The Total Synthesis of Chlorophyll

Abstract

Chlorophyll a, the major green pigment of the plant world, is certainly the most widespread and conspicuous of organic natural products. Few can be unaware of its decorative function, and all are beneficiaries of its central role in transforming sunlight into substance and sustenance. Yet the fruitful chemical study of this green badge of life did not commence until fairly recent times. For chlorophyll a is a very reactive, sensitive, and complicated substance. Only when, early in this century, the genius of Willstatter applied itself to the problem, were the first sure steps taken. That great investigator isolated the pigment-and its closely related frequent minor concomitant chlorophyll b as well-in the pure state, established correctly the empirical formula of the substance, and laid down a sound and extensive preliminary basis of transformation and degradation. These achievements can be measured against the fact that the isolation of chlorophyll in a state of purity is even now, after more than fifty years, no mean feat, and, further, that the empirical formula defined by Willstatter, repeatedly called into question by subsequent investigators, has stood the test of time. For some period after this solid foundation had been laid by Willstatter there was little activity until three new groups took the field late in the 1920s. Stoll, who had played a prominent role in the early studies as a collaborator of Willstatter, took up the work anew, and made important contributions, as did Conant in the United States. But by far the greatest contribution was made by Hans Fischer and his collaborators at Munich. Fresh from his dramatic conquest of the blood pigment, Fischer hurled his legions into the attack on chlorophyll, and during a period of approximately fifteen years, built a monumental corpus of fact. As this chemical record, almost unique in its scope and depth, was constructed, the molecule was transformed and rent asunder in innumerable directions, and the fascination and intricacy of the chemistry of chlorophyll and its congeners was fully revealed. These massive contributions were crowned by the proposal, in 1940, of a structure which was complete except for stereochemical detail. Finally, in a series of elegant investigations completed only during the last few years, Linstead and his associates at Imperial College were able to solve the stereochemical problem and to provide definitive confirmatory detail in respect of the number and disposition of saturated carbon atoms within the nuclear framework. Half a century of structural study had culminated in the complete formula (I) for chlorophyll aI, 8. Our active interest in chlorophyll was initiated four years ago, in 1956. The first questions we asked were very general ones. The structural investigations had been carried out almost entirely during the twilight of the classical period of organic chemistry. Only the very simplest basic * A brief communication recording the results on which this lecture is based has appeared in

Cycloaddition reactions of porphyrins

“…20 The reactivity of β-imino derivatives of meso-tetraphenylporphyrin 30 as heterodienes was recently investigated. For instance, in the presence of La(OTf) 3 it reacts with 3,4-dihydro-2H-pyran to afford tetrahydropyridine-fused porphyrins 31 and pyridoporphyrins 32 (Scheme 6). …”

“…Two landmarks have to be mentioned: the synthesis of protoporphyrin-IX (1) by Fischer in 1929, 2 and the synthesis of chlorophyll a (2) by Woodward and his seventeen postdoctoral colleagues in 1960. 3 Research has also been centred on physical chemistry and biochemical aspects of such macrocycles. 4,5 More recently several groups have reported promising applications for porphyrin derivatives.…”

Exciton/Charge-Transfer Electronic Couplings in Organic Semiconductors

“…50 Porphyrins have an absorption onset near 450 nm 51 and have an important role in photosynthetic systems. 52,53 a) b) Figure 7: Attachment/detachment density plots for Zn-porphyrin:PTCBI illustrating a) nonlocal CT-like and b) localized exciton-like TDDFT states. Red (green) regions have excess (deficient) density compared to the ground state.…”

Extração e purificação de clorofila a, da alga Spirulina maxima: um experimento para os cursos de química

“…Cabe salientar que os coeficientes de absortividade molar das clorofilas e dos carotenos são da mesma ordem de grandeza (1-1,5 x 10 5 L mol -1 cm -1 ) 6 confirmando que a presença de carotenos no extrato é de fato pequena e sequer pode ser detectada pelo espectro na região do UV-Vis do extrato bruto. Deve-se destacar ainda dois dados importantes de identificação da clorofila a. O primeiro é a presença da principal banda de absorção de um derivado do tipo porfirina/clorina (banda Soret) encontrada geralmente entre 400 e 420 nm.…”

“…6 Pouco tempo depois Fleming 7 determinou a configuração absoluta da clorofila a não deixando dúvidas quanto a quaisquer aspectos estereoquímicos e, ao mesmo tempo, contribuindo para uma série de estudos no que diz respeito a este tipo de pigmento. 8 Os métodos descritos na literatura para a extração/purificação da clorofila a proveniente de plantas e algas verdes podem ser considerados como processos complicados e, em alguns casos, ineficientes e dispendiosos.…”