Study On The Switch Of Reactivity Of AllylSmBr And The Application In The Selective Synthesis Of Bicyclo[3.3.0]-and[3.1.0]-1-ols

Samarium diiodide as a single electron transfer reagent is widely used in various reductive coupling reactions,however,few reports of the reductive coupling reactions promoted by divalent organosamarium reagent,such as allylsamarium bromide(allylSmBr),have been reported.In order to broaden the application of allylSmBr and develop more efficient new divalent organosamarium reagents,this paper investigated the influences of various additives on the reactivity of allylSmBr.The results show that with different additives,selective synthesis of bicyclo[3.3.0]-and[3.1.0]-1-ols could be achieved promoted by allylSmBr.The research focuses on two aspects.In the first part,the real reducing species was probed for the reductive coupling of aryl ketones mediated by Sm/alkyl halide/MeOH.Thus a variety of carbonyl compounds were treated by allylSmBr in the presence of a variety of additives,such as MeOH,EtOH,tBuOH,Pydine,Et3N,et al.The results demonstrate that the real reducing species in Sm/alkyl halide/MeOH system should be allylSmBr,and MeOH has elegantly switched the reactivity of allylSmBr from being nucleophilic to being good reductive coupling reagent.Besides,H2O was also found to be a useful additive to realize the pinacol coupling of aliphatic aldehydes and ketones promoted by allylSmBr.In the second part,selective synthesis of bicyclo[3.3.0]-and[3.1.0]hexan-l-ols were realized via switch of the reactivity of allylSmBr by different additives.Substrates containing hex-5-enoate moiety were designed and treated by allylSmBr/HMPA/additive.By screening of the additives and temperatures,pyrrole as the additive and 5-10 ℃ was found to be the optimal conditions to achieve the cascade reductive cyclization affording bicyclo[3.1.0]hexan-l-ols.Unexpectedly,the same substrates when treated by allylSmBr/HMPA(without the presence of proton source),afforded bicyclo[3.3.0]hexan-l-ols in good to excellent yields.The reaction is highly diastereoselective and the relative configuration is ascertained by single-crystal X-ray crystallographic analysis.The generation of bicyclo[3.3.0]hexan-l-ols has elegantly made use of both the nucleophilicity and reducing ability of allylSmBr,and showed its characteristic performance.