FORSCHUNG 1 CHI MIA 44 (1990) Nr.I·-2 (Januar-Februar)
Chimia44 (1990) 1-21 direct insight into the chemical processes © Schweizerischer Chemiker- Verband; ISSN 0009-4293 of nature. And finally, in the 1950's, there was that enormous enthusiasm with which Ruzicka, soon to retire, regarded the struc- tural and mechanistic connections between the sesqui-, di- and triterpenes, which were to find their formulation in the «Biogenetic Leopold Ruzicka - Isoprene Rule» 15.61.That was to be the cul- mination of his life's work and of the work of his school, which would unlock the por- From the Isoprene Rule tal between structure determination of ter- penes and biochemistry. Ruzicka's life and scientific work have to the Question of already been summarized in the above- mentioned Biographical Memoir by PreJog and leger. In addition, the occasion of Ru- Life's Origin zicka's lOOth birthday has been marked by publications dealing with specific aspects of his life's work 171, such as an essay by Gunther Ohloff on his accomplishments as Albert Eschenmoser* an inventorl7R1. The object of the present article is to depict Ruzicka as the - so to say - romantic This is an English version of the article «Leopold Ruzicka: Von der Isoprenregel zur Frage scientist, shrewd realist though he was in nach dem Ursprung des Lebens» that appeared in the Leopold Ruzicka Centenary issue of his research, the one who never missed an Rad, the journal of the lugoslavian Academy of Sciences and Arts, Zagreb (Rad, lugoslav. opportunity to put his work on structure akad. znan. umjet. kem. [443) 7,21-68 (1989)). The German text was translated by determination into the context of a Professor Dr. Eli Loewenthal, Department of Chemistry, Technion Haifa. Permission for «Chemistry of Life»181,the one who as a publishing an English version was provided by Professor K. Balenovic, Zagreb. boy had wanted to become a priest 191,who had then read Hackel's «Weltriitsel» in his student daysllOl, and who, though eventu- ally fulfilled with the success of his scien- tific achievements, never lost the ability of being amazed at life's mysteries. These lines are motivated by the author's remi- «In his later years he (Ruzicka) became All of us who were fortunate to have niscences of collaboration with Ruzicka af- greatly interested in aspects of evolution and been in close personal and professional ter the doctorate and of Ruzicka's in- of tl'e origin of life which in his view en- contact with him in those days can well fluence on his own academic career. They dorsed his general philosophy, as expressed remember the single-mindedness with are meant not only to express the author's in many articles and books which he had which he started to pursue the biochemical opinions but above all to provide a forum collected and in the comments he made on literature. It was as if at long last there for Ruzicka's views, as formulated in pub- this topic.» came to the surface that innate driving lished and unpublished lectures, notes and (From L. Ruzicka, Biographical Memoirs force that was behind his work on terpenes comments, and as shown by reproduction of Fellows of the Royal Society, by V. Pre- - its biological significance, that aspect of in his own characteristic handwriting. log and O. Jeger[lJ) chemistry which to him was of primary It would be naive and misleading, how- import in what he considered the order of ever, to attempt to find a direct connection things in the universe. between aspects of Ruzicka's personality This short quotation hints at an underly- as described above and his choice of scien- ing theme in Ruzicka's scientific work. «Die wunderbare Harmonie, die zwischen tific problems to tackle. In that he was first Barely perceptible as an undertone in some den biochemischen Reaktionen in der leben- and foremost a solid and down-to-earth of his early publications, it later developed den Zelle herrscht, ware eines Gottes wur- realist, with a healthy dose of opportunism into a full-blown motif, if not basic moti- dig» when it was appropriate. In his own words vation, after his formal retirement. We find (a Belgrade lecture towards the end of his this borne out by a wealth of material left - is an authentic remark made by Ruzicka careerllll): after his death: in the form of hand-written at that time. notes and comments, marginal observa- There is an inner connection between «In all my work, there was at the beginning tions scribbled in books and other publica- Ruzicka's experimental work and his later a common objective - to elucidate the struc- tions, and in addition, manuscript drafts of inclination towards biochemistry that is tures of natural products which were of in- lectures, mostly left unpublished, under discernible already in his early investiga- dustrial importance ... » (see Fig. 1). headings such as «Biosynthese heute und tions on sesquiterpenes. They were not wiihrend der biologischen und chemischen merely determination of constitution but In the sequel we shall see just how accu- Evolution» 121,or «Biochemie und Welt- the vehicle for recognizing the importance rately that described the beginning of his bild» 131.The overall impression is that of an of a unifying principle in structure determi- independent scientific work. intense preoccupation on his part. nation - the «Isoprene Rule», and ulti- mately for first indications of the signifi- cance of that rule for the recognition of the path by which nature constructs these mo- lecules. In his 1933 address to the Zurcher In the Beginning • Correspondence: Prof. Dr. A. Eschenmoser Naturforschende Gesellschaft 141he referred In 1916, after nearly six years as research Laboratorium fUr Organische Chemie Eidgeniissische Technische Hochschule Zurich to the role of the chemistry of organic na- assistant to Hermann Staudinger, his for- ETH-Zentrum, Universitiitstrasse 16 tural products as the way by which the mer mentor, Ruzicka felt that it was time CH-8092 Zurich natural products chemist could gain an in- for his «Habilitation» and the right to em- FORSCHUNG 2 CHIMIA 44 (1990) Nr. 1 2 (Januar Februar)
bark on his own independent scientific ca- reer.
« Toward the end of 5Yz years of mainly synthetic work on the pyrethrins I had come to the firm conclusion that we were barking up the wrong tree» [121.
Since he now had to find financial sup- port of his own, he decided to start on a synthesis of irone, an olfactory principle isolated in 1893 by Tiemann and Kruger [131 from iris root. To this end, and with Staudinger's help, he arrived at an agree- ment of collaboration with Haarmann and Reimer in Holzminden, the oldest estab- Fig. I. From Ruzicka's manuscript for IIIJ. lished perfumery company in Germany 1141. On the basis of the (wrong) empirical formula C13H200 for irone, Tiemann and Kruger had effected a condensation reaction between citral and acetone they then informed me that the structure Nothing came out of that irone project,
(CIOHI60 + C3H60 = C13H200 + HP!), suggested for irone was in error since cata- nor out of subsequent hectic pursuit of thus arriving at their well-known discovery lytic hydrogenation gave only a dihydro other supposedly irone-like compounds 1141. of «ionone» (131, an artificial «essence of vi- compound and therefore it would have to be What did result, in 1917, was Ruzicka's olets» whose subsequent manufacture by bicyclic. I then immediately changed my first independent publication entitled «The the Holzminden company was a great in- plans and decided to embark on the synthesis Total Synthesis of Fenchone»[19l which fi- dustrial success. On the basis of the sup- of a bicyclic compound of ionone type, start- nally and conclusively confirmed the posed constitution of' this «ionone» ing from the bicyclic terpene ketone fen- Semmler structure. Tiemann had then suggested one for irone chone 2*. For the latter Semmler had sug- Again and again in his later years, Ruz- which was simply that of a double bond gested a formula with three methyl groups icka returned in one context or another to isomer of «ionone» (Fig. 2) 1*. In his retro- situated as in the ionones and irone. On the that odoriferous constituent of the iris root spective publication «Rolle der Riech- other hand Wallach, in his 1910 Nobel lec- which marked the onset of his career-a for stoffe in meinem chemischen Lebens- ture, had defined all monoterpenes as him troublesome affair de creur fittingly werb)112jRuzicka made the following com- derivatives of p-cymene, thus rejecting characterized as a «comedy of errors» in ments: Semmler's formula as being inconsistent the Prelog-Jeger Biographical Memoirill. with that definition 3*. Since I was inclined This continued well into the 1940's. « As I had no money of my own and could not to agree with Semmler I then decided to put Olfactory substances played a decisive retain my position of assistant I was glad to an end to this controversy through a total role in Ruzicka's career, as he himself win the support of this company. However, synthesis. » stressed at the age of841121and, it is of some
I. Ironically enough, at the time of that historically important discovery, not only was there a mistake in the empirical formula ascribed to irone (corrected only 40 years later to CI4H220 by Ruzicka, Seidel, and Schinz 1151),but also in the constitutional formula for citral (both carbon-carbon double bonds conjugated with the aldehyde group). As a result of the latter, the constitutional formulae of thc primary condensation . TIEMANN'SCHE TIEMANN'SCHE RUZICKA'SCHE product pseudoionone and of «ionone» were also in- IONONFORMEL [13J IRONFORMEL [13] ARBEITSHYPOTHESE correct (cf. Fig. 2). The discovery of 20 « That is where we wanted to hang on an aldehyde group. condense the product with acetone and see WALLACH'SCHE CHRYSANTHEMUM· ARTEMISIA- whether we get something that smells like ironei>(from a SEMMLER'SCHE FENCHON- FENCHON - SAEURE [22] KETON [25][26] recording of Ruzicka's "Chemical Autobiography" FORMEL[17] FORMEL[18] lecture, Swiss Chemical Society, Basel, 13 September ]957). 3. This is where Ruzicka (at the age of84!) is mistaken: there is no mention of fenchone in Wallach's Nobel lecture[161. In his book "Terpenes and Camphor"[17J, pp.53]-539, Wallach justifics his own structure for fenchone by the fact of its conversion into meta-c~- mene (Fig.2), admits that the Semmler structure I 81 might be correct, but suggests that for the time being there is not enough experimental evidence to justify «abandonin~ the old structure». Ruzicka's own paper on fenchone 191makes no mention of Wallach's para- Fig. 2. Constitutionalformulae which played an important role at the beginning of Ruzicka's cymene rule. scientific career. FORSCHUNG 3 CH[MIA 44 (1990) Nr. 1 2 (Januar Februar) interest to reflect on this. There is no indi- alytical deduction. The latter, so contrary stated [25J: cation in his doctoral dissertation (< nearer to the isoprene rule. They did serve as basis for his «Habilitation» dissertation (submitted in 1918), and for the second of his classical achievements, in 1919, the to- tal synthesis of linalool (with V. Fornasir, his first doctoral student) [JII. It is also likely that they contributed two years later to the beginning of his lifelong and mutually val- artemizia- keton krizanlemna kiselina ued association with the Chuit & Naef Co. Kao karakteristicno za ondasnje gledanje i razvoj (later Firmenich & Co.) in Geneve[l·71. pojma »terpenski spoj« navodim, da sam sa S t a u- d i ill g e r - 0 m gotovo 6 godina radio sa krizantem- In one of his retrospective lectures [241 nom kiselinom, a da nismo ni pomislili, da imamo Ruzicka divided the history of the isoprene posla s jednim tipicnim monoterpenom. rule into three periods, the first of which, spanning the time between Wallach and Fig. 4. Reproduction from Lavoslav Ruzicka, «ad dalmatinskog buhaca do seksualnih the 1920's, laid the groundwork for the hormona», Lijecnicki Vjesnik (Zagreb) 62 (1940) 1 (Nobel lecture given in Croatian in systematic study of the monoterpenes. Zagreb on 14 March 1940, cffootnote 7*). hydrocarbon CIOHI4' the so-called cymol IJrst&Prognoserliiher den Bau rkrTerpem. Konslitldiqn. S)fIlIksen 177 (isopropyl-p-methylbenzene). Pinene, cam- phor, and other compounds can be converted by appropriate chemical processes to cymol or cymol derivatives, e.g. carvacrol.» Not a word here of either isoprene or of the isoprene rule. Twenty years earlier Wallach had expressed some bold views on the constitutional link between isoprene and the «sesquiterpenes and polyterpe- nes» [281,but now it seemed that Wallach, the cautious experimentalist, did not con- sider them significant enough for inclusion in his Nobel Prize lecture. On the other hand, one year previous to this in his book «Terpenes and Camphof)} [17]he went to great lengths to reiterate his previous views on the subject. Fig. 5 demonstrates this by reproduction from Ruzicka's own per- sonal copy (with the latter's hand-written comments). These two pages not only show the essence of Wallach's ideas on constitutional relationships among the ter- penes, they also vividly document Ru- zicka's appreciation of his great predeces- sor whose life and work were the subject of his 1932 Chemical Society Pedler lec- ture[291.There is little doubt that in his par- ticular personal attributes, those of courage and imagination in putting for- ward daring working hypotheses while adamantly insisting on solid experimental proof, Ruzicka saw in Wallach a kindred spirit. In his search for a synthesis of irone (see Fig.2 and Note2*) Ruzicka had performed the reaction of fenchone with methyl mag- nesium iodide 1121and, after dehydration of the product, obtained «a hydrocarbon» which, upon ozonolysis, yielded not only fenchone, as might have been expected, but also camphor. This important observation "B~i einem derartigen Aufbau wiirden den Sesauiterpenen und [ID~ started off his studies of the Wagner re- "man leicht findet) auch den Polyterpenen je zwei doppelte Kohlenstoff· arrangement (first so named by him) 1301 and I,~dungen zukommen, was den bisher bekannten Tatsachen entspricht of the structure of «pinene». These studies were indeed relevant to what were at that ,,[} 238 (1887), 78]. time urgent problems but there are no indi- cations of any kind that they brought him Fig. 5. From Ruzicka's personal copy of Otto Wallach's « Terpene und Campher» [171. FORSCHUNG 5 CHIMIA 44 (1990) Nr. I 2 (Januar Februnr) Concerning the sesquiterpenes he wrote as HELVETICA CHJMICA ACTA follows: Volumen V Fasciculus Sextus. «In 1887 Wallach had suggested that monoterpenes were formed by combination Die Konstitutionsauiklarung des Cadalins ww:de wesentlich gefor- of 2 isoprene units, and had tentatively ex- dert durch unsere Arbeitshypothese, dass die entsprechenden Sesqui- tended this to proposing a basis of three such terpene aus einer Kette dreier regelmassig aneinandergereihter Isopren- units for the sesquiterpenes. The first two sesquiterpenes whose structures were then molekeln besteben. established unequivocally: a-santalene (Semmler, 1910) and famesol (Kersch- baum, 1913) are indeed constructed from three isoprene units, but that fact seemed to have escaped notice altogether. At any rate there was certainly no such thing then as an isoprene rule serving as a working hypo- thesis for structure elucidation.» Drf~i-i;;opren·kette :::esquiterpenskelett Any objective examination of the ter- (Cadinentypus) pene literature of that period must con- II clude that these comments depicted a true picture of the situation (see also Fig. 5). Moreover, there is no doubt that it was 32 from then on (1920)1 ] that the work of Ruzicka and his co-workers, based on the DTei-isopren.ket.t.e Sesquiterpenskl'Jett Eudalin use of dehydrogenative degradation in (Eudesmoltypus) T. structure determination of sesquiterpenes of the cad alene and eudalene series, led to the breakthrough toward what later was termed the empirical isoprene rule (cf. Fig. 6. From Ruzicka's seminal papers on the isoprene rule, 1922 (33], Fig. 6). The crucial statement, dating from 1922 (33) reads as follows: I27 « The main conclusion so far from our It was characteristic of Ruzicka that he himself pointed out ], was his concurrent sesquiterpene work is that practically all did not leave his conclusions to rest in the work on abietic acid [lSI, a diterpene acid compounds whose carbon skeleton is now form of a «Cadalene and Eudalene Rule» the structure of whose dehydrogenation known show a close mutual relationship, be- applicable merely to that particular corner product retene (see Fig. 7) was then already ing derivable from a regular three-isoprene of sesquiterpene chemistry, but that he ad- known. The deductive path leading from chain, as produced in nature in the case of vanced from there toward the isoprene rule the latter to that of the natural product famesol.» to serve as a generally valid aid to structure itself was here much more difficult to tra- determination of the higher terpenes. Of verse and took more than ten years (38] - no It is striking how the analogy in molecu- decisive importance in this regard, as he wonder, since abietic acid is a diterpene lar build between the cyclic mono- and the sesquiterpenes is reflected in the way their constitution was uncovered. In either case recourse to studying their relationships with the familiar corresponding aromatic systems was essential. In one case it was p-cymene, in the other cadalene and eu- dalene. It was fortunate, and is in retro- spect quite significant, that it was those terpenes that had a regular isoprene ar- rangement which at the beginning served to reveal such a relationship 5*. RETEN, KONSTITUTIONSVORSC HLAG OEHYORIERUNGSPROOUKT FOR ABIETINSAURE VON OER ABIETINSAURE ASCHAN UNO VIRTANEN AUS OEM JAHRE 1921 (39J 5. The term «regular isoprene chain» «(head-to-tail union of isoprene units» according to Robert Robin- son 134]) was meant to describe that isoprene unit arran- gement to be found in the aliphatic mono- and sesqui- terpenes (Fig. 8) as well as in natural rubber (see Har- ries 136], 1914). In the case of natural rubber this arran- gement was interpreted as resulting from linking of isoprene molecules at their] - and 4-positions, in line with Thiele's theory of partial valencies. Incidentally, it seems of historical significance that Ruzicka's seminal papers on the constitution of the sesquiterpenesl33J, and Staudinger's imRortant publication on the structure of RUZICKA'S EXPERIMENTELL RUZICKA'S ARBEITSHYPOTHETISCHES natural rubber I 71 appeared in the same 1922 volume of ERARBEITETES KOHLENSTOFF - KOHLENSTOFFGEROST FOR Helvetica Chi mica Acta. To be sure, Ruzicka did not GEROST OER ABIETINSAURE ABIETINSAURE AUS OEM create the isoprene rule as such; what he did was to AUS OEM JAHRE 1932 [38] JAHRE 1922 (35] recognize and to demonstrate its importance for the structure determination of the cyclic sesqui-, di-, and triterpenes. Fig. 7. On the role of the isoprene rule in the determination of constitution of abietic acid. FORSCHUNG 6 CHI MIA 44 (1990) Nr.l- 2 (Januar Februar) with an irregular isoprene pattern. Fig. 7, Das 5 Kohlenstoffatome umfassende Bauelement der showing Ruzicka's initial working hypoth- Polyterpene besteht aus der verzweigten Kette des Iso- esis as compared with the finally estab- pen tan s, die sich in regelmaBiger oder unregelmaBiger lished structure, hints at how difficult it \Veise im Kohlenstoffgertist wiederholen kann, wie folgende was to arrive at the correct solution once it Beispiele zeigen: became clear that the isoprene chain ar- rangement was not a regular one (Fig. 8). C C C I I I c-c-c-c-,-c-c-c-c-.-c-c-c-c RegelmiiJ3ige Isopentankette (Isoprenkette) C C C The Isoprene Rule as an Instrumentfor I . I _ I Structure Elucidation C-c-c-C-;-_c-c-c-c---c-c-c-c Another aspect of Fig.7 is the way it illustrates the manner in which Ruzicka unregelmiiLlige Isopentankette (Isoprenkette) initially utilized the isoprene rule - this from the juxtaposition of his own 1922 Fig. From the introduction to Ruzicka's review article «Die Architektur der Polyter- ideas with the 1921 structure for abietic 8. pene» [45J. acid suggested by Aschan and Virtanen 139] which, as specially emphasized in his Fara- day lecturel27J, was at variance with the rule. For the whole of what he termed the rule's «second period», from 1921 to 1953, tion of how in actual fact these molecules we find it difficult to look at structures of it was basically a device for the formalistic were constructed by Nature. natural products from the perspective of a dissection of a carbon skeleton into isopen- For us in this day and age, having been classical terpene chemist. We tend to forget tane units. No thought was given to what conditioned by half a century of mechanis- that he in turn had been conditioned by the implications this might have on the ques- tic enlightenment and biosynthetic insight, fact that essentially nothing was known ~ *J ~"( -Li -L~ ~ ~ RUZICKA [67] RUZICKAY168J BLAIR [40J RAMAGE [691 1935 1935 1935 1935 RYDON 170) KIPPING (71) SORM 172] 1950 ESCHENMOSER (64)[74) 1951 1938 1938 BARTON (7311951 RAMAGE (75\1951 BARTON [76) 1951152 *) VON RUZICKA ALS ARBEITSHYPOTHESE BENUTZT Fig. 9. The various constitutional formulae put forward in connection with the elucidation of constitution of fJ-caryophyllene. FORSCHUNG 7 CHIMIA 44 (1990) Nr. I 2 (Janua, Feb,ua,) ctvjf()CfCl~ ~) ~) y:l ~:l SEMMLER[55] 1914 DEUSSEN [57] TREIBS [58] SHORT [59] ROBINSON [60] RUZICKA [56] 1929 1927 1935 1935 1935 ro J30l w RUZICKA [61] RUZICKA [61] HARPER [62] PLATTNER [63] ESCHENMOSER [64] 1951 1936 1936 1947 1948 PLATTNER, FURST [65] 1953 STORK [66] 1953 Fig. 10. The various constitutional formulae put forward in connection with the elucidation of constitution of cedrene. about biosynthesis of natural products, that both these sesquiterpenes exhibit a we had derived these structures .from far- and by the view that natural processes in regular (i.e. head-to-tail) three-isopentane nesol, and that subsequent to this we were biosynthesis might quite conceivably pro- array, as had been demonstrated in 1922 able to suggest.further examples confirming ceed by reaction types fundamentally dif- for the classical sesquiterpenes of the this scheme of construction. However, we do ferent from those known to chemists. The cad alene and eudalene type (see Fig. 6). not think it permissible to apply it a priori to more we realize this fact, the better can we One exception was that put forward in all sesquiterpenes. In all other sub-groups comprehend Ruzicka's elation when the 1935 by Blair [401 who used this very crite- there are cases where the rule of regular curtain began to rise on the «third period», rion in the case of caryophyllene, on the isoprene unit construction is not valid, such that of the biogenetic isoprene rule, as basis of a comment by Ingold[411, but still as in the cases of fenchone and camphor brought about by better understanding of for purely topological reasons. Ruzicka's among the monoterpenes, abietic and dex- reaction mechanisms in organic chemistry reaction [42] to this is illustrative of what in tropimaric acids among the diterpenes, and and the advent of lanosterol. 1936 were his views: the pentacyclic triterpenes. One must on no Well into the 1950's there is no lack of account cling to this rule as a working hypo- examples in the literature, both by Ruzicka « Blair has written 'it was pointed out by thesis where it is not supported by clear and and by other workers in the field, of the Ingold (Ann. Reports Chem. Soc., 1924, unambiguous degradation results.» purely formalistic use of the isoprene rule. 103) that the sesquiterpene skeleton is not Here and there, however, we do find at- merely composed of three isoprene units but This little altercation is significant not tempts to break out of this mold, partic- is, so to speak, afoldedfarnesol chain'. He only because it again manifests Ruzicka's ularly where sesquiterpenes such as has overlooked the fact that our early work thinking at the time but also because we see ~-caryophyllene and cedrene were con- on the carbon skeleton of the sesquiterpenes in Blair's proposal an inkling of the bio- cerned, and where dehydrogenative degra- of cadalene and eudalene type had derived genetic isoprene rule to appear fifteen dation was of little use. Figs. 9 and 10 show these from famesol; cJ, Helv. Chim. Acta, years later. By then both caryophyllene the variety of often weird working hy- 1922, 5 534, 968, where these conclusions and cedrene would indeed be seen as potheses suggested by various groups dur- were clearly expressed by structural formu- derived from a regular three-isoprene ar- ing that period for these two natural prod- lae» ([42],footnoteon p. 348). rangement as in farnesol, but mainly this ucts. All of them took the isoprene rule was to come about through a new concep- into account but were in retrospect little With this remark, relegated to footnote tion of the rule, not as a result of new more than topological manipulations with- status, Ruzicka claims priority for the idea degradation data, and above all from an out regard to mechanistic reasoning. This of regular isoprene connectivity while at insight into reaction mechanisms, for was true not just for Ruzicka but even for the same time (but now in the text of the which the time was not yet ripe in the someone like Robert Robinson, the cham- paper) he is explicit about the limitations: 1930's. Ruzicka had in fact taken the first pion of his time in the field of organic reac- step in that direction already in 1925, in tion mechanisms. «It is true that at the time of our work on connection with his work on the acid-cata- None of the early hypotheses suggested cadalene and eudalene type sesquiterpenes lyzed cyclization of the sesquiterpene FORSCHUNG 8 CHIMIA 44 (1990) Nr.I-2 (Januar··Februar) nerolidol to give products of the mono- principles of construction by Nature, albeit viate postwar hardship, particular in his cyclic bisabolene type. At the time he com- in the absence so far of any clue as to the native Yugoslavia. In addition, he began to mented that «perhaps such a transforma- mechanism of such a widespread biochemi- invest the considerable sums accumulated tion of acyclic precursors into cyclic cal pathway.» during the war from industrial utilization sesquiterpenes could similarly occur in na- of his research in his notable collection of ture» [43J6*. However, as we see from his Dutch paintings. All of this he did with the 1935 retort to Blair, he had become a great same singleness of purpose that had been deal more cautious in the interim. What in Time for Change characteristic of his scientific endeavours. retrospect he was to call the «Farnesol In retrospect the above observations As a result he was often away from the Rule» [5Jwas not yet to be developed into seem prophetic. They date from a period laboratory and out of touch with his doc- the refined tool of structure determination during which the first steps had already toral students. Nonetheless, his institute which it was destined to become. The prob- been taken in Ruzicka's laboratory toward continued to function perfectly, thanks to lems he had with caryophyllene, cedrene, the structure elucidation of lanosterol [47J. the capable hands of Vlado Prelog and and elemol hindered, rather than helped, in The two guidelines expressed above: the Placidus Plattner who had become associ- this respect. Moreover, the more complex basically conditional validity of the empiri- ate professors in 1949. The triterpene work di- and triterpenes, which by then he was cal isoprene rule, and the absolute priority which had been dearest to his heart was working on at full steam, were convincing to be accorded to experimental results, continuing at full steam under the increas- him to an increasing degree of the overrid- were to prove their worth later, on the oc- ingly independent direction of Oskar ing importance of experimental work, casion of the victory of the Ruzicka-leger leger. Andor Furst, together with Plattner since he was being confronted with numer- group[48Jin their race with Barton [49Jto fi- who was looking after the administrative ous carbon skeletons clearly based on nally solve the lanosterol structure. It side, were continuing with sesquiterpenes irregular isoprene arrangements. emerged that it was the carbon skeleton of in which Ruzicka's interest had waned These circumstances may account to that particular natural product, of funda- considerably, while both Hans Heusser some extent for the relative lack of pro- mental biological importance, which could and Plattner were busy with steroids. Hans gress in the development of the rule during not be perceived in terms of isoprene unit Gunthard had been given the task of estab- the period 1925-1935, but they were not construction at all 8*.And as for Ruzicka's lishing infra-red spectroscopy in the insti- the only reason. Ruzicka's interests were expectations that the isoprene rule would tute, and Emil Hardegger had branched then steadily veering toward subjects, un- emerge as a harbinger of our conception of out into the sugars. As for the monoterpe- connected with the isoprene rule, that pro- the way Nature constructs terpene nes, in particular odoriferous substances, pelled him into public attention and es- molecules, it was at that very time that the Ruzicka - having at long last solved the teem. These were the large-ring com- «count-down» had begun: Bloch and Rit- irone problem - left this field in the hands pounds and odoriferous principles of the tenberg found in 1945 [52Jusing, 14C-isotope of his veteran associates Hans Schinz and musk type (from 1924) and steroids and tracers, that cholesterol is biosynthesized Casimir Seidel. male sex hormones (from 1933). In the from acetic acid. It was to Schinz's special credit that he meantime he was basking in the satisfac- In their search for constitutional rela- used the freedom Ruzicka had granted tion of seeing how his Zurich colleague and tionships, to serve in understanding the him 9* to embark on a systematic study of «rival» Paul Karrer had to accord serious way molecules are constructed by Nature, the acid-catalyzed cyclization of polyenes attention to the isoprene rule in his work there is much similarity between Ruzicka using model systems related to the mono- on the biologically important squalene, and his great British contemporary Robert and sesquiterpenes, a reaction type discov- vitamin A and the carotenoids[45J. Robinson. What Ruzicka did from 1920 on- ered by Tiemann [77J.It was this work which By 1945 Ruzicka had attained a matured wards in the terpene field was paralleled by first brought the present author into con- and somewhat detached attitude toward Robinson's work starting in 1917 [53Jin the tact with this reaction in his own diploma the isoprene rule, best expressed in his field of alkaloids which occupied the cen- and doctoral work, leading to years of Nobel Prize lecture [4617*: tral position in natural product chemistry study on the mechanism and steric course at that time. Robinson summarized his of polyene cyclization and therefore, in a «Even ifon occasion we were to encounter a decades of achievement in 1953 in the no- way, contributing to the decision to write natural product whose structure does not table Weizmann Memorial Lectures [53bJin; this article in the first place. strictly obey the isoprene rule, that would that same year Ruzicka delivered his lec- not at all diminish its importance as a work- ture on the biogenetic isoprene rule at the The year 1951 marked Ruzicka's chemi- ing hypothesis. Experience has taught us IUPAC congress in Stockholm [51.Robinson cal «renaissance». It was also then that he that there are no rules of molecular con- had the upper hand in his intuitive insight decided to re-marry. The concurrent re- struction to which there are no exceptions, on reactivity and mechanism [54]and in per- turn to chemistry came just in time. To- and that we are duty bound to examine the ceiving biosynthetic relationships between gelher with Pre log and Plattner 10*,both applicability of such rules in each and every natural products, especially alkaloids. But, fresh from their first visit to the USA, a case irrespective of whether their validity in a comparison of the relevance of their new generation with a new outlook on had in the past seemed absolute or whether work to the chemistry of life, it was Ru- exceptions had already beenfound.» zicka, the experimentalist, working on the «It is worth considering whether the iso- terpenes which in all their variety turned 8. This was the second example of a terpene whose prene rule is valid not only as a working out to be of biologically greater impor- constitution contradicted the empirical isoprene rule. hypothesis in the laboratory, but beyond tance than the alkaloids, who was in the The first was eremophi[one whose constitution was that also as a basis of the workings of Na- end to have the best of it. Whatever he may solved by Simonsen in 1937[50J,Here Robinson [51Jsug- ture. The regularities observed in the struc- have lacked in mechanistic insight he made gested that its carbon skeleton could be derived from a normal one by assuming a I -+ 2 methyl shift. This was tures of the higher terpenes point to the up for by the fact that by the time he later considered by Ruzicka to have been a first step likelihood of similarly regular and uniform reached the critical early 1950's he was not toward the biogenetic isoprene rule. left to fend for himself, but had managed to establish a framework of cooperation 9. Sclrinz was an external employee of Firmenich & Co. in Geneve. He took his doctorate with Ruzicka in 1924, 6. For a later, and in retrospect more extended propo- with the younger generation in his circle. remained in his group at the ETH, and toward the end sal cf. [44J, At the end of the second world war there of the war was permitted to work independently with was the period during which Ruzicka had doctoral students. 7. Ruzicka was awarded the Nobel Prize ("for his work began to detach himself from chemistry to on polymethylenes and higher terpenes") in 1939,but 10. In 1952 Plallllef left the Organic Chemical Labora- because of the war his (official) Prize lecture had to be an increasing degree, in order to devote his tory and assumed the position of Director of Research postponed until 1945 (in Zurich). efforts to organizations established to alle- in Basel. FORSCHUNG 9 CH[MIA 44 (1990) Nr. [2 (Janaar Februar) Fig. II. Excerptfrom Ruzicka's notes on Prelog's lecture «Organic Chemistry I», summer term 1952. ~~~------_ .. Fig. 12. Excerptfrom Ruzicka's notes on Prelog's lecture «Organic Chemistry II», winter term 1952/53. FORSCHUNG 10 CHIMIA 44 (1990) Nr.12 (Januar Februllr) organic chemis try was en trenching itself at THE the ETH; the age of reaction mechanism ELECTRONIC THEORY and of the electronic theory of organic chemistry had begun. The work on the OF constitution of lanosterol was reaching its ORGANIC CHE:~nSTRY climax. In the field of the pentacyclic triter- penes of by then known structure, leger had in the preceding year put forward a BY first, albeit hesitant, hypothesis on a pos- ~!.J. S. DEWAR sible biogenetic connection between the carbon skeletons of ~- and rx-amyrin, lupeol, and germanicol [781. In the sesquiter- pene field the present author had used a theory of the mechanism of acid-catalyzed polyene cyclization to derive new struc- tures for cedrene, ~- and rx-caryophyllene, lanceol, and elemol (see Fig. 9 and Fig. 10) from the structure of famesol or the re- lated famesene, and proposed that such mechanistic derivation of the constitution of terpenes represents an additional selec- tion criterion for using the isoprene rule in structure determination of such com- pounds (64). Finally, during that year Bloch Fig.l3. Title page of Dewar's text book (801, 1949, with Ruzicka's note. in Chicago was on his way to the proof, published a year later, that squalene is an intermediate in the biosynthetic conver- sion of acetic acid to cholesterol [791. It was remarkable how Ruzicka, then aged 64, reacted to these developments. During the summer semester of 1952 and the winter semester of 1952/53 he was to be found on most days sitting and listening attentively to Prelog's course - organic chemistry I and II - for beginners. It goes without saying that this was not, God for- bid, to keep a watch on Prelog's teaching, but in order to take in a new outlook on organic chemistry. Among the papers left by Ruzicka we can find a document of a kind singular in the history of the Institute of Organic Chemistry at the ETH Zurich (and probably in that of any other univer- sity): Prelog's basic course in Organic Chemistry, painstakingly taken down in Ruzicka's handwriting on carefully num- bered and dated loose sheets of notepaper, as exemplified by Figs. 11 and 12. The new Ruzicka was not slow in draw- ing his conclusions from the impact of the new age of reaction mechanism on the fate of the isoprene rule (perhaps with the im- petus of developments in the sesquiterpene field). This is vividly clear from Figs. 13 and 14: during the winter of 1950/51 he made an exhaustive and practically word- for-word study of Michael Dewar's new 1 book (80 , then considered sensational (see Fig. 14), to which he devoted his entire Christmas and Easter holidays as we see from Fig. 13. Fig. 14. A page in Dewar's text book (80] with characteristic Ruzicka traces. ~ FORSCHUNG 11 CHIMIA 44 (1990) Nr.1 2 (Januar Februar) «Is Lanostadienol (Lanosterol) a Triter- verwandte Verbindungen, \\-ie z. B. San ten mit 9. die Jonone mit 13. Iron pene or aSteroid?» - this emphatic ques- mit 14 Kohlenstoffatomen, die man als Terpenoide bezeichnen kann. Deren tion was central to the 1952 Jeger-Ruzicka 1-2 paper [48] presenting proof of structure of Kohlenstoffgerust Hisst sich formal urn die fehlenden Kohlenstoffatome this now crucially prominent natural zu einem der Isoprenregel gehorchenden Gerust erganzen. product. Their considered reply was the following: «In spite of its having 30 carbon atoms, and Die heutige CilOlesterin- of the identity of rings A and B with those Formel durch 3 Methyl- found in the pentacyclic triterpenes, lano- l(ruppen (-) zu einen sterol does not obey the isoprene rule and hypothetischen Triter- hence cannot be considered a triterpene.» pen erganzt Both question and answer were to repre- sent the last vestiges of a static mental atti- tude which had typified the «classical» pe- riod of triterpene and steroid research. It Das Cholesterin k6nnte darnach als em Triterpenoid betrachtet werden. was to come to an end in the following year when Woodward and Bloch published :Kach der in den Jahren 1930/32 - zur Zeit aIs ",-ir mit der Deutung der im their proposal [81], supported by carefully vorigen Abschnitt erwahnten ResuItate der Dehydrierung der Triterpene planned experimental work, that lano- beschaftigt waren - noch alIgemein angenommenen Konstitutionsformel fUr sterol is an intermediate in the biosynthesis of cholesterol. That began a new era, one in which Ruzicka saw himself as having Fig. 15. Reproductionfrom Ruzicka's Nobel lecture, 1945146]. reached a goal which could not have given him grater satisfaction: lanosterol as the link between the two major areas in his life's work: terpenes and steorids. That this possibility had been on his mind for a long time 11*, giving rise to the prescient remarks in his Nobel Prize lecture in 1945 [46], is indi- cated by Figs. 15 and 16. HO HO The Biogenetic Isoprene Rule For Ruzicka and his group lanosterol RUZICKA'S HYPOTHETISCHES KONSTITUTIONSFORMEL TRITERPEN AUS DEM JAHRE 1945 DES LANOSTERINS [48J was both victory and defeat. The paper by [46] (vgl. Abb. 15) 1952 Woodward and Bloch, interpreting the con- stitutional relationship between squalene and cholesterol without however providing any mechanistic detail, was to be the first step in a dramatic turn of events in terpene chemistry. Instinctively Ruzicka acted in the right direction. In the first place he tried to in- duce Konrad Bloch to come to the ETH as HO its first Professor of Biochemistry. In 1953 KONSTITUTIONSFORMEL Bloch had spent half a year on sabbatical OES CHOLESTERINS leave in Ruzicka's institute and had with 1932 his own hands provided experimental proof that C-7 in cholesterol was indeed Fig. 16. Ruzicka's hypothetical triterpene of 1945 (Fig. /5) compared to constitutional derived from acetic acid, in accord with the formulae of lanosterol and cholesterol. squalene conformation Woodward had proposed to him [83], and not with Robinson's earlier (1934) suggestion [84]. However, when faced with the choice ETH The second step was to write and publish nism. Different structural types of cyclic or Harvard he decided for the latter [85]. the paper on the biogenetic isoprene rule, mono-, sesqui-, di- and triterpenes were to parallel to the talk he gave that summer at be derived from acyclic precursors via the IUPAC congress in Stockholm (5]. In a cationic addition, cyclization, and 1,2-re- chapter of this publication entitled «Bio- arrangement reactions (see Fig. 17). The genesis of Steroids and Terpenic Com- central paragraph reads as follows: pounds» he presented, with A. Eschen- 11. In a lecture before the Swiss Chemica] Society in moser and H. Heusser as coauthors, the « ... the deduction of structures of natural Zurich in summer ]923 on bile acids, Heinrich Wieland had raised the possibility that these were biogenetically ideas known at that time on the mechanis- terpenic compounds by accepted reaction derived from lipids. In the ensuing discussion Ruzicka tic derivation of terpenes from aliphatic mechanisms from the hypothesized single pointed to a possible common origin for sterols and precursors, as the epitaph of the empirical precursors squalene, geranylgeraniol, far- higher terpenes. This hmothesis is described in the isoprene rule. The biogenetic isoprene rule nesol, geraniol ("biogenetic isoprene rule") dissertation of Rudolph ( ], one of Ruzicka's doctoral students, which appeared in printed form in 1925; cf. was enunciated as a comprehensive consti- not only serves to outline possible biogenetic also (5.9). tutional theory based on reaction mecha- routes, but also represents a new helpful tool FORSCHUNG 12 CHIMIA 44 (1990) Nr.) 2 (Januar Februar) Admitted]y Ruzicka's 1953 paper was decisively influenced and abetted by the work of others, but only he was capable and authoritative enough to give it the standing of a manifesto which was to re- ceive genera] recognition as signifying the rarnesol changes wrought in terpene chemistry by the new concepts. The biogenetic isoprene ,/ rule would now make obsolete all the old structural distinctions previously made on the basis of either regular or irregular iso- prene array, that is, of obeying or not e) obeying the old rule. This transition is surely among the most beautiful examples Bisabolene III of the evolution of a chemica] theory. Per- haps the profoundest aspect of the new rule was as a further step forward in the demystification of the origin of natura] products which began with Wohler's ]828 urea synthesis. It served to demolish the last vestige of the myth of a «Force Vita]e» - this time in the form of a belief that the enzymatic course of biosynthesis can go by XVlJI.-Ionie mochanisms in way of reactions of a type unknown to the tho biogonesis of sosfJuit.erpe- chemist. In this connection it is historically nos (6-mcmbercd ring intermo- wrong to make light of (mechanistically) Hexah~drocadalene it. -Jan/alene diato). the squalene folding concept put forward by Robinson in 1934[841(something which the author of these lines has himself been Fig. 17. Excerpt from « The Isoprene Rule and the Biogenesis oj Terpenic Compounds» lSI, guilty of). The Robinson concept should 1953 ((f also Fig. 19). be looked at as an historically important document, describing as it does the preva- lent view at that time on the then unknown territory of biosynthesis, and that against in the structure elucidation oj terpenic com- tion of terpenoid polyene chains[64I.In this the background of his standing in mecha- pounds. The new tool limits the number oj the hydroxy group in ring A was to be seen nistic organic chemistry at the time. His carbon skeletons which can be proposed on as a crucial functional group, and its origin approach to the isoprene rule (see Fig. ]0) the basis of the original isoprene rule ("em- a potentia] hydroxy] cation as cyclization is a similar document. pirical isoprene rule") alone.» initiator. This latter concept inspired The immediate universal acceptance ac- Bloch's experiments on the role of atmo- corded to the biogenetic isoprene rule was Impetus and background for the bio- spheric oxygen in cholesterol biosynthe- clearly due to the way it managed at a genetic isoprene rule paper were provided sis [86](cf.[83.87])and some years later the stroke to put order among the overwhelm- by leger's and Ruzicka's lanosterol work, search for squalene oxide by Corey [88J and ingly large number of cyclic terpenes which the Woodward-Bloch proposal, and the by van Tamelen [891. A final impetus was had become known during the preceding author's concepts of deriving the constitu- provided by the tricyclic triterpene alcoho] 70 years - this on the basis of just a few tional formulae of terpenes by coherent ambrein, whose structure was solved in organic reaction concepts. Later, in ]955, schemes involving cationic cyclizations 1947 by Lederer [90] working in competition it became all the more convincing in the and rearrangements 13*: complex sesquiter- with the ETH group 1911. This structure was way it was instrumental in deriving not penes (cedrene, caryophyllene) from far- clearly to be derived by squalene cycliza- only the constitution but the configuration nesene (64]12*; abietic acid and related diter- tion (this time by a non-oxidative mecha- of all known triterpenes derived from penes from the naturally occurring precur- nism). squalene, via definition of the steric course sor manoo]!64I; and, dating from 1953, all triterpenes of then known constitution from squalene. The derivation of the triter- pene formulae was based on the new con- Zur Kenntnis d('r'$rpel~ cept of the oxidative initiation of cycliza- 190. Mitteilung 12. These reaction schemes were published in detail for the first time in[51;see e.g. Fig. 17. Eine ste~e~~sch~nterEetati~n _ 13. According to the author's recollection the «rate- limiting step» in devising this scheme lay in overcoming tischen Isoprenregeloei den Triterpenen the «activation barrier» represented by having to disre- gard Markovnikov's rule where the cyclization to ring C of the triterpenes was concerned. Had it not been for von this, the whole scheme might have come to fruition three years earlier. It was H. Heusser (K. Bloch was then a guest in his laboratory) who first placed before Ru- zicka a scheme which represented an attempt to derive A. Eschenmoser, L. Ruzicka, O. Jeger und D. Ar~oni triterpenes from squalene. This scheme was quite diffe- ~ -':- ~ rent from that presented in lSI,but, as far as the author remembers, did include anti-Markovnikov cycliza- tions; it is quite likely that this helped in overcoming the «activation barrier». Fig. 18. Ruzicka's personal copy oJthe 1955 paper [61. FORSCHUNG 13 CH IM IA 44 (1990) Nr, I 2 (Jonuar Februar) of acid-catalyzed polyene cyclizations and could now be shown to lead to the com- ganic chemistry, and in forging the link Wagner-Meerwein shifts. In later years 1921 plete structure and shape of anyone of all between organic chemistry and biology. Ruzicka indicated that it was he who had the known triterpenes, all on the basis of Initially the rule had been mainly con- induced his younger colleagues (A. E. and an amazingly small number of strictly de- cerned with the cyclic terpenes. The scope Jeger's rising star student Duilio Arigoni) fined reaction postulates. changed in the course of the discovery of to solve the stereochemical aspects of the Things were again developing rapidly: at acetyl coenzyme A by Lynen 199in1 1951, problem. Be that as it may, solved it was Columbia University Stork, independently that of mevalonic acid by Folkers in indeed by these younger colleagues, and of the author's work at the ETH[6,95,96had1, 195611001and, the subsequent work of Ly- the result was a publication in 1955 in Hel- initiated experiments on the steric course nen 11011,Bloch [1021and, of Cornforth and vetica Chi mica Acta 16Jwhich was greeted of polyene cyclization using a working hy- Popjak 1103which1, all resulted in the com- as «the apotheosis of the isoprene rule» [94/ pothesis of his own and had moved toward plete picture of the biosynthesis of by John W. Cornforth, a competent witness a rationalization of the biogenetic path squalene starting from acetic acid, An im- on the strength of his fundamentally im- from squalene to the triterpenesl971. At portant «fall-out» of all this was the recog- portant contributions to the study of the Harvard University Woodward had (also nition ofisopentenyl pyrophosphate as the biosynthesis of cholesterol [93).Among the independently) lectured [98]during the win- true incarnation of the legendary «iso- papers left by Ruzicka we can find a reprint ter semester 1954/55 on the constitutional prene unit», thus laying the biochemical of this article with parts underlined by him and configurational derivation of lano- foundation of the biogenetic isoprene rule. (see Fig. 18). sterol and euphol from squalene, likewise Further subsequent tests of its validity The essence of this development was the using the argument of chair versus boat were the continuing work on the biosyn- establishment of two principles: one, on conformation of the squalene chain, thesis of cholesterol (Cf.188.89,104,1051),and the the preferential stereoelectronic course of As a theory covering both molecular ar- incisive investigations of selected cyclic ter- electrophilic addition and substitution re- chitecture and natural origin of the ter- penes, mainly by Arigoni (cf.11061)and his actions at a C=C double bond, in conjunc- penes the biogenetic isoprene rule has since schooPI071. tion with accompanying or subsequent its inception in 1953-1955 changed the Wagner-Meerwein rearrangement, the course of study of these natural products. Fig. 19 reproduces a selection of hand- other the conformational one based on the The number of new sesqui-, di- and triter- written notes made by Ruzicka for his lec- dichotomy of chair versus boat form of the penes since studied and found to be new ture at the Nobel Laureates' meeting in polyene chain. For those who were in- variants within the original concept has be- Lindau in 1967(21,They give us an idea of volved in this work it was an extraordinary come virtually limitless (at least so it seems how he himself viewed all these develop- experience to follow up how different per- to this author). As a result the rule has ments. The photograph in Fig. 20 showing mutations of constitutional and conforma- become instrumental in the metamorpho- Ruzicka and Lynen together most proba- tional variation in the squalene molecule sis of natural product chemistry into bioor- bly also dates from that meeting. ~~ l~Ujl , - .- - ..----- 2 Fig./9. Excerptsfrom manuscript draftsfor a lecture by Ruzicka on the biogenetic isoprene rule 1 /. FORSCHUNG 14 CHIMIA 44 (1990) Nr.12 (JanuarFebruar) notably his famous Faraday Lecturel27J, «History of the Isoprene Rule», which par- ticularly contributed to the propagation of the new concepts. Reading once again that lecture, as well as many unpublished manuscripts of other lectures (cf. Fig. 19), one cannot but sense the feeling of satisfac- tion experienced by Ruzicka in his seven- . ties and eighties, at his extraordinary good fortune in seeing the essence of his life's work, the isoprene rule, coming to final fruition. The Turn to Biochemistry The encounter between natural products chemistry and biochemistry, within the arena provided by the advent of the study of lanosterol, was the high point of terpene chemistry in the 1950's. For Ruzicka, who set the stage, it signified a common future course for both disciplines, and he lost no time in translating his convictions into concrete action. This was to take the form of establishing an Institute of Biochemistry at the ETH and of raising the necessary financial support by an appeal to chemical industry in Basel. Moreover, he personally took in hand the creation of a Chair in Biochemistry, and for him the first choice would have had to be between Konrad Bloch, Feodor Lynen, and George Popjak - at that time the leading figures in the field of cholesterol and terpenoid biosynthesis. Concurrently, and well into the sixties, Ruzicka made an extraordinary effort to familiarize himself with the biochemical literature. It is clear from the notes and sketches left after his death that, had he been able to complete his autobiography, it would have been under the heading «My Fig. 20. Leopold Ruzicka (with Feodor Lynen in the background) at the meeting of Nobel Predilection for the Chemistry of Life» \109). laureates in Lindau (probably 1967) ..Photo-Pfeiffer, D-7993 Kressbronn. The following typifies his intense preoccu- pation with that subject: «I would define as "Chemistry of Life" the In 1956 Ruzicka contributed his article minutest detail in the structures of the collaboration between the chemistry of «The Significance of Theoretical Organic steroids and triterpenes can be accountedfor organic natural products (= bio-organic Chemistry for the Chemistry of the Terpe- by the principles of the biogenetic isoprene chemistry) and biochemistry. The task of nes»IIOSIto «Perspectives in Organic Chem- rule. For the way the true picture has bio-organic chemistry is to elucidate in the istry», Todd's valedictory volume for emerged from behind thefacade of the struc- laboratory the structures and reactions of Robert Robinson. In this he surveyed the tural formula we have to thank modern de- naturally occurring organic compounds, entire story, from Wallach's «Terpenes velopments in reaction mechanism - a tri- whereas that of bioche11'}istryis to study and and Camphom['7] to the stereochemical umph over the rigid system of mere struc- clarify the course of chemical reactions implications of the biogenetic isoprene tural concepts. No other group of natural within living organisms)) [31. rule; from his early 1918 work on Wagner products has so far lent itself to the same rearrangements in the pinene series IlO) via extent to rationalization of structural and For Ruzicka natural product chemistry the views of Meerwein and Winstein on the configurational detail by simplest means as had always represented an essential pre- nature of carbocation reactions, to the lat- . that of the triterpenes and steroids. The abil- requisite for biochemistry_ He had none of est experimental results on the course of ity of so many living organisms to biosyn- the misgivings that presently underlie the acid-catalyzed polyene cyclization. This thesize all these compounds with always the debate on the relationship between organic was the new Ruzicka at the height of his same configuration, out of hundreds of alter- chemistry and biochemistry. In fact, he powers. He was in no way less enthusiastic natives, must surely be one of the most could express himself on that subject in no about reaction mechanisms and biosyn- remarkable chemical achievements of Na- uncertain terms Ill: thesis than his junior colleagues who were ture.)) assisting him. At the end of the article we « There are some small-minded organic find the following observations: Ruzicka was fortunate in witnessing the chemists who on occasion bewail the facl consequences and approbation of his suc- that a good part of organic chemistry has « The enormous progress we have seen in cess for many more years of his life. After been expropriated by biochemistry. They elucidation of structure since the days of his formal retirement in 1957 he continued have forgotten that organic chemistry origi- Wallach is evidentfrom the way in which the giving lectures to international audiences, nally set oul to clarify the chemistry of car- FORSCHUNG 15 CHIMIA 44 (1990) Nr.1 2 (Januar Februar) bon compounds found in living organisms. It 1953 was not only the year of the bio- for pre biotic chemistry, a subject first pro- is the level of perfection achieved by organic genetic isoprene rule; it also marked the pounded in 1924 by the Russian bio- chemistry after ]50 years which is enabling advent of the structure model of DNA by chemist Oparin through his hypothesis on biochemistry to penetrate the innermost Watson and Crick [1111. We can well imagine the origin of life on earth 11I3J. secrets of life processes on a molecular basis, Ruzicka's excitement during the following and this will more and more blur the distinc- years at the unexpected and extraordinary tion between the two disciplines. It brings to developments taking place in the realm of mind my favourite paradox: "To under- natural products outside the traditional stand biochemistry you need to know at least territory of purely organic chemistry. Ifwe Questions on the Origin as much organic chemistry as for organic accept his above-mentioned definition of The «First International Symposium on chemistry itself".» bio-organic chemistry at face value - and the Origin of Life on Earth» (1141,organized in the long run it may well be more valid by Oparin, took place in Moscow in 1957; Ruzicka's convictions on this subject are than the current version - then it would its Proceedings appeared in print two years eloquently clear both from the title of his certainly encompass a discovery such as later. The 500 or so scientists who took last autobiographical publication (< 14. The first three pages, as well as pages 6 and 10 of the Fig. 23. Excerpt from 11141; Ruzicka's marginal notes on W.M. Stanley's paper «On the copy or this manuscript are missing. Nature of Viruses, Genes, and Life». FORSCHUNG 16 CHJMIA 44 (1990) Nr.1 2 (Januar Fcbruar) proach drew the terse observation: lctS? My answer to these «Ch. commits the opposite mistake to that -~lieve that our science has of the Origin-of-Life scribblers» (Fig. 24) . hors in order 0 onc I a judgment illuminative not only of Char- Letnlstry or Vi ic we lack gaffbut of Ruzicka himself - his both open and critical attitude to the origin-of-life ~~~·.·~~~~ig.~~ Issue, rently fashionable template ~-~ It was not entirely surprising that Ru- zicka should have begun in his later years or comp e es of a nts in ,;.J to evince interest in the origin-of-life prob- ~rr:mgement. of theconsti- (~ . lem. The search for the origin of biosyn- the tic pathways was a logical outcome of .c acids, blood-group sub- ~ his endeavours to gain an insight into the : agents must be able both \ molecular construction principles of na- ture: first on the level of organic natural illthe constituent sequence ) products chemistry and then on the bio- operate) :--.... organic level of biosynthesis. His life-long -lfuWtheseagems fascination with the topic «Chemistry of • --i1.lIV ..•.... - ---=- ct.t, Life» led naturally to his preoccupation with that of the «Origin of Life». Pre- sumably he agreed with Oparin's postulate (~'~,~~~ that «the real nature of life is comprehen- sible only through understanding of its origin» ([114J, p. 368). Fig. 24. Excerptfrom [l14J; Ruzicka's marginal notes on E. Chargaff's paper «Nucleic Acids In the manuscript of his «Biochemie und as Carriers of Biological Information». Weltbild» lecture[3JRuzicka devoted nearly 3 out of23 pages to the subject of the origin of life and evolution, but struck a rather «Life transcending Physics and Chem- Polanyi. For Ruzicka, organic chemistry cautious note, perhaps in view of the na- istry» [1191, a significant article by the physi- was the epicenter of his private universe; ture of his audience. (Bio)chemical evolu- cal chemist and philosopher Michael his approval of other authors' work was in tion 15*, in his view, ought to be considered an essential prerequisite of biological evo- lution, the latter being now almost univer- sally accepted; even its opponents could hardly object to the idea of (bio)chemical evolution since it would apply only to the creation of inanimate organic chemical compounds. He then wrote: «The crux of the concept of evolution as a whole lies in the fact that somewhere in be- tween (bio) chemical and biological evolu- tion we must assume a point where life was created no matter whether its origin is re- garded as "natural" or "supernatural"» [31. We should not overlook the fact that eventually Ruzicka decided to strike out the end of the last sentence (see Fig. 21). That act was in line with innumerable notes and comments from the 1960's which leave no doubt that from both a scientific and emotional viewpoint he considered the creation of life as a consequence of the intrinsic properties or organic matter on earth. His perusal of the publications of experimentally active protagonists of pre- biotic chemistry such as Calvin [115.116J, Miller [117J, and Ponnamperuma [118J is lit- tered by marginal notes expressing em- phatic approval ("very good", "very inter- esting and important", "excellent"). Com- ments in a different vein adorn his copy of 15. Ruzicka here used the term «biochemicaJ evolu- tion» in the sense of Calvin's use of «chemical evolu- tiOnl,[115J. Fig. 25. Ruzicka's sketches on the subject of «prebiotic chemistry». FORSCJ:UNG 17 CHIMIA 44 (1990) Nr. I 2 (J,mu"r Fehru"r) proportion to the extent to which it would ~~wearenow rcady to comment on the fmal uestion: correlate the structures and properties of organic compounds with their biological' C\I) I VIlI.6. 0uUhLJ'/''''lOmwon of Lil_ t \I ~xpla.ned by our Presenl Concepts 01 P"ysj~ significance. For him as an organic chemist A S!mple "yes" may bring us into a difficult position, the discipline of pre biotic chemistry was because we may be asked to prove this answer. e.g. by complete induction. It may be_wiser_to turn the closest to his heart, judging by the way he ~i~n~~nd, ~~~t systematically kept himself up-to-date in DnvSiCs does IiOfOffer an ion I e I t. h v etter I ve the (' y this field by hand-written excerpts from r e. ' Do we have a c nter-example? This may depend Annual Reports on the Progress of Chem- very much on whether we agree about the definition istry 1967-1968 (see Fig. 25). of life. None of this quite accounts for the way A--J_' 0Earin r 1281 once proposed the foUowing list of ~ Ruzicka, then already in his eighties, was metabolism, selfreproductivity. fascinated by the young Manfred Eigen. ~ - We must marvel at Ruzicka's attempts to as a basis for the definition of the word "liv·ng". ~ come to terms with a theory of evolution this definition cou d III ed by a machine like which was largely based on mathematical principles. From his hand-written excerpts Fig. 26. Excerpts from Ruzicka's copy of the journal Naturwissenschajien containing from Eigen's Nobel lecture on «Immeasur- 20 Eigen's publication (1 1. ably Fast Reactions» we can gather how much he was impressed by Eigen's bril- liance of exposition well beyond the nar- row confines of chemistry. One of the im- portant items among the Ruzicka papers is the issue of Naturwissenschaften from the e.g., marginal comments which appear in Postscript year 1971 with Eigen's article: «SelfOrga- the 1975 Symposium Volume on «Evolu- It was on the occasion of the Ruzicka nization of Matter and the Evolution of tion» issued by the Akademie der Deut- Centennial Symposium in Zurich that the Biological Macromolecules» [1201, anno- schen Naturforscher Leopoldina (cf. author agreed to accede to Professor K. tated by Ruzicka. In retrospect this is a Fig.27), specifically on Kuhn's essay «On Balenovi(;'s request to write an article touching token of an 84-year old Ruzicka the Evolution of a Self-organizing Pre- about Ruzicka. What I had in mind was a still determined to add to his fund of biotic System» (125,1261. short essay on the theme «Leopold Ru- knowledge and understanding of the zicka and the problem of the origin oflife». «Chemistry of Life» (cf. Fig. 26). Among the many loose sheets found What came about in the end, as a result of What was the effect Eigen eventually among the Ruzicka papers there is one on closer preoccupation with that subject and had on Ruzicka? An answer is provided by which he constructed a sentence around an with the Ruzicka papers, was an attempt to an incident which occurred at the 1971 important concept in the history of science present this theme in conjuction with an Biirgenstock conference [121]. Eigen had just - «Force Vitale» - which is a valid reinter- account of the history of the isoprene rule. finished giving his lecture on «Evolution of pretation of that concept and at the same In particular it also resulted in an account Biological Macromolecules», whereupon time an enunciation of Ruzicka's scientific of the origins of the biogenetic isoprene an agitated Ruzicka stood up and said: credo: rule, from the special viewpoint of the au- thor who saw its beginnings, and believes «It has taken all my life's work to convince «The Life Force, abused so often in days that after 35 years he has possibly attained myself that life is chemistry .. and now you gone by as a deus ex machina, is not de- a balanced perspective. come along and tell us it is physics.» throned: it is as mighty as ever; but the What seems certain is that whoever will irrevocable evolution of the sciences reveals chance to acquaint himself with Ruzicka's Apart from causing much amusement that force to be molecular and physico- scientific legacy in the future cannot but among the audience, this rejoinder went chemical in essence» (Fig. 28). come under the spell of his extraordinary right to the heart of the matter. At a time personality, and will regret, as the author when experimental endeavour in the area Ruzicka's work on the isoprene rule in- does, that it was not to be in his power to of the origin-of-life problem was overshad- deed contributed to that revelation. complete his autobiography. owed by the brilliance of physical theories - the kinetic concepts of evolution by Ei- gen (120.122] and the thermodynamic percep- tions of Prigogine (1231 - it was fitting that Ruzicka the chemist should intuitively have made a point that today has moved to ih the forefront of interest: the problem of the ~ ~~rrJ' 141--o- 't0 origin of life on earth is above all a chemi- cal one, and specifically also a problem of synthetic organic chemistry. A chemical Ju.w. Fi~. ~J - Reproduction of . elected puge\ from the mUIII/ -cript ((Biochemie IlIld Weltl itt/l die D It. unter B It. be~ dar sp. %i- 1\h"ItJ fi.schen Lenkung der bl010g1schen Prot 1.nsynthese sp eLt. dareD Aufklirung in erater Linie Wat on und Cr %u verdaDken ilt. Es war vorher schon bekannt. 8S8 die Prot inl these nicht La Zellkern durch die DNAsondern ausserhalb des Z erns 1m Cytopla a unter Betel1igung der aogenannten Hikro durch- gefUhrt wlrd. Dt Hikro OIIIeD sind kleine rundl1che KO~uchen die dar r 1.ner ziemLicb hochmolekularen RN B n- enfols- zu genannt. da 8ie ala ft dar D It. in. a trigt und Itch don an d Oberfllcbe dee Ribo OllIS niederlis.t. In di 8 Zustand reprllentiert die m-RNAden Be! 1 dar D A. der bel dar Frot Iynthes. e.ing .lten arden mu • Dab r wlrd dl Ribosoms litze.nde I FORSCHUNG 19 CHIMIA 44 (1990) Nr. 12 (Junuar Februar) - 18 - Nac~en heute die Methodik d Konst tionsa ng der Proteine sehr v rentacht ist, werd n solch Untersuchungen Leicht auf zahlreiche Organismen von den einfachst werden konnen. Man wird so Zus el'lhange zwischen der bioch ischen und der bi010~~ kennenl rnen, allerdings nur am B spiel des L bens. Blochem at' a en seit etwa 30 Jahren egonnen Bicb ernst- lich Gedanken zu llIacben. niche nur iiber den Ursprung des Lebens auf Erden, sondern auch tiber die Vorgeschichee des Lebena. Oparin und Haldane stelken die plausible Hypothes auf. dass die Vorg schichte des Lebens in einer biochemischen Evolution• be- sta-nd. Bevor Leben entstehen konnte, llIussten die lebenswicbti en chenis Evolution muss a1 eine se1bstverstandliche Voraussetzung -.---...... ::..der biologischen Evolution angenommenwerden. Die biologische Evolution von d n ni.edrigsten Lebewesen aufwarts kann heute kaUlllmehr bestritten M eo'), werden. Aber auch ein Gegner der biologischen Evolution wird kaUlllEinwendungen machen konnen gegen die bioch sche Evolution ff~ ~F denn lie besteht ja nur in der Annahmeder Entstehung und Evo- ~ 5') l~ion organischer Verbin~ung~ ~hne Leben. Der heikle Punkt ~ tv d r Gu t-Evolutlon be teht darin. dan zwischen der b10- ~C eh isehen un dar 010g1 chen Evolut1on di Entstehung des ebens lie&en MUS Fur di gesamte Evolution beglnnend mit der Entstehung der erst n organischen Mol kiil~ auf Erden steht eine Periode von ungefihr 4000 Million n Jahren zur Verfu!ung. Damals be- stand die Atmos hire wobl hautplachlich au~O~e~in,ha. ~t ~A{ und Wasserltoff, auch war viel Waseer vorhanden. Besonders ,'Vi Cl FORSCHUNG 20 CHIMIA 44 (1990) Nr. 1 2 (Januar Februar) Leopold Ruzicka Centennial Symposium, ETH Ziirich, 20 March 1987; b) L. Trueb, «Ein Leben fiir Naturstoffchemie, Alpenblumen und nieder- liindische Malerei - Zum 100. Gcburtstag ~~ Leopold Ruzickas», Neue Zlircher Zei/Ung, II September 1987, p.25; c) C. Klemm, «Professor Dr. Leopold Ruzicka und seine Stiftung nieder- liindischer Gemiilde im Kunsthaus Ziirich»; ~ ;Uv/ d) H. Gerson, «Leopold Ruzicka als Bilder- sammler», catalogue of the exhibition «1m Lichte Hollands», Kunstmuseum Basel, 14 ~ .~(Y\~~ ~~~ June-27 September 1987. [8] Fragments for a biography entitled: «Vorliebe fiir die Chemie des Lebens - Leopold Ruzicka im zweiten Halbjahrhundert, 1908-1958, der Struk- Ml~ ~ tW1\~ ~'Jk,awUtvuA- turchemie», see also the title of one of Ruzicka's Lindau lectures: «Nobelpreise und Chemie des Lebens», Meeting of Nobel Laureates, Lindau, 29 June 1970; see Naturwiss. RlIIldsch. 24 (1971) ~ ----~ ~~ ;1'1WVVW 50. - Cf. facsimile from the manuscript in Fig. I. [9] L. Ruzicka, «In the Borderland between Bioor- ganic Chemistry and Biochemistry», Anll. Rev. Biochem. 42 (1973) I. [10] Cf.[1I, p.441. ~cUr~~ [II] Handwritten introduction for a lecture given at a congress in Beograd «Biogenetische Iso- prenregel» (approx. 1965). [12] L. Ruzicka, «Rolle der Riechstoffe in mcinem ~ ;4lu ~d ~ )v1v ehemischen Lebenswerk», Helv. Chim. Acla 54 (1971) 1753. [13] F. Tiemann, P. Kriiger, «Ober Veilchenaroma», Ber. DIsch. Chem. Ges. 26 (1893) 2675, 2691. [14] L. Ruzicka, exchange of letters with Haarmann ~dWvV~~tL und Reimer, chemische Fabrik zu Holzminden G.m.b.H., 1916 (Ruzicka Archives, ETH Ziirich). [15] L. Ruzicka, C.F. Seidel, H. Schinz, «Ober die Bruttoformel und einige Umsetzungen des Irons», Helv. Chim. Acta 16 (1933) 1143. [16] O. Wallach, «Alicyclic Compounds», in Nobel Lectures, Chemistry /901-/92/, Elsevier, Am- sterdam (1966), p. 178. [17] O. Wallach: Terpene lmd Compher, Veit & Co., Leipzig (1909). ~ ~\ [18] F. W. Semmler, Chem. ZIg. 29 (1905) 1313. [19] L. Ruzicka, «Die Totalsynthese des Fenchons», Ber. DIsch. Chem. Ges. 50(1917) 1362. [20] «Uber Phenylmethylketen. Von der Grossher- Fig. 28. Reproduction of a loose leaf among the Ruzicka papers. zoglieh Technischen Hoehschule Fridericiana zu Karlsruhe genehmigte Dissertation zur Erlan- gung der Wiirde eines Doktor-I ngenieurs, vorgelegt von Leopold Ruzicka, Dipl.-Ing. aus Vukovar (Slavonien)>>, Karlsruhe 1911. [21] H. Staudinger, 1. Ruzicka, «)nsektcntotende Stoffc I. Uber Isolierung und Konstitution des Acknowledgement: Many people have Received: Deecmber 8, 1988 [FR 69] wirksamen Teils des dalmatinischen Inscklcnpul- contributed their efforts to this manuscript. vcrs», Helv. Chim. Acta 7 (1924) 177. [22] H. Staudinger, L. Ruzicka, (dnsektentotende Dr. Dorothee Felix helped me in searching Stoffe H. Zur Konstitution der Chrysanthemum- through the voluminous archive of Ruzicka's [I] V. Pre log, O. Jeger, «Leopold Ruzicka», Biogr. monocarbonsiiure und -dicarbonsiiure», Helv. personal papers, prepared numerous repro- Mem. Fellows R. Soc. 26 (1980) 411; a German Cllim. Acta 7 (1924) 201. ductions, and checked all of the literature version was published in Helv. Chim. Acta 66 [23] W. H. Pcrkin,jun., J. F. Thorpe, J. Chem. Soc. 75 (1983) 1307. (1899) 48. citations. Dr. Jakob Schreiber and Dr. En- [2] «Biosynthese heute und wiihrend der biologi- [24] L. Ruzicka, «Perspektiven der Biogenese und der gelbert Zass prepared theformulasfor pub- schen und chemischen Evolution», type- and Chemie der Terpene», Pure Appl. Chem. 6 (1963) lication, Dr. Claude Wintner 'helped in the manuscript for a lecture presented at the Mceting 493. preparation of the final manuscript, and of Nobel Laureates, Lindau, July 1967 (Ruzicka [25] L. Ruzicka, T. Reichstein, R. Pulver, «Synthese Archives, ETH Zurich). des Tetrahydro-artemisiaketons», Helv. Chim. Mrs. Hermie Zass-Gachter produced it, [3] «Biochemie und Weltbild», lecture presented on Acta /9 (1936) 646. During our lunches at the Dozentenfoyer, 18 February 1964 in Bern, organized by the [26] Y. Asahina, S. Tagaki, J. Pharm. Soc. Japan my colleague Vlado Prelog has shared with Freistudentenschaft Bern (Ruzicka Archives, (1920) 873. me his nostalgic reminiscences of Leopold ETH Ziirich). [27] L. Ruzicka, «History of the Isoprene Rule» [4] L. Ruzicka, «Ober das Bauprinzip und die neu- (Faraday Lecture), Proc. Chem. Soc. (1959) 341. Ruzicka; he also gave me the photograph este Ausdehnung der Terpenchemie in biologisch [28] O. Wallach, «Zur Kenntnis der Terpene und der reproduced in Fig. 20. Drs. K. Schenker wichtige Richtungen», typescript for a lecture iitherischen Ole», Liebigs Anll. Chem. 239 (1887) (Basel), H. Westen (ETH) , and W. Acklin presented before the Naturforschende I. (ETH) all provided valuable information. I Gesellschaft, Ziirich 1933 (Ruzicka Archives, [29] L. Ruzicka, «The Life and Work of Otto Wal- ETH Ziirich). lach» (3rd Pedler Lecture, 10 March 1932), J. owe all of these people my very special [5] L. Ruzicka, «The Isoprene Rule and the Biogen- Chem. Soc. (1932) 1582. thanks. I also wish to take this opportunity esis of Terpenic Compounds» with the chapter [30] L. Ruzicka, «Zur Kenntnis der Wagnerschen to thank once again Firmenich SA, Geneve, «Biogenesis of Steroids and Terpenic Com- Umlagerung», Helv. Chim. Acta / (1918) 110. for the generous support which has made Dr, pounds» together with A. Eschenmoser and H. [31] L. Ruzicka, V. Fornasir, «Uber die Totalsyn- Heusser, Experientia 9 (1953) 357, 362. these des Linalools», Helv. Chim. Acta 2 (1919) Felix a memher of my research group, Fi- [6] A. Eschenmoser, L. Ruzicka, O. Jeger, D. 182. nally, my collaboration with Professor Eli Arigoni, «Eine stereochemische Interpretation [32] L. Ruzicka, J. Meyer, «Uber Sesquiterpcn- Loewenthal in connection with his transla- dec biogenetischen Isoprenregel», Helv. Chim. verbindungen I. Uberfiihrung des Cadinens in tion of the German version of this article was Acta 38 (1955) 1890. einen Naphthalinkohlenwasserstoff», Helv. [7] a) G. Ohloff, «Der Forscher Leopold Ruzicka als Chim. ACla 4 (192/) 505. for me an especially enjoyable experience. I Erfinder», Chimia 4/ (1987) 181, lecture by the [33] L. Ruzicka, M. Stoll. «Hiihere Terpen- am very much in his debt. presidcnt of the Swiss Chemical Society at the verbindungen VII. Konstitution des Eudalins, FORSCHUNG 21 CHIMIA 44 (1990) Nr.1 2 (Jalmar Febru"r) des Selinens und des 0:- Santalens. Das Kohlen- [63] P. A. Plattner, Chimia 2 (1948) 248. [97] G. Stork, A. W. Burgstahler, «The Stereochem- stoffgeriist der Sesquiterpene», Helv. Chim. Acta [64] A. Eschenmoser, «Zur siiurekatalysierten Zy- istry of Polyene Cyclization», J. Am. Chem. Soc. 5 (1922) 923; see also 1. Ruzicka, J. Meyer, M. klisierung bei Mono- und Sesquiterpen- 77 (1955) 5068. Mingazzini, ibid. 5 (1922) 345, 353. verbindungen», thesis ETH Ziirich No.2018 (98] Lecture notes of Dr. Karl Schenker, postdoctoral [34] R. Robinson, Annu. Rep. Prog. Chem. 20 (1923) (February 1951), Juris-Ver]ag, Zurich (1952). fellow with Prof. R. B. Woodward, August 1953- 100. [65] P.A. Plattner, A. Furst, A. Eschenmoser, W. November 1955 (personal communication K. [35] 1. Ruzicka, J. Meyer, «Zur Kenntnis der Abie- Keller, H. Klaui, S. Meyer, M. Rosner, Helv. Schenker). tinsiiure. Oberfiihrung der Abietinsiiure ins Chim. Acta 36 (1953) ]845. (99] F. Lynen, E. Reichert, 1. Rueff, Liehigs Ann. Methylrcten», Helv. Chim. Acta 5 (1922) 315, [66] G. Stork, R. Breslow, J. Am. Chem. Soc. 75 Chem. 574 (1951) I. 58!. (]953) 329!. [100] 1. D. Wright, E.L. Cresson, H. R. Skeggs, [36] C. Harries, Liebigs Ann. Chem. 406 (19]4) 173; [67] 1. Ruzicka, W. Zimmermann, «Poly terpene und G. D. E. MacRae, C. H. HotTman, D. E. Wolf, K. C. Harries: Untersuchungen liber die natlir/ichen Polyterpenoide XCII. Zur Kenntnis del' Folkers, J. Am. Chem. Soc. 78 (1956) 5273. und klinstlichen Kautschukarten, Springer, Berlin Caryophyllcnsiiure und del' Nor-caryophyllen- [101] F. Lynen, H. Eggerer, H. Henning, I. Kessel, (1919). saure. Uber das Additionsprodukt von Malein- Angell'. Chem. 70 (1958) 738. [37] H. Staudinger, 1. Fritschi, Helv. Chim. Acta 5 siiure-anhydrid an Caryophyllen», Helv. Chim. [102] S. Chaykin, J. Law, A. H. Phillips, T. T. Tchen, (1922) 785. Acta 18 (1935) 2]9. K. Bloch, Proc. Natl. A cad. Sci. USA 44 (1958) [38] 1. Ruzicka, G. B. R. de Graaff, H. J. Miiller, [68] 1. Ruzicka, Chem. Ind. (1935) 509. 998. «Ober cine neue Wendung bei del' Aufkliirung [69] G.R. Ramage, J.L. Simonsen, J. Chem. Soc. [103] J. W. Cornforth, R. H. Cornforth, G. Popj