Organoaluminum-promoted

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). 

The organoaluminum-promoted Claisen rearrangement139 of allyl vinyl ethers such as (f )-( )-6-methyl-4-vinyloxy-2-heptenc leads preferentially to Ry(E)- or (S)-(Z)-1 depending on the reagent employed 14°. The rearrangement products have been assigned by correlation (see p 437). i... 

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. 

For further examples of regioselective rearrangements with organoaluminum reagents see refs 123 and 129. For organoaluminum-promoted rearrangements of allylphenyl ethers see ref 140. 

Homologation of carbonyl compounds. The very bulky organoaluminums promote reaction of aldehydes and ketones with diazoalkanes. Aldehydes afford ketones. 

The salient feature of the synthesis is the organoaluminum-promoted stereospecific pinacol-type 1,2-rearrangement common to both segments. The 338—>339 rearrangement (Scheme 46) proceeds with preservation of alkene geometry, and is completely enantiospecific. The other 1,2-rearrangement, 345 —> 346 (Scheme 47), also proceeds with no EjZ isomerization of the olefin and with complete enantiospecificity. 

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