Stereospecificity of the addition

Recent synthetic applications of the photochemical [2 + 2] cycloaddition of unsaturated sulfones have been noted. Musser and Fuchs84 have effected an intramolecular [2 + 2] addition of a 6-membered ring vinyl sulfone and a five-membered ring vinylogous ester in excellent yield, as part of a synthetic approach to the synthesis of the mould metabolite, cytochalasin C. The stereospecificity of the addition was only moderate, however, and later problems with this synthetic approach led to its abandonment. Williams and coworkers85 have used the facile [2 + 2] photoaddition of 73 and... 

Figure 10-9 Representation of the course of enzyme-induced hydration of fumaric acid (trans-butenedioic acid) to give L-malic acid (L-2-hydroxy-butanedioic acid). If the enzyme complexes with either—C02H (carboxyl) group of fumaric acid, and then adds OH from its right hand and H from its left, the proper stereoisomer (l) is produced by antarafacial addition to the double bond. At least three particular points of contact must occur between enzyme and substrate to provide the observed stereospecificity of the addition. Thus, if the enzyme functions equally well with the alkenic hydrogen or the carboxyl toward its mouth (as shown in the drawing) the reaction still will give antarafacial addition, but o,L-malic acid will be the product.

The addition of azetidine (15) to DMAD proceeded exothermically to give dimethyl (l-azetidinyl)maleate (17) as a crystalline solid88 with MP, compound 16 was obtained, and the NMR spectrum of this compound was informative of the stereochemical course of the reaction, which is solvent-dependent. In aprotic solvents, only cis addition occurred, giving the trans-acrylate, whereas in methanol a mixture of cis and trans products in the ratio 2 1 was obtained. The effect of solvents on the stereospecificity of the addition reaction agrees with that usually observed for secondary amines87 but contrasts with aziridine where the trans addition competes to some degree with the cis addition even in aprotic solvents,39,88... 

The reactions of 9, 10 and 11 can be carried out under neutral conditions. The decomposition of 13 under phase transfer catalyzed (PTC) conditions usually gives the adducts in yields better than those obtained in the alkoxide-induced decomposition of 12. The reactions of 2,2-disubstituted vinylidenes are rather sensitive to steric hindrance. The stereospecificity of the addition has been demonstrated in the reaction of 12. Intramolecular reactions giving particularly strained methylenecyclopropanes have also been reported " . 

As the two components do not react in the dark, a triazoline intermediate is not probable. Other A -carbethoxyaziridines were formed via carbethoxy nitrene when ethyl azidoformate was irradiated in dihydropyran and in enolacetates . The stereospecificity of the addition is high. The photolysis of cis and trans 2-butene at — 20° yielded mainly the cis (130) and trans (131) aziridines respectively This stereospecificity has been found to diminish upon dilution... 

Yamamoto has also examined the reactions of substituted E- and Z-enol stan-nanes derived from cycloalkanones and acyclic ferf-butylethyl and -propyl ketones (Eq.49). The addition reactions of cyclic enolates afforded the adducts in excellent yields (92-96%) and 89/11 to 93/7 anti/syn diastereomeric ratio with the enantiopurity of the major diastereomer 234 uniformly high (92-95% ee). The use of acyclic Z-enol stannanes delivered the complementary iyn adducts in superb diastereoselectivities (<1/99 anti/syn) and enantioselectivity (91-95% ee). The high degree of stereospecificity of the addition process with respect to the... 

It is interesting to note that trans isomers, whereas, a ratio of 1 9 is obtained from c s,sulfur dichloride may be attributable to a concerted 1,4-addition followed by chlorine migration to the double bond. The high... 

Purpose. The bromination of frans-cinnamic acid is carried out to obtain en/flzro-2,3-dibromo-3-phenylpropanoic acid, the direct precursor to 2 -bromo-styrene, which is the synthetic target of Sequence D. You wiU review the stereospecificity of the addition of molecular bromine to an alkene. 

Ketones can present a problem in specificity. Under basic conditions, they may react with two or more molecules of the electrophile to give a mixture of products. Furthermore, unsymmetric ketones may present a choice of two enolate sites so that control is necessary to direct to the desired one. Many alternatives have been developed for this problem. One solution is to incorporate a temporary group on one enolate site to render that site more acidic so that the electrophile will react there. The familiar p-ketoester reactions (acetoacetic ester synthesis) are widely used. For another alternative, the ketone is first converted to an imine (Section 6.2.3) or a dimethyl hydrazone, and the enolate of that derivative is used with electrophiles [28]. Stereospecificity of the addition is obtained by forming a derivative with (5)-l-amino-2-methoxymethyl-pyrrolidine (SAMP) as shown in Equation 7.15 [29]. Without derivati-zation, alkylation of unsymmetric ketones will occur mostly at the more substituted enolate site under reversible deprotonating conditions. Using a base such as EDA will give alkylation primarily at the least substituted enolate. 

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