Polyene unit, isomerization

With aluminum-chloride catalysts, the polymer structure is 80% cyclobutane units and about 5% cyclopropane units, the remainder being polyene units. With stannic chloride, we do not observe any cyclopropane units the polymer consists mainly of cyclobutane units (75%). The presence of methyl groups on cyclohexane shows that the other units possess partially isomerized and cyclized polyene structures. When using titanium chloride or ethyl aluminum chloride with traces of water as cocatalyst, the polymer consists mainly of cyclobutane units—that is, 60%, the demainder being cyclized see Equation 17. 

For example, the ion of [M-69]+, which is observed in the tandem mass spectra of lycopene, neurosporene, and y-carotene but not a-carotene, p-carotene, lutein, or zeaxanthin, indicates the presence of a terminal acyclic isoprene unit. Elimination of a hydroxyl group or a molecule of water, [M-17]+ or [MH-18]+, from carotenoids such as astaxanthin or zeaxanthin is characteristic of the presence of a hydroxyl group. Also, tandem mass spectrometry can be used to distinguish between isomeric carotenoids such as a-carotene and p-carotene, or lutein and zeaxanthin. For example, the ring of a-carotene containing the double bond that i s not conj ugated to the rest of the polyene chain shows unique retro-Diels-Alder fragmentation to form the ion of [M-56]+. In a similar manner, isomeric lutein and zeaxanthin differ by the... 

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