Trisubstituted alkenes structures

A number of chiral ketones have been developed that are capable of enantiose-lective epoxidation via dioxirane intermediates.104 Scheme 12.13 shows the structures of some chiral ketones that have been used as catalysts for enantioselective epoxidation. The BINAP-derived ketone shown in Entry 1, as well as its halogenated derivatives, have shown good enantioselectivity toward di- and trisubstituted alkenes. 

Borane may react sequentially with 3 mol of alkene to form mono-, di-, and trialk-ylboranes. Both the alkene structure and reaction conditions affect product distribution. Trialkylboranes are usually formed from terminal olefins [Eq. (6.57)] and unhindered disubstituted alkenes such as cyclopentene irrespective of the reactant ratio.340 The reaction cannot be stopped at the mono- or dialkylborane stage. In contrast, hindered disubstituted olefins (e.g., cyclohexene) and trisubstituted alkenes are converted mainly to dialkylboranes [Eq. (6.58)]. Careful control of... 

Even though it is a trisubstituted alkene, owing to its strained structure, 1-methylcyclobutene reacts under mild metathesis conditions 122... 

Condensation of propiolic esters with alkenes.2 Methyl propiolate or tetrolate, when activated by complcxation with the C5H5Fe(CO)2 cation (Fp+), condense with alkenes to form cyclobutenes, 1,3-dienes, and lactones. The type of products formed depend on the structure of the alkene. 1,2-Disubstituted alkenes yield cyclobutenes and 1,3-dienes mainly, whereas 1,1-disubstiluted or trisubstituted alkenes form mainly lactones. 

The hydroxyl group at the allylic position has a significant effect on the syn/anti methyl stereoselectivity [67,68] and the diastereoselectivity [63,64] of the photo-oxygenation ene reaction (see Sec. II.B). To assess the effect of the hydroxyl at the more remote homoallylic position, the reaction of O with the geminal dimethyl trisubstituted homoallylic alcohols (85, 86, 89) and the cis dis-ubstituted 90 was examined in nonpolar solvents [116], The regioselectivity trend was compared with that of the structurally similar trisubstituted alkenes (87, 88, 91) [105], The results are summarized in Table 12. 

One important elastane polymer that can be made by polymerization in a Ziegler-Natta fashion is rubber. Natural rubber is a polymeric terpene (Chapter 51) made from mevalonic acid and has a branched structure with regular trisubstituted alkenes, which are all in the Z-configuration. 

White peach scale. Several scale sex pheromones have now been elucidated each of them possesses an asymmetric center and usually a trisubstituted alkene link within an isoprenoid framework (43). The structure of the white peach scale pheromone, R,Zb-II (Figure 8), lent itself to synthesis with another chiral starting material, namely limonene (44). Selective ozonlysis followed by workup with dimethyl sulfide-methanol provided a ketoacetal, III. Wittig methylenation followed by hydrolytic cleavage of the acetal gave a dienaldehyde, IV. Conversion of the aldehyde via the acid to an amide (45) with enantiomerically pure ot-methylbenzylamine permitted chromatographic assessment of the purity of the diene aldehyde (and the limonene). The required R-isomer of the diene aldehyde was >48% ee. 

A more complex picture was painted in a further study by Rapoport, which indicated that both the mechanism and reactivity sequence are dependent upon the alkene structure and reaction conditions 1,2-disubstituted alkenes (1) reacting via an oxaselenocyclobutane intermediate with a reactivity sequence CH > CH2 > CH3 geminally disubsdtuted alkenes (2) with a reactivity sequence CH > CH2 > CH3 and trisubstituted alkenes (3) with a reactivity sequence CH2 > CH3 > CH, ( )-allylic alcohols being the preferred products as established by Blichi types (2) and (3) reacting via carbenium ion intermediates (4) without four-membered ring closure or by unspecified cyclic transition states. Rapoport s evidence also showed the final step to occur by 5n1 or 5n1 processes and not by 5n2. Monosubstituted alkenes, particularly arylpropenes, commonly react with rearrangement. ... 

Since our direct route to angularly fused triquinanes from cycloaddition of l-(4-pentynyl)cyclopen-tenes is limited to trisubstituted alkenes and simple terminal alkynes, bisnorisocomene, but not iso-comene itself, could be prepared (Scheme 22). However, this limitation is not a factor for most other compounds in this class of natural products, and the steric interactions described earlier worked to our advantage in a diastereocontrolled synthesis of pentalenene (see structure. Scheme 20). The natural product was obtained by subjecting the product of Scheme 14 to the sequence i, Li, NH3, MeOH ii, MeLi, Et20 iii, p-TsOH, benzene, reflux. ... 

If we look closely at Table 6.1 (p. 183), we find that the heats of hydrogenation of alkenes having similar structures are remarkably constant. For monosubsti-tuted alkenes (RCH CH2) the values are very close to 30 kcal/mole for disubsti-tuted alkenes (R2C CH2 or RCH CHR), 28 kcal/mole and for trisubstituted alkenes (R2C CHR), 27 kcal/mole. For a compound containing more than one double bond we might expect a heat of hydrogenation that is the sum of the heats of hydrogenation of the individual double bonds. 

Carbometalation of alkynes is a widely used method for the synthesis of stereodefmed di- and trisubstituted alkenes. These structural features are often encountered in... 

The chiral diol moiety is only rarely present as such in the final compound to be synthesized. Usually, the 1,2-diol unit is further functionalized and sometimes, can be quite difficult to find in the end-product. Nevertheless, a few natural products possess, embedded in their complex structures, an optically active vicinal diol function which has been created using the AD reaction. Such is the case for the alkaloids indicine 2 and intermedine 1, which were obtained by condensation of the nitrogen heterocycle 4 with (-1-)- and (-)-trachelanthic acid 3 and 5 respectively (Scheme 12) [79]. The desired optically active esters 8 and 9 were rapidly accessed by the AD reaction of the trisubstituted alkene 6. Dihydroxyla-tion using the AD-mix-P generated ester 7 in 83% yield and 85% ee whilst AD-mix-a gave the antipode 8 with slightly better enantioselectivity (90% ee). Hydrolysis and recrystallization furnished the optically pure acids 3 and 4 which were further transformed into the desired natural products. 

Ethylene is the most reactive alkene, and the rates of reaction can decrease with the size and number of alkene substituents. However, the rate-limiting step can be the initial oxidative addition of R-X with Pd°, making alkene structure less Important in the overall rate. Successful substitution is general for monosubstituted alkenes and common even with 1,2-disubstituted alkenes. Trisubstituted alkenes are largely useless in the Heck reaction the intermediate R-Pd-X species... 

It is likely that the ( )-alkene selective reactions of anionic ylides are due to equlibration of the betaine lithium halide adduct as discussed earlier. However, the balance is delicate and small structural changes can have surprising consequences. Thus, Corey s stereospecific trisubstituted alkene synthesis via /3-oxido ylides (Table 10) is clearly under dominant kinetic control, even though lithium ion is present and aromatic aldehydes can be used as the substrates (54,55). The only obvious difference between the intermediates of Table 10 and oxido ylide examples such as entry 11 in Table 21 is that the latter must decompose via a disubstituted oxaphosphetane while the stereospecific reactions in Table 10 involve trisubstituted analogues. Apparently, the higher degree of oxaphosphetane substitution favors decomposition relative to equilibration. There are few easy and safe generalizations in this field. Each system must be evaluated in detail before rationales can be recommended. 

The terminal alkene that was previously suggested as a possible structure for A has five sets of non-equivalent hydrogens. The trisubstituted alkene that was suggested has four sets of non-equivalent hydrogens. Hence the only structure from part (a) that is consistent with the nmr spectrum is the tetrasubstituted alkene, 1,2-dimethylcyclopropene ... 

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