Tin-mediated cyclization

Tin-mediated cyclizations were then carried out on halides 49D and 49E (Bu3SnH, AIBN, heat), followed by Tamao oxidation (for 49D KF, KHCO3, H2O2) or fluoride-induced desilylation (for 49E TBAF) [121, 124]. Here,... 

From halides. Halides are easily converted into selenides. Since halides are also suitable radical precursors, this transformation is usually done when side reactions of halides with nucleophiles can occur. An example of this type is reported in Eq. (1). The bromide 5 was converted into a phenyl selenide, which could stand DIBALH reduction and imine formation. Tin mediated cyclization of 6 afforded the cyclopentylamine 7 in 72% yield [5]. 

Fused indoles have been synthesized using various radical strategies. Some of the more renowned works include Ziegler s Photochemically-induced cyclization of aUcyl, vinyl, oxiranyl, and aziridinyl radicals [87-89], Chuang s benzenesulfonyl radical induced oxidative cyclization of l-(4-allylsulfonylbutyl)indole [90], and Moody s tin-mediated cyclization of l-co-iodoalkylindoles [91]. 

Hies L, Tsuji H, Sato Y, Naktunura E (2008) Modular synthesis of functionalized benzosUoles by tin-mediated cyclization of (o-alkynylphenyl)silane. J Am Chem Soc 130 4240-4241 Hies L, Tsuji H, Nakamura E (2009) Synthesis of benzo[b]siloles via KH-promoted cyclization of (2-alkynylphenyl)silanes. Org Lett 11 3966-3968... 

Another tin-mediated cyclization reported this year is the formation of, for example, (198) from (1S>9) with SOClg-pyridine. This and related cyclizations are thought to proceed via the formation of a y-carbo-cation followed by destan-nylation. Finally, BuaSnH converts thiocyanates RSCN into thiostannanes (200) which are air-stable and high-boiling protected forms of the parent thiols. ... 

Tin-mediated-radical cyclization of isonitriles provides a useful strategy for the preparation of indoles (Fukuyama reaction).90 This radical cyclization is used for synthesis of 6-hydroxy-indole-3-acetic acid, which is the aromatic subunit of Nephilatoxin. The requisite isonitriles are prepared from nitroarenes via amines (Eq. 10.66).91... 

Tributyl tin radical mediated cyclization of the glucose derived exo-methylene furanose derivatives led to highly functionalized cA-fused bicyclic ethers. The product could subsequently be transformed into optically active tricyclic nucleoside analogue or oxepine derivative (Fig. 51).67... 

Radical cyclizations based on tin mediation present an additional method for effecting intramolecular additions to unsaturated sugar derivatives, the processes commonly occurring with both regio- and stereo-specificity. 

Stable 2,5-dihydropyridines have also been formed from the tin-mediated radical cyclization of /3-allenylbenzo-yloximes 37 but only when the oxime substituent was electron withdrawing (R = Ph or P(0)(0Et)2). When the substituent is not electron withdrawing (e.g., R = H, Me) the 5-o o-cyclization occurs to form cyclopentenes (Scheme 9) <2000EJ0275>. The cyclization has also been reported utilizing the tosyl radical as a substitute for the tributyltin radical <1999TL4547>. 

As an alternative to tin-mediated conditions, thiyl radicals are an attractive alternative they generate carbon-centered radicals upon reversible addition to a multiple bond. This was found to be effective for cyclizations of 49E and 49F (Scheme 13). Treatment with AIBN and PhSH, followed by fluoride-induced desilylation, afforded vinyl and 2-(thiophenyl)vinyl adducts 50E and 50F, respectively [122, 123, 125]. Interestingly, during fluoride-induced (TBAF) desilylation of the intermediate formed... 

Vinyl radicals also add to carbon-carbon double bonds intramolecularly to give 2,6-cw-disubstituted cyclic ethers (Equation (5)).41 In the tin hydride-mediated cyclization of the substrates including alkynes, alkyl radicals attack to carbon-carbon triple bonds leading to uco-alkylidene allylic alcohols (Equation (6)).42 The coupling reaction between alkyl radicals may afford cyclization products. Thus, the reduction of 1,3-diiodopropane derivatives with a tin hydride provides substituted cyclopropanes.4... 

Wnuk, S.F., Bergolla, L.A., and Garcia, P.L, Jr., Studies toward the synthesis of a-fluorinated phosphonates via tin-mediated cleavage of a-fluoro-a-(pyrimidin-2-ylsulfonyl)alkylphosphonates. Intramolecular cyclization of the a-phosphonyl radicals, J. Org. Chem., 67, 3065, 2002. 

Two type Ib approaches to indoles were reported involving tin-mediated radical cyclization reactions. Treatment of either 2-alkenylthioanilides <05H(66)241> or 2-(l-alkenyl) imidoyltellurides <05SL1893> provided 2,3-disubstituted indoles. The former precursor was also utilized in a synthesis of tetrahydro- 3-carbolines. 

Very recently, a variant of this methodology was developed by the same author through tin-radical-mediated cyclization of ort/zo-alkenyl-substituted thioanilides [20], Moreover, Rainier et al. demonstrated that a very efficient synthesis of indoles can be carried out also with urz/to-alkynyl-substituted aryl isonitriles, provided that a TMS group is linked to the alkyne moiety [21] (Scheme 13). 

A particularly convenient procedure for the enantioselective total synthesis of (-)-a-kainic acid also employed tin-radical-mediated cyclization of an alkenyl monothioformimide (See Ref. 22e). 

Miscellaneous reactions. The Heck reaction is accomplishable in perfluorinated solvents while employing a fluorous triarylphosphine ligand. A minimally fluorous diaryl diselenide used to assist tin hydride mediated cyclization can be recovered. ... 

A radical-mediated two-step synthesis of spiroketals has recently been reported the strategy (an example is shown in Scheme 8) involves an intramolecular radical cyclization of a ketal precursor (14), which could be easily prepared from 2-methylene-3,4-epoxyoxolanes (13). As alternative to tin-hydride mediated cyclization of alkynyl halides, samarium(II) iodide has been used to generate the alkyl radical which adds intramolecularly to the triple bond. ... 

Tin-mediated tandem cyclization can be useful for the construction of heterocyclic rings as intermediates in natural product synthesis a series of enynes like 55 has been recently used and the reaction also represents a general route to allyl stannanes (equation 44). 

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