Racemic heterocyclic product

Alternatively, as shown in Scheme 8, we envisioned that styrenyl allylic ethers, in the presence of an appropriate catalyst, might undergo a net skeletal rearrangement to yield the desired isomeric heterocyclic products [14]. Rearrangement substrates would be synthesized in the non-racemic form by the Zr-catalyzed kinetic resolution [5c]. 

The Chi group recently achieved the first NHC-eatalyzed diastereo- and enantioselective [3 + 4] cycloaddition of azomethine imines and enals via 1,4-dipolarophile intermediates generated by oxidative catalytic remote y-car-bon aetivation of enals. Dinitrogen fused seven-membered heterocyclic products were produced with high enantiomerie purity (up to 81% yield and 99% ee). Racemic azomethine imines ean be used as 1,3-dipolar substrates to afford dinitrogen-fused seven-membered heterocyclic compounds with high enantiomeric purity (s-factor up to 339). The key vinyl enolate intermediate is generated by oxidative y-carbon activation of enal via NHC catalysis as the reactive 1,4-dipolarophile (Scheme 7.116). 

One active field of research involving the Heck reaction is asymmetric Heck reactions (AHR). The objective is to achieve enantiomerically-enriched Heck products from racemic substrates using a catalytic amount of chiral ligands, making the process more practical and economical Although intermolecular Heck reactions that occurred onto carbocyclic arenes are rare, they readily take place onto many heterocycles including thiophenes, furans, thiazoles, oxazoles,... 

As a result of 1,3-induction one expects that the main alkylation product will occur by attack of the electrophile from the opposite side to the R group on C-2, i.e., 8 from 6 and 10 from 7, with 8 and 10 being enantiomers. Hydrolysis of the alkylated heterocycles reestablishes the a-hydroxy acids which have been a-alkylated without racemization. Thus, the preparative value of this concept75-78 is dependent on two factors ... 

A very elegant approach has been developed by Kanerva et al. DKR of N-heterocyclic a-amino esters is achieved using CAL-A [29]. Racemization occurs when acetaldehyde is released in situ from the acyl donor. In this case, aldehyde-catalyzed racemization of the product cannot occur (Scheme 5.15). This is one of the few reported examples of DKR of secondary amines (see also Reference [69]). 

Scheme 61, yielded thiazole 200 as the major product, along with minor amounts of carbinol 201 [152]. On the other hand, treatment of the imine formed from 199 and p-methoxyphenylamine with catalytic tetrabutylammonium cyanide, produced suc-cinimide derivative 202. In both cases, the process is initiated by nucleophilic attack to the carbaldehyde C=0 (or azomethine s C=N) group, which is followed up by an anionic rearrangement. A variation of the above process using as catalysts /V-heterocyclic carbenes (NHC) derived from base treatment of azolium, imidazo-lium, or triazolium salts, has also been developed to access gem-disubstituted succinimides [153, 154]. Unfortunately, an attempt of kinetic resolution of racemic 4-formyl (3-lactams by using chiral NHC resulted in moderate selectivities only [154]. 

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