Olefins, alkyl-substituted trisubstituted

Chiral l,T-diphosphetanylferrocene Et-FerroTANE serves as an effective ligand for the rhodium-catalyzed hydrogenation of y9-aryl- and /9-alkyl-substituted monoamido ita-conates (Eqs. 19 and 20) [54]. The Et-DuPhos-Rh catalyst was utihzed for the asymmetric hydrogenation of the trisubstituted olefin derivative in the preparation of an important intermediate for the drug candoxatril (>99% ee) [110]. 

S l substitution of different compounds by nitronate anions followed by elimination of nitrous acid is a good method for the synthesis of various substituted olefins. A clear example is the base-promoted HN02 elimination from the C-alkylation products formed by the S l reaction between 2-chloromethyl-3-nitroimidazo[l,2-a]pyridine (11) and various aliphatic, cyclic and heterocyclic nitronate anions87, affording new potential pharmacological derivatives with a trisubstituted double bond at the 2-position. A representative example is shown in equation 32. 

More recently this procedure has been expanded to include some trisubstituted olefins (Asveld and Kellog, 1980). The wide applicability of these procedures to the preparation of dioxetanes of diverse structure, and what proved to many workers to be a surprising stability of most alkyl- and alkoxy-substituted dioxetanes, have facilitated their study and contributed to the tremendous amount of activity in the field. To date well over 100 dioxetanes have been prepared and scrutinized with the objective of elucidating the mechanistic details of their remarkable transformation to excited state carbonyl-containing products. 

Bauld and coworkers, especially, developed the analogous Diels-Alder (4 + 2) cycloaddition reactions. These reactions are conveniently catalyzed by tris(4-bromophenyl)aminium hexachloroantimonate (78) or by photosensitization with aromatic nitriles. The radical cation-catalyzed Diels-Alder reaction is far faster than the uncatalyzed one, and leads to some selectivity for attack at the least substituted double bond for the monoene component (Scheme 18, 79 —> 80), but only modest endo selectivity (e- and x-80) [105]. Cross reactions with two dienes proved to be notably less sensitive to inhibition by steric hindrance of alkyl groups substituted on the double bonds than the uncatalyzed reactions, as cyclohexadiene adds detectably even to the trisubstituted double bond of 2-methylhexadiene (82), producing both 83 and 84. Dienes such as 85 react with donor-substituted olefins (86) to principally give the vinylcyclobutene products 87, but they may be thermally rearranged to the cyclohexene product 88 in good yield [105]. Schmittel and coworkers have studied the cation radical catalyzed Diels-Alder addition of both... 

Das et al. have explored the reaction of MBH acetates with unactivated alkyl halides in the presence of Zn in saturated aqueous NH4CI solution to afford stereoselective trisubstituted olefins. The reactions of 3-hydroxy-2-methylene-alkanoates 220 gave (2 )-2-substituted-alk-2-enoates 222 exclusively, whereas the reactions of 3-hydroxy-2-methylene-alkanenitriles 221 afforded (2Z)-2-substituted-alk-2-enenitriles 223 as major products with high ( -selectivity (Scheme 3.88). The methodology was successfully applied to the synthesis of (2 )-2-butyloct-2-enal 224, an alarm pheromone component of the African weaver ant, Oecophylla longinoday ... 

The obvious first test of methylene-bis(oxazolines) 6 was the copper-catalyzed cyclopropanation. Among the various derivatives that we had prepared (6 R = benzyl, c-alkyl, phenyl, tert-hutyl the bulky t rt-butyl-substituted derivative proved to be the most effective ligand, giving similarly high enantiomeric excesses as the semicorrin Ic (R = CMe20H) (Scheme 14) [25]. The same results were obtained by Masamune and coworkers in an independent parallel study [28]. More recently, Lowenthal and Masamune showed that the method works also well for certain trisubstituted and unsymmetrically disubstituted cis-olefins if the di-t rt-butyl-substituted ligand is replaced by structurally modified bis(oxazolines) [32]. 

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