Cycloaddition to 1,3-Diene

Nitroethene undergoes rapid cycloaddition to 1,3-dienes the subsequent Nef reaction gives cyclohexenones, which are formally produced by the Diels-Alder reaction of ketene with... 

The highly strained double bond in methylenecyclopropane displays enhanced reactivity in cycloaddition reactions. In addition to normal [4+2] cycloaddition to 1,3-dienes (e.g. equation 13)32, methylenecyclopropane and its derivatives have a pronounced tendency to undergo thermal [2+2] cycloaddition reactions. For example, thermal dimerization of methylenecyclopropane in the gas phase results in formation of isomeric dispirooctanes 16 and 17 (equation 14)33. This unusual cyclization is considered to proceed via a stepwise radical mechanism involving the intermediacy of biradical 18 (equation 15)34. Equation 15 demonstrates that methylenecyclopropanes possessing substituents capable of stabilizing intermediate radicals undergo efficient [2+2] dimerization even... 

The A -hydroxycarbamate 1, prepared from 2-exo-3-cx -2-fm-butoxy-3-bornanol, is oxidized to the nitroso compound 2 which undergoes cycloaddition to 1,3-dienes, e.g., 3133. The cycloadducts are obtained with good yields and high diastereoselectivity (determined by HPLC) (Table 4). The configuration of the prevalent diasteromers was assigned by correlation with known compounds. 

The synthesis and chemistry of thiirane-1-oxides (episulfoxides) was reviewed in 1997 by Abu-Yousef and Harpp <1997SR1>. One of the many interesting processes discussed there is the thermolysis of thiirane-l-oxide 122, at temperatures exceeding 100 °C, to generate sulfur monoxide. The latter is very reactive and can be trapped by cycloaddition to 1,3-dienes such as 2,3-dimethyl-l,3-butadiene 123 (Scheme 21). 

Dienophiles substituted with appropriate heteroatoms may offer a number of advantages such as (1) provide dienophilic equivalents of C-—C and moieties which do not undergo [4 + 2] cycloadditions to 1,3-dienes because of low (e.g. isolated alkenes, alkynes, allenes) or different reactivities (e.g. ketenes) (2) enhance or invert the regiochemistry of the Diels-Alder process (3) permit facile removal of the activating and/or regiocontrolling group after cycloaddition with or without introduction of further functionalities. The use of nitroalkenes as dienophiles demonstrates these issues most strikingly. 

Nitroethylene undergoes rapid cycloaddition to 1,3-dienes subsequent conversion of the nitro to a carbonyl group, e.g. (1) (2) -+ (3), exemplifies its application as a dienophilic ketene equivalent (Scheme... 

The role of sulfur functionalities in activating alkenic and alkynic dienophiles, and directing their cycloadditions to 1,3-dienes, has been recently reviewed and only some essential points are outlined here. Depending on the oxidation state of sulfur, its vinyl derivatives range from strongly or weakly electron-deficient (sulfones or sulfoxides) to electron-rich (sulfides) dienophiles. 

Tetrahalocyclopropenes are considerably more stable than their 1,2-dihalo counterparts and undergo cycloaddition to 1,3-dienes at ca. 100 C. The twofold dchydrochlorination of the adducts provides access to 1,1-dihalobenzocyclopropenes and cycloproparenes. The original synthesis of 1,1-difluorobenzocyclopropene (2a) from buta-1,3-diene and 1,2-dibromo-3,3-difluorocyclopropene (la) is typical. ... 

Addition of aluminium chloride to B-ethylthio nitroalkenes resulted in formation of thienium cations, e,g. (64) which underwent [4+2]-type cycloadditions to 1,3-dienes. Cleavage of the cycloadducts yielded 1,4-substituted olefins (Scheme 41). 

Cycloadditions.—Cycloaddition to 1.3-Dienes. The addition of simple alkylmethylene-cyclopropenes to cyclopentadiene has been presented (Scheme 22 p. 38). ... 

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