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Expansion of the scope of the 1,3-diaza-Claisen rearrangement beyond bridged-bicyclic tertiary allylic amines has been investigated through a tethering strategy. Isothioureas tethered to tertiary allylic amines are converted to carbodiimides through a reaction with AgOTf/Et3N. Intramolecular cyclization of the tertiary allylic amine to the carbodiimide equilibrates with a zwitterionic intermediate. Heating the carbodiimide/zwitterion affords a rearrangement product. Heating carbodiimide/zwitterion with a deuterated allyl group results in the scrambling of the deuterium label, which is consistent with an ionic mechanism involving heterolytic cleavage of the allylic C-N bond, followed by trapping of the allyl cation at either terminal carbon. The ionic mechanism is attributed to silver salt contamination since pushing deuterium-labeled carbodiimide/zwitterion through silica gel prior to heating results in clean deuterium transposition consistent with a sigmatropic mechanism, and adding back silver salts results in deuterium scrambling. Overall, the tethering strategy broadens the scope of the rearrangement to simpler allylic substrates. Density functional theory (DFT) calculations of the sigmatropic rearrangement are in agreement with reactivity trends observed with reactions run under silver-free conditions.

Isothioureas tethered to bridged-bicyclic tertiary allylic amines can be converted to carbodiimides through reaction with Hg(II) salts. Intramolecular cyclization of the tethered tertiary allylic amines to the carbodiimides afford zwitterionic intermediates that undergo 1,3-diaza-Claisen rearrangements, affording highly substituted tricyclic guanidines.

Strained, tertiary, allylic, amine 2-benzyl-2-azabicyclo[2.2.1]hept-5-ene reacts with in situ generated carbodiimides in a 1,3-diaza-Claisen rearrangement to afford structurally interesting bicyclic guanidines. Use of more electron deficient carbodiimides makes these rearrangements more facile; however, there are not sufficient methods for the synthesis of highly electron deficient carbodiimides. Herein reported is the exploration of the synthesis of such carbodiimides from parent ureas and isothioureas and their use in 1,3-diaza-Claisen rearrangements.

Bridged bicyclic tertiary allylic amines aza-norbornene 1 and isoquinuclidene 2 add to isocyanates, isothiocyanates and in situ-generated carbodiimides to form zwitterionic intermediates that undergo 1,3-diaza-Claisen rearrangements to afford highly substituted ureas, thioureas and guanidines respectively. Aza-norbornene 1 is significantly more reactive towards 1,3-diaza-Claisen rearrangements than isoquinuclidene 2. This reactivity difference is most likely due to the inherent ring strain in the aza-bicyclo[2.2.1]heptene ring system of aza-norbornene 1. The most apparent reactivity trend of the heterocumulenes is that the most electron-deficient heterocumulenes are more reactive toward 1,3-diaza-Claisen rearrangements. The introduction of a new stereocenter α- to the nucleophilic nitrogen in aza-norbornene 1 and isoquinuclidine 2 decreases the reactivity towards 1,3-diaza-Claisen rearrangements, while the exo-diastereomers 3b and 4b are less reactive than the corresponding endo-diastereomers 3a and 4a. Isocyanates that bear an electron-withdrawing group react with allylic amines 1-3b to afford mixtures of ureas and isoureas, however, with excess isocyanate and heat, thermodynamic equilibration is possible affording ureas. Inspired by this observation a one-pot reaction of isocyanates with amines 1, 2, and 3b followed by BF3•OEt2-catalyzed isomerization of the urea/isourea mixture was developed that affords the corresponding ureas in excellent yields.

A methodology is presented for the facile synthesis of Arg-containing peptides modified at the guanidine headgroup as substituted amidine cores. This process allows for the iterative construction of these Arg isosteres while the peptide is being built out on the solid support, providing a high potential for diversity in substitution pattern in the resulting peptide. A series of N-Pmc-substituted thioamides were condensed with deprotected δ-N Orn-bearing peptides while attached to the solid support using Mukaiyama's reagent as coupling reagent, yielding isosteric Arg-containing analogs. Peptides were cleaved using trimethylsilyl trifluoromethanesulfonate/TFA and analyzed in their crude form in order to illustrate the amenability of this process toward production of peptide isolates in high crude purity. Arg-containing peptides having a single Arg isostere were utilized to show the general utility of this approach as well as a multiple-Arg-containing construct, illustrating the amenability of this method toward stepwise construction of differently substituted amidine headgroups within the same peptide.

A convenient approach is presented for the preparation of substituted amidines through the fusion of N-Pmc-substituted thioamides with amines, facilitated by a variety of thiophilic reagents. Assorted thiophiles were evaluated for their effectiveness as condensation reagents between the two partners, with the Mukaiyama reagent emerging as the optimal candidate for this process. The scope and limitations of this method were explored, revealing interesting synthetic challenges and constraints. Many of these limitations were overcome through the application of alternative reaction pathways in the construction of troublesome products. Treatment of the resultant Pmc-amidines with trifluoroacetic acid and trimethylsilyl trifluoromethanesulfonate cleanly afforded the deprotected substituted amidines.

Condensation of methyl 3-benzyl-1,2,3,4,5,6-hexahydroazepino[4,5-b]indole-5-carboxylate (3) with methyl Z-4-formylhex-3-enoate (6) gave cis-fused dimethyl 5-benzyl-4-ethyl-2,4a,5,6,7,12-hexahydro-1H-benzo[2,3]azepino[4,5-b]indole-2,12b-dicarboxylate and its trans-fused diastereomer. Selective reduction of the less hindered ester group with sodium borohydride to an alcohol ester, tosylation, debenzylation, and cyclization gave racemic catharanthine (1).

An overview of the facile and high-yielding EDCI-mediated reaction of amines with N-Pmc-N′-alkyl thioureas to afford guanidines is presented, in which the general scope and limitations of the reaction are probed. It was found that the N-sulfonyl- N′-substituted thioureas cannot possess internal nucleophiles or disubstitution, and that the incoming amine must possess adequate nucleophilicity in order for the reaction to be viable. However, it is noted that the guanidine products can be accessed through two possible synthetic approaches, and that a simple reversal of amine and thiourea substituents allows the reaction to proceed successfully.

The synthesis of two conformationally constrained lysine analogues is reported. The synthesis of the novel analogue 1 based on the 3-aza-bicyclo[3.1.0]hexane system is accomplished from the known tricycle 3 in eight steps. The synthesis of the analogue 2 is accomplished in eight steps from 4-hydroxy proline. Both analogues are synthesized appropriately protected for Fmoc/Boc solid-phase peptide synthesis.

Bridged bicyclic tertiary allylic amines aza-norbornene 1 and isoquinuclidene 2 add to isocyanates, isothiocyanates, and in situ-generated carbodiimides to form zwitterionic intermediates that undergo 1,3-diaza-Claisen rearrangements to afford highly substituted ureas, thioureas, and guanidines, respectively. Aza-norbornene 1 is significantly more reactive toward 1,3-diaza-Claisen rearrangements than isoquinuclidene 2. This reactivity difference is most likely due to the inherent ring strain in the aza-bicyclo[2.2.1]heptene ring system of aza-norbornene 1. The most apparent reactivity trend of the heterocumulenes is that the most electron-deficient heterocumulenes are more reactive toward 1,3-diaza-Claisen rearrangements. The introduction of a new stereocenter α- to the nucleophilic nitrogen in aza-norbornene 1 and isoquinuclidine 2 decreases the reactivity toward 1,3-diaza-Claisen rearrangements, while the exodiastereomers 3b and 4b are less reactive than the corresponding endodiastereomers 3a and 4a. Isocyanates that bear an electron-withdrawing group react with allylic amines 1-3b to afford mixtures of ureas and isoureas; however, with excess isocyanate and heat, thermodynamic equilibration is possible affording ureas. Inspired by this observation, a one-pot reaction of isocyanates with amines 1, 2, and 3b followed by BF3·OEt2-catalyzed isomerization of the urea/isourea mixture was developed that affords the corresponding ureas in excellent yields.

The reaction of N-enoyloxazolidinones with diphenylsulfonium isoproylide in the presence of Lewis acids is explored. The diastereoselectivity of the reaction of N-enoyloxazolidinones 1a and 1b with diphenylsulfonium isopropylide could be mediated by the addition of some Lewis acids. The reaction of N-enoyloxazolidinone 4 with diphenylsulfonium isoproylide was also explored with chiral Lewis acids. When this reaction was run with bis(oxazoline) ligand 6 and a number of Lewis acids, product was obtained in as high as 95% ee. A loss of stereoselectivity was observed with less than stoichiometric amounts of Lewis acid.

This paper reports the synthesis of a small library of peptide-derived ruthenium complexes as potential oxidation catalysts. Evaluation of the solid-supported complexes identified several potentially promising epoxidation catalysts. Liberation of the promising catalysts from the solid-support resulted in catalysts with improved activity.

Evaluation of polymer-supported metal complexes as oxidation catalysts revealed that product distribution is not dependent on metal stoichiometry, or resin but is dependent on the oxidant, oxidant stoichiometry and the metal incubation times. The model systems investigated under the optimized conditions afforded highly reproducible results, indicating that this methodology can be used to reliably screen libraries of metal complexes.

Improvements in the stereoselective alkylation of a bicyclic lactam derived from pyroglutamic acid are described. This methodology is used to synthesize a conformationally constrained homo-glutamic acid analog.

The cyclopropanation of unsaturated lactams 1 and 3 derived from pyroglutamic acid with nucleophilic alkylidene transfer reagents is investigated. Good-to-modest yields of cyclopropanes were obtained with most sulfur ylides explored. Syn/anti selectivity was found to be dependent on the synthon as well as the sulfur ylide. This cyclopropanation methodology is used in the synthesis of arginine and glutamic acid analogues.

A strategy for the synthesis of peptide mimics of the poly-l-proline type II secondary structure from 4-substituted prolines is presented. Dimeric and trimeric oligomers composed of 4-substituted prolines are shown by NMR to preferentially populate the poly-l-proline type II secondary structure in both CDCl3 and D2O. Oligomers composed of 4-substituted prolines thus imitate the desired backbone conformation and are able to incorporate non-prolyl side chains on the proline backbone.

Reexamination of the solution conformation of a series of N-Boc O-benzyl protected proline oligomers (dimer-hexamer) by 500 MHz NMR reveals that these oligomers adopt the poly-L-proline type II conformation in solution and do not require a critical chain length for conformational stability as has been previously reported.

A series of novel C11-substituted derivatives of azaelliptitoxin (azatoxin) have been synthesized and tested for their inhibitory activity against human DNA topoisomerase II. Incorporation of a C11 polyamine or amine resulted in an increase in the intercalation properties of the drug and a decrease of topoisomerase II activity. The structure-activity relationship (SAR) profile of the nonintercalating C11 anilino azatoxin class follows the SAR of the (anilino)acridine family. 11-(4-Cyanoanilino)azatoxin (14) was found to be the most active analog in this series, exhibiting approximately 10-fold higher activity than azatoxin 12 and etoposide.

A series of 1-aryl-1,2,3,4-tetrahydro-β-carbolines were synthesized, as “truncated” analogs of the novel agent, azatoxin, and assayed in vitro for their ability to induce proteinassociated DNA strand breaks in the presence of DNA and human DNA topoisomerase II.

A novel methodology for the synthesis of substituted amides is presented using The condensation step can be followed by removal of the protecting group i the presence of formic acid or TFA.

10-Aryl-3a,4,10,10a-tetrahydrofuro[3,4-b]carbazol-1-ones were synthesized by an intramolecular Diels-Alder reaction. Exo / endo product ratios were found to be sensitive to both solvent and temperature. The synthesis of iso elliptitoxin 2 , a hybrid molecule of ellipticine and 4'-demethylpodophyllotoxin, is presented.

10-Aryl-3a,4,10,10a-tetrahydrofuro[3,4-b]carbazol-1-ones were synthesized by an intramolecular Diels-Alder reaction. Exo / endo product ratios were found to be sensitive to both solvent and temperature. The synthesis of iso elliptitoxin 2 , a hybrid molecule of ellipticine and 4'-demethylpodophyllotoxin, is presented.

A series of novel C11-substituted derivatives of azaelliptitoxin (azatoxin) have been synthesized and tested for their inhibitory activity against human DNA topoisomerase II. Incorporation of a C11 polyamine or amine resulted in an increase in the intercalation properties of the drug and a decrease of topoisomerase II activity. The structure-activity relationship (SAR) profile of the nonintercalating C11 anilino azatoxin class follows the SAR of the (anilino)acridine family. 11-(4-Cyanoanilino)azatoxin (14) was found to be the most active analog in this series, exhibiting approximately 10-fold higher activity than azatoxin 12 and etoposide.

The reaction of N-enoyloxazolidinones with diphenylsulfonium isoproylide in the presence of Lewis acids is explored. The diastereoselectivity of the reaction of N-enoyloxazolidinones 1a and 1b with diphenylsulfonium isopropylide could be mediated by the addition of some Lewis acids. The reaction of N-enoyloxazolidinone 4 with diphenylsulfonium isoproylide was also explored with chiral Lewis acids. When this reaction was run with bis(oxazoline) ligand 6 and a number of Lewis acids, product was obtained in as high as 95% ee. A loss of stereoselectivity was observed with less than stoichiometric amounts of Lewis acid.

Tertiary allylic amines with an electron-deficient alkene react with isocyanates and isothiocyanates to give highly substituted ureas and thioureas arising from formal 1,3-diaza-Claisen rearrangements. Isocyanates and isothiocyanates with strong electron-withdrawing groups are more reactive. Similarly, the data suggest that a stronger electron-withdrawing substituent on the alkene favors a faster reaction, but this may be offset by sterics in the cyclic transition state.

Tertiary allylic amines with an electron-deficient alkene react with isocyanates and isothiocyanates to give highly substituted ureas and thioureas arising from formal 1,3-diaza-Claisen rearrangements. Isocyanates and isothiocyanates with strong electron-withdrawing groups are more reactive. Similarly, the data suggest that a stronger electron-withdrawing substituent on the alkene favors a faster reaction, but this may be offset by sterics in the cyclic transition state.

A strategy for the solid-phase diversification of PPII mimic scaffolds through guanidinylation is presented. The approach involves the synthesis N-Pmc-N'-alkyl thioureas as diversification reagents. Analogues of Fmoc-Orn(Mtt)-OH can be incorporated into a growing peptide chain on Wang resin. Side chain deprotection with 1% TFA/CH2Cl2 followed by EDCI-mediated reaction of N-Pmc-N'-alkyl thioureas with the side chain amine affords arginine analogues with modified guanidine head groups. The scope, limitations, and incidental chemistry are discussed.

An overview of the facile and high-yielding EDCI-mediated reaction of amines with N-Pmc-N′-alkyl thioureas to afford guanidines is presented, in which the general scope and limitations of the reaction are probed. It was found that the N-sulfonyl- N′-substituted thioureas cannot possess internal nucleophiles or disubstitution, and that the incoming amine must possess adequate nucleophilicity in order for the reaction to be viable. However, it is noted that the guanidine products can be accessed through two possible synthetic approaches, and that a simple reversal of amine and thiourea substituents allows the reaction to proceed successfully.

Based on the X-ray crystal structure of cAMP-dependent protein kinase (PKA) with the endogenous inhibitor PKI and the X-ray crystal structure of cyclin-dependent kinase 2 (CDK2) with a substrate peptide, a proposal is put forth that some protein kinases bind peptide substrates in their active sites in the poly-L-proline type II (PPII) conformation. In this work, PPII peptide mimics are evaluated as pseudosubstrate inhibitors of cGMP-dependent protein kinase (PKG) to explore if PKG also binds peptide substrates in the PPII conformation. Inhibition data of our PPII mimetics provide evidence that the P-1, P-2, and P-3 residues of substrate peptides bind in the PPII conformation (phi approximately -75 degrees, psi approximately 145 degrees). In addition, the inhibition data also suggest that the P-1, P-2, and P-3 residues in substrate peptides bind with a gauche(-) chi1 angle.

[reaction: see text] N-Alkyl-N'-tosylthioureas activated by EDCI react with azanorbonenes at room temperature through a 1,3-diaza-Claisen rearrangement, affording highly substituted, bicyclic guanidines in moderate to good yields.

This paper describes conformational studies of proline-templated amino acids (PTAAs) based on the 3-azabicyclo[3.1.0]hexane system as well as conformational studies on short peptides composed of these PTAAs. NOE data, coupling constants, and molecular modeling are consistent with a flattened boat conformation for monomeric and oligomeric residues based on this bicyclic system. NMR studies on dimeric and trimeric oligomers are consistent with a populated poly-L-proline type II conformation in CDCl3 and D2O. Solution studies and molecular modeling predicts phi approximately -70 degrees, psi approximately 131 degrees, chi 1 approximately -57 degrees, and chi 2 approximately -158 degrees for oligomeric residues.

[reaction: see text] Solid-phase guanidinylation of proline-templated amino acids is studied as a diversification strategy of poly-L-proline type II scaffolds.

This paper reports the synthesis of a small library of peptide-derived ruthenium complexes as potential oxidation catalysts. Evaluation of the solid-supported complexes identified several potentially promising epoxidation catalysts. Liberation of the promising catalysts from the solid-support resulted in catalysts with improved activity.

Eighteen analogues of the nonintercalative DNA topoisomerase II (topo II)-active epipodophyllotoxin-ellipticine hybrid, azatoxin, were synthesized and evaluated for their ability to induce topo II-mediated DNA strand breaks in vitro. In general, the SAR profile of the azatoxins showed more homology with that of the epipodophyllotoxins than with the ellipticines. Of the compounds studied, only fluoro substitution at the 8-, 9, and 10-positions of azatoxins enhanced activity, with 9-fluoroazatoxin being the most active compound in this series.

Azatoxin was rationally designed as a DNA topoisomerase II (top2) inhibitor [Leteurtre et al. , Cancer Res 52 : 4478–4483, 1992] and was also found to inhibit tubulin polymerization. Its cytotoxicity is due to action on tubulin at lower concentrations and on top2 at higher concentrations. At intermediate concentrations, the combination of the two mechanisms appears antagonistic [Solary et al. , Biochem Pharmacol 45 : 2449–2456, 1993]. The aim of this study was to design azatoxin derivatives that would act only on tubulin or on top2. Selective targeting of top2 or tubulin was tested using top2-mediated DNA cleavage assays, and tubulin polymerization and tubulin proteolysis assays, as well as COMPARE analyses of cytotoxicity assays in the National Cancer Institute in vitro Drug Screening Program. Selective inhibitors of top2 and tubulin polymerization have been obtained. Top2 inhibition, abolished by methylation at position 4′, was enhanced by the addition of a bulky group at position 11. Bulky substitution at position 11 determined different patterns of top2 cleavage sites and suppressed the action on tubulin. Selective inhibition of tubulin was obtained with 4′-methylazatoxin that was found to bind to the colchicine site. These results are consistent with those obtained in the podophyllotoxin family to which azatoxin is structurally related. Some azatoxin derivatives are under consideration for further preclinical development.