A novel Prins cascade process for the stereoselective synthesis of oxa-bicycles.

E- and Z-9-Methyldeca-3,8-dien-1-ols undergo smooth cyclization with aldehydes in the presence of 20 mol% AgSbF6 under extremely mild conditions to generate the corresponding oxa-bicycles in good yields with excellent selectivity. In fact, E-olefin affords the trans-product exclusively, whereas the Z-olefin gives the cis-product predominantly. In the case of E- or Z-8-methylnona-3,8-dien-1-ol, the product is formed via the termination of Prins cyclization with an allylic C-H bond through olefin migration. The termination of Prins cyclization with tethered olefin is an unprecedented reaction, which provides a useful motif of various natural products.

A novel domino cyclization for the stereoselective synthesis of indeno[2,1-c]pyran and cyclopenta[c]pyran derivatives.

A novel bicyclization of 2-(2-(hydroxymethyl)-1-methylene-2,3-dihydro-1H-inden-2-yl)ethanol with aldehydes in the presence of 10 mol% BF3·OEt2 in dichloromethane at 0-25 °C affords the biologically relevant indeno[2,1-c]pyran scaffolds in good yields with high selectivity. Similarly the bicyclization of 2-(1-(hydroxymethyl)-2-methylenecyclopentyl)ethanol with aldehydes generates the corresponding cyclopenta[c]pyran derivatives under similar conditions. This method is very useful to produce hematoxylin and brazilin like scaffolds.

Thia-prins bicyclization approach for the stereoselective synthesis of dithia- and azathia-bicycles.

A novel thia-Prins bicyclization approach has been developed for the first time for the synthesis of hexahydro-2H-thieno[3,2-c]thiopyran derivatives from the coupling of homoallylic mercaptans such as hex-3-ene-1,6-dithiol with various aldehydes using 10 mol % InBr3 in dichloromethane with high selectivity. In addition, the coupling of (E)-N-(6-mercaptohex-3-enyl)-4-methylbenzenesulfonamide with aldedydes affords the corresponding N-tosyloctahydrothiopyrano[4,3-b]pyrrole derivatives in good yields. This reaction is a stereoselective affording trans-fused product from E-homoallyllic mercaptan and cis-fused product from Z-homoallyllic mercaptan.

Oxidative Prins and Prins/Friedel-Crafts cyclizations for the stereoselective synthesis of dioxabicycles and hexahydro-1H-benzo[f]isochromenes via the benzylic C-H activation.

1-Benzyl ethers of (E)- and (Z)-hex-3-en-1,6-diols and hept-3-en-1,7-diols undergo a smooth oxidative cyclization with DDQ in the presence of In(OTf)(3) through a sequential C-H bond activation and an intramolecular Prins cyclization to afford the corresponding trans- and cis-fused hexahydro-2H-furo[3,2-c]pyrans and octahydropyrano[4,3-b]pyrans respectively in good yields with an excellent stereoselectivity. Aryl tethered homoallylbenzyl ethers such as benzyl ethers of (E)- and (Z)-6-arylhex-3-enyl alcohols undergo a tandem Prins/Friedel-Crafts cyclization in the presence of stoichiometric amounts of DDQ and SnCl(4)via the benzylic C-H bond activation to furnish the corresponding trans- and cis-fused hexahydro-1H-benzo[f]isochromenes in good yields with complete stereoselectivity.

The stereoselective synthesis of cis-/trans-fused hexahydropyrano[4,3-b]chromenes via Prins cyclization trapping by a tethered nucleophile.

A novel intramolecular Prins cyclization of (Z)-2-(5-hydroxypent-2-enyl)phenol with various aldehydes has been achieved using 10 mol% In(OTf)(3) and 30 mol% TsOH to produce the cis-fused hexahydropyrano[4,3-b]chromene derivatives in good yields, while the coupling of (E)-2-(5-hydroxypent-2-enyl)phenol with aldehydes under similar conditions affords the corresponding trans-fused hexahydropyrano[4,3-b]chromene derivatives.

Differentiation of three pairs of Boc-beta,gamma- and gamma,beta-hybrid peptides by electrospray ionization tandem mass spectrometry.

A new series of Boc-N-beta(3), gamma(4)-/gamma(4), beta(3)-isomeric hybrid peptides (containing repeats of beta(3)-Caa and gamma(4)-Caa's, Caa = C-linked carbo beta(3)-/gamma(4)-amino acids derived from D-xylose) have been differentiated by both positive and negative ion electrospray ionization (ESI) ion-trap and high resolution quadrupole time-of-flight/tandem mass spectrometry (Q-TOF MS/MS). MS(n) of protonated isomeric peptides and [M+H-Boc+H](+) produce characteristic fragmentation involving the peptide backbone, the Boc-group, and the side chain. The positional isomers are differentiated from one another by the presence of y(n)(+), b(n)(+), and other fragment ions of different m/z values. It is observed that the peptides with beta-Caa at the N-terminus produce extensive fragmentation, whereas gamma-Caa gave rise to much less fragmentation. Peptides with gamma-Caa at the N-terminus lose NH(3), whereas this process is absent for the carbopeptides with beta-Caa at the N-terminus. Two pairs of dipeptide diastereomers are clearly differentiated by the collision-induced dissociation (CID) of their protonated molecules. The loss of 2-methylprop-1-ene is more pronounced for Boc-NH-(R)-beta-Caa-(R)-gamma-Caa-OCH(3) (6) and Boc-NH-(R)-gamma-Caa-(R)-beta-Caa-OCH(3) (12), whereas it is insignificant or totally absent for its protonated diastereomeric pair Boc-NH-(S)-beta-Caa-(S)-gamma-Caa-OCH(3) (1) and Boc-NH-(S)-gamma-Caa-(S)-beta-Caa-OCH(3) (7). Further, ESI negative ion tandem mass spectrometry has also been found to be useful for differentiating these isomeric peptide acids.

Palladium-Catalyzed Double Allylation of Sugar-Imines by Employing Tamaru-Kimura's Protocol: Access to Unnatural Iminosugars.

Conversion of vinyl pyranosylamine and vinyl furanosylamines to 2,6- and 2,5-disubstituted pyrrolidine and piperidine iminosugars, respectively, in one pot was developed using Kimura and Tamaru's procedure, where a Pd salt in the presence of Et2Zn was used for the domino reaction. In this procedure, double allylation, which involves nucleophilic allylation-heterocyclization, took place to give desired nitrogen heterocycles. This strategy was further elaborated to synthesize some unnatural deoxycalystegines, hydroxylated pyrrolidines, piperidines, and indolizidine analogues.

Total Synthesis of the Proposed Structure of Maltepolide C.

The first total synthesis of the proposed structure of cytotoxic macrolide maltepolide C has been achieved via an E-selective intramolecular Heck cyclization as a key step. Other key features of the synthesis are Z-selective Wittig olefination, Sharpless asymmetric dihydroxylation followed by Williamson-type cyclo-etherification, Brown asymmetric allylation, and Noyori reduction of an alkynone. Detailed NMR study confirms the structure and stereochemistry of the synthetic maltepolide C unambiguously. However, the deviation of the spectra of the synthetic maltepolide C from those of the natural maltepolide C indicates a possible error in the original structural assignment.

Unusual cyclo-tetra and hexa peptidation of bis-boc-cystine with cystine-di-OMe: one step preparation of the novel 32- and 48-membered cyclotetracystine and cyclohexacystine.

The unprecedented formation of 32- and 48-membered macrocycles that inscribe 4 and 6 cystine units, in the peptidation of bis-Boc-cystine with cystine di-OMe is reported.

Design, Synthesis, and Characterization of Tyrosinophanes, a Novel Family of Aromatic-Bridged Tyrosine-Based Cyclodepsipeptides.

A simple two-step design strategy has been developed for the synthesis of a large variety of a new class of tyrosine-based aromatic (Ph or Pyr) bridged cyclodepsipeptides (tyrosinophanes). The design is flexible with respect to the size of the ring and the nature of the bridging unit and permits the incorporation of a variety of amino acid residues inside or outside or both inside and outside the ring as illustrated here with the preparation of tyrosine-based macrocycles with aromatic (Ph or Pyr), cage-like alicyclic (adamantane) or simple polymethylene bridging units in ring sizes varying from 26-membered to 78-membered and containing leucine residues as part of the ring or as pendants on the exterior or both inside and outside the macrocyclic ring. (1)H NMR, FT-IR, and CD studies have indicated open-ring structures for these macrocycles. A noteworthy feature of the strategy is the formation of the 1 + 1 + 1 + 1 catenane arising from the interlocking of sebacoyl-bridged tyrosine rings. The potential of tyrosinophanes to serve as simple aromatic hosts in the study of pi-cation type interactions was illustrated with Pyr-bridged macrocycles (6b-8b) using N-methylacridinium hexafluorophosphate as the pyridinium guest. The K(assoc) value with 6b was found to be 8.95 x 10(3) M(-)(1).

A 13C-NMR study of non-planar distortions in amides.

The technique of 13C-NMR spectroscopy of oriented systems to problems of biological importance has been suggested and used to investigate non-planar distortions in substituted amides--models for peptides. The studies in conjunction with the proton magnetic resonance data on 15N-[13C]methyl[13C]formamide oriented in a nematic solvent provide all the direct dipolar couplings between the interacting nuclei in the system. When the 13C- and the 1H-NMR experiments are performed under non-identical conditions, 22 different direct dipolar couplings are obtained. It is demonstrated that they can be used to determine unambiguously non-planar distortions around the nitrogen atom together with other geometrical data and the molecular order.

NMR study of dideoxynucleotides with anti-human immunodeficiency virus (HIV) activity.

The molecular structures of 3'-azido-2',3'-dideoxyribosylthymine 5'-triphosphate (AZTTP), 2',3'-dideoxyribosylinosine 5'-triphosphate (ddlTP), 3'-azido-2',3'-dideoxyribosylthymine 5'-monophosphate (AZTMP) and 2',3'-dideoxyribosyladenine 5'-monophosphate (ddAMP) have been studied by NMR to understand their anti-HIV activity. For ddAMP and ddITP, conformations are almost identical with their nucleoside analogues with sugar ring pucker equilibriating between C3'-endo (approximately 75%) and C2'-endo (approximately 25%). AZTMP and AZTTP on the other hand show significant variations in the conformational behaviour compared with 3'-azido-2',3'-dideoxyribosylthymine (AZT). The sugar rings for these nucleotides have a much larger population of C2'-endo (approximately 75%) conformers, like those observed for natural 2'-deoxynucleosides and nucleotides. The major conformers around C5'-O5', C4'-C5' and the glycosidic bonds are the beta 1, gamma + and anti, respectively.

NMR study of the peptide present in the principal neutralizing determinant (PND) of HIV-1 envelope glycoprotein gp120.

The peptide sequence Gly-Pro-Gly-Arg-Ala-Phe (GPGRAF) is present in many principal neutralizing determinants (PND) of the human immunodeficiency virus type-1 (HIV-1). It has been shown that peptides from the PND sequence contain a significant beta turn in the conserved Gly-Pro-Gly-Arg sequence. In order to find out whether or not the smaller subunits also contain this turn, we have studied the NMR of a hexapeptide [GPGPRAF, peptide (I)], a heptapeptide Gly-Pro-Gly-Arg-Ala-Phe-Cys [GPGRAFC, peptide (II)] and a dodecapeptide [GPGRAFGPGRAF, peptide (III)], retaining the side chain protecting groups. Although the majority of conformations for these peptides are disordered, there is a considerable propensity of structures with beta turn in the GPGR sequence. While peptide (I) and peptide (III) seem to have both type I and type II beta turn conformations, peptide (II) shows a propensity of only type II beta turn. The nascent structures obtained in these peptides may get stabilized as the receptor binding conformation in the presence of the receptors, thus playing a significant role in vaccine development against HIV.

NMR study of cyclic peptides with renin inhibitor activity.

Several cyclic analogues of renin inhibitors, based on Glu-D-Phe-Lys motif have been investigated by NMR spectroscopy and molecular dynamics calculations (MD). The 15 membered macrocycle, resulting from Glu and Lys side-chain cyclization, exhibits conformational preference. The structural evidence from NMR shows the presence of hydrogen bond between Lys NH and Glu side-chain carbonyl, resulting in a 10 membered pseudo beta-turn-like structure. The structure of the cyclic moiety is similar in all the peptides, which takes at least two conformations around Calpha-Cbeta in Glu side chain. The restrained MD calculations further support such observations and show that the macrocycle is fairly rigid, with two conformations about the Glu Calpha-Cbeta bond. The linear peptide appendages, which are essential for activity in cyclic peptides, show an extended structure in the beta-region of Ramchandran plot. These calculations also demonstrate that for the most active peptide, two major conformers each exist about the Calpha-CO bond of the Lys, D-Trp and Leu residues. In this peptide, the cyclic moiety presents a negatively charged surface formed due to the carbonyl oxygens, which are thus available to form hydrogen bonds with the receptor. The linear fragment presents further binding sites with a surface which has the hydrophobic side chains of D-Trp, Leu and D-Met on one side and carbonyls on the other side.

2,5-Anhydro sugar diacid and 2,5-anhydro sugar diamine based C 2 symmetric peptidomimetics as potential HIV-1 protease inhibitors.

Conformationally constrained molecular frameworks of the 2,5-anhydro sugar diacid (9) and 2,5-anhydro sugar diamines (10, 11) were used to construct architecturally beautiful novel C 2 symmetric peptidomimetics 1-8. Although none of these compounds showed any significant HIV-1 protease inhibitory activity, further refinements in design may lead to protease inhibitors based on these rigid carbohydrate-derived scaffolds.

Differentiation of three pairs of positional isomers of hybrid peptides with repeats of phenylalanine-beta3-h-valine/beta3-valine-phenylalanine by electrospray ionization tandem mass spectrometry.

Electrospray ionization ion trap mass spectrometry has been used to distinguish three pairs of positional isomers of a new series of N-blocked hybrid peptides derived from repeats of phenylalanine(D)-beta3-h-valine/beta3-h-valine-phenylalanine(D) (FbetaV/betaVF) non-natural amino acids. MSn of protonated isomeric peptides produces characteristic fragmentation involving the peptide backbone, the Boc group and the side chain. FbetaV-peptides can be distinguished from betaVF-peptides by the loss of R-OH from [M+H-Boc+H]+, which is either of relatively low abundance or totally absent for the latter peptides. In contrast, betaVF-peptides show abundant Mannich base characteristic ions by the elimination of ammonia, and imine due to a retro-Mannich cleavage. This fragmentation is absent for FbetaV-peptides. When beta-valine is at the C-terminus, abundant b+(n-1) ions are produced. This is ascribed to the probable formation of a stable diketopiperazine structure, and this has been supported by the loss of H2O and CO in the CID spectra of b+(n-1) ions. The hybrid dipeptide acids have also been distinguished in negative ion mass spectrometry.

Furanoid sugar amino acids as dipeptide mimics in design of analogs of vasoactive intestinal peptide receptor binding inhibitor.

In this study we describe the development of peptidomimetic analogs of the potent vasoactive intestinal peptide receptor binding inhibitor, Leu(1) -Met(2) -Tyr(3) -Pro(4) -Thr(5) -Tyr(6) -Leu(7) -Lys(8) -OH 1, by incorporating furanoid sugar amino acids (SAAs) 2-4 into the molecule. The furanoid SAAs 2-4 were used as dipeptide isosteres to replace Tyr(3) -Pro(4) or Pro(4) -Thr(5) in sequence 1. The resulting analogs 5-9 were tested for their anti-cancer activities in vitro, following the standard MTT assay on a panel of human cancer cell lines. One of the potent analogs, 6a was tested in vivo for tumor regression on primary colon tumor xenografted nude mice. Our experimental results suggest that many of these analogs show either retention or enhancement of biological activity.

1H NMR study of the sugar pucker of 2',3'-dideoxynucleosides with anti-human immunodeficiency virus (HIV) activity.

The sugar ring conformations of 2',3'-dideoxyribosyladenine (ddA), 2',3'-dideoxyribosylcytosine (ddC), 2',3'-dideoxyribosylguanine (ddG), 2',3'-dideoxyribosylhypoxanthine (ddI), 3'-azido-2',3'-dideoxyribosylthymine (AZT), 3'-azido-2',3'-dideoxyribosyluracil (AZU) and 3'-fluoro-2',3'-dideoxyribosylthymine (FddT) have been investigated by 1H NMR spectroscopy. While the sugar ring in FddT exists almost totally in C2'-endo geometry, other nucleosides show equilibrium between sugar puckers of C3'-endo family (N-type) and C2'-endo family (S-type). For unsubstituted dideoxynucleosides C3'-endo conformer is favoured (congruent to 75%), whereas for AZT and AZU both the conformers have almost equal populations. Unlike X-ray diffraction studies, the NMR results do not support the suggestion that C3'-exo sugar puckers are desirable for the anti-HIV activity of these nucleosides.

N.M.R. of 'oriented' monosubstituted amides--cis and trans N-methyl formamide.

The n.m.r. spectrum of N-methyl formamide oriented in a lyotropic liquid crystal solvent shows the presence of the cis as well as the trans forms. Analysis of the spectra due to both the species has been carried out. Possible non-planar distortions at the nitrogen atom for the trans species have been computed using the derived dipolar couplings and the available structural data.