Learning Organic Chemistry Through Natural Products pdf

SERIES I ARTICLE Learning Organic Chemistry Through Natural Products 1. Natural Products - A Kaleidoscopic View N R Krishnaswamy wasunna~dmtoilieworld of natural products byT R SeshadriatUmveni~of Delhi and has carried on ilie glorioustradinons of his mentor. He has taught at BangaloreUmveni~, Calicut Umveni~ and Sri Sailiya Sai Insntute of Higher Learning. Generanons of students would vouch for ilie fact iliat he has ilie uncanny abili~ to present ilie chemistry ofnatural products logicallyand wiili feeoog. 1 Empirical formula is deter- mined from combustion analysis. 2 Thin layer chromatography (TLC)is an analytical tool to separate compounds based on their differential interactions with a stationary phase and a moving solvent. Aspecies with a greater relative affinity for the solvent moves faster (higher R, value! on a TLCplate. 56 N R Krishnaswamy Anaturally occurring organic compound has been chosen to illustrate (a) structure determination by chemical and spec- troscopic methods, and (b) synthesis and chemical transfor- mations. Organic chemists isolate or synthesize new compounds all the time. How does one identify their structures? Some may answer - "X-ray"! However, X-ray diffraction canbe used to determine structures onlyforcompounds which formsinglecrystals. For the vast majority of organic compounds, structure elucidation is carried out by a combination of chemical transformations and spectroscopic analyses. In this article we discuss the general approach that is followedusing a natural product as an example. In order to keep the focuson chemistry, wedefergiving its trivial name and plant source till the end of the discussion. An addi- tional bonus of such an approach is that the student logically deduces the structure from the given s~t of data and does not merely recallthe structure from memory. Further, whereverpos- siblewelookat a problem from different angles thus covering a wider ground. As our first example we choose a molecule, designated as A. We describe two different analytical approaches and a synthetic approach for elucidating and confirming its structure. The classicalmethod which has developedover the yearsfrom alarge volume of experimental work will be described first. The first step in structure elucidation isthe determination of the correct molecular formula, which for compound A is CloHlSO.l RESONANCEI January 1996 SERIES I ARTICLE The Series on "learning Organic Chemistry Through Naturol Products" Nature is a remarkable and excellent teacher. For effective learning, one needs a suitable language and the language of organic chemistry appears Ideal forunderstanding nature atthe molecular level. The molecules of nature, the small and the big, Individually and collectively give form, shape and substance to the living organisms in which they occur. The key to their biological functions is their chemistry which inturn is intimately associated with their structures. Therefore, the firststep inthe study of biomolecules is to find out their structures and stereochemistry. Next, the reaction profiles of a molecule need to be established as they enable us to understand the biological activity. An organic chemist does not just stop at that, but goes further by creating If ingeniously in the laboratory using tools of her own. Thissynthesis Isan integral part of organic chemical research and the inspiration forIfis provided by nature. Thus, the chemisfry of natural products forms a wide canvas portraying every kind oforganic chemical activityand going beyond. Ifacts .A structure Is likea skeleton. Whatgives it'life', like flesh and blood, are the bonding and the non-bond- Ing electrons Incorporated Inthe structure. as a bridge for transferring principles and con~pts of chemisfry to the biosciences thus providing a molecular basis for biological phenomena. How does effectively learns a subject of such vast dimensions within a short time is an intriguing question. At present organic chemistry is taught in a narrative form and the student is compelled to memorise a vast amount of descriptive data and a wide variety of apparently unconnected structures. If is therefore not surprising that the subject is not favoured by serious students seeking Intellectual contents. This is unfortunate since half a dozen carefully chosen natural products can take a stu- denffo every nook and comer of organic chemistry, and illustrate and highlight important guiding prin- ciples of the subject. This can be done by removing artificial barriers which at present divide natural products into various structural categories. -There- fore, when one shifts the focus from the gross skeletal structure to the interior electronic configu- ration, the need for classification based on struc- tural types loses Importance, and the emphasis shifts to fundamental chemical principles which are few and unifying. N Ii Krishnaswomy What isthe next step? The functional groups aretobe determined now. The natUreofthe oxygenfunctionality is shown by asimple reaction which can even be demonstrated on a thin layer chro- matogram.2The adjacent picture showsthe TLC behaviour ofA, and the product obtained after treatment with aceticanhydride or sulphuric acid. o Develop o . with CHCla AS AS From the chromatogram it can be inferred that A has a hydroxyl RESONANCEI January 1996 57 SERIES I ARTICLE 3 la) Mn02 is a specific oxidant for allyllcand benzylicalcohols, . Ib)DNP derivatives of saturated carbonyl compounds are pale yellow. whereas highly conju- gated compounds like benzo- phenone give brick-red prod- ucts with DNP. · Ozonolysis cleaves a double bond to produce two carbonyl compounds. Oxidative workup converts an aldehyde to the carboxylic acid. Ketones are unaffected. group since it undergoes acetylation. B moves higher with the solvent (higher Rrvalue) indicating its weaker interaction with the silica surface. Acanbeoxidisedwith Mn02 to yieldC(CloIiI60)which formsan orange-redderivative with 2,4-dinitrophenylhydrazine (DNP).3 These observations suggestthatC isana, p-unsaturated aldehyde or ketone. Compound C does not answer the iodoform test (IjOH-), but reduces Tollen's reagent, suggesting that it is not a methyl ketone, but an aldehyde. A on catalytic hydrogenation gives a tetrahydro derivative D (CiolinO), showing thereby that it has two double bonds. The molecular formula of D also shows that it is a saturated alkanol. Therefore, A is a doubly unsaturated, acyclic aUylic primary alcohol. Ozonolysis of A followed by oxidative workup4 yields one mol- ecule each of acetone, glycolic acid, and a keto carboxylic acid E H CRa CRa H I 1')( Ot I I\}I I 1\ }I 2 I I CH OH ~~o ~H cYc~c"c""c"c~c"c- ~ 2 3 I " , OH , ' H H ' D ' 'C' H H 'A' (Geraniol) . ! ozonolysis ~Ra CRa COtH C + ~ J + I RacY ~O HOtC' ~ o CHzOH Acet.one (LevuUc acid) Glycolic acid llz/0H- HOt~C02H Succinic acid 58 RESONANCE I January 1996 SERIES I ARTICLE (CsHsOz).Compound E on treatment with I/OH- gives succinic acid.These reactions areshown inScheme1.The ketoacidEcould therefore be identified as levulicacid. The structure ofA can thus be logically derived as shown in the scheme. This compound is Geraniolwhich is a component of several essential oils including the oil of Geranium.s We will now describe an alternative approach based on modem instrumental methods for finding out the struCture of geranio1.6 Among the advantagesofthis approach arethe factthat unlike the classicalchemical method, it is by and large non-destructive and canbeoperated on microquantities ofthe compound. But the data obtained from the two approaches are not the same and, taken together, they givea more complete picture of the molecule than that availablefrom either set ofdata. Therefore the best approach to structure determination would be a judicious combination of spectroscopic methods with a fewselected chemical transforma- tions. In order to determine the chromophoric ('colour producing') groups a UVIVis spectrum can be taken.7The UV spectrum of a methanolic solution ofAshowsanintense absorption at about 205 nm. In the UV spectrum ofC, there are twoabsorption maximaat 205 and 232 nm. The inferences are: A has two similar, if not identical chromophores. The position ofthe absorption maxima indicates that they are trisubstituted ethylenic double bonds. In C, one ofthese chromophores isretained while the other, respon- sible for the maximum at232nm, is ana, p-unsaturated aldehyde. The IR spectra of A and C provide important information.s The spectrum ofAhasabroad band at 3350cm-Iwhich is absent in the spectrum of C. This band is due to a hydroxyl group. The spectra ofboth A and C have bands at 3040(=C-H), 2920,2860and 1350 (CHz and CH3 groups) cm-I. These bands show that A and C possess an aliphatic skeletal framework with one or more double bonds. The most prominent absorption band in the spectrum of C is seen at 1680cm-Iwhich is absent in the spectrum of A. This RESONANCE I January 1996 5 Biosynthetlcally.geraniol is the first member of the terpe- noid family arising from two units of mevalonic acid. HO CH3 ~ HOCH2 C02H Mevalonic Acid · Thefollowing five paragraphs contain advanced material. and will require some knowl- edge of spectroscopy. 7 UltraViolet absorption arises due to electronic excitation of functional groups called chro- mophores. For example, in an olefin it is an electronic transi- tion from the n-orbital to the n*-orbital. "InfraRed spectroscopy is used to characterize vibrational (stretching, bending etc) energy levels of molecules. Groups such as O-H, C-H, c=o can be easily identified from IR spec- tra. The nature ofthe c=o le.g., acid chloride, amide, ester etc.! can also be determined. 59 SERIES I ARTICLE 9 Nuclearmagneticresonance Is a technique In which the en- ergy levels of magnetic nuclei can be observed by looking at the absorption (resonance) of electromagnetic radiation Inthe presence of an external mag- netic field. Hydrogen Is one such nucleus, and the chemi- cal environment of the hydro- gen affects the resonance fre- quency making it a valuable analytical tool for chemists. This resonance frequency, con- verted to a parts per million di- mensionless scale, Is reported as the d value. Typically,higher the partial positive charge on a hydrogen atom, higher Is Its d value. band is due to the a, /I-unsaturated carbonyl group. The presence of the CHO group in C is also indicated by a band at 2800em-I. The IH-NMR spectra of A and C provide the most compelling evidencefortheir structures.9The spectrumofA has signals at 152.2 (s,6H), 2.3(s,3H),2.4-2.5 (m, 4H), 3.5 (d, 2H), 5.1 (I, IH) and 5.3 (I, IH). These data show the presence of three methyl groups on sp2 carbons, two olefinic hydrogens(each on acarbon atom next to aCHz group as the signals appear as triplets), and a CHz-CHz unit. The signal at 153.5is due to the -CHzOH end group next to an sp2carbon. This signal shifts to 154.5upon acetylation of A. From these data it is possible to deduce the structUre of A asshown in Scheme2. In the NMR spectrum of C, the two proton signal at 153.5is missing, and instead, there is a one proton doublet at 159.3,due to the aldehydic proton. 'A' 'C' A~H.OH -e A~t-H M+. (m/z 154) H - A~'OH \J -H. - H A~-H m/z 153 - 60 RESONANCE I January 1996 SERIES I ARTICLE Scheme3givesthe mass spectral fragmentation ofA.IOBesidesthe molecular ion signal at m/z 154,there is a prominent peak at m/z 153.This can be accounted for in mechanistic terms, and lends strong suppon to the structUre. Finally, the structUre of geraniol is confirmed by synthesis. Whereas the analytical approach involves the dissection of the molecule into smaller recognizable units, the synthetic approach involvesthe reconstruction ofthe proposed structure fromsmaller molecules using predictable and unambiguous reaction path- ways.One synthetic approach is shown in Scheme4. Explanatory notes have been deliberately omitted as the students should themselves logically determined each and every step of the reac- tion sequences. We conclude this brief account of the chemistry of geraniol with a shon note on its stereochemistry. This compound is optically inactive and non-resolvable, but can assume several conforma- tions. On acid catalyzed dehydration, geraniol yieldsdipentene as one of the products. The formation of this cyclic terpene which can be rationalized, as shown in Scheme5, illustrates the impor- tance of appropriate conformations in intramolecular reactions. This example can also be used for demonstrating the formation, stability and fate of a carbocation. 10 Mass spectrometry, In Its slm- plestform, InvolvesIonizationota molecule with high energy elec- trons, and detecting Ions,or their fragmentedunitsaccordingtotheir mass to charge ratio. This Is a valuable tool tor determination ot the molecularmass,aswell as for structure elucidation - since the fragmentation pattern Is unique for a given molecule. The analytical approach involves the dissection of the molecule into smaller recognizable units; in the synthetic approach we reconstruct the proposed structure from smaller molecules using predictable and unambiguous reaction pathways. CHaCOCH2C02Et CeHaO-Na+ · Hydrolysis . HeC F'L 0 H ~C-Mg Br ~ H3C~S 3 I . H 7" OH Scheme4 Linalool Geraniol RESONANCE I January 1996 61 SERIES I ARTICLE CHi"0H . . ,. - - + Dipentene Geraniol Nerol Ascan be seen from the structure (box in Scheme 5),geraniol has the E configuration at the unsymmetrically substituted double bond. The Z isomer also occurs in nature and is known as nerol. Nature converts geraniol into nerol via geranyl pyrophosphate. The two configurational isomers (diastereomers) have separate existence. However,upon oxidation, both geranial and neral give a mixture ofaldehydes (citral) which is an inseparable mixture of geraniol and neral. (Can you explain why the two diastereomers ofthe aldehyde co-existin contrast to those ofthe parent alcohol? A related problem is how you would prove that geraniol has the E configuration?) Address for correspondence N R Krlshnaswamy. VIsitIng Professor. NMKRV College forWomen with PG Centre. Jayanagar IIIBlock. Bangalore 560011. India. 62 RESONANCE I January 1996 . SERIES I ARTICLE Learning Organic Chemistry Through Natural Products 1. Natural Products - A Kaleidoscopic View N R Krishnaswamy wasunna~dmtoilieworld of natural products byT R SeshadriatUmveni~of Delhi. " ;learning Organic Chemistry Through Naturol Products& quot; Nature is a remarkable and excellent teacher. For effective learning, one needs a suitable language and the language of organic chemistry. Sailiya Sai Insntute of Higher Learning. Generanons of students would vouch for ilie fact iliat he has ilie uncanny abili~ to present ilie chemistry ofnatural products logicallyand wiili feeoog. 1