Configuration, olefinic

The authors did not report on trisubstituted diphenylphosphine oxides, but apparently, the results are excellent with what is usually a difficult 1,1 unsubstituted pattern about the olefin. The investigation found near perfect selectivity and yields for most of the reported compounds when using 26 as a ligand. However, a very slow reaction was noted for the E olefin 59h while hydrogenation of E configured olefin 59j preceded smoothly, indicating a delicate balance between structure and reactivity. Other anomalous behavior of the substrates 59h-j is that both E- and Z-configured olefins produce the same enantiomer. 

In the case of the (1 )-configured olefinic ketones high conversion rates and enantiomeric excesses were achieved on the analytical and on the semi-preparative scale. With TBADH, which requires a slightly higher reaction temperature, decomposition of the halogenated substrates was observed. In the case of CPCR, which was used as a partially purified enzyme preparation from the parental strain, the formation of up to 50% of the fully saturated alcohols was observed (Scheme 2.2.7.18). Substrate (l )-33 could be converted into enantiopure (S)- and (R)- E)-32, respectively, by recLBADH- and HLADH/CPCR-catalyzed reaction. 

Cis- 13-Vitamin A acid was synthesized by Bestmann and Ermann 2565 starting from the aldehyde synthon 514, one endocyclic double bond of which already exhibits cw-configuration. Olefination of 514 with the ylide 513 affords the 2/7-pyran-2-one derivative 515, which is reduced and hydrolyzed to the cis-13-vitamin A acid 516. Applying the same synthetic technique also the aromatic analogs 517, 518 and 519 are obtained 256) (Scheme 87). 

On the other hand, stabilized ylides react with aldehydes almost exclusively via trans-oxaphosphetanes. Initially, a small portion of the cw-isomer may still be produced. However, all the heterocyclic material isomerizes very rapidly to the fnms-configured, four-membered ring through an especially pronounced stereochemical drift. Only after this point does the [2+2]-cycloreversion start. It leads to triphenylphosphine oxide and an acceptor-substituted fnms-configured olefin. This frara-selectivity can be used, for example, in the C2 extension of aldehydes to /ran.v-con figured aj8-unsaturated esters (Figure 9.11) or in the fnms-selective synthesis of polyenes such as /1-carotene (Figure 9.12). 

An asymmetric variant of this kind of allylic amination, based on their phenylcyclohexanol-derived chiral N-sulfinyl carbamates, was developed by Whitesell et al. (see also Sect. 3.2) (Scheme 34) [85]. After the asymmetric ene reaction with Z-configured olefins (not shown) had occurred, nearly di-astereomerically pure sulfinamides 127 were obtained which were found to be prone to epimerization. Their rapid conversion via O silylation and [2,3]-a rearrangement dehvered the carbamoylated allyhc amines 128 with around 7 1 diastereoselectivity as crystalline compounds that can be recrystallized to enhance their isomeric purity to 95 5. Obviously the imiform absolute configuration at Cl in the ene products 127 was difficult to transfer completely due to the already mentioned ease of epimerization. Unhke the sulfonamides of Delerit (Scheme 33) [84], the carbonyl moiety can easily be cleaved by base treatment. 

Julia-Lythgoe olefmation is probably the most important method for synthesizing acceptor-free, -configured alkenes, starting from an aldehyde and a primary alkylphenyl sulfone. In this two-step procedure, first the sulfone reacts with the aldehyde to form an acetyl-protected alcoholate and second this species undergoes Elcb elimination to afford the desired alkene. (Sylvestre) Julia olefination is a one-step procedure. It also affords -configured olefins from an aldehyde and an alkylsulfone as substrates, but is limited to base-resistant aldehydes. The most advanced variant is (Sylvestre) Julia-Kocienski olefination, which is also a one-step procedure and is applicable to all kinds of aldehydes. The mechanism is shown below. 

Forbidden reactions can be catalytically broken down into steps each of which is allowed. A catalyst can in this way effect the facile interconversion of substrates otherwise restricted to fixed-bonding configurations. This kind of catalytic intervention usually involves intermediate ionic species (e.g., carbonium ions) or intermediates composed of catalyst and substrate in distinctly different bonding configurations. Olefin... 

The study of chiral 1,4-threitol-derived dioxolanes was also extended to a series of medium and large ring olefins. Very high diastereoselectivities were obtained for both (Z)- and ( -configurated olefins in ring sizes of five to sixteen63. 

Other types of hydrocarbons are formed during the petroleum refining process. Important among these are olefins and acetylenes. Olefins are unsaturated hydrocarbons with at least one double bond in the molecular structure, which may be in either an open chain or ring configuration olefins are highly reactive. Acetylenes are also unsaturated and contain at least one triple bond in the molecule. 

Farwick and Helmchen [28] prolonged the alkyl chain of chiral allylamines by a hydroformylation-(Wittig olefination) sequence (Scheme 5.139). Particular attention was paid to the choice of the JV-protective group. As expected in the Wittig olefination step, mainly the Z-configured olefins were formed. After selective removal of only one Af-protective group, the obtained Af-Boc protected methyl esters were converted via a diastereoselective aza-Michael reaction into the corresponding fi-proline derivatives. 

Highest turnover numbers of 400 - 500 h were observed with a ratio of sub-strate/rhodium of 910. Mainly 9- and 10-formyl octadecanoic acid methyl esters were produced. Detailed investigations showed that the cis double bond isomer-izes rapidly to a trans-configured olefin. The latter undergoes hydroformylation only slowly. Below 50 "C, isomerization did not take place. Microwave irradiation could accelerate the hydroformylation of these substrates [24]. 

The asymmetric epoxidation of all four isomeric allylic-homoallylic alcohols of the type 26 and the subsequent hydride reduction of each epoxide to both possible dideoxyheptitols has been reported. " Only three isomers of 26 undergo a diastereoselective epoxidation and it was concluded that the direction of epoxidation for E-alkenes was controlled by the chirality of the allylic alcohol, whereas for Z-configurated olefins the relative stereochemistry between the two alcohols is important. 

The carbenoid intermediate which contains an E-configured olefin gives then intramolecular cyclopropanation of the allylic unit leading to the final... 

---