Synthesis of propargylamines with functional side chains

4. Results and Discussion

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4. Results and Discussion

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A recent publication from 2017 by the group of Evansdescribes partial epimerization as a general feature of the Bestmann-Ohira reaction.¹²² The chiral lability of urethane protected α-amino aldehydes in aldol or Wittig reactions is further documented in the literature.¹²³ Aliphatic protecting groups, which offer a positive inductive effect, might be used as a substitute for carbamates to decrease the acidity of the alpha proton. This issue was addressed by the group of Reetz^{124, 125} with the usage of N, N-dibenzyl aminoaldehydes, which are accessible and isolable aldehydes and configurationally stable at room temperature (Scheme 32).

Scheme 32. Synthesis of dibenzyl protected α-amino aldehydes. Starting with the benzylation of free amino acids to N, N-dibenzyl amino benzylesters, which are reduced with LiAlH4 to the respective dibenzylated amino alcohols, followed by a Swern oxidation to obtain the aldehydes.

These have been shown to react without loss of stereoinformation in several electrophilic reactions with organometal species, as well as aldol- and Wittig reactions.^{124, 125} However, since the benzyl group is cleaved hydrogenolytically, it cannot be cleaved on the propargylamine stage, which would result in a reduction of the triple bond. Furthermore, it would not be orthogonal to a benzylester in solution phase peptide synthesis.

The required properties are provided with the application of the acid labile N-trityl group, which has been successfully used by Dellaria Jr. et al. to add the anion of dimethyl methylphosphonate to N-trityl-phenylalaninal, without any racemization of the C³-atom of the product (Scheme 33).¹²⁶

Scheme 33. The trityl protected phenylalaninal proved to be configurationally stable during nucleophilic attacks at basic conditions.

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In a further publication they mention the comparison of the configurational stability of highly epimerization prone Boc-phenylalaninal, with N-trityl phenylalaninal, in presence of TEA in chloroform. The optical rotation did not change after 19 h, for the trityl protected aldehyde.¹²⁷

Since the addition of a phosphonate anion represents the first reaction step in the Seyferth-Gilbert reaction, the trityl-group was used for the Bestmann-Ohira reaction as N-protecting group (Scheme 34).

Scheme 34. Synthesis of the Boc- and Fmoc-protected propargylamine (S)-33 and 34;

a) LiAlH4 (2.0 eq), THF, 0°C 2 h, reflux, overnight; b) Trt-Cl (1.0 eq), TEA (2.0 eq), DCM, rt, overnight; c) Swern‘s procedure;⁵³ d) p-ABSA (1.2 eq), DOP (1.2 eq), K2CO3 (3 eq), ACN/MeOH (5:1), rt, overnight; e) DCM/MeOH/TFA (2:1:1), rt, 30 min; f) Fmoc-OSu (1.1 eq), NaHCO3 (4 eq), THF/H2O (1:1), rt, overnight; g) i) Boc2O (2 eq), THF/H2O (1:1), K2CO3 (4 eq), rt, overnight; ii) imidazole (3 eq), rt, 30 min.

Starting from H-Phe-OH, the phenylalaninol ((S)-28) can be obtained on multiple gram scale in good yields of 88% after reduction with LiALH4 and a simple extraction. On bigger scale, if desired, the inflammable LiAlH4 can also be exchanged by the NaBH4/I2-system.¹²⁸ For the trityl protection step it is worthwhile to mention, that only the amino group reacts with Trt-Cl, when used in equimolar ration and the less nucleophilic alcohol moiety remains unprotected. The trityl protected amino alcohol (S)-29 can be obtained in sufficient crude purity for the following Swern reaction, which likewise proceeds without side products. However, caution is advised concerning the high acid lability of the trityl group.

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For the aqueous workup, a 10% citric acid solution is tolerated to remove residual free basic compounds, whereas commonly used dilute HCl or KHSO4 solutions lead to a partial deprotection. If column chromatography becomes necessary, although most of the side products and remaining reagents can be removed by aqueous workup and the quantitative reaction process can be monitored by TLC, adding 1 vol% TEA into the eluent is recommended.

The Bestmann-Ohira reaction using dimethyl diazomethylphosphonate (25) according to the original procedure¹¹⁹ works in a very clean manner. On a bigger scale, the one-pot procedure was performed successfully, where the dimethyl diazomethylphosphonate is formed in situ.¹²⁰ P-ABSA (35), which is considered a safe diazo-transfer reagent, was utilized instead of Tos-N3.¹²⁹ However, compared to Tos-N3 it reacts more slowly, generally requiring stirring overnight, compared to the stated 2 h of the original procedure. The storable p-ABSA was synthesized on multi-gram scale from inexpensive starting materials (Scheme 35).^{129, 130}

Scheme 35. Synthesis of p-ABSA (35), starting from the sulfonyl-chloride via addition-elimination mechanism with NaN3, the tert-butyl ammonium chloride serves as a phase transfer reagent.

The Bestmann-Ohira reagent 25 was obtained as a green oil after column chromatography, starting from dimethyl 2-oxopropylphosphonate and using shelf-stable p-ABSA as a diazotransfer reagent in high yields of 82% in acetonitrile, utilizing K2CO3 as a mild base by stirring overnight (Scheme 36).

Scheme 36. Synthesis of the Bestmann-Ohira reagent (25), utilizing p-ABSA and K2CO3 in acetonitrile at rt.

This method avoids the usage of sodium hydride in dry THF, which is commonly used for the deprotonation of the activated methylene moiety,^{131, 132} followed by reaction with p-ABSA. Another advantage is the convenient nature of a one-pot process.

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However, utilizing trityl protected propargylamines (S)-31 in the RuAAC resulted in lacking conversion of the starting materials, which could be recovered after the failed reaction attempts. Therefore, the trityl group was cleaved with dilute TFA and methanol as a scavenger, followed by Boc- or Fmoc-protection without prior purification of the free amine. The Fmoc or Boc protected propargylamines (S)-33 and 34 were purified by a final column chromatography and obtained in acceptable yields. Comparing the Boc-protected enantiomer via chiral HPLC with the data obtained by Fröhr,¹⁰⁴ we found it to be enantiomerically pure.

The synthesis of the ^D-Phenylalanine analogue propargylamine (R)-33 was performed with the same method (Scheme 37).

Scheme 37. Synthesis of the Fmoc-D-Phe≡ ((R)-33); a) LiAlH4 (2.0 eq), THF, 0°C 2 h, reflux, overnight; b) Trt-Cl (1.0 eq), TEA (2.0 eq), DCM, RT, overnight; c) Swern‘s procedure;⁵³ d) p-ABSA (1.2 eq), DOP (1.2 eq), K2CO3 (3 eq), ACN/MeOH (5:1), rt, overnight; e) DCM/

/TFA/TES (95:2.5:2.5), rt, 30 min; f) Fmoc-OSu (1.2 eq), NaHCO3 (4 eq), ACN/H2O (1:1), overnight.

In contrast to the L-configured propargylamine, the column chromatography of the Trt protected propargylamine (R)-33 was skipped, which resulted in an overall yield of 32%

over four steps after Fmoc protection and only a single column chromatography in total (averaging 77% per step starting from ^D-phenylalanine).

Additionally, the possibility to apply this route for amino acids with functional side chains was investigated. Regarding solid phase peptide synthesis, a general method would be desirable, which converts commercially available Fmoc protected amino acids (bearing orthogonal sidechain protecting groups) into Fmoc protected propargylamines.

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This was addressed utilizing trityl protected aminoaldehydes for Fmoc-Lys(Boc)-OH (Scheme 38).

Scheme 38. a) Carboxylic acid reduction.¹³³ CDI (1.3 eq), THF, rt, 10 min, NaBH4 (1.7 eq), THF/H2O, 0°C->rt, 30 min; b) Fmoc cleavage.¹³⁴ 1-octanethiol (10 eq), DBU (3 mol%), THF, rt, 4 h; c) Trt-Cl (1.0 eq), TEA (3.0 eq); d) Swern oxidation⁵³;e) p-ABSA (1.2 eq), DOP (1.2 eq), K2CO3 (3.0 eq), ACN/MeOH (5:1), rt, overnight; f) DCM/TFA/MeOH (92.5:2.5:5); g) Fmoc-OSu (1.2 eq), NaHCO3 (4.0 eq), H2O/THF (1:1), rt, overnight.

Due to the N-terminal Fmoc group, or ester protecting groups for several sidechain functionalities, a reduction of the weinrebamide with DIBAL-H, as described by Dickson et al., is not feasible. Cleavage of the Fmoc-group followed by reduction of the carboxylic acid with LiAlH4 under reflux condition, as in the case of the phenylalanine, would also lead to cleavage of most sidechain protecting groups. A convenient protocol of Hwang et al.¹³³ which converts the carboxylic acid with CDI into an activated acylimidazole and subsequently reduce it under mild conditions with NaBH4 into the Fmoc- protected aminoalcohol 36 was used. Alternatively, an activation with DCC and N-hydroxysuccinimide, followed by NaBH4 reduction would be feasible, as demonstrated by Jadhav et al.¹³⁵

In terms of product isolation in solution phase the cleavage of the Fmoc group is not as straightforward as for its solid phase pendant, where the free amine is immobilized on solid phase and the dibenzofulvene-adduct can be simply washed off. A procedure from Sheppeck II et al.¹³⁴ which cleaves the Fmoc group with catalytic amounts of DBU and uses thiols to trap the dibenzofulvene, followed by removing the dibenzofulvene-thiol adduct by trituration with petroleum ether, was used. The free amine remains as a colourless oil in sufficient crude purity.

The subsequent trityl protection is performed utilizing equimolar ratios of Trt-Cl, leaving the remaining hydroxyl group unprotected 37. The propargylamine 38 is formed by a Bestmann-Ohira reaction of the trityl protected aminoaldehyde, which is obtained by a previous Swern oxidation.

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Overall the Fmoc-protected propargylamine 39 was obtained in 13% yield, starting from Fmoc-Lys(Boc)-OH over seven steps (averaging 75% per step), utilizing just two column chromatographies. A possible improvement might be the application of polymer bound thiols, which would improve the execution of the Fmoc deprotection, by avoiding the trituration step and possibly, lead to improved yields at this step.

A further development and alternative to the trityl group is the phenyl fluorenyl (Pf) group, introduced in the 1980s.¹³⁶ The Pf group resembles the trityl group but is up to 6000 times more solvolytically stable towards acids and has been proven to effectively prevent racemization of α-amino acid derivatives in enantiospecific synthesis. However, stochiometric amounts of lead are required for the introduction of the Pf group (Scheme 39).

Scheme 39. The Pf group can be introduced to amino acid esters and silylesters. The lead forms a salt with the bromide to enhance its leaving group capability.

The Pf group is commonly cleaved hydrogenolytically or under strong acidic conditions.

Depending on the situation, it could be used with benzyl or allyl type sidechain protecting groups.

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