Radical cyclization of beta-aminoacrylates: synthesis of (-)-indolizidine 223AB

[reaction: see text] (-)-Indolizidine 223AB was synthesized via radical cyclization of the beta-aminoacrylate derivative of a trans-2,5-disubstituted pyrrolidine. The trans-2,5-disubstituted pyrrolidine substrate was prepared by radical cyclization of a Ses-protected beta-aminoacrylate.     

A general approach to 3-n-butyl-5-alkylindolizidines: total synthesis of (-)-indolizidine 195B

Indolizidine type alkaloids have been attractive synthetic targets due to their biological activity. The total synthesis of (-)-indolizidine 195B via a general route, which could potentially be used to prepare other indolizidine alkaloids such as (-)-gephyrotoxin 223AB and (-)-myrmicarin 237A, is described.     

A general and efficient strategy for 7-aryloctahydroindole and cis-3a-aryloctahydroindole alkaloids: total syntheses of (+/-)-gamma-lycorane and (+/-)-crinane

A general and efficient approach to both 7-aryloctahydroindole and cis-3a-aryloctahydroindole alkaloids has been developed. The key step involves Michael additions of the corresponding kinetics and thermodynamics lithium enolates of ketone 9 to the versatile building blocks: nitroethylene 10. Two representative members, (+/-)-gamma-lycorane and (+/-)-crinane, have been synthesized in 22 and 36% overall yields, respectively.     

Asymmetric synthesis of alpha-substituted beta-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.     

Total synthesis of the neotropical poison-frog alkaloid (-)-205B

[reaction: see text]. A stereocontrolled total synthesis of the neotropical poison-frog alkaloid (-)-205B (1) has been achieved, employing a dithiane three-component linchpin coupling, a one-pot sequential construction of the embedded indolizidine ring, and ring-closing metathesis (RCM) to arrive at the novel 8b-azaacenaphthylene ring system comprising the alkaloid. The synthesis proceeded with a longest linear sequence of 19 steps, affording (-)-1 in 5.6% overall yield.     

Synthesis of alkylated indolizidine alkaloids via Pummerer mediated cyclization: synthesis of (±)-indolizidine 167B, (±)-5-butylindolizidine and (±)-monomorine I

The syntheses of indolizidine alkaloids, i.e., (±)-coniceine, (±)-indolizidine 167B, (±)-5-butylindolizidine and (±)-monomorine I via Pummerer cyclization are described. The key step is the transformation of lactam sulfoxide to bicyclic lactam via the Pummerer cyclization.     

New access to indolizidine and pyrrolizidine alkaloids from an enantiopure proline: total syntheses of (-)-lentiginosine and (1R,2R,7aR)-dihydroxypyrrolizidine

A new approach to the synthesis of indolizidine and pyrrolizidine skeletons is reported. (-)-Lentiginosine and (1R,2R,7aR)-dihydroxypyrrolizidine have both been synthesized in 13 steps from di-O-isopropylidene-d-mannitol. The common key intermediate is (-)-dihydroxyproline benzyl ester 10.     

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 novel and general synthetic pathway to strychnos indole alkaloids: total syntheses of (-)-tubifoline, (-)-dehydrotubifoline,and (-)-strychnine using palladium-catalyzed asymmetric allylic substitution

A method of palladium-catalyzed asymmetric allylic substitution for synthesizing 2-substituted cyclohexenylamine derivatives was established. Treatment of a 2-silyloxymethylcyclohexenol derivative with ortho-bromo-N-tosylaniline in the presence of Pd(2)dba(3).CHCl(3) and (S)-BINAPO in THF afforded a cyclohexenylamine derivative with 84% ee in 80% yield. The Heck reaction was carried out to produce an indolenine derivative in good yield. Using this method, we synthesized indolenine derivative 7, which was recrystallized from EtOH to give an optically pure compound. From this compound, tetracyclic ketone 13, which should be a useful intermediate for the synthesis of indole alkaloids, could be synthesized. The total syntheses of (-)-dehydrotubifoline, (-)-tubifoline, and (-)-strychnine were achieved from 13. All ring constructions for the syntheses of these natural products were achieved using a palladium catalyst.     

A new and general synthetic pathway to strychnos indole alkaloids: total syntheses of (-)-dehydrotubifoline and (-)-tubifoline by palladium-catalyzed asymmetric allylic substitution

[see reaction]. A novel procedure for the synthesis of an indole skeleton was developed. Treatment of a cyclohexenol derivative having a silyloxymethyl group at the 2-position with N-tosyl-o-bromoaniline in the presence of Pd2dba3*CHCl3 and (S)-BINAPO gave compound 6a with 84% ee in 75% yield. Compound 6a was converted into 11, which was treated with Pd(OAc)2 and Me(2)PPh in the presence of Ag2CO3 to give indoline derivative 12. From 12, we succeeded in the total syntheses of (-)-dehydrotubifoline and (-)-tubifoline.     

Enantioselective total syntheses of (+)-arborescidine A, (-)-arborescidine B, and (-)-arborescidine C

Described are the first enantioselective total syntheses of (+)-arborescidine A ((+)-1), (-)-arborescidine B ((-)-2), and (-)-arborescidine C ((-)-3), via routes that proceeded in five steps and 50% overall yield, eight steps and 61% overall yield, and nine steps and 51% overall yield, respectively, from 6-bromotryptamine (7). The syntheses feature the use of the Noyori catalytic asymmetric hydrogen-transfer reaction to introduce chirality in dihydro-beta-carbolines 6 and 8. On the basis of an ample precedent from Noyori's work, the reduction produces dihydro-beta-carbolines, and ultimately the natural products, possessing the R absolute configuration. The synthetic arborescidines displayed optical rotations that were opposite in sign those of the natural products, thereby supporting the S configuration for natural arborescidines A (1) and B (2) and the (3S,17S) configuration for natural arborescidine C (3). Our results are in agreement with the initial stereochemical assignment by Pa?s and co-workers, and are counter to their recently revised assignment.     

An expedient route to Montanine-type Amaryllidaceae alkaloids: total syntheses of (-)-brunsvigine and (-)-manthine

The first total syntheses of (-)-brunsvigine (1) and (-)-manthine (2) were accomplished in 10 and 18 steps, respectively. (-)-Quinic acid was converted to enone 12 in five steps. Iodination of enone 12 followed by stereoselective reduction yielded alpha-iodo allylic alcohol 16. Conversion of alcohol 16 into Weinreb amide 11 followed by anionic cyclization gave bicyclic enone 10. Stereoselective reduction of enone 10 and subsequent protection afforded pivaloate 9. Grignard addition of 8 to 9 and detosylation afforded amine derivative 19. Pictet-Spengler cyclization of 19 with Eschenmoser's salt and subsequent hydrolysis gave enantiomerically pure (-)-brunsvigine (1). For the total synthesis of (-)-manthine (2), the key intermediate 7 was hydrolyzed to diol 21. Conversion of 21 into 22 followed by regioselective cleavage with DIBAL furnished alcohol 25. Alcohol 25 was converted to the corresponding triflate 26, which on treatment with CsOAc and 18-crown-6 gave stereoinverted acetate 27. Hydrolysis of acetate 27 followed by methylation afforded compound 29. Detosylation of 29 afforded amine derivative 30. Pictet-Spengler cyclization of 30 followed by debenzylation gave alcohol 33. Finally, methylation of alcohol 33 afforded (-)-manthine (2).     

First asymmetric total syntheses of (-)-subincanadines a and B, skeletally rearranged pentacyclic monoterpenoid indole alkaloids in aspidospermasubincanum

We achieved the first asymmetric total syntheses of novel Aspidosperma indole alkaloids, (-)-subincanadines A and B, which involve an intramolecular diastereoselective Pictet-Spengler cyclization and an intramolecular Nozaki-Hiyama-Kishi reaction as key steps in the total syntheses.     

First total syntheses of (+/-)-penicillones A and B

The first total syntheses of (+/-)-penicillones A (1) and B (2) have been accomplished from 2-methoxy-4,6-dimethylphenol (7) in 9 and 8 synthetic steps, respectively. Intramolecular Diels-Alder reaction of masked o-benzoquinone 8 and aqueous acid-catalyzed intramolecular aldol reaction are the key steps.     

A unified route to the welwitindolinone alkaloids: total syntheses of (-)-N-methylwelwitindolinone C isothiocyanate, (-)-N-methylwelwitindolinone C isonitrile, and (-)-3-hydroxy-N-methylwelwitindolinone C isothiocyanate

As part of a comprehensive strategy to the welwitindolinone alkaloids possessing a bicyclo[4.3.1]decane core, we report herein concise asymmetric total syntheses of (-)-N-methylwelwitindolinone C isothiocyanate (2a), (-)-N-methylwelwitindolinone C isonitrile (2b), and (-)-3-hydroxy-N-methylwelwitindolinone C isothiocyanate (3a) from a common tetracyclic intermediate. The crucial vinyl chloride moiety was installed through electrophilic chlorination of a hydrazone, but only after adjustment of reactivity to circumvent a facile skeletal rearrangement. Selective desulfurization and oxidation of 2a provided access to 2b and 3a, respectively. Notably, this work provides corrected (1)H and (13)C NMR spectral data for 3a.     

The lyconadins: enantioselective total syntheses of (+)-lyconadin A and (-)-lyconadin B

A full account of the enantioselective total syntheses of (+)-lyconadin A (1) and (-)-lyconadin B (2) is presented. Central to this venture was recognition and deployment of a key strategy-level intramolecular aldol/conjugate addition cascade that led, in a single operation, to two new carbon-carbon sigma-bonds, three new stereogenic centers, and two new rings, albeit with the incorrect stereogenicity at C(12) for the lyconadins. Correction of the C(12) stereogenicity was achieved via innovative use of a protonated intramolecular aminal. An aminoiodo olefin cyclization, in conjunction with alpha-pyridinone and 3,4-dihydropyridinone annulation protocols, permitted completion of the syntheses of (+)-lyconadin A (1) and (-)-lyconadin B (2), respectively.     

Formal total syntheses of the (-)-salicylihalamides A and B from D-glucose and L-rhamnose

[reaction: see text] Two formal total syntheses of the (-)-salicylihalamides, based on chiral pool approaches, are reported. D-glucose and L-rhamnose were used to prepare advanced intermediates 23 and 54, which can be converted in three or four steps, respectively, to the target compounds. The synthesis of 23 from a known D-glucose-derivative was accomplished in 12 steps and 17% overall yield, and the synthesis of 54 from a known L-rhamnose-derivative was done in nine steps and 6% overall yield. A key step in the synthesis was a ring-closing metathesis reaction to prepare the macrocyclic ring system. It was demonstrated that the phenolic protecting group was critical for inducing the preferential formation of the desired E isomer. It was further shown that the protecting group at the C13 hydroxyl group had no significant influence on the E:Z ratio during the ring-closing metathesis reaction.