Michael oxygenated analogous compounds

Intermediates such as 224 resulting from the nudeophilic addition of C,H-acidic compounds to allenyl ketones such as 222 do not only yield simple addition products such as 225 by proton transfer (Scheme 7.34) [259]. If the C,H-acidic compound contains at least one carbonyl group, a ring dosure is also possible to give pyran derivatives such as 226. The reaction of a similar allenyl ketone with dimethyl mal-onate, methyl acetoacetate or methyl cyanoacetate leads to a-pyrones by an analogous route however, the yields are low (20-32%) [260], The formation of oxaphos-pholenes 229 from ketones 227 and trivalent phosphorus compounds 228 can similarly be explained by nucleophilic attack at the central carbon atom of the allene followed by a second attack of the oxygen atom of the ketone at the phosphorus atom [261, 262], Treatment of the allenic ester 230 with copper(I) chloride and tributyltin hydride in N-methylpyrrolidone (NMP) affords the cephalosporin derivative 232 [263], The authors postulated a Michael addition of copper(I) hydride to the electron-... 

Synthesis of pyrrolizidines by Dieckmann condensation continues to receive wide attention. It is a useful method for producing pyrrolizidines with oxygen substituents and has been exploited in a number of syntheses by Viscontini and co-workers. For their attempted preparation of 1,7-dioxopyrrolizidine, the very unstable 3-oxopyrrolidine was required as an intermediate.29 This compound was stabilized by ketalization (37), and by this means N-substituted 3-oxopyrrolidines are readily accessible. The 1,7-dioxopyrrolizidine was synthesized as the sodium enolate (38) as shown in Scheme 3. The dione itself is unstable and could not be isolated. Analogous Michael addition of the pyrrolidine (37) to a-chloroacrylic ester gave the corresponding 5,6-dihydropyrrolizine as the sodium salt (39).30 Treatment of this salt with acid produced a very stable H-bonded aromatic species. 

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