Chlorophyll isocyclic ring reactions

An alternative route to the reaction sequence applied by Woodward for the formation of the isocyclic ring E of chlorophyll a in his synthesis has been developed.40 55,56 It was shown that... 

The oxidative scission of the isocyclic ring on the nucleus of chlorophyll and its derivatives, via allomerization, consists of a series of well documented reactions (6-7) and leads to such products as chlorin (e.g. -ea.-eg.-PeT nd purpurin (e.g. 

If the aUomerization reaction takes place in a slightly alkaline medium and under inert atmosphere, there is only solvolysis of the isocyclic ring, and the resulting components are denominated esterified chlorin es and rhodin g-j, depending on whether they come from the a- or fc-series, respectively. Chlorophyll a yields Mg-phytol-chlorin while pheophytin a or pheophorbide a yields phytol-chlorin eg or free chlorin eg, respectively. The same series of transformations can be described for chlorophyll b. The structural difference between these compounds and the purpurins 7 is solely in the substituent of C-15, which, in place of an O, is an H, so that their coloration, polarity, and electron absorption properties are very similar. Other techniques, such as mass spectroscopy (MS), must be used for their differentiation [28]. 

In contrast, de-esterification of phytol in the chlorophyll molecule does not affect the chromophore stmcture, so the electron adsorption spectmm is unaltered. Thus, chlorophyUides and pheophorbides have the same spectroscopic properties as then-respective precursors, chlorophylls and pheophytins. The same is tme of some of the reactions that affect the isocyclic ring, such as epimerization and decarbometh-oxylation on C-13. ... 

Primary allomerizalion reactions that give rise to hydroxy- or methoxy-chlorophylls also do not affect the spectroscopic properties. In contrast, other oxidative reactions involving opening of the isocyclic ring or the formation of a lactone ring do cause important changes in the electron absorption spectmm. 

Pigments like XVII, XVIII and XIX bearing a chlorin macrocycle characteristic for plant chlorophylls, do not exchange under the reaction conditions. The same is true for pigments like (XV, XVI) in which the isocyclic ring V is absent. 

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