Dipolar chiral rhodium catalysts

Hashimoto and coworkers [69] have recently begun to explore the use of chiral rhodium catalysts in the intramolecular dipolar cycloadditirai reactions of indoles, and have applied their methodology to the synthesis of the Aspidosperma ring system. Thus, the cycloaddition of the cyclopropyl carbonyl ylides derived from cyclopropyl diazo-5-imido-3-ketoesters 135 upon treatment with dirhodium (11) tetrakis[Af-tetrachlorophthaloyl-(5)-ferf-leucinate] gave cycloadducts 136 along with the spiro[2.3]hexanes 137 in only moderate yields (Scheme 34). Although the reaction proceeds with exclusive endo diastereoselectivity, only moderate enantioselectivities of up to 66% enantiomeric excess (ee) could be obtained. 

Another successful catalytic enantioselective 1,3-dipolar cycloaddition of Qf-diazocarbonyl compounds using phthaloyl-derived chiral rhodium(II) catalysts has been demonstrated [ill]. Six-membered ring carbonyl ylide formation from the a-diazo ketone 80 and subsequent 1,3-cycloaddition with DMAD under the influence of 1 mol % of dirhodium(II) tetrakis[M-benzene-fused-phthaloyl-(S)-phenylvaline], Rh2(S-BPTV)4 101 [112], has been explored to obtain the cycloadduct 102 in up to 92% ee (Scheme 31). 

The rhodium carbenoid (55) generated from diazocompound (50) has been shown to react efficiently with 1,3-dipolar nitrones in a formal [3 + 3] manner to produce heterocyclic cycloadducts (56) in high yields. Using chiral dirhodium catalysts such 0 as (51) or equivalent (R = methyl instead of adamantyl), the process exhibits a very high level of enantioinduction. 

The rhodium-catalyzed tandem carbonyl ylide formation/l,3-dipolar cycloaddition is an exciting new area that has evolved during the past 3 years and high se-lectivities of >90% ee was obtained for both intra- and intermolecular reactions with low loadings of the chiral catalyst. 

Good yields of the bridged tetrahydropyran-3-one 38 are obtained when the a-diazoketones 37 are decomposed by chiral Rh(II)-catalysts in the presence of DMAD. It is proposed that an enantioselective intermolecular 13-dipolar cycloaddition follows the generation of a carbonyl ylide which is bound to the rhodium (Scheme 21) <99JA1417>. 

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