Chlorophyll-a derivatives

A famous use of electrochemical reduction of porphyrin systems is that used to convert a chlorophyll a derivative, chlorin-e6 trimethyl ester (64), into a chlorophyll b derivative, rhodin-g7 trimethyl ester (65) (71LA(749)109). Electrolysis of chlorin-e6 trimethyl ester gave the chlorin-phlorin which was photolyzed in dioxane/water to give the trans-d o (Scheme 7) this was simply transformed into rhodin-g7 trimethyl ester (65) to accomplish the first ever interconversion of a chlorophyll a series pigment into one of the companion b series. 

Cyclopheophorbide enol (122) a nonmetalated chlorophyll a derivative was isolated from a New Zealand sponge (Darwinella oxeata) and its structure determined by X-ray measurements (113). Although 132,173-cyclopheophorbide enol (122) was first isolated from natural sources, it had previously been synthesized during a study of ring E enolization of chlorophyll derivatives (114). A new pheophorbide a-related compound named chlorophyllone a (123) was isolated from extracts of the short-necked clam Ruditapes philippinarum (115). Compound 123 exhibits antioxidative activity. 

The losses of magnesium and phytol through the combined effects of cellular senescence and predation (i.e. aerobic heterotrophy) in the water column lead to pheophytin-a and pheophorbide-a becoming the primary chlorophyll-a derivatives deposited in marine sedimentary environments. Though it is not known at present, the heterotrophic processes which cleave phytol more than likely also affect the 10-carbomethoxy group. Studies are underway to investigate the amounts of pyro-pheophorbides in water column detritus and surface sediments. 

Figure 12. Proposed diagenesis of chlorophyll-a derivatives. Code MESO = 2-vinyl reduced to ethyl PYRO = lacks C 10 carbo-methoxy group OD = 9-oxydeoxo asterisk = possible 6,y-cyclo-etheno intermediate LMWCC = low molecular weight colorless compounds (cf. Figure 1).

Fig. 5. 9 Major anoxic sedimentary diagenetic routes for chlorophyll-a derivatives subsequent to those in Fig. 5.6 (after Barwise Roberts 1984 Baker Louda 1986 Louda et al. 1998, 2000).M= cations (e.g.Ni2+,V02+, Cu2+, GaOH2+).

The reactivity of the porphyrins and metalloporphyrins will be described mainly using octaethylporphyrin and protoporphyrin derivatives. Reactions of TPP and chlorophyll a derivatives are mentioned only on the occasion of P-formylation and the reduction of nitro groups, which yield important educts for synkinetic reactions. The reversible reactions described for -substituted reactions also work with meso-tetraphenylporphyrins. 

Progress in the chemical reactions of chlorophyll-a derivatives and synthesis of polysubstituted chlorin or porphyrin 05CJO1353. 

Figure 3.19 A methyl ester chlorophyll-a derivative and the helical structure it adopts in the solid state.

Among porphyrins, chlorophyll - a derivative of chlorin (dihydroporphyrin-7,8) attracts the greatest attention. 

The investigation was carried out with an in vitro digestion method which simulates both the gastric and small intestinal phases of the process. During the digestion, the native chlorophylls were converted to Mg-free pheophytin derivatives and the miceUaiization of chlorophyll a derivatives was significantly more efficient... 

Chlorins 10 (Figure 10.2) were prepared using a similar route, from methyl pyropheophorbide-a (a chlorophyll-a derivative) by functionalization of the vinyl group using various alcohols ROH (R = methyl, propyl, pentyl, heptyl, nonyl), followed by cleavage of the ester group, activation with oxalyl chloride, and reaction with the tetramethylammonium salt of Chlorins 10... 

Since Woodward s work on the synthesis of chlorophyll a (60JA3800) it is known that the intrinsic unstable thioformyl moiety can be stabilized by the delocalization effect of heterocyclic systems. Recently the synthesis of 2-amino- and 3-aminothioformylthiophenes (and furans) and the corresponding benzo derivatives (Scheme 19) has been reported (96S1185). These compounds exist as amino tautomers (91S609 96S1185). 

Chlorophyll a, the green photosynthesis pigment, is the prototype of the chlorin (2,3-dihydro-porphyrin) class of products. It was first isolated by Willstatter1 at the turn of the century. The common structural unit in this class is the chlorin framework named after chlorophyll. The chromophore with a partially saturated pyrrole ring, which is formally derived from the completely unsaturated porphyrin, is less symmetric than the latter and systematically named according to IUPAC nomenclature as 2,3-dihydroporphyrin. 

Another tetrahydroporphyrin derivative, porphodimethene (7), which consists of two dipyr-rylmethene halves is obtained in the photoreduction of magnesium-containing chlorophyll a (6) with hydrogen sulfide3211 42 as reductant and pyridine as base. 

Several chemical transformations in the chlorin series were discovered during the course of Woodward s total synthesis of chlorophyll a.3a d An important reaction in the final steps of this total synthesis is the removal of an a-oxo acid ester residue from the 17-position of the chlorin 22, which proceeds very smoothly in the presence of base by a retro-aidol-type fragmentation to yield the chlorin isopurpurin methyl ester (23) which is also available by degradation of chlorophyll a, so that at this point of the synthesis synthetically derived material could be compared with an authentic sample prepared from natural chlorophyll a. 

From these stractural features it is interesting to note that each molecule of chlorophylls a and b consists of a hydrophilic part (tetrapyrrole macrocycle) and a hydrophobic portion (long terpenoid chain of phytol esterifying the acid group at C-17). Figure 2.1.2 shows the structures and nomenclature of chlorophylls a and b and their major breakdown derivatives. 

Despite the ubiquitous distribution of chlorophylls in all photosynthetic plants, quantitative information exists only for a few vegetables. The most common edible plants lack definitive data and consequently no information is available about chlorophyll distribution in current food composition tables. Still more difficult is to find analytical data in literature about the individual amounts of chlorophyll a and b and their respective derivatives. 

Amotf was the first to develop a set of equations for acetone to simultaneously calculate chlorophyll a and chlorophyll b in 1949. Several authors later proposed different new equations based on more adjusted and accurate extinction coefficients due to the development of higher resolution spectrophotometers adapted to each special condition. Moreover, besides 80% acetone, coefficients for diethyl ether and ethanol were also established and their respective equations developed, as reviewed by Schwartz and Lorenzo and Eder. Solvents chosen should be those for which specific absorbance coefficients have been published to derive equations and updates should be carefully tracked for new values. 

Spectrofluorometry presents sensitivity and selectivity greater than the absorbance spectroscopy, being more suitable for chlorophyll estimates in the nmol range and for residual amounts of derivatives in food products. Absorbance spectroscopy is satisfactory for concentrations > 1 xMP Spectrofluorometry is also more accurate for a wide range of chlorophyll a-to-chlorophyll b ratios, but it is less accurate when applied to complex sample matrices because of unpredictable quenching effects. 

All the analytical methods mentioned to separate, identify, and quantify chlorophylls and derivatives consume time, money, and samples. As alternatives, industries have been employing non-destructive methods for surface color measurements that are not only indirectly related to chlorophyll content, but may also estimate the pigments directly in tissues, leaving the sample intact and enabling serial analyses in a relatively short time. Eood color affects consumer acceptance and is an important criterion for quality control. Color vision is a complex phenomenon that depends on both the total content and number of pigments and also on absorption, reflectance and emission spectra of each compound present. 

Ergun, E. et al.. Simultaneous determination of chlorophyll a and chlorophyll b by derivative spectrophotometry. Anal. Bioanal. Ghent., 379, 803, 2004. 

FIGURE 13.3 The structures of chlorophylls a and b. Other chlorophylls and their decomposition products can be derived from these structures as described in the text. 

Hynninen and coworkers <99JCS(PT1)2403> used a similar approach to prepare phytochlorin-C6o diad 38 (Scheme 11). The protocol employed the pyrolysis of the natural chlorophyll a molecule 35, followed by transesterification and demetallation to furnish derivative 36. Subsequent oxidation of 36 with OsCU and NaI04 has allowed the synthesis of the formyl derivative 37, which was further used as precursor of the azomethinic ylide intermediate in the 1,3-DC reaction with Cm leading to the formation of diad 38. Photochemical studies revealed that this diad underwent a fast intramolecular photoinduced electron transfer in polar solvents such a benzonitrile <99JACS9378>. 

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