Cyclopropanes opening

Cuprates add directly to the carbonyl group however, promoted by the Lewis acid BF3, ring cleavage is facilitated and a 1,5-addition affords the bicyclic compound 32. This mode of cyclopropane opening is a key step in the short preparation of the monoterpene eucalyptol22). 

Not surprisingly, two trimethylsilylmethyl groups further increase reactivity of cyclopropanes. Therefore 262 opens to 263 even at —78 °C yielding a new functionalized allylsilane as the product. An attempt to transform 262 into the corresponding P-ketosulfone 264 leads to the unusual cleaved compound 265, which carries an allyl and vinylsilane unit as well as the P-ketosulfone moiety. It is evident that this mode of cyclopropane opening is initiated under the basic reaction conditions by deprotonating the intermediate 264 in the side chain a to one of the trimethylsilyl groups 113). 

In the early seventies Stork and his students have shown that the intermediate enolate can be trapped by suitable electrophiles, e.g. alkyl halides, giving substituted ring-opened products. These modes of cyclopropane opening have been used for the synthesis of several natural compounds ". A tetra-activated cyclopropane is cleaved by sodium naphthalene in a preparation of a specifkally substituted semibullvalene (equation 36) . ... 

In general, cyclopropanes open under the influence of acidic reagents to form the most stable carbenium ion, which then may rearrange, eliminate in one or several directions, react with solvent, or give rise to any one of a number of products Still more readily,... 

Other complementary methodologies include the preparation of substituted cyclopropanes from glycals using rhodium acetate carbenoid additions [65,66]. Additionally, acid catalyzed cyclopropane opening reactions in alcoholic solutions afford the 2-C-branched-glycosides. These combined reactions were used to prepare a key intermediate in marine diterpene norrisolide synthesis from D-mannose [67]. 

An efficient system for the construction of spirocyclic quartenary centers based on cyclopropane opening is described in Scheme 7 [10]. 

This silyl-transfer cascade involved initial cyclopropane-opening-assisted 1,2-Brook rearrangement of lithium alkoxide 169. The resulting primary alkyllithium 170 underwent retro-1,5-Brook silyl migration to the ketone enolate 171. 

Vinylcyclopropane (VCP) can be viewed as the homologue of 1,3-butadiene, which is supported by theoretical computations and spectroscopy analysis [1], However, it s the experimental fact that VCP can participate in cycloadditiOTis as does 1,3-butadiene that may make the viewpoint more convincing. The vinyl substituent of VCP is usually essential to activate or direct the cyclopropane opening, but it does not have to participate in cycloadditions. As a result, VCP can act as either a five-carlxMi synthon or a three-carbon synthon. Combination of many other components with VCP (as a five- or three-carbon synthon) under transition metal catalysis or mediation has led to development of numerous new cycloaddition patterns. The cycloadditions of VCP introduced will be classified according to its role as a five- or three-carbon synthon. Under each category, further elaboratifMi is arranged by different cycloaddition patterns and transition metals. 

Reaction of the cyclopropylalkynes (151) with water (or methanol), catalysed by (Tf2N)3Fe, proceeds with the cyclopropane opening, which generates the homo allenic cation (153), whose hydration affords (154). By contrast, catalysis by (Tf2N)Au, (Tf2N)Ag, and/or (Tf2N)2Hg proceeds as an uneventful hydration to produce ketones (155). This example clearly demonstrates the subtle electtonic differences in the intermediate metal-alkyne complexes 152). ... 

Cycloadditions of TCNE with cyclopropylethylenes, in which new bonds to the cyclopropane carbons form, have been reported further. Bicyclo[6,l,0]-nonatriene and the eJco-9-chloro-derivative undergo cycloaddition with TCNE but, though the cyclopropane opens, the stepwise polar additions are across C-3—C-4 to give adducts (424) see also p. The structures of (424) have been proven by 2f-ray crystallography and the trans junction was confirmed in both cases. 

Scheme 79 1,3-Acyl shift followed by cyclopropane opening and cyclization...

Gharpure and coworkers also found that the oxocarbenium ion from the cyclopropane opening reacts with carbon nucleophiles. C-Aryl furan 41 resulted from the use of 1,3,5-trimethoxybenzene (40) as the nucleophile in the ring opening (Scheme 12). 

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