Selenyl radical, cyclization

Tsuchii et al. reported a very interesting four-component domino process where an alkyne, two olefins and diphenyl diselenide sequentially react to form a highly functionalized cyclopentane derivative, after a linear addition sequence and 5-exo-trig cyclization [136]. This reaction can be seen as an interrupted polymerization process initiated by the addition of selenyl radical to an electron-deficient alkyne in the presence of a large excess of a Michael acceptor. The identity of each reaction partner is important for the outcome of the reaction. For instance, use of (PhS)2 instead of (PhSe)2 leads to the polymerization product rather than to the cyclization one, while (PhTe)2 did... 

An interesting application of the asymmetric alkoxyselenenylation of alkenes to natural product synthesis was reported recently by Wirth, who described a short procedure to obtain some furofuran lignans 147]. The total synthesis of (+)-Samin 53 [47 a] is shown in Scheme 7. The protected allylic alcohol 50 was treated with the selenyl triflate derived from diselenide 29 in the presence of 2,3-butadien-l-ol, and afforded the addition product 51 in 55% yield and with a diastereomeric ratio of 15 1. The favored 5-exo-trig radical cyclization of the major isomer afforded the tetrahydrofuran derivative 52 from which the final product was obtained through few classical steps. 

Upon irradiation, thiyl radicals are generated and add to the alkyne 148 (the corresponding selenyl radicals are also formed but are less reactive than the thiyl radicals). The vinyl radical 149 cyclizes to 150. Finally, 150 reacts with diphenyl diselenide, the best radical trap present in the reaction mixture, to afford the product 151. 

There are many synthetic examples that use radical cyclization as a key step, and the radical precursor is not limited to iodides or bromides. In Pattenden s synthesis of pentalenene, conjugated selenyl ester 156 was treated with Bu3SnH and AIBN to give a 45% yield of tricyclic ketone 159. Loss of PhSe generated the acyl radical 157, which exists in equilibrium with the ketene radical 156. Radical cyclization via the latter intermediate leads to 159. Cyclization via aryl radicals is also possible. In Schultz s synthesis of hexahydro-phenanthren-2-one derivatives, " aryl bromide 160 was cyclized to 161 in 78% yield under standard conditions. Radical cascade reactions have become quite popular for the synthesis of polycyclic ring systems. In these reaction, polyenes are subjected to radical cyclization, generating tricyclic or even tetracyclic ring systems. 5 Chiral auxiliaries have been used effectively in radical cyclization reactions. ... 

Scheme 10.12 gives other examples of cyclization, including examples in which radicals are generated by sulfuryl and selenyl abstraction and by Mn(02CCH3)3 oxidation. 

Pattenden et al. applied his expertise in polycyclic ring constructions based on radical-mediated cyclizations of polyolefin selenyl esters for the total synthesis of cytotoxic ( )-spongian-16-one, isolated from both New Zealand and Australian specimens of the sponge Chelonaplysilla viola-cea (Scheme 25.38). Treatment of selenoester 85 with Bu3SnH led to tetracycle lactone 86, which after methyle-nation gave the corresponding exocyclic methylene (not shown) in 42% overall yield. 

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