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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

[reaction: see text] The stereocontrolled synthesis of malayamycin A, a novel naturally occurring bicyclic C-nucleoside of the perhydrofuropyran type, is described.

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.

A stereocontrolled synthesis of 1-cytosinyl-N-malayamycin A, an N-analogue of the naturally occurring malayamycin A with fungicidal activity, is reported. The approach was designed to rely solely on substrate control for introduction of the required stereochemistry, avoiding the use of chiral reagents or auxiliaries. Formation of the N-nucleoside was achieved through the activation of a thioglycoside, proceeding via sulfonium and thionium intermediates. Ring closure metathesis was used to build the bicyclic perhydrofuropyran heterocycle.

The reductive opening of 2-methoxyethylidene acetals of vicinal diols in uridine and 1,4-anhydro-D-ribitol in the presence of TiCl 4 and Et 3 SiH was investigated. The 3¢-O-(2-methoxyethyl) ether of uridine and the 2¢-O-(2-methoxyethyl) ether of 1,4-anhydro-D-ribitol were isolated and characterized. The results were rationalized based on coordination effects involving proximal substituents.

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