Download full text in PDF Opens in a new window. Article suggestions will be shown in a dialog on return to ScienceDirect. ... Please enable JavaScript to use all the features on this page. ... The title structures can be prepared by Diels-Alder cycloaddition of activated acetylenic or olefinic dienophiles tethered to a proximally-methylated butadiene unit. ... All chiral substances were produced as racemates; a single enantiomer is shown for simplicity. All yields cited are for ...