Effect of steric bulk on the absolute reactivity of allene oxides.
Alpha-alkyl substituted fluorenylidene allene oxides were generated photochemically from appropriately substituted fluorenol derivatives in 2,2,2-trifluoroethanol (TFE) in order to assess the affect of steric bulk on the absolute reactivity of the allene oxides. The absolute reactivities of the allene oxides were measured using nanosecond laser flash photolysis, and were found to change little in TFE upon going from alpha-methyl to alpha-butyl to alpha-t-butyl. However, all three alpha-alkyl substituted allene oxides were considerably more reactive than the alpha-hydrogen derivative, indicating that steric bulk did not result in a decrease in reactivity as is typically found in other allene oxides. The contrary reactivity of the fluorenylidene allene oxides is explained by examining in detail the reactions of these allene oxides, determined from product studies.
Stereoselektive Bildung von Halbacetalen des cis‐2,3‐Dimethylcyclopropanons aus 2‐Halogen‐3‐pentanonen. [4 + 3]‐Cycloadditionen an konjugierte Dien‐Systeme
2-Chlor- und 2-Brom-3-pentanon (1,2) reagieren in Methanol, Ethanol und 2-Propanol mit den entsprechenden Natriumalkoxiden hochstereoselektiv zu Halbacetalen des cis-2,3-Dimethylcyclo-propanons (4aα–4cα). Das Benzoat von 4aα wurde durch Rontgenstrukturanalyse charakterisiert. cis-2,3-Dimethylcyclopropanon kann durch Aldoladdition mit Malonsaure-dimethylester abgefangen werden. Es erleidet eine disrotatorische retro-elektrocyclische Ringoffnung zu einem W-konfigurierten Allylium-2-olat (Oxallyl) 10, welches hochstereoselektive [4 + 3]-Cycloadditionen mit Isopren, (E)-1,3-Pentadien, Furan und 2-Methylfuran eingeht. Mit Methanol und 2,2,2-Trifluorethanol entstehen die 2-Alkoxy-3-pentanone. N-Methylpyrrol wird in 2-Stellung oxoalkyliert. 
 
 
 
Stereoselective Formation of cis-2,3-Dimethylcyclopropanone Hemiacetals from 2-Halo-3-pentanones. — [4 + 3] Cycloadditions with Conjugated Diene Systems 
 
 
 
2-Chloro- and 2-bromo-3-pentanone (1, 2) react in methanol, ethanol, and 2-propanol with the corresponding sodium alkoxides in a highly stereoselective manner to provide the hemiacetals of cis-2,3-dimethylcyclopropanone (4aα – 4cα). The benzoate of 4aα was characterized by an X-ray crystal structure determination. cis-2,3-Dimethylcyclopropanone can be trapped by an aldol type addition with dimethyl malonate. It undergoes a disrotatory retro-electrocyclic ring opening to form a W-configured allylium-2-olate (oxyallyl) 10 which combines with isoprene, (E)-1,3-pentadiene, furan, and 2-methylfuran in highly stereoselective [4 + 3]cycloadditions. With methanol and 2,2,2-trifluoroethanol, the 2-alkoxy-3-pentanones are formed. N-Methylpyrrol is oxoalkylated at the 2-position.
Electrosynthesis of cyclopropanone adducts
Electroreduction of α,α′-dihalo ketones to cyclopropanones (isolated as the hemiacetals or hemiacylals) is accomplished with good yields only with highly alkylated α,α′-dihalo ketones. Attempts to prepare little substituted or unsubstituted cyclopropanone (derivatives) by electroreduction of α,α′-dihalo ketones in which the carbonyl group is protected as an acylal or aminal resulted in the isolation of the desired compounds in low yields. The cyclisation reaction cannot compete successfully with side-reactions. With acetals as the protecting group, the reductive cyclisation proceeds smoothly. Contrary to the acetals, the cyclopropanone acylals are excellent precursors for cyclopropanone.