Cyclopropylidene ketones

Unprotected 2-acylcyclobutanones 21 are only stable in solution. They are obtained from the corresponding cyclopropylidene ketones 17 if the double bond is fully substituted. If not, they must be prepared from the corresponding cyclopropylidene alcohols 18 through a sequence of epoxidation, oxidation and rearrangement.53 54... 

Cyclopropylidene ketones exhibit high reactivity in conjugate addition of nucleophiles . Dicyclopropylideneethane, 31 29,591 cyclopropylideneacetic ester show high reactivities in Diels-Alder reactions. 

K-Cyclopropylidene ketones can be obtained in the same way by the reaction of ct-keto ketals with cyclopropylidenetriphenylphosphorane (2, 95-96). ... 

Sulfur ylides can also transfer substituted methylene units, such as isopropylidene (Entries 10 and 11) or cyclopropylidene (Entries 12 and 13). The oxaspiropentanes formed by reaction of aldehydes and ketones with diphenylsulfonium cyclopropylide are useful intermediates in a number of transformations such as acid-catalyzed rearrangement to cyclobutanones.285... 

The Doering-Moore-Skattebol method including a cyclopropylidene-allene rearrangement is often used for the synthesis of allenes. However, the reaction conditions applied are often not compatible with acceptor substituents. One of the rare exceptions is the transformation 76 —> 77 (Scheme 7.11) [122]. The oximes 77 are not accessible by the classical route starting from allenyl ketone and hydroxylamine (see Section 7.3.2). 

Oxaspiropentanes have been obtained from the cyclopropylide 103, prepared by treatment of cyclopropyldiphenylsulfonium tetrafluoroborate 102 either with sodium methylsulfmyl carbanion in dimethoxyethane at —45 °C or with potassium hydroxide in dimethylsulfoxide at 25 °C. While the reaction of the ylide 103 with a,p-unsaturated carbonyl compounds has resulted in selective cyclopropylidene transfer to the a, 3-carbon-carbon double bond leading to spiropentanes, condensation of 103 with non-conjugated aldehydes and ketones led to oxaspiropentanes such as 104, which have been isolated in 59-100% yields, Eq. (30) 57). 

Table 1. Additions of Nucleophiles to a-Cyclopropylidene-Substituted Aldehydes/Ketones...

Organometallic reagents exhibit their expected typical reaction pattern towards a, S-unsaturated carbonyl compounds. Whereas methyllithium reacted cleanly at — 10°C with 3-cyclo-propylidenebutan-2-one (3) to give 3-cyclopropylidene-2-methylbutan-2-ol (4) in 80% yield via 1,2-addition, methylmagnesium iodide converted the ketone 3 to a 60 40 mixture of the 1,2- and 1,4-addition products 4 and 5 in 55% yield. The ratio depended on the substrate structure and was totally reversed when a phenyl substituent was present on the double bond. Soft methylcuprates only gave the 1,4-adducts 2l-o upon reaction with 1 in diethyl ether for 20 minutes at — 10°C to — 30 °C and subsequent warming to 20 °C (Table 1). 

A general method for the preparation of a-cyclopropylidene-aldehydes and -ketones (26) involves reaction of cyclopropylidenetriphenylphosphorane with the requisite a-keto-aldehyde or a-diketone monoketal followed by deketalization. ... 

On the basis of the 1,3-dipolar cycloaddition of 5-membered ring carbonyl ylides, syntheses of oxabicyclo[2.2.1]heptan-2-one ring systems were successfully accompUshed. For example, the reactions of the cyclopropyl-substituted five-membered ring carbonyl ylide 48 derived from the a-diazo ketone 47 with different dipolarophiles have been investigated [78-80]. The compound 47 undergoes cycloaddition in the presence of Rh2(OAc)4 with dimethyl maleate, dimethyl fumarate, cyclopentenone, 1,1-dimethoxyethylene and bi-cyclopropylidene furnishing the expected cycloadducts 49-53, respectively... 

Dicyclopropyl ketone (21) proves to be a valuable starting material for the synthesis of dispiro [2,0,2,1] heptanes as shown in Scheme 2, and the cyclopropylidene compound (23) has been used in the synthesis of various rotanes (see pp. 20 and 71). The synthesis of dimethyl bicyclo[l,l,0]butane-l,3-dicarboxylate from dimethyl 1 -halogenocyclobutane-1,3-dicarboxylate is readily effected by base, but it is surprising to find that dehydrochlorination is more efficient than dehydrobromination. Py-Epoxycarbanions have proved to be particularly useful precursors to cyclo-... 

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