Synthesis of 2-Azaindolizines by Using an Iodine-Mediated Oxidative Desulfurization Promoted Cyclization of N-2-Pyridylmethyl Thioamides and an Investigation of Their Photophysical Properties

Iodine-mediated, oxidative desulfurization promoted cyclization of N-2-pyridylmethyl thioamides serves as an efficient and versatile method for the preparation of 2-azaindolizines (imidazo[1,5-a]pyridines) and rare 2-azaindolizine sulfur-bridged dimers. The 2-azaindolizines prepared in this manner are readily converted to a variety of fluorescent compounds by using transition-metal-catalyzed cross-coupling reactions. [reaction: see text].

Asymmetric [2 + 2] Cycloaddition: Total Synthesis of (−)-Swainsonine and (+)-6-Epicastanospermine

[reaction: see text] An asymmetric total synthesis of (-)-swainsonine and (+)-6-epicastanospermine is described from a common intermediate, which is obtained through diastereoselective [2 + 2] cycloaddition of dichloroketene to a chiral enol ether.

Synthesis of 1-homoaustraline

The 1,3-dipolar cycloaddition of a five-membered cyclic nitrone derived from malic acid and unsaturated D-threo-hexonolactone leads to a single adduct, which was transformed into 1-homoaustraline via a reaction sequence involving rearrangement of the six-membered lactone ring into the five-membered one, removal of the terminal carbon atom from the sugar chain, reduction of the lactone fragment into triol, protection of primary hydroxy groups, mesylation of the secondary one, cleavage of the N-O bond, and the intramolecular alkylation of the nitrogen atom.

Au(I)-Catalyzed Annulation of Enantioenriched Allenes in the Enantioselective Total Synthesis of (−)-Rhazinilam

Highly stereoselective Au(I)-catalyzed pyrrole additions to enantioenriched allenes afford a unique entry to optically active heterocycles. Asymmetric quaternary carbons can be installed with concurrent heterocycle annulation utilizing this methodology. The enantioenriched allenes are conveniently obtained by catalytic asymmetric acyl halide-aldehyde cyclocondensations and SN2' ring opening of the resulting enantioenriched beta-lactones. An enantioselective total synthesis of (-)-rhazinilam highlights the potential utility of this reaction technology in target-oriented synthesis.

Synthesis of 2,4,6-Trisubstituted Chiral Piperidines and (−)-Dendroprimine by One-Pot Asymmetric Azaelectrocyclization Protocol

[reaction: see text] Stereocontrolled synthesis of 2,4,6-trisubstituted piperidine diastereomers has been realized from common intermediates, obtained by a one-pot azaelectrocyclization protocol. Based on the method, the asymmetric synthesis of an indolizidine alkaloid, (-)-dendroprimine, was achieved.

Synthesis of Pentahydroxy Indolizidine Alkaloids Using Ring Closing Metathesis: Attempts To Find the Correct Structure of Uniflorine A

Ring closing metathesis of d-glucose derived diene-substrate containing nitrogen functionality followed by asymmetric dihydroxylation afforded sugar substituted dihydroxylated pyrrolidines 8a-c which on 1,2-acetonide deprotection and reductive amination afforded putative uniflorine A 2a and its analogues 2b-c, respectively.

Antitumor agents 251: synthesis, cytotoxic evaluation, and structure-activity relationship studies of phenanthrene-based tylophorine derivatives (PBTs) as a new class of antitumor agents

Polar phenanthrene-based tylophorine derivatives (PBTs) were designed, synthesized and evaluated as potential antitumor agents. These compounds contain a core phenanthrene structure and can be synthesized efficiently in excellent yield. The newly synthesized PBTs were evaluated for cytotoxic activity against the A549 human cancer cell line. Among them, N-(2,3-methylenedioxy-6-methoxy-phenanthr-9-ylmethyl)-L-2-piperidinemethanol (34) and N-(2,3-methylenedioxy-6-methoxy-phenanthr-9-ylmethyl)-5-aminopentanol (28) showed the highest potency with IC50 values of 0.16 and 0.27 microM, respectively, which are comparable to those of currently used antitumor drugs. A structure-activity relationship (SAR) study was also explored to facilitate the further development of this new compound class.

Intramolecular 5-endo-Trig Aminomercuration of β-Hydroxy-γ-alkenylamines: Efficient Route to a Pyrrolidine Ring and Its Application for the Synthesis of (+)-Castanospermine and Analogues

The intramolecular aminomercuration reaction of sugar-derived beta-hydroxy-gamma-alkenylamines 8a-c undergoes 5-endo-trig cyclization in high yield. The sugar-substituted pyrrolidines thus obtained were elaborated to the synthesis of polyhydroxylated indolizidine alkaloids, namely, castanospermine 1a, 1-epi-castanospermine 1b, and 8a-epi-castanospermine 1c, having promising glycosidase inhibitory activities.

Syntheses of ficuseptine, juliprosine, and juliprosopine by biomimetic intramolecular Chichibabin pyridine syntheses

[structure: see text] Biomimetic intramolecular Chichibabin pyridine syntheses using two molecules of an aldehyde and 4-aminobutanal dimethyl acetal (6) proceed efficiently in AcOH at 95 degrees C to give 2,3-dihydro-1H-indolizinium salts. Reaction occurs at 25 degrees C if 1-pyrroline (5) is used instead of 6. This reaction has been used for a one-step synthesis of ficuseptine (1) and the first syntheses of juliprosine (2) and juliprosopine (17t), which is now assigned as the trans stereoisomer.

Total synthesis and biological evaluation of 22-hydroxyacuminatine

A total synthesis of 22-hydroxyacuminatine, a cytotoxic alkaloid isolated from Camptotheca acuminata, is reported. The key step in the synthesis involves the reaction of 2,3-dihydro-1H-pyrrolo[3,4-b]quinoline with a brominated phthalide to generate a substituted pentacyclic 12H-5,11a-diazadibenzo[b,h]fluoren-11-one intermediate. Despite its structural resemblance to camptothecin and luotonin A, a biological evaluation of 22-hydroxyacuminatine in a topoisomerase I-deficient cell line P388/CPT45 has confirmed that the observed cytotoxicity is not due to topoisomerase I inhibition, even though 22-hydroxyacuminatine has a hydroxyl group that can theoretically hydrogen bond to Asp533. This result is consistent with the hypothesis that pi-pi stacking is more important than hydrogen-bonding interactions in determining topoisomerase I inhibitor binding in the ternary cleavage complex.

Synthesis and biological evaluation of B-ring analogues of (−)-rhazinilam

Three macrocyclic analogues of rhazinilam 1 having a 11- or 12-membered B-ring with an endocyclic carbamate group or an amino-acid residue were synthesized from the natural product. These analogues 3 and 4 displayed a very low activity on tubulin. Thirty N-1 and C-16 substituted analogues of rhazinilam were also synthesized regioselectively from rhazinilam. Stereochemical analyses showed that N-1 and C-16alpha analogues have the same conformation as rhazinilam, whereas C-16beta analogues adopt a different conformation for rings B and D. All N-1 and C-16 analogues were less active than rhazinilam on tubulin, though analogues 5a, 6aalpha, 6balpha, and 6f having the less bulky substituents retained close affinities. A few analogues either active (like 6f) or inactive (like 5o) on tubulin showed significant inhibition of the growth of KB cancer cells.

A Versatile Approach for the Asymmetric Syntheses of (1R,9aR)-Epiquinamide and (1R,9aR)-Homopumiliotoxin 223G

[reaction: see text] Using 5b as a common intermediate, the first asymmetric synthesis of (-)-epiquinamide (4) and a formal asymmetric synthesis of (-)-homopumiliotoxin 223G (2) is described. A key feature of our approach is the flexible introduction of a functionalized C(4) side chain to (S)-3-benzyloxyglutarimide 7 in a regio- and diastereoselective manner. Utilization of a tandem Swern oxidation-Grignard addition strategy efficiently prevented racemization. An unexpected NaN(3)-promoted methanesulfonic acid elimination yielded 17, a reaction which could be useful for the syntheses of 8-dehydrodesmethylpumiliotoxins such as alkaloid 235C (3).

Synthesis of Enantiopure Indolizinones by Cascade Ring Enlargements of 4‘-Chlorospiro[cyclopropane-1,5‘-isoxazolidines]

2-Chloro-2-cyclopropylideneacetates (1-Me and 1-Et) and their spiropentane analogues 2 cycloadd enantiopure five-membered cyclic nitrones to give the corresponding adducts (quantitatively, four examples), which undergo cascade ring enlargements to yield indolizinone derivatives (53-70%, four examples). The ring enlargement process is triggered by the abstraction of a bridgehead proton induced by a base and can be suppressed by the presence of a bulky substituent nearby, such as a (triisopropylsilyl)oxy group.

A General, Convergent Strategy for the Construction of Indolizidine Alkaloids: Total Syntheses of (−)-Indolizidine 223AB and Alkaloid (−)-205B

N-Toluenesulfonyl aziridines comprise effective second electrophiles in the solvent controlled three-component linchpin union of silyl dithianes for the stereocontrolled convergent elaboration of protected 1,5-amino alcohols. This tactic, in conjunction with a one-flask sequential cyclization, constitutes an effective general strategy for the construction of indolizidine and related alkaloids, illustrated here with the total syntheses of (-)-indolizidine 223AB (1) and alkaloid (-)-205B (2).

Expedient synthesis and structure-activity relationships of phenanthroindolizidine and phenanthroquinolizidine alkaloids

The total synthesis of alkaloids phenanthroindolizidine 1a, tylophorine 1b, and phenanthroquinolizidine 1c, has been achieved in 46%, 49%, and 42% overall yield, respectively, starting from the corresponding phenanthrene-9-carboxaldehyde. Compound exhibited potent inhibition activity in three human cancer cell lines, with IC(50) values ranging from 104 to 130 nM. The structure-activity relations of these alkaloids and some of their synthetic intermediates against the three cell lines were also described.

Syntheses of Biologically Active Natural Products and Leading Compounds for New Pharmaceuticals Employing Effective Construction of a Polycyclic Skeleton

Cascade reactions are useful methods for the construction of polycyclic skeletons, which are important cores for biological activities. A variety of cascade reactions carried out under multiple reaction conditions, such as pericyclic, polar, radical, and transition metal-catalyzed reaction conditions, have been investigated. Culmorin, pentalenene, pentalenic acid, deoxypentalenic acid, longiborneol, cedrandiol, 8,14-cedranoxide, atisirene, atisine, and estrane-type steroids were synthesized via the intramolecular double Michael reaction. Aza double Michael reaction was applied to the syntheses of tylophorine, epilupinine, tacamonine, and paroxetine. Furthermore, sequential Michael and aldol reactions were performed in both intramolecular and intermolecular manners, leading to the formation of polycyclic compounds fused to a four-membered ring. Synthesis of paesslerin A utilizing a multicomponent cascade reaction revealed an error in the proposed structure. Unique cascade reactions carried out under radical and transition metal-catalyzed reaction conditions were also investigated. With the combination of several cascade reactions, serofendic acids and methyl 7beta-hydroxykaurenoate, both of which have neuroprotective activity, were synthesized in a selective manner.

Total synthesis of two novel 5,6,7,8-tetrahydroindolizine alkaloids, polygonatines A and B

The structures of polygonatines A and B, two simple but structurally novel alkaloids, have been substantiated by synthesis. Cyclisation of 4-(1H-pyrrol-1-yl)butanoic acid or its ethyl ester produced 6,7-dihydroindolizin-8(5H)-one , formylation of which at C-3 followed by reduction afforded polygonatine A . Acetylation of this alkaloid followed by displacement of the acetate with ethanol yielded polygonatine B , possibly via an azafulvenium cation.

Divergent Cycloaddition and Ring-Closing Metathesis Approaches to Indolizidine and Pyrrolo[1,2-a]azepine Skeletons from a Chiral Precursor: An Expeditious Route to (−)-8-epi-Swainsonine Triacetate

[reaction: see text] A divergent strategy for the synthesis of diverse azabicyclic ring systems has been developed in which a chiral N-allylpyrrolidine derivative, obtained from a carbohydrate precursor was converted to (-)-8-epi-swainsonine triacetate by RCM and to a pyrrolo[1,2-a]azepine derivative and a 3-hydroxymethyl-substituted indolizidine by N-allylcarbohydrate nitrone and nitrile oxide cycloadditions.

Concise synthesis of 22-hydroxyacuminatine, cytotoxic camptothecinoid from Camptotheca acuminata, by pyridone benzannulation

A short, efficient synthesis of 22-hydroxyacuminatine, starting from a readily accessible hydroxy pyridone, is presented; key steps include a Heck coupling with methyl pentadienoate, a flash vacuum pyrolytic cyclization, and a Friedländer condensation.

Intramolecular Additions of Various π-Nucleophiles to Chemoselectively Activated Amides and Application to the Synthesis of (±)-Tashiromine

[reaction: see text] Vilsmeier-Haack type cyclizations proved to be particularly efficient for generating parts of the polycyclic cores of many alkaloids, although only monocyclizations have so far been reported. With the goal of rapidly and efficiently constructing polycyclic alkaloids, we decided to exploit the Vilsmeier-Haack reaction by utilizing iminium ions successively generated and trapped with tethered nucleophiles. To develop such a strategy, we had to set the first cyclization. This constitutes a great challenge in itself because amide activation conditions are usually not compatible with tethered nucleophiles, except for indoles and aromatic rings which have already been reported. This paper describes the comprehensive study of intramolecular addition of silyl enol ethers, allylsilanes, and enamines to chemoselectively activated formamides, aliphatic amides, and lactams. Good to excellent yields were obtained for the 5-exo, 6-exo, and 6-endo modes of cyclization. Moreover, we demonstrated that the species in solution after the cyclization are iminium ions. This is highly encouraging for the development of bis-cyclization strategies. An expeditious total synthesis of (+/-)-tashiromine is also reported.

Preparation of new nitrogen-bridged heterocycles. 58. Syntheses and intramolecular arene-pi interactions of 3-(allylthio)- and 3-(propargylthio)thieno[3,4-b]indolizine derivatives

Various thieno indolizine derivatives having an allylthio or propargylthio group at the 3-position were prepared and their intramolecular arene-pi interactions were investigated. Their 1H-NMR spectra showed significant low-field shifts (delta 0.10-0.34 ppm) to the 5-proton on the thieno indolizine ring, and this effect was the reverse to that observed in 3-(arylmethylthio)thieno indolizines. However, their UV spectra exhibited a characteristic absorption band due to the arene-pi interaction near 430 nm and these values were almost similar to those for arene-arene interaction of 3-arylmethylthio derivatives though their molar extinction coefficients were largely varied by the 3-substituents. Furthermore, both types of gauche conformations in which the intramolecular arene-pi interactions are possible in one form and impossible in the other were confirmed by X-ray analyses of some compounds.

Concise Synthesis of (±)-Rhazinilam through Direct Coupling

[reaction: see text] A concise synthesis of rhazinilam through direct, palladium-catalyzed, intramolecular coupling is described.

The 5-endo-trig cyclization of D-glucose derived gamma-alkenylamines with mercury (II) salts: synthesis of 1-deoxycastanospermine and its 8a-epi-analogue

The intramolecular aminomercuration of gamma-alkenylamines 1a, 1b and 4 was shown to afford the 5-endo-trig cyclized product exclusively in good yield. The utility of pyrrolidine derivatives thus obtained from D-glucose derived gamma-alkenylamines 1a and 1b was demonstrated in the synthesis of 1-deoxycastanospermine (3a) and 1-deoxy-8a-epi- castanospermine (3b).

Direct Synthesis of Monofunctionalized Indolizine Derivatives Bearing Alkoxymethyl Substituents at C-3 and Their Benzofused Analogues

[reaction: see text] Treatment of (Z)-2-pyridine and quinoline silylated vinylacetylenes at reflux with several alcohols in the presence of a suitable inorganic base (KOH, K2CO3, CsF, or KF) serendipitously gave 3-alkoxylmethylindolizines and the corresponding 1-alkoxymethylpyrrolo [1,2-a]quinolines and not the anticipated desilylated vinylacetylene derivatives. A mechanistic possibility for this unexpected chemical transformation is suggested.

1,3-Dipolar Cycloaddition Chemistry for the Preparation of Novel Indolizinone-Based Compounds

[Chemical reaction: See text] Starting from methyl 5-oxo-6-trifluoromethanesulfonyloxy-1,2,3,5-tetrahydroindolizine-8-carboxylate, obtained by a Rh(II)-catalyzed 1,3-dipolar cycloaddition reaction of 1-(2-benzenesulfonyl-2-diazoacetyl)pyrrolidin-2-one and methyl acrylate, several indolo- and furano-fused indolizinones were efficiently prepared. In the first case, a palladium-mediated C-N coupling of the triflate with a variety of substituted anilines provided the desired methyl 5-oxo-6-(arylamino)-1,2,3,5-tetrahydroindolizine-8-carboxylates in high yield. Methyl 6-(2-bromophenylamino)-5-oxo-1,2,3,5-tetrahydroindolizine-8-carboxylate as well as its decarboxylated analogue, 6-(2-bromophenylamino)-2,3-dihydro-1H-indolizin-5-one, were synthesized in excellent yield and were found to undergo an intramolecular Heck cyclization to give 1,2,3,6-tetrahydroindolizino[6,7-b]indol-5-ones. To prepare furano-fused indolizinones, methyl 6-hydroxy-5-oxo-1,2,3,5-tetrahydroindolizine-8-carboxylate was etherified with different allyl halides, and the resultant allyl ethers were subjected to a thermal Claisen rearrangement to give the corresponding methyl 7-allyl-6-hydroxy-5-oxo-1,2,3,4-tetrahydroindolizine-8-carboxylates. Cyclization under Wacker oxidation conditions afforded methyl 2-methyl-8-oxo-5,6,7,8-tetrahydro-1-oxa-7a-aza-s-indacene-4-carboxylates in near-quantitative yield.