Enolate, II. Einfache Synthese von 3,4‐Dihydro‐2,4‐dioxo‐2H‐pyranen und 2‐Pyronen Enolate als 1,3‐Dianion‐Äquivalente Kristall‐ und Molekülstruktur von 6‐Ethoxy‐3,4‐dihydro‐3,3‐dimethyl‐2,4‐ dioxo‐2H‐pyran‐5‐carbonsaure‐ethylester

Enolate Ions, I11). – Convenient Synthesis of 3.4-Dihydro-2.4-dioxo-2H-pyrans and 2-Pyrones. – Enolate Ions as Synthetic Equivalents of 1.3-Dianions. – Crystal and Molecular Structure of Ethyl 6-Ethoxy-3,4-dihydro-3,3-dimethyl-2,4-dioxo-2H-pyran-5-carboxylate Reaction of dimethylmalonyl chloride (Sa) or cyclic 1,l-dicar- bony1 dichlorides 6b – d with two equivalents of lithium enol- ates 7, 22, and silyl enol ethers 8, respectively, yields 3,4- dihydro-2,4-dioxo-2H-pyrans 10, 24, or spiro compounds 11 – 13,25- 27. The spiro[2S]octenes 11 and 25 isomerize spontaneously to give 2-pyrones 14, 28, whereas spiro[3S]nonenes 12, 26 rearrange only on heating in the presence of aluminium trichloride to form the corresponding 2-pyrones 15 and 29.However, 10, 13 as well as 24, 27 do not isomerize under these conditions. Reaction of dicarbonyl dichlorides 6a, c with one equivalent of ketene acetal 16 leads to hydroxy-dioxo-2H-pyr- ans 17a, c. On the contrary, treatment of tetraethoxyallene (18) with dicarbonyl dichlorides 6a, b leads to dihydro-2,4-dioxo-2H-pyran 19a (X-ray structure analysis) and spiro[2.5]octene 19b, respectively. Isomerization of 19 b yields a mixture of 2-pyrone 20 and 4-pyrone 21.