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Alternatywne mediatory reakcji Mitsunobu

100%

Falkiewicz B. , Wiśniewski K. , Kołodziejczyk A. S.

Wiadomości Chemiczne

2001

tom [Z] 55, 9-10

849--858

The Mitsunobu reaction is a versatile method for the alkylation of various Bronsted-Lowry acids (HA) by alcohols, proceeding in neutral media in a presence of the redox system which traditionally consists of diethyl azodicarboxylate and triphenylphosphine. The key step of the process proceeds according to the SN2 mechanism and results in one of the most useful attributes of the reaction, namely complete configurational inversion at the carbinol carbon. The reaction, however, has a serious limitation - the acidic component has to have pKa smaller than 13 for the reaction to proceed smoothly. Moreover, the classical methodology of the reaction suffers from low yields when applied to secondary alcohols. In recent years, in order to overcome these drawbacks and expand the versatility of the original combination of the Mitsunobu mediators, significant progress in the reaction methodology has been made, mainly due to the work of Tsunoda and Itô. Two types of new mediators have been developed to replace the azodicarboxylate-TPP system. The first one is an N,N,N',N'-tetrasubstituted azodicarboxamide - tributyl phosphine system. All azodicarboxamide derivatives were found to be more efficient than traditional DEAD in the Mitsunobu reaction, especially for less acidic HX. N,N,N',N'-Tetramethylazodicarboxamide, TMAD, gives the best overall results among acyclic amides, whereas 4,7-dimethyl-3,5,7-hexahydro-1,2,4,7-tetraazocin-3,8-dion, DHTD, in combination with TBP was found to be unique in mediating the formation of the C-C bond with sec-alcohols at room temperature. The other type of new mediators in the Mitsunobu reaction, structurally based on betaine, is cyanomethylenetrialkylphosphorane. The phosphorane reagents are generally less active at room temperature, but in higher temperatures they are, esp. CMMP, better than DHTD-TBP and afford satisfactory alkylation of secondary alcohols. Furthermore, the phosphorane reagents mediate the reaction of acids of pKa up to 23.5 [33]. Comparative studies of the C-alkylation revealed the general reactivity of new mediators as TMAD-TBP ( DHTD-TBP ( CMBP ( CMMP.

Zastosowania reakcji Mitsunobu w chemii aminokwasów

75%

Wiśniewski K. , Kołodziejczyk A. S. , Falkiewicz B.

tom [Z] 52, 3-4

243--267

The Mitsunobu reaction has been knowm since the late sixties. It is mediated by the redox system : triaryl - or trialkylphosphine/dialkyl azodicarboxylate and brings about the nucleophilic substitution of an alcoholic hydroxyl group by the conjugate base of an acidic reactant, with inversion of configuration at the alkohol carbon. The Mitsunobu reaction is widely used in organic chemistry and its mechanism (Scheme 1) has been intensively studied (for review - see [2-5]). This article deals with the application of the reaction in the chemistry of amino acids. The reaction was proposed as an effective method of a-amino acid synthesis using hydroxy acids as substrates. As the amino group synthons phtalimide [10, 11], (Scheme 2), hydrazoic acid [14], (Scheme 5) or t-butyl-2(trimethylsilil)ethylsulphonylcarbamate [18], (Scheme 7) were used. The procedure using HN3 was profoundly improved by the introduction of a stable bis-pyridine complex of zinc oxide [16]. The use of phtalimide as an amino group precursor in Mitsunobu-type reaction was successfully applied to the synthesis of 2-2H-labelled chiral glycine [13], (Scheme 4). In the model studies on the synthesis of 15N-labelled N-protected chiral amino acids Degerbeck et al. [17] found that the yield of the Mitsunobu conversion (Scheme 6) depends on the acidity of the NH function in the imidocarbonate or sulphonylcarbamate used. The Mitsunobu reaction has also been applied to the synthesis of many unnatural or modified amino acids such as protected 2,3-diamino butyric acid [19], 3- or 4- mercaptoproline derivatives [20, 21], (Scheme 8), N5-acetyl-N5-hydroxy-L-ornitine [22], (Scheme 9) and a-N-hydroxyamino acids [23], (Scheme 10). Wojciechowska et aal[24] have reported the preparation of dehydroamino acids from protected serine and threonine derivatives under the intramolecular Mitsunobu dehydration condition (Scheme 11). A general approach to the preparation of N-monoalkylated amino acids based on the Mitsunobu reaction has been developed [3-] using N-tosylamino acid esters as acidic components of the reaction (Scheme 15). Since the removal of tosyl group is difficult, a modification of the N-alkylation procedure has recently been devised [32, 33]. The Mitsunobu reaction is also an excellent procedure for transforming hydroxy acids or hydroxy amino acids into esters whose subsequent hydrolysis leads to a stereoisomer of the initial compound with the inverted configuration at the carbinol centre and was very often used in this way [37, 38], (Schemes 8 and 17). The Mitsunobu reaction provides also an interesting method of esterification in which an alcohol, not a carboxylic component, is activated. It was used to the synthesis of diphenylmethyl esters of N-trityl amino acids [43], to the attachment of a first amino acid to the polymer support [44] or to active ester synthesis [45], (Scheme 18). The applications of the Mitsunobu reaction include also the preparation of amny cyclic derivatives of amino acids such as B-lactams [51], (Scheme 20), aziridines [49, 54], (Schemes 19, 21) or B-lactons [60]. The last cyclic derivatives are valuable intermediates for the synthesis of B-substituted alanines (Scheme 22). B-Lactonization proceeds easily in case of serine derivatives whereas in threonine derivatives B-elimination is the dominant reaction [61], (Scheme 23). The review deals also with the application of the title reaction to the synthesis of peptide (polyamide) nucleic acids (PNA) [31, 39, 40, 68-70].