Enone addition

Allylstannanes are unreactive toward weakly electrophilic enones such as cyclohexenone, but enone additions have been established with the more electrophilic 2-ethoxycarbonylallylstannane (Equation (108)).290... 

Remarkable solvent effects have been reported for select silyl ketene acetal-enone additions for example, acetonitrile suffices to promote additions in the absence of a Lewis acid.86 Alternatively, addi-... 

An alternative disconnection of the alkoxide requires the addition of a silyllithium reagent to an enone. Addition of stoichiometric base to the alcohol 51 produces an alkoxide 52, but no evidence of Brook rearrangement to generate 53 was found on protonation of the product. However, alkoxide 52 must exist in equilibrium with some of the organolithium 53, since alkylation with a soft electrophile (Mel) produced 54.41 The equilibrium concentration of the organolithium 53 is lessened in this case by the impossibility of O-Li coordination. 

The intramolecular 1,3-cycloaddition of azidoalkyl cyclohexenones occurred smoothly when the temperature range was between 80-110 °C, however, aziridines were obtained from the intermediate dihydrotriazoles il the correct substitution pattern and regiochemical arrangement were present, cf. formation of 13 and 14 vs. IS131,132. The reaction was applied to the formal total synthesis of ( )-desamylperhydrohistrionicotoxin132. Direct nitrene enone addition can not be ruled out when the reaction is performed in refluxing xylene. 

This strategy was recently applied to the synthesis of scopadulcin24 114. They had already made the enone 115 and wished to use it as starting material so means had to be found to bolt ring A onto the bottom left hand side of the enone. Addition of the lithium derivative 117, made from the chloride 116 with the aid of ultrasound (often a help in heterogeneous reactions) to the enone 115 gave the allylic alcohol 118 and this duly rearranged on Cr(VI) oxidation. 

Conjugate addition of O-silyl ketene acetals to enones Addition of 1 -methoxy-1 -(r-butyldimcthylsilyloxy)ethylene to cyclohcxcnone proceeds in low yield when catalyzed by TiCI4 or TiCI4/Ti(0-/ -Pr)4, but is effected in 95% yield when catalyzed by 1.0 M LiCI04 in diethyl ether. 

A trans-annular complex is postulated to rationalize the high stereoselectivity of a cuprate-enone addition in the example of Scheme 3.52. 

Biradical reversion appears to be a much more important process in enone additions to electron-deficient alkenes. Thus, Schuster and co-workers found that, in the reaction of 3-methylcyclohexenone (3-MCH) with Z- and E-l,2-dicyanoethene (maleo- and fumaronitrile), isomerization of the alkenes accompanies formation of cycloadducts. Based upon quantum yields for aU processes and the rate constants for quenching of the enone triplet by these alkenes, Schuster et al. concluded that alkene isomerization occurred by reversion of 1,4-biradical intermediates (i.e., a Schenck-type mechanism) rather than by triplet-triplet energy transfer from the enone to the alkenes, although the latter was a distinct possibility due to the relatively low triplet energies of these particular alkenes.The full significance of biradical reversion as a critical factor in affecting the course of enone photocycloadditions is discussed below. 

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