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A series of new aliphatic 3-oxazolines was obtained in moderate to good yields via oxidation of the corresponding 1,3-oxazolidines by N-chlorination with sodium hypochlorite followed by elimination in basic medium. Some of the 3-oxazolines obtained present interesting properties in the field of fragrance chemistry. © 2000 Elsevier Science Ltd. All rights reserved. The synthesis and use of 2-oxazolines has been widely developed, 1 particularly as chiral ligands in the field of asymmetric catalysis. 2 In contrast, the practical utilization of 3-oxazolines has been relatively neglected. Certain 3-oxazoline derivatives (or 2,5-dihydro oxazoles) have been isolated and characterized as volatile flavor compounds in food and present important organoleptic properties. 3 Thus, 2,4,5-trimethyl-3-oxazoline was found in various foods such as cooked beef, roasted peanuts or fried chicken, and it has also been used as flavoring agent. 4a-e We have been interested in the synthesis of aliphatic and volatile 3-oxazolines of potential applications in food and fragrance chemistry. Reported preparations of 3-oxazolines include photochemically-induced ring opening of substituted 2H-azirines, 5a-c reaction of a-iminoper- fluoronitriles, 6 or diazo derivatives. 7 The condensation of an a-hydroxyketone with a carbonyl compound in liquid ammonia has also been described. 8a-d However, these methods were mainly applied to the preparation of aromatic oxazolines, and no general and efficient procedure is available for the selective access to aliphatic 3-oxazolines. Only one example reports the treatment of oxazolidines with t-butylhypochlorite and potassium superoxide. 9 However, only three alkyl substituted oxazolines were synthesized from ketones and the use of t-BuOCl and KO2 as reagents, presented some drawbacks and a limited industrial use due to their low stability. We report here a selective synthesis of aliphatic and volatile 2-mono- and 2,2-disubstituted-3- oxazolines, based on the oxidation of 1,3-oxazolidines with usual reagents. This strategy tends |
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