Organometallics and analogues

Enantiopure (R)-N-tert-butylsulfinamide 155, when reacted in diethyl ether at room temperature with 1.5 equiv of racemic allenylzinc 156, afforded a mixture of trans- and ds-aziridines (Rg)-157 with a selectivity of 79 21 in favour of the trans isomer [94]. This selectivity is lower than that recorded (90 10) when racemic imine 155 was reacted with 1.5 equiv of racemic 156. According to Hoffmann s statement on the configurational stability of organometaUic reagents, allenylzinc 156 should be regarded as at least partially configurationally stable with respect to 

Davies resolved o-anisaldehyde(tricarbonyl)chromium 158 subjecting the racemic valinol-derived imines 159 and 160 adsorbed onto neutral alumina to slow elution with ether [95]. A single enantiomer of 158 was first eluted, resulting from hydrolysis of imine 159, followed by the imine 160 of the second enantiomer (dichloromethane-methanol 5 1 as eluent). This latter imine may be prevented from hydrolysis through reversible formation of the oxazoHne 161. 

The BF3 Et20-promoted addition of racemic y-aUcoxyallylstannane 162 to a norephedrine-derived 2-methoxy-oxazolidine 163 afforded only two diastereomeric adducts 164, out of the eight possible (two stereogenic centres plus a stereogenic double bonds are generated in the condensation) in a 95 5 ratio [96]. The reaction showed a strong kinetic preference for consumption of the (S)-y-stannane 162. 

The reaction of the racemic enolate 165 with racemic styrene oxide 166 in the presence of diethylaluminium chloride afforded only the RRR(SSS) complex 167, 

Yamamoto reported the discrimination of enantiomers of the racemic organoa-luminium reagent 172 through reaction with terpene-derived chiral ketones [98]. The resulting mixture served as a catalyst for the hetero-Diels-Alder reaction of benzaldehyde and the diene 173. ds-174 was obtained as the major stereoisomer in 

---