These are poisons, which include atropine, various other alkaloids, some snake and spider venoms and nerve gas, which disrupt the workings of the nervous system.

This consists of billions of nerve cells or neurones [3]. Neurones are special elongated cells which can carry electrical impulses along their length.  The nervous system is divided into two sections:

• Central Nervous System (CNS) which consists of the brain and spinal cord. The CNS co-ordinates and integrates information it receives and organises responses

• Peripheral Nervous System (PNS) which consists of nerves running from the CNS to the peripheries - arms, legs, abdomen, etc. The role of the PNS is to collect information about changes in the environment registered by receptors and pass this on to the CNS and to pass impulses from the CNS to the effectors, for example skeletal muscles, which respond accordingly.

Neurones themselves fall into three classes:

• Motor neurones are nerve cells which transmit impulses from the CNS to the effectors, for example muscles.

• Sensory neurones are nerve cells which transmit impulses from the receptors to the CNS.

• Intermediate neurones which are nerve cells found in the CNS and which connect sensor and motor neurones with each other and with other cells in the CNS

Neurones can be very long, perhaps up to a metre in length and impulses travel along them from one end to the other at speeds of up to 100 m s^-1 (over 220 m.p.h.!)

The long, cylindrical section of the neurone along which the impulse passes is known as the axon. This membrane of the axon does not conduct electricity and  is normally impervious to the passage of ions such as Na⁺, and K⁺.   However it is possible to force cations through this membrane by an energy absorbing "sodium pump". The system so arranges itself so that the majority of sodium ions reside outside the membrane and most of the potassium ions lie within the cell plasma.  The numbers of sodium and potassium ions transferred is not equal and this results in an electrical potential of about 60 mV being set up across the membrane, the inside being negative and the outside being positive. This potential remains constant while the axon is at rest and so is referred to as the resting potential.

When an impulse passes down the axon the situation changes for an instant. As the impulse reaches a particular point on the axon the membrane becomes more permeable to sodium ions. The sodium ions, being in higher concentration on the outside of the membrane than in the plasma, rush into the interior of the cell and carry their charge with them. This suddenly reverses the potential and the interior becomes positive for an instant. This is known as the action  potential.  After this,  the potassium ions rapidly migrate through the membrane to the outside, so reducing the positive charge within the cell and re-establishing the resting potential. The sodium and potassium pumps then re-establish the original concentrations of cations so that the process may be repeated. As the process occurs at one point in the axon, so it initiates a similar process at an adjacent site and so the impulse propagates along the axon in a wave like motion.

The point at which neurones meet is called a synapse. The end of an axon is a somewhat bulbous region known as a synaptic knob. The synaptic knob is full of mitochondria and small sac-like vesicles containing neurotransmitters such as acetylcholine and it is sealed by a membrane. There is a small gap approximately 20 nm wide, between this membrane and that of the next cell and this is known as the synaptic cleft.  The membrane of the cell through which the impulse is being transmitted is called the pre-synaptic membrane while that of the cell to which the signal is to be transferred is called the post-synaptic membrane.

When the impulse reaches the synaptic knob it initiates an inflow of Ca²⁺ ions which causes the vesicles to move towards, and become attached to, the pre-synaptic membrane. The vesicles then discharge their neurotransmitter substance (acetylcholine) into the synaptic cleft. This material rapidly diffuses to the post-synaptic membrane where it initiates a process which generates an impulse in this cell. If the neurotransmitter remained bound to the site on the post-synaptic membrane it would maintain constant stimulation of the cell resulting in the repeated generations of impulses in the cell. The cell would become fatigued and cease to transmit pulses and so would become ineffective. To prevent this situation from occurring the acetylcholine receptor has an enzyme attached to it which rapidly decomposes the neurotransmitter once it has initiated a response. The enzyme acting on acetylcholine is called cholinesterase and it converts acetylcholine into ethanoic (acetic) acid and choline. The choline then diffuses back to the pre-synaptic membrane where it is reabsorbed and converted back into acetylcholine in the vesicles. If this enzyme ceased to act, not only would the post-synaptic cell become inactive due to constant stimulation resulting in fatigue, but the store of acetylcholine in the pre-synaptic cell would rapidly become exhausted and this cell too would become inactive.

The chemical nature of synaptic transfer makes them very susceptible to disruption by toxins. Many alkaloid poisons, snake and spider venoms and nerve gases act in this way [4], [5].

The structure of atropine was investigated by R Willstätter in the late 1890's. It was found that on hydrolysis, atropine gave (±)-tropic acid and tropine, which was shown to be an alcohol.

Tropic acid was shown to have a molecular formula of C₉H₁₀O₃ and lost a molecule of water to yield atropic acid on strong heating. Atropic acid, C₉H₈O₂, had the structure (1) implying that tropic acid was either: compound (2) or (3):

Mackenzie and Wood showed that tropic acid was compound (2), 3-hydroxy-2-phenylpropanoic acid, by unambiguous synthesis in 1919 while Fodor, et al, demonstrated the absolute configuration by its correlation with (-)-alanine in 1961:

Tropine, C₈H₁₅ON contains a hydroxyl group and behaves as a saturated compound. Ladenburg (1883, 1887) demonstrated that the molecule contained a reduced pyridine nucleus. His work lead him to propose two possible structures for tropine, (4) and (5):

Merling (1891) obtained tropinic acid by oxidation of tropine using chromium trioxide. Tropinic acid was shown to be a dicarboxylic acid which contained the same number of carbon atoms as tropine:

Since tropinic acid is a dicarboxylic acid and no carbon is lost in the oxidation, the alcohol group oxidation must have involved ring cleavage and so the hydroxyl group had to be within a ring. This made the Ladenburg structures untenable and so Merlin proposed either structure (6) or (7) for tropine:

Willstätter (1895 - 1891) notes that tropinone was formed during oxidation prior to ring cleavage. This substance was a ketone which was demonstrated to contain the CH₂COCH₂moiety by the formation of dibenzylidene and di-oximino derivatives. This, in turn, made Merling's structures untenable and so Willstätter proposed structure (8) for tropine:

The structure of tropine has been confirmed to be (8) by synthesis by Willstätter (1900 - 1903) and by Robinson (1917) - see section on synthesis.

The first synthesis of atropine was achieved by Richard Willstätter in 1901 [7]. Before continuing the discussion of his work it is perhaps useful to remind ourselves of the situation which faced organic chemists at this time. Recent innovations such as mass spectrometry, infrared spectroscopy and nuclear magnetic resonance spectroscopy which are taken for granted today, were simply not available to assist in the determination of structure. Instead chemists had to rely on hard won information based upon simple chemical tests. This information was often inadequate and incomplete and the determination of structure was a process of detective work which often required great intuition and creativity. At this time structure determination was not always equivocal and final proof could only be established by unambiguous synthesis of the compound with the suspected structure followed by comparison with an authentic sample of the natural product. Thus synthesis was often a matter of utilitarian necessity rather than the creative, elegant art form illustrated by the work of many of the great synthetic chemists such as Woodward and Corey.

It was relatively straightforward to reduce tropinone to tropine and hence form its ester atropine.

His preparation of tropinone is a competent but long synthesis which demonstrates one of the fundamental difficulties involved in the preparation of complex organic molecules. Although the individual steps in the synthesis generally give good to excellent yields, there are many of them which means that the overall yield becomes diminishingly small, of the order of 1%.  As a result the early steps in the synthesis have to be carried out on inconveniently large quantities of material,  and despite this, usually have to be repeated several times in order to obtain sufficient material to carry out the later stages on an acceptable scale.

In 1917 Robinson [9] approached the synthesis in a totally radical way.  In his own words:

The initial product is a salt of tropinone dicarboxylic acid,  and this loses two molecules of carbon dioxide with the formation of tropinone when the solution is acidified and heated" [10, click on image to link to Nobel site].

This resulted in the formation of  tropinone in one step.   More recent work by Schöpf has allowed the yield for this reaction to be raised to about 90%,  mainly by carrying out the processed under buffered conditions.

For a more detailed account of these syntheses it is worth reading the account by Ian Fleming in  his book Selected Organic Syntheses, A Guidebook for Organic Chemists [11].

Tropinone may then be converted into tropine by metal in acid reduction, the best yields being obtained  using zinc in HI.   It may be noticed that the final precursor in the Willstätter synthesis appears to be tropine.  This is not the case as the material is its geometric isomer,   j -tropine,  and thus tropine is formed by oxidation of j -tropine to tropinone followed by stereoselective reduction of the carbonyl group.

Tropic acid

Mackenzie and Ward [12] proved the structure of tropic acid by synthesis from acetophenone in 1919:

Note that the addition of HCl in step 4 contravenes Markownikoff's rule.  This is presumably due to the electron withdrawing effect of the carboxyl group which destabilises the tertiary carbonium ion intermediate relative to the primary carbonium ion.  It is tropic acid which introduces the stereocentre into the atropine molecule.  The racemic mixture formed from this reaction sequence may be resolved by reaction with quinine followed by fractional crystallisation of the diastereoisomers..

More recently Blicke et al have prepared tropic acid from phenylacetic acid via a Grignard reagent and formaldehyde:

Atropine

The final problem in the synthesis,  the combination of tropine and tropic acid,  was overcome by a Fischer-Speier esterification [13].  The acid and alcohol were heated together in the presence of HCl to yield atropine

References

1     Finar, I.L., Organic Chemistry Vol. 2 4th. Edn.,   Longman 1965, p. 601

2     http://www.staffs.ac.uk/schools/sciences/chemistry/tebby/alkaloids.html

3     Roberts, M. B. V.,  Biology,  A Functional Approach 2nd. Edn.,  Thomas Nelson and Sons 1979,  pps. 262 et seq

4     http://web.indstate.edu/thcme/mwking/nerves.html#synaptic

5     http://www.pharmcentral.com/neurotransmitters.htm#1-neurotransmitters

6     Finar, I.L., Organic Chemistry Vol. 2 4th. Edn.,   Longman 1965,  pps. 631 - 639

7     Willstätter, R.,  Ber.,  34, (1901),  pp. 129 and 3163

8     http://www.nobel.se/chemistry/laureates/1915/

9     Robinson,  R.,  J.  Chem.  Soc., (1917),   p762

10   http://www.nobel.se/chemistry/laureates/1947/index.html

11     Fleming, I.,  Selected Organic Syntheses,  John Wiley 1973,  pps. 17 - 23

12    Finar, I.L., Organic Chemistry Vol. 2 4th. Edn.,   Longman 1965, pps. 631 et seq.

13    Finar, I.L., Organic Chemistry Vol. 2 4th. Edn.,   Longman 1965, p. 636

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