Vinyl cation synthons

Phenyl 2-(trimethylsilyl)ethynyl sulfone (118) can act as a vinyl cation synthon (equations 93 and 94)78 79. Thus, the reaction of enolates with 118 and subsequent desulfonylation of the adduct gives a-vinyl ketone, such as 119 and 120. 

Few reagents are available to the synthetic organic chemist which function as vinyl cation synthons. At present, these include a-... 

Because of their high reactivity, these complex salts react rapidly and regiospecifically, at low temperature, with a number of carbon and heteroatomic nucleophiles, including thiols, amines, and alcohols. Finally, exposure of the double bond takes place under particularly mild conditions so that isomerization of the (3,Y-unsaturated carbonyl system may be avoided. The present scope of reactions with these vinyl cation synthons is summarized in [able I. 

Vinyl acetate- Acetic acid vinyl ester Acetic acid ethenyl ester, (108-05-4), 65, 135 VINYL CATION EQUIVALENT, 66, 95 Vinyl cation synthons, 66, 102 Vinyl radical cyclization, 66, 81, 82 VINYLAT10N OF EN0LATES. 66, 95, 104 Vinyl lithium, 66, 53, 55... 

More relevant to this chapter is the realisation of a vinyl cation strategy which was in fact chosen as the development route for the product. Adjusting the oxidation state of the benzene ring reveals a promising enone 37 and requires a reagent for the vinyl cation synthon 38. 

Z)-/( )-olefins (47) regiospecifically, in excellent yield (80 %). The trimethysilyl ethynynl sulphone (49) has been developed as a vinyl cation synthon by Barr and co-workers. The addition of carbanions (48) to the synthon (49) furnishes the vinyl sulphones (50), which can be converted into the adducts (51) either by... 

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. 

Three-Carbon Synthon for [3 + 2] Annulations. Danheiser and co-workers have exploited allenylsilanes as the three-carbon components in a [3 + 2] annulation strategy for the synthesis of a variety of five-membered carbocycles and heterocycles. The pathway by which a typical annulation proceeds is shown in eq 6. Reaction of the 2-carbon component (the allenophile ) at C-3 of the allenylsilane is followed by rapid rearrangement of the silicon-stabilized vinyl cation. Ring closure then affords the five-membered product. 

Allenylsilanes lacking a C-1 alkyl substituent do not function efficiently as three-carbon synthons in the [3 + 2] annulation. This phenomenon is attributable to the relative instability of the terminal vinyl cation intermediate required according to the proposed mechanism for the annulations (eq 6). Fully substituted five-membered rings result from annulations emplo3fing allenylsilanes substituted at both C-1 and C-S. ... 

N of nitrogen belongs to the set IB. The synthons 1 to n are unstable except of one mesomeric structure of the synthon n

L-Iduronyl synthons catalyzed by a vinyl cerium reagent have been shown to open a way to an efficient preparation of l,2,4-tri-0-acetyl-3-0-benzyl-Z,-iduronyl derivatives.1097 The synthetic utility of the cationic lanthanide complex [Cp 2Ce][BPh4] as an effective Lewis acid catalyst for the hetero-Diels-Alder reaction between Danishefsky s diene and substituted benzaldehydes has been demonstrated (Scheme 307). 

Anions from the Schiffs base (78) can be C- or A -alkylated with ethyl iodide or diethyl sulphate. The ratio of the products depends both on the solvent and on the presence of 18-crown-6. In non-polar solvents, the crown ether increases the solubility of the base, and C-alkylation is the major pathway. In dipolar aprotic solvents, the 18-crown-6 breaks up ion pairs by solvation of the Na" cation, and favours A -alkylation. A nerylsulphonamide, formed from (79), undergoes regiospecific reductive desulphonylation to give nerol (80), which implies that (79) is an effective synthon for cisoid iso-prenoids. Chiral complexes of crown ethers, e.g. (81), catalyse the Michael addition reaction of j3-keto-esters and methyl vinyl ketone to give adducts in high optical yields. ... 

---