1,5dimethyl-7-oxabicyclo[2.2.1]-hept-5-en-2-one ((+)-ti). This chiron and its enantiomer (-)-8 are obtained readily.2 5 u 3 4 u 7