On the addition of lithiated 2-alkyl- and 2-(chloroalkyl)-4,5-dihydro-1,3-oxazoles to nitrones - A mechanistic investigation

The addition of 2-(lithioalkyl)-4,5-dihydro-1,3-oxazoles 2a-c and 2-[chloro(lithio)alkyl]-4,5-dihydro-1,3-oxazoles 2d,e to nitrones 3 has been studied. While lithiated 2-methyl-4,5-dihydro-1,3-oxazole 2a adds stereoselectively to nitrones 3, resulting after long reaction times (3 h) in the formation of 2-[(E)-alkenyl]-4,5-dihydro-1,3-oxazoles 8a-h, lithiated 2-(chloromethyl)-4,5-dihydro-1,3-oxazole 2e affords 2-[(Z)-alkenyl]-4,5-dihydro-1,3-oxazoles 26a-d and 26f-h. α-Lithiated 2-ethyl-4,5-dihydro-1,3-oxazole 2b adds to 3a to give the 1,6-dioxa-2,9-diazaspiro[4.4]nonane 9 and 2-alkenyl-4,5-dihydro-1,3-oxazole 14 after treatment with oxalic acid. Quenching after short reaction times shows that the conversions of 2a to 8 and of 2b to 14 go through spirocyclic compounds 7 and 9, while the reaction between 2e and 3a, quenched even at short reaction times, gives a mixture of the 1,6-dioxa-2,9-diazaspiro[4.4]nonanes 21-H and 22-H and the 2-(1,2-oxazetidin-4-yl)-4,5-dihydro-1,3-oxazoles 25a and 27a. The addition of 2c to 3a furnishes the 1,6-dioxa-2,9-diazaspiro-[4.4]nonane 15 and then isoxazolidin-5-one 16 upon hydrolysis with oxalic acid. The addition of 2d to 3a gives the 1,6- dioxa-2,9-diazaspiro[4.4]nonanes 17b and 18b after short reaction times and the 2-(1,2-oxazetidin-4-yl)-4,5-dihydro-1,3-oxazole 19 after long reaction times.