Syn-directing effect

In 1963, Dauben and Berezin published the first systematic study of this syn directing effect (Scheme 3.15) [37]. They found that the cyclopropanation of 2-cyclohexen-l-ol 32 proceed in 63% yield to give the syn isomer 33 as the sole product. They observed the same high syn diastereoselectivity in a variety of cyclic allylic alcohols and methyl ethers. On the basis of these results, they reasonably conclude that there must be some type of coordinative interaction between the zinc carbenoid and the substrate. 

Substituted indenes provide other examples of substituent directive effects. Over Pd-alumina, the indenols 6a-c show both cis stereoselectivity and a syn directive effect. The directive effect is reinforced by steric effects as the alkyl group becomes larger.7... 

In the case of a sterically hindered allylic methoxycarbonyl group, the epoxida-tion of substituted cyclohexenes-1,4-dienes occurs on the opposite side. Further studies on the epoxidation of j3,7-unsaturated cyclohexenecarboxylic acids and esters indicate that the steric and polar effects of the COaMe group result mainly in anh -epoxidation while the carboxyl group in inert solvent exerts a syn directing effect. ... 

With the Ir-catalyst noted in reactions (31 to 33), Scheme 12 [31] the hydrogenation of cyclohexenols is totally syn-controlled. In acyclic systems such as reaction (34) [31] and reactions (35 and 36) in Scheme 13 [32] a Rh-catalyst is employed which has a syn directing effect in reaction (34) whereas reactions (35... 

Using Simmons-Smith type reagents, hydroxy and alkoxy groups in the allylic or homoallylic position exhibit a very strong syn-directing effect on methylene transfer, which usually overrides the steric influence of further substituents in an alkene2,5 (see Table 2 for the transformation of simple cyclic alkenes)29 32. 

It was suggested that epoxidation of 64 occurs via conformer 66 having the sulfur-oxygen bond quasi-axial. As discussed above, this conformer is apparently more stable than 65 which has a quasi-equatorial sulfur-oxygen bond. A syn directing effect of the oxygen in 66 would lead to the... 

A related smdy of 3,4-disubstituted oxymethylcyclobutenes showed moderate syn-directive effects on MCPBA epoxidation. In this case, the effect was attributed to interaction of the relatively electron-rich peroxide oxygens with the positively charged methylene hydrogens, but the electrostatic effect of the bond dipoles would be in the same direction. 

Winstein showed that carboxamides display a strong syn directing effect, as demonstrated in the epoxidation of amide 21, furnishing cis-epoxide 22 as the only observed diastereomer (Equation 8) [63]. The directing effect of carboxamide substituents was studied by Sable, who found that triol 24 is converted diastereoselectively into epoxide 25 (syn to the amide group), despite the presence of one allylic and two homoallylic hydroxyl substituents on the opposite face (Equation 9) [64],... 

Hydroxyl groups favor syn stereoselectivity.127 Amino groups have a similar directing effect.128 This is similar to the substituent effects observed for peroxy acids and suggests that the substituents may stabilize the transition state by acting as electron donors. 

The axial-directing effect of the C-l substituent depends on the total stereochemistry and mode of substitution and falls roughly in the order of polarity of the substituents. The axial-directing effects of the C-l substituent is augmented when acetates at the other ring positions are replaced by benzoates possibly there are attractive syn-diaxial interactions between benzoate groups. 

Bromination of /-oxygenated dehydroamino acids 133 gives predominantly the syn product 134. This outcome has been attributed to an unspecified yyw-directing effect of oxygen (135)217. However, the reviewer is of the opinion that the nitrogen lone pair... 

In the HMO if> of styrene, the coefficients at the a and / carbon atoms are ca. 0-4 and 0-6 respectively. In the HMO ifi7 of acenaphthylene, the coefficients at carbon atoms 1 and 2 are ca. — 032 and 0-32. Therefore, the directive effect for addition is syn in styrene and anti in acenaphthylene Second, MO calculations may be helpful in new or different systems. With respect to eliminations, Fukui and Fujimoto (1965) used frontier electron theory to provide reactivity indices for two model... 

Syn hydroxyladon is most commonly performed using osmium tetroxide or potassium permanganate, and addition usually occurs hx)mrhe less hindered side of the double bond in the absence of other directing effects. With Aese reagents, the steieospecificity of syn addition results from formation of cyclic ester intermediates. Support for these arises from isolation of solid addition complexes (1) on reaction of... 

In contrast to the anti addition observed with acyclic and monocyclic alkenes, bicyclic alkenes frequently show syn addition. Norbornene gives exclusively exo-syn addition. Even 7,7-dimethylnorbornene gives the exo-syn product, in sharp contrast to the usual ent/o-directing effects of 7-substitution. These results are difficult to reconcile with a bridged mercurinium ion and suggest that intramolecular transfer of the nucleophile from mercury occurs. In norbornene derivatives, the competing anti addition is sterically restricted by the endo bridge. 

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