1,3-Dimethoxybenzene: Applications in Synthesis, Enantioenrichment, and Peptide Chemistry

1,3-Dimethoxybenzene is widely used in chemistry and is an important initiator in the synthesis of polymers and resins with specific properties. It is also used as a solvent and extractant in a number of organic syntheses and chromatographic analyses. 1,3-Dimethoxybenzene can be produced by reacting phenol with methanol. The reaction of phenol with methanol in the presence of acidic conditions or catalyzed by an acidic catalyst produces p-methylphenol, which is then methylated to produce 1,3-Dimethoxybenzene.

Enantioenrichment of a tungsten dearomatization agent utilizing chiral acids

A method is described for the resolution of the versatile dearomatization reagent TpW(NO)(PMe3)(η2-benzene), in which the 1,3-dimethoxybenzene (DMB) analogue of this complex is synthesized. In turn, the coordinated arene of TpW(NO)(PMe3)( 1,3-dimethoxybenzene) is protonated with either d or l dibenzoyl tartaric acid (DBTH2) in a butanone/water or 2-pentanone/water solution. Sustained stirring of this mixture results in the selective precipitation of a single form of the diastereomeric salt [TpW(NO)(PMe3)(DMBH)](DBTH). After isolation, the salt can be redissolved, and the 1,3-dimethoxybenzene ligand can be deprotonated and exchanged for benzene to produce the desired product TpW(NO)(PMe3)(η2-benzene) in either its R or S form. The absolute configuration of the tungsten stereocenter in TpW(NO)(PMe3)(η2-benzene) can be determined in either case by substituting the naturally occurring terpene (S)-β-pinene for benzene and evaluating the 2D NMR spectrum of the corresponding β-pinene complex. To generate the benzene complex 1, the salt 3A was stirred with triethylamine in 1:1 THF:benzene solution. A white precipitate (believed to be triethylammonium dibenzoyltartrate) was removed by flushing the heterogeneous solution through activated basic alumina, followed by a THF rinse. The resulting golden filtrate was then stirred for 16 h to allow for the conversion of enriched 1,3-dimethoxybenzene complex (R)-2 to the benzene complex (R)-1. The final product, obtained by concentrating the filtrate and inducing precipitation with hexanes, was the enantioenriched benzene complex (R)-1 (er = >92:8;). Here we note that the degree of resolution was retained between 3A and (R)-1.[1]

C-terminal N-alkylated peptide amides resulting from the linker decomposition of the Rink amide resin

Decomposition of the resin linkers during TFA cleavage of the peptides in the Fmoc strategy leads to alkylation of sensitive amino acids. The C-terminal amide alkylation, reported for the first time, is shown to be a major problem in peptide amides synthesized on the Rink amide resin. This side reaction occurs as a result of the Rink amide linker decomposition under TFA treatment of the peptide resin. The use of 1,3-dimethoxybenzene in a cleavage cocktail prevents almost quantitatively formation of C-terminal N -alkylated peptide amides. Oxidized by-product in the tested Cys- and Met-containing peptides were not observed, even if thiols were not used in the cleavage mixture. In this work, we report on the identification of the C-terminal N -alkylated amide by-product and the use of 1,3-dimethoxybenzene (DMB) as a component of the scavenger cocktail consisting of 92.5% TFA:2.5% TIS: 5% 1,3-dimethoxybenzene (Reagent I) suitable for preventing its formation. It is demonstrated that formation of the C-terminal N -alkylated amide by-product result exclusively from decomposition of the Rink amide resin linker. Reagent I suppresses almost completely alkylation of the C-terminal amide group by preventing the linker decomposition during peptide cleavage from the resin. Numerous model peptides or peptides synthesized for other purposes were studied under various cleavage conditions. The optimal mixture of scavengers was tested in the presence of various side-chain protecting groups in order to estimate the limits of its application.[2]

To define the optimal composition of the cleavage mixture, we varied the percentage of 1,3-dimethoxybenzene from 2.5 to10% while keeping unchanged at 2.5% the percentage of TIS. The cleavage mixtures consisting of 95% TFA:5% 1,3-dimethoxybenzene (v/v) and 92.5% TFA: 7.5% 1,3-dimethoxybenzene (v/v)were also tested. As is shown in Figures 4 and 5,and in Table 2, the cleavage mixture 92.5% TFA:2.5% TIS: 5% 1,3-dimethoxybenzene suppresses almost quantitatively the formation of C-terminal alkylated amide peptides. Although very good results can be obtained with the cleavage mixture 92.5% TFA: 7.5% 1,3-dimethoxybenzene (v/v) , it is not recommended for Cys-containing peptides. If the Cys residue is present in the peptide sequence, then the presence of TIS in the cleavage mixture (Reagent I) is required, since, otherwise, the incomplete deprotection of the sulfydryl group becomes a major problem. Incomplete deprotection of the-Asn(Trt)- residue was also observed (<5%), but only in the case when it occupies the N-terminal position in the sequence.

In this work, we have demonstrated that decomposition of the Rink amide resin linker during the TFA cleavage step leads to the formation of C-terminal N-alkylated amide by-products. This sequence-independent side reaction can be suppressed by using as cleavage mixture Reagent I. 1,3-dimethoxybenzene was estimated to contribute to this result by preventing the formation of carbocations that originated from the inappropriate Rink amide linker decomposition. The use of thiols was not required in the cleavage mixture of the Cys- and Met-containing peptides since no oxidation reactions were observed in the tested peptides.

References

[1]Lankenau, Andrew W et al. “Enantioenrichment of a tungsten dearomatization agent utilizing chiral acids.” Journal of the American Chemical Society vol. 137,10 (2015): 3649-55. doi:10.1021/jacs.5b00490

[2]Stathopoulos, Panagiotis et al. “C-terminal N-alkylated peptide amides resulting from the linker decomposition of the Rink amide resin: a new cleavage mixture prevents their formation.” Journal of peptide science : an official publication of the European Peptide Society vol. 12,3 (2006): 227-32. doi:10.1002/psc.706