The influence of chain-branching on the steric outcome of some olefin-forming reactions

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Abstract
When applied to the construction of internal disubstituted double bonds in compounds such as pentadec-7-ene and 2,6,11,15-tetramethylhexadeca-2,6,8,10,14-pentaene (5), the reductive elimination of vicinal benzoyloxy-sulphones gives products with a trans : cis ratio of ca. 4 : 1. When applied to the construction of disubstituted olefins in which branching occurs adjacent to the newly formed double bond, much higher trans-stereoselectivity is observed, e.g, for 5-ethyl-2-methylhept-3-ene (21) and 3,6-dimethylocta-2,4,6-triene (25) it is at least 98%. A similar effect of chain-branching is also observed in some Wittig and Homer reactions using primary allylic phosphorus derivatives. When a Z-allylic sulphone is used as the starting material in the benzoyloxy-sulphone method, the original Z-geometry is preserved in the product; (Z)-2-methylbut-2-enyl p-tolyl sulphone and tiglic aldehyde give almost exclusively (2E,4E,6Z)-3,6-dimethylocta-2,4,6-triene (27).

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