Cycloalkenone ring

Ring expansion, cleavage of cyclic ketals, and cycloalkenone ring closure s. 11, 859... 

In the case of cycloheptenone and larger rings, the main initial photoproducts are the -cycloalkenones produced by photoisomerization. In the case of the seven- and eight-membered rings, the double bonds are sufficiently strained that rapid reactions follow. In nonnucleophilic solvents dimerization occurs, whereas in nucleophilic solvents addition occurs. ... 

Cycloalkenones and/or their derivatives can also behave as dienic partners in the Diels-Alder cycloaddition. It is well documented [41] that cyclic acetals, for example, can interconvert with ring-opened enol ether forms, in a reversible manner the latter compounds can then be trapped by various dienophiles. Thus dienes 119 and 120 reacted with [60]-fullerene (Ceo) at high pressure, affording highly thermally stable products [42] (Scheme 5.16). Ketones 123 and 124 could be directly obtained by cycloaddition of enol forms 121 and 122 of 2-cyclopen-ten-and 2-cyclohexen-l-one, respectively. 

Reduction, carboxyl groups, 56,83 Reduction of a,0-unsaturated p-toluene-sulfonyl-hydrazones to alkenes, 59,42 Reductive alkylation, 56,52 Reductive cleavage, 56, 101 Resolution of amines, 55,80, 83 Rexyn 201,55,4 Rhodium(III) oxide, 57, 1 Ring contraction, 56, 107 Ring expansion of cycloalkanones to cycloalkenones, 59, 113... 

ONE-CARBON RING EXPANSION OF CYCLOALKANONES TO CONJUGATED CYCLOALKENONES 2-CYCLOHEPTEN-l-ONE... 

Preparation of 2-Cycloalkenones and Cycloalkane-1,3-diones BY Iron(III) Chloride-Induced Ring Opening of 1-Trimethyi,-siLYLOxY- AND 1,2-Bis(trimethylsilyloxy)bicyclo[w.1.0]alkanes... 

In the presence of a strong acid as catalyst, alkenoic acids will add internally to furnish a valuable route to lactones (Scheme 4).460 Regardless of the position of the double bond in the chain, the product obtained is always the y- and/or 8-lactone due to acid-catalyzed double bond migration to a position favorable for ring closure.461 The choice of catalyst and the reaction temperature are important in minimizing the amount of cycloalkenone side product that usually accompanies lactone formation.462... 

The procedure described is a simple, rapid, and convenient method for conversion of n-sized cycloalkenones into n+4 alkenolides. Significant but limited progress has been reported in the recent literature toward the preparation of medium and large ring lactones via ring-expansion reactions. One of the most notable and useful developments in this area involves conversion of a cycloalkanone into a bicyclic vinylic ether which is oxidatively cleaved to form a ring-enlarged keto lactone.3... 

Based on the data in the Table and on our published results,7 it is clear that five-, six-, and seven-membered cycloalkenones undergo this 4-atom ring enlargement reaction to produce medium ring, unsaturated lactones in overall yields... 

Fig. 14.25. Ring expansion of cydoheptenone via a car-benoid intermediate. The elimination of LiBr from the car-benoid occurs with or is followed by a [l,2]-alkenyl shift. The enolate C is formed and, upon agueous workup, it is converted to the ring-expanded cycloalkenone B.

Simple diastereoselectivities are observed in the reaction of cyclo-alkenones with cyclic alkenes, e.g., cyclopentene, wherein the transoid tricycle is formed preferentially. Whereas cyclopent-2-enone (7) affords 36 selectively, the more flexible cyclohex-2-enone (8) gives a 3 1 mixture of diastereoisomers 37 and 38 [7,61]. A stereogenic center in the cycloalkenone also has a strong impact on the product ratio as shown for the reactions of 4-alkylcyclohex-2-enones 39 and 40 with acyclic alkenes wherein the major diastereoisomer formed is the one in which the enone-alkyl group is trans to the new ring forming C-C bonds, i.e., 41t and 42t, respectively (Sch. 12) [62,63]. Cycloadducts 42 have been further converted to the pheromone periplanone-B. 

Ito, Y, Fujii, S., and Saegusa, T. 1976. Reaction of l-silyloxybicyclo[n.l.OJalkanes with Fe Clj. A facile synthesis of 2-cycloalkenones via ring enlargement of cyclic ketones. J Org Chem 41, 2073-2074. 

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