A hetero Diels-Alder access to (Z)-zeatin and (Z)-isozeatin

(Z)-6-(4-Hydroxy-3-methylbut-2-en-1-ylamino)purine, (Z)-zeatin, and the iso meric (Z)-isozeatin, both free from the E isomers, were prepared using the cycloaddition of the bl situ generated tert-butyl nitrosoformate on isopren e. A mixture of tert-butyl 5(and 4)-methyl-3,6-dihydro-2H-1,2-oxazine-2-car boxylates thus formed was deprotected and reductively cleaved to (Z)-4-amin o-2(and 3)-methylbut-2-en-1-ols, which were chromatographically separated a nd finally alkylated with 6-chloropurine to give the title compounds.