Laboratory of Organic Chemistry

Chemistry of natural tetrapyrroles and quinones

Project leader: Prof. Emer. Paavo H. Hynninen

The long-term research, initiated 1969, on the synthesis, isolation, and structural analysis of porphyrin and chlorophyll compounds, has been continued and expanded into new directions with emphasis on the application possibilities of the compounds in photoinduced energy and electron transfer ( cf . the mechanism of natural photosynthesis) and in the diagnosis and photodynamic therapy of cancer. The opportunities offered by purified natural quinones as textile dyes have been investigated. Lately, research activity has been focused on the following topics.

Structure and reactivity of bonellin dimethyl ester

Bonellin ( 1 ) is a natural chlorin that occurs in the echurian worm Bonellia viridis , living in Mediterranean strands, buried in sand. Physiologically bonellin is unique in that it determines the sex of B. viridis . Also, bonellin has been found to be effective in inhibiting the growth of many bacteria and has shown antitumoral activity in tumor cell cultures. The biochemical activity of bonellin requires the presence of oxygen and light, suggesting that this chlorin is an effective photosensitizer. However, its potential use as a photosensitizer for photodynamic therapy (PDT) requires that the fine structure of the compound should be known in every detail. We have previously studied the NH tautomerism in bonellin dimethyl ester (DME) ( 2 ) by variable temperature NMR. Our continued structural analysis, using 2-dimensional NMR techniques (HMQC, HMBC, ROESY) and spin simulation, has now enabled us: (1) to fully assign the 1 H and 13 C spectra of bonellin DME; (2) to provide evidence that the aromatic 18 p -electron [18]diazaannulene delocalization pathway includes the ß -pyrrolic carbons of rings A and C but leaves outside those of ring B; (3) to assign the stereochemical configuration at C-17 as S ; and (4) to estimate the population values for the 17 1 -17 and 17 2 -17 1 staggered rotamers of the C-17 propionate side-chain. A relatively high population value 0.41 was found for the 17 1 -17 g - rotamer, which was interpreted as explaining the exceptionally high tendency of bonellin to form anhydrobonellin ( 3 ) via an intramolecular Friedel-Crafts acylation. (Hynninen, Helaja, Montforts, Müller)

Asymmetric synthesis: Stereoselectivity in the regioselective 13 2 -hydroxylation of chlorophylls a and b with the (-) and (+)-(camphorsulfonyl)oxaziridines

13 2 ( R/S )-Hydroxychlorophylls are relatively stable chlorophyll (Chl) derivatives, which have attracted much interest for several reasons. The 13 2 ( R/S )-hydroxyChl a diastereomers (epimers) have been found to form in the Willstätter allomerization reaction of Chl a , in the horseradish peroxidase catalysed oxidation of Chl a , and in the early stage of plant senescence. The 13 2 ( R/S )-hydroxyChl a epimers can easily be demetallated and dephytylated to 13 2 ( R/S )-pheophorbide a epimers, which are good photosensitizer candidates for photodynamic therapy (PDT). However, the stereoselectivity is low in all the above reactions. Recently, we have studied the stereoselectivity in the hydroxylations of Chl a and Chl b (Chl b has a formyl group instead of the methyl at C-7 in 1 ) using sterically hindered reagents, i.e. the (-) or (+)-enantiomer of (camphorsulfonyl)oxaziridine ( 5 / 6 ) as an oxidant and 1,8-diazabicyclo[5.4.0]undec-7-ene as a base. With the (-)-enantiomer ( 5 ), a very high diastereoselectivity for 13 2 ( R )-hydroxyChl a was achieved (d.e. = 94%), whereas that for 13 2 ( R )-hydroxyChl b was somewhat lower (d.e. = 66%). When the (+)-enantiomer ( 6 ) was used in the hydroxylations of Chl a and Chl b , the diastereoselectivity remained low in both cases (d.e. = 10% for 13 2 ( R )-hydroxyChl a and 8% for 13 2 ( R )-hydroxyChl b ). (Hynninen, Leppäkases, Mesilaakso)

Regioselective oxidation of 13 2 ( R/S )-hydroxychlorophyll a to 13 2 -demethoxy-carbonyl-13 2 -oxochlorophyll a

A facile pyrolysis method for the regioselective oxidation of 13 2 ( R/S )-hydroxychlorophyll a to 13 2 -demethoxycarbonyl-13 2 -oxochlorophyll a is described. Fully assigned 1 H and 13 C NMR spectra are presented for the product and a reaction mechanism involving an endiol intermediate is proposed. The attempts to apply the same method for the preparation of 13 2 -demethoxycarbonyl-13 2 -oxochlorophyll b from 13 2 ( R/S )-hydroxychlorophyll b were unsuccessful. (Hynninen, Leppäkases, Mesilaakso)

Synthesis and structural characterization of enol derivatives and analogs of chlorophylls a and b

The enol derivatives of chlorophylls (Chls) are of special interest for several reasons. The enolization of the b -ketoester system in the isocyclic ring E causes noticeable changes in the whole macrocyclic p -system, making it more easily oxidizable. Hence, the Chl enol derivatives have been proposed to participate as reaction center (RC) Chls in the primary events of photosynthesis. This proposal has now become more significant, since the recent crystallographic studies have shown that the RC of the photosystem I of oxygenic organisms contains a Chl special-pair, P700, consisting of one 13 2 ( R )-Chl a and one 13 2 ( S )-Chl a (= Chl a ') molecule. (Note that the epimerization at C-13 2 of the isocyclic ring can only take place via the enol intermediate). Also, studies of the precise electron structure of the macrocyclic p -system of the Chl enolates are highly interesting in relation to one of the most important theories in organic chemistry, the theory of electron delocalization and aromaticity. These motives have inspired us to synthesize and structurally characterize several Chl enol derivatives, such as the tert -butyldimethylsilyl enol ether of Chl a ( 3a ), 13 1 -deoxo-13 2 -dehydroChl a ( 4a ) and the enolate anions of Chl a and Chl b ( 5a,b ). The measured 1 H and 13 C NMR spectra suggest that the aromatic 18 p -electron, [18]diazaannulene delocalization pathway is retained in these derivatives. However, as compared with the NMR spectra of Chl a /Chl b , these derivatives show noticeable changes in the chemical shifts, which can be attributed largely to the weakening of the ring current and partly to conformational alterations. (Hynninen, Kavakka, Mesilaakso)