236 CH HCH3 278 CH2 16 CH3 2 CH3CD2-NH-CH CD2 1NH -CH1CD2-N CD20H 677 17518 26031 MW293 20 236 260 .294 m/e 100 140 180 220 260 3U0 Figure 2 s spectrum of the derivative of tripeptide (Leu-Ala-Pro).Figure reprinted from Reference 12 3.ALKALOIDS Naturally occurring substances,mainly from tropical plants,had been used for centuries in folk medicine and later in academic studies.Atropine,morphine,and yohimbine are good examples In the first half of the twentieth century,the pharmaceutical industry began the systematic search for active substances.An early success was an alkaloid isolated in 1932 from the roots of Rauolfia serpentina and named reserpine.Its structure was not determined until 12 years later.Reserpine was one of the first antihypertensive drugs and became a huge financial success for CIBA(Basel, Switzerland)in the early 1950s.Research laboratories in both the pharmaceutical industry and academia raced to dupl e this achi names and melting points,but determining their structures was a laborious,time-consuming process. In the course ofmy work on muscopyridine Ihad learned much about alkaloidsand the problems involved in the determination of their molecular structures.I remembered one case,where three independent laboratorieshad proposed Structure Ifor the alkaloid sarpagine,but only one ofthem suggested proving it by correlation with a degradation product(Structure ID ofanother alkaloid of ajmaline.However,three years hadg one by without success.This was probably elation,the to the one it should be comparedh buthssevera eperimentallc steps.It now occurred to me that by using mass spectra for the comparison,one needed to prepare only a similar comparison product.The reasoning was that because these indole alkaloids consist of an aromatic system and a polycyclic unit,only the latter will fragment;the former will remain relatively intact.Therefore,the mass spectra of molecules with identical polycyclic structures,but differently substituted aromatic portions,will display the same patterns will be shifted by the mass differen of the s substituentso the aromatic por ry to convert sarpagin (Structure mpond of sterure .hicoed on thre very simple chemical raction Checking the product of each step by mass spectrometry eliminated the need for conventional 6
AC08CH01-Biemann ARI 10 June 2015 13:25 Figure 2 Mass spectrum of the derivative of a tripeptide (Leu-Ala-Pro). Figure reprinted from Reference 12. 3. ALKALOIDS Naturally occurring substances, mainly from tropical plants, had been used for centuries in folk medicine and later in academic studies. Atropine, morphine, and yohimbine are good examples. In the first half of the twentieth century, the pharmaceutical industry began the systematic search for active substances. An early success was an alkaloid isolated in 1932 from the roots of Rauwolfia serpentina and named reserpine. Its structure was not determined until 12 years later. Reserpine was one of the first antihypertensive drugs and became a huge financial success for CIBA (Basel, Switzerland) in the early 1950s. Research laboratories in both the pharmaceutical industry and academia raced to duplicate this achievement, and shelves began to fill with alkaloids, which had names and melting points, but determining their structures was a laborious, time-consuming process. In the course of my work on muscopyridine I had learned much about alkaloids and the problems involved in the determination of their molecular structures. I remembered one case, where three independent laboratories had proposed Structure I for the alkaloid sarpagine, but only one of them suggested proving it by correlation with a degradation product (Structure II) of another alkaloid of known structure, ajmaline. However, three years had gone by without success. This was probably the case because for such a correlation, the sarpagine molecule has to be converted to a compound identical to the one it should be compared with, but this involves several experimentally difficult steps. It now occurred to me that by using mass spectra for the comparison, one needed to prepare only a similar comparison product. The reasoning was that because these indole alkaloids consist of an aromatic system and a polycyclic unit, only the latter will fragment; the former will remain relatively intact. Therefore, the mass spectra of molecules with identical polycyclic structures, but differently substituted aromatic portions, will display the same patterns, except that some peaks will be shifted by the mass difference of the substituents on the aromatic portions of the molecules. According to this reasoning, it was only necessary to convert sarpagine (Structure I) to a compound of Structure III, which involved only three very simple chemical reactions. Checking the product of each step by mass spectrometry eliminated the need for conventional 6 Biemann Annual Rev. Anal. Chem. 2015.8:1-19. Downloaded from www.annualreviews.org Access provided by 45.58.110.168 on 08/27/16. For personal use only
H CH;C Structures I-III characterization as described for the m mined by Pro Robert B.W e,MA),just up the river from MIT.In the course of that work he had prepared compound II.I knew him very well and he gladly provided me with a sample.Woodward,the preeminent natural products chemist of the time(and a 1965 Nobel laureate)was curious about how my novel approach to structure proof would work out.The mass spectra of compounds II and III indeed showed almost identical patterns with the predicted shift in mass(Figure 3)(19).This method then became as the in August 1960 in Melbourne,Australia.Apparently,word the room was so crowded that the eminent Bob Woodward had to sit on the steps of the lecture room.After my talk.Professor Carl Dierassi of Stanford University (Stanford.CA)came over and invited me to come to his laboratory to help set up his mass spectrometer,which was already on order.and teach his students and postdocs how to interpret the spectra.I agreed and spent January and February of 1961 at Stanford.Later, mme nted that it was 11 powerful. logy,one should also he around.Carl indeed competed with me in the alkaloid field,but we became good friends in later years.In a retrospective article(20)he paid tribute to my contributions,referring to a seminar talk I gave while at Stanford in 1961:"It was the elegant rationalization by Biemann et al.of the mass spectral fragmentation behavior of alkaloids of the aspidospermine class that stimulated a serious effort at Stanford on organic chemical applications of mass spectrometry"(p.1341).A
AC08CH01-Biemann ARI 10 June 2015 13:25 Structures I–III characterization, as described for the muscopyridine work. The structure of ajmaline had been determined by Professor Robert B. Woodward at Harvard University (Cambridge, MA), just up the river from MIT. In the course of that work he had prepared compound II. I knew him very well and he gladly provided me with a sample. Woodward, the preeminent natural products chemist of the time (and a 1965 Nobel laureate) was curious about how my novel approach to structure proof would work out. The mass spectra of compounds II and III indeed showed almost identical patterns with the predicted shift in mass (Figure 3) (19). This method then became known as the mass spectrometric shift technique. I presented this work at the International Symposium on the Chemistry of Natural Products in August 1960 in Melbourne, Australia. Apparently, word of my work had gotten around and the room was so crowded that the eminent Bob Woodward had to sit on the steps of the lecture room. After my talk, Professor Carl Djerassi of Stanford University (Stanford, CA) came over and invited me to come to his laboratory to help set up his mass spectrometer, which was already on order, and teach his students and postdocs how to interpret the spectra. I agreed and spent January and February of 1961 at Stanford. Later, some of my colleagues commented that it was a great mistake to help a powerful, well-funded man like Djerassi become my competitor. However, I had thought that if one develops a useful new methodology, one should also help to spread it around. Carl indeed competed with me in the alkaloid field, but we became good friends in later years. In a retrospective article (20) he paid tribute to my contributions, referring to a seminar talk I gave while at Stanford in 1961: “It was the elegant rationalization by Biemann et al. of the mass spectral fragmentation behavior of alkaloids of the aspidospermine class that stimulated a serious effort at Stanford on organic chemical applications of mass spectrometry” (p. 1341). A www.annualreviews.org • Structure Determination of Natural Products 7 Annual Rev. Anal. Chem. 2015.8:1-19. Downloaded from www.annualreviews.org Access provided by 45.58.110.168 on 08/27/16. For personal use only