SYNTHETIC ROUTES TO ALLENIC ACIDS AND ESTERS AND THEIR STEREOSPECIFICCONVERSION TO BUTENOLIDES

The synthesis of allenic acids and esters and their conversion to bute nolides has been examined in some detail. Racemic butenolides 10 are e fficiently prepared from the esters 8 through treatment with BCl3 and exposure of the derived acid 9 to catalytic AgNO3 in acetone. Conversi on of the enantioenriched allenylstannane (S)-17 to the acid 18 throug h lithiation and subsequent carboxylation with CO2 afforded racemic pr oduct, The enantioenriched propargylic mesylates 16 and 22 afforded th e allenic esters 19 and 23 with inversion of configuration through tre atment with Pd(Ph(3)P)(4), CO, and the appropriate alcohol in THF. The se reactions proceeded with ca. 10% or less of racemization. The allen ic esters 23 yielded the iodobutenolides 24 by reaction with IBr, Hydr ogenolysis to the butenolide 25 was achieved with Pd(PPh(3))(4) and Bu (3)SnH, Alternatively, the allenic acids 27 could be prepared directly from mesylates 22 with Pd(PPh(3))(4) and CO in aqueous THF. Cyclizati on to the butenolides 25 was achieved, as before, with catalytic AgNO3 .