Reactions of Achiral Oxabutadienes with Chiral Vinylethers

Reactions of Chiral Oxabutadienes with Achiral Vinylethers 

1-Oxa-1,3-dienes with acyloxy functions at the C-4 position react with enolethers at room temperature, furnishing the cycloadducts with moderate selectivities. After removal of (/ )-0-methyl mandelic acid by treatment with Me0H/K2C03 the major isomer was transformed into (-)-T-olivose. 

Dihydropyrans are obtained with excellent selectivities starting from oxabutadienes with an acyloxy function at the C-2 position bearing a chiral oxazolidinone. A reversal of the endo/exo selectivity is achieved by appropriate choice of the Lewis acid. 

The [4+2] cycloaddition of A -crotonyl oxazolidinones with alkenes at low temperature in the presence of Me2AlCl as Lewis acid leads to 8-lactones stereoselectively. 

Cycloaddition reactions of the chiral exocyclic methylene compounds A and B derived from menthone and of related chiral 1-oxa-l,3-dienes (C) with alkenes as dienophiles, e.g. ketene acetals, proceed smoothly and are highly stereoselective.  

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