Cyclopropanes, preparation

Bromo-l-propenyl phenyl sulfone (104) can serve as a Michael acceptor to Grignard reagents to give cyclopropyl phenyl sulfones in good yields, (equation 85)70. Cyclopropanes prepared by this method are listed in Table 8. However, with methyl, ethyl or t-butyl... 

Few examples of the formation of cyclopropanes by intramolecular C-H insertion of electrophilic carbene complexes have been reported. This methodology for cyclopropane preparation seems only to be suitable for polycyclic compounds with little conformational flexibility. Illustrative examples are listed in Table 4.3. 

Table 2. Alkoxy siloxy cyclopropanes prepared by Et2Zn/CH2I2.

It is possible that a dialkylzinc-methylene iodide reagent may further improve the yield as in the case with cyclopropanes prepared from vinyl ethers (Fura-kawa, J., Kawabata, N., Nishimura, J. Tetrahedron 24, 53 (1968)). 

Two pcntasubstituted cyclopropanes prepared by Perkin (cf.page72) are of some interest. As seen fror.1 fwrtoilae I and IJ they are substitution products... 

Olefination and cyclopropanation. Preparation of alkenyl allyl ethers by the Wittig reaction is readily achieved. Tellurium and arsonium ylides tend to produce cyclopropyl ketones on reaction with enones. ... 

Another approach to the synthesis of the parent compounds involves 35) the reaction of 1-ethoxy-1-trimethylsilyloxy cyclopropane prepared according to the Ruhlman method 112) and selenols, in the presence of zinc chloride (Scheme 31 a). The reaction is quite rapid with methylselenol35) but much more difficult with phenylselenol3> 35). It takes a completely different course when an alkyl substituent is present on the cyclopropane ring114). Surprisingly thus methylselenol cleaves n4> the cyclopropane ring of 1-ethoxy-l-trimethylsilyloxy-2-ethyl cyclopropane 11S) on addition of zinc chloride and lead114> to 1,1 -bis(methylseleno)pentane by reductive selenoacetalisation (Scheme 31b). 

The appropriate alkene (1 mol) and benzal chloride (10.6 g, 0.066 mol) were placed in a flame-dried, 250-mL three-neck flask, equipped with a dry-ice condenser, low-temperature thermometer, addition funnel and magnetic stirrer. All operations were carried out under a small positive N2 pressure. MeLi (0.10 mol) in EtjO was added over a period of 2.5 h at — 40"C. After completion of the addition, the mixture was stirred for an additional and then poured onto ice. The product was washed with H O and dried (Na2S04), and the excess alkene was allowed to evaporate. The EtjO and remaining alkene were removed under reduced pressure, and the residue was distilled from bulb-to-bulb under vacuum. The analysis of the product was carried out by GC. Cyclopropanes prepared in this manner are compiled in the above table. 

A mixture of alkene (8-10 equiv) and halide (0.8 equiv) was stirred at rt (with a small amount of EtjO when the alkene was norborn-2-ene) as a solution of LTMP (1 equiv prepared from 2,2,6,6-tetramethyl-piperidine and MeLi) in EtjO was added dropwise. After 20 min, H O was added and the organic phase was separated, washed with HCl and dried (Na2S04). The solvents were removed by rotary evaporator, and the residue was either distilled or chromatographed on silica gel. The ratios of products were determined by GC prior to workup. The cyclopropanes prepared in this way are included in Table 1. 

There are only a few examples of cyclopropanes prepared via the addition of dichlorocarbene to alkenenitriles. Depending on the structure of the unsaturated nitrile, the chloroform/base/phase-transfer catalyst or dichlorohalomethyl(phenyl)mercury methods are used in the former case, the formation of the adducts of trichloromethyl anion competes considerably. Thus, acrylonitrile, but-2-enenitrile and 3-phenylpropenenitrile gave, by the phase-transfer catalytic method, Michael adducts exclusively. The same occurs if acrylonitrile reacts with trihalomethyl(phenyl)mercury and sodium iodide mixture. An example is the formation of 1 and... 

A large number of cyclopropanes have been prepared by synthetic routes where the last step is hydrolysis of a lithiocyclopropane and therefore, formally, a substitution of lithium with hydrogen. Almost without exception, however, the cyclopropyllithium species involved in these syntheses are generated in situ because they are too unstable to be stored in a practical way for any length of time even at low temperature. Cyclopropanes prepared in this way are therefore not regarded as being obtained by a substitution reaction. A notable exception is 1,1-dideuterio-2,2,3,3-tetramethylcyclopropane (2), which was obtained in almost quantitative yield when the stable compound l,l-dilithio-2,2,3,3-tetramethylcyclopropane (1) was treated with deuterium oxide. 

The absolute configuration of (61) was established by further chemical degradation. Ozonolysis of (61) produced a ketoacid (69), followed by methylation to give a methyl ester (70), C11H18O4, [a]o —25.1 , whose spectral data were identical with those of (+)-c/.y-l-methoxycarbonyl-2,2-dimethyl-3-(3-oxo-n-butyl) cyclopropane, prepared by degradation of (+)-car-2-ene (71) and (-l-)-bicyclogermacrene (228). Thus, the structure and... 

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