Synthesis of Optically Active Bifunctional Isoprenoid Building Blocks by Rhodium(I)-Catalyzed Asymmetric Allylamine to Enamine Isomerization

Article abstract of DOI:10.1002/hlca.19900730516

The application of the known asymmetric allylamine to enamine isomerization methodology to bifunctional C5-isoprenoid allylic amines of types IId and IIe (Scheme 1) is described.It is shown that a number of such substrates can be isomerized with enantioselectivities of >90percent ee using cationic Rh^I complexes containing (6,6'-dimethylbiphenyl-2,2'-diyl)bis(diphenylphosphine) (BIPHEMP; 9) as asymmetry-inducing ligand (Scheme 2, Tables 1 and 2).Synthetically most useful is the isomerization of the benzyloxy derivative 10a into the (E)-enamine 11a.This isomerization proceeds with very high enantioselectivity (98-99percent ee) and affords, after enamine hydrolysis, the optically active 4-(benzyloxy)-3-methylbutanals ((R)- or (S)-12) in chemical yields of ca. 90percent.In conjunction, a short synthetic route to the starting material 10a has been developed which has a Pd-catalyzed amination of isoprene epoxide (30) as the key step.Thus, convenient and practical access to the optically active aldehydes (R)- and (S)-12 is now at hand.These aldehydes are useful optically active bifunctional building blocks for isoprenoid homologation.