Allene-1,3-dicarboxylates, Diels-Alder

As a first approach to the synthesis of nagilactone 296, a norditer-penoid isolated from Podocarpaceae, which inhibit the expansion and mitosis of plant cells, an intramolecular Diels-Alder reaction of allene 1,3-dicarboxylic acid esters was used. The cyclization of 297 afforded the 8-lactone 298, rather than the y-lactone 299 [85JCS(P1)747]. 

An interesting variation of the above process involved the addition of optically active (/ )-di-(—)-menthyl allene-1,3-dicarboxylate (28) onto furan in an asymmetric Diels-Alder reaction [27]. Three equivalents of titanium tetrachloride at 40 °C gave the highest diastereofacial selectivity and afforded a 53% isolated yield of the optically active adduct 29, which represents a very useful chiral intermediate for synthetic applications (Scheme 13.11). 

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