(Z)-6-(4-Hydroxy-3-methylbut-2-en-1-ylamino)purine, (Z)-zeatin, and the isomeric (Z)-isozeatin, both free from the E isomers, were prepared using the cycloaddition of the in situ generated tert-butyl nitrosoformate on isoprene. A mixture of tert-butyl 5(and 4)-methyl-3,6-dihydro-2H-1,2-oxazine-2-carboxylates thus formed was deprotected and reductively cleaved to (Z)-4-amino-2(and 3)-methylbut-2-en-1-ols, which were chromatographically separated and finally alkylated with 6-chloropurine to give the title compounds.
A survey of the synthetic pathways leading to the fluorine-containing analogues of amino acids is given. From the great number of syntheses the typical examples are selected and divided into two groups: classical syntheses and the modern ones. The classical ammonolysis of halogeno acids and equivalent reactions are discussed as first, followed by a few examples of oxo → amino group transformation. Conversion of the oxo compounds into amino acids richer by one carbon atom is realized by the Strecker and hydantoin syntheses. For the prolongation by two carbons, the Erlenmeyer azlactone method and alkylation of CH-acidic esters are applied. The modern syntheses are represented by direct fluorination by elemental fluorine and other electrophilic fluorinating reagents. Further examples include the applications of the Yarovenko reagent, sulphur tetrafluoride and its derivative DAST. The use of trifluoropyruvates as the fluoro synthons is mentioned briefly. Finally, the examples of the amidocarboxylation method and the syntheses of diverseω-fluorinated methionines are shown.
A method is described for the synthesis of 2-fluoropropenoic acid esters, amides, and nitrile from the corresponding derivatives of 2-fluoro-3-(4-toluenesulfonyloxy)propanoic acid by heating with potassium phthalimide in vacuo. The free acid Ia and its chloride have also been synthesized. The reactivity of esters and nitrile of acid Ia has been verified by the Michael addition of diethyl acetamidomalonate to these compounds.
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