Stereoselective 3,3-disubstituted cyclopropene

Cycloalkene Derivatives Cyclopropenes readily interact with nitrile oxides. Reactions of a broad series of 3,3-disubstituted cyclopropenes with 4-substituted benzonitrile, methoxycarbonyl- and cyanoformonitrile oxides (229) as well as with di(isopropoxy)phosphorylformonitrile oxide (230) give 2-oxa-3-azabicyclo[3.1.0]hexene derivatives 62. Stereoselectivity of the cycloaddition is governed by both steric and polar factors. In particular, steric factors are supposed to prevail for 3-methyl-3-phenylcyclopropene affording 62 [R1 =... 

Similar conversions of cyclopropenes can be achieved with other metal complexes. In the presence of trialkylphosphane-modified Pd(0) catalysts, 3,3-dimethylcyclopropene was quantitatively and stereoselectively cyclotrimerized to give 3,3,6,6,9,9-hexamethyl-c ,c/i,/ran -tetra-cyclo[6.1.0.0 0 ]nonane (14). The structure was derived from spectroscopic data and X-ray crystal analysis. The product undergoes thermal rearrangement to form 3,3,6,6,9,9-hexamethyl-trun. -tricyclo[6.1.0.0 ]non-4-ene (15). ° This method has been extended to other 3,3-disubstituted cyclopropenes. ... 

A little-used route to stereoselectively labeled cyclopropanes is nucleophilic addition to the corresponding cyclopropene. Access to both labeled and unlabeled cyclopropenes from the corresponding 1-alkynes allows preparation of both diastereomers of the labeled product. This route is one of the few that would allow easy preparation of a 1,1-disubstituted-cyclopropane-2-d. Cyclopropanation of the corresponding alkene would in principle achieve the same goal but preparation of such alkenes with a label in a defined stereochemical position is not easy. An example of the approach is shown in Figure 10. ... 

The addition of phenyllithium to cyclopropene occurred with 99% stereoselectivity to give cw-l-lithio-2-phenylcyclopropane, albeit in 3 /o yield. With 3-methylcyclopropene, the reaction was more efficient and, after protonation of the lithiocyclopropane, 2-methyl-1-phenyl-cyclopropane was obtained in 44.5% yield. The product is about 94% in the trans form thus attack of the organolithium occurs on the opposite face to that bearing the larger substituent at C3. When the cyclopropene is 3,3-disubstituted, e.g. 17, both faces are sufficiently hindered to slow the addition process and metalation of a vinylic hydrogen predominates. ... 

In a subsequent study, it has been shown that the phosphine-stabilized zircona-cyclopropene species 304 were capable of the metallacycle expansion on reaction with mono- and disubstituted alkynes [84]. With this methodology, l-boryl-1,3-butadienes were prepared in good chemical yields and with moderate to excellent stereoselectivity (Scheme 10.102). 

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