Nickel-Mediated Ring Closure

A nickel-chromium catalyst prepared from chromous chloride and (p-diphenylphos-phinopolystyrene)nickel dichloride mediates the ring-closure of the ene-allene 236 (R = H) to a mixture of 3.4 parts of 237 and 1 part of 238 (equation 120)121. An analogous reaction of the t-butyldimethylsilyl ether of 236 yields solely the (E)-isomer 237 (R = t-BuMeaSi). Cyclization of the ene-allene 239 affords the perhydroindane 240 in 72%... 

Exan les of intramolecular addition reactions of alkenylchromium reagents to aldehydes have ap-peared. In the course of synthetic studies in the brefeidin structural series, the nickel(II)/chro-miiun(II)-mediated intramolecular addition reactions of ( )-iodoalkenes (227) and (230) were studied by Schreiber and Meyers (equations 72 and 73). Treatment of (227) with CrCh and a catalytic portion of [Ni(acac)2] in DMF produced a 4 1 mixture of 4-epibrefeldin C (228) and (+)-brefeldin (229) in 60% yield. In a similar fashion, precursor iodide (230) afforded a >10 1 mixture of cyclized hydroxy lactones (231) and (232) in 70% yield. An explanation of the stereochemical preference observ has been elo-quendy offered in a discussion of loc conformational preferences found in the starting material and the product lactone, as each is relevant to a transition structure for a 13-membered ring closure. 

Crevisy, C. Beau, J.-M. The esperamicin-calicheamicin aglycones ring closure of a simple strained system mediated by chromium(II)-nickel(ll) salts. Tetrahedron Lett. 1991, 32, 3171-3174. 

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