Methyl 5-deoxy-5-iodo-pentofuranosides are reductively ring-opened and propargylated in a tandem fashion in the presence of zinc. The 1,7-enynes thus obtained are subjected to ring-closing enyne metathesis with catalyst B to produce functionalized 1-vinyl cyclohexenes. By adding BnNH(2) to the tandem reaction, an amino group can be introduced in the 1,7-enyne products. Addition of 2-TMS-ethynylcerium(III) chloride after the reductive ring-opening produces the corresponding 1,6-enynes. Further annulation of the product 1,3-dienes can be achieved through a Diels-Alder reaction with good control of stereochemistry. These procedures constitute efficient methods for rapid carbocyclization and annulation of carbohydrates to produce a variety of functionalized five- and six-membered ring systems.
Carbohydrates
Carbohydrates U 0500Regioselective Conversion of Primary Alcohols into Iodides in Unprotected Methyl Furanosides and Pyranosides. -Two methods are described for the alcohol conversion into iodides. For the first method, reverse phase column is used to separate the iodoglycosides (II) from POPh 3 . The second method includes sulfonylation in pyridine with sterically hindered trichloro-or tribromobenzenesulfonylchloride. The sulfonates are effective leaving groups, thus, iodide substitution can be carried out in a one-pot process. Products (II) as well as ethers of type are required for Zn-mediated fragmentations. -(SKAANDERUP, P. R.; POULSEN, C. S.; HYLDTOFT, L.; JOERGENSEN, M. R.; MADSEN*, R.; Synthesis 2002, 12, 1721-1727; Dep. Chem., Tech. Univ. Den., DK-2800 Lyngby, Den.; Eng.) -C. Herrmann 02-177
Two methods are described for the regioselective displacement of the primary hydroxy group in methyl glycosides with iodide. The first method is a modification of a literature procedure employing triphenylphosphine and iodine, where purification has been carried out on a reverse phase column in order to efficiently separate the desired iodoglycosides from triphenylphosphine oxide. The second method employs a new procedure using sulfonylation in pyridine with sterically hindered 2,4,6-trichloro-and 2,4,6-tribromobenzenesulfonyl chloride. The sulfonates thus formed are effective leaving groups and substitution with iodide can be carried out in a one-pot process. Protection of the iodoglycosides is also described either by benzylation with benzyl trichloroacetimidate or silylation with triethylsilyl chloride.
Enyne metathesis combines an alkene and an alkyne into a 1,3-diene. The first enyne metathesis reaction catalyzed by a ruthenium carbene complex was reported in 1994. This review covers the advances in this transformation during the last eight years with particular emphasis on methodology development and synthetic applications. Concluding Remarks
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