1- Cyclopentenes, 1,2-disubstituted

An interesting probe of reactivity was presented by Burton in his study of cycloadditions of l,2-disubstituted-3,3,4,4-tetrafluorocyclobutenes and 1,2-disub-stituted-3,3,4,4,5,5-hexaf1uorocyclopentenes with butadiene, 2-methylbutadiene, and 2,3-dimethylbutadiene [86], On the basis of the extent of their conversions to adducts, the relative reactivities of the cyclobutenes and of the cyclopentenes are as shown in equation 74. A typical reaction is shown in equation 75. 

Recent advances in gas chromatographic separations of enantiomers allow precise determination of the enantiomeric purity of the algal pheromones. The czs-disubstituted cyclopentenes, such as multifidene, viridiene, and caudoxirene, are of high optical purity [ 95% enantiomeric excess (e.e.)] whenever they have been found (32,33). The situation is different with the cyclopropanes and the cycloheptadienes, as shown in Table 2 and Figure 1. Hormosirene from female gametes or thalli of... 

The reaction of 1,3-disubstituted bicyclo[2.1.0]pentanes with tris(4-bromophenyl)ammoniumyl hexachloroantimonate (the latter in catalytic amounts) leads to the corresponding cyclopentene after 1,2-hydrogen or 1,2-alkyl migration in the intermediary 1,3-cation-radicals (Adam and Sahin 1994 Scheme 7.47). 

Polymers containing rings incorporated into the main chain (e.g., by double-bond polymerization of a cycloalkene) are also capable of exhibiting stereoisomerism. Such polymers possess two stereocenters—the two atoms at which the polymer chain enters and leaves each ring. Thus the polymerization of cyclopentene to polycyclopentene [IUPAC poly(cyclopen-tane-l,2-diyl)] is considered in the same manner as that of a 1,2-disubstituted ethylene. The... 

The cycloaddition of substituted acrylates has been investigated with cyclic nitronate 24 (Table 2.49) (14). The cycloaddition of a 1,1-disubstituted dipolar-ophile (entry 2), proceeds in good yield, but both 1,2-disubstituted alkenes fail to react. The effect of substitution pattern on the dipolarophile was investigated with a slightly more reactive nitronate (Table 2.50) (228). Less sterically demanding alkenes such as cyclohexene, cyclopentene, and methyl substituted styrenes react, albeit at elevated temperature. The only exception is the 1,1-disubstituted alkene (entry 4), which reacts at room temperature. Both stilbene and dimethyl fumarate fail to provide the desired cycloadduct. In a rare example of the dipolar cycloaddition of tetra-substituted alkenes, tetramethylethylene reacts at 50 °C over 3 days to give a small amount of the cycloadduct (entry 7). 

A number of examples involving nitrile oxide cycloadditions to cyclic cis-disubstituted olefinic dipolarophiles was presented in the first edition of this treatise, notably to cyclobutene, cyclopentene, and to 2,5-dihydrofuran derivatives (15). The more recent examples discussed here also show, that the selectivity of the cycloaddition to 1,2-cis-disubstituted cyclobutenes depends on the type of substituent group present (Table 6.8 Scheme 6.41). The differences found can be explained in terms of the nonplanarity (i. e., pyramidalization) of the double bond in the transition state (15) and steric effects. In the cycloaddition to cis-3,4-diacetyl-(197) and cis-3,4-dichlorocyclobutene (198), the syn-pyramidalization of the carbon atoms of the double bond and the more facile anti deformability of the olefinic hydrogens have been invoked to rationalize the anti selectivity observed. 

The stereochemistry with respect to the ketene iminium salt in cases of monosubstituted or unsymmetrically disubstituted derivatives in cycloadditions to alkenes show differences from the ketene counterpart. In contrast to ketene cycloadditions of monosubstituted ketenes with alkenes where the substituent in the bicyclic derivative ends up in the endo position, the cycloaddition of two monosubstituted ketene iminium salts with cyclopentene and cyclohexenc gives the e.wp-substituted derivatives 4.6... 

The ROMP of alcohol or acetate disubstituted cyclopentene monomers is not possible by catalysts such as... 

Borane may react sequentially with 3 mol of alkene to form mono-, di-, and trialk-ylboranes. Both the alkene structure and reaction conditions affect product distribution. Trialkylboranes are usually formed from terminal olefins [Eq. (6.57)] and unhindered disubstituted alkenes such as cyclopentene irrespective of the reactant ratio.340 The reaction cannot be stopped at the mono- or dialkylborane stage. In contrast, hindered disubstituted olefins (e.g., cyclohexene) and trisubstituted alkenes are converted mainly to dialkylboranes [Eq. (6.58)]. Careful control of... 

The ring-opened polymer of norbomadiene consists of 3,5-disubstituted cyclopentene units (94). When the concentration of these units is kept below 0.2 M the polymer remains soluble, but above this concentration, in the presence of WClg/Me4Sn (1/2), it gels. This is caused by cross-linking, brought about by the ROMP of the enchained cyclopentene... 

Carboxylates behave as an O-nucleophile and are allylated. Reaction of AcONa with cyclopentadiene monoxide (139) proceeds with retention of stereochemistry as shown by 140, to give the 3,5-c -disubstituted cyclopentene 141 [74], Although alcohols are rather unreactive nucleophiles for the Pd-catalysed allylation, the alkoxide anions generated by the treatment of silyl ethers with TBSF are easily allylated. Desilylated alcohol from 142 reacts intramolecularly with the cis and trans vinyl epoxides to give the cis and trans pyrans 143 and 144 regio- and stereoselectively, and... 

Reductive cyclization.1 Reduction of the unsaturated aldehyde 1 with Sml2 in THF/HMPT (20 1) at 0° effects a tandem radical cyclization of the fram-3,5-disubstituted cyclopentene system to a linear triquinane unit (2) with surprisingly high cw-ann-di-stereoselectivity. 

The first reaction is between epoxycyclopentadiene and adenine, one of the heterocyclic building blocks of nucleic acids, and follows the course we have just described to give a cis-1,4-disubstituted cyclopentene. The alcohol is then activated by conversion into the carbonate, which reacts with phenylsulfonylnitromethane, which could later be converted into an alcohol. Once again, retention of stereochemistry during the palladium-catalysed substitution gives the cis product. 

TABLE 3.16 Percent Cis Isomer in Saturated Product from Hydrogenation of Disubstituted Cyclopentenes and Related Cyclopentylidenes , gf... 

Wadamoto M, Phillips EM, Reynolds TE, Scheldt KA (2007) Enantioselective Synthesis of a,a-disubstituted cyclopentenes by an A-heterocyclic carbene-catalyzed desymmetrization of 1,3-diketones. J Am Chem Soc 129 10098-10099... 

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