Alkylidene cyclopropanes

The catalyst exerts some influence on the bonds broken in hydrogenolysis of saturated cyclopropanes (775), but in vinyl and alkylidene cyclopropanes the effect is pronounced. Platinum or palladium are used frequently. In one case, Nishimura s [124a) catalyst, rhodium-platinum oxide (7 3), worked well where platinum oxide failed (.75). An impressive example of the marked influence of catalyst is the hydrogenation of the spirooctane 42, which,... 

Bei der selektiven Reduktion geminaler Dibrom-alkyliden-cyclopropane mit Tributyl-zinnhydrid zu Monobrom-Derivaten wird die C=C—Doppelbindung nicht ver-schoben, aber ein Stereoisomeren-Gemisch erhalten4 ... 

Die weitere Reduktion der erhaltenen 3-Brom-l-alkyliden-cyclopropane kann ahnlich durchgefiihrt werden5 ... 

The two articles in this current volume describe recent developments with small ring compounds which have not teen compiled in such a context before. T. Hirao discusses selective transformations initiated by transition derivatives in the construction of functionally substituted five-, six- and seven-membered rings as well as open-chair compounds. Cycloadditions onto methylene- and alkylidene-cyclopropane derivatives, described by A. Goti, F. M. Cordero and A. Brandi, not only yield products with spirocyclopropane moieties which can be desirable as such or as potential mimics of gem-dimethyl groupings, but also intermediates which can undergo further transformations with ring-opening of the cyclopropane units. 

Goti, A., Cordero, F. M., and Brandi, A. Cycloadditions Onto Methylene- and Alkylidene-cyclopropane Derivatives. 178,1-99(1996). 

Cycloadditions Onto Methylene- and Alkylidene-cyclopropane Derivatives... 

Under mUd conditions, norbomene and phenylacetylene were selectively coupled to give alkylidene cyclopropane 123 (Scheme 9.19). 

The dehydrohalogenation of monohalogenated cyclopropanes under phase transfer conditions has been of greater preparative importance Monoalkylated bromo-cyclopropanes have been converted to alkylidene cyclopropanes, and 3,3-disub-stituted cyclopropylhalides provide the corresponding cyclopropenes in good yield [158]. 

The required oxaspiropentanes are obtained by epoxidation of alkylidenecyelopropanes (A Section 3.2.2.1.), by reacting aldehydes and/or ketones with diphenylsulfonium cyclopropanide (B Section 3.2.2.2.), and by reacting ketones with 1-bromo-l-lithiocyclopropanes (C Section 3.2.2.3.) or diazocyclopropane (D Section 3.2.2.4.). Most widely used is diphenylsulfoni-um cyclopropanide, but for higher than 2,2-disubstituted cyclobutanones the use of alkylidene-cyclopropanes or 1-bromo-l-lithiocyclopropanes may be advantageous. Once formed, oxaspiropentanes rearrange with extreme ease. 

Lithiosilanes derived from cyclopropane dithiocetals add to aldehydes to give precursors for Peterson olefinations - one of the best ways of making alkylidene cyclopropanes. In the example below, the lithiated allyl sulfide 72 adds cleanly to a ketene dithioacetal to give cyclopropane 73. Successive reductive lithiations give silane 74 and then a mixture of... 

Schweitzer has synthesized alkylidene cyclopropanes by Wittig reaction with a cyclopropyl phosphonium salt. If, however, the electrophilic character of the cyclopropane is enhanced by a second acceptor group (e.g. CO2R, SR), suitable nucleophiles are able to attack the three-membered ring. The resulting ylid can form hetero- and carbocycles by an intramolecular Wittig reactionas demonstrated in equations 62 and 63. These annulation reactions have been applied to terpene and alkaloid preparations. 

Preparation of alkylidene cyclopropanes is feasible by alkylation of lithium carbenoids followed by base-promoted elimination (equation 13or by addition of carbonyl compounds, silylation and reductive elimination (equation 132). Similarities with the chemistry displayed by sulphur-substituted cyclopropanes (Section VI.D.l) are obvious. 

Following these examples, methylene cyclopropene and certain 5,6-dihydrocali-cenes are now obtainable. Adding lithio trimethylsilylcyclopropanes to aldehydes also provides alkylidene cyclopropanes (Peterson olehnation route) . An approach to a-methylene y-butyrolactones starting with dibromocyclopropanes has been opened by a suitable manipulation of intermediate cyclopropyl anions (equation 133). ... 

Vinylcyclopropanols can be prepared either from the readily available cyclopro-panone hemiacetaP, from 1-hydroxycyclopropanecarboxaldehyde derivativesfrom a,a -dichloroacetone, from the silver Simmons-Smith cyclopropanation of a-ethylenic ketone silyl enol ethers from the dye-sensitized photo-oxygenation of alkylidene-cyclopropanes or from the ring-opening of oxaspiropentanes (cf. Section III.C). Consequently, they become participants of choice in a number of useful chemical transformations (see also Section IV.A). 

Alkylidene cyclopropanes.4 These compounds (3) can be prepared by reaction of aldehydes and ketones with 1-methylseleno-l-lithiocyclopropane (1) to give /J-hydroxycyclopropyl selenides (2). Elimination of CH3SeOH is then effected with N,N -carbonyldiimidazole or PI, for substrates derived from ketones. 

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