Cyclopropanone ketals: ring opening

The reaction with optically active hydrazones provided an access to optically active ketones. The butylzinc aza-enolate generated from the hydrazone 449 (derived from 4-heptanone and (,S )-1 -amino-2-(methoxymethyl)pyrrolidine (SAMP)) reacted with the cyclopropenone ketal 78 and led to 450 after hydrolysis. The reaction proceeded with 100% of 1,2-diastereoselectivity at the newly formed carbon—carbon bond (mutual diastereo-selection) and 78% of substrate-induced diastereoselectivity (with respect to the chiral induction from the SAMP hydrazone). The latter level of diastereoselection was improved to 87% by the use of the ZnCl enolate derived from 449, at the expense of a slight decrease in yield. Finally, the resulting cyclopropanone ketal 450 could be transformed to the polyfunctional open-chain dicarbonyl compound 451 by removal of the hydrazone moiety and oxymercuration of the three-membered ring (equation 192). 

The spiro ketal derivatives studied by Giusti 105> are remarkably uniform in their behavior toward acid hydrolysis (Table 17). Ring opening usually occurs at the site of the less-substituted carbon and, in all cases, the ester is the sole product. The overwhelming preference shown by cyclopropanone ethylene ketals for path b) in Scheme 28 may be attributed to (a) the reversibility of path a) due to rapid intramolecular ketalization at the incipient Ci-carbonium ion 131 (R = CH2CH2OH) and (b) the stability of the intermediate dioxocarbonium ion 131 a generated in path 6).106>... 

TABLE 10. Ring-opening reactions of cyclopropanone ketals under acidic conditions ... 

Tishchenko and coworkers obtained dihydrofuran derivatives when 2-benzoyl-1,1-dichlorocyclopropane (102) was treated with alkoxides (Scheme 40) . Product (107) is highly reminiscent of butenolide (70) obtained by the addition of chloromethoxycarbene to acrolein as discussed earlier (Scheme 29) and may result from ring-opening of cyclopropanone ketal (104). Perhaps a more likely mechanism involves addition of alkoxide to intermediate 103 followed by cyclization 106 - 107. When treated with thiophenoxide, 102 gave cyclopropanone dithioketal (108). A similar result has been obtained by Ban well (Scheme 41) . Treatment of 109 with thiophenoxide gave dithioketal 110 in 98 % yield. In contrast to the results of Tishchenko, however, keto ester products (112) were obtained from 2,2-dichloro-l-acylcyclopropanes (111) and alkoxides (Scheme 42) ... 

Use of the Simmons-Smith reagent for the formation of cyclopropanone precursors has been reported in the case of ethoxy vinyl carboxylates (1) and ketene ketals (2) (Scheme 2). Using 1-ethoxyvinyl acetate and benzoate, it was found that the zinc iodide generated in the reaction tends to favor opening of the cyclopropane ring. However use of glyme as a solvent prevents attack on the ring by this Lewis acid since the zinc salt is insoluble in that medium . 

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See also in sourсe #XX -- [ , ]

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