3- Chloro-4-hydroxy-2-cyclopentenones

Tropone (125) and the 2-substituted tropones showed a different reactivity in the cycloaddition with 2-cyclopentenone (28). Whereas tropone itself (125) and the 2-methoxytropone (126) reacted at lOkbar, giving a mixture of four and three products, respectively (Scheme 5.18), 2-hydroxy- and 2-chloro-tropone failed to react at all [43b]. Compound 127 does not have the expected dihydro-homobarrelenone framework it is probably derived from the cycloaddition of 125 and 1,4-cyclopentadien-l-ol, the enol form of 28. 

Other 3-substituted l-lithio-l-(methoxymethoxy)allenes such as compounds 747-749 have been used in the synthesis of cyclopentenones. The intermediate 747 and an amide have been used for the synthesis of A7-10-chloro-15-deoxy PGA1 ethyl ester1079 and, by reaction with a trifluoromethyl dienone, for the synthesis of 15-deoxy-12-hydroxy-10-(trifluoromethyl)- A7 PGA1 ethyl ester1080. A combination of allenyllithiums 740, 748 and 749 with amides allowed the parallel chemical synthesis of cyclopentenones1081. 

Chloro-2-methoxy-2-cyclopentenone or 2,3-dimethoxy-2-cyclopentenone refluxed 2 hrs. with excess K-carbonate in dil. methanol -> 3-hydroxy-2-methoxy-2-cyclopentenone. Y 80-90%. G. Hesse and K. Mix, B.92, 2427 (1959). 

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