Beckmann rearrangements organoaluminum promotion

Organoaluminum-promoted Beckmann rearrangement/methylation of cyclohexanone oxime mesylate, followed by allylation of ketimine 40a and Mannich cyclization of the intermediate iminium-allylsilane, provides piperidine 40b possessing cxo-unsaturation (08BKC1669). 

Another synthetic route via the Beckmann rearrangement, which is promoted by organoaluminum reagent along with alkylation, involves a new stereoselective reduction of the imino group. The starting oxime sulfonate (228) was synthesized from cyclopentanone (226) in three steps Reaction of 226 with 1-undecene in the presence of silver oxide produced the a-undecylcyclopentanone (227) which on successive treatment with hydroxylamine and methanesulfonyl chlo-ride-triethylamine gave the mesylate (228). Treatment of the oxime mesylate... 

Treatment of a wide variety of oxime sulfonates with several equivalents of alkyl-aluminum reagents in CH2CI2 resulted in formation of the imines, which were directly reduced with excess DIBAH to give the corresponding amines, as shown in Sch. 19. This organoaluminum-promoted Beckmann Rearrangement of oxime sulfonates has been successfully applied to the stereoselective synthesis of naturally occurring alkaloids, pumiliotoxin C, and solenopsin A and B, as illustrated in Sch. 20 [43]. 

Combination of silyl enol ethers with the organoaluminum-promoted Beckmann rearrangement of oxime sulfonates resulted in a novel reaction system that leads to the formation of enaminones [44]. Treatment of a mixture of anfr-2-methylcyclohexa-none oxime sulfonate (33) and 2-(trimethylsiloxy)-l-octene in dry CH2CI2 with Et2AlCl at -78 °C for 30 min, and at 20 °C for additional 1 h resulted in formation of the enaminone 34 in 90 % yield (Sch. 21). 

Gephyrotoxin (106), a constituent of the skin extraas of the poison-dart hrog, was synthesized as outlined in Scheme 11. The organoaluminum-promoted Beckmann rearrangement produced an imine, which was stereoselectively reduced (essentially total selectivity) with DIBAL to a ord piperidine (109 41% yield). 

N.S. Mani and co-workers utilized the organoaluminum promoted modified Beckmann rearrangement during their efficient synthetic route to chiral 4-alkyl-1,2,3,4-tetrahydroquinoline. (4R)-4-Ethyl-1,2,3,4-tetrahydroquinoline was obtained by rearrangement of the ketoxime sulfonate of (3R)-3-ethylindan-1-one. The resulting six-membered lactam product was reduced to the corresponding cyclic secondary amine with diisobutylaluminum hydride. 

---