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I am a biologist, and organic chemistry lessons are a distant memory now. I am interested in synthesizing a small amount of octopine. Octopine is a natural molecule resulting from an enzyme-catalyzed reaction between pyruvate and arginine, as shown below:

Octopine biosynthesis

Sato et al. [1] synthesize octopine as follows:

Octopine was prepared by synthesis from ʟ-arginine and ʟ-α-bromopropionic acid according to the method of Abderhalden. Purification of octopine was done by ion-exchange chromatography using Dowex 50W-x8 (H+ type) and recrystallization from aqueous ethanol. The purity (>99%) of the synthesized octopine was checked by high performance liquid chromatography.

I could not find the original Abderhalden paper (E. Abderhalden and R. Haase, Z. Physiol. Chem., 202, 49-55 1931). From what I understand based on other papers mentioning this name, the "method of Abderhalden" is simply a nucleophilic substitution between the α-amine and the halogenoalkane.

However, my understanding is that in this case it should lead to two issues:

  1. Multiple reactions of the alkyl halide on the target α-amine, leading to e.g. tertiary amine products.
  2. Reactions between the alkyl halide and the guanidino group of the amino-acid.

Are there reasons why these would not be a problem? Should one use special reaction conditions to obtain the correct product? Are there alternative easy ways to synthesize this molecule, other than purifying the enzyme and using the biosynthetic reaction?

Reference

  1. Sato, M.; Nakano, T.; Takeuchi, M.; Kanno, N.; Nagahisa, E.; Sato, Y.; Kobatake, Y. Effects of Octopine on the Serum Cholesterol Level in Rats. Biosci. Biotechnol. Bioch. 1996, 60 (1), 154–156. DOI: 10.1271/bbb.60.154.
  2. Abderhalden E.; Hase E. Gewinnung von Iminodicarbonsäuren aus Aminosäuren und Halogenfettsäuren. Z. Physiol. Chem., 1931, 202 (1), 49-55. DOI:10.1515/bchm2.1931.202.1-3.49
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    $\begingroup$ Both issues are realistic concerns. I can only presume that the purification steps referred to removed the side products which are likely to form. $\endgroup$
    – Waylander
    Commented Jun 28, 2022 at 11:55
  • $\begingroup$ I have a vague recollection that sodium cyanide borohydride will selectively reduce imines over ketones, as long as the imine is protonated. You might be able to use that to do a biomimetic synthesis $\endgroup$
    – Andrew
    Commented Jun 28, 2022 at 16:33
  • $\begingroup$ @Andrew you are correct, though it will give diastereomers. You probably don't want to do it with pyruvic acid as it may be strong enough to liberate HCN from cyanoborohydride $\endgroup$
    – Waylander
    Commented Jun 28, 2022 at 17:03

1 Answer 1

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Arginine methyl ester with a nitro group on the guaninidine is commercially available. A reductive amination with the commercially available methyl pyruvate (ethanol or methanol with sodium cyanoborohydride or sodium triacetoxyborohydride at room temperature) will give you the dimethyl ester of nitrooctopine. This should be a good reaction and it is easy to purify by chromatography at this point. Mild basic hydrolysis of the esters followed by hydrogenation over Pd/C to remove the nitro group will give octopine.

edit: As noted by @user55119 this will give a mixture of diastereomers.

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    $\begingroup$ The reductive amination is likely to give diastereomers, one of which is the undesired epimer of the dimethyl ester of nitrooctopine. This could be a serious problem if the epi-diastereomer dominates. How confident are you about the hydrogenation? $\endgroup$
    – user55119
    Commented Jun 28, 2022 at 15:23
  • $\begingroup$ @user55119 Hydrogenation of the nitro to deprotect the guanidine of arginine is the first method mentioned in Greene's "Protection Groups in Organic Synthesis", see J. Org. Chem. vol 44 (1979) 3442. You are correct about the possible diasteromers, $\endgroup$
    – Waylander
    Commented Jun 28, 2022 at 15:52
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    $\begingroup$ I have several questions: (1) There are 2 C=O's on (methyl)pyruvate; is there a reason to expect only one of them to react? (2) What is the purpose of the esters? Solubility in the solvents used for the reaction, maybe? (3) Wouldn't the hydrogenation of the nitro group give an aminoguanidine instead of regenerating the desired guanidine? (4) Wouldn't the secondary amine is the guanidine (the guanidine nitrogen closest to the carboxyl) also react and create secondary products? $\endgroup$
    – Mowgli
    Commented Jun 28, 2022 at 15:58
  • $\begingroup$ 1) One of the C=O's is in the ester group, its presence activates the ketone C=O to attack by the -NH2 2) The reductive amination can be done on pyruvic acid but the conditions are more complex - this is an easy reaction with the esters. 3) The hydrogenation cleaves the N-N bond (see the ref. in my comment above). 4) The presence of the nitro group withdraws electron density from the whole guanidine group making it less nucleophilic. $\endgroup$
    – Waylander
    Commented Jun 28, 2022 at 16:09

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