Enones as templates

Ammonium ions have also been used as templates for creating pockets capable of stabilizing positively charged transition states. For example, antibodies raised against hapten 77 accelerate the dehydration of P-hydroxy ketone 75 to give enone 76. The ammonium group in 77 served as a surrogate for the protonated alcohol formed in the acid-catalyzed elimination reaction, and evidence obtained with... 

Surfaces. Preference in orientation of the substrates on the surfaces of silica, due to cavity wall-guest interactions, may also lead to some regio- and stereoselectivity. Examples of photoreactions of steriodal enones provided by de Mayo and co-workers illustrate the principles involved [173-175]. In these examples, the surface serves as a template when guest molecules are adsorbed on it. 

The chiral bicyclic enones, levoglucosenone, isolevoglucosenone, and new functionalized L-arabinose enone possess excellent reactivity and functionality. Their properties mid application as convenient precursors in the synthesis of many attractive templates or intermediates of complex natural products are reviewed. These compounds are attracting increasing interest due to their structural rigidity and ability for stereoselective functionalization without protection, deprotection sequences necessary in many synthetic organic methodologies. 

In summary, we have shown that squaric acid, 3-hydroxy-2-methylidene propionic acids, 5-(2-bromoacetyl)pyrroles and enones are useful polymer-bound key intermediates for the synthesis of a large number of different core structures. Squaric acid was used as a fluid template, because cores structures with different ring sizes could be synthesized. All other examples started from linear templates and afforded linear core structures as well as cyclic core structures. The examples shown here demonstrate the advantages of polymer-bound templates for this type of synthesis. This strategy also reduces the optimization time needed for developing the synthetic route for a specific structure because the synthesis of the polymer-bound educt has to be evaluated only once for the variety of core structures derived from this educt. 

Several review articles have dealt with various aspects of cycloaddition reactions. A short review has presented examples to illustrate the use to which tethered alkenes can be put in the synthesis of cyclobutenes. The photochemical (2 + 2)-cycloaddition of enones to ethene has been studied from a theoretical standpoint. A review has highlighted the use of linear templates to control photochemical reactions such as (2 + 2)-photocycloadditions. The stereochemical control of photochemical reactions in clay-intercalated compounds has been discussed. A review has highlighted the interdisciplinary nature of photochemistry. ... 

The examples cited above under cyclic and acyclic transfer , provide ample evidence for the use of sugar nuclei as stereochemical templates. Rather less well utilized are the latent functionalities which may be unveiled, and one of the most valuable of these is the a, P-unsaturated carbonyl system. The most readily available is enone (83) (Scheme 20) however, the alkylated analogues (161) and (162) (Scheme 38) have been prepared (38) as have the congeners (163), (164) and (165) 23). 

As a 4-Alkyl Enone Scaffold. PhS02CHD-epoxide can be used as a template for the construction of enantiopure 4-alkyl cyclohex-2-enones (eq 4). Addition of alkyl groups in an exclusive trans-1,4-fashion effectively transfers chirality from the epoxide to the newly formed carbon-carbon bond etherification is quantitative. Isomerization of the //-methoxy vinyl sulfone to an aUyhc... 

Lewis acid promoted reactions of silicon enolates, /.e., silyl enol ethers and ketene silyl acetals with various electrophiles have yielded a wealth of novel and selective synthetic methods. This combination of reagents has been used in the past to perform such reactions as aldol-condensations with aldehydes and acetals, imine-condensations, conjugate additions to a,P-enones, alkylations, electrophilic aminations, and Diels-Alder/cyclocondensations. Our own interest in this field has involved the use of titanium tetrachloride to promote the reaction of ketene silyl acetals with non-activated imines as an efficient route to P-lactams. This reaction has been applied to the asymmetric synthesis of P-lactams via a chiral imine-titanium tetrachloride template. We have also found that both ketene silyl acetals and vinylketene silyl acetals oxidativelly dimerize or cross-couple, in the presence of titanium tetrachloride to conveniently yield various diesters . Our present study concerns reactions of vinylketene silyl acetals with non-activated imines and vinylimines promoted by titanium and zirconium tetrachlorides. 

Cu(OTf)2/t-Bu-BOX (13)-catalyzed conjugate addition of carbamates to a -hydroxy enones has been demonstrated by Palomo and coworkers (Scheme 17.50) [69]. Conjugate additions of carbamates (235) to a -hydroxy enones (29) bearing aliphatic P-substituents proceed in moderate to good yield with excellent enantioselectivity. However, enones bearing -aryl substituents lack sufficient reactivity. The impact of the a -hydroxy enone template in this methodology bears mention, as an a,P-unsaturated oxazolidinone substrate is completely unreactive under identical conditions. 

Tricyclic templates such as 246 have been used to obtain stereocontrol in the synthesis of important cyclopentanoic natural products [85]. Normally, the endo-face of a cage-like structure is strongly shielded, and all kinds of additions occur from the exo-side. Thus, cuprate addition to enone 246 selectively gives the exoadduct 247. If, however, an additional substituent is introduced onto the exo-face as in the ether 246b or the sulfide 246c, considerable amounts of the endo-adduct 248 are generated. For X= SPH, the endo-selectivity is completely reversed, yielding exclusively compound 248. 

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