Abstract
Abstract
1,3-Dipolar cycloadditions of racemic 3-substituted 1-butenes to nitrones derived from (-)- or (+)-menthone occurred via EXO-approach of the alkene onto the nitrone’s less hindered face. This process afforded bicyclic spiroheterocycles as epimers, with selectivities £ 4:1, depending on the 3-substituent (R) on the alkene. The selectivity appeared to be influenced by hydrogen bonding (R = OH) or the bulkiness of the R group (R = OBz, Br), as a result of kinetic resolution. The cycloadducts obtained led, after a reductive step and cleavage of the chiral auxiliary, to enantiopure dihydroxylated non-natural amino acids in high overall yield.