Double stereodifferentiation in aldol reactions on protected pyroglutamate esters

Excellent stereoselectivity is observed at both chiral centres created in the aldol condensation of the matched pairs of (2S)-pyroglutamate ester urethane 1 and either (R)-glyceraldehyde acetonide 2 or the (S)-Garner aldehyde 6. Stereoselectivity was not so impressive with the mismatched pairs 1+4 or 1+8 or when the C-2 reduced bromide 12 was used in the corresponding reaction with aldehyde 2.