Substituted methylene group

The formation of six-membered or larger rings by intramolecular C-H bond insertion normally requires the attacked position to be especially activated towards electrophilic attack [1157,1158]. Electron-rich arenes or heteroarenes [1159-1162] and donor-substituted methylene groups can react intramolecularly with electrophilic carbene complexes to yield six- or seven-membered rings. Representative examples are given in Table 4.8. 

P-Keto acids484 are easily decarboxylated, but such acids are usually prepared from p-keto esters, and the esters are easily decarboxylated themselves on hydrolysis without isolation of the acids.485 This decarboxylation of p-keto esters involving cleavage on the carboxyl side of the substituted methylene group (arrow) is carried out under acidic, neutral, or... 

LIVa), (LVa), and (LVIa) are not formed. Formally these would result from rotation of the unsubstituted methylene group instead of the methyl-substituted methylene group, or in the case of (LVa), of the trans- and not the cis-methyl-substituted methylene group. 

The reaction of 1-arylsulfonylaziridines 217 with dimethylsulfoniumethoxycarbonyl methylide 218 is a fairly general approach for stereoselective synthesis of 1-arylsulfonylazetidines 219 bearing an ethoxycarbonyl functionality (Equation 58) <1995J(P1)2605>. However, the products are obtained in moderate yields. The reaction involves a regioselective transfer of an ethoxycarbonyl-substituted methylene group from the ylide to 1-arylsulfonylaziridines. 

Molecular dynamics are time-consuming because the nonbonded interactions scale as n where n is the number of atoms. To save time, one may implement the united atom approach, substituting some atomistic detail with an imaginary entity that represents the essential features of what has been substituted. For example, it is common to substitute methylene groups with an imaginary spherical atom with mass 14. Therefore a polyethylene chain would look like a chain of spherical atoms, appropriately rescaled, terminated by similar entities with mass = 15 for the methyl groups. 

Whangbo and Wolfe (251), in the case of 127, considered the highest occupied orbitals of the ethyl group as a whole, namely ffcc and instead of orbitals of a substituted methylene group (Figure 28, cf. Figure 27). The energies of orbitals Ucc and are nearly the same and from this point... 

Allylic alcohols are also cyclopropanated over 100 times faster than their unfunctionalized alkene equivalents. Coordination between the zinc atom and the hydroxyl group in the transition state explains both the stereoselectivity and the rate increase. Unfortunately, while the Simmons-Smith reaction works well when a methylene (CH2) group is being transferred, it is less good with substituted methylene groups (RCH or R2C ). 

Extension of the synthetic versatility of sulfur ylides so that substituted methylene groups, as well as methylene itself, can be transferred has been accomplished by modification of the sulfur substituents. Entry 6 in Scheme 2.9 illustrates a transfer of an isopropylidene unit. Even greater structural flexibility has been achieved by use of nucleophiles derived from sulfoximines." The N-p-toluenesulfinyl sulfoximines appear to be particularly promising reagents. They are prepared from sulfoxides by reaction with p-toluenesulfonyl azide. Strong base... 

Cyclic acetals are five- and higher-membered monomers with at least one unit in which two oxygen atoms flank an tmsubsti-tuted or substituted methylene group. The bridge connecting two acetal oxygen atoms may be either an aU-carbon chain... 

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