Molybdenum-promoted cycloisomerization

If photochemical apparatus is not available, the cycloisomerization reaction can be conducted using trimethylamine N-oxide to promote oxidative decarbonylation of molybdenum hexacarbonyl in a mixture of EtjN and EtgO, followed by addition of 1-phenyl-3-butyn-1-ol (1). In the submitters hands, this procedure required somewhat higher loading of molybdenum hexacarbonyl, and purification of the 2-phenyl-2,3-dihydrofuran (2) product required silica gel chromatography. 

Based on these studies and choosing to focus on Ru for practical considerations such as cost, we envisioned a possible catalytic cycle shown in Scheme 1.1, wherein an oa-alkynyl alcohol 1 would cycloisomerize to the dihydropyran. Although McDonald s group has pioneered the use of molybdenum- and tungsten-mediated processes, several issues related to chemoselectivity and the common need for stoichiometric amounts make the development of other catalysts for such processes desirable [8]. Using CpRu(PAr3)2Cl-based complexes, less electron-rich arylpho-sphine ligands such as m- or p-fluorophenylphosphines promote such processes (Equation 1.2) [9]. 

Asymmetric synthesis of nucleosides via molybdenum-catalyzed alkynol cycloisomerization coupled with stereoselective glycosylations of deoxyfuranose glycals and 3-amidofuranose glycals. Journal of the American Chemical Society, 118, 6648-6659 (b) McDonald, F.E. and Chatterjee, A.K. (1997) Group VI metal-promoted endo-azacyclizations via alkyne-derived metal vinylidene carbenes. Tetrahedron Letters, 38, 7687-7690 ... 

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