Molecular stereoselectivities

The stereoselectivity of elimination of 5 bromononane on treatment with potassium ethox ide was described in Section 5 14 Draw Newman projections or make molecular models of 5 bromononane showing the conformations that lead to cis 4 nonene and trans 4 nonene respec tively Identify the proton that is lost in each case and suggest a mechanistic explanation for the observed stereoselectivity... 

The stereoselectivity of this reaction depends on how the alkene approaches the catalyst surface As the molecular model m Figure 6 3 shows one of the methyl groups on the bridge carbon lies directly over the double bond and blocks that face from easy access to the catalyst The bottom face of the double bond is more exposed and both hydrogens are transferred from the catalyst surface to that face... 

In other work Rozen added molecular fluorine to a steroidal ene-one dissolved in ethanol at low temperatures to produce a vicinal difluonde in a cleaner, better yield reaction than previously obtainable [55] Although the reaction was not general, the stereoselectivity was very high, and contrary to addition of other halogens, addition was r>ii, characteristic of an electrophilic addition pathway... 

In transforming bis-ketone 45 to keto-epoxide 46, the elevated stereoselectivity was believed to be a consequence of tbe molecular shape — tbe sulfur ylide attacked preferentially from tbe convex face of the strongly puckered molecule of 45. Moreover, the pronounced chemoselectivity was attributed to tbe increased electropbilicity of the furanone versus the pyranone carbonyl, as a result of an inductive effect generated by tbe pair of spiroacetal oxygen substituents at tbe furanone a-position. ... 

A rather unexpected discovery was made in connection to these investigations [49]. When the 1,3-dipolar cycloaddition reaction of la with 19b mediated by catalyst 20 (X=I) was performed in the absence of MS 4 A a remarkable reversal of enantioselectivity was observed as the opposite enantiomer of ench-21 was obtained (Table 6.1, entries 1 and 2). This had not been observed for enantioselective catalytic reactions before and the role of molecular sieves cannot simply be ascribed to the removal of water by the MS, since the application of MS 4 A that were presaturated with water, also induced the reversal of enantioselectivity (Table 6.1, entries 3 and 4). Recently, Desimoni et al. also found that in addition to the presence of MS in the MgX2-Ph-BOX-catalyzed 1,3-dipolar addition shown in Scheme 6.17, the counter-ion for the magnesium catalyst also strongly affect the absolute stereoselectivity of the reac-... 

Tab. 6.1 Dependence of absolute stereoselectivity on molecular sieves and counter ion in the reaction of la with 19b catalyzed by 10 mol% MgX2-Ph-BOX catalysts 20...

Compound 16, the projected precursor of 15, could conceivably be assembled from bishomoallylic alcohol 17 via a pathway that features the oxidative functionalization of the A20,21 double bond with participation by the C-17 secondary hydroxyl. Compound 17 is an attractive retrosynthetic precursor for compound 16 because the A20-21 double bond, which could permit the introduction of the adjacent C-20 and C-21 stereocenters in 16, provides a convenient opportunity for significant molecular simplification. Thus, retrosynthetic cleavage of the A20 21 double bond in 17 furnishes compounds 18 and 19 as potential building blocks. The convergent union of the latter two compounds through a Wittig reaction would be expected to afford 17 stereoselectively. 

The emergence of the powerful Sharpless asymmetric epoxida-tion (SAE) reaction in the 1980s has stimulated major advances in both academic and industrial organic synthesis.14 Through the action of an enantiomerically pure titanium/tartrate complex, a myriad of achiral and chiral allylic alcohols can be epoxidized with exceptional stereoselectivities (see Chapter 19 for a more detailed discussion). Interest in the SAE as a tool for industrial organic synthesis grew substantially after Sharpless et al. discovered that the asymmetric epoxidation process can be conducted with catalytic amounts of the enantiomerically pure titanium/tartrate complex simply by adding molecular sieves to the epoxidation reaction mix-... 

An interesting and stereoselective synthesis of 1,3-diols has been developed which is based on Lewis acid promoted reactions of /f-(2-propenylsilyloxy (aldehydes. Using titanium(IV) chloride intramolecular allyl transfer takes place to give predominantly Ag/r-l,3-diols, whereas anti-1,3-diols, formed via an / / /-molecular process, are obtained using tin(IV) chloride or boron trifluoride diethyl ether complex71. 

Optically active 2-allylpiperidines and -pyrrolidines arc obtained by treating hydroxylactams containing the l-[(S)-l-arylethyl]substituent as an auxiliary (see Appendix) with tin(IV) chloride and trimethyl(2-propenyl)silane46. Interestingly, the moderate diastereoselection when the aryl group is phenyl decreases when 2-chlorophenyl is used, whereas the sense of the stereoselectivity reverses for 2,6-dichlorophenyl or pentachlorophcnyl. These results are rationalized by application of molecular orbital theory and substrate conformational preferences46. 

The epoxyketone 3 is a versatile precursor for a variety of tricycloundecane systems having a p,y-unsaturated carbonyl chromophore, which are not so readily accessible. The contiguous epoxy ketone functionality and the double bond present in the five-membered ring provide opportunities for further manipulation. Adduct 3 may be transformed into a variety of molecular frameworks such as linearly fused cis anti cis tricyclopentanoids, protoilludanes, and marasmanes in a stereoselective fashion after suitable chemical and photochemical manipulation.13... 

Cyclodextrins can solubilize hydrophobic molecules in aqueous media through complex formation (5-8). A nonpolar species prefers the protective environment of the CDx cavity to the hulk aqueous solvent. In addition, cyclodextrins create a degree of structural rigidity and molecular organization for the included species. As a result of these characteristics, these macrocycles are used in studies of fluorescence and phosphorescence enhancement (9-11), stereoselective catalysis (.12,13), and reverse-phase chromatographic separations of structurally similar molecules (14,15). These same complexing abilities make cyclodextrins useful in solvent extraction. 

Singlet molecular oxygen ( A is an electron acceptor powerful enough to react with olefins in the pseudoexcitation band. The [2h-2] cycloaddition and ene reactions and the stereoselectivities are reviewed in this subsection. 

Sp and sp-carbon atoms such as a carbonyl group and an alkene are reactive cent-ersl in many kinds of reactions. The n orbitals of a simple molecule such as ethylene or formaldehyde are symmetric in magnimde and antisymmetric in sign with respect to reflection in the molecular plane. Various attempts have been made to rationalize stereoselectivities, and several general ideas have emerged, including a steric basis and an orbital basis. 

R. Noyori and T. Ohkuma, Asymmetric Catalysis by Architectural and Functional Molecular Engineering Practical Chemo- and Stereoselective Hydrogenation of Ketones , Angew. Chem. Int. Ed. Engl, 2001, 40, 40. 

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