(A) convenient method for the preparation of cyclo-heptenones

Abstract

Cyclohept-3-3nones were prepared in four to five steps starting from cyclopentanone sily enol ether in high overall yields(42-75\%). Silyl enol ethers of cyclopentanones were converted to dichlorobicyclo [3.2.0] heptanones(75-88\%) via [2+2] cycloaddition with dichloroketene generated from trichloroacetyl chloride. Dechlorination of the dichlorobicyclic ketones was performed by treatment with tributyltin hydride in the presence of AIBN to give bicyclic ketones(89-94\%). The ketones were either reduced with sodium borohydride or treated with methylmagnesium bromide to give bicyclic secondary or tertiary alcohols(95-98\%). The secondary alcohols were subjected to mesylation ($CH_3SO_2Cl$, $Et_3N$/DCM) and fragmentation (n-$Bu_4NF$/THF) condition to give cyclohpt-3-enones(59-64\%). On the other hand the tertiary bicyclic alcohols underwent fragmentation spontaneously during mesylation condition to give the corresponding cyclohept-3-enones almost quatitatively. All the cyclohept-3-enones were converted into the corresponding cyclohept-2-enones quatitatively by treatment with p-TsOH. Similary, thermal [2+2]cycloaddition of silyl enol ethers with dithisubstituted ketene gave the corresponding cycloadducts. However, when the cycloadducts were treated with $Bu_4NF$, $C_1$-$C_7$ bond cleavage occurred exclusively resulting diketones in almost quantitative yield. 6-Methylbicyclo [5.3.0] dec-10-en-2-one having trans configuration between $C_6$-methyl and $C_7$-proton was synthesized by applying the above methodology for cycloheptenone synthesis. The 5-methyl-1-(trimethyl siloxy) bicyclo [3.2.0]heptanone was treated with Grignard reagent of 2-(2-bromoethyl)-1,3-dioxane, and subsequent fragmentation produced the corresponding cyclohept-3-enone. Stereoselective hydrogenation of cycloheptenone was achieved in the presence of rodium on alumina affording a stereoisomer mixture of cis and trans(15:1). Acid catalyzed cyclization of cycloheptanone derivative was carried out by Hea...