Stereospecific Synthesis of Conjugated Dienes by Carbenoid Eliminative Cross‐Coupling Using Lithiated Allylic Carbamates

Abstract

Abstract1,2,4‐Trisubstituted 1,3‐butadienes were prepared in a stereocontrolled manner by carbenoid eliminative cross‐coupling between stereodefined lithiated secondary allylic carbamates and α‐carbamoyloxyalkyl boronates. All four geometric isomers of 4,7‐dimethyl‐1‐phenylocta‐3,5‐diene were stereospecifically generated [yields 44–73 %, dr (Δ3,4)=84 : 16 to 97 : 3] by appropriate combinations of (S,E)‐ or (S,Z)‐i‐PrCH=CHCLi(Me)OCb with (R)‐Ph(CH2)2CH(Bneo)OCb. A conjugated cyclohexenylidene and a model diene representing the C5‐C12 domain of cytotoxin callystatin A, were similarly elaborated in a stereochemically programmed manner. The configuration of the 1,2‐disubstituted alkene within the Li‐carbenoid was preserved in the diene product and the newly generated trisubstituted alkene component of the diene could be selectively installed in either (E)‐ or (Z)‐configuration by variation in carbenoid stereochemical pairing (like or unlike) and/or the mode of the elimination stage (syn or anti).