| Abstract
| - The synthesis of allenic acids and esters and their conversion tobutenolides has been examined insome detail. Racemic butenolides 10 are efficientlyprepared from the esters 8 through treatmentwith BCl3 and exposure of the derived acid 9 tocatalytic AgNO3 in acetone. Conversion oftheenantioenriched allenylstannane (S)-17 to theacid 18 through lithiation and subsequentcarboxylation with CO2 afforded racemic product. Theenantioenriched propargylic mesylates 16 and22afforded the allenic esters 19 and 23 withinversion of configuration through treatment withPd(Ph3P)4, CO, and the appropriate alcohol inTHF. These reactions proceeded with ca. 10%orless of racemization. The allenic esters 23 yielded theiodobutenolides 24 by reaction with IBr.Hydrogenolysis to the butenolide 25 was achieved withPd(PPh3)4 and Bu3SnH.Alternatively, theallenic acids 27 could be prepared directly from mesylates22 with Pd(PPh3)4 and CO inaqueousTHF. Cyclization to the butenolides 25 was achieved, asbefore, with catalytic AgNO3.
|
