Diastereoselectivity substitution

Scheme 25 N-a-methoxylation of carbamates and diastereoselective substitution of the methoxy group.

It has been demonstrated that in Bi(OTf)3-catalyzed alkylation reactions the optical activity of enantiopure benzyl alcohols is lost and a racemic product is isolated. This can be explained by a SA-l-type reaction mechanism and the existence of a carbocationic intermediate. However, diastereoselective substitutions of benzyl alcohols with a chiral centre in close neighborhood to the electrophilic carbon should be feasible (Scheme 23). 

Organ MG, Winkle DD, Huffman J (1997) Tandem transformations involving allylic silanes. 2. Highly diastereoselective substitutions involving [(trialkylsilyl) methyl]cyclohexene... 

Reduction of stable 2,3-dihydroisoxazoles 107 afforded diastereoselectively substituted / -amino ketones and /3-amino alcohols, depending on the reaction conditions. Treatment of 107 with NaBH4 gave the /3-amino alcohol 108 with high diastereoselectivity, whereas treatment with zinc powder afforded amino ketones 109 (Scheme 26) <20050L5741>. 

In another application, the totally regioselective direct anodic methoxylation of (45)-pyrimidinone-4-methyl carboxylate obtained from L-2,4-diamino butyric acid followed by diastereoselective substitution of the methoxy group by allyl trimethylsilane can be combined with the follow-up decarboxylative methoxylation of the carboxy function after hydrolysis. A second methoxy group exchange by allyl trimethylsilane yields the C2-sym-metrical (4i ,6i )-4,6-diallyl-pyrimidin-2-one enantiomerically pure [Eq. (50)] [246]. 

An efficient DABCO-mediated regio- and diastereoselective substitution of MBH acetates with -toluenesulfonylmethylisocyanide (TosMIC) has been developed by Krishna et al. via a tandem Sifl -Sifl mechanism. The products 277 were obtained in excellent yields (80-92%) with moderate diastereoselee-tivities (66-80%) in favor of the iyn-isomer (Scheme 3.109). ... 

Alkenyl C—H bonds were also directly functionalized with aryl bromides using this catalytic system, yielding both regio- and diastereoselectively substituted alkenes (Scheme 9.25) [35]. 

Methylation by MeOTf can also be carried out asymmetrically. Treatment of a-lithio benzyl 2-quinolyl sulfide with MeOTf in the presence of a bis(oxazoline) as an external chiral ligand yielded chiral 1-phenylethanethio quinolyl sulfide (eq 17). Lithiation adjacent to nitrogen of chiral 3,4-substituted piperidines and pyrrolidines followed by diastereoselective substitution with MeOTf offered general routes to 2,4,5- and 2,4,5,6-substituted piperidines as well as 2,3,4- and 2,3,4,5-substituted pyrrolidines. ... 

A. Alexakis, J.-F. Normant - Chirals Acetals in Enantio- and Diastereoselective Substitution or Elimination Reactions, Pure Appl. Chem. 60,49,1988. 

Trimethylsilyl azide can be employed as an azide source for a diastereoselective substitution of hydroxy or acetoxy groups in a-position to the diene system in the presence of scandium(lll) triflate or boron trifluoride etherate. A 1,5-nucleo-... 

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