E-stereoselectivity

Thus unsubstituted (R=H) and substituted (R = alkyl) non-stabilized diyiides 1 react with phenylisocyanate and dicyclohexylcarbodiimide (R NCX), leading to the formation of new monoylide type intermediates. These last ones react in situ with carbonyl compounds through a Wittig type reaction leading respectively to a,)8-unsaturated amides 2 and amidines 3, with a high E stereoselectivity, the double bond being di- or tri-substituted [48,49]. By a similar reactional pathway, diyiides also react with carbonic acid derivatives, with the synthesis as final products of -a,/l-unsaturated esters 4 and acids 5 [50]. 

The benzotriazolyl derivative of acrolein acetal, compound 882, is lithiated, treated with chlorodiphenylphosphine, and the obtained intermediate is oxidized with hydrogen peroxide to phosphine oxide 883 (Scheme 145). The relatively acidic proton in derivative 883 is easily removed by a base, and the obtained anion adds to a carbonyl group of aldehyde or ketone. Subsequent rearrangement and elimination of the phosphorane group generates diene 884. For the derivatives of aldehydes (884, R2 = H), (E)-(E) stereoselectivity of the elimination is observed. Acidic alcoholysis of dienes 884 affords esters of P,y-unsaturated carboxylic acids 885 < 1997JOC4131>. 

The presence of EWG on 2,4-dioxopentane-3-thione favours the formation of thiophilic adducts with different allyl substrates.178 In all cases the reaction affords single thiophilic ene adducts and formation of C=C occurs with high E stereoselectivity (Scheme 17). 

The mode of reaction of titanacydobutenes with carbonyl compounds is largely dependent on steric factors (Scheme 14.31) [72]. Ketones and aldehydes tend to insert into the titanium—alkyl bond of 2,3-diphenyltitanacydobutene, and homoallylic alcohols 70 are obtained by hydrolysis of the adducts 71 [65a,73]. On the contrary, when dialkyl-substi-tuted titanacydobutenes are employed, the reaction with aldehydes preferentially proceeds through insertion into the titanium—vinyl bond. Thermal decomposition of the adducts 72 affords conjugated dienes 73 with E-stereoselectivity as a result of a concerted retro [4+2] cycloaddition [72]. 

Acyl-allylboronates are smoothly provided through a palladium-mediated three-component assembly reaction of an acyl chloride, an allene and a diboron with high regioselectivity and E-stereoselectivity (Scheme 16.56) [62]. 

The high E-stereoselectivity can be explained by the face-selective coordination of an allene to an acylpalladium complex (Scheme 16.57). 

Silyl homoallylic alcohols are obtained with high y-regioselection and E-stereoselection on reaction of chiral alkoxy- and aminomethyl-substituted a -silylallyl carbanions with aldehydes factors which influence the diastereomeric excess have been identified. 

Other examples of the formation of six-membered rings by means of an intramolecular alkylation of an ester enolate are given in Table 7. Entry 6, i.e., stereoselective transformation of the epoxy ester into the cyclohexane derivative, should be discussed briefly as a representative for the other cases. The probable reason for the unexpectedly high selectivity i.e., the nonappearance of the diastereomer 8, can be demonstrated by the two transition-state-like conformations 9 and 10. 9 displays a very severe 1,3-diaxial interaction in comparison to 10, thus, formation of the diastereomer 7 from conformation 10 is highly favored113. 

E. Stereoselective Cyclopropanation of Alkenes using Stoichiometric Chiral... 

The reactivity of the dimetallic compounds 186a and 186b was then investigated and these species were less reactive than their formulas would lead to predict, as no reaction occurred with ketones, esters, Michael acceptors or trimethylsilyl chloride. In the presence of BF3 OEt2, addition occurred to aldehydes and led after /3-elimination to the corresponding olefination products 189 with good to excellent (E) stereoselectivity (equation 92)127. 

Palladium-catalyzed coupling of allylic acetate 113 (Scheme 23) with diethyl malonate, or with acetylated diethyl tartronate, yielded the Boc-protected allylic amines 114 in 99 1 E stereoselectivity.1 "1 Saponification of 114 (R1 = H) and decarboxylation gave the Tyr- Ala alkene isostere 116 in quantitative yield, whereas 114 (R = OAc) was converted into the Tyr-Gly alkene isostere 115 in a two-step procedure in 62% overall yield. Considering the number of steps and the overall yield, this procedure is among the most efficient to prepare Xaatp, CH=CH]Gly dipeptide isosteres. 

More recently, Kitazume and co-workers found that allylic alcohols 7 undergo the Johnson Claisen rearrangement in higher yields when the reaction is run in a sealed tube at 130 C for 12 hours.35 Under these conditions. (/ )-7 is converted into 8 with high E stereoselectivity and complete 1,3-chiral transfer. 

In the presence of an InCb catalyst, substituted indoles were found to react with several substituted allylic acetates via an. S N2 mechanism giving yields of >79% with (g) E-stereoselectivity (Scheme 3).8... 

Addition to allylic mesylates.6 Conjugate addition of organocyanocuprates to acyclic allylic mesylates substituted at the P-position with a chiral sulfoxide group involves an SN2 -substitution with high Z/E stereoselectivity and high asymmetric induction. This reaction provides a route to chiral trisubstituted vinyl sulfoxides. 

In contrast with the catalytic system based on RuCl(rf-C9H7)(PPh3)2 in micellar solutions [32], the reaction of secondary propargylic alcohols in 2-propanol/ H20 at 100 °C in the presence of 5 mol % of RuCl(Cp)(PMe3)2 leads to conjugated enals with E stereoselectivity (Eq. 12) [88]. 

Furthermore, in (Z)-l,3-disubstituted allylic sulfoxides the transoid transition state is free of severe 1,3-allylic strain which destabilizes the corresponding cisoid transition state12. Such systems show nearly complete chirality transfer as well as absolute E stereoselectivity. 

Carbonyl compound MO energy levels (au) MuUiken atomic charges (e) Stereoselectivities (boron enolate 122a)... 

Gawley, R. E. Stereoselective Addition of Chiral a-Aminoorganometallics to Aldehydes, Adv. Asym. Synth., Vol. 3, Hassner, A., Ed. JAI Press, Stamford, CT 1998. 

E. Stereoselective Syntheses via Esters of Arsinous and Arsinthious Acids... 

Chen, Q, Kong, E, Stereoselective glycosylation using fuUy henzylated pyrimidin-2-yl 1-thio-p-D-glycopyranosides, Carbohydr. Res., 272, 149-158, 1995. 

Morrow, D. F., Butler, M. E. Stereoselectivity in the Neber rearrangement-synthesis of a steroidal spiroazirine. J. Heterocycl. Chem. 1964,... 

Breczinski, P. M., Stumpf, A., Hope, H., Krafft, M. E., Casalnuovo, J. A., Schore, N. E. Stereoselectivity in the intramolecular Pauson-Khand reaction towards a simple predictive model. Tetrahedron 1999, 55, 6797-6812. 

Gahman, T. C., Overman, L. E. Stereoselective synthesis of carbocyclic ring systems by pinacol-terminated Prins cyclizations. Tetrahedron 2002, 58, 6473-6483. 

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