Exocyclic olefin

The C-nor-D-homosteroid rearrangement was discovered by Hirschmann and co-workers at the time that Wintersteiner and his collaborators established that the steroid alkaloids jervine and veratramine incorporate a 14 (1312) abeo-nng system. This was the predecessor of the family of simultaneous ring contraction-expansion reactions. Solvolysis of the 12j5-methanesulfonate (144a) gives mainly the kinetic reaction product, the C-nor-D-homo exocyclic olefin (145) along with some 13(17)-ene (146a). 

Several generalities can be formulated regarding selective reduction of polyolefins. Usually the least hindered double bond is hydrogenated pre ferentially (123), and, with steric hindrance about equal, the most strained bond will be reduced first. Exocyclic olefins are reduced more easily than those in the ring (R)-(+ )-Limonene, 190 g, was shaken with W-4 Raney nickel (12 g) under hydrogen at atmospheric pressure. After 31.9 1 of hydrogen had been absorbed, the solution was filtered. Essentially, pure (R)-( -i- )-carvomenthene was obtained in 96% yield (58). 

Macrolactones 77 and/or 78 can be prepared from the reductive cyclisation of ynals 76 in the presence of NHC-nickel complexes (Scheme 5.21) [21], This maaolactonisation occnrs with different selectivity depending on the ligands attached to the nickel. If carbenes snch as IMes or IPr are nsed, the exocyclic olefin 77 is preferentially obtained, however when phosphine ligands are nsed, the endocyclic adducts 78 are preferentially obtained. 

NMR Spectra - The proton NMR spectrum of poly(N-pheny1-3,4-dimethy-lenepyrroline) (VII) had three singlet absorptions at 6 2.56, 4.81 and 7.60 respectively (Figure 10). The integration of these peaks showed a ratio of 4 4 5. The presence of exocyclic olefinic protons was not observed, indicating that 1,4- addition was predominant in the polymerization with little or no 1,2 addition taking place. 

Chakraborty has described the highly diastereoselective. Barrero and his group developed an approach to functionalized six-membered rings with exocyclic olefins from a-oxygenated derivatives of geraniol. The diastereo-selectivity observed is reasonable and thus the method holds promise for natural product synthesis [105]. 

Cyclic 0,y-unsaturated ketones undergo a photochemical rearrangement thought to involve a-cleavage and radical rearrangement. The products of these cyclic ketones are j8,y-unsaturated ketones possessing exocyclic olefinic linkages,<41,4a)... 

A concerted four-center cis addition leads to (52) and a trans adduct a trans addition, possibly via protonium species, leads to (53) and a cis adduct a stepwise cationic addition leads to (54) and a mixture of cis and trans adducts. Recent studies by Marshall and Wurth strongly indicate that intermediate (54) is correct. Irradiation of octalin (55) in aqueous /-butyl alcohol (DaO)-xylene results in formation of the equatorially deuterated alcohols (56) and (57) and the equatorially deuterated exocyclic olefin (58) ... 

The selectivity in the Heck reaction of allylic alcohol 111 is interesting, and the factors that lead to the observed preference for (3-hydride elimination toward nitrogen in this system are unclear, although a combination of steric effects and stereoelectronic factors (i.e., alignment of C-H and C-Pd bonds, nN a c H interactions) is likely involved. Examination of related examples from the literature (Scheme 4.20) reveals no clear trend. Rawal and Michoud examined substrate 115, which lacks the influence of both the amine and hydroxyl substituents and also seems to favor (3-hydride elimination within the six-membered ring over formation of the exocyclic olefin under standard Heck conditions [18a]. However, under... 

A rhodium-catalyzed allenic Alder ene reaction effectively provides cross-conjugated trienes in very good yields (Scheme 16.70) [77]. The reaction most likely involves ft -hydride elimination of an intermediate rhodium metallacycle to afford an appending olefin and ensuing reductive elimination of a metallohydride species to give the exocyclic olefin. 

In Bettolo and co-workers approach to (+)-methyl trachyloban-18-oate (16), enone 13 was subjected to a photocycloaddition with 1,2-propadiene (1) to afford the [2 + 2]-cycloadduct 14 as a single product in 67% yield (Scheme 19.3) [5]. The addition proceeded exclusively from the /3-face. The resulting exocyclic olefin was eventually converted to a ketone using osmium tetroxide and NaI04 and taken on to 15, constituting a formal total synthesis of 16. 

A particularly clear example is shown in Fig. 8, the hydrogenation of 4-ler<-butylmethylenecyclohexane (64). In this instance, the most likely initial product of isomerization is the endocyclic isomer, 4-leri-butyl-l-methylcyclohexene, which would yield mainly trans-4-tert-butyl-l-methylcyclohexane. Further, the exocyclic olefin, the original substrate, is reduced selectively in competition with its endocyclic isomer so that the latter cannot he an intermediate in the hydrogenation of the former. 

Compound D was transformed into the exocyclic olefin E by a oxidation-elimination procedure. 

Similar results are observed in the intramolecular double silylation of alkynes.59cd Both terminal and internal alkynes lead to exocyclic olefin formation. Interestingly, the reaction is not successful for internal alkynes tethered to a disilanyl functionality by a four atom chain, but is accomplished when four atoms link a disilanyl to a terminal alkyne. Internal alkynes with ester or olefin in conjugation with the C-C triple bond undergo chemoselective double silylation, and alkynes with substituents in the tether are also good substrates for the reaction. 

Reaction of p-nitrobenzenesulfonyl azide with alkylidenecycloalkanes 22-25187 does not yield isolable triazolines as expected, but the reaction products derived from alkenes 22 and 23 suggest a single triazoline intermediate, whereas in the case of tetrasubstituted derivative 24 both possible reaction modes are present,187 owing to weak double bond dissymmetry. Product analysis from 25 indicates some conflict between electronic and steric control in the addition, but provides evidence that electronic factors are much more important than steric effects in controlling regioselectivity.187 Reaction of the exocyclic olefins 22 and 23 appears to be controlled more by the interaction of the LUMO of the azide and the HOMO of the alkene.187 p-Nitrobenzenesulfonyl azide is reported to react with members of the novel... 

Similarly, spirotriazoline intermediates from the addition of p-nitrobenzenesulfonyl azide to exocyclic olefins 22 and 23 give a single ring-expanded sulfonimide (97),18 7 190-19 2 whereas those derived from 24 yield two sulfonimides, 97 from ring expansion and 98 from methyl migration.187 Four... 

Reductions of imines in similar indole alkaloids under neutral conditions often result in the cleavage of C3-C7 bond and acidic conditions solved this problem. On the other hand, reduction of 15 under acidic conditions proceeded by elimination of the "OSEM" moiety to give C16-C17 exocyclic olefin. After testing numerous neutral or acidic conditions, we determined that treatment with 5 equiv of TiCfi at -78 °C before the addition of NaBHjCN effectively prevented the ring opening... 

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