Isoprene monoepoxide

Both rat and mouse liver microsomes are able to catalyse the formation of isoprene monoepoxides (Bogaards et al., 1996). 

Bogaards, J.J.P., Vanekamp, J.C. van Bladeren, P.J. (1996) The biotransformation of isoprene and the two isoprene monoepoxides by human cytochrome P450 enzymes, compared to mouse and rat liver microsomes. Chem-biol. Interact., 102, 169-182... 

When natural amino acids are used as the nucleophile, the alkylation of isoprene monoepoxide exhibits a matched and mismatched phenomenon (Scheme 8E.29). Good control of diastereose-lectivity by the catalyst was observed with the naphthyl ligand 128 [159]. The complementary selectivity could be increased to 12 1 and 14 1 when the ligands used were 71 and ent-71 respectively, albeit with somewhat lower yields (45% and 50%, respectively). 

Scheme 8E.29. Deracemization of isoprene monoepoxide with a-amino ester.

Trost has reported enhanced enantioselectivity in the desymmetrization of mero-biscarbamates in the presence of triethylamine. Under these conditions, high yields (>80%) and enantiomeric excesses (93-99% ee) are obtained. This methodology has been applied to the synthesis of (—)-swainsonine. a-Amino esters have been used as nucleophiles in the reaction with acyclic allylic esters and isoprene monoepoxide, providing access to diastereoselective N-alkylated a-amino esters. By employing the feature ligand, asymmetric palladium(0)-catalyzed cychzation of 2-(tosylamino)phenol with ( -l,4-bis[(methoxycarbonyl)oxy]but-2-ene provides 2-vinylbenzomorpholine in 79% ee. A number of alternative diphosphine ligands were studied and found to be inferior. 

Reaction of isoprene monoepoxide (20) with acetoacetate afforded the allylic alcohol 21. The pheromone 24 was synthesized after second allylation of acetoacetate with 22 to provide 23 [8]. The regioselectivity is controlled by ligands. The 1,4-adducts such as 21 are formed when achiral ligands are used. Interestingly, the same reaction of 20 afforded the 1,2-adduct 25 (64% yield) and the 1,4-adduct 26 in a ratio of 79 16 when Trost L-1 as a bulky chiral ligand (even racemic) was used [10]. 

Similar conditions were apphed to a palladium-catalysed asymmetric aUyhc alkylation, effecting a DKR transformation of isoprene monoepoxide and a... 

Trost and Jiang recently reported on an asymmetric allylic alkylation for the synthesis of the quaternary center of the cyclopentyl core of viridenomycin 158 (Scheme 13.42). Palladium-catalyzed dynamic kinetic asymmetric transformation of racemic isoprene monoepoxide with ketoester 155 furnishes lactol 156 in 71% yield as a mixture of diastereo-mers. The alkylation was performed with chiral ligand 148... 

Reaction of RCHO with 1,3-diene monoepoxides.4 The monoepoxides of butadiene or of isoprene after treatment with CrCl2 and 1A1 (2 l) react with aldehydes to form cis-l,3-diols with a quaternary center at C2. 

Earlier in the chapter you saw the epoxidation of a diene to give a monoepoxide only one of the double bonds reacted. This is quite a usual observation dienes are more nucleophilic than isolated alkenes. This is easy to explain by looking at the relative energy of the HOMO of an alkene and a diene—this discussion is on p. 138 of Chapter 7. Dienes are therefore very susceptible to protonation by add to give a cation. This is what happens when 2-methylbuta-l,3-diene (isoprene) is treated with add. Protonation gives a stable delocalized allylic cation. 

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