Cascade molecules in which the branch points are phosphonium ion sites have been synthesized bearing phosphoms in a variety of oxidation states at the core. In addition to the previously reported phosphonium ion core species, these new cascade molecules include those bearing phosphine, phosphine oxide, and phosphorane cores. INTR 0 D U CTI ONA recent interest of our laboratory has been in the preparation and investigation of an intriguing category of molecules, cascade molecules (dendrimers, chemical fractals) [l] in which phosphorus has been incorporated at the core and branch points of the structure. Our efforts have been particularly concerned with the preparation and study of species in which phosphorus is present as phosphonium ion sites, rendering the covalent structure positively charged with "free floating" associated anions [2,3]. These species represent the first dendrimers bearing (charged sites within the covalent framework.The current efforts have been concerned with the preparation and investigation of dendrimers which retain the phosphonium ion branch points but are modified at the core to contain phosphorus in other oxidation states. Of specific interest are those with phosphine cores (accessible through the phosphine oxide or phosphine sulfide species) and those with phosphorane cores. Phosphine cores provide entry into the investigation of a wide range of chemical interactions, including those as nucleophile and metal complexation agent, modified by the presence of a large, highly charged ancilliary structure. A phosphorane core provides the first dendrimer involving pentadirectional elaboration from a single atom. RESULTS AND DISCUSSIONThe elaboration of all of the dendrimers considered in the present work begins with the previously reported trib-methoxymethy1)phenylphosphine. [2,3] The preparation of the phosphine oxide core dendrimer beginning with 1 is accomplished as shown in Scheme 1.Oxidation of the phosphine 1 to the phosphine
1,3-Dioxan-5-yl pyrimidine nucleoside analogues, higher homologues of antiviral and anticancer 1,3-dioxolanes, were prepared from bis-1,3-tritylglycerol and 3-benzoylated bases (uracil, 5-fluorouracil, thymine). Mitsunobu condensation, deprotection, and cycloacetalization gave cis/trans mixtures of 2,5-disubstituted-1,3-dioxanes in which the desired cis stereoisomers predominated. Cytosine derivatives could not be obtained in this manner; N 4-benzoylcytosine afforded an O-2 alkylated Mitsunobu product that rearranged to an O 2-(2,3-dihydroxypropyl)cytosine on detritylation with aqueous acetic acid. Cytosine and 5-fluorocytosine nucleosides were therefore prepared from the corresponding uracils via their 1,2,4-triazole derivatives. 1H NMR data established the conformational preference for equatorial 2‘-hydroxymethyl and axial 5‘-base in the cis isomers; the trans compounds were diequatorial. Despite their conformations, the cis nucleosides showed no antiviral activity.
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