Claims
- 1. A compound of the formula II ##EQU4## in which R.sub.1 is lower alkyl, R.sub.2 is lower alkyl or phenyl and R.sub.3 is phenyl and wherein phenyl is unsubstituted or substituted by lower alkyl, phenyl or lower alkylphenyl and X stands for phenyloxy or p-nitrophenyloxy.
- 2. A compound of claim 1, wherein R.sub.1 and R.sub.2 each represents a methyl or ethyl group, R.sub.3 has the same meaning as in claim 1, and X is phenyloxy.
- 3. A compound of claim 1, wherein R.sub.3 represents a phenyl, tolyl or biphenylyl group, and R.sub.2 stands for lower alkyl, and X is phenyloxy.
- 4. A compound of claim 1, wherein R.sub.1 stands for methyl or ethyl, R.sub.2 and R.sub.3 each stands for phenyl, and X is phenyloxy.
- 5. A compound of claim 1, wherein the ##EQU5## group is the 2-phenyl-isopropyloxycarbonyl group or the 1,1-diphenylethyloxycarbonyl group.
- 6. A compound of claim 1, wherein the ##EQU6## group is the 2-para-tolylisopropyloxycarbonyl group, and X is phenyloxy.
- 7. A compound of claim 1, wherein the ##EQU7## group is the 2-(para-biphenylyl)-isopropyloxycarbonyl group, and X is phenyloxy.
Priority Claims (3)
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1073/67 |
Jan 1967 |
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12038/67 |
Aug 1967 |
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17108/67 |
Dec 1967 |
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Parent Case Info
The present invention provides a new process for the temporary protection of amino groups in peptide syntheses by acylating the amino group and then eliminating the acyl group introduced.
As is known it is in general necessary when synthesizing peptides by coupling aminoacids or peptides to block any functional groups not participating in the coupling reaction, for example the terminal amino group or the terminal carboxyl group, the amino and carboxyl groups of the sidechains and possibly further groups such as mercapto, guanidino or hydroxyl groups. While at this stage a considerable choice of protective groups are available, they are still not quite satisfactory, especially so when long-chain and delicate peptides are to be synthesized. The problem involved is above all that in such cases hydrogenolytic elimination is out of the question (because of sulfurous aminoacids present) and the peptides are not sufficiently stable towards hydrolytically acting acid or especially alkaline agents so that the hydrolytic elimination of all protective groups in the last stage of the synthesis entails considerable losses of costly material.
It is another difficulty that the protective group located at the coupling end must be eliminated again after every coupling reaction, whereas the other protective groups should be retained until the synthesis is complete. These two types of protective groups must therefore, on one hand, be selectively eliminable with respect to each other and, on the other hand, both types must be eliminable under mild conditions.
In the peptide field, there exists therefore a need for protective groups which
No ideal protective groups for all kinds of peptides are available. Depending on the peptide to the synthesized, it is necessary to use a different combination of protective groups.
As side-chain protective groups eliminable under mild acidic conditions the tertiary butyloxycarbonyl group (BOC) has proved particularly suitable for protecting the amino groups and the tertiary butyl ester group (tBu) for protecting the carboxyl groups, and in addition the tertiary butyl ether group is available for protecting hydroxyl groups. Apart from these groups the trityl group for example when used an an .alpha.-amino protective group, can be selectively eliminated, since in a weakly acidic medium it is much faster eliminable, for example in 80% acetic acid at room temperature 20,000 times faster. Unfortunately, however, the suitability of the trityl group as amino protective grouping is very limited. Owing to a steric hindrance it cannot be used in coupling according to the method of the activated esters and of the mixed anhydrides (except in the case of glycine) and even in the carbodiimide method it gives poor yields. Therefore, it is unsuitable for use in the synthesis of peptides starting from the carboxyl end, for example by the new method of the solid phase synthesis (cf. Merrifield, J.AM.CHEM.SOC. 85, 2149 [1963]). The trityl group has also other disadvantages because it is difficult to introduce and compounds protected by the trityl group are not very stable.
The present invention is based on the observation that protective groups of the formula ##EQU2## where R.sub.1 is lower alkyl, R.sub.2 lower alkyl or phenyl and R.sub.3 represents phenyl - can be used with advantage in the synthesis of long-chain, delicate peptides, in fact with better results than known protective groups, for example the trityl group. The phenyl radicals in the above formula represent an unsubstituted phenyl ring or a phenyl ring substituted by one or two lower alkyl, phenyl or lower alkylphenyl groups. The substituents are above all in para-position, though they may also be in ortho-position or in the ortho- and parapositions. The lower alkyls contain at most 5 carbon atoms and are in the first place methyl or ethyl or, for example, propyl or butyl radicals. The lower alkyl groups are linear, though - especially in the case of the phenyl substituents - they may also be branched.
The new groups are distinguished by the fact that they are eliminable under very mild acidic conditions, for example at room temperature in about 60 to 90% aqueous acetic, chloracetic or formic acid or in a mixture of at least two of these acids and 10 to 40 % of water. Since the speed at which they are eliminated is at least 600 times greater than that of the BOC group, they can be eliminated selectively with respect to this group and also to the tertiary butyl ester group, the tertiary butyl ether group and the trityl-mercapto protective group. They are therefore particularly suitable as .alpha.-amino protective groups in the synthesis of delicate peptides containing acid-labile sidechain protective groups, such as the groups just mentioned. Since they display no signs of a steric hindrance, they can be used in any desired coupling method and especially also in the solid phase synthesis.
An object of the invention is therefore a process for the temporary protection of amino groups in peptide syntheses by acylating the amino group to be protected before carrying out the coupling step(s) by a protective group of the formula I ##EQU3## in which R.sub.1 represents lower alkyl, R.sub.2 lower alkyl or phenyl, and R.sub.3 stands for phenyl, and wherein the phenyl radicals are unsubstituted or substituted by lower alkyl, phenyl or lower alkylphenyl groups and splitting off the protective group after the coupling step(s) has (have) been carried out.
An object of the invention are aminoacids and peptides and derivatives thereof which are protected by a group of the formula I as defined above. A further object of the invention are means for introducing the new acyl groups.
As mentioned above, the new protective groups are especially useful for protecting aminoacids or peptides and derivatives thereof used in peptide syntheses. By peptides are to be understood in the first line those peptides which occur in nature and are, for instance, described in the text book "The Peptides" by Schroder and Lubke, Academic Press, New York and London, Volumes I and II, 1965-66, and also synthetic analogues of such peptides which differ from them by having one or more aminoacids exchanged by other aminoacids, those exchange aminoacids being known in the peptide field. By peptides are further to be understood partial sequences of the natural or synthetic peptides just mentioned. As aminoacids occurring in the peptides as building elements or structural units there may be mentioned the 20 code aminoacids, cf. for instance Sci. American October 1960, pg. 55, and homologues, structural isomers and optical isomers thereof, for instance amino - lower alkanoic acids with at most 7 carbon atoms other than those occurring as code aminoacids, for instance .alpha.-amino butyric acid, norvaline, norleucine, .beta.-alanine, .gamma.-aminobutyric acid, .alpha.,.beta.-diaminopropionic acid, further, for instance, hydroxyproline, normethionine, phenylglycine, ornithine, citrulline, N-methyl-tyrosine and other N-lower alkyl amino-acids, further racemic and D-aminoacids.
Derivatives of peptides are especially those which have substantially the same activity as the peptides themselves, for instance C-terminal amides, especially N-unsubstituted amides, but also N-mono- or N-disubstituted amides, such as N-monoalkyl or N-dialkyl amides having up to 18, especially 1 to 5 carbon atoms in the alkyl group, or C-terminal esters, for instance alkyl esters having in the alkyl group up to 18, especially 1 to 5 carbon atoms. Other derivatives are those which are generally used as intermediates in the synthesis of peptides, for instance activated esters, hydrazides azides, mixted anhydrides and peptides or derivatives in which, besides the amino group protected according to the invention, one or more functional groups selected from amino, carboxyl, hydroxy, mercapto, and guanidino groups are protected in known manner by protective groups.
Such derivatives are also described in the above-mentioned text-book of Schroder and Lubke or in publications of Wieland et al. "Peptidsynthesen", I to V, Angew. Chem. 63 (1951) 7-14; l.c. 66 (1954), 507-512; l.c. 69 (1957), 362-372; l.c. 71 (1959), 417-425, l.c. 75 (1963), 539-551. Derivatives especially to be mentioned are lower alkyl esters for instance methyl ester, ethyl ester, tertiary butyl ester, tertiary amyl ester, benzyl ester, p-nitrobenzyl ester, p-methoxybenzyl ester, 2,2,2-trichloroethyl ester, 2-iodoethyl ester, p-nitrophenyl ester, 2,4-dinitrophenyl ester, 2,4,6- or 2,4,5-trichlorophenyl ester, 2,3,4,5,6-pentachlorphenyl ester, N-hydroxy- succinimide ester, and other activated esters as mentioned for instance in U.S. Pat. No. 3,035,041; further C-terminal hydrazides and azides, further mixed anhydrides, for instance anhydrides with carbonic acid lower alkyl ester such as carbonic acid methyl ester or anhydrides with lower alkanoic acids which may be halogen-substituted, for instance with formic acid, pivaloic acid, trichloracetic acid; further derivatives of aminoacids or peptides in which one or more functional groups such as amino, carboxyl, hydroxy, mercapto and/or guanidino groups are protected.
It should be noted that the protective groups of the invention can be used in any method of peptide synthesis, and that they are also especially useful in the solid phase method. They can be used in connection with any protective groups, but it is of particular advantage to use them in connection with protective groups which can be split off by means of acids, for instance the BOC group, the tert. butylester group and the tert. butyl ether group and analogous groups, because the new groups have an excellent selectivity which respect to these known groups, which are preferably used in the synthesis of sensitive peptides.
The new protective groups are introduced in known manner, similar to the BOC group, for example by way of the azide method or the method of the activated esters (for example phenyl esters, p-nitrophenyl ester, hydroxysuccinimide ester) or by reaction of carbinols of the formula R.sub.1 R.sub.2 R.sub.3 C--OH with the isocyanic acid esters corresponding to the aminoacids.
The following Examples illustrate the introduction and elimination of the new group in the case of some natural .alpha.-aminoacids and of peptides synthesized from natural .alpha.-aminoacids. In the same manner the groups are used with other amino-acids and peptides.
US Referenced Citations (4)
Divisions (1)
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239721 |
Mar 1972 |
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Continuation in Parts (1)
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698118 |
Jan 1968 |
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