An easy entry into berbane and alloyohimbane alkaloids via a 6-exo radical cyclization

[reaction: see text] The pharmacologically important tetracyclic berbane and pentacyclic alloyohimbane structures were prepared efficiently in four steps including a stereoselective 6-exo radical cyclization using xanthates as the radical source.     

A new entry to the synthesis of 2,3-disubstituted indoles

[reaction: see text] The Stille coupling of N-acyl-2-iodoanilines with the 1-(tributylstannyl)-1-substituted allenes affected the successive one-step formation of the 2-methyl-3-substituted indoles. Alternatively, the other type of 2-alkyl-3-substituted indoles could be synthesized in a one-pot operation, which consists of the Stille coupling reaction with the 1-(tributylstannyl)-1,3-disubstituted allenes, followed by TBAF treatment. This procedure could be applied to the synthesis of indomethacin.     

Synthesis of 2,3-disubstituted benzo[b]selenophenes via electrophilic cyclization

2,3-Disubstituted benzo[b]selenophenes have been prepared by the electrophilic cyclization of various 1-(1-alkynyl)-2-(methylseleno)arenes by Br2, NBS, I2, ICl, PhSeCl, PhSeBr, and Hg(OAc)2. This method tolerates a wide variety of functional groups, including alcohol, ester, nitrile, nitro, and silyl groups, and proceeds under exceptionally mild reaction conditions.     

Metathesis reaction of formaldehyde acetals: an easy entry into the dynamic covalent chemistry of cyclophane formation

The acid-catalyzed transacetalation of formaldehyde acetals (formal metathesis) is a suitable reaction for the generation of well-behaved Dynamic Libraries of cyclophane formals. The composition of the equilibrated mixtures solely depends on concentration, and is totally independent of whether the feedstock is any of the pure cyclic oligomers, or a mixture of oligomers/polymers. Effective Molarities related to the formation of the lower cyclic oligomers were directly measured as their equilibrium molar concentrations above the critical monomer concentration. The finding that silver cation acts as a selective binder toward the cyclic dimer C2, coupled with the "proof reading and editing" capability of our quickly equilibrating system, translated into significant amplifications of C2 when the equilibrated mixtures were exposed to the action of the silver template. These results highlight the potential of Dynamic Combinatorial Chemistry as a powerful tool for the preparation in synthetically useful amounts of an otherwise elusive macrocyclic compound. The possibility of using a mixture of high molecular weight byproducts as feedstock adds considerably to the practical value of the procedure.     

Synthesis of 2,3-disubstituted indole using palladium(II)-catalyzed cyclization with alkenylation reaction

The reaction of ethyl 2-ethynylphenylcarbamate derivative with alkenes in the presence of a palladium(II) catalyst, copper dichloride and tetrabutylammonium fluoride (TBAF) produced 2-substituted 3-ethenylindoles during refluxing. The intramolecular cyclization reaction of ethyl 2-ethynylphenylcarbamates, which have an ethenyl part in the ethynyl group, was also used to produce carbazole derivatives.     

Synthesis of 2,3-disubstituted benzo[b]thiophenes via palladium-catalyzed coupling and electrophilic cyclization of terminal acetylenes

2,3-Disubstituted benzo[b]thiophenes have been prepared in excellent yields via coupling of terminal acetylenes with commercially available o-iodothioanisole in the presence of a palladium catalyst and subsequent electrophilic cyclization of the resulting o-(1-alkynyl)thioanisole derivatives. I(2), Br(2), NBS, p-O(2)NC(6)H(4)SCl, and PhSeCl have been utilized as electrophiles. Aryl-, vinyl-, and alkyl-substituted terminal acetylenes undergo this coupling and cyclization to produce excellent yields of benzo[b]thiophenes. (Trimethylsilyl)acetylene also undergoes this coupling/cyclization process with I(2), NBS, and the sulfur and selenium electrophiles to afford the corresponding 2-(trimethylsilyl)benzo[b]thiophenes. However, cyclization of the silyl-containing thioanisole using Br(2) affords 2,3-dibromobenzo[b]thiophene.     

(R)-2,3-Cyclohexylideneglyceraldehyde: a novel template for simple entry into both cis- and trans-2,5-disubstituted tetrahydrofurans

Sharpless asymmetric dihydroxylation at the terminal olefin of benzoates 3a and 3b, using both AD-mix α and AD-mix β afforded only one diastereomer of diols 5a and 5b, respectively. Diols 5a and 5b were easily transformed into cis- and trans-2,5-disubstituted tetrahydrofurans 7 and 14, respectively, which were subsequently converted into known compounds 12 and 19.     

Electrophilic cyclization of 2-geranylthiophene and its 5-carboxy derivative

Conclusions An allylic thienyl group can serve as a terminator of electrophilic cyclization, which in this case proceeds by a stepwise scheme.See [1] for preliminary communication.Deceased.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2578–2583, November, 1980.     

Palladium-catalyzed beta-allylation of 2,3-disubstituted indoles

Given the prevalence of the indole nucleus in biologically active compounds, the direct C3-functionalization of 2,3-disubstituted indoles represents an important problem. Described is a general, high-yielding method for the palladium-catalyzed beta-allylation of carba- and heterocycle fused indoles, including complex natural product substrates.     

Stereochemistry of 2,3-disubstituted 1,3,2-dioxaborinanes

According to¹H NMR spectroscopic data, molecules of 2,4-disubstituted 1,3,2-dioxaborinanes exist in the preferred sofa conformation with equatorial alkyl groups at C₍₄₎. This conclusion has been confirmed by calculation of the optimal geometry of the ring of the cyclic boron esters by the MM2 molecular mechanics method. A. V. Bogatskii Physicochemical Institute, Ukraine National Academy of Sciences, Odessa 270080. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1574–1578, November, 1997.     

Highly regioselective synthesis of 2,3-disubstituted indenes via a novel palladium-catalyzed cyclization reaction of propargylic carbonates with carbon nucleophiles

Palladium-catalyzed reaction of propargylic carbonates with carbon nucleophiles offers an efficient, direct route to highly substituted indenes. The reaction conditions and the scope of the process are examined, and a possible mechanism is proposed. [reaction: see text]     

Electrophilic cyclization of (Z)-selenoenynes: synthesis and reactivity of 3-iodoselenophenes

We present here our results of the electrophilic cyclization reaction of (Z)-selenoenynes with different electrophiles such as I(2), ICl, PhSeBr, and PhSeCl. The cyclization reaction proceeded cleanly under mild reaction conditions, and 3-substituted selenophenes were formed in moderate to excellent yields. We observed that the nature of solvent and structure of (Z)-selenoenyne were important to the cyclization reaction. In addition, the obtained 3-iodoselenophenes were readily transformed to more complex products using a metal-halogen exchange reaction with n-BuLi and trapping the intermediate formed with aldehydes, furnishing the desired secondary alcohols in good yields. Conversely, using the palladium or copper catalyzed cross-coupling reactions with terminal alkynes or alkyl alcohols, we were able to convert 3-iodoselenophene to Sonogashira or Ullmann type products, respectively, in good yields.     

An easy one-pot stereoselective synthesis of 4-substituted and 4,5-disubstituted oxazolidin-2-ones from N-Boc-2,3-aziridino alcohols

A novel and efficient one-pot stereoselective transformation of N-(t-butoxycarbonyl)-2,3-aziridino alcohols into 4-substituted and 4,5-disubstituted oxazolidin-2-ones has been developed; these functionalized products are amenable to other elaborations, some of which are described.     

High-yield syntheses of 2,3-disubstituted furans and thiophenes

The course of lithiation of furan- and thiophene-2-carboxylic acids is critically dependent on the identity of the lithium base allowing high-yielding syntheses of 2,3- and 2,5-disubstituted thiophenes: the proposed use of a trimethylsilyl blocking group to control metallation in the furan acid provides a useful route to 2,3-disubstituted furans.     

Oxidative cyclization of beta-hydroxyenones with palladium(II): a novel entry to 2,3-dihydro-4H-pyran-4-ones

[reaction: see text] A palladium(II)-mediated oxidative cyclization was found to be effective for the preparation of structurally diverse 2,3-dihydro-4H-pyran-4-ones from the corresponding beta-hydroxyenones. Attractive features of this transformation include the ready availability of the starting enones, the regiocontrol, and the easy access of enantiopure 2,3-dihydro-4H-pyran-4-one from the corresponding enantiopure enone.     

A new synthesis of 2,3-disubstituted indoles

A novel synthesis of 2,3-disubstituted indoles starting from N-alkyl-N-propvnyl anilines is described. The reactions employed are analogous to the thermal rearrangement of aryl propynyl sulfoxides.     

Stereocontrolled chemoenzymatic synthesis of 2,3-disubstituted piperidines

(R,R)-2,3-Disubstituted piperidines were readily synthesized starting from (R)-(+)-5-bromo-2-hydroxypentanenitrile (R)-2. An enantioselective (R)-oxynitrilase-catalyzed transcyanation was used to prepare the starting cyanohydrin (R)-2.     

Thiol mediated 8-endo-trig radical cyclization: an easy access to medium-sized cyclic ethers

An efficient new method for the synthesis of eight-membered heterocycles has been developed via a thiophenol mediated intramolecular 8-endo radical cyclization reaction. Alkenyl radicals are generated from easily available terminal alkynes and thiophenol.     

Iminium salts of omega-dithiafulvenylpolyenals: an easy entry to the corresponding aldehydes and doubly proaromatic nonlinear optic-phores

A short, high-yielding route to omega-dithiafulvenylpolyenals (1) via the corresponding iminium salts (2) and starting from trimethyl-1,3-dithiolium tetrafluoroborate is reported. The Knoevenagel reactions of either 1 or 2 with isoxazolone-containing acceptors afford merocyanines 7 and 9, in a process that is often accompanied by a vinylene-shortening side reaction. Experimental and theoretical studies reveal that compounds 7 and 9, featuring two proaromatic end groups, are strongly polarized and show good second-order nonlinear optical responses.