Pyroglutamates, synthesis

Pyroglutamic acid is a useful starting material for the synthesis of several natural products, such as pyrrolidine alkaloids, kainoids, and other unnatural amino acids. Interesting chemose-lective Michael additions of anions derived from pyroglutamates have been reported (see Eqs. 4.54 and 4.55).69... 

In the course of their investigation of the enantioselective total synthesis of avarainvillamide and the stephacidins, the Baran group has developed a method for the dehydrogenation of tryptophan as well as a one-step tryptophan synthesis from pyroglutamate and substituted aniline precursors <06JACS8678>. 

A diastereoselective synthesis of 3,5-disubstituted indolizidines based on an intermol. addition of an allylsilane on an acyliminium ion derived from (d )-pyroglutamic acid, is described <2006TA1253>. 

HE Klieger. On peptide synthesis I. Synthesis of glutamic acid peptides using car-bobenzoxy-L-pyroglutamic acid. Justus Liebig s Ann Chem 640, 145, 1961. 

In connection with the enantioselective alkylation of Pro or 4-hydroxy-proline, the azabicyclo[3.3.0]octane system 81 was obtained after reaction with pivaldehyde (81HCA2704 85HCA155). In a more complex transformation A-protected L-Pro was transformed into the same bicyclic system (Scheme 49) (81JA1851 84JA4192). The product was prepared as a model substance in the total synthesis of pumiliotoxin. A related compound 82 was prepared from 5-(hydroxymethyl)-2-pyrrolidinone (prepared from L-pyroglutamic acid) by an acid-catalyzed condensation with benzaldehyde (86JOC3140). 

In the synthesis of the alkaloid gephyrotoxin, L-pyroglutamic acid was used as a starting material for the preparation of intermediate 85 (81TL4197). A related pyrrolooxazine 86 was also obtained from pyroglu-tamic acid after A-substitution and ring closure in the presence of silica... 

For the synthesis of (+)-preussin, commercially available L-pyroglutamic acid (154) could serve as an enantiomerically pure starting material. Given the key reaction we had in mind, the bond set for the synthesis of preussin was straightforward (Scheme 40). Attachment of the alkyl chain to the core... 

A total synthesis of (—)-stemoamide is achieved from (—)-pyroglutamic acid using ruthenium-catalyzed enyne metathesis as a key step in 14 steps in 9% overall yield [Eq. (6.79)]... 

Vamos M, Ozboya K, Kobayashi Y (2007) Synthesis of bicyclic pyroglutamic acid featuring the Ugi reaction and a unique stereoisomerization at the angular position by Grob fragmentation followed by a transannular ketene [2-1-2] cycloaddition reaction. Synlett 1595-1599. Kreye O, Westermann B, Wessjohann LA (2007) A stable, convertible isonitrile as a formic acid carbanion [-COOH] equivalent and its application in multicomponent reactions. S3mlett 20 3188-3192... 

During synthesis of the hormone rf-aldosterone, 20,21-dihydroxy-lip,18-epoxy-5a-pregnan-3-one diacetate was oxidised to the 3-oxo acid by stoich. RuOy CCl [78, 79], Oxidation of a substituted pyrrolidine to a pyroglutamate, part of the total synthesis of the antibiotic biphenomycin B, was effected by RuO /aq. Na(lO )/ EtOAc [102], Oxidation of a number of A-5 steroids to enones was effected by RuClj/TBHP/cyclohexane [391] safety aspects of these large-scale reactions were examined, as in the preparation of the antibacterial squalamine [186],... 

When solid-phase peptide synthesis was initially being developed, the question of whether or not a separate neutralization step is necessary was considered. Since it was known from the work of others that the chloride ion promotes racemization during the coupling step in classical peptide synthesis, and since we were deprotecting the Boc group with HC1, it seemed advisable to neutralize the hydrochloride by treatment with TEA and to remove chloride by filtration and washing. This short, additional step was simple and convenient and became the standard protocol. Subsequently, we became aware of three other reasons why neutralization was desirable (1) to avoid weak acid catalysis of piperazine-2,5-dione formation, 49 (2) to avoid acid-catalyzed formation of pyroglutamic acid (5-oxopyr-rolidine-2-carboxylic acid), 50 and (3) to avoid amidine formation between DCC and pro-tonated peptide-resin. The latter does not occur with the free amine. 

The high tendency of glutamine and to a lesser extent of glutamic acid and related derivatives to form pyroglutamic acid which is exploited for the synthesis of pyroglutamyl peptides (Section 6.7.1.1.1), at the same time represents a source of undesirable side reactions in peptide chemistry (for reviews, see refs 16-18 ). 

The key step is the construction of a bicyclo[4.2.1] system. To this end, pyrrolidine derivative 95 having fif-substituents is synthesized from (+)-pyroglutamic acid and subjected to enyne metathesis using Ig to result in formation of 96 in 84% yield. From 96, synthesis of (+)-anatoxin-a is straightforward and successfully achieved. 

An enantiospecific synthesis of the potent anti-Alzheimer s agent (-t-)-ferruginine 1 and two enantiospecific syntheses of the potent anti-Alzheimer s agent anatoxin-a 2 were submitted essentially simultaneously earlier this year. In each case, the starting material used was the inexpensive pyroglutamic acid 3, and in each case the synthesis depended on Ru-catalyzed alkyne-alkene metathesis. 

Samuel Danishefsky of Columbia University has also described (J. Am. Chem. Soc. 2005, 127, 8298) a total synthesis of 1, starting from the pyroglutamate derivative 10. Conjugate addition followed by alkylation established the lactam framework. Intramolecular cyclization of 12 gave 13, establishing the aminated quaternary center. The oxygenated quaternary center was then constructed by phenylselenyl-mediatcd cyclization of 14. The end game of this synthesis used the already-established cyclohexenyl zinc addition, which worked as well with 16 as it had with 7. 

Several other methods have been utilized to produce the aminoalcohol 81. A ring expansion of prolinol 84 (derived from pyroglutamic acid (83) in eight steps) gave the trisubstituted chloropipendine 85. Dechlorination of 85 was accomplished under reductive radical conditions to give the aminoester 86, which was reduced to the aminoalcohol 81, thus completing the formal synthesis of (-)-paroxetine (6) (Scheme 15)." ... 

The (R)-enantiomer of (242) has also been prepared and used as a chiral auxiliary in an enantioselective aldol synthesis of (+)-(S )-gingerol (79CB3703). (R )-Glutamic acid (246) was thus converted into (i )-pyroglutamic acid by simply heating in water. Conversion of (247) to its methyl ester and LAH reduction delivered alcohol (248). Ethyl nitrite treatment of (248) gave nitrosoamine (249), which was methylated to furnish (250). Exposure of (250) to LAH completed the synthesis of the required chiral auxiliary RAMP [(R)- l-amino-2-(methoxymethyl)pyrrolidine]. The hydrazone (252), derived from RAMP and acetone, was... 

Pyroglutamic add can be used as an alternative for glutamic acid during the synthesis of y-substituted glutamic acids.1 95 Selective y-alkylation of protected pyroglutamic acid 57 has been utilized for the synthesis of (25,45)-2-amino-4-(2,2-diphenylethyl)pentanedioic acid hydrochloride (61, Scheme 21). 

Scheme 21 Asymmetric Synthesis of a /-Substituted Glutamic Acid from Pyroglutamic Acid[951...

An additional interesting example is the conjugate addition of 1 to activated allylic acetates 18 under the chiral phase-transfer catalysis of 4d, and subsequent elimination reaction, as reported by Ramachandran and coworkers, as this enables the synthesis of various enantiomerically enriched glutamic add derivatives [39]. The utility of this process has been demonstrated by the transformation of (S)-19 (R = Ph) into 4-substituted pyroglutamate (2S,4S)-20, as illustrated in Scheme 2.17. 

The addition of sulfur radicals to isocyanides produces imidoyl radicals as intermediates. The stereocontrolled 5-exo-trig cyclization of imidoyl radicals in the synthesis of substituted (alkylthio)pyrrolines, pyroglutamates, and thiopyroglutamates has been reported [95JOC6242], The intermediate 137 is not isolated but undergoes further thermal reaction to furnish a pyroglutamate derivative. 

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