Stereoselectivity aliphatic nucleophilic

Chiral hydrazines, supported on Merri-field resin, were reacted with various aldehydes, affording the corresponding hydrazones. These compounds allowed stereoselective preparation of a-branched amines, through 1,2-addition of both aromatic and aliphatic nucleophiles to the C=N double bond of the hydrazones. Reductive cleavage released the desired amine from the resin. Moderate to good enantiomeric excesses (50-86%) were achieved. 

With Sulfur Nucleophiles N-Carboxy-protected aziridine-2-carboxylates react with thiols to give P-mercapto-ot-amino acid derivatives. The reaction is usually catalyzed by BF3 and the yields range from fair to excellent [15, 16, 108-111]. With N-unprotected 3-substituted aziridine-2-carboxylates, the ring-opening with thiols usually takes place with anti stereoselectivity, especially in the case of the C-3 aliphatic substituted substrates. In cases in which C-3 is aromatic, however, the stereoselectivity has been found to be a function of the substitution pattern on the aromatic ring 3-p-methoxy ph eri yl-su bs li In led aziridines 143a (Scheme 3.51) and... 

Moreover it has been shown that PV0CC1 prepared by free-radical polymerization of vinyl chloroformate (V0CC1) is an atactic polymer having a Bernouillian statistical distribution as expected (J[9). In order to extend our studies on the chemical modification of PV0CC1, the stereoselective character of the nucleophilic substitution of the chloroformate units with phenol has been examined by the study of the 13c-NMR spectra of partly modified polymers in the region of the aliphatic methine carbon atoms. The results obtained in this field are presented here. 

In recent years, the importance of aliphatic nitro compounds has greatly increased, due to the discovery of new selective transformations. These topics are discussed in the following chapters Stereoselective Henry reaction (chapter 3.3), Asymmetric Micheal additions (chapter 4.4), use of nitroalkenes as heterodienes in tandem [4+2]/[3+2] cycloadditions (chapter 8) and radical denitration (chapter 7.2). These reactions discovered in recent years constitute important tools in organic synthesis. They are discussed in more detail than the conventional reactions such as the Nef reaction, reduction to amines, synthesis of nitro sugars, alkylation and acylation (chapter 5). Concerning aromatic nitro chemistry, the preparation of substituted aromatic compounds via the SNAr reaction and nucleophilic aromatic substitution of hydrogen (VNS) are discussed (chapter 9). Preparation of heterocycles such as indoles, are covered (chapter 10). 

The excellent ability of late transition metal complexes to activate alkynes to nucleophilic attack has made them effective catalysts in hydroamination reactions. The gold(l)-catalyzed cyclizations of trichloroacetimidates 438, derived from homopropargyl alcohols, furnished 2-(trichloromethyl)-5,6-dihydro-4f/-l,3-oxazines 439 under exceptionally mild conditions (Equation 48). This method was successfully applied to compounds possessing aliphatic and aromatic groups R. With R = Ph, cyclization resulted in formation of 439 with complete (Z)-stereoselectivity <2006OL3537>. 

It was found that the ring opening of phenyl-substituted epoxides, as well as of aliphatic epoxides, can be achieved in high yield in a regio- and stereoselective manner with a mixture of 1 3 of diisopropylamine and hydrogen fluoride (diisopropylamine trishydrofluoride).9 1 m-2,3-Diphenyloxirane. for example, gives pure. >w-2-fluoro-l, 2-diphenylcthanol (1). indicating a nucleophilic reaction mechanism of the SN2 type. 

Nucleophilic addition of tiiethyl phosphite to 3-acetoxy-2-methylenealkanenitriles and methyl 3-acetoxy-2-methylenealkanoates provides (2 )-2-(diethoxyphos-phoryhnethyl)alk-2-enenitriles and methyl (2Z)-2-(diethoxyphosphorylmethyl)alk-2-enoates, respectively, with good stereoselectivity (Scheme 3.155). Notably, nitriles provide allylphosphonates with high stereoselectivity ( /Z = >90/< 10) particularly for aliphatic substituents (R = alkyl), thus complementing the work of Janecki and Bodalski.i i ... 

In organocatalytic cross-aldol reactions of two different aldehydes through the enamine intermediate first reported by MacMillan and Northrop, the a d-cross-aldol adduct could be obtained in a highly stereoselective fashion. However, most such reactions required the use of sterically hindered aliphatic aldehydes, from which the enamine intermediates are rather difficult to form, or aromatic aldehydes as electrophile. In the direct aldol reaction between simple aliphatic aldehydes (enolisable aldehydes), both aldehydes can perform the double role of nucleophile and electrophile, and consequently, two cross-aldol adducts and two homo-aldol adducts would be possible products with each having four stereoisomers. To differentiate two... 

In 2008, Procter and coworkers described the synthesis of this spirocyclic system using a connective Pummerer activation followed by intramolecular nucleophilic addition f Scheme 20.311. Different aliphatic and aromatic thiols were used, and the reaction showed some stereoselectivity, explained by minimization of the interaction between the R thiol group and... 

For the reaction between two enolizable aldehydes in which one bears an a-chloro-substituent, a more nucleophilic catalyst 120 was developed. This catalyst is effective even with aliphatic aldehydes, and affords the desired iy -aldols with excellent chemo- and stereoselectivity (Chart 3.17) [144]. 

A more significant growth on enantioselective hetero Diels-Alder reactions has been pioneered by the Jacobsen group [44-46], The HD A reactions of less nucleophilic dienes than Danishefsky s diene were successfully realized by employing Jacobsen s chiral tridentate Schifif base chromium(lll) complex 1.42 [46-48], This powerfiil method has been utilized on multiple occasions in total synthesis. For example, Patterson and coworkers applied this concept to a synthesis of leucascandrolide A [49], Aliphatic aldehyde 1.40 coupled with diene 1.41 using Jacobsen s chromium catalyst furnished the 2,6-cw-tetrahydropyranone 1.43 with high stereoselectivity (Scheme 1,7),... 

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