Stereoselective synthesis of cis-2,6-disubstituted piperidines from 1,2-cyclic sulfamidates

Diastereoselective synthesis of cis-2,6-disubstituted piperidines from 1,2-cyclic sulfamidates is described. Regioselective ring-opening reactions of 1,2-cyclic sulfamidates derived from L-phenylalanine, alanine, valine, norvaline with the ketal protected acetylide with a phenyl substituent proceed smoothly to form the N-sulfamate intermediates which on acidic hydrolysis give alkynylated amines with the ketal group intact. Hydrogenation of the alkynylated amines, debenzylation, ketal deprotection, subsequent cyclization (of aminoketones) and stereoselective hydrogenation of the cyclic iminium ion intermediates afford the corresponding cis-2,6-disubstituted piperidines in high diastereoselectivity (98%?≥?d.e.) with good chemical yields (68–86%). The present approach provides a novel route for the stereoselective synthesis of cis-2,6-disubstituted piperidines.     

TiF4-mediated biomimetic alkylation-cyclization cascade reaction of 2-trimethylsilylmethyl-1,5-dienes with aldehydes

TiF₄ has proven to be the Lewis acid of choice for promoting the biomimetic addition of 2-trimethylsilylmethyl-1,5-dienes to aliphatic aldehydes with concomitant cyclization. 1,3-cis-Disubstituted methylenecyclohexanes are thus produced in good yields and high diastereoselectivity. The reaction appears to proceed via a highly concerted mechanism involving a chair-like transition state.     

Efficient and stereoselective synthesis of novel cis-4-substituted proline analogues

A series of novel cis-4-substituted proline analogues were designed and synthesized. Highly stereoselective alkylations at the γ-position of glutamic ester 2 were achieved, followed by reduction, mesylation, and cyclization to afford the title compounds 1 in good yields and high diastereoselectivity.     

Cyclization into perhydronaphthalenones using samarium diiodide

Samarium(II) diiodide has been employed to promote the intramolecular cyclization reactions of aldehydes or ketones onto α,β-unsaturated ketones. The cyclization reactions described herein provide a general approach to the syntheses of perhydronaphthalenones with a cis-relationship between the OH at C-5 and the proton or methyl group at C-4a with good diastereoselectivity under mild reaction conditions.     

A concise synthesis of (−)-cytoxazone and (−)-4-epi-cytoxazone using chlorosulfonyl isocyanate

A concise synthesis of (−)-cytoxazone and its stereoisomer (−)-4-epi-cytoxazone, novel cytokine modulators, has been accomplished each in six steps from readily available p-anisaldehyde with good diastereoselectivity. Key steps in the synthesis include the regioselective and diastereoselective amination of anti- and syn-1,2-dimethyl ethers with chlorosulfonyl isocyanate and the subsequent regioselective cyclization of the diol to construct the oxazolidin-2-one core. The diastereoselectivity of amination reaction using CSI was explained by the Cieplak electronic model via S_N1 mechanism and neighboring group effect, leading to the retention of the configuration.     

Formal total synthesis of (±)-fragranol via template catalyzed 4-exo cyclization

A novel approach to the synthesis of (±)-fragranol is described that relies on a radical 4-exo cyclization. This key step is catalyzed by a cationic titanocene complex with a pending amide ligand. In this manner the radical and its acceptor are bound to the titanocene center in a two-point mode. By this interaction the 4-exo cyclization that is not supported by gem-dialkyl substitution is rendered thermodynamically and kinetically favorable. Moreover, the crucial intermediates and transition structures become highly ordered. This results in a good diastereoselectivity of cyclobutane formation. From the key-intermediate, the formal total synthesis of the natural product can be completed in a few steps.     

Sequential Baylis-Hillman reaction and radical cyclization of furanose derivatives: expeditious approach to enantiopure benzo-fused nine-membered oxacycles

A regioselective 9-endo-trig aryl radical cyclization of d-glucose derived diastereomeric Baylis-Hillman reaction products with Bu₃SnH led to highly functionalized tricyclic benzannulated ethers incorporating cis- and trans-9,5 bicyclic systems in good yields. Degradation of one of the products afforded an enantiopure multifunctionalized benzoxonine derivative.     

Synthesis of 9-arylamino- and (Z)-9-arylimino-9H-pyrrolo[1,2-a]indoles by reactions of 2-(pyrrol-1-yl)benzaldehydes with aryl amines

Heating mixtures of 2-(pyrrol-1-yl)benzaldehydes and aryl amines under argon afforded 9-arylamino-9H-pyrrolo[1,2-a]indoles, via cyclization of the resulting 2-(pyrrol-1-yl)benzaldimine intermediates. Heating in the presence of oxygen afforded (Z)-9-arylimino-9H-pyrrolo[1,2-a]indoles, which were successfully hydrolyzed with hydrochloric acid to give pyrrolo[1,2-a]indol-9-ones.     

Selectivities in the reaction of vicinal diimines and acyl chlorides

The reaction of vicinal diimines and acyl chlorides in the presence of triethylamine produces 3-imino-β-lactams and/or bis-β-lactams chemo-, regio-, and stereoselectively, which are important intermediates in pharmaceutical and organic synthesis. The selectivities in the reaction have been investigated. The results indicate that all diimines react with various ketenes generated from acyl chlorides in the presence of triethylamine to give rise to cis-4-imino-β-lactams (mono-cis-β-lactams) diastereoselectively due to the electron-withdrawing property of the imino group in the vicinal diimines. Bis-β-lactams were obtained from diimines via the mono-cis-β-lactams as intermediates. Only ketenes with strong electron-donating substituents can react with the mono-cis-β-lactams to yield bis-β-lactams, affording a pair of C2-symmetric cis-bis-β-lactams with symmetric diimines, two or four pairs of diastereomeric bis-β-lactams with ketoaldehyde-derived unsymmetric diimines depending on the steric hindrance of their N-substituents. The current investigation provides very important information for the selective preparation of mono- and bis-β-lactams from vicinal diimines.     

Synthesis of substituted pyrrolidines and piperidines from endocyclic enamine derivatives. Synthesis of (±)-laburnamine

Functionalization of the α- and β-positions of readily available endocyclic enamine derivatives provides a convenient method for the formation of substituted pyrrolidines and piperidines. α-Alkoxy-β-iodopyrrolidines are formed by the electrophilic addition of iodine to the endocyclic enamine double bond of an N-substituted 2-pyrroline, and nucleophillic attack by an alcohol on the intermediate iodonium ion. The resultant α-alkoxy-β-iodopyrrolidines can be used in radical cyclization reactions to give bicyclic hemiaminal compounds, which can be further elaborated using N-acyliminium chemistry to form α,β-cis-dialkylsubstituted pyrrolidines. A strategy for the incorporation of amino functionality at the β-position was also established by using iodoamination of the enamine double bond, followed by migration of the amine functionality through an aziridination/methanolysis protocol. An alternative method uses an azidomethoxylation protocol using ceric ammonium nitrate (CAN) in the presence of NaN₃ and methanol. Formation and trapping of the N-acyliminium ions derived from these substrates, afforded the 3-carbamate and 3-azido-2-substituted products with good diastereoselectivity, with the preferential formation of the trans and cis stereoisomers, respectively. Using the sequential iodoamination, aziridination in methanol and N-acyliminium transformation, trans-3-NHCO₂Me-2-allyl-pyrrolidine was prepared, which was used as the key precursor in a synthesis of the natural 1-amidopyrrolizidine alkaloid, (±)-laburnamine.     

The importance of vinylsilane stereochemistry and σ - π stabilization in iminium ion-vinylsilane cyclizations. A short total synthesis of the amaryllidaceae alkaloid (−) −epielwesine.

$\text{cis}$-3a-Aryl-2,3,3a,4,5,7a-hexahydro-1H-indoles 5 and 10 are formed in excellent yield from acid promoted cyclization of (Z)-vinylsilane imines 3 and 9. The failure of the corresponding (E)-vinylsilane isomer 4 to cyclize under similar conditions demonstrates that the β-silylcation intermediates formed in these reactions derive significant stabilization from σ - π delocalization.     

Protecting group directed cyclization: asymmetric synthesis of both cis- and trans-dihydrexidine from a common precursor

Protection group of amino- and tethered o-arene functionality of 1,4-aryl-2-amino-1-butanol derived from l-serine dictates the cyclization mode under acidic conditions leading to reverse diastereoselectivity. N-Boc and acetal protected amino alcohol undergo cascade cyclization providing exclusively cis-dihydrexidine via reduction, where formation of C-ring (isoquinoline unit) prior to Friedel–Crafts cyclization control the cis-stereochemistry of the B-ring. N-Cbz and O-benzyl protection direct first F–C cyclization yielding the trans-1-aryl-2-aminotetralin and subsequent deprotection-cyclization forming the C-ring afforded dihydrexidine.     

N-Arylmethyl-7-azabicyclo[2.2.1]heptane derivatives: synthesis and reaction mechanisms

N-Arylmethyl-7-azabicyclo[2.2.1]heptane (I) derivatives have been synthesized by deprotection of N-protected, N-(arylmethyl)cyclohex-3-enamines, bromination of the resulting secondary cyclohex-3-enamines, followed by base-promoted cyclization (route a), or by bromination of N-protected, N-(arylmethyl)cyclohex-3-enamines followed by deprotection and base-mediated cyclization (route b). In these protocols we have observed that the bromination of the key intermediates (12, 13, and 19) is stereoselective leading to major trans-3-cis-4-dibromides (14, 17, and 20), whose mild base-mediated heterocyclization (on compound 14), or the two-step acid hydrolysis plus base-promoted cyclization (on compounds 17 and 20), gave products 6 and 7 in good yield. A mechanistic investigation using DFT has been carried out to explain the results observed in this work.     

Lewis acid mediated diastereoselective keto-ene cyclization on chiral perhydro-1,3-benzoxazines: synthesis of enantiopure cis-3,4-disubstituted 3-hydroxypyrrolidines

Chiral 2-acyl-3-allyl-perhydro-1,3-benzoxazines derived from (−)-8-aminomenthol were easily cyclized in the presence of Lewis acids at 0 °C. The diastereoselectivity of the cyclization was dependent on the nature of the Lewis acid. The cyclization compounds can be transformed into enantiopure cis-3,4-disubstituted 3-hydroxypyrrolidines by ring opening of the N,O-acetal moiety and subsequent elimination of the menthol appendage.     

Synthesis of 2-trifluoromethyl-1(substituted aryl)-4(1H)-quinolones using trifluoroacetamidoyl chlorides

A simple two-step procedure for the preparation of 2-trifluoromethyl-1(substituted aryl)-4(1H)-quinolones utilizing electrophilic trifluoroacetamidoyl chlorides as the starting materials is described. The cyclization of trifluoromethyl enaminone intermediates with potassium carbonate in hot dimethylformamide produce the trifluoromethylated 4(1H)-quinolones in good to excellent yields.     

A convenient synthesis of iminosugar-C-glycosides via organometallic addition to N-benzyl-N-glycosylhydroxylamines

N-Benzyl-N-glycosylhydroxylamines were prepared in very good yield via condensation of furanoses and pyranoses with N-benzylhydroxylamine at 110°C for 30 min under solvent-free conditions. These anomeric sugar-hydroxylamines exist in equilibrium with the open-chain nitrone form. In fact upon treatment with various organometallic reagents, the corresponding adducts were obtained with good to high diastereoselectivity. These adducts were converted into iminosugar-C-glycosides by reductive dehydroxylation and intramolecular cyclization.     

Aerobic oxidative desymmetrization of meso-diols with bifunctional amidoiridium catalysts bearing chiral N-sulfonyldiamine ligands

Asymmetric aerobic oxidation of a range of meso- and prochiral diols with chiral bifunctional Ir catalysts is described. A high level of chiral discrimination ability of Cp^∗Ir complexes derived from (S,S)-1,2-diphenylethylenediamine was successfully demonstrated by desymmetrization of secondary benzylic diols such as cis-indan-1,3-diol and cis-1,4-diphenylbutane-1,4-diol, providing the corresponding (R)-hydroxyl ketones with excellent chemo- and enantioselectivities. Enantiotopic group discrimination in oxidation of symmetrical primary 1,4- and 1,5-diols gave rise to chiral lactones with moderate ees under similar aerobic conditions.     

Asymmetric synthesis of β-lactams by [2+2] cycloaddition using 1,4:3,6-dianhydro-d-glucitol (isosorbide) derived chiral pools

Highly diastereoselective synthesis of cis-β-lactams via [2+2] cycloaddition reactions of imines derived from a chiral bicyclic aldehyde and ketenes is described. The chiral bicyclic aldehyde as well as chiral acids were prepared from commercially available inexpensive isosorbide. The cycloaddition reaction was found to be highly diastereoselective; in some cases giving a single diastereomer of cis-azetidin-2-one in very good yields. A moderate diastereoselectivity was observed with chiral ketenes derived from isosorbide.     

An Enantioselective β-Lactam Synthesis Starting from L-(S)-Glyceraldehyde Acetonide

Complete diastereoselectivity is observed during the cyclocondensation of activated glycine derivatives with aldimines derived from L -(S)-glyceraldehyde acetonide. 3,4-cis-β-lactams are isolated in high optical and chemical yields. They are converted into key intermediates used in the syntheses of various mono- and bicyclic β-lactam antibiotics. A mechanism is suggested to explain this remarkable diastereoselectivity.     

Nucleophilic Reactions at Tertiary Carbon. Part 3. σ- and π-routes to the 9-decalyl cation

The generation of the 9-decalyl cation by solvolysis of cis- and trans-9-decalyl chloride ( 1 ) has been reinvestigated. The results of product, rate and isomerization studies implicate stereoisomeric ion pairs as intermediates, as in the case of the solvolysis of other stereoisomeric tertiary chlorides (Parts 1 and 2). On the other hand, both symmetrically and unsymmetrically solvated 9-decalyl cations are indicated in the cyclization of 4-(cyclohexen-1-yl)butyl tosylate. No evidence was obtained that conformational isomers of the 9-decalyl cation play a role as product determining intermediates.