Enones vinyl groups

Organoboron reagents ate pariictdarly well suited for 1,4-additions of aryl and vinyl groups to enones. Hayasbi et al. developed a highly enantioselective RliQ)/ BlNAP-catalyzed 1,4-addilion of pbenylbotonic add lo cyclic and acyclic enones [24] fSclieme 7.5) and 1-alkenylpbospbonales [25]. 

The Nazarov cyclization of vinyl aryl ketones involves a disruption of the aromaticity, and therefore, the activation barrier is significantly higher than that of the divinyl ketones. Not surprisingly, the Lewis acid-catalyzed protocols [30] resulted only in decomposition to the enone derived from 46,47, and CO. Pleasingly, however, photolysis [31] readily delivered the desired annulation product 48 in 60 % yield. The photo-Nazarov cyclization reaction of aryl vinyl ketones was first reported by Smith and Agosta. Subsequent mechanistic studies by Leitich and Schaffner revealed the reaction mechanism to be a thermal electrocyclization induced by photolytic enone isomerization. The mildness of these reaction conditions and the selective activation of the enone functional group were key to the success of this reaction. 

Vinyl cuprates.17 Vinyl cuprates can be prepared conveniently by in situ trans-metallation of vinylstannanes, available by hydrostannylation of alkynes, with a cuprate such as (CH3)2Cu(CN)Li2 (equation I). Reagents prepared in this way effect conjugate addition of the vinyl group to an enone with essentially no transfer of the methyl group. 

But this strategy is doomed too as the allylic bromide will almost certainly react at its less hindered end on the vinyl group. However, we might choose the alternative branchpoint disconnection 38a and consider conjugate addition of some vinyl-metal (copper ) derivative to the enone 41 that could be made by some aldol process from the ketone 42 and an enol(ate) of acetone 43. 

Cyclopropyl carbonyl compounds, SbF, promotes condensation of a, -enones with a-diazo carbonyl compounds to form 1,2-disubstituted cyclopropyl carbonyl compounds. BF3 etherate or TiF4 is much less efficient. The reaction is subject to steric hindrance by substituents on the vinyl group. SbFs also promotes isomerization of the initial trans-cis mixture of products to the more stable isomer. ... 

Stereoselective introduction of the angular vinyl grouping. Divinylcopperlilhium is useful for the introduction of a fi- and even an angular S-vinyl substituent. Thus addition of the enone (I) to a solution of 1.3 moleeq. of divinylcopperlilhium in THF at —72" under argon leads to an exothermic reaction with formation of a single / -vinylated... 

A lithium atom may also be replaced by copper to make a cuprate 161. As in the previous example, using pentynyl copper ensures that no vinyl group is wasted. Such cuprates are superior to simple copper derivatives in conjugate addition to give 162 with enones. All these reactions happen with retention at the vinylic carbon. 

These annulations are governed by conformational factors. In 1968 Johnson predicted that the dominant conformation for C(3),C(4)-dialkyl-substituted cyclohex-enones, such as 5, has the C(4) substituent oriented axially because of A-strain (Eq. 7)P In addition, the diene moiety of S can exist in either a transoid or a cisoid conformation, cf. conformations lii and iv, respectively. In simple acyclic 1,3-dienes, the planar transoid conformation is more stable than the cisoid form. However, examination of a Drieding model of iii suggests that nonbonded steric interactions tween the equatorial C(4) methyl substituent and the C(3) vinyl group would force the dienone into a cisoid conformation. This orientation clearly precludes 1,6-nucleophilic attack by the silicate species thus, 1,4-addition occurs to generate a cyclopentane ring. 

Even cyclopropanes lacking a methylene or vinyl group can be involved in formal cycloadditions. Cyclopropyl ketones 11.137 undergo cycloaddition to enones under nickel catalysis with an NHC ligand (Scheme 11.47) in the absence of the enone, their dimerization is observed. The reaction is proposed to proceed via a metallacyclic enolate complex 11.140, perhaps after initial oxidative addition to one of the cyclopropyl C-C bonds. 

Additions to Enones via the Cuprates. ( -(Trimethyl-silyl)vinyllithium undergoes Michael addition to enones, in good yields, in the presence of copper salts (eq 8). The vinyl group can be oxidized to the epoxide, which is then converted to the acyl group. The overall transformation is the addition of an acyl anion equivalent. Q -(Trimethylsilyl)vinyllithium compares favorably with other acyl anion equivalents, particularly with the vinyl ether analogs which do not add well to hindered enones. 

Photochemical reaction of several steroidal P,y-enones have been examined. Thus, the ketones 67 and 70 underwent oxa-di-7i-methane reaction on sensitized irradiation, while 68 and 69 gave ODPM products upon direct irradiation (Scheme 12). The modified steroidal ketone 70 gave the ODPM product 74 on direct irradiation as well It appears that the dihedral angle between the carbonyl and vinyl groups in the enones controls the efficiency of population of the triplet excited state on direct irradiation and, hence 1,3-acyl shift vs. 1,2-acyl shift (oxa-di-Ji-methane) selectivity. ... 

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