2- Melhyl-2,3-pentadiene

Bei der Umsetzung von N-(2,3,4,5,6-Pentaacetoxy-thiohexanoyl)-acetamid mit (Ethoxycarbonyl-methyli-den)-triphenyl-phosphoran wird in niedriger Ausbcute 2-( 1,5-Diacctoxy-l,3-pentadien-y )-5-etht>xyi arbo-nyl-4-melhyl-l 3-thiazol (18%) erhalten756. Der Mechanismus dieser offensiehllich komplexen Reaktion ist noch ungeklart. 

With substrates bearing two different substituents at the 3-position, such as 3-ary 1-3-melhyl-1.4-pentadienes, tm/rs-products are favored under thermodynamic control (transjeis 4 1). Preferential formation of c/s-products (up to transicis 1 2) can be achieved under kinetic control using lower reaction temperatures, shorter reaction times or variations in the catalytic system. However, epimerization of stereogenic carbon centers adjacent to the carbonyl group is observed under the reaction conditions15. Preliminary experiments u ith chirally modified rhodium catalysts show only low asymmetric induction in the cyclopentanone products19. 

The two structures cannot be superposed. 4-Melhyl-2,3-hexadiene is chiral. Rotation of either representation 180° around an axis that passes throngh the three carbons of the C=C=C unit leads to superposition of the groups at the bottom carbon but not at the top. 2,4-Dimethyl-2,3-pentadiene is achiral. Its two mirror-image forms are superposable. 

Recently, Cataldo and co-workers reported polymerization reactions of isoprene and 3-melhyl-1,4-pentadiene in DCA inclusion compounds, and compared the polymerization reactions of 2,3-dimethylbutadiene within the DCA and thiourea host structures. The polydimethylbu-tadiene prepared by inclusion polymerization in the DCA host structure was found to have a lower degree of regularity and crystallinity than that prepared by polymerization in the thiourea host structure. 

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