Cyclohexene substituted, from Diels-Alder reaction

Dimerization.—Unsaturated acids, whether monoenoic or polyenoic, furnish dimers which are in demand because of the valuable surface-active properties of their various derivatives. Methods of dimerization have therefore been extensively examined, but understanding of the reaction and the structural identification of the products have lagged behind. Dimerization is effected in several ways but clay catalysts are the most widely employed, and it is now recognized that such catalysts operate in several ways. They may promote modification of monoenoic and dienoic acids to more reactive monomers in addition to assisting both the dimerization process and the subsequent changes in the dimer molecules. In particular, hydrogen transfer seems to be important monoenoic acids are thereby converted to more reactive dienoic acids and the dimer (probably a cyclohexene derivative resulting from Diels-Alder reaction) is converted to a substituted aromatic compound. ... 

Cycloaddition involves the combination of two molecules in such a way that a new ring is formed. The principles of conservation of orbital symmetry also apply to concerted cycloaddition reactions and to the reverse, concerted fragmentation of one molecule into two or more smaller components (cycloreversion). The most important cycloaddition reaction from the point of view of synthesis is the Diels-Alder reaction. This reaction has been the object of extensive theoretical and mechanistic study, as well as synthetic application. The Diels-Alder reaction is the addition of an alkene to a diene to form a cyclohexene. It is called a [47t + 27c]-cycloaddition reaction because four tc electrons from the diene and the two n electrons from the alkene (which is called the dienophile) are directly involved in the bonding change. For most systems, the reactivity pattern, regioselectivity, and stereoselectivity are consistent with describing the reaction as a concerted process. In particular, the reaction is a stereospecific syn (suprafacial) addition with respect to both the alkene and the diene. This stereospecificity has been demonstrated with many substituted dienes and alkenes and also holds for the simplest possible example of the reaction, that of ethylene with butadiene ... 

Cycloadditions of conjugated dienes to alkenes (dienophiles) provide rapid access to substituted cyclohexenes. In view of the widespread occurrence of carbocyclic compounds in nature, and the recent demands for the synthesis of enantiomerically pure compounds, it is not surprising that carbohydrates were examined as substrates for Diels-Alder reactions. Both carbohydrate-derived dienophiles and dienes have been described in the literature. The products of reactions involving these types of compounds, referred to as annulated sugars, are most often formed stereoselectively. The hi levels of both diastereofacial selectivity and endo/exo selectivity sometimes result in the formation of a single cycloadduct from among several possibilities. 

The enormous synthetic potential of the Diels-Alder reaction for the construction of a wide array of substituted cyclohexenes is mirrored in the versatility of the hetero-Diels-Alder reaction for the formation of 6-membered heterocycles. The mechanistic possibilities of the hetero-Diels-Alder reaction span the range from concerted to stepwise ionic processes, which is likely a function of the particular catalysts and reaction conditions employed. Although the hetero-Diels-Alder cycloaddition reaction was historically slower than its all-carbon counterpart in gaining widespread recognition, its utilization has been the subject of intense investigations in more recent years [13, 36, 37]. 

Cycloaddition reactions result in the formation of a new ring from two reactants. A concerted mechanism requires that a single transition state, and therefore no intermediate, lie on the reaction path between reactants and adduct. The most important example of cycloaddition is the Diels-Alder (D-A) reaction. The cycloaddition of alkenes and dienes is a very useful method for forming substituted cyclohexenes.1... 

Interestingly, treatment of the allylic carbonate 23, which had proven problematic in the previous study, under analogous reaction conditions with the copper enolate derived from 24 furnished the a,/9-disubstituted ketone. Subsequent ring-closing metathesis furnished the 1,2-cyclohexenes 25a/25b in 75% overall yield favoring the trans-dia-stereomer 25a (2° 1°=30 1, ds=10 l) [14]. Overall, this reaction provides an alternative approach to an exo-selective Diels-Alder cycloaddition, and indicates that a-substituted enolates are even more tolerant nucleophiles than the unsubstituted derivatives. 

The classic de Mayo reaction involves the [2 + 2] photocycloaddition of an alkene to the hydrogen-bonded enol tautomer of a P-dicarbonyl compound as exemplified by the formation of 1,5-diketone 9 from pentane-2,4-dione and cyclohexene (vide supra). In addition to alkenes, allenes are also used as the olefinic component. For example, irradiation of a mixture of dimedone and allene results in the formation of 3,3-dimethyl-7-methylenecycloocta-l,5-dione 12 via the cyclobutane intermediate 11, together with the corresponding head-to-tail product 13, which spontaneously dimerizes to the hetero Diels-Alder adduct 14. Diketone 12 is a versatile building block for the preparation of substituted cyclooctadienones and 8-valerolactones. 

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