合成路线1

The formation of the free carboxyl group derivative (IV), as shown in Scheme 22578901b, was suggested to occur through formation of the diasteromeric lactone intermediates (VIIIa, b). Although there is no evidence to support the following mechanism, it has been hypothesized that only the syn-lactone can eliminate in the presence of one equivalent of LiOEt to give the free carboxyl derivative which is then transformed into the final compound (IV). Conversely, as far as the anti-lactone derivative is concerned, it seems that, for steric reasons, this intermediate is unable to reach the necessary antiperiplanar conformation to evolve towards the Z-elimination product. In the latter case, LiOEt, acting this time as a nucleophile, could afford lithium aldolate (VII). The final retroaldolic reaction could regenerate the starting materials (II) and (III), repeating the aldol-elimination-lactonization process.