Allyl chiral

Enamines derived from ketones are allylated[79]. The intramolecular asymmetric allylation (chirality transfer) of cyclohexanone via its 5-proline ally ester enamine 120 proceeds to give o-allylcyclohexanone (121) with 98% ee[80,8l]. Low ee was observed in intermolecular allylation. Similarly, the asymmetric allylation of imines and hydrazones of aldehydes and ketones has been carried out[82]. 

An allylic chiral center between the nitrile oxide and alkene groups can also affect the stereochemistry of INOC reactions. For example, the production of only the cycloadduct (191) in the reaction of the (Z)-nitroalkene (190) (Scheme 90), compared to the formation of a 3 1 mixture of the isoxazolines (193) and (194) from the ( )-isomer (192) (Scheme 91) (84ACR410) is a dramatic example of the influence of allylic 1,3-strain (89CRV1841) on these processes. 

A similar effect is observed in the osmylation of allylamides bearing a bis-homoallylically located sulfoxide group94. In this case the asymmetric 1,5-induction of the stereogenic sulfur atom totally overwhelms the weak bias of the allylic chirality. The concomitant sulfoxide-to-sulfone transformation suggests sulfoxide involvement in the oxidation mechanism. In this example, as well as in the previous one, replacement of the sulfur-based directing group by a sulfone moiety leads to a drop in diastereoface differentiation. 

Nishibayashi and Uemura attempted the diastereoselective [2,3]sigmatropic rearrangement via a chiral selenium ylide bearing an optically active ferrocenyl-selenium moiety, but the selectivity was low (up to 25% de) [54]. The treatment of an allylic chiral ferrocenyl selenide with ethyl diazoacetate in the presence of a catalytic amount of Rh2(OAc)4 afforded a diastereomeric mixture (around 2 1) of ethyl 2-ferrocenylseleno-3-phenylpent-4-enoates (III) with low diastereo-selectivity (Scheme 35). 

Up to three chiral centers can directly be involved in the Claisen reairangonent (Equation 27). The configuration of the allylic chiral center at C-4 in (269) specifically relates to the configuration of the newly formed chiral centers in (271) and (273). Moreover, the ratio of diastereomeric transition states (270) and (272) greatly depends on die relative steric bulk of substituents R and Thus, generally a highly selective course of the reaction via pathway A or pathway B can be predicted. It is important to note that the primary chiral center is destroyed in die rearrangement ( self-immolative process ) and... 

Exo radical cyclizations of bromoacetals that contain allylic chiral centers are often referred to as Ueno-Stork reactions [50]. It was recently discovered that stereochemistry in these cyclizations could also be controlled by the acetal center, as shown in Eq. (13.39) [51]. A chair-like transition state (125) is envisaged where the alkene occupies a pseudoequatorial position leading to predominantly cis product 126. Yields of approximately 70% and cis trans ratios of up to 98 2 are obtained. 

Emerging at a fast rate, but not examined in this book, are the syntheses of allylic chiral amines [2] and propargylic chiral amines [3]. In addition, the very recent development of efficient catalytic hydroaminoalkylation, albeit racemic, holds great promise [4]. 

Allylation. Chiral products are obtained from o -acetamido- 3-ketoesters. ... 

The terminal substituent on the allylic chiral acetal (2) also exerts some conu-ol on the Lewis acid catalysed cyclisation to the tetrahytopyran (94CC1953). 

In addition, theoretical frameworks for Diels-Alder reactions of dienes, such as cm, had been adumbrated by Houk [30] and Hehre [31]. Therefore, it was an opportune time for experimental clarification of 7c-facial selectivity in the Diels-Alder reaction of homochiral dienes containing an allylic chiral center. 

Chiral /3-amino sulfoxides served as chiral ligands in Pd-catalyzed allylations. Chiral j8-aminoethyl sulfoxides 31a-f-palladium complexes catalyzed asymmetric ally-lations of 33 to give (5)-34 with 29% ee. o-Aminophenyl sulfoxides 32a,b provided much higher enantioselectivity (50%), presumably due to the sterically fixed structure of the intermediary palladium complex 35 (Scheme 17). 

Choice of the Model Substrates. In this context, we decided to use of 3-substituted C-2 vinyl glycals (Figure 15) as model compounds for 2-substituted dienes bearing a chiral centre in the 2-substituent (C). Prior studies have addressed 1-substituted dienes carrying an allylic chiral centre on the 1-substituent (A in Figure 15). 

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