A One-Pot Formal [4 + 2] Cycloaddition Approach to Substituted Piperidines, Indolizidines, and Quinolizidines. Total Synthesis of Indolizidine (−)-209I

[reaction: see text] Heating a mixture of substituted N-benzyl gamma-chloropropylamines, conjugated alkynoates or alkynones, sodium carbonate, and a catalytic amount of sodium iodide in i-PrOH at 70-83 degrees C delivers substituted piperidines in good yields. This transformation goes through a cascade Michael addition/alkylation process and represents a facile one-pot formal [4 + 2] cycloaddition approach to piperidine ring. Using secondary cyclic gamma-chloropropylamines as substrates, this process produces substituted indolizidines or quinolizidines. On the basis of this approach, indolizidine (-)-209I is elaborated in 11 steps from methyl 2-hexenoate.

Novel Synthesis of Castanospermine and 1-Epicastanospermine

[reaction: see text] Polyhydroxylated indolizidines have potential for treatment of HIV, hepatitis C and HSV infection, multiple sclerosis, angiogenesis, cancer, and diabetes. A new synthetic approach to the title compounds from a 5-C-methoxypyranosyl azide has been developed. The route incorporates the aldol reaction and a novel catalytic reductive amination cascade to generate the indolizidine ring.

Asymmetric Synthesis of α-Substituted β-Amino Ketones from Sulfinimines (N-Sulfinyl Imines). Synthesis of the Indolizidine Alkaloid (−)-223A

Of the possible four stereoisomers, addition of the lithium enolate of 4-heptanone to sulfinimines resulted in only the syn- and anti-alpha-substituted beta-amino ketones. The formation of the major syn-beta-amino ketone was rationalized in terms of addition of the E-enolate to the C-N double bond of the sulfinimine via a six-member chelated chairlike transition state. The enolates of 4-heptanone were generated using LiHMDS in THF where a 1:2.5 E:Z enolate ratio was noted. In diethyl ether the E:Z ratio was 15:1 in favor of the E-enolate and explained in terms of Ireland's transition state model. Here increased steric interactions between the ethyl group and the carbonyl-LiN(TMS)(2) moiety destabilize the transition state leading to the Z-enolate in the poorly coordinating diethyl ether solvent. This new synthesis of syn-alpha-substituted-beta-amino ketones was applied to the concise enantioselective total synthesis of indolizidine (-)-223A, a 5,6,8-trisubstituted alkaloid isolated from the skin of the dendrobatide frog.

A reinvestigation of 4-hydroxyindole-6-carboxylate synthesis from pyrrole-2-carboxaldehyde: a facile synthesis of indoles and indolizines

Cyclization of the Stobbe product 3 under the literature conditions (acetic anhydride/sodium acetate) affords both the indole 6 and the indolizine 8. The presence of base promotes the formation of indolizine products, and using acetic anhydride/triethylamine leads to indolizine products in good yield. Improved conditions for conversion to indoles 2, 5, and 6 are reported, and inconsistencies in some of the literature structure assignments and characterization data are noted.

CoMFA and CoMSIA investigations revealing novel insights into the binding modes of dopamine D3 receptor agonists

As an extension of a series of dopamine D(3) receptor agonists involving FAUC 54, ex-chiral pool synthesis, and biological evaluation of 3-substituted 7-aminotetrahydroindolizines was performed. Considering the structural features of both series of enantiomers, we developed a novel alignment hypothesis for D(3) agonists, allowing for the placement of the aromatic moieties on two alternative, adjacent positions. CoMFA and CoMSIA analyses yielded significant cross-validated q(2) values of 0.726 and 0.590, respectively, when a newly invented program application (IRAS) controlling the alignment selection proved to be useful. Employing the CoMFA/CoMSIA contribution maps, we were able to transform a previously constructed homology model of the D(3) receptor from an inactive into an activate state. Besides the established ionic interactions, we propose pi-stacking with Phe6.51 and a hydrogen bond between His6.55 and the acyl moiety to be primarily involved in the D(3) receptor binding of FAUC 54 and its analogues.

Indolizine 1-sulfonates as potent inhibitors of 15-lipoxygenase from soybeans

A number of indolizine 1-sulfonates have been prepared by cyclization of cyclopropenones with pyridines followed by trapping of the intermediate 1-indolizinol with a sulfonyl halide, and examined as inhibitors of 15-lipoxygenase (15-LO). The compounds display IC(50) values between 15 and 42 microM; all are more active than the well-known 15-LO inhibitor quercetin (IC(50) 51 microM). A wide variety of substituents are well tolerated. The enzyme inhibition was not affected by preincubation or the presence of a detergent and no significant particle formation was observed. Hence, inhibition from aggregates of indolizines, promiscuous inhibition, is highly unlikely.

Herbicidal indolizine-5,8-diones: photosystem I redox mediators

6,7-Disubstituted indolizine-5,8-diones showed activity as herbicides, giving rapid desiccation symptomology in whole-plant tests and bleaching in leaf-disc assays. In isolated chloroplasts, such compounds initiated rapid uptake of oxygen in Photosystem I. A redox mediator mode of action was further supported by electrochemical studies. Several compounds described had low photostability, high volatility, hydrolytic instability or reactivity with glutathione. The detrimental effects that these factors may have had on expression of herbicidal activity are discussed.

Synthesis of a library of benzoindolizines using poly(ethylene glycol) as soluble support

A library of benzoindolizines (pyrrolo [1,5-a] quinolines 10 and pyrrolo [1,5-a] quinolines 9) has been synthesized using poly(ethylene glycol) (PEG) as soluble polymer support. The PEG-supported isoquinolinium salt 4 reacted, respectively, with active alkenes 11 using tetrakispyridinecobalt(II) dichromate (TPCD) as oxidant or alkynes 12 to give 10, of which yields were from moderate to high. By analogy, the reaction of PEG-supported quinolinium salt 3 with 12 was to produce 9. However, in the presence of TPCD the reaction of 3 with 11 afforded indolizines 8, which was discovered firstly.

Studies towards the enantioselective synthesis of 5,6,8-trisubstituted amphibian indolizidine alkaloids via enaminone intermediates

Investigations aimed at the enantioselective total synthesis of indolizidine 223A, a recently described 5,6,8-trisubstituted indolizidine alkaloid from a dendrobatid frog, are described. tert-Butyl (2R,3R)-3-amino-2-ethylhexanoate and its (2S,3R)-diastereomer, prepared in several steps from lithium N-benzyl-N-[(1R)-1-phenylethyl]amide and tert-butyl (2E)-hex-2-enoate by the Davies protocol, served as chiral building blocks from which two complementary suites of diastereomeric intermediates were made en route to pivotal tert-butyl 3-[2-(alkoxycarbonylmethylene)pyrrolidin-1-yl]-2-ethylhexanoate intermediates 20 and 21. Cyclisation of these enaminones, achieved by acid hydrolysis of the tert-butyl esters and activation of the liberated carboxylic acids as mixed anhydrides, afforded 6-ethyl-7-oxo-5-propyl-1,2,3,5,6,7-hexahydroindolizine-8-carboxylate esters 28 and 29. Several further transformations of these potential scaffolds for the synthesis of the target alkaloidal systems are also reported.

Synthesis of heteroarenes using cascade radical cyclisation via iminyl radicals

Cascade radical cyclisation involving homolytic aromatic substitution has been used to synthesise new tetracycles. Treatment of vinyl iodide radical precursors with Me(3)Sn. radicals (from hexamethylditin) yielded intermediate vinyl radicals which undergo 5-exo cyclisation onto suitably placed nitrile groups to yield intermediate iminyl radicals. The iminyl radicals undergo aromatic homolytic substitution via 6-endo cyclisation (or 5-exo cyclisation followed by neophyl rearrangement) with loss of hydrogen (H.) in a H-abstraction step. We propose that this abstraction was facilitated by tert-butoxyl (t-BuO.) radicals from di-tert-butyl peroxide or methyl radicals, generated from breakdown of trimethylstannyl radicals (Me(3)Sn.). The biologically active alkaloids mappicine and luotonin A were synthesised using the new methodology. A novel radical conversion of nitriles to primary amides is proposed.

Functional divergency oriented synthesis of azoninones as the key intermediates for bioactive indolizidine alkaloids analogs

A functional divergency oriented synthetic approach to the azoninone (9-membered lactams), key intermediate for the indolizidine alkaloids library, using amide enolate induced aza-Claisen rearrangement has been achieved.

Stereoselective Synthesis of Polyhydroxylated Indolizidines Based on Pyridinium Salt Photochemistry and Ring Rearrangement Metathesis

Ruthenium-catalyzed ring rearrangement metathesis (RRM) reactions of stereochemically diverse, differentially protected 4-N-allylacetamidocyclopenten-3,5-diols, prepared by using pyridinium salt photochemistry, have been explored as part of a program to develop novel routes for the synthesis of polyhydroxylated indolizidines. The RRM reactions, which produce selectively protected 1-acetyl-2-allyl-3-hydroxy-1,2,3,6-tetrahydropyridines, were found to take in high yields and with high levels of regioselectivity. The significance of RRM reactions of 4-N-allylacetamidocyclopenten-3,5-diols in the context of polyhydroxylated indolizidine synthesis is demonstrated by an application to the concise preparation of the potent glycosidase inhibitor, (-)-swainsonine.

A Straightforward and Versatile Synthetic Approach to 1-Azabicyclic Alkaloids

A very straightforward route to 1-azabicyclo alkaloid scaffolds with several ring sizes is reported. The final bicyclic structures were built through a synthetic scheme that involved (i) the construction of dienic 4-piperidone systems by an imino-Diels-Alder reaction between aminotrienes and N-omega-vinylimines, in the presence of Yb(OTf)(3), and (ii) the ring-closing metathesis reaction of these cyclic dienes, under the influence of the first-generation Grubbs' Ru-complex catalyst. During this investigations, various polysubstituted azabicyclic ring skeletons, including several examples of the quinolizidine alkaloids, are reported, and their relative stereochemistry is adequately discussed.

A new synthetic method for an indolizidine skeleton by C-N bond formation via a pi-allylpalladium complex

Pd(0)-catalyzed intramolecular cyclic reaction via a pi-allylpalladium complex provided an indolizidine skeleton in satisfactory high and reproducible yields by using allylic compound having an acetoxyl group as a leaving group. These results must be available for syntheses of various functional indolizidine alkaloids.

Epoxide-initiated cationic cyclization of azides: a novel method for the stereoselective construction of 5-hydroxymethyl azabicyclic compounds and application in the stereo- and enantioselective total synthesis of (+)- and (-)-indolizidine 167B and 209D

A novel and general method has been developed for the stereoselective construction of 5-hydroxymethyl azabicyclic ring skeletons based on epoxide-initiated cationic cyclization of azides. The key cyclization reaction was systematically studied with the model compound, 3-(1-oxa-spiro[2.4]hept-4-yl)propyl azide 3a, and EtAlCl(2) was found to be an ideal choice as the catalyst. The generality of this transformation was further tested with different ring sizes, where six- and seven-membered epoxyazides 3b,c underwent smooth cyclization to give 5-hydroxymethyl azepine 4b and 5-hydroxymethyl azocine 4c, respectively, as a single detectable diastereomer. This novel methodology was elegantly applied in the stereoselective total synthesis of indolizidine alkaloids 167B and 209D. Further, the enantioselective total synthesis of natural and unnatural indolizidine alkaloids 167B and 209D was accomplished by using Sharpless asymmetric dihydroxylation as a key step.

Enantioselective synthesis of (-)-dendroprimine and isomers

An enantioselective and diastereoselective synthesis of six 5,7-dimethylindolizidine isomers is described via an intramolecular cyclization that involves an allylsilyl nucleophilic group and an acyliminium ion. The first total synthesis of naturally occurring (-)-dendroprimine has been achieved in five steps.

Palladium-catalyzed asymmetric allylic substitution of 2-arylcyclohexenol derivatives: asymmetric total syntheses of (+)-crinamine, (-)-haemanthidine, and (+)-pretazettine

Much interest has been shown in Amaryllidaceae alkaloids as synthetic targets due to their wide range of biological activities. Over 100 alkaloids have been isolated from members of the Amaryllidaceae family; most of them can be classified into eight skeletally homogeneous groups. We have succeeded in the first asymmetric total syntheses of the crinane-type alkaloids (+)-crinamine (1), (-)-haemanthidine (2), and (+)-pretazettine (3). The starting cyclohexenylamine 14 was obtained from allyl phosphonate 11c by palladium-catalyzed asymmetric amination in 82% yield and with 74% ee. The product was recrystallized from MeOH. Interestingly, (-)-14 with 99% ee was obtained from the mother liquor (74% recovery). Intramolecular carbonyl-ene reaction of (-)-10 proceeds in a highly stereoselective manner to give hexahydroindole derivative 9 as the sole product. In the Lewis-acid-catalyzed carbonyl-ene reaction, an interesting rearrangement product, 20, was isolated in high yield. From 9, (+)-crinamine was synthesized. Thus, the asymmetric total synthesis of (+)-crinamine was achieved in 10 steps from 11c, and the overall yield is 19%. The total synthesis of (-)-haemanthidine was also achieved from 9 by a short sequence of steps.

Asymmetric Total Synthesis of Dendrobatid Alkaloids: Preparation of Indolizidine 251F and Its 3-Desmethyl Analogue Using an Intramolecular Schmidt Reaction Strategy

Total syntheses of alkaloid 251F (1), a natural product detected from the skin extracts of the dendrobatid frog species Minyobates bombetes, and its racemic 3-desmethyl derivative (2) are reported. A Diels-Alder reaction initiated both syntheses and established four consecutive stereogenic centers. Important to the synthesis of 2 was a first-generation ozonolysis/olefination/aldol strategy to convert a [2.2.1] bicyclic acid to the [3.3.0]bicyclooctane diquinane 4b. Further elaboration to an appropriate keto azide allowed for a key intramolecular Schmidt reaction to deliver the tricyclic core of the target molecule. In a second-generation approach, a tandem ring-opening/ring-closing metathesis reaction effected an overall [2.2.1] --> [3.3.0] skeletal rearrangement to deliver diquinane 4a. In similar fashion, 4a was manipulated to an appropriate keto azide, and an intramolecular Schmidt reaction generated the core cyclic architecture of 251F.

Synthesis of Putative Uniflorine A

A diastereoselective synthesis of the putative structure of the natural product uniflorine A has been achieved by using the Petasis borono-Mannich reaction and ring-closing metathesis as key steps. The NMR data of the synthetic material did not match that reported for the natural product. The structure of the final synthetic product was unequivocally determined by single-crystal X-ray study of its pentaacetate derivative. Thus it was concluded that the proposed structure of uniflorine A is incorrect.

Multicomponent Linchpin Coupling of Silyl Dithianes Employing an N-Ts Aziridine as the Second Electrophile: Synthesis of (−)-Indolizidine 223AB

[reaction: see text] An efficient, stereocontrolled assembly of the indolizidine alkaloid, (-)-indolizidine 223AB, exploiting a three-component linchpin coupling employing an N-Ts aziridine as the second electrophile, followed by a one-pot sequential construction of the indolizidine ring system, has been achieved. The longest linear sequence was 10 steps, proceeding in 10% overall yield.

A pair of epimeric 13-hydroxy-2,3,6,7-tetramethoxy-8b,11,12,12a,13,13a-hexahydro-9H-9a-azacyclopenta[b]triphenylene-10-ones

The structures of two compounds which are intermediates in the synthesis of phenanthroindolizidine alkaloids have been determined. (8bS,13aS,14R,14aR)-8b,9,11,12,13,13a,14,14a-Octahydro-14-hydroxy-2,3,6,7-tetramethoxydibenzo[f,h]pyrrolo[1,2-b]isoquinolin-11-one acetone solvate, C24H27NO6.C3H6O, (II), crystallizes in a chiral space group with one solvent molecule (acetone) present in the asymmetric unit. On the other hand, (8bS,13aS,14S,14aR)-8b,9,11,12,13,13a,14,14a-octahydro-14-hydroxy-2,3,6,7-tetramethoxydibenzo[f,h]pyrrolo[1,2-b]isoquinolin-11-one, C24H27NO6, (III), crystallizes in a centrosymmetric space group with two molecules in the asymmetric unit and with no solvent present. The two molecules in the asymmetric unit of (III) are structurally the same. Compounds (II) and (III) are epimers at the C atom carrying the OH group; otherwise they are very similar in structure.

Total synthesis of mappicine ketone (nothapodytine B) by means of sulfur-directed 5-exo-selective aryl radical cyclization onto enamides

Enamides 5, on treatment with Bu(3)SnH-AIBN, underwent aryl radical cyclization in a 5-exo manner to give 1-[bis(phenylthio)methyl]dihydroisoindoles 6, which were partially desulfurized with Bu(3)SnH-AIBN to give 1-mono(phenylthio)methyl congeners 7. Formation of 6 from 5 may be explained by the presence of two phenylthio groups at the terminus of the N-vinylic bond of 5, since enamide 8a having no phenylthio group underwent aryl radical cyclization in a 6-endo manner. Compound 7d (R = CF(3)) was transformed into sulfoxide 16, which was treated with (CF(3)CO)(2)O and then with 10% NaOH to give a model compound 20 of mappicine ketone (MPK) (1) through aldol condensation of aldehyde 18. An attempt to synthesize MPK using this method with sulfoxide 28 prepared from 25, however, was unsuccessful, and, instead, photochemical cyclization of enamide 38 prepared from 25 furnished MPK.

A Three-Component, One-Pot Synthesis of Indolizidines and Related Heterocycles via the [3+2] Cycloaddition of Nonstabilized Azomethine Ylides

Nonstabilized azomethine ylides (i.e. those bearing only hydrogens or alkyl groups) can be generated from (2-azaallyl)stannanes and (2-azaallyl)silanes through an intramolecular N-alkylation/demetalation cascade. The resulting ylides undergo [3+2] cycloaddition with electron-poor or electron-rich dipolarophiles yielding indolizidines and related 1-aza[m.3.0]bicycloalkane systems in good yield. An in situ protocol allows for a one-pot, three-component synthesis of indolizidines. The (2-azaallyl)stannanes tolerate enolizable hydrogens in these cycloadditions, while (2-azaallyl)silanes do not. The mechanism of the cycloaddition cascade is clarified by a series of control experiments. The same (2-azaallyl)stannanes may be transmetalated by n-butyllithium to generate 2-azaallyllithiums, which also may undergo a [3+2] cycloaddition/N-alkylation cascade to form indolizidines.

Cyclopropane-Annelated Azaoligoheterocycles by Ti-Mediated Intramolecular Reductive Cyclopropanation of Cyclic Amino Acid Amides

Starting from pyrrole- and indole-2-carboxylic acids 5 a and 5 b, the tri- and tetracyclic N,N-dibenzylcyclopropylamines 7 a and 7 b have been synthesized in 52 and 33 % overall yield, respectively. The synthesis of the enantiopure tetracyclic diamine 10 has been achieved applying the established set of reactions to N-tert-butoxycarbonylindoline-2-carboxylic acid (8) in 46 % overall yield. The amide 15 could not be prepared in the same way starting from the N-tert-butoxycarbonylproline 11. In fact, in the allylation step the stereogenic center was deprotonated and the doubly alkylated amide 13 was formed. However, the desired intermediate 15 could be obtained from L-proline in 49 % yield performing first the N-allylation step, then the introduction of the amide function. From 15, the cyclopropane-annelated pyrrolizidine 16 was obtained in 70 % yield as a mixture of (1aS,6aS,6bR)-16 and (1aR,6aS,6bS)-16 diastereoisomers in a ratio of 1:2.9.

Direct Asymmetric Synthesis of β-Amino Ketones from Sulfinimines (N-Sulfinylimines). Synthesis of (−)-Indolizidine 209B

N-Sulfinyl beta-amino ketones, prepared directly from the potassium enolates of methyl ketones and enantiopure sulfinimines, are transformed in one pot to protected amino ketones, which are valuable chiral building blocks for the assembly of piperidines. The utility of this methodology is illustrated in a concise asymmetric synthesis of (-)-indolizidine 209B. [reaction: see text]