Enolate, II. Einfache Synthese von 3,4‐Dihydro‐2,4‐dioxo‐2 H ‐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‐2 H ‐pyran‐5‐carbonsaure‐ethylester

Enolate Ions, I1 1) . – 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‐2 H ‐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‐2 H ‐pyr‐ ans 17a , c. On the contrary, treatment of tetraethoxyallene ( 18 ) with dicarbonyl dichlorides 6a, b leads to dihydro‐2,4‐dioxo‐2 H ‐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 .