Stereoselectivity lateral

Focken, T. Hopf, H. Snieckus, V. Dix, I. Jones, P. G. Stereoselective lateral functionalization of monosubstituted [2.2]paracyclo-phanes by directed ortho metalation-homolo-gous anionic Fries rearrangement. Eur. J. Org. Chem. 2001, 2221-2228. 

The substrate scope of the reaction is shown in Scheme 10.2. Aliphatic and aromatic enals, branched enolizable aldehydes, and different substituted unsaturated 2-indolones could be used to afford the corresponding spirooxindoles in excellent yields and high stereoselectivity. Later, Chen and coworkers expanded the substrate scope of this reaction using maleimides and imines instead of enals to afford the spiro products in excellent yields and stereoselectivities [11]. 

Alkylation of aldol type educts, e.g., /3-hydroxy esters, using LDA and alkyl halides leads stereoselectively to erythro substitution. The erythro threo ratio of the products is of the order of 95 5. Allylic and benzylic bromides can also be used. The allyl groups can later be ozonolysed to gjve aldehydes, and many interesting oligofunctional products with two adjacent chiral centres become available from chiral aldol type educts (G. Prater, 1984 D. Seebach, 1984 see also M. Nakatsuka, 1990, p. 5586). 

About ten years later in J. Am. Chem. Soc., Vol. 113, No. 23 (1991) and Vol. 114, No, 21 (1992) we still find simple target molecules. Stereoselective reactions, mostly alkylations, prevail now. How can the problems of regio- and stereoselectivities be solved Read the papers and learn ... 

Cyclopentene derivatives with carboxylic acid side-chains can be stereoselectively hydroxy-lated by the iodolactonization procedure (E.J. Corey, 1969, 1970). To the trisubstituted cyclopentene described on p. 210 a large iodine cation is added stereoselectively to the less hindered -side of the 9,10 double bond. Lactone formation occurs on the intermediate iod-onium ion specifically at C-9ot. Later the iodine is reductively removed with tri-n-butyltin hydride. The cyclopentane ring now bears all oxygen and carbon substituents in the right stereochemistry, and the carbon chains can be built starting from the C-8 and C-12 substit""" ... 

Other Borohydrides. Potassium borohydride was formerly used in color reversal development of photographic film and was preferred over sodium borohydride because of its much lower hygroscopicity. Because other borohydrides are made from sodium borohydride, they are correspondingly more expensive. Generally their reducing properties are not sufficiently different to warrant the added cost. Zinc borohydride [17611-70-0] Zn(BH 2> however, has found many appHcations in stereoselective reductions. It is less basic than NaBH, but is not commercially available owing to poor thermal stabihty. It is usually prepared on site in an ether solvent. Zinc borohydride was initially appHed to stereoselective ketone reductions, especially in prostaglandin syntheses (36), and later to aldehydes, acid haHdes, and esters (37). 

Some stereospecific reactions are listed in Scheme 2.9. Examples of stereoselective reactions are presented in Scheme 2.10. As can be seen in Scheme 2.9, the starting materials in these stereospecific processes are stereoisomeric pairs, and the products are stereoisomeric with respect to each other. Each reaction proceeds to give a single stereoisomer without contamination by the alternative stereoisomer. The stereochemical relationships between reactants and products are determined by the reaction mechanism. Detailed discussion of the mechanisms of these reactions will be deferred until later chapters, but some comments can be made here to illustrate the concept of stereospecificity. 

In most cases, the addition of prostereogenic benzyl anion reagents with prostereogenic aldehydes (or ketones) proceeds with low stereoselectivity. Some notable exceptions are reviewed later. 

A report of an unusually high stereoselectivity (93% ee) in the addition of menthyl acetate to acetophenone was later corrected by Solladie et al. The real ee is only 58% see S. Mitsui, K. Konno, 1. Onuma, K. Shimizu, Nippon Kagaku Zasshi 85, 437 (1964) Chem. Ahstr. 61. 13167 f. (1964). 

Immobilized PLE was applied to promote stereoselective acetylation of prochiral bis(hydroxymethyl)methyl-phenylgermane 106 (R = Me) with vinyl acetate as a solvent and acyl donor. Later on, the same group reported that each enantiomer of hydridogermane monoacetates 107 (R = H) was obtained either via acetylation of the bis-hydroxy derivative 106 (R = H) or hydrolysis of the corresponding diacetate 108 (R = H). In both methods, porcine pancreatic lipase was used and, obviously, each reaction led to a different enantiomer of 107 (Equation 51). ... 

Pyrolysis of the parent thiirane oxide 16a monitored by microwave spectroscopy led to the conclusion that the sulfur monoxide is generated in its triplet ground state, although the singlet state ( A) cannot be excluded completely (equation 8). A later study presented evidence, based on the stereoselective addition to dienes of sulfur monoxide generated from thiirane oxide as well as thermochemical data, that the ground state S is formed exclusively ° . ... 

This quantity represents the energy of the multiple-site charge-transfer interaction which will later play an important role in the theory of stereoselection. It is to be remarked that, although any MO may involve an arbitrary constant of which the absolute value is unity, the value of the numerator in each term of the right side of this equation is always definite. 

Corradini et al. examined in some detail by molecular mechanics15 and density functional studies100 the polymerization mechanism proposed by Cossee and the catalytic sites on TiC surfaces, including those proposed by Arl-man and Cossee13 and by Allegra.14 According to the calculations, for all these octahedral active sites a similar general mechanism of stereoselectivity occurs which is very similar to the one established several years later for stereospecific metallocenes (see previous section). The chirality of the site would determine a chiral orientation of the first C-C bond of the chain (for a A site,... 

---