Intramolecular reductive cyclization

Both 1,6- and 1,7-enynes could be cyclized, but carbonylation proceeded only in the presence of CHCI3. 

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Conjugated nitrones are formed by intramolecular reductive cyclizations of nitro groups onto ketones the resulting nitrones give starting materials for preparing azasteroids. An example is shown in Eq. 8.43.66... 

Bicyclic y-lactones,7 These lactones can be obtained by intramolecular reductive cyclization of unsaturated keto esters with Sml2 in THF-HMPT. Example ... 

Scheme 12 Nickel cyclam mediated intramolecular reductive cyclization.

The previous chapter covered radical cation cyclization reactions that were a consequence of single-electron oxidation. In the following section, radical anion cyclization reactions arising from single-electron reduction will be discussed. In contrast to the well documented cyclization reactions via carbon-centered free radicals [3, 4], the use of radical anions has received limited attention. There are only a few examples in the literature of intramolecular reductive cyclization reactions via radical anions other than ketyl. Photochemi-cally, electrochemically or chemically generated ketyl radical anions tethered to a multiple bond at a suitable distance, have been recognized as a promising entry for the formation of carbon-carbon bonds. 

Table 10. Intramolecular Reductive Cyclization of Aliphatic Ketones 16,, 7> 0.1 M (C4H,)4NBF4 in DMF, 2 F mol"1...

To examine whether TAA+ cations other than DMP+ can catalyze intramolecular reductive cyclizations, the reduction of <59 was performed with 0.005 M (C2H5)4NBF4 in solution. Like DMP+, (C2HS)4N+ had a catalytic effect, 69 could be reduced at —2.80 V(SCE) and the cyclic alcohol 70 was obtained in high yield (94%). However, (C2H5)4N+ was less efficient and 6 F mol-1 were consumed for completion, while the DMP+ mediated reduction of 69 was complete after transfer of 2 F mol-1. 

While some of the reactions mentioned above can be used for the synthesis of 1,1-difunctionalized siloles30, none of them can afford 2,5-difunctionalized siloles. 2,5-Difunctionalized siloles can be directly prepared only by route B. Wrackmeyer has reported that a reaction of bis(stannylethynyl)silanes 8 with BR3 (R = Me, Et, i-Pr, and n-Bu) affords 2,5-distannyl-3-borylsiloles 9 (equation 3)31. We have recently reported a general synthesis of 2,5-difunctionalized siloles by the intramolecular reductive cyclization of diethynylsilanes (equation 4)32. Thus, the reaction of bis(phenylethynyl)silanes 10 with lithium naphthalenide affords 2,5-dilithiosiloles 11, which can be further transformed into a variety of 2,5-difunctionalized siloles 12. In addition, starting from bis(phenylethynyl)diaminosilanes 13, various 1,1-difunctionalized siloles 15 are also accessible through 1,1-diaminosiloles 14 (equation 5)33. With these functionalized siloles in hand, various silole-based <7- and jr-con jugated compounds have been prepared as described below. 

The development of a new synthesis of 1-hydroxyindoles 131, via a lead-promoted intramolecular reductive cyclization of the o-nitrobenzyl ketones (or aldehydes) 132 in the presence of tetraethylammonium formate (TEAF), offers a new useful route to this interesting class of compounds <03JOC9865>. 

Intramolecular reductive cyclization of 7 with sodium dithionite gives 3-phenyl-5,7-dimeth-oxyquinoxalin-2(l//)-one (8) in good yield. ... 

Intramolecular reductive cyclization involving a conjugated ester and a 8-keto group leads to the formation of cyclopropanolactone. ... 

Another hydroxymethyl equivalent is the silylmethyl group. Tamao oxidation of the product obtained from the samarium iodide-promoted intramolecular reductive cyclization of bromosi-lyloxy derivatives leads to the hydroxymethyl group (eq 48). ... 

Nagarai M, Boominathan M, Manikannan R, Muthusubramanian S, Bhuvanesh N. Synthesis of highly substituted N-hydroxy piperedine via intramolecular reductive cyclization of l-keto-5-ketoxime. Synthetic Comm 2013 43(7) 930-40. 

TTie first general synthetic route to 2,5-difunctionaUzed siloles is the intramolecular reductive cyclization of diethynylsilanes [10]. Thus, the reduction of bis(phenylethynyl)silane 1 using an... 

Thus, the intramolecular reductive cyclization of diethynylsilane, followed by quenching with the remaining lithium naphthalenide with a bulky chlorosilane and transmetalation with ZnCl2(tmen) (tmen = MAf.JVW -tetramethylethylenediamine), affords 2,5-dizinc silole 5, which is subsequently treated with the appropriate aryl bromides in the presence of a Pd catalyst to give the corresponding 2,5-diarylsiloles 6 in high yields. 

Aryl-2-methylene-4-nitroalkanoates, obtained by 5- 2 nucleophilic reaction of the acetate of MBH products with nitroalkanes, also offers opportunities to construct highly substituted pyrrolidin-2-ones by intramolecular reductive cyclization. ° As shown in Scheme 4.129, the indium-FICl reductive system can successfully achieve the reductive cyclizations of adducts 403 to pyrrolidin-2-ones 404 and 405 in moderate yields. Flowever, SnCF 2H2O-promoted reduction mainly produced oxime derivatives 407, with one exception that formed 406 the oxime derivatives 407 could be further converted into 177-pyrroles 409. 

A recent publication described a short enantioselective synthesis of (+)-L-733,060, a selective and potent nonpeptide neurokinin substance P receptor antagonist. The key chirality-inducing step involved a Shi epoxidation of homoallylic carboxylate 89. Subsequent intramolecular reductive cyclization of azidolactone constructed the piperidine ring. 

Reductive Cyclization of (<>-Azidoalkenes. Hydroboration of < >-azidoalkenes with chloro(thexyl)borane followed by hydrolysis affords, via an intramolecular reductive cyclization, the corresponding p)UTolidine (78%) or the piperidine (53%) (eq 7). ... 

Tamao K, Yamaguchi S, Shiro M (1994) OligosUoles first synthesis based on a novel endo-endo mode intramolecular reductive cyclization of diethynylsilanes. J Am Chem Soc 116 11715-11722... 

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