On the Synthesis and Selective Deprotection of Low‐Generation Dendrons with Orthogonally Protected Peripheral Amine Groups and a Possible Impact of the Deprotection Conditions on the Stability of Dendronized Polymers' Skeletons

The synthesis of first‐ and second‐generation dendrons with defined ratios of orthogonally protected amine groups in the periphery ((benzyloxy)carbonyl (Cbz) and (tert‐butoxy)carbonyl (Boc) protection) and the degree to which they can be selectively removed are described. The reaction conditions required for these deprotections were applied to methacrylic acid (= 2‐methylprop‐2‐enoic acid) based dendronized polymers carrying the same peripheral protecting groups to investigate whether they have any detrimental interference with the polymer skeleton. Specifically it was explored whether dendrons attached to the backbone could possibly be cleaved off as a whole (de‐dendronization). Finally it was investigated how de‐dendronizations can be used for quantifying both the dendron‐structure perfection and the polymer‐backbone configurations.     

Facile Synthesis of bis‐α‐Aminoalkylphosphinates

Herein we report a facile synthesis of esters of bis‐α‐aminoalkylphosphinic acids obtained by an addition of Cbz‐protected phosphinic analogues of amino acid methyl esters to an appropriate imine in refluxing benzene. Complete deprotection of the esters could be achieved in one step by the action of 30% HBr in acetic acid.     

Selective tert-butyl ester deprotection in the presence of acid labile protecting groups with use of ZnBr2

Chemoselective hydrolysis of tert-butyl esters in the presence of other acid-labile groups has been explored by employing alpha-amino esters and ZnBr(2) in DCM. Although N-Boc and N-trityl groups were found to be labile, PhF protected amines were compatible with these Lewis acid deprotection conditions such that a variety of N-(PhF)amino acids were prepared in good yields from their corresponding tert-butyl esters.     

Enzymatic resolution of N-protected-β3-amino methyl esters,using lipase B from Candida antarctica

Racemic β³-amino methyl esters bearing the amine function protected with Bz, Cbz, Boc, Fmoc and as aminobenzamide, were resolved by enantiospecific transesterifications catalyzed by lipase B from Candida antarctica. The reactions proceeded with a high conversion and yielded enantiomerically pure enantiomers.     

Highly Enantioselective Synthesis of Orthogonally Protected (2S)‐2,3‐Diaminopropanoates through Catalytic Phase‐Transfer Aza‐Henry Reaction

The syntheses of enantiomer‐enriched orthogonally protected different (2S)‐2,3‐diaminopropanoates and unnatural furyl‐substituted (tert‐butoxy)carbonyl (Boc) as well as (benzyloxy)carbonyl (Cbz) protected amino acid esters are accomplished by means of an enantioselective aza‐Henry reaction. A key feature of this protocol is organocatalysis as a genesis of chirality to ensure high enantioselectivity.     

Solution-phase synthesis of aminooxy peptoids in the C to N and N to C directions

[structure: see text] Aminooxy peptoids, which are potential peptidomimetics, were synthesized by a stepwise monomer assembly. Ns-protected N-substituted aminooxyacetate tert-butyl esters were used as a monomer in both the C to N and the N to C directions. Submonomer synthesis of aminooxy peptoids is also described.     

Dendron rodcoils: synthesis of novel organic hybrid structures

Convergent and divergent syntheses of novel organic hybrid structures termed dendron rodcoils (DRC) containing dendritic, rodlike, and coillike segments are described. The aryl ester dendron masked with 32 trifluoromethyl groups is prepared via a convergent approach using 5-(tert-butyldimethylsiloxy)isophthalic acid as the monomer unit. The activation of the focal point of the dendron allows for successful coupling between the dendron and the diblock rodcoil molecules synthesized separately. In another example, the dendritic block is grown via divergent strategy from the terminus of rodcoil using 3,5-bis(tert-butyldimethylsiloxy)benzoic acid as an AB(2) monomer. A combination of catalyzed esterification reactions and silyl deprotection chemistry proved to be a very efficient method for construction of these nanosized structures with unusual molecular architecture. Both synthetic strategies allowed for the preparation of DRCs with nearly monodisperse dendritic blocks as demonstrated by NMR, MALDI-TOF, and GPC measurements.     

9-(4-Bromophenyl)-9-fluorenyl as a safety-catch nitrogen protecting group

[reaction: see text] The 9-(4-bromophenyl)-9-fluorenyl (BrPhF) group has been developed as a novel safety-catch amine protection. This relatively acid-stable protecting group can be successfully activated by palladium-catalyzed cross-coupling reaction of the aryl bromide with morpholine and then cleaved effectively under mild conditions using dichloroacetic acid and triethylsilane. Complementary conditions are reported for selective removal of the BrPhF group in the presence of tert-butyl esters and carbamates as well as deprotection of tert-butyl esters and carbamates in the presence of BrPhF amines.     

Phenols-useful templates for the synthesis of bi-functional orthogonally protected dendron building blocks via solid phase Mitsunobu reaction

Bi-functional dendritic building blocks for convergent dendrimer growth were successfully synthesized from phenolic templates in the solid phase via a Mitsunobu reaction. Each arm of the dendron building block carries an orthogonally protected secondary amine along the arm, and a peripheral primary amine or phenol group (building block type 1) or a tertiary amine junction with orthogonally protected peripheral primary amine or carboxyl groups (building block type 2). The synthetic routes reported in this work are general and applicable for the preparation of diverse building blocks, controlling protection, arm length, and peripheral moieties. These novel dendron units can form unusual dendritic architectures by solid-phase chemistry, which may be incorporated into specific complex structures expanding the scope of dendrimer science.     

Efficient functional molecule incorporation method to functionalized peptide nucleic acid (PNA): use in synthesis of labeled PNA oligomers

A novel efficient synthetic method for a functionalized PNA (peptide nucleic acid) is described, in which a functional molecule is incorporated in place of a nucleobase. Novel ω-AA-^BocPNA-OH (20-24, AA=amino acid) were designed as PNA precursor monomer units into which functional molecules could be incorporated efficiently. Compounds 20-24 reacted quantitatively with OSu (N-hydroxysuccinimidyl) active ester derivatives and isothiocyanate derivatives of commercial functional molecules to give target functionalized PNA monomer units 25-53. Various types of functionalized PNA monomer units could be efficiently incorporated into multiple predetermined positions in a PNA oligomer by SPPS (solid phase peptide synthesis) in the same way as for the four A(Cbz), G(Cbz), C(Cbz), and T PNA monomer units.     

Synthesis of chiral nonracemic 4-trans-substituted pipecolic acid derivatives

The syntheses and resolutions of enantiomerically enriched 4-phenyl, 4-tert-butyl, and 4-isopropyl pipecolic acids are described. Optically active diastereomers were prepared by diastereomeric salt formation with the chiral base, l-tyrosine hydrazide, to provide Cbz or Boc protected 4-cis-d-pipecolic acid derivatives in >98% ee. Subsequent esterification followed by sodium methoxide catalyzed epimerization provided the isomeric 4-trans-l-pipecolic esters. In addition, an efficient synthesis of 4-phenyl-cis-pipecolic acid is described.     

Synthesis and cytotoxic properties of derivatives of the tert-butyl ester of 7-alkylidene-3-methyl-3-cepheme-4-carboxylic acid

Sulfones of the tert-butyl esters of 7-arylmethylene-and 7-(2-furyl)methylene-3-methyl-3-cepheme-4-carboxylic acid were obtained by the condensation of the tert-butyl ester of 3-methyl-7-oxo-3-cepheme-4-carboxylic acid with arylmethylene-and 2-furylidenetriphenylphosphoranes and subsequent oxidation of the intermediate products by meta-chloroperbenzoic acid. The combination of the tert-butyl esters of 7E-bromomethylene-and 7,7-dibromomethylene-3-methyl-1,1-dioxo-3-cepheme-4-carboxylic acids with trimethylsilylacetylene under conditions of the Sonogashira reaction gave the tert-butyl esters of 3-methyl-1,1-dioxo-7E-(3-trimethylsilyl-2-propynylidene)-3-cepheme-4-carboxylic acid and 3-methyl-1,1-dioxo-7-[1,5-bis(trimethylsilyl)-1,4-pentadiyn-3-ylidene]-3-cepheme-4-carboxylic acid. The Vilsmeier reagent was used to incorporate the dimethylaminomethylene group at C-2 of the 7Z-and 7E-isomers of the tert-butyl ester of 7-(4-chlorophenyl)methylene-3-methyl-1,1-dioxo-3-cepheme-4-carboxylic acid. The cytotoxic properties of the derivatives of the tert-butyl ester of 7-alkylidene-3-methyl-3-cepheme-4-carboxylic acid in regard to cancer and normal cells in vitro depends on the structure and 7Z-or 7E-isomerism of the substituent in the 7-alkylidene group as well as the presence of a dimethylaminomethylene group at C-2 of the 3-cepheme system.     

Novel Dendritic alpha-Sialosides: Synthesis of Glycodendrimers Based on a 3,3'-Iminobis(propylamine) Core

The cluster or multivalent effect has been recognized as an effective means by which to increase binding interactions between carbohydrates and proteins. In fact, it has been demonstrated that sialylated multibranched L-lysine dendrimers were potent inhibitors of hemagglutination of human erythrocytes by Influenza viruses. In a continuation of these studies, the synthesis of novel glycodendrimers with even valencies from 2 to 16 and ending with equidistant thiosialoside residues is described. These symmetrical dendrimers were more readily characterized by standard NMR spectral techniques than previously reported nonsymmetrical dendrimers of this general type. The synthesis of the dendritic core was based on the regioselective protection of the primary amines of 3,3'-iminobis(propylamine) (4) using benzyl cyanoformate. The resulting secondary amine 5 was alkylated with tert-butyl bromoacetate to provide divalent core structure 6 with Cbz protected amines and acid protected tert-butyl ester. Sequential deprotection by trifluoroacetolysis or hydrogenation afforded acid 7 or diamine 8 as key precursors, respectively. The two fragments were coupled using HOBt/DIC strategy to provide Cbz-protected dendrimers with valencies of 2, 4, 8, and 16 in the first, second, third, and fourth generations, respectively, in reasonable to good yields (42-82%). Cbz-protected precursors were efficiently transformed into N-chloroacetylated dendrimers by hydrogenolysis and treatment of the resulting amines with chloroacetic anhydride (82-91%). N-Chloroacetylated dendrimers were then treated with an excess of 2-thiosialic acid derivative 3 to give fully protected sialodendrimers in 76-96% yields. Deprotection of sialodendrimers under Zemplén conditions followed by methyl ester saponification and purification by gel permeation chromatography afforded symmetrical dendritic alpha-thiosialosides 21, 23, 25, and 27 in fair yields (47-58%). These novel sialodendrimers, in keeping with their design, are currently being evaluated as inhibitors of human erythrocyte hemagglutination by Influenza viruses.     

New pyrrole-based amino acids for the synthesis of peptidomimetic constrained scaffolds

A new family of pyrrole-based amino acids have been prepared through the microwave assisted Paal-Knorr reaction of 1-4 ketoesters derived from the corresponding β-ketoester with a functional homologation. The carboxylic group is located in position 3 of the pyrrole, whereas the amino group, protected with the Cbz moiety, is present on the side chain in positions 1 or 2. These compounds were used to prepare constrained oligopeptides.     

(R)-tert-Butoxycarbonylamino-fluorenylmethoxycarbonyl-glycine from (S)-benzyloxycarbonyl-serine or from papain resolution of the corresponding amide or methyl ester

The enantiospecific synthesis of (R)-Boc-(Fmoc)-aminoglycine 7 was achieved. (S)-Cbz-serine 1 was reacted with diphenylphosphoryl azide in the presence of triethylamine to yield cyclic (S) carbamate 2. The ring nitrogen of 2 was protected with a Boc group (3). The cyclic carbamate of 3 was hydrolyzed with benzyltrimethylammonium hydroxide to yield the (R)-enantiomer of alcohol 4. The oxidation of 4 with pyridinium dichromate yielded the enantiomerically pure (97% ee) (R)-Boc-(Cbz)-aminoglycine 5, which was converted to 7 with retention of optical purity. Similarly, starting from (S)-Boc-serine 9, cyclic (S) carbamate 10 was obtained. The ring nitrogen of 10 was protected with a Cbz group (11) with retention of configuration. The cyclic carbamate of 11 was base hydrolyzed to yield 12, the (S)-enantiomer alcohol. Independently, Boc-(Fmoc)-aminoglycine amide 13 and Boc-(Fmoc)-aminoglycine methyl ester 14 were resolved using papain. The stereochemistry of the isolated acid was determined to be (R) by coelution on HPLC of its derivative with Marfey's reagent and that of an authentic sample (7) obtained by enantiospecific synthesis.     

Synthesis of multi(metallo)porphyrin dendrimers through nucleophilic aromatic substitution on meso-pyrimidinyl substituted porphyrins

A convergent synthetic strategy toward novel all-porphyrin dendrimers, which can be regarded as synthetic model systems of the natural photosynthetic light-harvesting antennae, was successfully explored. The dendron propagation, based on nucleophilic aromatic substitution reactions on a meso-dichloropyrimidinyl substituted porphyrin AB2 monomer, was carried out up to the second-generation dendron, and the used approach was proven to be compatible with the stepwise introduction of different metals in the successive generations. Three different metals (Pt, Zn, and Ni) could be introduced in the second-generation heptaporphyrin dendron. By applying analogous meso-dichloropyrimidinyl substituted porphyrin cores, novel multiporphyrin dendrimers were obtained.     

An improved method for the protection of carboxylic acids as 1,1-dimethylallyl esters

1,1-Dimethylallyl (DMA) esters of various N-protected amino acids have been synthesized using prenyldimethylsulfonium tetrafluroborate, a reagent that can be readily made and stored, in conjunction with catalytic CuBr. These reactions were complete within several hours and afforded DMA esters in high yields. As has been previously shown in our group, DMA esters represent a palladium-labile proctecting group for carboxylic acids that resists nucleophilic attack as a tert-butyl ester would.     

Tertiary-butoxycarbonyl (Boc) – A strategic group for N-protection/deprotection in the synthesis of various natural/unnatural N-unprotected aminoacid cyanomethyl esters

A number of cyanomethyl esters of natural/unnatural aminoacids with un-protected amino functionality were synthesized because of their synthetic and medicinal importance. Critical N-Boc deprotection methods in the presence of labile (hydrolytic sensitivity) cyanomethyl functionality were screened thoroughly and it was found that readily available 4M HCl in 1,4-dioxane solution (2–4 equiv); acetonitrile, 0?°C, 2–4?h was a suitable condition. This condition was generalized and successfully applied to a variety of alkyl, alkynyl, aryl, heteroaryl, benzyl, azido, spiro amino acid cyanomethylesters irrespective of the nature of the amine (primary or secondary) and the distance between the amine and ester group to achieve final deprotected amino esters with high yield, and purity compared to other commonly known N-protecting groups (Cbz, Fmoc, Ac, Bn, Bz etc.). It was also demonstrated that N-Boc protected aminoacid cyanomethylesters are stable enough to carry out further functionalization compared to N-unprotected counterparts.     

Synthesis of a Methacrylic Monomer Containing an <Emphasis Type="Italic">L</Emphasis>-Lysine-Based Dendron

Procedures were developed for preparing a polylysine dendron of second generation, containing terminal Boc-protected amino groups and a free focal amino group; from this substance, a previously unknown methacrylic monomer linked to the L-lysine-based dendron was prepared.__________Translated from Zhurnal Prikladnoi Khimii, Vol. 78, No. 6, 2005, pp. 1024–1028.Original Russian Text Copyright © 2005 by Khimich, Tennikova.     

A convenient, general synthesis of 1,1-dimethylallyl esters as protecting groups for carboxylic acids

[reaction: see text] Carboxylic acids were converted in high yield to their 1,1-dimethylallyl (DMA) esters in two steps. Palladium-catalyzed deprotection of DMA esters was shown to be compatible with tert-butyl, benzyl, and Fmoc protecting groups, and Fmoc deprotection could be carried out selectively in the presence of DMA esters. DMA esters were also shown to be resistant to nucleophilic attack, suggesting that they will serve as alternatives to tert-butyl esters when acidic deprotection conditions need to be avoided.