Stereoselectivity, alkenes from

Vinyllithium [917-57-7] can be formed direcdy from vinyl chloride by means of a lithium [7439-93-2] dispersion containing 2 wt % sodium [7440-23-5] at 0—10°C. This compound is a reactive intermediate for the formation of vinyl alcohols from aldehydes, vinyl ketones from organic acids, vinyl sulfides from disulfides, and monosubstituted alkenes from organic halides. It can also be converted to vinylcopper [37616-22-1] or divinylcopper lithium [22903-99-7], which can then be used to introduce a vinyl group stereoselectively into a variety of a, P-unsaturated systems (26), or simply add a vinyl group to other a, P-unsaturated compounds to give y, 5-unsaturated compounds. Vinyllithium reagents can also be converted to secondary alcohols with trialkylb o r ane s. 

This procedure illustrates a general method for the preparation of alkenes from the pal 1 adium(Q)-cata1yzed reaction of vinyl halides with organo-lithium compounds, which can be prepared by various methods, including direct regioselective lithiation of hydrocarbons. The method is simple and has been used to prepare a variety of alkenes stereoselectively. Similar stoichiometric organocopper reactions sometimes proceed in a nonstereoselective... 

Stereoselective construction of tetrasubstituted exocyclic alkenes from the [4 + 2] cycloaddition of vinylallenes [155]... 

Several methods for stereoselective alkene synthesis are based on boron intermediates. One approach involves alkenylboranes, which can be prepared from terminal alkynes. Procedures have been developed for the synthesis of both Z- and E-alkenes. 

Both E- and Z-isomers of the terpene y-bisabolene have been isolated from natural sources. The synthesis of these compounds can be achieved by stereoselective alkene syntheses using borane intermediates. An outline of each synthesis is given below. Indicate the reaction conditions that would permit the stereoselective synthesis of each isomer. 

From these results it can be seen that in the hydrogenation of 1,2-dialkylcyclohexenes the expected cis-1,2-dialkylcyclohexane is not the sole product. Similarly, in the hydrogenation of the 1,4-dialky lcyclohexenes where both the cis- and trans-cyclohexanes are expected, the trans-isomer being the thermodynamically more stable, the stereoselectivity varies from metal to metal. Thus with palladium, the cis/trans ratio approaches the equilibrium composition, whereas with platinum and rhodium, the equilibrium composition is never approached. It is also instructive to note that in the palladium-catalysed reactions, hydrogenation is accompanied by extensive alkene isomerisation [220—223], whereas with rhodium and platinum, little or no isomerisation is observed [220,... 

Sophorolipid lactone (34) was obtained from an open-chain bisalkyne precursor (concomitant formation of 2-butyne) in a macrocyclization yield of 78% [29], The epothilones have been obtained through non-stereoselective alkene metathesis [26d], making them worthwhile targets for the combination alkyne metathesis/Lindlar hydrogenation to assemble the C12-C13 Z double bond. Furthermore, epothilone C (35) shows the most complex array of functional groups among the substrates of alkyne metathesis. The macrocyclization yield, starting from the expected OTBS-protected precursor (not shown) was 81% [23, 30]. Neither... 

A, vy/ -elimination of Ph2MeP=0 and simultaneous stereoselective alkene formation from an oxaphosphetane are shown in Figure 4.45 (note that this oxaphosphetane is not produced via a P-ylide). In Figure 4.45, this elimination is part of a an alkene inversion in which, via a four-step reaction sequence, an alkene such as cw-cyclooctene, a molecule with little strain, can be converted into its trans- and, in this case, more highly strained, isomer. 

Hetero-l,l-dimetallo-l-alkenes and hetero-1,1-dihalo-1-alkenes could be obtained in a stereoselective manner from 2-lithio-2,3-dihydrofuran via a dyotropic-type rearrangement of the derived cuprate. An example is depicted below <03SL955> <03S2530>. 

A reaction closely related to the [2 + 2] cycloaddition of thiocarbonyl S.S-dioxides is the addition of sulfur trioxide-dioxane complex to alkenes. This occurs stereoselectively and from (Z)- and (E)-2-butene the corresponding cis- and /ra .s -3,4-dimethyl-1,2-oxathietane 1,1-dioxides 11 are obtained121. 

The second cyclopropanation occurs at the only remaining alkene with a carbene generated a diazoester. The stereoselectivity comes from attack on the opposite side of the ring to the r membered ring already present. 

Addition of bromine to aikenes is both stereoselective and stereospecihc. We say it is completely stereoselective since, from a given alkene, we obtain only one diastereomer (or one pair of enantiomers). We say it is stereospecific, since just which stereoisomer we obtain depends upon which stereoisomeric alkene we start with. 

The cyclic phosphate and phosphoramidate derivatives of v/c-diols undergo a two-step reductive elimination on treatment with Li/NHs, Na-naphthalene, TiCl4-Mg(Hg) or TiClt-K. In acyclic systems the reaction is only moderately stereoselective. The reaction has particular value in the synthesis of tetra-substituted alkenes from highly hindered v/c-diols. Scheme 20 illustrates its use in the synthesis of [10,10]-betweenanene (49). ... 

Sano, S., Takehisa, T., Ogawa, S., Yokoyama, K., Nagao, Y. Stereoselective synthesis of tetrasubstituted (Z)-alkenes from aryl alkyl ketones utilizing the Horner-Wadsworth-Emmons reaction. Chem. Pharm. Bull. 2002, 50, 1300-1302. 

Stereoselectivity comes from a stereospecific syn or anti addition to an alkene of fixed and known geometry. These last reactions, applied to cyclohexene, lead to anti bromohydrin 14 while epoxidation occurs stereospecifically and syn. It doesn t matter which end of the epoxide 12 or bromonium ion 13 is attacked by the nucleophile anti addition occurs in both cases since inversion is demanded by the mechanism of the SN2 reaction. Cyclohexene must be Z but in open chain compounds syn addition to the -isomer would lead to the same diastereoisomer of the product as anti addition to the Z-isomer. In this chapter we explore more advanced versions of these reactions in which usually several types of selectivity will be combined and show how they are used in synthesis. 

As the product with the anti- stereochemistry is formed in excess over the syn- addition product (in which the two Br groups add to the alkene from the same side), the reaction is stereoselective (i.e. one particular stereoisomer of the product is formed in excess). 

Epoxide opening. A stereoselective alkene synthesis starts from reaction of triethylsilyloxirane with an organocuprate reagent, and it is concluded by oxidation of the j8-silyl alcohol to the aldehyde, Grignard reaction and elimination of [Et Si/OH]. The elimination leads to either the ( )- or the (Z)-alkene by using different reagents. 

In Fig. 4, reaction A is a highly stereoselective reduction of 1-aryl alkanones with (-)-chlo-ro diisopinocampheylborane [21]. Upon co-ordination of the ketone oxygen with the Lewis acidic chirotopic and non-stereogenic [22] boron atom of the chiral reagent, two diastereo-isomeric complexes arise. The sterically less hindered one is preferentially formed and leads the major (,S)-enantiomer, which is isolated after a work-up that allows recovery of a-pinene, the chiral alkene from which the borane is prepared. 

Alkenes from 1,2-diols. The catalytic elimination using Cp ReOj in the presence of PhjP in hot chlorobenzene is stereoselective. This method is potentially competitive with the standard Corey-Winter procedure. 

Kadota, I. et ah, A simple and practical method for the stereoselective synthesis of (Z)-l-iodo-l-alkenes from 1,1-diiodo-l-alkenes, Tetrahedron Lett., 44, 8645, 2003. 

The most obvious method for the formation of alkenes from alkynes is by partial reduction. This reaction can be effected in high yield with a palladium-calcium carbonate catalyst that has been partially deactivated by addition of lead(II) acetate or quinoline (Lindlar s catalyst). It is aided by the fact that the more electrophilic alkynes are adsorbed on the electron-rich catalyst surface more strongly than the corresponding alkenes. An important feature of these reductions is their high stereoselectivity. In most cases the product consists very largely of the thermodynamically... 

The stereoselectivity of the Wittig reaction depends, not only on the substituents, but also on the conditions under which the reaction is effected. The presence of lithivun salts tends to favour the fJ-alkene, so reactions in which the Z-alkene is desired are often carried out using sodium or potassium bases. It is possible to obtain high yields of the fJ-alkene from a non-stabilized phosphonium ylide by deprotonation, then reprotonation of the intermediate oxaphosphetane or betaine. An especially useful application of the Wittig reaction is in the formation of exo-cyclic double bonds. Thus, cyclohexanone and (methylene)triphenylphosphorane give (methylene)cyclohexane, whereas the use of the Grignard reaction followed by dehydration leads to the endocyclic isomer. 

---