Sintesi

Lo studio della chimica organica espone uno studente ad una grande varietà di reazioni intercorrelate. Gli alcheni, per esempio, possono essere convertiti in composti strutturalmente simili come alcani, alcoli, alogenuri alchilici, epossidi, glicoli, e borani; possono essere scissi per ottenere aldeidi, chetoni ed acidi carbossilici più piccoli; possono essere ingranditi per ottenere da carbocationi e addizioni radicaliche come pure da cicloaddizioni. La totalità di questi prodotti può essere successivamente trasformata in un mucchio di nuovi prodotti inglobando una larga varietà di gruppi funzionali, e perciò pronti ad ulteriore manipolazione. Di conseguenza, la pianificazione razionale di una sintesi a più stadi per la realizzazione di un dato prodotto a partire da sostanze specifiche, diventa una delle sfide più impegnative che si possano affrontare.
Una sequenza in uno o due passi di semplici reazioni non è difficile da dedurre. Se, per esempio, viene chiesto di preparare il meso-3,4-esandiolo a partire dal 3-esino, la maggior parte degli studenti si rendono conto che sarà necessario ridurre l'alchino a cis o trans-3-esene prima dell'inizio della formazione del glicole. L'idrossilazione del cis-3-esene con permanganato o con tretrossido di osmio produrrebbe il meso composto desiderato. Dal trans-3-esene sarebbe necessario dapprima epossidare l'alchene con un peracido, seguito dall'apertura dell'anello con ione idrossido. Questo esempio illustra una proprietà comune delle sintesi: esiste spesso più di una procedura efficace per realizzare il prodotto desiderato.  

Sintesi multistadio più complesse richiedono analisi e riflessioni accurate, dal momento che occorre prendere in considerazione molte scelte. Come un esperto giocatore di scacchi valuta nel lungo termine i pro e contro di possibili mosse, il chimico deve valutare il potenziale successo di vari possibili percorsi di reazione, focalizzando gli scopi ed i limiti a cui ogni specifica reazione è sottoposta. Questo può essere un compito che intimidisce, per il quale si acquisisce abilità con l'esperienza, spesso tra tentativi ed errori.
I tre esempi mostrati sotto sono illuminanti. Il primo è un problema di semplice conversione di un gruppo funzionale, che può sembrare all'inizio difficoltoso. E' spesso utile lavorare con simili problemi andando all'indietro, partendo, cioè, dal prodotto. In questo caso dovrebbe essere evidente che il cicloesano deve essere sostituito per ottenere cicloesanone, poiché quest'ultimo potrebbe essere ottenuto per semplice ossidazione. Inoltre, poiché il cicloesano (e gli alcani in genere) è relativamente inerte, la bromurazione (o la clorurazione) sembrerebbe essere un primo ovvio passo. A questo punto, lo sperimentatore è indotto a convertire il bromocicloesano in cicloesanolo con una reazione S_N2 con ione idrossido. Questa reazione potrebbe essere senza dubbio seguita da una eliminazione E2, percorso che la rende più pulita, sebbene con uno stadio più lungo, rispetto al primo che prevede la preparazione del cicloesene da idratare con qualunque metodo (es. ossimercurazione e idroborazione) incluso quello che viene mostrato cliccando sul diagramma.

Appariranno, a questo punto, anche soluzioni plausibili del secondo e terzo problema. Nel problema 2 il prodotto desiderato ha sette atomi di carbonio e la sostanza di partenza ne ha quattro. Chiaramente, i due intermedi derivati dal composto di partenza devono essere uniti insieme e un atomo di carbonio deve essere perduto, e l'una o l'altra cosa deve avvenire prima o dopo che il legame abbia luogo.La funzione alcolica terziaria nel prodotto suggerisce la formazione di una addizione di Grignard ad un chetone, e l'isobutene sembra essere un buon precursore ad ognuno di questi reagenti, come è mostrato. I composti che costituiscono il reagente ed il prodotto nel terzo problema sono isomeri, ma è chiaramente necessario qualche tipo di sequenza di rottura e di formazione di legami affinché avvenga questo cambiamento strutturale. Una procedura possibile è mostrata sopra. Potrebbe anche essere utile un riarrangiamento catalizzato da acidi dell'ossido di cicloesene, seguito da riduzione.

Un vantaggioso metodo di lavoro per la messa a punto delle sintesi, che si attua partendo dalla molecola finale per arrivare alle più semplici sostanze di partenza, è stato formalizzato dal Prof. E. J. Corey (Harvard) ed è denominato analisi di retrosintesi. 

The useful approach of working out syntheses starting from the target molecule and working backward toward simpler starting materials has been formalized by Prof. E. J. Corey (Harvard) and termed retrosynthetic analysis. In this procedure the target molecule is transformed progressively into simpler structures by disconnecting selected carbon-carbon bonds. These disconnections rest on transforms, which are the reverse of plausible synthetic constructions. Each simpler structure, so generated, becomes the starting point for further disconnections, leading to a branched set of interrelated intermediates. A retrosynthetic transform is depicted by the => symbol, as shown below for previous examples 2 & 3. Once a complete analysis has been conducted, the desired synthesis may be carried out by application of the reactions underlying the transforms.

The above diagram does not provide a complete set of transforms for these target compounds. When a starting material is specified, as in the above problems, the proposed pathways must reflect that constraint. Thus the 4-methyl-2-pentanone and 3-methylbutyrate ester options in example 2, while entirely reasonable, do not fit well with a tert-butanol start. Likewise, a cyclopentyl intermediate might provide an excellent route to the product in example 3, but does not meet the specified conditions of the problem.
Retrosynthetic analysis is especially useful when considering relatively complex molecules without starting material constraints. If it is conducted without bias, unusual and intriguing possibilities sometimes appear. Unfortunately, molecular complexity (composed of size, functionality, heteroatom incorporation, cyclic connectivity and stereoisomerism) generally leads to very large and extensively branched transform trees. Computer assisted analysis has proven helpful, but in the end the instincts and experience of the chemist play a critical role in arriving at a successful synthetic plan. Some relatively simple examples, most having starting material restrictions, are provided below.

Problem 1

A synthesis of N-ethyl-2-aminomethylspiro[3.3]heptane from starting compounds having no more than three contiguous carbon atoms is required. This provides a good example of the importance of symmetry in planning a synthesis. First, it should be recognized that the amine group is best introduced at the end of the synthesis, by reacting ethylamine with an ester (or acyl chloride derivative) of spiro[3.3]heptane-2-carboxylic acid, followed by LiAlH₄ reduction. This approach avoids the necessity of protecting a nucleophilic nitrogen from undesired participation in other reactions. Second, the symmetry of the remaining carbon skeleton suggests its disconnection into 1,3-difunctionalized propane units, as shown below. All of these have a common origin in diethyl malonate, which can be reduced to a 1,3-glycol and then converted into 1,3-dibromopropane.

Problem 2

A synthesis of 2,7-dimethyl-4-octanone from starting compounds having no more than four contiguous carbon atoms is required. The structural formula and a first-stage retroanalysis of this ketone are displayed in the following diagram. Three straightforward disconnections are shown, as drawn by the dashed lines. The first (magenta arrow) is undoubtedly the simplest, since a Grignard reagent addition to a suitable nitrile gives the product directly. However, one or more of the reactants is larger than C₄ and must therefore be prepared independently before use. A two-step procedure involving Grignard addition to an aldehyde, followed by oxidation of the 2º-alcohol product, also suffers the same requirement, as do the epoxide opening routes presented in the second row (cyan arrow). Secondary preparations of these intermediates are easily conceived by way of cyanide substitution of a 1º-halide, coupling of a Gilman reagent with allyl bromide, or Grignard addition to ethylene oxide.
The last disconnection (green arrow) creates the desired carbon skeleton by sequential alkylations of terminal alkynes (first acetylene and then 4-methyl-1-pentyne). Mercury catalyzed hydration of the symmetrical octyne product generates the desired ketone. All the necessary reactants are C₄ or less, so the synthesis is accomplished in three steps (not counting the formation of alkyne salts).

Three more first-stage analyses will be displayed above by clicking on the diagram. The first of these (red arrow) is a two step sequence initiated by isobutyl magnesium bromide addition to acetonitrile, followed by isobutyl bromide alkylation of the resulting 4-methyl-2-pentanone. Regioselective control might be a problem in the last step. The second disconnection (orange arrow) suggests an α, α'-dialkylation of acetone. Since acetone itself is prone to base-catalyzed condensation, this might be difficult to accomplish directly. However, the use of ethyl acetoacetate avoids this problem for the first step, and the second alkylation is the same one proposed as part of the first disconnection synthesis. Both of these sequences would provide efficient routes to the target ketone.
Finally, the last disconnection is a four component assembly consisting of two conjugate additions and a Grignard addition. This would most likely result in a longer and lower yield procedure than the previous two.

Problem 3

A synthesis of 1,4,6--trimethylnaphthalene from para-xylene and other starting compounds having no more than four contiguous carbon atoms is required. Plausible transforms for the attachment of the second ring carbons to para-xylene are Friedel-Craft alkylation or acylation (acylation is usually better), nucleophilic attack of an aryl metal reagent derived from 2-bromo-para-xylene on carbonyl or epoxide electrophiles, or possibly by cycloaddition to a aryne intermediate. A palladium catalyzed coupling reaction might also prove useful. Because of their simplicity and broad scope, we shall consider only the first two transforms.
The following diagram shows retrosynthetic analyses based on the Friedel-Craft transform for both bond formations to the aromatic ring. Of these, the first seems to offer the most efficient synthesis route, consisting of Friedel-Craft acylation, Wolff-Kischner reduction, a second Friedel-Craft acylation and methylation of a ketone enolate. In all cases the substituted tetralone precursor of the desired naphthalene must be reduced to an alcohol and dehydrated. The resulting dihydro naphthalene is then aromatized by Pt catalyzed dehydrogenation, or mild oxidation by heating with sulfur or selenium.

By clicking on the diagram, a new set of disconnections, starting from 2-bromo-para-xylene, will be displayed. A derived Gilman or lithium reagent is used for conjugate addition to an unsaturated carbonyl compound or ring opening of an epoxide. Further lengthening of the side chain is effected by cyanohydrin formation (top example), malonic ester alkylation (middle example), and Arndt-Eistert homologation (bottom example). The final steps must then parallel those used for the first examples.

Problem 4

A synthesis of 2-acetyl-2-methylbicyclo[2.2.2]octane from cyclohexene and other starting compounds having no more than four contiguous carbon atoms is required. The target molecule has two bridged six-membered carbon rings, and cyclohexene is one of the starting materials. Whenever a six-membered carbon ring must be formed, possible Diels-Alder transforms should always be considered. For such a construction one needs a conjugated diene and a dienophile. Cyclohexene might be considered a dienophile, but acting as such would lead to a fused ring product, not a bridged ring structure. Also, commonly used electron-rich dienes are not expected to react well with an unstrained, electron-rich alkene.
If the role of cyclohexene is changed to that of a diene, these objections are overcome. This alteration is easily managed by addition of bromine to cyclohexene, followed by a double elimination, yielding 1,3-cyclohexadiene.

The possible use of cyclohexadiene in this synthesis is shown above. A Diels-Alder cycloaddition to a dienophilic double bond generates the desired bicyclooctane ring system, and the task is to identify a reasonable intermediate for this purpose. Among the many reactions that form ketones, the addition of a Grignard reagent to a nitrile is particularly efficient. If we choose this as the last step, the dienophile becomes 2-methylacrylonitrile, and the retrosynthetic path is complete. The isolated double bond produced by the cycloaddition is reduced by catalytic hydrogenation, so distinction between exo and endo-addition products is lost (the endo-adduct shown predominated).

Problem 5

A synthesis of 2-benzyl-3,3-dimethylcyclohexanone from benzene derivatives having no more than seven carbons and other starting compounds having no more than four contiguous carbon atoms is required. Since conjugate addition of a methyl group to 2-benzyl-3-methyl-2-cyclohexen-1-one should proceed in good yield, this unsaturated ketone provides a good alternative target, as shown. Once again, the cyclohexane ring suggests a Diels-Alder transform. Three such disconnections are depicted in the following diagram along with a possible aldol cyclization (example 4). Diels-Alder approach 1 is the most promising, since it features an electron-rich diene reacting with an electron deficient dienophile. Chloroacrylonitrile is a useful surrogate to ketene as a dienophile (ketene normally reacts by [2+2} cycloaddition). Hydrolysis of the α-chloronitrile unit in the adduct converts it to a carbonyl group. Unfortunately, the regioselectivity of this cycloaddition is likely to be poor, with 5-benzyl-4-methyl-2-cyclohexen-1-one (orange box bottom left) being formed in significant or possibly major amount. Also, the diene, (3E)-3-methyl-5-phenyl-1,3-pentadiene, needed for this reaction may be difficult to obtain as the desired stereoisomer (the Z-isomer will be relatively unreactive because of steric hindrance in the cisoid conformation).
Diels-Alder synthesis 2 does not have a regioselectivity problem, but the reaction of an electron-rich diene with an electron-rich dienophile is often sluggish and incomplete. Also the initial adduct has a methyl ether where a carbonyl function is needed. The third Diels-Alder proposal in the gray-shaded area has even more problems. As in reaction 2, electronic factors make the cycloaddition poor, and the regioselectivity will likely favor the wrong adduct (circled in orange). Even if the desired 3,3-dimethylcyclohexanone were obtained, benzylation at the desired α-position (green) will have to compete with that at the less hindered α'-position (magenta).

By clicking on the diagram, a new set of disconnections will be displayed. The first of these (top line) is a cyclic aldol transform similar to the last case discussed. Here, however, the symmetry of the 1,5-diketone (after decarboxylation) permits only one cyclohexenone product, 3-methyl-2-cyclohexen-1-one (drawn in the light gray box). This key synthetic intermediate, known as a synthon, may lead to the target molecule in two ways, depending on the order in which conjugate addition and α-alkylation are conducted. Another useful concept, revealed by the disconnections in the last two rows, is that benzene derivatives may serve as precursors to cyclohexane compounds.
By clicking on the diagram a second time, the reactions which may be used to achieve the proposed constructions will be shown above. Note the use of a Birch reduction in the second line. All three approaches should produce the target compound, the most efficient arguably being the third.

Problem 6

A synthesis of all-cis-1,2,3,4-tetrakis(hydroxymethyl)cyclopentane from simple starting materials (six or fewer contiguous carbons) is required. Since carboxylic acids, esters, aldehydes and 1º-alcohols are easily interconverted, this target may be changed to the corresponding tetracarboxylic acid, as shown in the following diagram. Constructing the cyclopentane ring becomes a primary goal, and this may be done by condensation reactions (first two disconnections), cycloaddition (third disconnection) or by starting with a cyclopentane reagent (last example). Although there is precedent in known chemistry for all these approaches, some turn out to have serious flaws.

By clicking on the diagram, chemical reactions corresponding to each of the disconnection paths will be shown above. The first example, which takes advantage of symmetry, turns out to suffer from subsequent rapid Michael addition of a second acetonedicarboxylic acid moiety to the intermediate cyclopentadienone. This is, in fact, a general synthesis of bicyclo[3.3.0]octane-3,7-diones, known as the Weiss reaction. The second approach constructs the five-membered ring by a Dieckmann condensation of a tetra-carboxylic ester prepared from triethyl aconitate. Addition of the fourth carboxyl group by way of a cyanohydrin should be straightforward, but a mixture of stereoisomers will result, with the all-cis compound being a minor component. The cycloaddition proposed for the third approach is allowed by orbital symmetry, but only a few examples have been observed. Pursing this synthesis would be unwise, because it suffers from the same lack of stereoselectivity as the second case. Finally, The last approach, involving sequential [2+2] cycloaddition of ketenes to cyclopentadiene, is longer and has an inherent problem associated with the regioselectivity of the conventional Baeyer-Villiger oxidation. This problem may be overcome by using chiral catalysts (enzymes or transition metal complexes) with hydrogen peroxide, but a 50% conversion is the best that can be achieved and stereoselectivity may still be a problem.

A careful examination of the tetracarboxylic acid target reveals a possible precursor in which the cis carboxyl groups at C1 and C4 are masked by incorporation in a double bond. Such a bicyclo[2.2.1]heptene structure is readily achieved from 1,3-cyclopentadiene by way of a Diels-Alder reaction, as shown in the following retrosynthetic disconnection. With this as a guide, a simple three step synthesis may be proposed (shown by clicking on the diagram). The borohydride workup of the ozonolysis in the last step will convert aldehydes to 1º-alcohols.

Practice Problems

The following problems examine many aspects of organic synthesis. They are roughly organized by increasing difficulty.

One of the earliest, and perhaps most significant-although accidental-examples of synthesis was reported by Friederich Wöhler in 1828. In an experiment designed to prepare ammonium cyanate from silver cyanate, he heated the latter with ammonium chloride expecting the outcome shown below.

The product Wöhler obtained did not correspond to the expected cyanate salt, but was identified as urea, NH₂CONH₂, an organic compound isolated from urine fifty years earlier. This result was revolutionary in two respects. First it provided another example of isomerism, in that ammonium cyanate, ammonium fulminate (NH₄O-N=C) and urea are all isomers, a novel concept for the time. Second it cast doubt on the widely held doctrine of vitalism, which maintained that all living organisms were endowed with a vital or life force that rendered them and their component parts uniquely different from ordinary "inorganic" matter. Thus, strongly heating organic substances such as carbohydrates and proteins yielded water, ammonia and carbonaceous solids (all inorganic), with loss of the vial essence. Wöhler's experiment was acclaimed as the first conversion of an inorganic substance into an organic compound.
Less than twenty years later, the German chemist Adolf Kolbe provided an even more convincing synthesis of organic from inorganic substances. The two equations written below outline his experiment. First, carbon disulfide, obtained by reaction of carbon with sulfur, was converted to carbon tetrachloride by heating with chlorine, and the simultaneous pyrolysis of CCl₄ yielded a mixture of products which included tetrachloroethene, presumably formed from dichlorocarbene (:CCl₂). Treatment of tetrachloroethene with aqueous chlorine (think HOCl) gave trichloroacetic acid, which Kolbe reduced electrolytically to acetic acid. This ended the reign of vitalism as a scientific theory.

During the 1850's, the French chemist Pierre Berthelot synthesized scores of simple organic compounds, ranging from ethanol to acetylene and benzene, setting the stage for more ambitious attempts. Just as the alchemists sought to transmute base metals into gold, early organic chemists were drawn to the isolation or preparation of rare dyes, exotic perfumes and unusual spices, often worth more than their weight in gold. A notable example of this interest is William Perkin's attempt to synthesize quinine.
Quinine, an important drug for the treatment of malaria, was available only from the bark of the South American tree Cinchona officinolis, and in the mid 1850's a decline in the native tree population had caused a large rise in the price of the drug. Very little was known about the compound, other than its molecular formula C₂₀H₂₄N₂O₂. Nevertheless, in the spring of 1856, William H. Perkin, a student (age 18) at the Royal College of Chemistry in London, attempted its synthesis in his home laboratory. Perkin reasoned that oxidation of a suitable 10-carbon amine, such as allyl toluidene, C₁₀H₁₃N, might generate quinine, as shown in the following equation.

This simple approach failed, and from our vantage point a century and a half later it is easy to see why. Many thousands of isomers having the molecular formula of quinine are possible, but only one unique configuration of these 48 atoms constitutes a molecule of quinine. That the atoms of allyltoluidine should, in the course of one reaction, selectively reorganize and combine in this specific fashion is beyond all reasonable probability.
Perkin's experiment was a failure only in the respect it did not yield quinine, and his subsequent study of aromatic amine oxidations demonstrates the value of persistence. From an impure sample of aniline he obtained a purple dye he called aniline purple (also called mauve), which became the cornerstone of the synthetic dyestuff industry in Europe and made a fortune for its dicoverer.
A total synthesis of quinine was achieved in 1944 by R. B. Woodward and W. E. Doering (Harvard), and improved syntheses continue to be reported.