Novel synthesis of fused indoles and 2-substituted indoles by the palladium-catalyzed cyclization of N-cycloalkenyl-o-haloanilines

A new palladium-catalyzed cyclization of N-alkenyl-o-haloanilines with selective isomerization of a double bond followed by formal 5-endo-trig cyclization was developed. A variety of fused and 2-substituted indoles were synthesized from enaminoesters prepared by condensation of β-ketoesters and o-iodoaniline.     

Synthesis of 13a-methylphenanthroindolizidines using radical cascade cyclization: synthetic studies toward (±)-hypoestestatin 1

A radical cascade involving 6-endo cyclization of aryl radicals generated from N-acryloyl-N-(1-methylethenyl)-9-bromophenanthren-10-ylmethylamines, followed by 5-endo-trig cyclization of the resulting α-amidoyl radicals afforded phenanthroindolizidines bearing a methyl substituent at the angular C13a position. 2,3,6-Trimethoxy derivative was synthesized by using this method, but its spectral data were not in accord with those of literature values reported for hypoestestatin 1. Further synthetic study toward hypoestestatin 1 is demonstrated.     

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.     

Me3SiCl-promoted intramolecular cyclization of aromatic compounds tethered with N,O-acetals leading to the facile preparation of 1,4-benzodiazepine skeletons

The Me₃SiCl-promoted intramolecular aminomethylation of a novel type of N,O-acetals, which were prepared via a facile three-step synthesis from N-alkylaniline derivatives and N-alkyl-2-oxazolidinones that leads to the production of pharmaceutically useful 1,4-benzodiazepine skeletons with a variety of functional groups is described. This method was successfully applied to the facile preparation of both tricyclic benzodiazepine derivatives and a 1,4-benzoxazepine derivative via 7-exo-trig cyclization.     

Palladium-catalyzed synthesis of indane and cyclobuta[a]indenes from homoallylic alcohols derived from Baylis-Hillman adducts: base-dependent stereoselectivity for the benzylidene group in cyclobuta[a]indene

Various indane and cyclobuta[a]indene derivatives were synthesized by palladium-catalyzed cyclization of homoallylic alcohol derivatives prepared from Baylis-Hillman adducts. Especially, cyclobuta[a]indene derivative was synthesized stereoselectively by palladium-catalyzed 5-exo-trig/4-exo-trig cascade cyclization, albeit in moderate yield. The Z isomer was formed exclusively in the presence of Et₃N by usual Heck-type carbopalladation process while E isomer with Cs₂CO₃ most likely by a concerted metalation/deprotonation (CMD) process.     

Manganese(III)-mediated facile synthesis of 3,4-dihydro-2(1H)-quinolinones: selectivity of the 6-endo and 5-exo cyclization

The 4,4-bis(ethoxycarbonyl)-3,4-dihydro-2(1H)-quinolinones 2 were easily synthesized by the oxidative 6-endo-trig cyclization of 2-[2-(N-arylamino)-2-oxoethyl]malonates 1 with manganese(III) acetate in good to excellent yields. The same reaction of N-(2,4-dimethoxyphenyl)-substituted malonate 1t exclusively produced the 5-exo-cyclized 4,4-bis(ethoxycarbonyl)-1-azaspiro[4,5]deca-6,9-diene-2,8-dione 5t instead of the corresponding dihydroquinolinone. The regioselectivity during the cyclization could be explained by the difference in the activation energy of the transition state of the 6-endo/5-exo cyclization.     

Synthesis of cyclopropyl silyl ethers and their facile ring opening by photoinduced electron transfer as key step in radical/radical cationic cascade reactions

Various ring-fused cyclopropyl silyl ethers with an benzylic, olefinic or acetylenic side chain have been synthesized. Upon oxidative photoinduced electron transfer (PET) the cyclopropane ring opens and forms a reactive β-keto radical, which undergoes intramolecular cyclization. In some cases we observed only formation of ring opened non-cyclized products. With olefinic side chain 5-exo-trig mode of cyclization rather than 6-endo-trig mode of cyclization takes place whereas in case of acetylenic side chain we observed 6-endo cyclization.     

Alkyl radical cyclization to vinylogous carbonates for the stereoselective synthesis of unsymmetrical dioxa-cage compounds: effect of conformation on the rate of cyclization versus reduction

An efficient strategy for the stereoselective construction of unsymmetrical dioxa-cage compounds containing ether linkages employing a 6-exo-trig alkyl radical cyclization to vinylogous carbonates is developed. The radical precursors are prepared from the diols obtained from the Diels-Alder adducts via iodoetherification followed by addition of the alcohol to the ethyl propiolate. The geometrical constrains play important role in deciding the outcome of the reaction as cyclization versus simple reduction. Formation of the mono-oxa-cage compounds via a 5-exo-trig intramolecular alkyl radical cyclization to olefin is also described. The dioxa-cages could also be assembled employing a tandem oxymercuration reduction-radical cyclization to vinylogous carbonates protocol with equal efficiency and with reduced number of steps.     

π-Annulation reactions of 4-thiazolidinone enaminones in the synthesis of fused bi- and tri-cyclic compounds

The α-C position in 4-oxo thiazolidinones functions as one of the three possible nucleophilic sites in these molecules. We used the inherent reactivity of α-C of the exocyclic double bond (a so called push pull system) to obtain bicyclic fused thiazolidinones via π-annulation cyclization. Appropriate reaction conditions to selectively activate this position and secondary nitrogen towards N,π-annulation were found. Furthermore, the intramolecular vinylogous iminium ion 6-exo-trig cyclization was used to access fused bicyclic precursors for the π-annulation in order to obtain the novel tricyclic structures stereoselectively.     

DBU-mediated regioselective intramolecular cyclization/dehydration of ortho diketo phenoxyethers: a synthesis of 2,3-substituted γ-benzopyranones

The regioselective cyclization/dehydration sequence of ortho diketo phenoxyethers induced by DBU has been explored. The results demonstrated a high degree of selectivity with preference for 6-exo-trig cyclization leading to the formation of γ-benzopyranone derivatives in good yield.     

Facile access to a benzoazepinoquinazolinone via a free radical cyclization

An efficient synthetic protocol based on a free-radical cascade reaction via 6-exo-trig and 7-endo-trig cyclization processes is described for a new substituted benzoazepinoquinazolinone system which could in principle be used to form the cyclopropanequinone system found in duocarmycins.     

Synthesis of enantiomerically pure perhydrofuro[3,4-b]pyrans and perhydrofuro[3,4-b]furans

Olefinic diols, prepared from (R)-(+)-2,2-dimethyl-1,3-dioxolane-4-carboxaldehyde via olefination and hydrolysis, were converted into enantiomerically pure hydroxy substituted tetrahydrofuran derivatives by cyclization using N-phenylselenophthalimide and BF₃. The PhSe group in the C-4 position of these tetrahydrofurans was then substituted by an allyl group using allyltributylstannane in the presence of AIBN. The selenium promoted cyclizations of the allyl tetrahydrofurans in which the OH and the allyl groups are trans to each other formed the enantiopure perhydrofuro[3,4-b]pyrans, while the cyclization of the allyl tetrahydrofurans in which the OH and the allyl groups are cis gave rise to the perhydrofuro[3,4-b]furans. These bicyclic products were finally deselenenylated with triphenyltin hydride and AIBN.     

Stereoselective synthesis of contiguous THF–THF and THF–THP units via Pd-catalyzed tandem reaction with 1,3-chirality transfer

The Pd^II-catalyzed tandem cyclization of chiral allylic alcohols possessing an internal epoxide and a terminal alcohol provided a contiguous THF–THP and THF–THP ring units stereospecifically. The cyclizations take place via a 5-exo-tet-5-exo-trig mode, however, the cyclization of methyl substituted epoxy diols proceeded via 6-endo-tet-6-exo-trig fashion in a part to construct the oxygen-fused THP–THP ring. The different reaction rates of precursors, which are different stereochemistry at allylic alcohol have been elucidated.     

Nucleophilic 5-endo-trig cyclization of 2-(trifluoromethyl)allylic metal enolates and enamides: Synthesis of tetrahydrofurans and pyrrolidines bearing exo-difluoromethylene units

Ketones and imines bearing a 2-(trifluoromethyl)allylic moiety successfully underwent nucleophilic 5-endo-trig cyclization via their metal enolates and enamides. O- or N-Cyclization proceeded exclusively in each case to afford the corresponding five-membered heterocycles with both exo-difluoromethylene and exo-alkylidene units. On treatment with potassium hexamethyldisilazide (KHMDS) or lithium diisopropylamide (LDA), 2-(trifluoromethyl)allylic ketones or imines provided the corresponding tetrahydrofurans or pyrrolidines bearing a Z-alkylidene group with perfect or substantial stereoselectivity, respectively.     

Intramolecular addition of carbon radicals to aldehydes: synthesis of enantiopure tetrahydrofuran-3-ols

A simple and efficient substrate-controlled asymmetric synthesis of enantiopure tetrahydrofuran-3-ols by a 5-exo-trig radical cyclization is described. This cyclization occurs when a δ-carbon radical adds intramolecularly to the carbonyl group of an aldehyde. The δ-carbon radicals can be efficiently produced from the tin hydride mediated deselenenylation of 5-phenylseleno-3-oxapentanals, which were easily prepared starting from commercially available enantiopure epoxides or chlorohydrins.     

Synthesis of aza-aromatic hydroxylamine-O-sulfonates and their application to tandem nucleophilic addition-electrophilic 5-endo-trig cyclization

Hydroxylamine-O-sulfonic acid (HOSA) was used as an efficient nucleophilic amination reagent for 2-chloropyrimidines, 2-chloroquinolines, and 1-chloroisoquinoline. The newly obtained heteroaromatic hydroxylamine-O-sulfonates subjected to the reaction with acyl isothiocyanates underwent tandem nucleophilic addition-electrophilic 5-endo-trig cyclization. The mechanism of the cyclization was investigated with use of the long-range corrected hybrid density functional ωB97X-D/6-31+G^∗ and SM8 (DMF) solvation model. The structures of the heteroaromatic hydroxylamine-O-sulfonates and N-(5-methoxy-2H-[1,2,4]thiadiazolo[2,3-a]pyrimidin-2-ylidene)benzamide were confirmed by single crystal X-ray analysis. N-(2H-[1,2,4]thiadiazolo[3,2-a]isoquinolin-2-ylidene)benzamide exhibited a pronounced in vitro cytostatic activity against human tumor cell lines SISO, LCLC, A-427, DAN-G, and RT-4 (IC₅₀ in the range 1.47-2.97 μM).     

Spiro cyclisations of N-acyliminium ions involving an aromatic π-nucleophile

Spiro 2-pyrrolidin-5-ones were obtained from N-substituted succinimides by a two-step procedure, involving 5- or 6-endo-trig cyclisation of N-acyliminium ion intermediates with a tethered aromatic π-nucleophile.     

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.     

A new and practical PIFA-promoted olefin amidohydroxylation: six- versus five-membered ring formation

A novel access to the isoindolinone and isoquinolin-2-one skeletons from adequately substituted aromatic precursors is described. The key intramolecular cyclization step was performed by the action of phenyliodine(III)bis(trifluoroacetate) (PIFA) on the corresponding vinyl or allyl substituted N-(p-methoxyphenyl)benzamide derivatives leading to the heterocyclic compounds through 5-exo-trig and 6-exo-trig processes, respectively.     

Palladium-catalyzed intramolecular 5-endo-trig oxidative Heck cyclization: a facile pathway for the synthesis of some sesquiterpene precursors

An efficient and convenient method for the construction of substituted cyclopentenones via palladium-catalyzed intramolecular 5-endo-trig oxidative cyclization has been introduced as a powerful new strategy for the synthesis of sesquiterpenes.