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This review describes available methods for the preparation of α-aminoboronic acids in their racemic or in their enantiopure form. Both, highly stereoselective syntheses and asymmetric procedures leading to the stereocontrolled generation of α-aminoboronic acid derivatives are included. The preparation of acyclic, carbocyclic and azacyclic α-aminoboronic acid derivatives is covered. Within each section, the different synthetic approaches have been classified according to the key bond which is formed to complete the α-aminoboronic acid skeleton. [ABSTRACT FROM AUTHOR]

The (2S,5R) stereoisomer of 5-benzylproline, i.e. the l-proline analogue that bears a δ-benzyl substituent cis to the carbonyl function, has been prepared in enantiomerically pure form and excellent global yield. The procedure involves the construction of the pyrrolidine ring through intramolecular cyclization and uses as starting material the enantiopure β-amino acid obtained by homologation of l-phenylalanine. The generation of an intermediate vinyl triflate with full regiochemical control followed by a stereoselective hydrogenation reaction allowed the isolation of the target δ-substituted l-proline analogue in optically pure form and 43% overall yield from the initial β-amino acid. The trans stereoisomer of (2R,5R) configuration is obtained as a minor product through a less stereoselective hydrogenation reaction. [ABSTRACT FROM AUTHOR]

An overview of synthetic methods developed to build [ c]-fused bicyclic proline analogues is presented. The focus is on the preparation of azabicycles that bear a carbocyclic ring fused to the [ c] face of the pyrrolidine unit. Attention is paid both to procedures that afford the desired compounds in racemic form and to asymmetric strategies. Procedures are organized according to the size of the carbocycle that is fused to the pyrrolidine moiety. Strategies able to provide multigram quantities of enantiopure compounds that have application in the synthesis of marketed drugs are highlighted. [ABSTRACT FROM AUTHOR]

A versatile methodology for the preparation of racemic δ,δ-dimethylproline derivatives has been developed. Methyl N-Boc-δ,δ-dimethylprolinate was synthesized from a β-amino acid in six steps and 55 % overall yield. The route is amenable to the preparation of a broad range of δ,δ-disubstituted prolines by starting with the adequate β-amino acids. In addition, one of the intermediate compounds in the synthetic route has been used for the preparation of a δ,δ-dimethylproline derivative that is substituted at the β-position with a phenyl group. This has been achieved by coupling phenylboronic acid with a regioselectively generated vinyl triflate followed by a stereoselective hydrogenation. [ABSTRACT FROM AUTHOR]

ABSTRACT The preparation of all four stereoisomers of the proline analog that bears a phenyl group attached to the β carbon either cis or trans to the carboxylic acid ( cis- and trans-β-phenylproline, respectively) has been addressed. The methodology developed allows access to multigram quantities of the target amino acids in enantiomerically pure form and suitably protected for use in peptide synthesis. Racemic precursors of cis-β-phenylproline and trans-β-phenylproline were prepared from easily available starting materials and subjected to high-performance liquid chromatography enantioseparation. Semipreparative columns (250 × 20 mm) containing chiral stationary phases based on amylose (Chiralpak IA) (Daicel-Chiral Technologies Europe, Illkirch, France) or cellulose (Chiralpak IC) were used respectively for the resolution of the cis- and trans-β-phenylproline precursors. Chirality, 24:1082-1091, 2012. © 2012 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Applications of the boron–zinc exchange reaction to make new carbon&z.sbnd;carbon bonds are reviewed. Functionalized chiral secondary alkylzinc reagents can be prepared by this exchange reaction and allows to perform formal enantioselective Michael-additions with umpolung of reactivity. The scope of substrate controlled diastereoselective hydroborations can be considerably enhanced by this methodology. A chemoselective approach to difunctionalized arylsilanes is also described. [Copyright &y& Elsevier]

Discusses the enantiospecific synthesis regarding N-(9-phenylfluoren-9-yl)-...-amino ketones. How were these ketones prepared; Why is the preparatory method not applicable for the acylation of Grignard reagents; Reference to the reports of several different methods for the synthesizing of ...-amino ketones.

Review: [procedures that afford the desired compounds in racemic form and asymmetric strategies; 92 refs. [ABSTRACT FROM AUTHOR]

The conformational propensities of the proline analogue bearing a phenyl substituent attached to the β carbon, in either a cis or a trans configuration relative to the carbonyl group, have been investigated. The behaviour of cis- and trans(βPh)Pro has been compared with that of proline in homochiral and heterochiral dipeptide sequences. NMR and IR studies as well as X-ray diffraction analysis provide evidence that the β-phenyl substituent does not disrupt the tendency of proline to occupy the i+1 position of a β-turn. The puckering of the pyrrolidine ring is significantly affected by the presence of the aromatic substituent, which tends to occupy positions that minimize steric repulsions. As a consequence, this substituent adopts specific well-defined orientations, which are more restricted for the cis derivative. Interactions between this aromatic group and that in the adjacent phenylalanine residue may be responsible for some of the conformational differences observed among the different peptides studied.

An efficient methodology for the preparation of the α-tetrasubstituted proline analog (S,S,S)-2-methyloctahydroindole-2-carboxylic acid, (S,S,S)-(αMe)Oic, and its enantiomer, (R,R,R)-(αMe)Oic, has been developed. Starting from easily available substrates and through simple transformations, a racemic precursor has been synthesized in excellent yield and further subjected to HPLC resolution using a cellulose-derived chiral stationary phase. Specifically, a semipreparative (250 mm × 20 mm ID) Chiralpak® IC column has allowed the efficient resolution of more than 4 g of racemate using a mixture of n-hexane/tert-butyl methyl ether/2-propanol as the eluent. Multigram quantities of the target amino acids have been isolated in enantiomerically pure form and suitably protected for incorporation into peptides., (Copyright © 2011 Wiley-Liss, Inc.)

Peptides and proteins find an ever-increasing number of applications in the biomedical and materials engineering fields. The use of non-proteinogenic amino acids endowed with diverse physicochemical and structural features opens the possibility to design proteins and peptides with novel properties and functions. Moreover, non-proteinogenic residues are particularly useful to control the three-dimensional arrangement of peptidic chains, which is a crucial issue for most applications. However, information regarding such amino acids--also called non-coded, non-canonical, or non-standard--is usually scattered among publications specialized in quite diverse fields as well as in patents. Making all these data useful to the scientific community requires new tools and a framework for their assembly and coherent organization. We have successfully compiled, organized, and built a database (NCAD, Non-Coded Amino acids Database) containing information about the intrinsic conformational preferences of non-proteinogenic residues determined by quantum mechanical calculations, as well as bibliographic information about their synthesis, physical and spectroscopic characterization, conformational propensities established experimentally, and applications. The architecture of the database is presented in this work together with the first family of non-coded residues included, namely, alpha-tetrasubstituted alpha-amino acids. Furthermore, the NCAD usefulness is demonstrated through a test-case application example.

This review describes available methods for the diastereoselective and asymmetric synthesis of quaternary prolines. The focus is on the preparation of alpha-functionalized prolines with the pyrrolidine moiety not embedded in a polycyclic frame. The diverse synthetic approaches are classified according to the bond which is formed to complete the quaternary skeleton.

An improved strategy for the effective synthesis of enantiomerically pure (2R,3aS,7aS)-octahydroindole-2-carboxylic acid (Oic), based on the formation of a trichloromethyloxazolidinone derivative, has been developed. Additionally, the completely diastereoselective α-alkylation of such oxazolidinone provides a very convenient and concise route to enantiopure α-tetrasubstituted derivatives of this Oic stereoisomer.

[reaction: see text] The reaction of R(3)SnLi with carboxylic acid derivatives proceeds through a novel, very fast stanna-Brook rearrangement that generates alpha-alkoxyorganolithium compounds as intermediates. The outcome of these reactions depends on the nature of the carboxyl derivatives. Reaction of R(3)SnLi with ester derivatives gives rise to coupled products through a novel C-C bond formation reaction. Experimental evidence of the detailed reaction mechanism is provided.

[reaction: see text] Polyfunctional mixed lithium arylcuprates of the type (FG-Ar)(PhMe(2)CCH(2))CuLi obtained via iodine- or bromine-copper exchange with (PhMe(2)CCH(2))(2)CuLi react with cyclic 2-iodoallylic acetates with high S(N)2 selectivity. The addition of ZnBr(2) changes this selectivity and allows the performance of highly regioselective and enantioselective anti S(N)2' substitutions using open-chain allylic pentafluorobenzoates.

The scope of substrate-controlled diastereoselective hydroborations can be considerably enhanced by a boron-zinc exchange reaction, providing organozinc derivatives that react with a broad range of electrophiles. Even normally unreactive boronic esters, obtained by Rh-catalyzed hydroboration with catecholborane, react readily with iPr2Zn providing the corresponding zinc reagents in high diastereoselectivity.

[reaction: see text] Allylic substitution reactions of zinc-copper organometallics on (Z)-allylic pentafluorobenzoates proceed with very high regioselectivity and excellent anti selectivity. The high fidelity in transfer of stereochemical information allowed a short synthesis of (+)-ibuprofen (97% ee).

[reaction: see text] Functionalized allylic electrophilic reagents such as chiral 2-iodo-1-cyclohexenyl and -cyclopentenyl phosphates undergo highly stereoselective anti-S(N)2'-allylic substitution reactions with a wide range of organozinc reagents (R(2)Zn and RZnI) leading to chiral products with a transfer of the chiral information >95%. The use of functionalized organozinc iodides allows preparation of the bicyclic enones 8 and 9 in >or=93% ee.