Stereoselectivity electrophilic

Pale et al. have reported that the stereoselective electrophile trapping of alkynyl-stabilized lithiated epoxides 189, generated from the parent epoxide 188 and n-BuLi, gives substituted epoxides such as 190, in good yield and de (Scheme 5.44) [67]. 

Stereoselective Electrophilic Amination with Sulfonyloxycarbamates and Azodicarboxylates... 

In contrast to stereoselective C-C bond-forming reactions, the related electrophilic animation of chiral carbon nucleophiles is a recent conceptual advance in the repertoire of synthetic methodology. The problem is the correct choice of the nucleophile, and of the most efficient nitrogen electrophile. The chemistry of useful reagents such as sulfonyl azides 3 and chloro nitroso alkanes 5 was excellently reviewed in 1995 [Id]. This chapter outlines a collection of the major reports that have appeared in the field of stereoselective electrophilic animation using sulfonyloxy-carbamates 6 and azodicarboxylates 4. 

The chemistry of nitrogen electrophiles R R2NX (I) was excellently summarized in 1989Ia. Recent studies in the field of stereoselective electrophilic amination, however, have shown that azidation with sulfonyl azides III, reactions with di-tert-butyl azodicarboxylate (II) and reactions with 1-chloro-l-nitroso reagents IV, are superior to the previously mentioned nitrogen electrophiles. 

In the latter three cases, the initially formed adducts are easily transformed into the desired amines RNH2 with very high diastereoselectivity and/or enantioselectivity3. It is thus the main purpose of this section to summarize these stereoselective electrophilic aminations and to mention reactions performed with electrophiles of the type R R2NX, I. 

The stereoselective electrophilic azidation of 2-deoxyaldono-l,5-lactones as a route to 2-azido-2-deoxyaldopyranoses was also reported recently23. Thus, reaction of 25a as indicated in the following scheme leads to 26 a which, on further reaction, gives 27 a in 80% yield stereospecifically. Similarly, if the reaction is started with the stereoisomer 25b, 27b is formed exclusively in 65% yield. 

Azo-ene reactions. The ene reaction provides a powerful method for C-C bond formation with concomitant activation of an allylic C-H bond. A variety of functionalized carbon skeletons can be constructed due to the range of enophiles which can be used. For example, carbonyl compounds give homoallylic alcohols and imino derivatives of aldehydes afford homoallylic amines. The azo-ene reaction offers a method for effecting allylic amination by treatment of an alkene with an azo-diester to afford a diacyl hydrazine which upon N-N cleavage furnishes a carbamate. Subsequent hydrolysis of the carbamate provides an allylic amine. Use of chiral diazenedicarboxylates provides a method for effecting stereoselective electrophilic amination. 

Theoretical studies have led to the choice of (IS, 2S)-l-phenyl-2-benzyl-2-(2-propylamino)ethanol as the chiral auxilary in the 1,3,2-oxazaphospholane 295 for use in stereoselective electrophilic amination. The experimental results confirm the theoretical studies, de values up to 83% being obtained (Scheme 33). 

The Oppolzer sultam 35-1 (Scheme 35, reaction (101) [84] reacts with even higher stereoselectivies and is easier to remove. The main domains of the Oppolzer sultam are conjugate 1,4-additions or simple double bond additions [Scheme 35, reactions (102) and (103)] [85], which show diastereoselectivities of >95% in most cases. Scheme 36 presents examples of persistent, restorable and selfimmolative auxiliaries which are all based on amino acids or amino alcohols, finders RAMP-SAMP [86] is attached to ketones or aldehydes in form of a hydrazone 36-1 which is used for highly stereoselective electrophilic a-alkyla-tions. After the reaction the auxihary is removed via ozonolysis which generates the nitrosamine 36-2 first. In an ensuing step this is reduced to the original auxiliary. In Schollkopf s bislactim ether alkylations [Scheme 36, reaction (105)]... 

Regio- and stereoselective electrophilic fluorination at C19 of vitamin D-SO2 adducts was achieved with NFSi and bulky bases. Benzylic a,a-difluorosulfonates have been prepared by stepwise reaction of the corresponding benzylic sulfonate with r-butyl lithium and NFSi (eq 19). ... 

Trisyl azide was also successfully employed in the stereoselective electrophilic azidation of sugars (eqs 35 and 36). The corresponding 2-azido epimer was not detected in either example. These 2-azido sugars were subsequently transformed into 2-amino sugars. 

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