Stereoselective olefin synthesis

Carbonyls. The stereochemistry of the Wittig olefin synthesis has been reviewed. /i-a/u-Stereoselective olefin synthesis via /3-oxido-ylides is possible only in the presence of soluble lithium salts. Protonation of jS-oxido-ylides prepared from salt-free ylides leads to mixtures of erythro-and r/jr o-betaines and hence to mixtures of cis- and rm/i5-olefins. 

Warren continues to extend his phosphine oxide-based method of stereoselective olefin synthesis. The previously published method of acyl transfer followed by borohydride reduction to give selectively the threo-hydroxyalkylphosphine oxide intermediate has now been applied to more complex examples carrying other chiral centres.28 Optically active butyl(2-methoxyphenyOphosphine oxide (47) has been used to provide a chiral... 

Carbostannylation of alkynes generates cis-substituted alkenylstannanes and is one of the most useful reactions for stereoselective olefin synthesis. Treatment of tributyl-(phenylethynyl)tin 37 with a 1 2 mixture of [PdCl(7r-C3H5)]2-A -(2-diphenylphosphino benzylidene)-2-phenylethylamine (38) under an acetylene atmosphere in THF at... 

Scheme 7.16 Stereoselective olefin synthesis through Eschenmoser-Claisen rearrangement.

Cor mth stereoselective olefin synthesis 14,717 Corrins s. Semicorrins Conmarans... 

Cecropia Juvenile Hormone Stereoselective Olefin Synthesis... 

We will continue with olefin chemistry by considering lasonolide A (Introduction-5). Lasonolide A is a target that will reinforce how synthesis is still an important tool for determining structure. This target will allow us to briefly look at some modern organometallic chemistry as applied to the problem of stereoselective olefin synthesis. In addition it will be used to introduce the topic of macrolide synthesis (macrocyclic lactones). 

This synthesis illustrates one strategy for the preparation of acyclic molecules containing multiple stereogenic centers — use cyclic structures to control stereochemistry and then liberate the acyclic structure. The strategy is not unlike several of the Cecropia juvenile hormone syntheses we examined, where stereoselective olefin synthesis was the goal. Whereas three pieces are ultimately assembled, the synthesis of the central fragment is linear and there was a price to pay for this approach. It is long. 

Having read all the chapters, I hope that readers are left with the notion that stereoselective olefin synthesis is still a rapidly growing area with constant emergence of novel synthetic methods. 

Hydroboration - regloseiective and stereoselective (syn) addition of BH3 (RBH2, R2BH) to olefins. Synthesis of alcohol including optically active alcohols from olefins. Also useful In synthesis of ketones by stitching ot olefins and CO... 

This synthetic sequence for an olefin synthesis has been further developed by Kocienski who has shown that eliminative desulphonylations carried out on / -acyloxysulphones are remarkably stereoselective for the synthesis of trans-disubstituted-olefins. The method has wide applicability in that a-lithio phenyl sulphones are readily generated, and are readily coupled to aldehydes or ketones, to give j8-hydroxysulphones. The hydroxyl function of these is then esterified and the synthesis is completed by the reductive elimination with sodium amalgam. Kocienski has prepared two reviews that summarize his syntheses of a range of natural products - one of which is diumycinoP obtained... 

In many cases, the second side chain of the cyclopentane ring of prostaglandins possesses a double bond with ( )-geometry at C-13. The introduction of this side chain is usually achieved by olefination with phosphonate anions according to the Homer technique. However, because of the vicinal oxygen function at C-15 it is also recommended to introduce the side chain via ( )-stereoselective Wittig synthesis... 

A synthesis of the pear ester worked out in our laboratory uses the (Z)- and the ( )-stereoselective Wittig olefination for the generation of the two double bonds in the molecule 232165). Reaction of 2-ethoxyvinyl-triphenylphosphonium bromide 234 with one equivalent sodium ethanolate yields 2,2-ethoxyethylene-triphenyl-phosphorane 235166,167). (Z)-Stereoselective olefination of 235 with hexanal 236... 

Faulkner, D. J. Stereoselective synthesis of trisubstituted olefins. Synthesis 1971, 175-189. 

The Julia olefin synthesis is rather like the Wittig reaction with a sulfone instead of a phosphonium salt but with one other important difference the elimination step is stereoselective and both dia-stereoisomers of the intermediate can give the same isomer of the alkene. Treatment of the sulfone 147 with a strong base gives the anion 148 (or a metal derivative) that combines with an aldehyde to give a diastereomeric mixture of adducts 149. Elimination by various methods gives, in open chain compounds, mostly -150 but, in cyclic compounds, mostly the Z-alkene.29... 

Phosphoryl chloride-Stannous chloride-Pyridine [1, 881, before references], Johnson20 states that the stereoselectivity of the Cornforth olefin synthesis can be improved by carrying out the Grignard reaction (the first step) at a low temperature... 

A Sterling-Winthrop Research Institute group in cooperation with Johnson3 applied the new scheme of biogenetic-like stereoselective olefin cyclization to the synthesis of 19-norsteroids and so gained access to a steroid type of importance in medicine. 

Stereoselective olefination of carboxylic eaters or synthesis of allylamines from a-amino acid esters mediated by benzotriazole(Bt) derivatives. 

A one-pot synthesis of 4-hydroxycyclopent-2-en-l-ones (150), involving (Z)-stereoselective olefination of a-diketones to give (149) followed by intramolecular aldol condensation, has been reported. Wadsworth-Emmons reactions of bis(2,2,2-trifluoroethyl)phosphono sulfoxides (151) with aromatic aldehydes give predominantly (Z)-a,p-unsaturated sulfoxides while similar reactions with the corresponding sulfides (152) give ( )- or lower (Z)-selectivity. In olefinations using (5)-dimethyl phosphorylmethyl p-tolyl sulfoxide (153) substantial racemisation at sulfur occurs when -butyllithium is used as base. ... 

Terpenes.- The first total synthesis of the biologically active marine natural product metachromin A (241) has been achieved using an ( )-stereoselective olefination with the phosphonate (240) as the key step. The olefination reaction of the a-cyanophosphonate (242) with ( , )-farnesal (243) has been studied under a wide variety of conditions with a view to optimising (Z)-stereoselectivity.Excellent (Z)-stereoselectivity was achieved and the resulting procedure applied to a stereoselective synthesis of plaunotol (244). Epimerisation and P-elimination of base-sensitive substrates can cause problems in phosphonate- and phosphine oxide-based olefinations. Such reactions have been excluded in a recent report of enantiospecific syntheses of iridoid monoterpene lactones by intramolecular phosphonate-olefination through the use of mild bases such as DBU and di(wopropyl)ethyl amine/lithium chloride. 

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