Allenes radical cyclization

Grissom, J.W., and Khngberg, D., Tandem enyne allene-radical cyclization via base-catalyzed isomerization of enediyne sulfones, Tetrahedmn Lett., 36, 6607, 1995. 

Scheme 3.48. Thermolysis of benzoenynes-allenes initiating a domino radical cyclization.

Scheme 11.17 Diastereoselective formation of a tetrahydrofuran nucleus via 5-exo-trig alkyl radical cyclization on to an allene functionality in the course of a total synthesis of (+)-samin (56) [75], a)33 67 mixture of diastereomers.

Allenes have also been used as substrates in free radical cyclizations. Dener and Hart demonstrated that such entries are valuable in constructing pyrrolizidine and indolizidine ring systems [71]. In a total synthesis of pyrrolizidine base (+)-heliotri-dine (340), compound 338 possessing an allene functionality was used as a key intermediate (Scheme 19.62). Tri-n-butyltin radical-mediated carbon-selenium bond homolysis of 338 followed by the addition of the free radical to the allene moiety... 

The benzannulated analogs were also found to behave in a similar fashion. Attachment of a pendent olefin to the benzannulated enyne-allene system as depicted in 89 allowed the aryl radical in 90 to be captured in a 5-exo radical cyclization reaction leading to 91 and then the dihydrobenz[e]indene 92 (Scheme 20.20) [55, 56]. 

Scheme 20.20 Tandem enyne-allene and radical cyclizations.

Cascade Radical Cyclizations of Biradicals Generated from Enyne-Allenes 1119... 

Enamine carbaldehyde 76 (Scheme 20) by sequential photochemical cycloaddition and iminium ion-propargylsilane cyclimtion furnishes allenes 77a-c in good yield and with high diastereoselectivity (92X2081). (Gas chromatographic fR values and thin-layer chromatographic Rp values have been reported.) Radical cyclization of ca-iodoalkyl isoquinolone 78a under... 

It was reported that Pd(0)-catalyzed coupling reactions of allenic alcohols, amines and acids with hypervalent iodonium salts afforded cyclized heterocyclic tetrahydrofurans, tetrahydropyrans, pyrrolidines, piperidines, or lactones under mild conditions <99SL324>. Intramolecular 1,5-hydrogen atom transfer radical cyclization reaction of pyrrolidine derivatives was examined. Reaction of 3,4-dialiyloxy-JV-(0-bromobenzyl)pyrtolidine gave hexahydro-... 

Most of the recent synthetic developments in the field of radical cyclization have involved the reactions of carbon-centered radicals with alkenes and alkynes. Other useful acceptors include allenes,31 dienes30 and vinyl epoxides.32 The same methods are used for cyclizations to these acceptors as for radical additions, and the preceding chapter should be consulted for specific details on an individual method (the organization of this section parallels that of Section 4.1.6). Selection of a particular method to conduct a proposed cyclization is based on a variety of criteria, including the availability of the requisite pre-... 

The tandem radical cyclization of tetrayne (97) and its derivatives has been performed to generate the polycyclic pyran (98) via a biradical intermediate.238 The cycloaddition reaction of a biradical species (or diyl) and a multiply bonded species (the diylophile)239 has been observed with unique allene diylophiles.240 The short-lived biradical fonned by the irradiation of the diazene (99) is trapped by an allene diester to form a second biradical species (100). Intramolecular cyclization occurs such that all steric interactions are minimized and so enforces stereocontrol in the formation of the cycloadduct (101) see Scheme 14. A paper reports the rearrangement of 2-vinyhnethylenecyclopropane (102) to 3-methylcyclopentene (103) via the triplet biradical (104), which has been characterized for the first tune by IR spectroscopy.241... 

Regio- and stereoselective toluenesulfonyl bromide-mediated radical cyclization of bis(allene) (201) in the presence of AIBN also goes ahead to give rise to the transfused 5-membered ring (202) containing vinyl sulfone and vinyl bromide as shown in eq. 3.78. 

Stereoselective synthesis of (+ )-botryodiplodin was carried out by a radical cyclization of dibromoacetal (203) containing an allene group, with Bu3SnH initiated by Et3B, through the 5-exo-trig cyclization, and the subsequent debromination with bulky... 

Wang, K. K. Cascade Radical Cyclizations via Biradicals Generated from Enediynes, Enyne-Allenes, and Enyne-Ketenes. Chem. Rev. 

Ring annulation by radical cyclization of ene-diynes and (Z)-allene-ene-ynes in a thermal reaction to give aromatics (electrocyclization). 

The allene functionality is also useful in radical cyclizations. Radical precursor 867 (R=OH OAc SePh) is readily cyclized to a 5 1 mixture of 868 and 862 together with two other cyclic compounds. After separation of the desired product 868, the methyl group is oxidized to an alcohol and then acetylated. Finally, the carbonyl groups are reduced to give ( + )-heliotridine (850) [191]. 

Naturally occurring (l S, 25, 3i )-4-hydroxymethylcyclopent-4-ene-l,2,3--triol (950) plays a central role in the ability of a non-aristeromycin producing mutant strain of Streptomyces citricolor to support production of both aristeromycin and neplanocin. Swem oxidation of readily available 13 from L-tartaric acid provides the aldehyde 943 which, when treated with an excess of propargyl zinc bromide, leads to a 2.3 1 diastereomeric mixture of acetylenic alcohols 944. Silylation of the hydroxyl group with TBSOTf and subsequent saponification of the ester group yields the carboxylic acid 945 in 74% overall yield from 13. Interestingly, Dess-Martin oxidation of 943 provides the allenic ketone 946, which is unstable to base and cannot be used in the subsequent radical cyclizations. 

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