Cycloaddition precursor cyclobutanones

The Cj - and 54-symmetric tetraesters of tricyclo[3.3.0.0 ]octane (430 and 431) have been prepared by oxidation of diene 429 To access the parent hydrocarbon (435), acid chloride 432 was transformed to the derived ketene which undergoes intramolecular [2+2] cycloaddition The resulting cyclobutanone (433) serves as precursor to perester 434 whose thermal decomposition proceeds with chain transfer in competition with cleavage The unique arrangement of the carbon atoms in 435 is such that the smallest rings are all five-membered. The highly symmetric structure may be viewed as a constrained cisoid bicyclo[3.3.0]octane (as well as the symbol of NATO). 

In general, cyclobutanones are synthesized by either ketene cycloadditions or by ring expansions of cyclopropyl precursors. For the synthesis of simple a-substituted monocyclic cyclobutanones, the latter method is usually employed, and a variety of approaches have been used to prepare the required eyclopropyl intermediates. 

The fused cyclobutane derivatives produced via the above cycloaddition reactions have been utilized as synthetic precursors for the preparation of indole-2-acetonitriles (Scheme 6) [23]. First, the acetyl-substituted cyclobutanes 28 were converted to the cyclobutanone derivatives 29, which were in turn treated with hydroxylamine to provide the corresponding oximes. Beckmann fission of oximes 30 in the presence of thionyl chloride then produced the l-benzoylindole-2-aceto-nitrile derivatives 31 in good yield. 

DicWoroketene is particularly reactive, and reductive dechlorination of the product with zinc and acetic acid allows access to the cyclobutanone from formal addition of ketene itself. Thus, cycloaddition of dichloroketene with cyclopentadiene, followed by dechlorination and Baeyer-Villiger oxidation gave the lactone 173, a usehil precursor to various oxygenated cyclopentane products (3.117). Intramolecular cycloaddition reactions of ketenes can allow the formation of bicyclic and polycyclic products using otherwise unstable ketene intermediates. ... 

The enol precursor 151 was prepared in three steps by addition of Steri-col 150 onto trichloroethylene, which was followed by the formation of a sensitive ynol through addition of n-BuLi and excess allyl iodide (Scheme 43). The ynol was then partially hydrogenated with Pd on barium sulfate, furnishing the required enol ether 152 contaminated with 8% of the corresponding dihydro enol ether. [2+ 2]-Cycloaddition between 152 and dichloroketene, generated in situ from trichloroacetyl chloride, led to the cyclobutanone 153 as the major diastereomer with a 93 7 diastereocontrol. 

Systematic studies have shown that intramolecular [2+2] cycloadditions of unsaturated ketenes or ketiminium salts (278) constitute a simple and reasonably general approach to cyclobutanones (279), which are useful as precursors to butyrolactones following Baeyer-Villiger oxidation. The potential of this method is clearly shown by the efficient formation of the bicyclo[5.2.0]nonane (280) and of the... 

The utility of ketens in such cycloadditions is further exemplified by the regioselective addition of dichloroketen to conjugated silyl enol ethers (34), to give cyclobutanones (35). In addition, the novel keten (36), adds to cyclopenta-diene leading [via (37)] to the useful prostaglandin precursor (38). A review... 

Mixed ketene dimers are also obtained by generating haloketenes in the presence of dimethylketene, by mixing of solutions of two different ketenes, and by cogeneration of two different ketenes from the carboxylic acid precursors Bis(trifluoromethyl)ketene does not dimerize thermally, but it reacts with Me2C=C=0 to form cyclobutanone and /3-propiolactone-type dimers ". The cycloaddition always proceeds across the C=C bond in the dimethylketene. In the reaction with ketene and methylketene, only the jS-propiolactone-type mixed dimers are formed. 

Vinylketenes, generated from diazoketenes or acid chlorides, undergo an intramolecular [2+2] cycloaddition reaction to give cyclobutenone or cyclobutanones, depending on the precursors. For example, generation of vinylketene 153 affords the cyclobutenone 154 °. ... 

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