Alkenes, from aldehydes ethers

If a sample contains one or more members of a homologous series, identifications can be made using a plot of log tR against the number of carbon atoms, previously prepared from standards. The plot, which is valid for one temperature only, is linear and can be used for alkanes, alkenes, alcohols, aldehydes, ketones, esters and ethers. 

As shown in Figure 3-2, titanium is coordinated with the oxygen from both the aldehyde and the alkene enol silyl ether. When aldehyde approaches the enol species, intermediate A is favored to B, and anti-aldol is obtained as the major product. Table 3-4 presents some results of these reactions. 

The yields from aldehyde alkylidenation is somewhat lower due to the reductive dimerization of aldehydes with low-valent Ti. Alkylidenation of esters is possible by the reaction of 1,1 -dibromoalkane. TiCU and Zn in the presence of TMEDA to give (Z) vinyl ethers [60], Cyclic vinyl ethers are prepared from unsaturated esters in two steps. The first step is formation of the acyclic enol ethers using a stoichiometric amount of the Ti reagent, and the second step is ring-closing alkene metathesis catalysed by Mo complex 19. Thus the benzofiiran moiety of sophora compound I (199, R = H) was synthesized by the carbonyl alkenation of ester in 197 with the Ti reagent prepared in situ, and the subsequent catalytic RCM of the resulting enol ether 198 catalysed by 19 [61]. 

Scheme 5. General alkylidenation of esters to enol ethers 9 and synthesis of disubstituted ( )-alkenes 10 from aldehydes and chromium(II) reagents according to Takai and Utimoto.

Kobayashi et al. found that lanthanide triflates were excellent catalysts for activation of C-N double bonds —activation by other Lewis acids required more than stoichiometric amounts of the acids. Examples were aza Diels-Alder reactions, the Man-nich-type reaction of A-(a-aminoalkyl)benzotriazoles with silyl enol ethers, the 1,3-dipolar cycloaddition of nitrones to alkenes, the 1,2-cycloaddition of diazoesters to imines, and the nucleophilic addition reactions to imines [24], These reactions are efficiently catalyzed by Yb(OTf)3. The arylimines reacted with Danishefsky s diene to give the dihydropyridones (Eq. 14) [25,26], The arylimines acted as the azadienes when reacted with cyclopentadiene, vinyl ethers or vinyl thioethers, providing the tet-rahydroquinolines (Eq. 15). Silyl enol ethers derived from esters, ketones, and thio-esters reacted with N-(a-aminoalkyl)benzotriazoles to give the /5-amino carbonyl compounds (Eq. 16) [27]. The diastereoselectivity was independent of the geometry of the silyl enol ethers, and favored the anti products. Nitrones, prepared in situ from aldehydes and N-substituted hydroxylamines, added to alkenes to afford isoxazoli-dines (Eq. 17) [28]. Addition of diazoesters to imines afforded CK-aziridines as the major products (Eq. 18) [29]. In all the reactions the imines could be generated in situ and the three-component coupling reactions proceeded smoothly in one pot. 

If an organic chemist were allowed to choose ten aliphatic compounds with which to be stranded on a desert island, he would almost certainly pick alcohols. From them he could make nearly every other kind of aliphatic compound alkenes, alkyl halides, ethers, aldehydes, ketones, acids, esters, and a host of others. From the alkyl halides, he could make Grignard reagents, and from the reaction between these and the aldehydes and ketones obtain more complicated alcohols and so on. Our stranded chemist would use his alcohols not only as raw materials but frequently as the solvents in which reactions are carried out and from which products are recrystallized. 

Cyclopropyl trimethylsilyl ethers, which were readily prepared from aldehydes, ketones and esters, coupled with alkenes on treatment with mercury(II) acetate and, after reduetion with sodium borohydride, afforded chain-extended carbonyl compounds, e.g. 37, in moderate to good yields. The whole reaction could be carried out in one pot. 

Note that the aldehyde approaches the alkene from the direction anti to the silicon atom. Therefore, when a chiral allylsilane or allylstannane with a substituent in the a-position is used, chirality transfer takes place, to generate the homoallylic alcohol with essentially no loss in enantiomeric purity. For example, reaction of the aldehyde 157 with the chiral allylsilane 158, using boron trifluoride etherate as the catalyst, gave predominantly the syn product 159 (1.151). The absolute stereochemistry can be determined by using a model in which the hydrogen atom on the a-carbon of the allylsilane eclipses the alkene (the so-called inside hydrogen effect ) in order to minimize steric interactions (1.152). 

The detection and identification of polar components is also possible using simultaneous hydrolysis and alkylation. These are normally difficult or impossible to determine because they tend to remain on the column, cause peak tailing, show poor reproducibility, or lose their identity due to the formation of low molecular weight fragments. Thus, for example, methyl esters of polybasic acids and long-chain fatty acids and methyl ethers of polyhydric alcohols are formed from alkyd resins with TMAH, whereas in conventional PyGC, aldehydes are formed from polyols, and alkanes and alkenes from fatty acids (Figure 6). 

The sequence of chapters and topics in Organic Chemistry do not differ markedly from that of other organic chemistry textbooks. Indeed, the topics are presented in the traditional order, based on functional groups (alkenes, alkynes, alcohols, ethers, aldehydes and ketones, carboxylic acid derivatives, etc.). Despite this traditional order, a strong emphasis is placed on mechanisms, with a focus on pattern recognition to illustrate the similarities between reactions that would otherwise appear unrelated (for example, ketal formation and enamine formation, which are mechanistically quite similar). No shortcuts were taken in any of the mechanisms, and all steps are clearly illustrated, including all proton transfer steps. 

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