Acylhydrazones, cycloaddition

Inspired by the previous results, Leighton et al. reported the enantioselective [3 + 2] acylhydrazone-enol ether cycloaddition reaction by employing the same pseudoephedrine-based chiral silane. The pyrazohdine product was obtained in 61% yield with 6 1 dr and 77% ee in 24 h. The use of tert-butyl vinyl ether led to an improvement in both diastereoselectivity and enantioselectivity as shown in Scheme 34 [108]. 

Gilchrist and co-workers (91TL125) have reported on the inter- and intramolecular cycloaddition of a,/3-unsaturated acylhydrazones 58 (Scheme 15). Heating at 160°C a mixture of ALphenylmaleimide and aza-dienes 58 in mesitylene furnished annulated pyridines 59 in yields higher than 90%. Significantly, the intermolecular cycloaddition to this deficient... 

The ability of related acylhydrazones 94 (Scheme 24), derived from a,/3-unsaturated aldehydes, to participate in intramolecular Diels-Alder cycloaddition reactions has been proven to occur by Gilchrist and coworkers (91TL125). They reported that azadiene 94 (n = 1) cyclized to 95, whereas compound 94 (n = 0) did not undergo intramolecular cycload-... 

A simple chiral silane Lewis acid has been used in the highly diastereo- and (g enantio-selective 3 + 2-cycloaddition of acylhydrazones with enol ethers.73 The 1,3-dipolar cycloaddition of isocyanides (CNCH2EWG) with electron-deficient alkynes ... 

Acryloyl isocyanates, reaction with amino-sugars, uridine analogs from, 55, 139 A-Acyldehydroaminoacids, addition of nucleophiles, 57, 230 Acylhydrazones, a/3-unsaturated, cycloadditions, 57, 14 A-Acyliminium (ions), generation from alkylidenepiperazine-2,5-diones, 57, 230... 

Enantioselective [3+2] cycloaddition of halohydrazones 692 with dipolarophile 691 in the presence of ligand 693 under Lewis acid conditions, followed by reduction, provided an entry to chiral dihydropyrazole scaffolds 694 in excellent yields and good ees (Scheme 86) <2005JA8276>. Highly enantioselective synthesis of pyrazolidines 698 was achieved by the [3+2] acylhydrazone-enol ether cycloadditions of 695 and 696 in the presence of a chiral silicon Lewis acid 697 (Equation 148) <2005JA9974>. 

Asymmetric inverse-electron-demand 1,3-dipolar cycloaddition of C,A-cyclic azomethine imines with c-rich dipolarophiles was accomplished with a high stereo-selectivity by using an axially chiral dicarboxylic catalyst (40)." The metal-free silicon Lewis-acid-catalysed 3-1-2-cycloadditions of A-acylhydrazones with cyclopentadiene provides a mild access to pyrazolidine derivatives in excellent... 

Shrrakawa, S., Lombardi, P. J., Leighton, J. L. (2005). A simple and general chiral silicon Lewis acid for asymmetric synthesis Highly enantioselective [3+2] acylhydrazone-enol ether cycloadditions. Journal of the American Chemical Society, 127, 9974-9975. 

Kobayashi and coworkers have reported that the [3 + 2] cycloaddition between hydrazones (23) and olefins is accelerated in the presence of a stoichiometric amount of BF3 OEt2, under mild conditions affording cyclic five-membered ring compounds (Equation 17) [23]. The reaction of acylhydrazone with cyclopentadiene gives the expected product (24) in high yield and reasonable diastereomeric ratio. 

Related to azomethine imines, the acylhydrazone 58-olefin 59 cycloaddition also yields pyrazolidines 60 (Scheme 41.13). ... 

SCHEME 41.14. Chiral silane promoted [3+2] cycloaddition with acylhydrazones. 

Tran K, Leighton JL. A simple, efficient, and highly enantioselective synthesis of MS-153 employing a chiral silane Lewis acid-promoted acylhydrazone-enol ether [3-1-2] cycloaddition. Adv. Synth. Catal. 2006 348 2431-2436. 

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