Diastereoselection during 1,2-Addition of the Allylindium Reagent to alpha-Thia and alpha-Amino Aldehydes in Aqueous and Organic Solvents.
The stereochemistry of the indium-promoted reaction of allyl bromide with alpha-thia (PhS and MeS), disubstituted alpha-amino (Bn(2)N, Me(2)N, isoindolyl), and protected alpha-amino aldehydes (Ac and Boc) in water has been evaluated. The reactions involving the sulfur derivatives are minimally diastereoselective, indicating that the allylindium reagent is not thiophilic. Chelation is not observed and pi-facial discrimination is achieved via Felkin-Ahn transition states under the steric control of the substituents. The Garner aldehyde is also anti-diastereoselective. Interestingly, N-acetylmannosamine is appreciably responsive to chelation control and is capable of generating 90% of the syn beta-amino alcohol when reacted in a 0.5 M NH(4)Cl solution. While the alpha-dibenzylamino substituent is too bulky to enter into complexation, the alpha-dimethylamino group is not and can lead to high levels (99%) of syn diastereomer. The size of other neighboring substituents does have an impact on pi-facial discrimination in these systems and can erode the stereoselectivity accordingly.
Paquette
Leo A.
LA
Evans Chemical Laboratories, The Ohio State University, Columbus, Ohio 43210.
Mitzel
Thomas M.
TM
Isaac
Methvin B.
MB
Crasto
Curtis F.
CF
Schomer
William W.
WW