STEREOSELECTIVE PREPARATION OF POLYFUNCTIONAL CYCLOPENTANE DERIVATIVES BY RADICAL NICKEL-CATALYZED OR PALLADIUM-CATALYZED CARBOZINCATIONS

The reaction of 5-hexenyl iodides with diethylzinc (2 equiv) and catal ytic amounts of a Pd-II or Ni-II complex like PdCl2(dppf), PdCl2(MeCN) (2), or Ni(acac)(2) results in an efficient ring closure (THF, RT, 2-1 2h) affording cyclopentylmethylzinc iodides, which, after transmetalat ion with CuCN . 2LiCl, can be further functionalized by treatment with a range of electrophiles like allylic halides, acyl chlorides, enones , nitro-olefins, ethyl propynoate, and alkynyl halides to yield polyfu nctional cyclopentane derivatives. The ring closures occur via radical intermediates, and the stereochemistry of the products can be explain ed according to the rules for radical cyclizations developed by Beckwi th. The preparation of several di- and trisubstituted cyclopentanes ha s been achieved with high stereoselectivity. Tandem ring closures can be performed to construct bicyclic or tricyclic ring systems, Cyclizat ions of iodo-ethylenic and acetylenic esters and ketones can be accomp lished, although the high reactivity of acetylenic ketones leads to un expected cyclization products. The synthetic utility of this method ha s been demonstrated by an enantioselective synthesis of (+)-methyl epi jasmonate and (-)-methyl cucurbate.