A face-selectivity

The simplest nitroalkene, nitroethene, undergoes Lewis acid-promoted [4+2] cycloaddition with chiral vinyl ethers to give cyclic nitronates with high diastereoselectivity. The resulting cyclic nitronates react with deficient alkenes to effect a face-selective [3+2] cycloaddition. A remote acetal center controls the stereochemistry of [3+2] cycloaddition. This strategy is applied to synthesis of the pyrrolizidine alkaloids (+)-macronecine and (+)-petasinecine (Scheme 8.33).165... 

The reduction of 2-oxoacids bound to different chiral auxiliaries gave the 2-hydroxyacid derivatives in a 64 to 76% yield and 42 to 86% de depending on solvent, proton donor, supporting electrolyte, temperature, and substituent R in the oxoacid. The results are in accordance with an ECE reduction of the 2-oxoamide to an enolate anion, which subsequently undergoes a face-selective protonation to the hydroxy acid [346, 347]. 

Hence, a reaction of Type I will involve a racemic or achiral/me,t(9 nncleophile which will react enantioselectively with an achiral acyl donor in the presence of a chiral catalyst, while on the other hand, a reaction of Type II will associate an achiral nncleophile and a racemic or udm lmeso acyl donor in the presence of a chiral catalyst. In both cases, when a racemic component is implicated the process constitntes a KR and the maximum theoretical yield of enantiomerically pure product, given perfect enantioselectivity, is 50%. When an achiral/mera component is involved, then the process constitutes either a site-selective asymmetric desymmetrisation (ASD) or, in the case of tt-nucleophiles and reactions involving ketenes, a face-selective addition process, and the maximum theoretical yield of enantiomerically pure product, given perfect enantioselectivity, is 100%. 

There are two (limiting) possibilities that could explain the enantioselectivity a group-selective metal insertion distinguishing the enantiotopic allylic protons, or a face-selective addition that distinguishes the enantiotopic double bond faces. Figure... 

Reduction of (140a) and (140b) separately with H2/PdCl2 in ethanol afforded the epimeric pyr-rolizines (143a) and (143b) (Equation (14)) the stereochemistry at C-5 is presumably due to a face-selective reduction of an intermediate cyclic iminium ion <89JCS(P1)2415>. 

Snpramolecular host-guest systems could be formed in ball-milling conditions by self-assanbly as well. For instance, Hapiot rationalized a face-selective tosylation of cyclodextrins by the formation of supramolecular inclusion complex between CD and the reagent (see chapter Carbon-Oxygen and Other Bond Formation Reactions) [9],... 

This work demonstrates the first case where both a non face-selective and a face-selective addition to the tri-substituted olefinic portion of DMAPP has occurred within the same molecule. A related observation with respect to the loss of stereochemistry in the reverse prenylated indole alkaloids echinulin and roquefortine have been reported and are discussed elsewhere in this article. 

Two highly efficient and very practical alternatives have emerged in recent years (Scheme 1.2). One of these approaches consists of activating the acceptors by reversible conversion into a chiral iminium ion. Thus, the reversible condensation of an a,p-unsaturated carbonyl compound with a chiral secondary amine provides a chiral a,p-unsaturated iminium ion. A face-selective reaction with the nucleophile provides an enamine, which can be either reacted with an electrophile and then hydrolysed, or just hydrolysed to a p-chiral carbonyl compound. The second approach is the enamine pathway. If the nucleophile is... 

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