Allyl relative stereoselection

In acyclic systems the 1,4-relative stereoselectivity was controlled by the stereochemistry of the diene. Thus, oxidation of (E,E)- and (E,Z)-2,4-hexadienes to their corresponding diacetates affords dl (>88% dl) and mesa (>95% me so) 2,5-diacetoxy-3-hexene, respectively. A mechanism involving a t vans-accto xy pal I adation of the conjugated diene to give an intermediate (rr-allyljpalladium complex, followed by either a cis or trans attack by acetate on the allyl group, has been suggested. The cis attack is explained by a cis migration from a (cr-allyl)palladium intermediate. The diacetoxylation reaction was applied to the preparation of a key intermediate for the synthesis of d/-shikimic acid, 3,... 

HeteFoatom-substituted allylic stannanes Relative stereoselection... 

An asymmetric synthesis of estrone begins with an asymmetric Michael addition of lithium enolate (178) to the scalemic sulfoxide (179). Direct treatment of the cmde Michael adduct with y /i7-chloroperbenzoic acid to oxidize the sulfoxide to a sulfone, followed by reductive removal of the bromine affords (180, X = a and PH R = H) in over 90% yield. Similarly to the conversion of (175) to (176), base-catalyzed epimerization of (180) produces an 85% isolated yield of (181, X = /5H R = H). C8 and C14 of (181) have the same relative and absolute stereochemistry as that of the naturally occurring steroids. Methylation of (181) provides (182). A (CH2)2CuLi-induced reductive cleavage of sulfone (182) followed by stereoselective alkylation of the resultant enolate with an allyl bromide yields (183). Ozonolysis of (183) produces (184) (wherein the aldehydric oxygen is by isopropyUdene) in 68% yield. Compound (184) is the optically active form of Ziegler s intermediate (176), and is converted to (+)-estrone in 6.3% overall yield and >95% enantiomeric excess (200). 

The isomerization of an allylic amine to an enamine by means of a formal 1,3-hydrogen shift constitutes a relatively small structural change. However, this transformation could be extremely valuable if it could be rendered stereoselective. In important early studies, Otsuka and Tani showed that a chiral cobalt catalyst, prepared in situ from a Co(ii) salt, a chiral phosphine, and diisobutylaluminum hydride (Dibal-H), can bring about the conversion of certain pro-chiral olefins to chiral, isomeric olefins by double bond migra-... 

It is well known that aziridination with allylic ylides is difficult, due to the low reactivity of imines - relative to carbonyl compounds - towards ylide attack, although imines do react with highly reactive sulfur ylides such as Me2S+-CH2-. Dai and coworkers found aziridination with allylic ylides to be possible when the activated imines 22 were treated with allylic sulfonium salts 23 under phase-transfer conditions (Scheme 2.8) [15]. Although the stereoselectivities of the reaction were low, this was the first example of efficient preparation of vinylaziridines by an ylide route. Similar results were obtained with use of arsonium or telluronium salts [16]. The stereoselectivity of aziridination was improved by use of imines activated by a phosphinoyl group [17]. The same group also reported a catalytic sulfonium ylide-mediated aziridination to produce (2-phenylvinyl)aziridines, by treatment of arylsulfonylimines with cinnamyl bromide in the presence of solid K2C03 and catalytic dimethyl sulfide in MeCN [18]. Recently, the synthesis of 3-alkyl-2-vinyl-aziridines by extension of Dai s work was reported [19]. 

The catalytic enantioselective addition of vinylmetals to activated alkenes is a potentially versatile but undeveloped class of transformations. Compared to processes with arylmetals and, particularly alkylmetals, processes with the corresponding vinylic reagents are of higher synthetic utility but remain scarce, and the relatively few reported examples are Rh-catalysed conjugate additions. In this context, Hoveyda et al. reported very recently an efficient method for catalytic asymmetric allylic alkylations with vinylaluminum reagents that were prepared and used in Thus, stereoselective reactions... 

Two catalytic cycles are proposed to explain the difference in selectivity. In both cases, catalytic cycle is initiated by the oxidative addition of an alkynylstannane to nickel(O) species, leading to the formation of alkynylnickel(ll) complex 77 (Scheme 24).92 Then, an allene is inserted into the nickel(ll) complex in a manner which avoids steric repulsion with the butyl group to afford the anti-ir-a y complex 80. The carbometallation of the terminal alkyne can take place at the non-substituted allylic carbon of the corresponding syn-Ti-a y complex 78. The stereoselectivity is determined by the relative rate of the two possible insertion modes which depend on the ligand used. A bidentate... 

As the t-butyl group can readily be removed upon acidic or basic hydrolysis, this method can also be used for //-hydroxyl acid synthesis. In analogy with allylation reactions, the enolate added preferentially to the Re-face of the aldehydes in aldol reactions. Titanium enolate 66 tolerates elevated temperatures, while the enantioselectivity of the reaction is almost temperature independent. The reaction can be carried out even at room temperature without significant loss of stereoselectivity. We can thus conclude that this reaction has the following notable advantages High enantiomeric excess can be obtained (ee > 90%) the reaction can be carried out at relatively high temperature the chiral auxiliary is readily available and the chiral auxiliary can easily be recovered.44... 

The asymmetric induction cannot be explained simply by steric interaction because the R group in the aldehyde is far too remote to interact with the tartrate ester. In addition, the alkyl group present in the tartrate ligand seems to have a relatively minor effect on the overall stereoselectivity. It has thus been proposed that stereoelectronic interaction may play an important role. A more likely explanation is that transition state A is favored over transition state B, in which an n n electronic repulsion involving the aldehyde oxygen atom and the /Mace ester group causes destabilization (Fig. 3-6). This description can help explain the stereo-outcome of this type of allylation reaction. 

In order to rationalize the factors determining the enantioselectivity of the hydrosilylation of the para-substituted styrenes, we have calculated the relative thermodynamic stabilities of all the intermediates of the catalytic cycle that are precursors of the two enantiomeric products as a function of the para-substituted substrates. Since, the 5 configuration product was formed in 64% ee from styrene, whereas 4-(dimethylamino)styrene afforded the R product with 64% ee [6], we have performed all calculations with these two different substrates. We shall demonstrate, in fact, that the relative thermodynamic stabilities of the fi3-allylic complexes are decisive for both the regio and the stereoselectivity. 

Both anodic oxidation reactions proceeded well. As illustrated in Scheme 44, an anodic methoxylation of menthyl pyroglutamate followed by the trapping of an incipient A-acyliminium ion with allyl-silane in the presence of Lewis acid led to (138) [86]. While the stereoselectivity of this reaction was not high, the major product from the reaction could be fractionally crystallized from hexane and the route used to conveniently prepare (138) on a scale of 10 g. In this case, a platinum wire anode was used in order to keep the current density high. This was required because of the high oxidation potential of the secondary amide relative to the methanol solvent used in the reaction. 

The Hiyama addition of allyl bromide 20 to (7 )-2-(methoxymethoxy)hexanal to give 21 with high stereoselectivity. Both the starting allyl bromide and the aldehyde are chiral and were used in nonracemic form. The relative configuration of the two stereogenic centers created in this reaction (c and d) had to be determined128 (see pp 463 and 483). 

Dehydrobromination of bromotrifluoropropene affords the more expensive trifluoropropyne [237], which was metallated in situ and trapped with an aldehyde in the TIT group s [238]synthesis of 2,6-dideoxy-6,6,6-trifluorosugars (Eq. 77). Allylic alcohols derived from adducts of this type have been transformed into trifluoromethyl lactones via [3,3] -Claisen rearrangements and subsequent iodolactonisation [239]. Relatively weak bases such as hydroxide anion can be used to perform the dehydrobromination and when the alkyne is generated in the presence of nucleophilic species, addition usually follows. Trifluoromethyl enol ethers were prepared (stereoselectively) in this way (Eq. 78) the key intermediate is presumably a transient vinyl carbanion which protonates before defluorination can occur [240]. Palladium(II)-catalysed alkenylation or aryla-tion then proceeds [241]. 

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