Highly diastereoselective cascade cyclizations of terpenoid polyalkenes via photo-induced electron transfer (PET) can be accomplished by means of a chiral auxiliary which can be remotely located from the initiation site of the reactions. In addition to the high chemo- and regioselectivities, the cyclizations are subject to exclusive diastereofacial differentiation giving ready access to enantiomerically pure cyclic terpenoids. The efficient remote asymmetric induction, constituting seven new stereocenters in a single step, is most likely effected by chiral folding of the polyalkene chain. This evidence ist the first experimental support in favour of the earlier proposed spontaneous pre-chair folding of terpenoid polyalkene chains in accord with the idea of „minimal enzymatic assistance„, i.e. in the present case in non-oxidative biosynthesis which is mimicked by photochemical radical cation formation. Notably, as a further consequence of such selective folding only two out of 256 possible stereoisomers are formed in the example depicted in Scheme 1.

This methodology has meanwhile given synthetic access to several natural products such as stypoldione, hydroxyspongianone and abietanes. Ongoing work concentrates on a related biomimetic path to the taxanes trying to steer the folding pattern of polyalkenes.

Scheme 1.Biomimetic access to enantiomerically pure steroidal polycycles via photo-induced electron transfer (PET).

1.2.Bioinorganic photochemistry - Metalloproteins

In nature the basic taxane skeleton, i.e. taxa-4,11-diene, is enzymatically produced by taxadiene cyclase, a metalloprotein with at least one Mg²⁺ ion, from geranyl orthopyrophosphate (diphosphate, OPP) (a). This stands in contrast to the biotransformation of squalene (b) which represents an analogous structural type of a polyalkene terpenoid as in (a), but interestingly the folding of the starting terpenoids are different. In the second case (b) there are no metal ions in the enzyme present. The metal in the binding site of the enzyme(s) seems to play a crucial role in the prefolding process en route to the taxadiene and the taxols.

Notably, taxanes are presently one of the most widely used anti-cancer drugs (e.g. against mamalian cancer). The major drawback, however, is the insufficient availability of these drugs from nature - synthetically there exist not yet practicable routes which could be used for production.

Our newest approach in this field concerns the synthesis of biomimetic precursors (as shown below) as future substrates for transformationswith taxol synthase following the natural route (a). Furthermore, modifications of E. coli are attempted by fixing gene encoding taxadiene synthase on a plasmid.

In parallel we are about to cyclise the above shown intermediate via biomimetic photochemical electron transfer cyclisations without the involvement of enzymes. The functional groups EWG (electron withdrawing group) and OR1 seem to prevent rotation around the central CC bond favouring the natural conformation as shown in (a) and (c).

2. Enzymatic light-driven processes

Baker´s yeast (Saccharomyces cerevisiae) has gained increasing importance in view of applications in asymmetric synthesis. Among the numerous enzymes in baker´s yeast, oxidoreductases play an important role for reductions of aldehydes, ketones, keto esters and keto acids.

For practical access to enantiomerically pure alcohols, the control of the stereoselectivity is a challenge. The enantiomeric excess of reductions of keto esters withS. cerevisiae can conveniently and drastically be altered in organic solvents such as diethyl ether, toluene, hexane and ethyl acetate, when compared to the corresponding transformations in water.

A further goal of this research concerns the immobilization of enzymes in e.g. polyacrylate and calcium alginate matrices (encapsulation technique) to enable repeated use of the enzymes.

By this way squalene epoxide has been cyclized to lanosterol in successive batches.

The enantioselectivity of NADH dependent oxidoreductases from yeast can be steered by light and by organic additives. Furthermore, the photochemistry of NADH has been investigated.

The stereodifferentiation in the baker’s yeast reductions of some a- and b-ketoesters is enhanced by selectively steering the equilibrium of the different oxidoreductases competing for the same substrate. This goal is achieved with 300-400 nm UV light and via a new route invoking a two-substrate application. The baker's yeast activity upon reducing ethyl acetoacetate is light inhibited and the irradiation changes significantly the conformation of isolated ADH I as documented by CD spectroscopy. Instead, corresponding in vitro reductions with the isolated, NAD(P)H dependant enzyme preparations alcohol dehydrogenase I (ADH I), L-lactate dehydrogenase (L-LDH), and b-ketoacyl reductase are light-resistant. It is demonstrated that the natural enzyme cofactor β-nicotinamide-adenine-dinucleotide (NADH) protects ADH I against the light by absorbing it for conducting its own photochemistry. Four baker's yeast oxidoreductases participate simultaneously in the reduction of ethyl acetoacetate with L-LDH being proposed as the light-sensitive one.

3. Solar photochemistry and technology

Prototypes of linearly focussing collectors and concentrators have been constructed and probed for the purpose of running photo- and thermochemical reactions in sunlight at ambient to elevated temperatures.

In parallel with these developments, a new and technically even simpler concept for the utilisation of solar energy and the application of photo­chemistry is being investigated in Mülheim. In addition to using the direct solar radiation, such as is required for operation of concentrators, the new

concept also enables utilisation of the diffuse sunlight which accounts for about 60% of the incident sunlight at German latitude. Initial experiments in this field with newly developed flat-bed reactors have far exceeded our expectations establishing an efficient technology by using regenerative energy for photochemical purpose.

4. Semiconducting materials for oxidation and reduction of water with solar radiation - Storage of hydrogen and oxygen

Efficient and durable generation of hydrogen andoxygen from water has been achieved using the semiconductor titanium disilicide and halogen light which closely mimics solar radiation. The reactions are carried out under non-aerobic conditions, i.e.,under nitrogen or argon. 160 mL of hydrogen is produced at 80 ^oC and 1.5 bar pressure (100 cm² aperture, 24 h). In the first phase (A) of the reactions the catalytically active domains (cd) are built up at 50-80 ^oC. During phase A, the kinetics of the water splitting process is growing in and leads ultimately to a linear dependence in the further course of the reactions which consists mainly of water splitting to yield hydrogen and oxygen in a 2:1 ratio.

Figure 1. Representative reaction of titanium disilicide (<325 mesh) at 55 ^oC under nitrogen. During phase-A hydrogen evolution the catalytic domains (cd, Figure 3) are built up. Dotted lines represent pressure release and flushing the gas phase with nitrogen (scale shift of hydrogen concentration to 0). Phases B represents hydrogen evolution from water splitting (>96%) showing linear kinetics. Liberation of dioxygen (blue) from storage is achieved by short heating to 100 ^oC in the dark; thereafter the reaction was continued at 55 ^oC.

Whereas hydrogen is partially and reversibly stored physically, oxygen behaves differently: It is stored entirely under the applied reaction conditions (50-80 ^oC, light) up to ca. 25 wt% and can be liberated from storage upon short heating of the reaction slurries to 100 ^oC in the dark. This allows elegantseparation of hydrogen and oxygen. The stability of titanium disilicide has been positively tested over several months. It is an abundant and inexpensive material which absorbs light in a broad range of solar radiation. XRD and XPS studies show that titanium disilicide is 80% crystalline and oxide formation is limited to a few molecular layers in depth. Finally, it has been shown that hydrogen production exceeds stoichiometry of potential sacrificial oxidation of the titanium disilicide by >50 % in still proceeding experiments. The ultimate proof for water splitting has been established by running reactions with ¹⁸O-labeled water: the evolved oxygen contained ¹⁶O¹⁶O and ¹⁸O¹⁸O.

Figure 2.Pressure dependent efficiency of water splitting with titanium disilicide (>325 mesh) as determined by hydrogen evolution (reactions under nitrogen). Dotted lines: pressure release and flushing the gas phase with nitrogen (scale shift of hydrogen concentration to 0). The reactions were run at 50 ^oC and in one example oxygen was released from storage in phase A after ca. 125 h by short heating to 100 ^oC in the dark (4x amplification of peak intensity); thereafter reaction was continued at 50 ^oC.

At present, the reaction mechanisms presented below in Figure 3 are proposed to be operative on the basis of the interpretation of our results, i.e., the catalytic domains (cd) for water splitting are built up within the initial hours of reaction.

Figure 3.Build-up of catalytic domains (cd)for water oxidation and reduction within Phase A of the reactions.