Rrans-pent

The same discrepancy is found in the abilities of metals to afford one of the other isomers in hydrogenation of cw-penta-1,3-diene. Half-reduction over Co leads to 90% yield of rrans-pent-2-ene, whereas over Cu 70% pent-l-ene is obtained even though the disubstituted double bond is more difficult to hy ogenate than the terminal one ... 

Figure 4. Catalytic activity of the Grubbs carbene 1 in the metathesis reaction of rran -pent-2-ene.

The mechanism of both the uncatalysed and W-catalysed cycloisomerisation of pent-4-yn-l-ol has been studied in detail. The endo reaction is complex but is favoured mainly as a result of stabilisation of a vinylidene intermediate <02JA4149>. The W-mediated cyclisation of alkynols has been used to synthesise glycals of the antibiotics vancosamine and saccharosamine <02OL749> and rrans-fused THP derivatives of 5 H-dibenzo[a,d]cycloheptene <02TL8697>. 

Stable metal carbene complexes, such as W[=C(OMe)Me](CO)5, were first prepared by Fischer, E.O. (1964). These 18-electron complexes can be activated as catalysts for the metathesis of pent-l-ene or the ROMP of cycloalkenes by the use of a cocatalyst, or by heat or UV irradiation see Table 2.1. For such complexes to become active as initiators of olefin metathesis it is necessary for a CO ligand to be displaced, allowing the substrate to enter the coordination shell and react with the metal carbene bond. For the ROMP of 1-methyl-rrans-cyclooctene initiated by W(=CPh2)(CO)5 at 50°C the Ph2C= end groups may be detected in the polymer by the UV absorption at 245 nm (Lee, S.J. 1976). 

The stereochemistry of the addition of chlorine to 3,4-dihydro-2/f-pyran has been found to be influenced by the polarity of the solvent in which the reaction is conducted. Stereoselective syn addition occurred in non-polar solvents e.g. n-pent-ane) to give a mixture of cis- and rrans-2,3-dichlorotetrahydropyrans in the ratio of ca. 4 1, whereas the ratio was 1 2 in polar solvents e.g. dichloromethane). cis-2,3-Dichlorotetrahydropyran yielded exclusively trans-products with various nucleophiles (N3, MeO, PhS, and AcO ) in DMF with inversion of configuration at C-2, whereas trans-2,3-dichlorotetrahydropyran yielded the cis-product with the thiophenate anion and a mixture of cis- and trans-products with the other anions. Hydrogen chloride and hydrogen bromide added to 3,4-di-O-acetyl-L-fucal (134) and -rhamnal (135) to give adducts (136) and (137), respectively, possessing the a-L configuration. ... 

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