Diene synthesis stereospecific

The stereospecific synthesis of an A ring synthon of la-hydroxy vitamin D has been carried out. The ( )-allcene is cyclized to give the (E -c.xo-diene 155, and the (Z)-allcene affords the (Z)-e,xo-diene 156 stereospecifically[125,126]. These results can be understood by the cis addition and syn elimination mechanism. 

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 ... 

Compounds 1, which bear in X an electrofugical leaving group, such as trialkylsilyl or diphenylphosphinyl, are of high value for the synthesis of stereochemically homogeneous 1,3-dienes via stereospecific anti or urn elimination. 

Methyl 2,5-dihydro-4-carbomethoxy-2-thiopheneacetate added at 180° to excess butadiene in the presence of hydroquinone, the intermediate refluxed 18 hrs. with Raney-Ni W 5 in methanol, then saponified by refluxing 40 hrs. with aq.-ethanolic KOH diacid. Overall Y 52%. - This synthesis can be used for the construction of trans-fused bicyclic isocyclics with angular methyl groups. F. e. s. G. Stork and P. L. Stotter, Am. Soc. 91, 7780 (1969) stereospecific diene synthesis, effect of substituents, s. P. C. Jain, Y. N. Mukerjee, and N. Anand, Chem. Commun. 1971, 303. 

Stereospecific diene synthesis with 2,5-dihydrothiophenes Ring opening by desulfuration... 

Stereospecific intramolecular diene synthesis with indoles... 

Asym. diene synthesis Reversal of stereospecificity Alcohols from carboxylic acid esters... 

Lactams by stereospecific double ring closure via N-acylation and intramolecular diene synthesis... 

Multzer s group has applied the dehydrative decarboxylation of (3-hydroxy-acids to the synthesis of buta-l,3-dienes. Thus, the addition of acetate anion to the a -unsaturated ketone (204) followed by reaction with dimethylformamide dimethyl acetal affords the ( )-dienes (205) stereospecifically, in high yields... 

Cuvigny T, Fabre JL, Du Penhoat CH, Julia M. Synthesis with sulfones. 31. The stereospecific reduction of 2-benzene-sulfonyl-1,3 dienes to conjugated Z,E-dienes-synthesis of (9Z,ll )-9,ll-tetradecadienyl acetate. Tetrahedron Lett. 1983 24(40) 4319 322. 

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