Alkene precursor

Allylboron compounds have proven to be an exceedingly useful class of allylmetal reagents for the stereoselective synthesis of homoallylic alcohols via reactions with carbonyl compounds, especially aldehydes1. The reactions of allylboron compounds and aldehydes proceed by way of cyclic transition states with predictable transmission of olefinic stereochemistry to anti (from L-alkene precursors) or syn (from Z-alkene precursors) relationships about the newly formed carbon-carbon bond. This stereochemical feature, classified as simple diastereoselection, is general for Type I allylorganometallicslb. 

The other commonly quoted industrial photochemical process is the production of vitamin D3 involving a photochemical electrocyclic ring opening followed by a thermal 1,7-hydride shift (Scheme 7.2). This is a further example of a successful low quantum yield process in this case there is no viable thermal alternative. Vitamin A acetate has also been produced commercially using a photochemical isomerization process to convert a mixed tetra-alkene precursor to the all-trans form. 

Jurenka R. A. and Subchev M. (2000) Identification of cuticular hydrocarbons and alkene precursor to the pheromone in hemolymph of the female gypsy moth, Lymantria dispar. Arch. Insect Biochem. Physiol. 43, 108-115. 

The stabilities of carbocations for which the appropriate alkene precursors are not available were determined from the gas-phase halide-transfer equilibria (29) of the corresponding benzylic halides (Mishima et al., 1995). 

The rate-determining step in catalytic osmylations is usually the oxidation of the osmium(VI) esters 2 or 735t 37. Depending on the steric requirements of the alkene precursor, the trioxo(l,2-diolato)osmate(VIII) 4, formed in the amine TV-oxide promoted catalytic osmylation, can be further transformed following two different pathways. 

The synthesis of oxetanes from alkenic precursors has been demonstrated and the range and scope of Patemo-Biichi photocycloadditions are broad. In general, both the reaction regioselectivity and stereoselectivity can often be predicted by considering the reaction mechanism (Section 2.4.2) for example, the directing effects of alkene substituents are readily imderstood. Synthetic applications are numerous owing to the rtq>id stereocontrolled assembly of multifimctional targets. 

In addition to the sulfoximines, Johnson has studied phosphinothioic amides as alkene precursors. This reagent has not achieved the popularity of the sulfoximines. It can be utilized for ketone methylen-ation with resolution, as well as for Ae synthesis of more highly substituted alkenes. Rigby has found that in the synthesis of guaianolides this reagent was effective where Peterson and Wittig reactions gave only p-elimination. ... 

A study carried out by Kocienski and Lythgoe flrst demonstrated the trans selectivity of the Julia coupling process. The authors found the i uctive elimination could best be carried out with the acet-oxy or benzoyloxy sulfones. If the lithio sulfone derivative is used for addition to the carbonyl, the reaction can be worked up with acetic anhydride or benzoyl chloride to obtain the alkene precursor. In cases where enolization of the carbonyl is a complication, the magnesium derivative can frequently be used successfully. A modification of the reductive elimination was found to be most effective. Methanol, ethyl acetate/methanol or THF/methanol were the solvents of choice and a temperature of -20 C was effective at suppressing the undesired elimination of the acetoxy group to produce the vinyl sulfone. With these modifications of the original procedure, the ability of the reaction to produce dienes as well as rran.r-disubstituted alkenes was demonstrated, llie diastereoisomeric erythro- and threo-acetoxy sulfones could be separated and it was demonstrated that both isomers were converted to the rrans-alkene. It... 

Despite the range of sources available, the majority of reactions described have used alcohol or alkene precursors. This reflects both the availability of such materials and their ease of reaction. Typical examples of alkene reagents are shown in equations (9), (10), (11) and (12). When two reactive groups are in close proximity in the substrate it is possible to combine transannular cyclization with Ritter reaction (equation 13). - ... 

Many of the same reagents that reduce vic-dihalides also reduce halohydrins with similar regiochemi-cal and stereochemical advantages. However, halohydrins are more useful alkene precursors because they can be prepared by a greater variety of methods. The Fischer-Zach synthesis of 3,4,6-tri-O-acetyl-D-glucal (11 Scheme 3) reported in 1913 is one of the first synthetically useful applications of reductive elimination reactions to the manipulation of natural products and the method is still widely used today. ... 

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