Synthesis of alkaloids by a diastereoselective allylation of chiral N-sulfinyl imines
The indium-promoted allylation of chiral N-sulfinyl imines represents a useful and versatile procedure to prepare chiral protected homoallyl amines in a diastereoselective manner. Desulfinylation under acidic conditions liberates easily the corresponding enantioenriched homoallyl amines. This type of compounds can be easily manipulated synthetically in order to prepare a series of natural alkaloids in an enantiopure form, using simple transformations.
Among the reactions involving the addition of an organometallic reagent to carbonyl compounds or imines, the corresponding allylation offers the advantage of making possible the further manipulation of the allylic moiety. In general, the allylation of imines has been far less studied than the corresponding reaction with carbonyl compounds (1, 2).
On the other hand, the asymmetric version of this reaction would afford chiral homoallylic amines, able to be transformed into interesting chiral nitrogen-containing compounds (3, 4). Some years ago (5), we found out that indium metal can promote the allylation of chiral N-tert-butylsulfinyl imines 1with allyl bromides 2 under mild reaction conditions, giving the expected protected homoallyl amines 3 with good yields and diastereoselectivities (Scheme 1).
The observed stereochemistry could be easily explained considering a chair-like transition state I in which the indium atom is coordinated to both the nitrogen and the oxygen atoms, and the group R1 is located in a pseudo-antiperiplanar position w ...