Allylic derivatives stereochemistry

Goering, H. L. Tseng, C. C. Alkylation of allylic derivatives. 7. Stereochemistry of alkylation of the isomeric trans-a,y-methyl(phenyl)-allyl acetates with lithium dialkylcuprates and alkylcyanocuprates. J. Org. Chem. 1983, 48, 3986-3990. 

Allyltributyltin (5) is the most commonly used reagent for carrying out allylation reactions via a free radical fragmentation process [5]. Keck reported the first practical use of allyltributyltin for free radical allylation reactions in 1982 in the context of a synthesis of perhydrohistrionicotoxin [6]. Heating bromide 4 with allyltributyltin in the presence of AIBN as a radical initiator gave the allylated derivative 6 (Scheme 3) in high yield with complete control of stereochemistry. Similar transformations had proven to be very difficult by standard ionic reactions. 

The most prominent use of ketenes is for [2 + 2] cycloaddition with imine for the construction of /3-lactam skeleton. When the Y group in Scheme 1 is vinyl or aryl group, the deprotonation of the activated a-proton is highly facilitated. In this context, the carbonylation of some allylic derivatives, for example, allyl bromide, allyl acetate, allyl phenyl ether, allyl methyl carbonate, allyl phenyl sulfone, and allyl phosphate, documented to form TT-allylpalladium intermediates is examined. It is interesting to note that only phosphate undergoes the cycloaddition to produce /3-lactam. The characteristic dependency of the stereochemistry on the reaction conditions, being contrary to the results in the usual base-induced cycloaddition is also intriguing. Scheme 2 presents the... 

A valuable feature of the Nin/Crn-mediated Nozaki-Takai-Hiyama-Kishi coupling of vinyl iodides and aldehydes is that the stereochemistry of the vinyl iodide partner is reflected in the allylic alcohol coupling product, at least when disubstituted or trans tri-substituted vinyl iodides are employed.68 It is, therefore, imperative that the trans vinyl iodide stereochemistry in 159 be rigorously defined. Of the various ways in which this objective could be achieved, a regioselective syn addition of the Zr-H bond of Schwartz s reagent (Cp2ZrHCl) to the alkyne function in 165, followed by exposure of the resulting vinylzirconium species to iodine, seemed to constitute a distinctly direct solution to this important problem. Alkyne 165 could conceivably be derived in short order from compound 166, the projected product of an asymmetric crotylboration of achiral aldehyde 168. 

Allylsilanes are available by treatment of allyl acetates and allyl carbonates with silyl cuprates17-18, with antarafacial stereochemistry being observed for displacement of tertiary allyl acetates19. This reaction provides a useful asymmetric synthesis of allylsilanes using esters and carbamates derived from optically active secondary alcohols antarafacial stereochemistry is observed for the esters, and suprafacial stereochemistry for the carbamates20,21. 

Thia-[2,3]-Wittig sigmatropic rearrangement of lithiated carbanions 47, obtained by deprotonation of the S-allylic sulfides 46, affords the thiols 48 or their alkylated derivatives 49. The corresponding sulfonium ylides 51, prepared by deprotonation of the sulfonium salts 50 also undergoes a [2,3]-sigmatropic shift leading to the same sulfides 49 [36,38] (Scheme 13). As far as stereochemistry is concerned, with crotyl (R R =H,R =Me) and cinnamyl (R, R =H,R =Ph) derivatives, it has been shown that the diastereoselectivity depends on the nature of the R substituent and on the use of a carbanion or an ylide as intermediate. 

The catalytic enantioselective desymmetrization of meso compounds is a powerful tool for the construction of enantiomerically enriched functionalized products." Meso cyclic allylic diol derivatives are challenging substrates for the asymmetric allylic substitution reaction owing to the potential competition of several reaction pathways. In particular, S 2 and 5n2 substitutions can occur, and both with either retention or inversion of the stereochemistry. In the... 

Although the allylation reaction is formally analogous to the addition of allylic boranes to carbonyl derivatives, it does not normally occur through a cyclic TS. This is because, in contrast to the boranes, the silicon in allylic silanes has little Lewis acid character and does not coordinate at the carbonyl oxygen. The stereochemistry of addition of allylic silanes to carbonyl compounds is consistent with an acyclic TS. The -stereoisomer of 2-butenyl(trimethyl)silane gives nearly exclusively the product in... 

The synthesis in Scheme 13.49 features use of an enantioselective allylic boronate reagent derived from diisopropyl tartrate to establish the C(4) and C(5) stereochemistry. The ring is closed by an olefin metathesis reaction. The C(2) methyl group was introduced by alkylation of the lactone enolate. The alkylation is not stereoselective, but base-catalyzed epimerization favors the desired stereoisomer by 4 1. 

Iridium-catalyzed transfer hydrogenation of aldehyde 73 in the presence of 1,1-dimethylallene promotes tert-prenylation [64] to form the secondary neopentyl alcohol 74. In this process, isopropanol serves as the hydrogen donor, and the isolated iridium complex prepared from [Ir(cod)Cl]2, allyl acetate, m-nitrobenzoic acid, and (S)-SEGPHOS is used as catalyst. Complete levels of catalyst-directed diastereoselectivity are observed. Exposure of neopentyl alcohol 74 to acetic anhydride followed by ozonolysis provides p-acetoxy aldehyde 75. Reductive coupling of aldehyde 75 with allyl acetate under transfer hydrogenation conditions results in the formation of homoallylic alcohol 76. As the stereochemistry of this addition is irrelevant, an achiral iridium complex derived from [Ir(cod)Cl]2, allyl acetate, m-nitrobenzoic acid, and BIPHEP was employed as catalyst (Scheme 5.9). 

Other factors which affect the case of electrocyclic ring opening include the nature of substituents which can stabilize or destabilize the development of possible charge and the release of strain in small cyclic systems. Thus different stereochemistries have been observed in the ring opening of cyclopropyl derivatives. All cis derivatives generate an all-cis allyl cation but the anti derivatives will form the all trans cation. 

Evidence derived from a study of the stereochemistry of hydrogenation of 1,2-cyclononadiene and 1,2-cyclodecadiene led Moore (108) to conclude that allyl complexes like those postulated above must be intermediates. He noted that, of the four ways in which either allene could be adsorbed on a surface, two, a and b, would yield via cis addition of hydrogen the cis-cycloalkene and two, c and d, the tram isomer. Examination of... 

The first attempts to develop reactions offering control over the absolute stereochemistry of a chiral center, created by y-selective substitution of an achiral allylic alcohol-derived substrate, involved the use of chiral auxiliaries incorporated in the nucleofuge. The types of stereodirecting groups utilized vary, and have included sulfoximines [15], carbamates [16], and chiral heterocyclic sulfides [17-19]. 

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