Cyclohexene trans stereoisomer

Reactions of 1 with epoxides involve some cycloaddition products, and thus will be treated here. Such reactions are quite complicated and have been studied in some depth.84,92 With cyclohexene oxide, 1 yields the disilaoxirane 48, cyclohexene, and the silyl enol ether 56 (Eq. 29). With ( )- and (Z)-stilbene oxides (Eq. 30) the products include 48, ( > and (Z)-stilbenes, the E- and Z-isomers of silyl enol ether 57, and only one (trans) stereoisomer of the five-membered ring compound 58. The products have been rationalized in terms of the mechanism detailed in Scheme 14, involving a ring-opened zwitterionic intermediate, allowing for carbon-carbon bond rotation and the observed stereochemistry. 

Methylation of the lithium enolates derived from cyclohexen-2-one and l,l-bis(methylseleno)ethylli-thium t es place regioselectively but requires the use of more than one equivalent of HI A (S equiv., 0 C, 1 h). The product consists quite exclusively of the trans stereoisomer (99%) with a trace (1%) of the cis (Scheme 140, d and e). The latter can be obtained as a single stereoisomer on hydrolysis of the adduct resulting from the reaction of the same organometallics with 2-methylcyclohexen-2-one... 

Prepare a model of cyclohexene. Note that chair and boat conformations are no longer possible, as carbon atoms 1, 2, 3, and 6 lie in a plane. Are cis and trans stereoisomers possible for the double bond Attempt to assemble a model of trani-cyclohexene. Can it be done Are cis and trans stereoisomers possible for 2,3-dimethylcyclohexene For 3,4-... 

One of the most useful features of the Diels-Alder reaction is that it is stereo-specific, meaning that a single product stereoisomer is formed. Furthermore, the stereochemistry of the reactant is maintained. If we carry out the cycloaddition with a cis dienophile, such as methyl ds-Z-butenoate, only the cis-substituted cyclohexene product is formed. With methyl P r///s-2-butenoate, only the trans-SLibstitLited cyclohexene product is formed. 

Figure 10. Synthesis of the stereoisomers of trans-6-methyl-3-cyclohexene carboxylic acid...

The addition of bromine and chlorine to a cycloalkene gives a trans dihalocycloal-kane. For example, the addition of bromine to cyclohexene gives trawfrl,2-dibromocyclo-hexane the cis isomer is not formed. Thus, the addition of a halogen to a cycloalkene is stereoselective. A stereoselective reaction is a reaction in which one stereoisomer is formed or destroyed in preference to all others that might be formed or destroyed. We say that addition of bromine to an alkene occurs with anti stereoselectivity. 

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