The synthesis of chiral aminoalcohols 4 derived from (S)-tryptophan is described, by reaction of the protected amino acid 3 with various Grignard reagents. The aminoalcohols were transformed into the corresponding oxazolidin-2-ones 2 and acylated with 2-butenoyl chloride and 2-hexenoyl chloride, affording the imides 5a-e. The conformational analysis of both the oxazolidin-2-ones 2 and the Michael accepters 5 was performed by means of 1H NMR, NOEDIF experiments and semiempirical AM 1 calculations.
Recently the potential utility of homochiral vicinal amino alcohols in asymmetric synthesis has been extensively demonstrated. 13-Amino alcohols are useful in various enantioselective syntheses for the preparation of chiral building blocks and valuable auxiliaries, l Furthermore 1,3,2-chiral oxazaborolidines can be easily obtained by reaction of 13-amino alcohols or or-amino acids with boron derivatives, such as BH3.THF or boronic acids and have recently gained prominence as catalyst for a variety of enantioselective reactions, such as enantioselective Diels-Alder reaction and reduction of carbonyl compounds. 2 As the three-dimensional arrangement of the catalyst is peculiar for the reaction stereoselectivity, several 1,2,3-oxazaborolidines containing the indolyl moiety have been designed. 3 Moreover amino alcohols are precursors of oxazolidin-2-ones, which have extensive use in the diastereomeric induction for the carbon-carbon or carbon-heteroatom bond formation in reactions such as aldol condensation, 4 alkylation and 1,4-asymmetric induction. 5 As the steric hindrance of the substituents at C4 and C5 is crucial for the diastereoselectivity of the reaction, good results have been obtained with bulky groups at C5.
In the last few years we have become interested in the use and preparation of chiral auxiliaries, and their use in the asymmetric conjugate addition to ~l~-unsaturated carbonyl compounds for the synthesis of optically active 13-substituted carbonyl compounds. 6 The origin of diastereoselection has in a rapidly increasing number of cases being attributed to a through-space intramolecular interaction between x electron system, that leads to compact structures that present only one stericaUy available face to their reaction pamaers. This phenomenon has been called n-stacking effect 7 and has been envisaged in the presence of naphthyl or indolyl moieties in the proximity of the reaction eentres. Those electron-rich substituents hinder one face of the system, producing high diastereoselection.