Olefins geometry

R is usually restricted to EWG such as C02F, C02Me, CN, S02Ph etc. and the olefin geometry is usually E. 

Stork-Eschenmoser Hypothesis- Olefin Geometry is preserved in the cyclization reaction, i.e. trans olefin leads to a trans fused ring jucntion A. Eschenmoser HCA 1955, 38, 1890 G. Stork JAGS 1955, 77, 5068... 

In the coupling with vinyl groups, the olefin geometry is usually retained E/Z-isomerization is only rarely observed. 

Because the olefin geometry in compound 9 will most certainly have a bearing on the stereochemical outcome of the hydroboration step, a reliable process for the construction of the trans trisubsti-tuted olefin in 9 must be identified. A priori, the powerful and predictable Wittig reaction28 could be used to construct E u, [3-unsaturated ester 10 from aldehyde 11. Reduction of the ethoxycarbonyl grouping in 10, followed by benzylation of the resulting primary alcohol, would then complete the synthesis of 9. Aldehyde 11 is a known substance that can be prepared from 2-furylacetonitrile (12). 

Rhodium complexes facilitate the reductive cydization of diyne species in good yield, although the product olefin geometry depends on the catalysts used. Moderate yields of -dialkylideneclopentane 169 resulted if a mixture of diyne 146 and trialkylsilane was added to Wilkinson s catalyst ClRh[PPh3]3 (Eq. 33) [101]. If, however, the diyne followed by silane were added to the catalyst, a Diels-Alder derived indane 170 was produced (Eq. 34). Cationic Rh complex, (S-BINAP)Rh(cod) BF4, provides good yields of the Z-dialkylidenecyclopentane derivatives, although in this case, terminal alkynes are not tolerated (Eq. 35) [102]. 

Soon afterwards [123] the bromo- (105) and iodo- (106) analogs of chlorovulone I (100) were also isolated from C. viridis in exceptionally low yield (ca. 0.01% of lipid extract). Their structures were established principally by spectroscopic means in comparison with chlorovulone I. Both of the new compounds possess the same olefin geometries as found in clavulone I and chlorovulone I. R Stereochemistry at Cl 2 was established in 105 by comparisons of CD spectra with those of chlorovulone I (100). These new halogenated clavulones showed levels of antiproliferative activity and cytotoxicity comparable to those of chlorovulone I (100). 

Meanwhile, the olefinic geometry is completely controlled by selection of suitable ligands (PPh3 vs. heterocyclic carbene) and nickel sources (Scheme 86). 

Stereoinduction was observed, as in the formation of 74 (Equation (46)) as a single diastereomer 1,3-stereo-induction was not successful. Most substrates contained only methyl-substituted olefins, leading to terminal alkenes. In the case of the cycloisomerization of an //-propyl-substituted enyne, a modicum of selectivity with respect to olefin geometry was exhibited 73 was produced in an isomeric ratio of 1 3.5. The authors do not specify whether the (E)- or (Z)-geometry was preferred. 

The tetraene precursor 14, assembled in a similar way to 11, underwent smooth cyclization using the ruthenium initiator 3 (0.1 equiv) to give macrolactone 15, again in good yield and with complete E-selectivity. Despite the incorrect olefin geometry, transformation into epoxides 16 provided further encour-... 

The vinylcyclopropenes that bear an alkyl group cis to the double bond relieve the strain of the three membered via prototropic shift. Because of the geometric constraints, olefin geometry is controlled (Eq. 99). Olefination of the bifunctional... 

Nerol can also be used as a substrate. The stereochemical outcome is shown in Scheme 1, which indicates that the BINAP-Ru species differentiates the C(2) enantiofaces. The C(6)-C(7) double bonds are left intact. Thus, both R and S enantiomers are accessible by either variation of ailylic olefin geometry or choice of handedness of the catalysts. 

The mechanistic hypothesis was tested with experiments involving a pair of substrates differing only in olefin geometry abont the a,[3-unsatnrated ester. If the assumption that proton transfer occnrs faster than the bond rotation of converting C to D is valid then the ( )- and (Z)-isomers are expected to prodnce opposite diastereomers. In the event, ( )-99 provides 42 1 dr while (Z)-99 provides 1 6 dr favoring the opposite diastereomer (Scheme 14). 

In contrast to reactions with vinyl epoxides and palladium catalysts, the reactions with rhodium retain the stereochemistry of the alkene fragment during the reaction [20]. This is illustrated by the reactions of trans-37a/h and cis-37a/b, which give only one product possessing the same olefin geometry as the starting epoxides (Eqs. 4 and 5). The retention of olefin stereochemisty has also been documented in allylic functionalizations with iridium catalysts, indicating that similar modes of action may be present [21, 22]. 

According to a deuterium-labeling experiment, Miyuara s hydroboration is actually a 1,1-addition process with concomitant 1,2-H-shift, rather than a true transaddition. The olefin geometry of the product boronate is presumably determined during an insertion of boron or hydrogen into the a-position of rhodium vinylidene intermediate 43 (Table 9.8). 

The self-immolative 1,3-chirality transfer from C-5 to C-3 the simple diastereoselection observed in the connection of C-2 with C-3 which results from the enol ester olefin geometry and the chair-like transition state of the [3.3]-sigmatropic rearrangement. 

In this case the absolute configuration at (at least) one of the chiral units needs to be determined. This situation is encountered, for example, in enantioselective addition reactions of olefins with known configuration when two stereogenic centers are created and the olefin geometry is translated to the product in a predictable way. For example, the enantioselective vicinal hydroxylation of (ZT)-l,2-disubstituted olefins97 98, such as tert-butyl (ZT)-3-(methoxycar-bonyl)-2-propenylcarbamate (for assignment, see p 439). 

Despite the remarkable success of olefin metathesis catalysts in organic applications, one major challenge that remains is the diastereomeric control of olefin geometry. Olefin stereoselectivity is an issue in all metathesis reactions. However, prior to the widespread use of CM processes, it was only pertinent to the RGM of large rings (>8 carbons) and in the backbone structure of ROMP-derived polymers. 

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