Para-Substituent-selective conversions

In regard to the distribution of the products, zeolitic catalysts yield a considerable firaction of p-VA, while this does not occur in the case of Nation. This indicates an effect of shape-selectivity which favours the formation of the para isomer, which is less bulky than the ortho isomer (o-VA). The para (p-VA) and the meta (m-VA) isomers have similar bulkiness, but the formation of the meta isomer is less favoured due to the electronic effects of the hydroxy and methoxy substituents. Nafion yields the highest amount of by-products, as does the H-Y zeolite (even though in the latter case the low conversion makes comparisons with other zeolites less reliable). 

In order to promote selective deprotonation at C-1 in 109, an electron attracting substituent is required at the para position, C-S. The nitrile group was chosen— also because it permitted ready conversion to phenolic hydroxyl at the end of the synthesis (Scheme 15). lodination of sulfone 109 followed by iodide-cyanide exchange gave the C-5 nitrile 115. The optically active acid (-f) 103 (Scheme 14), via its methyl ester (4-) 102, was converted in good yield into iodo olefin 116. 

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