Asymmetric hydrogenations for the synthesis of Boc-protected 4-alkylprolinols and prolines

The utility of 4-substituted prolinols and their corresponding prolines in peptides, peptidomimetics, and natural products has motivated researchers to find new and efficient routes for their preparation. Herein, we report a general approach to the synthesis of Boc-protected 4-alkylprolinols and prolines via a divergent asymmetric hydrogenation strategy. Intermediate exocyclic olefins were prepared by Wittig-type reactions with ketone 6 and subjected to hydroxyl and sterically directed reductions. The Crabtree catalyst (Ir[COD]PyPCy(3)PF(6)) proved to be highly effective in diastereoselective hydrogenations to give trans-substituted pyrrolidines (9). Good facial selectivities were also observed in heterogeneous hydrogenations with Raney-nickel to obtain cis-substituted pyrrolidines (11). Employing this strategy, we describe the synthesis of novel prolinol and proline-based building blocks for incorporation into biologically relevant peptidomimetics.

Rapid parallel synthesis of dipeptide diphenyl phosphonate esters as inhibitors of dipeptidyl peptidases

In this paper, we present a parallel synthesis of several series of dipeptide diphenyl phosphonates that are known to be irreversible inhibitors of serine proteases. Polymer-assisted solution-phase synthesis (PASP) is used for the rapid and clean coupling between various alpha-aminoalkyl diphenyl phosphonate ester building blocks and commercially available or easily accessible amino acids. These compounds were used for the rapid profiling of dipeptidyl peptidase II (DPP II) and the closely related dipeptidyl peptidase IV (DPP IV). A highly selective DPP II inhibitor was identified, N-cyclopentylglycyl-NHCH(C(6)H(5))PO(OPh)(2) (9.35), that will be useful to discriminate between DPP II and DPP IV in biological systems in order to further elucidate the biological function of DPP II.

Amino-zinc-enolate carbometalation reactions: application to ring closure of terminally substituted olefin for the asymmetric synthesis of cis- and trans-3-prolinoleucine

The amino-zinc-enolate cyclization reaction is a straightforward route for the synthesis of 3-substituted prolines. As classical intramolecular carbometalation reactions, the applicability of the addition of zinc to a double bond was limited to a substrate in which the terminal alkene carbon was unsubstituted. Being interested in the synthesis of cis- and trans-3-prolinoleucine derivatives for our structure-activity relation (SAR) studies, we focused our effort on the preparation of these compounds by amino-zinc-enolate cyclization of terminally substituted double bonds. Herein we report that the attachment of an activating group such as cyclopropyl to the terminal olefin carbon allows the amino-zinc-enolate cyclization of a terminally substituted alkene. The reaction is stereospecific, leading to a trans-3-substituted proline derivative, whereas a cis stereochemistry was observed with the amino-zinc-enolate cyclization of terminally nonsubstituted olefins. Absolute configurations obtained for the 3-prolinoleucine were established by X-ray analysis, NMR, and optical activity comparison of the cis and trans derivatives obtained by an unambiguous pathway.

Probing the stereoselectivity of the Heck arylation of endocyclic enecarbamates with diazonium salts. Concise syntheses of (2S,5R)-phenylproline methyl ester and Schramm's C-azanucleoside

[reaction: see text] The diastereoselectivity of the Heck arylation of several chiral, nonracemic, five-membered endocyclic enecarbamates with aryldiazonium tetrafluoroborates was evaluated. The cis selectivity observed for some enecarbamates bearing coordinating groups was explored in the concise synthesis of the (2S,5R)-(+)-phenylproline methyl ester, a scaffold for the nonpeptide cholecystokinin antagonist (+)-RP 66803, and in the synthesis of Schramm's potent antiprotozoan C-azanucleoside.

Stereoselective synthesis of beta-substituted phenylalanine-beta-phenylisoserine-derived tripeptides using N-cinnamoyl-L-proline as template: synthesis of structural analogues of HIV protease inhibitors

N-Cinnamoyl-L-proline can be used as a template on which beta-substituted phenylalanine and beta-phenylisoserine residues can be synthesized leading to tripeptide derivatives as structural analogues of HIV protease inhibitors.

Practical synthesis of Boc-protected cis-4-trifluoromethyl and cis-4-difluoromethyl-L- prolines

A short, efficient, and diastereomerically pure synthesis of N-Boc-cis-4-trifluoromethyl-L-proline (7) and N-Boc-cis-4-difluoromethyl-L-proline (9) from N-Boc-4-oxo-L-proline (4) is described. The reaction of 4 with Me(3)SiCF(3) and the conversion of the carbonyl group of 4 into the difluoromethylene group are the key steps for the synthesis of 7 and 9, respectively.

Dicarboxylic acid bis(L-prolyl-pyrrolidine) amides as prolyl oligopeptidase inhibitors

New dicarboxylic acid bis(L-prolyl-pyrrolidine) amides were synthesized, and their inhibitory activity against prolyl oligopeptidase from pig brain was tested in vitro. As compared with earlier described prolyl oligopeptidase inhibitors, these new compounds have in common an L-prolyl-pyrrolidine moiety, but the typical lipophilic acyl end group is replaced by another L-prolyl-pyrrolidine moiety connected symmetrically with a short dicarboxylic acid linker. These compounds are a new type of peptidomimetic prolyl oligopeptidase inhibitor.

Synthesis of 2,4-methanoproline analogues via an addition-intramolecular substitution sequence

A new two-step synthetic approach toward 3-(chloromethyl)cyclobutanone is described and used in the synthesis of 2,4-methanoproline analogues. The key step consists of a reversible addition of hydrogen cyanide onto the imines 12 in 50-74% yield under conditions that allow ring closure. 2-Alkyl-2-azabicyclo[2.1.1]hexane-1-carbonitriles 19, synthesized in four steps, can also be converted to the corresponding amines.

Complex-induced proximity effects. Temperature-dependent regiochemical diversity in lithiation-electrophilic substitution reactions of N-BOC-2-azabicyclo[2.1.1]hexane. 2,4- and 3,5-methanoprolines

[reaction: see text] Azabicycle 4 and sec-butyllithium/TMEDA afford the C(1) bridgehead alpha-lithio anion at 0 degrees C. Anion quenching with carbon dioxide, methyl chloroformate, or DMF provide the bridgehead acid 8a (N-BOC-2,4-methanoproline), ester 8b, or aldehyde 8c, respectively. By contrast, at -78 degrees C these same reagents give a mixture of regioisomeric methylene and bridgehead anions whose quenching leads to mixtures of regioisomeric methylene and bridgehead acids 6a/8a, esters 6b/8b, or aldehydes 6c/8c, respectively. The previously unknown 3,5-methanoproline was prepared as its N-BOC methyl ester 6b.

Drug testing in blood: validated negative-ion chemical ionization gas chromatographic-mass spectrometric assay for determination of amphetamine and methamphetamine enantiomers and its application to toxicology cases

BACKGROUND

Enantioselective analysis of amphetamine (AM) or methamphetamine (MA) in urine is already a well-established tool for differentiation of illicit from therapeutic ingestion of AM or MA derivatives. However, because of the increasing importance of plasma or serum in analytical toxicology, a method for enantioselective analysis of AM and MA in these matrices is needed.

METHODS

AM and/or MA were extracted from 0.2 mL of blood plasma or serum by mixed-mode solid-phase extraction. After derivatization with S-(-)-heptafluorobutyrylprolyl chloride, the resulting diastereomers were separated by gas chromatography on a HP-5MS column during a 15-min program and detected by mass spectrometry in the negative-ion chemical ionization mode (NICI-GC-MS). The method was fully validated and applied to >50 samples from authentic toxicology cases.

RESULTS

The derivatized AM and MA enantiomers were well separated and sensitively detected. The method was linear from 5 to 250 micro g/L per enantiomer with analytical recoveries, accuracy, and within- and between-run precision well within required limits. Extraction yields were 88.9-98.6%. Implications of concentrations and enantiomeric composition of AM and MA in the authentic samples were considered.

CONCLUSIONS

This sensitive, reliable, rapid NICI-GC-MS assay is suitable for enantioselective determination of AM and MA in blood plasma or serum samples.

Synthesis of N-glyoxyl prolyl and pipecolyl amides and thioesters and evaluation of their in vitro and in vivo nerve regenerative effects

The recent discovery that small molecule ligands for the peptidyl-prolyl isomerase (PPIase) FKBP12 possess powerful neuroprotective and neuroregenerative properties in vitro and in vivo suggests therapeutic utility for such compounds in neurodegenerative disease. The neurotrophic effects of these compounds are independent of the immunosuppressive pathways by which drugs such as FK506 and rapamycin operate. Previous work by ourselves and other groups exploring the structure-activity relationships (SAR) of small molecules that mimic only the FKBP binding domain portion of FK506 has focused on esters of proline and pipecolic acid. We have explored amide and thioester analogues of these earlier structures and found that they too are extremely potent in promoting recovery of lesioned dopaminergic pathways in a mouse model of Parkinson's disease. Several compounds were shown to be highly effective upon oral administration after lesioning of the dopaminergic pathway, providing further evidence of the potential clinical utility of a variety of structural classes of FKBP12 ligands.

Synthesis of ketone analogues of prolyl and pipecolyl ester FKBP12 ligands

The recently discovered small-molecule ligands for the peptidyl and prolyl isomerases (PPIase) of FKBP12 have been shown to possess powerful neuroprotective and neuroregenerative effects. Ketone analogues of the prolyl and pipecolyl esters, which mimic only the FKBP binding domain portion of FK506, are proposed and an efficient synthetic strategy is presented in this report, along with the preliminary results of in vitro and in vivo biological studies.

An effective new synthesis of 4-aminopyrrole-2-carboxylates

[reaction: see text] A series of 4-amino-1H-pyrrole-2-carboxylic acid benzyl esters has been synthesized in 61-84% yields on treatment of N-PhF-4-oxoproline benzyl ester and its 3-alkyl-substituted derivatives with different primary and secondary amines and a catalytic amount of TsOH in THF. 4-Hydroxy-1H-pyrrole-2-carboxylic acid benzyl esters were prepared in 59 and 70% yields by treatment of N-PhF-4-oxoproline benzyl esters with ammonium hydroxide in THF.

Toward a general strategy for the synthesis of 3,4-dihydroxyprolines from pentose sugars

A general strategy is proposed, wherein a pentose sugar gamma-lactone can be converted, via a series of nine reactions, to a 3,4-dihydroxyproline, suitably protected for use in peptide synthesis. Thus, D-ribonolactone (6) has been converted to N-fluorenylmethoxycarbonyl-3,4-di-O-tert-butyldimethylsilyloxy-D-2,3-cis-3,4-cis-proline (7) in 18.9% overall yield. Likewise, L-arabinonolactone (11) has been converted to N-fluorenylmethoxycarbonyl-3,4-di-O-tert-butyldimethylsilyloxy-L-2,3-cis-3,4-trans-proline (36) in 13.7% overall yield and L-lyxonolactone (12) to N-fluorenylmethoxycarbonyl-3,4-di-O-tert-butyldimethylsilyloxy-L-2,3-trans-3,4-cis-proline (37) in 11.2% overall yield. These building blocks have also been fully deprotected to give the free amino acids. We believe that this series of reactions ought to be applicable to the synthesis of any of the eight stereoisomers of 3,4-dihydroxyproline, by judicious selection of the pentose starting material.

Cytotoxicity activity of L-proline analogues of anthraquinone-2-carboxylic acid in breast cancer MCF-7 cells

Although prolidase [E.C.3.4.13.9] is found in normal cells, substantially increased levels are found in some neoplastic tissues. Prolidase evokes the ability to hydrolyse the imido-bond of various low molecular weight compounds coupled to L-proline. The synthesis of three proline analogues of anthraquinone-2-carboxylic acid (1-3) has been performed. Treatment of these prodrugs with prolidase generated L-proline and the free drug, demonstrating their substrate susceptibility prolidase. The concentrations of 1, 2 and 3 needed to inhibit [1H]thymidine incorporation into DNA by 50% (IC50) in breast cancer MCF-7 cells were found to be 185 +/- 5 microM, 107 +/- 6 microM and 87 +/- 6 microM, respectively, suggesting a lower cytotoxic potency of these compounds compared to Hoechst 33228 (IC50 = 55 +/- 6 microM).

Modulation of triple-helical stability and subsequent melanoma cellular responses by single-site substitution of fluoroproline derivatives

Collagen is a multifunctional protein, serving as a structural scaffold and a modulator of cellular responses. Prior work has identified distinct regions from several collagen types that promote cell adhesion, spreading, migration, and signal transduction. One of these regions, alpha1(IV)1263-1277 from type IV collagen, mediates these responses via melanoma cell CD44-chondrotin sulfate proteoglycan receptors. In the study presented here, we have used a triple-helical model of alpha1(IV)1263-1277 to evaluate (a) conformational stability and (b) cellular responses based on single-site incorporation of trans-4-fluoro-L-proline (trans-Flp) or cis-4-fluoro-L-proline (cis-Flp) for trans-4-hydroxy-L-proline (trans-Hyp). The structural effects of cis-Flp and trans-Flp substitution were studied by circular dichroism and NMR spectroscopies. The peptide containing a single trans-Flp instead of trans-Hyp was slightly more thermally stable than the parent peptide (T(m) = 37 vs 34 degrees C), while the peptide containing cis-Flp was considerably less stable than the parent peptide (T(m) = 30 degrees C). Melanoma cell adhesion and spreading were examined under conditions where the trans-Hyp-, trans-Flp-, and cis-Flp-containing ligands were approximately 15, <10, and approximately 65% denatured, respectively. Adhesion to each of the three ligands was remarkably sensitive to the respective ligand conformation, with EC(50) values of approximately 2.5, approximately 0.35, and >5.0 microM for the trans-Hyp-, trans-Flp-, and cis-Flp-containing ligands, respectively. Melanoma cell spreading was quantitated over a ligand concentration range of 0.01-50 microM and, in a fashion similar to adhesion, was more extensive on the trans-Flp ligand than on the trans-Hyp ligand. Very low levels of spreading were observed with the cis-Flp-containing ligand at all concentrations tested. Melanoma cell adhesion to and spreading on the three ligands suggested the dramatic biological consequence of even subtle changes in relative triple-helical content. Such subtle changes may model those occurring in the basement membrane during the tumor cell invasion process, and thus provide mechanistic insight into this stage of metastasis.

Syntheses of novel 4-tert-alkyl ether proline-based 16- and 17-membered macrocyclic compounds

Starting from N-Cbz-4-hydroxyproline methyl ester 1, a boron trifluoride-diethyl etherate-catalyzed reaction provided 4-tert-alkyl ether proline 4. Two deprotections and amide bond formations furnished the phenol alcohol 2. The macrocyclization of 2 was accomplished through a Mitsunobu reaction using triphenylphosphine and 1,1'-(azodicarbonyl)dipiperidine (ADDP), to afford novel 16- and 17-membered proline-based macrocyclic compounds of type 3.

SmI(2)-mediated cyclizations of derivatized beta-lactams for the highly diastereoselective construction of functionalized prolines

A series of C4-keto-functionalized 1-[(benzoyloxy)(ethoxycarbonyl)methyl]-2-azetidinones were prepared and studied for their tendency to undergo a Reformatsky-type cyclization to fused bicyclic or tricyclic beta-lactams with the single-electron reducing agent samarium diiodide. Whereas the azetidinone 21a underwent reductive cyclization, affording the potent antibiotic sanfetrinem's tricyclic [4.5.6] core structure as the major component, all other examples tested resulted in cyclization followed by an N to O acyl migration involving cleavage of the beta-lactam ring as the favored pathway. Highly functionalized proline derivatives were therefore accessed as single diastereomers through the reductive cyclization of benzoates 21b, 22, 23a,b, 24b, and 25-28. Pertinent for the success of these cyclizations was the addition of 1 equiv of tert-butyl alcohol, allowing for the protonation of the basic amide derivative obtained after the acyl migration step. The diastereoselectivities of these reactions deviate from those of similar cyclizations involving the corresponding lithium enolate. This divergence could be rationalized by the coordination of the metal ion of the samarium(III) enolate intermediate to the beta-lactam amide functionality in the cyclization step, which may not be possible for lithium enolates.

Influence of silaproline on peptide conformation and bioactivity

The analogue gamma-(dimethylsila)-proline, denoted silaproline (Sip), was synthesized in both enantiomerically pure forms by diastereoselective alkylation of a chiral glycine equivalent with use of Schöllkopf's bis-lactim ether method. The effect of replacing a proline residue in model peptides by this new proline surrogate has been examined in the crystal state by X-ray diffraction and in solution by IR absorption and NMR techniques. Silaproline and proline-containing sequences exhibit very similar conformational properties. Silaproline was also substituted for proline in a neurotensin (8-13) analogue that retained biological activity and exhibited enhanced resistance to biodegradation.

Effect of sequence on peptide geometry in 5-tert-butylprolyl type VI beta-turn mimics

The influence of sequence on turn geometry was examined by incorporating (2S,5R)-5-tert-butylproline (5-(t)BuPro) into a series of dipeptides and tetrapeptides. (2S,5R)-5-tert-Butylproline and proline were respectively introduced at the C-terminal residue of N-acetyl dipeptide N'-methylamides 1 and 2. The conformational analysis of these analogues was performed using NMR and CD spectroscopy as well as X-ray diffraction to examine the factors that control the prolyl amide (in this text, the term "prolyl amide" refers to the tertiary amide composed of the pyrrolidine nitrogen of the prolyl residue and the carbonyl of the N-terminal residue) equilibrium and stabilize type VI beta-turn conformation. The high cis-isomer population with aromatic residues N-terminal to proline was shown to result from a stacking interaction between the partial positive charged prolyl amide nitrogen and the aromatic pi-system as seen in the crystal structure of 1c. The effect of sequence on the prolyl amide equilibrium of 5-(t)BuPro-tetrapeptides (Ac-Xaa-Yaa-5-(t)BuPro-Zaa-XMe, 13 and 14) was studied by varying the amino acids at the Xaa, Yaa, and Zaa positions. High (>80%) cis-isomer populations were obtained with alkyl groups at the Xaa position, an aromatic residue at the Yaa position, and either an alanine or a lysine residue at the Zaa position of the 5-(t)BuPro-tetrapeptide methyl esters in water. Tetrapeptides Ac-Ala-Phe-5-(t)BuPro-Zaa-OMe (Zaa = Ala, Lys), 14d and 14f, with high cis-isomer content adopted type VIa beta-turn conformations as shown by their NMR and CD spectra. Although a pattern of amide proton temperature coefficient values indicative of a hairpin geometry was observed in peptides 14d and 14f, the value magnitudes did not indicate strong hydrogen bonding in water.

Enantioselective synthesis of (2S,3R)-3-hydroxy-3-methylproline, a novel amino acid found in polyoxypeptins

The enantioselective synthesis of (2S,3R)-3-hydroxy-3-methylproline (3) was achieved by the Sharpless AD, regioselective opening of cyclic sulfate by NaN(3) and intramolecular ring-closing reaction. The reported route has the advantage of a high overall yield and good enantiomeric purity, as well as starting from readily available chemical substrates and inexpensive reagents.

A new and short method for the synthesis of 2,4-methanoproline

2,4-Methanoproline, a supposed non-proteinogenic anti-feedant, was synthesised in 5 steps starting from allyl benzyl ether 3 in 10% overall yield with an intramolecular nucleophilic substitution as the key step for the formation of the bicyclic skeleton.

Application of L-threonine aldolase-catalyzed reaction to the preparation of protected 3R,5R-dihydroxy-L-homoproline as a mimetic of idulonic acid

A facile synthesis of 3R,5R-dihydroxy-L-homoproline as idulonic acid mimic, of which the carboxyl and 3-hydroxyl groups were protected, was attained using L-threonine aldolase-catalyzed reaction. Idulonic acid is a key acidic sugar of the b-FGF binding domain in heparin and heparan sulfate. Moreover, the synthetic precursor of N-acetyl-4-deoxy-D-mannosamine, which is a potent inhibitor of NeuAc synthase, was prepared from the side product of the enzymatic aldol condensation.