CHAPTER 1
Amino-acids
BY J. H. JONES
In one way or another, the whole of this Report is about the chemistry of amino-acid derivatives, but here we are concerned only with the amino–acids themselves. Emphasis is on α-amino-acids, and it is only for these that comprehensive coverage has been attempted. Furthermore, although many may think it retrogressive to draw demarcation lines between disciplines in these enlightened times, amino-acid biochemistry has been excluded. Amino-acid chemistry is under continuous scrutiny from every conceivable angle, and so papers on these compounds span a very diverse range of interests. The Reporter therefore makes no apology for the fact that this chapter is something of a miscellany.
1 Naturally Occurring Amino-acids
A. Occurrence of Known Amino-acids. — Pipecolic acid (of unstated configuration, but presumably L) has been isolated from azuki beans (Phaseolus angularis) in a yield of about 0.05%. N-Methyl-L-alanine, which has not been found in higher plants before, has been isolated from the leaves of Dichapetalum cymosum. This plant is toxic and causes considerable cattle loss in southern Africa because it produces fluoroacetate. Young leaves contain remarkably large amounts of N-methyl-L-alanine (up to 5.6% by weight on a dry-weight basis) and it was suggested that the metabolism of the amino-acid and the toxin might be linked. Erythro-γ-methyl-L-glutamic acid has been isolated from seeds of Lathyrus maritimus and distinguished from the other possible methylglutamic acids by nuclear magnetic resonance (n.m.r.) spectrometry: chromatographic evidence indicates that this amino-acid also occurs in other Lathyrus species. Nε-Trimethyl-L-lysine has been isolated from seeds of Reseda luteola and from chicken erythrocyte histones, where it occurs together with Nε-methyl- and Nε-dimethyl-L–lysine.6-Hydroxykynurenic acid has been obtained from tobacco leaves, thus providing the first example of the occurrence of a kynurenic acid derivative in plants. A survey of the distribution of fifteen non-protein amino-acids in about forty species of the genus Acacia shows that members of the Gummiferae series can be distinguished by their amino-acid content: they alone contain N-acetyldjenkolic acid, and they lack several amino-acids which are widely distributed among other Acacia species. The possibility that the toxicity of various species of Crotalaria may not be due solely to the presence of pyrrolizidine alkaloids in these plants has been demonstrated by the isolation of the neurotoxin α-amino-β-oxalylaminopropionic acid from seeds of C. incana and C. mucronata. Trans -3-hydroxy-L-proline has been isolated from seeds and vegetative tissue of Delonix regia, where it is a major component of the free amino-acid pool.
B. New Naturally Occurring Amino-acids. — A number of new natural amino-acids have been characterised during the year: these are listed at the end of this section, together with their sources. Those whose structure has been confirmed by synthesis are included in the list of newly synthesised amino-acids in section 2. The increasing usefulness of physical methods for the characterisation of amino-acids is apparent: n.m.r. and mass spectrometry (applications of mass spectrometry in amino-acid and peptide chemistry have been reviewed) have been particularly valuable. Especially noteworthy is the characterisation of the novel amino-acid (1) by spectroscopic methods.
C. A List of New Naturally Occurring Amino-acids. —
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2 Chemical Synthesis and Resolution of Amino-acids
The majority of new syntheses reported this year were performed by variations of well-established routes or involved elaboration of available amino-acids. Therefore, only those syntheses which have points of particular interest will be discussed in the following outline, the remainder being merely mentioned or incorporated into the appendix to this section (see p. 13).
A. Protein Amino-acids. — New syntheses have been reported for DL–lysine, DL-histidine, DL-cystine, and DL-tryptophan, and reactions for the conversion of serine to DL-cystine, L-tyrosine to L-phenylalanine, and L-ornithine to L-proline have been described. The resolution of racemic glutamic acid and alanine by preferential crystallisation has been studied in detail by Japanese workers.
B. Other α-Amino-acids. — A very brief preliminary description of a new general route to α-amino-acids has appeared: treatment of an NN-bis(trimethylsilyl)glycine ester (2) with base followed by reaction with an alkyl halide results in alkylation of the α-carbon atom giving (3) which is very easily hydrolysed to an α-amino-ester with dilute acid.
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A convenient synthesis of DL-α-aminosuberic acid (by the diethyl acetamidomalonate route) has been reported: resolution can be achieved enzymically or, preferably, by the method of Vogler et al. using optically active tyrosine hydrazide. An improved method for the conversion of tyrosine (D and L) to 3,5-dichlorotyrosine has been described. Synthesis of α,α-diaminopimelic acid by the method of Work et al. gives an equimolecular mixture of racemic and meso diaminodiacids: a simpler method for separating the isomers and resolving the racemate has been published. DL-α-Acetamido-β-methylaminopropionic acid has been obtained by an improved procedure. Stereospecific enzymic deacylation gave L-α-amino-β-methylaminopropionic acid which had the same specific rotation as a sample of the same amino-acid isolated in 1967 from Cycas circinalis, thus confirming the L configuration of the latter.
L-Felinine (4) can be prepared by acid-catalysed S-alkylation of L-cysteine with 2-methylbut-1-ene-4-ol or 2-methylbut-2-ene-4-ol.
Birch reduction of phenylalanine gives 3,6-dihydrophenylalanine (5), which can be partially hydrogenated to 3,4,5,6-tetrahydrophenylalanine (6). The position of the double bond in (6) was confirmed by isolation of the lactone (7) after treatment with hydrochloric acid, and hydrolysis of (7) gave the new amino-acid (8).
Allenic aldehydes such as (9) which are fully substituted at the 2-position give good yields in the Strecker synthesis, but poor yields of allenic amino–acids are obtained if this position is unsubstituted. The reaction of allenic bromides with diethyl formamidomalonate is a more general route to such amino-acids: the reaction is applicable to 1-bromoalka-1,2-, -2,3-, and -3,4-dienes. This reaction was used for the preparation of (10), which was treated with di-iodomethane and a zinc-copper couple followed by hydrolysis and decarboxylation to yield (±)hypoglycin A (11), a naturally occurring hypoglycaemic amino-acid. The final decarboxylation in this synthesis was stereoselective, and only one of the two racemates of (11) was obtained: this proved to be the required (±) amino-acid. The stereoselectivity was attributed to thermodynamic control, and models showed that the most stable racemate should consist of the (2S: 4S) and (2R: 4R) diastereoisomers. Since the susceptibility of natural hypoglycin A to enzymic oxidation had already established the configuration at the α-carbon as (S) it was predicted that the natural amino-acid would be found to have the absolute stereochemistry (2S: 4S), and this was confirmed by chemical correlation (see p. 15). The formamidomalonate route is also suitable for the preparation of aminoenynoic acids: thus alkylation of diethyl formamidomalonate with 5-bromopent-3-en-1-yne followed by hydrolysis and decarboxylation gave 2-aminohept-4-en-6-ynoic acid in good yield.
The diaminodiacid (12), which is one of the possible products of oxidative dimerization of 3,5-dihydroxyphenylalanine (DOPA), has been synthesised by the oxazolone route from the dialdehyde (13). This compound ('DOPA dimer') was shown to polymerise in the presence of tyrosinase and oxygen at the same rate as DOPA itself, which is consistent with the possibility of (12) as an intermediate in the melanogenesis of DOPA. Oxidative cyclisation of DOPA methyl ester with ferricyanide followed by dithionite reduction gives (14) which can be converted to 'cycloDOPA' (15) by anaerobic hydrolysis after acetylation.
DL-Capreomycidine (16), a guanidino-amino-acid obtained from acid hydrolysates of antibiotics of the capreomycin group, has been synthesised by catalytic reduction of the oxime (17) followed by saponification and separation of the mixture of diastereoisomers.
C. α-Dialkyl-α-amino-acids. — The hydantoin route is in frequent use for the synthesis of α-dialkyl-α-amino-acids but difficulties due to the resistance of 5,5-disubstituted hydantoins to hydrolysis are sometimes encountered. It has been reported that this difficulty can be circumvented by conversion of such hydantoins to their 3-tosyl derivatives (18). Alkaline hydrolysis (dilute sodium hydroxide) of (18) gives hydantoic acid derivatives (19) which can be hydrolysed further to amino-acids with dilute hydrochloric acid. Although developed primarily for α-dialkyl-α-amino-acids, this method of hydrolysing hydantoins under mild conditions may find application in the synthesis of amino-acids which would not survive the usual vigorous hydrolysis conditions.
A new synthesis of DL-α-methyltyrosine has been described: NN-dimethyl-p-hydroxybenzylamine reacts with ethyl α-nitropropionate in the presence of a catalytic amount of sodium hydride via a quinone methide intermediate to give (20), which can be reduced and hydrolysed to DL-α-methyltyrosine. The resolution of α-methyl-α-amino-acids is a wasteful process if only one of the isomers is required, because the lack of an α-hydrogen atom prohibits racemisation and recycling of the isomer which is not required. This difficulty is avoided if resolution is performed at an earlier stage in the synthesis, and an example of this approach is provided by a new route to L-α-methylDOPA (α-methyl-3,4,-dihydroxy-L-phenyl–alanine). The synthesis was performed by the Strecker method, with resolution at the aminonitrile stage: as the aminonitrile is formed in an equilibrium reaction, it is easily racemised, and it was therefore possible to recycle the D-aminonitrile.
The preparation of some α-methylserines is discussed in the next section.
D. Amino-acids with Aliphatic Hydroxyl Groups in the Side Chain. — A new synthesis of β-aryl-α-methylserines is shown in Scheme 1. The synthesis comprises reaction of an aryl Grignard reagent with a 4-carboalkoxy-2,4-dimethyloxazol-5-one at low temperatures, followed by reduction with sodium borohydride and hydrolysis. This gives a mixture of diastereoisomers in which the erythro form predominates. The interconversion of the erythro and threo series is easily accomplished with thionyl chloride.
An attempt to synthesise γ-hydroxyarginine by a multistage alternative to the direct preparation from γ-bydroxyornithine has so far met with little success. Ammonolysis of cis-epoxysuccinic acid gives exclusively threo-β-hydroxy-DL-aspartic acid (Scheme 2) but the trans epoxide was found to give the threo and erythro isomers in about 1:2 proportions. This last finding is surprising in view of the fact that the reaction of benzylamine with trans-epoxysuccinic acid is stereospecific, but it was admitted that the trans-epoxide used in the ammonolysis studies may have contained some cis isomer. N-Benzyl-threo-β-hydroxy-DL-aspartic acid is very conveniently (and almost quantitatively) resolved with ephedrine, and hydrogenolysis after resolution gave L- and D-hydroxy-amino-acids of specific rotation slightly greater than previously reported. Optically pure erythro-β-hydroxy-D-aspartic acid has been obtained from optically pure trans-L-epoxysuccinic acid. Threo- and erythro-β-hydroxy-DL-isoleucine have been synthesised by stereospecific routes from cis- and trans-3-methylpent-2-enoic acids respectively: the diastereoisomers can be dis– tinguished by n.m.r.
The recent isolation of γ,δ,δ'-trihydroxyleucine (21) has prompted the synthesis of the L-isomer by epoxidation of (22) (which had earlier been synthesised starting with L-asparagine), followed by hydrolysis (Scheme 3).
Dihydroxyprolines are discussed in the next section.
E. N-Substituted Amino-acids. — Nα-Methyl-L-histidine can be prepared from L-histidine by a modification of the procedure of Quitt et al. (formation of the Nα-benzyl derivative by reduction of the Schiff base, Nα-methylation by means of a Leuckart reaction, and finally removal of the Nα-benzyl group by hydrogenolysis). NαNα--Dimethyl-L-histidine has been prepared for the first time by subjection of L-histidine to catalytic hydrogenation conditions in the presence of formic acid. A new synthesis of N,β-dimethylleucine (23) has made use of the N-methylation procedure of Quitt et al. The synthesis of β-methylleucine (24) from the imidazolone (25) (which is readily available) shown in Scheme 4 gave both diastereoisomers, which were separated by fractional crystallisation and N-methylated.
These two diastereoisomers of (23) were identified as N,γ-dimethylisoleucine and N,γ-dimethy1alloisoleucine by spectroscopic examination, and the latter was resolved by way of its benzyloxycarbonyl derivative with ephedrine.
Treatment of 3,4-dehydro-DL-proline (26) with alkaline permanganate gives an equimolecular mixture of 2,3-trans-3,4-cis-3,4-dihydroxy-DL–proline (27) and the 2,3-cis-isomer (28). Separation of these isomers was difficult but was achieved by fractional crystallisation of their copper salts, and the two new amino-acids were characterised. When N-substituted 3,4-dehydroprolines are treated with osmium tetroxide, however, glycolation occurs exclusively from the less hindered side: e.g. benzyloxycarbonyl-3,4-dehydroprolineamide gave a single glycol (29), which could be converted to (27) by hydrogenation and hydrolysis, also in good yield.
N-Hydroxy-α-amino-acids (30) can be synthesised by a number of routes which were summarised in 1967, but the most general route is viaN-alkylation of the anti-benzaldoxime anion with an α-bromoester followed by hydrolysis of the resulting nitrone, as shown in Scheme 5. The syn-benzaldoxime anion undergoes alkylation on oxygen. Hydrazine can also be used for cleavage of the nitrone. This general route has been used for the synthesis of Nα-hydroxy-DL-asparagine, N-hydroxy-α-14C]-DL–phenylalanine, and also some N-hydroxy-β-amino-acids.
The L, D, and DL forms of the N-amino derivative (31) of histidine have been prepared by reaction of hydrazine with the chloroacids obtained from D, L, and DL-histidine respectively by deamination in concentrated hydrochloric acid.
F. β-Amino-acids.-A large number of N-benzoyl-β-amino-acids have been prepared by means of a Ritter reaction (treatment with acetonitrile and sulphuric acid) starting with 3-hydroxypropanoic esters. Aminopivalic acid is produced in poor yield when hydroxypivalic acid is catalytically hydrogenated in aqueous ammonia at high temperature and pressure.
G. Labelled Amino-acids. — A short route to [1-14C]-DL-arginine involving a Strecker synthesis withγ-guanidinobutyraldehyde has been reported : the synthesis uses radioactive intermediates only in the last two stages.83 Alternative syntheses of [γ-14C]-DL-aspartic and [δ-14C]-DL-glutamic acids have been claimed to be more convenient and less expensive than previous methods. Synthesis of isoleucine from [14C4]-2-bromobutane by the acetamidomalonate route gave [β,γ,γ',δ-14C]-DL-isoleucine with a completely labelled side-chain, and this was obtained free of the allo-isomer by preparative paper chromatography. The methods previously described for the 'cold' amino-acids have been used for the preparation of [α-14C]-β-(1-pyrazolyl)-DL-alanine, [α-14C]-β-(3-pyrazolyl)-DL-alaninaen, and [α-14C]-β-(2-furyl)-DL-alanine. A rapid and simple enzymic method for the resolution of [ε-14C]-α-aminoadipacicid has been briefly described. Syntheses of [α-14C]- and [15N]-DL-α-allylglycianned also [α-14C]- and [15N]-DL-homomethionine have been reported. The preparation of [γ-14C]-γ-aminobutyric acid (starting with radioactive cyanide) and its conversion to [4-14C]-DL-azetidine-2-carboxylic acid have been published.
