Surmnary
A simple asymmetric aldol condensation of acetophenone and benzaldehydes was performed via chiral vinylaminodichloroborane. The enatioselectivity was compared with that by a similar method using magnesium and lithium.
Recently, Enders and his co-workers* reported the first regiospecific and enantioselective aldol synthesis, which consists of a reaction series involving lithiation of chiral hydrazones followed by addition of carbonyl compounds, silylation of the condensation products and oxidative hydrolysis. We will describe here a much simpler enantioselective aldol synthesis as an extension of our simple directed aldol reaction using N-cyclohexylvinylaminodichloroborane. 3
Namely, (S)-(-)-N-(cc-methylbenzyliden)isobornylamine (la) was treated with boron trichloride in the presence of 2.5 equivalents of triethylamine in dichloromethane followed by addition of benzaldehyde. The reaction proceeded presumably via chiral vinylaminodichloroboranc 2. The optical purity (47.7%) and absolute configuration (R) of the aldol product (3a) obtained after acidic work-up followed by chromatography were determined by comparing the specific rotation of the corresponding acetyl derivative (4) with that of an authentic sample4
having the R configuration.
The similar reaction of chiral imines la, lb5 or lc with p-nitrobenzaldehyde and comparison of the reaction (run 2) with that using magnesium (run 3) and lithium (run 4) were carried out as shown in the Table . As noted, the best enantioselectivity observed was obtained by using boron atom and this may be rationalized by assuming that boron can provide more rigid intermediates by stronger coordination ability to carbonyl oxygen and facilitate the carboncarbon bond formation in a more concerted manner. However, the opposite enantioselectivity in run 4 and the poor enentioselectivity in run 5 can not be clearly understood at present. EXPERIMENTAL (S)-(-)-N-(a-Methylbenzyliden)isobornylamine (la) A solution of isobornylamine [ll g, 71.2 mmol, [a]? -46.4 (c 3, EtOH)] and acetophenone (8.6 g, 71.2 rmnol) in dry benzene (100 mL) was refluxed in the presence of molecular sieves (4A, 30 g) with a Dean-Stark apparatus for 17 h. The molecular sieves were filtered off and washed with benzene. The combined filtrate was concentrated and the residue was distilled