Nucleic acids U 0700Total Synthesis of N-Malayamycin A (I) and Related Bicyclic Purine and Pyrimidine Nucleosides. -The title compounds are prepared in order to test their biological activity. -(HANESSIAN*, S.; HUANG, G.; CHENEL, C.; MACHAALANI, R.; LOISELEUR, O.; J. Org. Chem. 70 (2005) 17, 6721-6734; Dep. Chem., Univ. Montreal, Montreal, Que. H3C 3J7, Can.; Eng.) -Jannicke 52-184
Methods are described for the total synthesis of bicyclic perhydrofuropyran nucleosides as N-analogues of the naturally occurring malayamycin A. Formation of the N-nucleosides relied on the activation of thioglycosides, proceeding via sulfonium intermediates. Ring closure metathesis was used in two approaches to build the bicyclic dioxa heterocycle. Another approach relied on the use of a sugar precursor and cyclization to the bicyclic thioglycoside.
Aliphatic azidonitriles separated by three or four carbon atoms undergo facile cyclo-additions in the presence of BF3·OEt2 at room temperature or lower, to give bicyclic tetrazoles. 1-Azido-[2-aryl-1,3-dioxolanyl]-glycerols afford oxabicyclic tetrazoles with trimethylsilyl cyanide (TMSCN). Aspects of these facile proximity-induced 1,3-dipolar cycloadditions are discussed with mechanistic interpretations.
Aliphatic azidonitriles separated by three or four carbon atoms undergo facile Lewis acid-induced cycloadditions to give bicyclic tetrazoles, even at 0 degrees C. Extension to 3-azido-2-aryl-1,3-dioxolanes and the corresponding 1,3-dioxanes in the presence of TMSCN and BF3.OEt2 leads to a series of diversely functionalized novel oxabicyclic tetrazoles. The reactions represent new aspects of proximity-assisted dipolar cycloadditions that afford thermodynamically controlled enantiopure products proceeding through discrete oxocarbenium ion intermediates.
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