Alkene derivatives reagents

CharactenshcaUy the reagents that react with the aromatic nng of benzene and its derivatives are electrophiles We already have some experience with electrophihc reagents particularly with respect to how they react with alkenes Electrophihc reagents add to alkenes... 

It is very important to keep the concentration of Br2 and HBr low to prevent side reactions derived from simple ionic addition with the alkene. These reagents are therefore generated in situ from NBS. The catalytically active species is Br2, which is almost always present in NBS samples (red colour). 

The hydroborating reagents described in Section B1.2 are generated from achiral alkenes. Addition of a source of borane to a homochiral alkene derived... 

Carbenes can be stabilized as transition metal complexes decomposition of phenyldiazomethane in the presence of a ruthenium(II) complex gives a carbene complex stable enough to be isolated and stored for months. These complexes are among the most important of carbene-derived reagents because of a remarkable reaction known as alkene (or more commonly olefin) metathesis. 

The increased steric requirement of the 2-(benzyloxy)ethyl side chain provides a somewhat more selective reagent. Similar results can be achieved by replacing the 2-methyl group of the a-pinene derived reagent with an ethyl or propyl group12. The latter reagents are prepared by hydroboration of the appropriate alkene, available from nopol12, with solid 9-BBN for 6 hours at 65 =C. 

The asymmetric a-sulfenylation of ketones is a particularly challenging reaction, as demonstrated by the poor success reported in the stereoselective variants via classical enolate/azaenolate reaction with an electrophilic sulfur reagent [71]. An umpolung approach has been devised by Coltart and co-workers [72] to effect the first asymmetric a-sulfenylation of ketones with arene thiols. Nitroso alkene derivatives, in i/tM-generated under basic conditions from a-chloro oximes, reacted with arene thiols in the presence of cinchona thiourea 27, which promoted the conjugate addition of thiophenol (Scheme 14.25). The chiral nonracemic a-sulfenylated oximes were directly hydrolyzed by IBX to ketones in high yield and good enantioselectivity. 

Other examples have been given of the preparation of branched-chain and extended-chain alkenes by application of Wittig reagents to sugar aldehydes and ketones,and the mass spectra of alkenes derived from 1,2 5,6-di-O-iso-propylidene-D-r/Z o-hexos-3-ulose have been described.Compound (25) was... 

The trisannulation reagent 7-acetoxy-l,l l-dodecadien-3-one (134) is derived from the bisannulation reagent 124 in four steps. This reagent is a synthetic equivalent of l-dodecene-3,7,11-trione, and the two ketone groups of the trione are masked as an acetoxy and a terminal alkene. The synthesis of optically active D-homo-19-norandrosta-4-en-3-one (135) by the trisannulation reaction... 

There are a wide variety of methods for introduction of substituents at C3. Since this is the preferred site for electrophilic substitution, direct alkylation and acylation procedures are often effective. Even mild electrophiles such as alkenes with EW substituents can react at the 3-position of the indole ring. Techniques for preparation of 3-lithioindoles, usually by halogen-metal exchange, have been developed and this provides access not only to the lithium reagents but also to other organometallic reagents derived from them. The 3-position is also reactive toward electrophilic mercuration. 

Mild acid converts it to the product and ethanol. With the higher temperatures required of the cyano compound [1003-52-7] (15), the intermediate cycloadduct is converted direcdy to the product by elimination of waste hydrogen cyanide. Often the reactions are mn with neat Hquid reagents having an excess of alkene as solvent. Polar solvents such as sulfolane and /V-m ethyl -pyrrol i don e are claimed to be superior for reactions of the ethoxy compound with butenediol (53). Organic acids, phenols, maleic acid derivatives, and inorganic bases are suggested as catalysts (51,52,54,59,61,62) (Fig. 6). 

---