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Synthesis of quaternary ?-methyl ?-amino acids by asymmetric alkylation of pseudoephenamine alaninamide pivaldimine.

ABSTRACT: The utility of pseudoephenamine as a chiral auxiliary for the alkylative construction of quaternary ?-methyl ?-amino acids is demonstrated. The method is notable for the high diastereoselectivities of the alkylation reactions, for its versatility with respect to electrophilic substrate partners, and for its mild hydrolysis conditions, which provide ?-amino acids without salt contaminants. Alternatively, ?-amino esters can be obtained by direct alcoholysis.

SUBMITTER: Hugelshofer CL

PROVIDER: S-EPMC3884032 | biostudies-literature | 2013 Jun

REPOSITORIES: biostudies-literature

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Synthesis of quaternary α-methyl α-amino acids by asymmetric alkylation of pseudoephenamine alaninamide pivaldimine.

Hugelshofer Cedric L CL Mellem Kevin T KT Myers Andrew G AG

Organic letters 20130607 12

The utility of pseudoephenamine as a chiral auxiliary for the alkylative construction of quaternary α-methyl α-amino acids is demonstrated. The method is notable for the high diastereoselectivities of the alkylation reactions, for its versatility with respect to electrophilic substrate partners, and for its mild hydrolysis conditions, which provide α-amino acids without salt contaminants. Alternatively, α-amino esters can be obtained by direct alcoholysis. ...[more]

PMID: 23746325

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Project description:A generalizable synthesis of higher L-alpha-vinyl amino acids is presented. The strategy pursued here involves the introduction of the amino acid side chain via the alkylation of a chiral, vinylglycine-derived dianionic dienolate, bearing the (-)-8-(beta-naphthyl)menthyl (d'Angelo) auxiliary. A model is presented that postulates a favored "exo-entended" conformation for this dienolate, leading to C(alpha)-alkylation at the si face. The model invokes internal amidate chelation to control ester enolate geometry and soft-soft interactions between the polarizable beta-naphthyl ring of the auxiliary and the extended pi-system of the dienolate to shield the re face. Heats of formation for four conformers of this dianion were calculated for their semiempirical optimized geometries (PM3). The results support the notion that in these vinylglycine-derived dianionic dienolates, "exo" conformations are considerable lower in energy than their "endo" counterparts, with the "exo-entended" conformation being most favorable. In fact, the d'Angelo auxiliary gives a greater degree of acyclic stereocontrol in this system when compared with the (-)-8-phenylmenthyl (Corey) and trans-2-(beta-naphthyl)cyclohexyl auxiliaries, using isobutyl iodide and benzyl bromide as model electrophiles. These dianions are generated from the corresponding dehydrobutyrine esters via sequential deprotonation with LDA and n-BuLi (2 equiv). When alkylations are carried out at -78 degrees C in THF-HMPA, they proceed in 65-81% yields, with both regiocontrol (deconjugative alpha-alkylation is preferred over gamma-alkylation) and a great degree of acyclic stereocontrol [91:9 to >/=98:2 diastereomeric ratios (10 examples)]. The auxiliary may be recovered in high yield (generally 90%) using a modification of Gassman's "anhydrous hydroxide" conditions, in which considerably higher temperatures are employed. Among the side chains introduced directly are those of butyrine, leucine, ornithine, phenylalanine, aspartate, valine, and norvaline. The lysine side chain is elaborated via a 4-step sequence from the alkylation product obtained with 1-chloro-4-iodobutane as electrophile. Importantly, to our knowledge, this work represents the first asymmetric synthesis of L-alpha-vinyl analogues of m-tyrosine, ornithine, and lysine, known time-dependent inhibitors for amino acid decarboxylases.

Dataset Information

Synthesis of quaternary ?-methyl ?-amino acids by asymmetric alkylation of pseudoephenamine alaninamide pivaldimine.

Publications

Synthesis of quaternary α-methyl α-amino acids by asymmetric alkylation of pseudoephenamine alaninamide pivaldimine.

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