Syntheses of (+)-2-exo-cyano-l -methyl-7-oxabicyclo[2.2.l]hept-5-en-2-endo -yl acetate (1) and of (i)-1 -methyl-7-oxabicyclo[2.2.l]hept-5-en-?-one (2) are reported. The additon of PhSeCl to 1 afforded (&)-5-endochloro-2-exo -cyano-l-methyl-6-exo-(phenylselenenyl)-7-oxabicyclo[2.2.l]hept-2-endo-yl acetate (6), whereas 2 added to PhSeCl with the opposite regioselectivity giving (~)-6-endo-chloro-l -methyl-5-exo -(phenylselenenyl)-7-oxabicyclo[2.2.l]heptan-2-one (7). These adducts were converted into 5-chloro-1 -methyl-7-oxabicyclo[2.2. Ilhept-5-en-2-one (9) and h-chloro-l-methyl-7-oxabicyclo[2.2.l]hept-5-en-2-one (lo), respectively.

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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

The Diels-Alder adduct of 2,4-dimethylfuran to I-cyanovinyl (1'R)-camphanate ( ( + ) cyano-l,5-dimethyl-7-oxabicyclo[2.2.1]hept-5-en-2-endo-yl (1'S)-camphanate ((-)-3)).All these cross-aldolisations are highly em-face selective for the bicyclic ketones. The best stereochemical matching is obtained when the lithium enolates and (x -methyI-substituted aldehydes can realize a 'chelated transition state' that obeys the Cmrn and Fefkin-Anh models (steric effects). Polypropionate fragments containing eleven contiguous stereogenic centres and tertiary-alcohol moieties are thus prepared with high stereoselectivity in a convergent fashion. The chiral auxiliaries ((1R)-and (1s)-camphanic acid) are recovered at the beginning of the syntheses.A large variety of natural products of biological interest contain polypropionate fragments (chain with alternating OH and Me substituents) and analogues involving tertiary-alcoholic centres [2]. Several methods and strategies have been developed to provide access to these systems which possess a high density of stereochemical information [3]. The control of the stereochemistry can rely on pericyclic reactions such as the Claisen rearrangement [4]