Synthons vinyl anion

Synthesis The vinyl anion synthon can either be the vinyl Grignard reagent or the acetylide arrion, in which case the synthesis becomes ... 

The vinyl anion synthon is best represented by an acetylide ion (frame 33). Synthesis ... 

On an industrial scale it was best to introduce the vinyl anion synthon as acetylene and then hydrogenate the alkyne. The unsaturated ketone was chosen as the starting material because its synthesis was already known, linalool synthesis... 

The carbonyl adducts of allyl phosphine oxides, e. g., 24 and 29 can also be used to make allyl alcohols32 by the reductive removal of the Ph2PO group with lithium aluminium hydride a reaction involving transposition of the double bond, e. g., 24 36. In this approach, the allyl anion of 23 is a reagent for the vinyl anion synthon 37. 

Introduction Reagents for the Vinyl Anion Synthon V inyl-Lithiums... 

Fresh from the discussion in chapter 15 on stereo-controlled alkene synthesis, did you notice that we avoided one disconnection entirely We never considered combining a vinyl anion 2 with a carbon electrophile. This chapter concerns reagents for the vinyl anion synthon and particularly those that allow control over the stereochemistry of the double bond in the product. Please notice from the start that we mean vinyl anions where the nucleophilic group is in the plane of the alkene 3. This means that we shall chiefly be discussing vinyl metals such as vinyl-lithiums. These are metal o-complexes 4 with the metal atom also in the plane of the alkene. 

If you want to make a diene one obvious place to disconnect is between the two alkenes 1. Though this has a pleasing symmetry, one of the synthons must be a vinyl cation 2 and the other a vinyl anion 3. We have already met stereochemically controlled reagents for the vinyl anion synthon such as vinyl metals 4 (chapter 16) and all we now need is a good reagent for the vinyl cation. That is the subject of this chapter. First we need to examine why you can t just use a vinyl halide and expect to get substitution of the halide ion. 

Disconnection at alkene a gives a fragment 130 with some functional group OR for the Wittig or Peterson reaction. There is a unique carbon atom between the nitrogen atom and alkene b so disconnection to an (unstable) iminium ion and a stereochemically controlled vinyl anion synthon 131 (chapter 16) was chosen by Overman,20 who rather specialises in this kind of iminium ion. 

Acetylenes are particularly useful for making dienes of type (22>. The double bond outside the ring comes from the acetylene and that inside the ring from the dehydration of an alcohol. This is very similar to the strategy outlined in Chapter 15—but here the acetylene is the reagent for the vinyl anion synthon. 

The protected version (61) of (59) can be made from acetoacetate and allylic halide (62) which can come from (58) (cf. Chapters 16 and 35) using acetylene for the vinyl anion synthon. 

Vinyl anions derived from vinylsilanes have continued to develop as useful synthons. The lithio derivative (201), prepared by lithium-bromine exchange shows an excellent level of stereointegrity if kept below 0°C and reacts with a wide range of electrophiles. The organomagnesium derivative (202) was also examined but this anion shows a higher propensity towards E/Z isomerisation.176 Configuration instability is a drawback to the use of anions such as (201) and... 

VINYL ANION SYNTHONS ACCESSIBLE VIA MISLOW-EVANS REARRANGEMENT... 

Dienal Vinyl Anion Synthon in a Double Mislow-Evans Process... 

The metallation of 3-methyl-4//-5,6-dihydro-l,2-oxazine has been shown to take place at the methyl group with hindered bases and at the methylene group with unhindered bases (81JA5916). Deprotonation of (753) with lithium dimethylamide at -65 °C followed by reaction with benzyl bromide gave (754) in 85% yield. This product was converted to enone (755) by reaction first with triethyloxonium tetrafluoroborate to produce an oxoiminium salt. The salt was stirred with trimethylamine and the resulting a,/3-unsaturated imine hydrolyzed with wet silica gel to the enone (Scheme 174). The lithiated derivative of (753) serves as a synthon for the unknown a-anion of methyl vinyl ketone. 

A CH-group, which bears vinyl and sulfide substituents, is acidic enough to be metallated by strong bases. Other d3-synthons may contain two activating functional groups in Imposition ( homoenolate -equivalents). Only one of the a-carbons is deprotonated under appropiate conditions (e.g. succinic diesters). Ano ther possibility is an acidic carbon and a non-acidic functional group in 1,3-positions (e.g. propargyl derivatives). Silyl ethers of a, -unsaturated alcohols can also be converted to anions, which react as d3-synthons (W. Oppolzer, 1976). 

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