Synthesis of spiroindolenine derivatives by a tandem radical-oxidation process

In the present work, a novel stereoselective radical cyclization/oxidation/spirocyclization cascade process using a mixture of n-Bu₃SnH/ dilauroyl peroxide is described. The proposed mechanism for this later process combines a 6-endo cyclization of an aryl radical onto an enamide double bond, and a consecutive oxidative-ionic spirocyclization at C-3 of an indole nucleus. All processes led to the construction of new spiroindolenine derivatives in a one-step synthesis starting from relatively simple starting materials. The organic peroxide appears to act as the initiator and the oxidant.     

An easy access to pyrimidine-fused azocine derivatives by thiophenol-mediated radical cyclization via 8-endo-trig mode

An efficient route for the synthesis of eight-membered nitrogen heterocycles has been developed via a thiophenol-mediated intramolecular 8-endo-trig radical cyclization. The radical precursors were prepared using BF₃·Et₂O-catalyzed aza-Claisen rearrangement followed by the reaction with propargyl bromide. The alkenyl radicals are generated from thiophenol initiated by the benzoyl peroxide instead of commonly used AIBN for easy and facile isolation of the pure products.     

Synthesis of nitrogen-containing heterocycles using exo- and endo-selective radical cyclizations onto enamides

The effect of positional change of the carbonyl group of enamides on Bu₃SnH-mediated alkyl radical cyclization leading to five-, six-, seven-, and eight-membered nitrogen-containing heterocycles was examined. A 5-exo cyclization is generally preferred over a 6-endo ring closure in systems having an alkyl radical center on the enamide-acyl side chain, whereas enamides having an alkyl radical center opposite to the acyl side chain predominantly gave 6-endo cyclization products. These results suggest that the exo or endo selectivity of radical cyclization onto the alkenic bond of enamides can be controlled by positional change of the carbonyl group. For an understanding of these selectivities, heat of formation for each transition state was calculated. 6-endo-Selective radical cyclization was applied to the radical cascade, enabling a concise synthesis of a cylindricine skeleton. A 7- or 8-endo alkyl radical cyclization, however, predominated over a corresponding 6- or 7-exo ring closure regardless of the positional change of the carbonyl group of enamides.     

Synthesis of 5-endo-, 5-exo-, 6-endo- and 6-exo-hydroxylated analogues of epibatidine

A convenient, high-yield synthesis of N-Boc-7-azabicyclo[2.2.1]hept-5-en-2-one (7) was developed by SmI₂-mediated desulfonylation of 6. Thus, 5-endo-, 5-exo-, 6-endo-, and 6-exo-hydroxylated epibatidine analogues 2a,b and 3a,b were synthesized from 7 by using a Pd(PPh₃)₄-catalyzed reductive Heck coupling reaction and SmI₂-mediated reduction of the carbonyl group as the key steps. Other reaction conditions for the reductive Heck procedure and the reduction step were also investigated.     

Substituent control in the diastereoselectivity of dipolar cycloadditions of nitrones and their Zn(II) complexes with N-arylmaleimides

The π-π stacking interactions between maleimide's and nitrone's aromatic rings during the 1,3-dipolar cycloaddition were assumed to control the exo-endo selectivity of the reaction. The exo-endo ratios change during the reactions until they reach a constant value, which depends on the substituent. Electron-withdrawing groups favour the exo adduct while electron-donating groups favour the endo adduct. The nitrone ZnBr₂ complexes react much more slowly than the free nitrone and the cycloaddition is exo selective in all cases independent of the substituents on the maleimide's aromatic ring. Thermal retrocycloaddition of the cycloadducts produce the corresponding nitrones. The ring opening in the presence of secondary amines did not induce imine formation. endo Adducts were shown for the first time to be the stable paramagnetic compounds.     

Reduction of 6/7-substituted 3-phenyltrop-3-en-2-ones: stereoselectivity and conformational analysis of the products

Reduction of 6/7-carboethoxy-3-phenyltrop-3-en-2-ones with H₂/Pd/C and NaBH₄ was studied in order to find a stereoselective route to the corresponding 3-phenyltropan-2-ones and 2α/2β-hydroxy-3-phenyltropanes. The 6/7-exo-carboethoxy-3-phenyltrop-3-en-2-ones were selectively reduced by Pd/C to 3β-phenyltropan-2-ones and 2α-hydroxy-3β-phenyltropanes. The corresponding 2β-hydroxy-3β-phenyl analogues were synthesized using NaBH₄, with a yield of 40%. Reduction of 6-endo-carboethoxy-3-phenyltrop-3-en-2-one yielded several products. The corresponding 7-endo-substituted analogue was selectively reduced with both Pd/C and NaBH₄ to 7-endo-carboethoxy-3β-phenyltropin-2-one. Analysis of stereochemically important ¹H NMR spectroscopy parameters was performed for all the products and used for conformational analysis in solution. X-ray analysis was performed for selected compounds.     

Design and synthesis of novel benzopyrazolodiazepinones via intra-molecular alkylation of α-alkylcarbonyl radicals mediated by dilauroylperoxide

Using a tin-free strategy, novel 4H-benzo[f]pyrazolo[1,5-a][1,3]diazepin-5(6H)-ones were synthesized in acceptable yields via intra-molecular alkylation over a benzene ring, of α-alkylcarbonyl radicals generated from ethyl pyrazolylbenzylaminoxanthates, using dilauroyl peroxide (DLP) as the radical initiator.     

One-pot synthesis of pyrrolidino- and piperidinoquinolinones by three-component aza-Diels-Alder reactions of in situ generated N-arylimines and cyclic enamides

An efficient synthesis of hexahydropyrrolo[3,2-c]quinolin-2-ones and hexahydropyridino[3,2-c]quinolin-2-ones has been developed in moderate to high yields by one-pot two-step aza-Diels-Alder reactions of N-arylimines, formed in situ from anilines and benzaldehydes, with cyclic enamides, formed in situ from 5-hydroxypyrrolidin-2-ones and 6-hydroxypiperidin-2-ones by BF₃·OEt₂-promoted dehydration in dichloromethane at room temperature. The hexahydropyrrolo[3,2-c]quinolin-2-ones were formed as a single exo-stereoisomer in most cases and hexahydropyridino[3,2-c]quinolin-2-ones were formed as a mixture of exo- and endo-isomers favoring the endo-diastereomer.     

Diverse synthesis of pyrimido[1,2-a]benzimidazoles and imidazo[2,1-b]benzothiazoles via CuI-catalyzed decarboxylic multicomponent reactions of heterocyclic azoles,aldehydes and alkynecarboxylic acids

We have developed a straightforward approach to diverse synthesis of 2,3-, 2,4-disubstituted pyrimido [1,2-a]benzimidazoles, 2,4,10-trisubstituted 2,10-dihydropyrimido [1,2-a]benzimidazoles and 2,3-disubstituted imidazo [2,1-b]benzothiazoles via multicomponent reactions (MCRs) of heterocyclic azoles, aldehydes with easily storable and handling alkynecarboxylic acids. In the presence of a catalytic amount of CuI and K₂CO₃, the pyrimido [1,2-a]benzimidazole or imidazo [2,1-b]benzothiazole scaffold could be rapidly constructed through a 6-endo-dig or 5-exo-dig cyclization, respectively. The preliminary mechanistic study suggested that the formation of 2,3- disubstituted pyrimido [1,2-a]benzimidazoles, which completes the assembly of the scaffold and its C-3 position functionalization in one pot, undergoes a novel cascade process involving a decarboxylation, A [3] coupling, 6-endo-dig cyclization, nucleophilic addition and dehydration.     

Regioselective oxidation of N-alkylpyrrolidines to pyrrolidin-5-ones by RuCl3/NaIO4

RuCl₃/NaIO₄ under EtOAc/H₂O biphasic conditions, selectively oxidizes the Nα-endo-methylene group of pyrrolidine derivatives, without affecting the exo-methylene group adjacent to the N-heteroatom.     

Regioselective radical cyclization initiated by the reaction of allylic hydroperoxides with iron(II) sulfate

Treatment of 1-methyl-2-methylene-1-cyclohexyl hydroperoxide with a mixture of FeSO₄/CuCl₂ yielded 1-(1-chlorocyclohexyl)ethanone as the major product consistent with 6-endo-trig cyclization of the intermediate 5-acetylhex-5-enyl radical. This strategy was extended to the ring enlargement of a series of 1-isopropenylcycloalkyl hydroperoxides. Regioselective 7- or 8-endo-trig cyclization reactions could be achieved by treatment of the corresponding cyclopentyl or cyclohexyl hydroperoxides with either a mixture of FeSO₄/CuCl₂ or with FeSO₄ only. The influence of substituents on the efficiency of the 8-endo-trig cyclization process was also explored.     

Synthesis of trans-perhydroisoquinolines by 6-endo-trig radical cyclization of amino-tethered vinyl bromides and cyclohexenes

Bu₃SnH-promoted cyclization of several N-(2-bromoprop-2-enyl)-N-[(4-oxocyclohex-1-enyl)methyl]alkylamines is reported. It has been found that the generated vinyl radicals evolve through a 6-endo-cyclization pathway giving rise to the corresponding 4,6-functionalized perhydroisoquinolines in a prevalent trans-relative configuration.     

5-Nitroisocoumarins from tandem Castro-Stephens coupling—6-endo-dig cyclisation of 2-iodo-3-nitrobenzoic acid and arylethynes and ring-closure of methyl 2-alkynyl-3-nitrobenzoates with electrophiles

Reaction of 2-iodo-3-nitrobenzoic acid with arylalkynyl copper(I) reagent gave 3-aryl-5-nitroisocoumarins. Castro-Stephens coupling was followed by in situ Cu-catalysed ring-closure. ¹H NMR and X-ray crystallography showed the cyclisations to be 6-endo, contrasting with reports of 5-exo cyclisation of analogous 2-iodobenzoate esters with alkynes. Sonogashira couplings of methyl 2-iodo-3-nitrobenzoate with phenylacetylene and with trimethylsilylacetylene gave the corresponding 2-alkynyl-3-nitrobenzoate esters. With HgSO₄, the phenylalkyne underwent 6-endo cyclisation to give 5-nitro-3-phenylisocoumarin. The disubstituted alkyne esters gave 4-phenylselenylisocoumarins with PhSeCl. 5-Nitro-3-phenyl-4-phenylselenylisocoumarin shows significant sterically-driven distortion of the isocoumarin ring. Reaction of methyl 3-nitro-2-phenylethynylbenzoate with ICl gave the 4-iodoisocoumarin. Thus the nitro group tends to direct these electrophile-driven cyclisations towards the 6-endo mode.     

Fluorolactonization of norbornenecarboxylic acids and their methyl esters with F-TEDA-BF4 and XeF2

The selective fluorolactonization was achieved by treatment of cis-5-norbornene-2,3-endo-dicarboxylic acid or its monomethyl and dimethyl esters with F-TEDA-BF₄ or XeF₂. The reactions of 5-norbornene-endo-2-carboxylic acid and its monomethyl ester with F-TEDA-BF₄ or XeF₂ proceed in a non-selective manner to give fluorolactonization, addition and rearrangement products. The basic factor responsible for selectivity of the fluorolactonization is the presence of two endo-oriented carboxyl groups in the substrate molecule. The electrophilicity and type of the fluorinating agent is of secondary importance in this regard. It is postulated that the fluorolactonization of norbornenecarboxylic acids and their methyl esters with F-TEDA-BF₄ or XeF₂ is realized mainly via “open” fluoronorbornyl carbocation intermediates which in the reaction with XeF₂ are postulated as the tight ion pairs.     

6-endo Versus 5-exo radical cyclization: streamlined syntheses of carbahexopyranoses and derivatives by 6-endo-trig radical cyclization

Three factors that can direct 6-endo radical cyclization over 5-exo ring closure: substitution at C-5, vinyl radical cyclization and ring strain, have been considered in the context of the preparation of carbapyranoses from carbohydrate derivatives. As a result, alkyl radicals in substrates containing a strain inducing 2,3-O-isopropylidene ring, and vinyl radical in non-strained compounds undergo a completely regioselective 6-endo-trig ring closure leading to carbasugar derivatives.     

Convenient access to isoindolinones via carbamoyl radical cyclization. Synthesis of cichorine and 4-hydroxyisoindolin-1-one natural products

An efficient and convenient access to 2-tert-butylisoindolin-1-ones via an oxidative radical cyclization process from stable carbamoylxanthates, derived from secondary tert-butylamines, is described. The proposed mechanism for this transformation involves, the generation of a carbamoyl radical, its cyclization to the aromatic system, and the dilauroyl peroxide (DLP) mediated rearomatization to generate the isoindolinone ring system. Additionally, the syntheses of cichorine and 4-hydroxyisoindolin-1-one natural products were carried out to underscore the synthetic potential of this xanthate-based carbamoyl radical-oxidative cyclization.     

8-Endo selective Friedel-Crafts cyclization of vinyloxiranes with Co2(CO)6-complexed acetylene

An 8-endo selective Friedel-Crafts cyclization of vinyloxirane 8 with Co₂(CO)₆-complexed benzeneacetylene was found to give poly-functional eight-membered cyclic compound 9 in high yields.     

Metal-free synthesis of benzimidazo[2,1-a]ellipticines via tandem inter and intramolecular cyclization

Synthesis of the new benzimidazo[2,1-a]ellipticine derivatives via tandem inter and intramolecular, 5-endo,6-endo cyclization of various 9-ethyl-2-(alkynyl)carbazole-3-carbaldehydes with different 1,2-aryldiamines is reported under metal free condition in good yields.     

Synthetic transformations of isoquinoline alkaloids. 1-alkynyl-3,6-dimethoxy-N-methyl-4,5α-epoxy-6,18-endoethenobenzo[i]isomorphinans and their transformations

The reaction of thebaine with 2-bromo-6-methyl-1,4-benzoquinone regioselectively afforded 6,18-endo-etheno-9-methyldihydrothebainehydroquinone. Iodination of 6,18-endo-ethenodihydrothebainehydroquinones with N-iodosuccinimide in trifluoroacetic acid was also selective, and the corresponding 1-iodo derivatives were formed. The main reaction pathway in the halogenation of 6,18-endo-ethenodihydrothebainehydroquinone with iodine chloride was chlorination of the fused hydroquinone fragment. The Sonogashira reaction of 1-iodo-6,18-endo-ethenodihydrothebainehydroquinones with trimethylsilylacetylene and subsequent desilylation gave 1-ethynyl-6,18-endo-ethenodihydrothebainehydroquinones. 1-[3-(4-R-Piperazin-1-yl)-propynyl]- and 1-{3-[2-(pyridin-3-yl)piperidin-1-yl]propynyl}-6,18-endo-ethenodihydrothebainehydroquinones were synthesized by the Mannich reaction of acetylenic derivatives of dihydrothebainehydroquinone with piperazine and anabasine in the presence of formaldehyde, catalyzed by copper(I) compounds. The reaction of 1-iodo-6,18-endo-ethenodihydrothebainehydroquinone with N-methylpiperazine and formaldehyde was accompanied by copper-catalyzed oxidative homocoupling.     

Diastereospecific synthesis of novel [3.6.6.4.7]-fused pentacyclic β-lactams by 6-exo-trig, 7-endo-dig tandem radical cyclization

An efficient synthesis of pentacyclic β-lactams has been achieved in high yield via a novel 6-exo-trig, 7-endo-dig tandem radical cyclization.