Isothiocyanate, reaction with amino acid

Tible 1 Color reactions of amino acids with fluorescein isothiocyanate — ninhydrin (extract from original table). 

Metal chelating amino acid derivatives of cellulose were recently obtained via modification of cellulose with 2,4-toluenediisocyanate, followed by treatment with amino acid ester derivatives [58,59]. Diisocyanates are able to crosslink cellulose chains and/or to yield reactive cellulose isocyanate, depending on the reaction conditions. Sato and his coworkers [60] examined the optimum conditions for the reaction between cellulose and 2,4-toluenediisocyanate and succeeded in introducing 0.30 mol of free isocyanate group per glucose unit. Cellulose isocyanate was further converted into isothiocyanate [61]. This derivative has also been synthesized by condensation of cellulose with 2,4-diisocyanototoluene, followed by hydrolysis and thiophosgene treatment [61]. 

The typical example of one-stage derivatization of amino acids is their treatment by isopropyl bromide in presence LiH with the formation of N-isopropy-lated isopropyl esters. Unfortunately, this reaction can take place only in high-boiling aprotic bipolar solvents like dimethyl sulfoxide (DMSO, = 189°C, RInonpoiar 90 18) tWs is a significant restriction for its application, in practice. A more important method is based on the reaction of amino acids with methyl or phenyl isothiocyanates with the formation of 3-methyl (phenyl) hydantoins ... 

As for the reaction of isothiocyanates with amino acids, the Edman degradation of proteins and/or peptides is well known [76]. 

Methyl isothiocyanate also reacts with sulfhydryl groups, but this complication can be turned into an advantage when the reaction is carried out at slightly acidic pH, under which conditions the reaction with amino groups is slow. The S-methylthiocarbamoyl derivatives (75) exhibit a weak uv band —(NH—CH—CO)—... 

The approach to nucleoside synthesis developed by Ogura et al. (Heterocycles, 911, 8, 125, and earlier references), which involved the reaction of glycosyl isothiocyanates with substituted hydrazines to give thiosemicarbazides which were cyclized with NBS, and finally desulphurized to give the required heterocycle, has now been applied to the synthesis of pyrazolopyrimidine nucleosides derived from D-glucopyranosyl, D-arabinopyranosyl, and d-ribofuranosyl isothiocyanates. Syntheses of nucleoside analogues from reactions of glycosyl isothiocyanates with amino-acids, enamines, and diamines have been summarized. ... 

Substitution of alkaline cyanates by isocyanates allows the preparation of 3-substituted hydantoias, both from amino acids (64) and amino nitriles (65). The related reaction between a-amino acids and phenyl isothiocyanate to yield 5-substituted 3-phenyl-2-thiohydantoiQS has been used for the analytical characterization of amino acids, and is the basis of the Edman method for the sequential degradation of peptides with concomitant identification of the /V-terminal amino acid. 

As shown by Oliveri-Mandalli,and by Lieber et al., 5-sub-stituted-amino-l,2,3,4-thiatriazoles are also formed by reaction of isothiocyanates with hydrazoic acid. When sodium azide is used instead of the free acid the isomeric mercaptotetrazoles are formed. 

Another main subject of research of Garcia Gonzalez, with the collaboration of J. Femandez-Bolanos, was the reaction of amino sugars with isothiocyanic acid derivatives. 2-Amino-2-deoxy-D-glucose hydrochloride and potassium thiocyanate were shown to give rise to 4-(D-arabino-tetritol-l-... 

A-Thiazolyl a-amino acids 56 have been prepared. The preferred route to these compounds would utilise the Hantzsch synthesis, however in this case the in situ formation of the required thiourea derivatives of a-aminoacids 52 failed. A variety of isothiocyanate reagents were tried, with the result being either no reaction, decomposition or the corresponding thiohydantoin 53. A modified version of the Hantzsch synthesis was developed. If the bromoketone 54 is initially treated with sodium thiocyanate an a-thiocyanatoketone 55 is formed, subsequent addition of the amino acid ester 51 yields A-thiazolyl a-amino acids 56 <00T3161>. 

Substituted derivatives of l-(tetrahydrobenzo[b]thiophen-2-yl-3-carboxylate)-5-phenyl-6-thio-l,2,4-triazin-4-one have been synthesized by heterocyclization reactions of different hydrazones obtained from 2-amino-tetrahydrobenzo[b]thiophene-3-carboxylate with phenyl isothiocyanate <00PS275>. Reaction of 5-methyl isothiosemicarbazide with a-amino acid vicinal tricarbonyl reactive substrates 1 and 2 yields 1,2,4-triazine substituted a-amino acids, as an equimolar mixture of regioisomers 3a/3b and 4a /4b, respectively <00JCS(P1)299>. 

In 1950 an alternative to the Sanger procedure for identifying N-terminal amino acids was reported by Edman—reaction with phenyl-isothiocyanate to give a phenylthiocarbamide labeled peptide. When this was heated in anhydrous HC1 in nitromethane, phenylthiohy-dantoin was split off, releasing the free a-NH2 group of the amino acid in position 2 in the sequence. While initially the FDNB method was probably the more popular, the quantitative precision which could be obtained by the Edman degradation has been successfully adapted to the automatic analysis of peptides in sequenators. 

Scheme 14 shows a typical example in a series of reactions in which a supported amino acid reacted with fluorenylmethoxycarbonyl isothiocyanate to give a supported (on Rink s amide)35 thiourea.36 Removal of the protection followed by 5-alkylation gave supported isothioureas. Reaction of these with amines, then cleavage from the resin, afforded substituted guanidines. For 10 examples the purities were between 40 and 92%. An aryl group separates the resin from the guanidine, just as in the sequences shown in Schemes 11 and 12. 

A powerful method of sequencing a peptide from the TV-terminal end is the Edman degradation in which phenyl isothiocyanate, C6HsN=C=S, reacts selectively with the terminal amino acid under mildly basic conditions. If the reaction mixture is then acidified, the terminal amino acid is cleaved from the peptide as a cyclic thiohydantoin, 8 ... 

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