Cyclic enone systems

Another example of a domino allylation reaction was published by Cook and Jarugumili [169]. Fe2(CO)g in combination with PPhj has been identified as a low-cost and environmentally benign catalyst system for the allylation of zinc enolates 158 generated in situ from copper-catalyzed asymmetric conjugate addition reactions. This catalyst system provides the allylated products 159 in modest to good yields at room temperature with unprecedented diastereoselectivity in cyclic enone systems (Scheme 12.74). 

A few interesting examples of allylations of alkynes have been described in the past decade. An intermolecular [2+2+1] carbonylative cocyclization between alkynes 175 and allyl halides 176 shows the versatile use of Ni catalysis for the construction of cyclic enone systems of type 177 (Scheme 12.83) [184]. 

Addition of azide ion to conjugated systems can be carried out by using NaN3 and acetic acid in water (Eq. 10.18).38 Some reactions were very rapid while others took 1 to 3 days to complete. Lewis base was found to catalyze such conjugate additions of azide ion to cyclic enones in water.39... 

Use of TMSCl in combination with HMPA, DMAP, or TMEDA all favored 1,2-addition over 1,4-addition. Sequential a-alkoxyalkylcuprate conjugate addition, enolate trapping with TMSCl, and silyl enol ether alkylation provides a one-pot synthesis of tetrahydrofurans (Scheme 3.35) [129]. Cyclic enones afford as-fused tetrahydrofurans, while acyclic systems give complex mixtures of diastereomers. a-Alkoxyalkylcopper reagents also participate in allylic substitution reactions with ammonium salts [127]. 

An interesting change of reactivity was observed (Scheme 4.44) when Muthu-samy et al. (83) reacted a similar cyclic ylide system with arylidenetetralones such as 184. This reaction did not generate the expected olefinic cycloadduct, but instead resulted in addition across the C=0 of the tetralone to produce a dioxolane 185. This behavior is unusual in that normal reactivity would preclude a reaction with the C=0 system and addition would occur only with the olefin of the enone. 

The intermolecular photocycloaddition of alkenes to cyclic enones was found to afford cis- and trans-fused bicyclic systems. This stereoselectivity and the diastereofacial selectivity of chiral alkenes and/or enones is discussed below. 

The proline-catalyzed conjugate addition of nitroalkanes was further developed by Hanessian and Pham, resulting in enantiomeric excesses up to 93% in the addition of a variety of nitroalkanes to cyclic enones (Scheme 4.14) [27]. In their catalytic system, L-proline (3-7 mol%) was employed together with equimolar amounts (relative to the substrate enones) of trans-2,5-dimethylpiperazine. The latter addi-... 

In conclusion, there have been many reports of the high synthetic potential of the intramolecular aldol reaction in the enantioselective construction of cyclic enones. In particular the proline-catalyzed desymmetrization of triketones has been widely used for formation of optically active bicyclic systems which are versatile building blocks for steroids and other biologically active compounds. 

Fukuzumi and Otera studied the reduction of electron-deficient alkenes in lactones and cyclic enones using Sml2.53 More recently, the use of Hilmersons Sml2 H20-amine system was shown to be effective for the selective reduction of a,p-unsaturated esters and conjugated double and triple bonds (Scheme 4.44).54,55... 

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