Unsaturated ketones conformations

The cyclobutanone 105, on thermolysis at 190°C, undergoes a retro-ene reaction yielding the crystalline 0,y-unsaturated ketone 115. This product in the solid regenerates 105 photochemically, in almost quantitative yield, whereas in solution the diketone 116 is the exclusive photoproduct. The ketone 115 is found in the solid to have the conformation 115a, a conformation that results, presumably, from the requirement for the bulky methyl at C-7 to adopt the pseudo-equatorial rather than the pseudoaxial position. This molecule in fact has a shape... 

Reaction with aji-unsaturated ketones and lactonesThe reactivity of a,/ -enones to singlet oxygen depends on the conformation. Systems that exist in s-trans-conformations (e.g., A4-3-ketosteroids) react slowly if at all. However, s-cis-enones react readily. For example, (R)-( + )-pulegone (1) reacts to give the products 2-4. The same products are obtained by oxidation with triphenyl phosphite ozonide (3, 324 325). 

Fig. 26 The elimination of HF from the 0-fluoroketone [68] is catalysed by antibody 1D4, elicited to hapten [70], to form the disfavoured (Z)-olefin [71]. This contrasts with the spontaneous process in which an anti-elimination reaction yields the ( )-<, 0-unsaturated ketone [69], The syn-eclipsed conformation of [70] is shaded.

In conclusion, in the case of 1,4-conjugated additions to a,B-unsaturated ketones, some substrates, in order to avoid steric interaction, react through a boat conformation to give an equatorially substituted product, but when there is no steric interaction, the axial attack through a chair conformation is energetically favored. Both processes are however stereoelectroni-cally controlled. 

The possibility of opening the cyclic unsaturated ketone ring was described for the reaction of (7 )-( + )-puligone 29 with substituted o-PDA 28 and benzodiazepine 30 annelated by the eight-membered ring which has a crown conformation is formed [44] (Scheme 4.9). 

Synthesis, structure and conformational behavior of cyclophanes are of interest. Synthesis of a novel chiral cyclophane consisting of indole as one of the core units was achieved [198]. The first bridge of the cyclophane 273 is formed by a conjugate addition of indole (2) to the unsaturated ketone 269. An intramolecular N-alkylation reaction of 271 resulted in the formation of... 

Steric effects that reduce the coplanarity of the conjugated system reduce the effect of conjugation. In the absence of steric hindrance, a conjugated system will tend toward a planar conformation. Thus, a,/3-unsaturated ketones may exist in s-cis and s-trans conformations. When both forms are present, absorption for each of the forms is observed. The absorption of benzalacetone in CS2 serves as an example both the s-cis and s-trans forms are present at room temperature. 

The photolyses of conformationally fixed 3,y-unsaturated ketones can also be performed in constrained media. This was described for the ODPM rearrangement of bicyclo[2.2.1]heptenone and bicyclo[2.2.2]octenone in MY zeolites [47]. It is remarkable that direct irradiation of the guest-incorporated thallium-Y (TIY) zeolites at 254 nm resulted in higher yields of ODPM products than the triplet-sensitized solution photolyses (Sch. 19). This indicates that the triplet carbonyl species are generated more efficiently in the supercages of the zeolites presumably because of the presence of heavy... 

It is proposed241 that the stereochemistry of photochemical addition of an olefin on to an a/3-unsaturated ketone is determined by the preferred conformation of the excited state of the enone. As a test case in the steroid series, allene adds to cholest-4-en-3-one (299) to give the 4a,5a-adduct (301). The excited enone is considered to have excess negative charge at C-5 (300), which is therefore pyramidal and results in a preference for formation of the product with the 5 a-configuration.241... 

Table 4.1-3 General infrared and Raman characteristics for estimation of the conformation of a, /3-unsaturated ketones and aldehydes (Oelichmann et al., 1982)...

A saturated ketone is obtained when an a, unsaturated ketone is treated with lithium in liquid ammonia, followed by water or a similar protonating species. Most steroidal A -3-ketones and similar compounds give products with the trnns-fused all-chair structure of minimum energy, which led Barton [32] to the generalisation that the more stable product is produced by protonation of an intermediate carbanion (14) in its thermodynamically preferred conformation (15) (see p. 54). Further studies, including the reduction of substituted... 

High quantum and chemical yields of ODPM rearrangements are obtained with substrates in which the ly-enone chromophore is part of a conformationally rigid molecular assembly which at the same time guarantees adequate orbital overlap of the C=C and rr-bonds. Whereas in bicyclic and bridged 3, y-unsaturated ketones these prerequisites are widely met, acyclic p,7-unsaturated ketones usually rearrange inefficiently since other channels of energy dissipation from the triplet state predominate. Exceptions to this rule are substrates in which the C= ir-bond is part of a styrene or an a, -enone moiety (Section 2.6.3.1). In this context it should also be noted that p,7-unsaturated aldehydes, except for one case, are ODPM unreactive (Section 2.6.3.2). 

Ylides can cyclopropanate unsaturated systems which are susceptible to Michael additions, i.e. a,jS-unsaturated ketones, esters, amides, nitriles, sulfones, sulfonamides, and nitro compounds. Enhancement of electron withdrawal from the carbon-carbon double bond facilitates the reaction. The reaction is non-stereospecific. The intermediacy of zwitterions has generally been accepted, and hence the stereochemistry of the product may be predictable on the basis of the stepwise mechanism. Namely, the Michael addition of the ylide will occur predominantly from the less hindered side of the double bond in a given molecule and the subsequent cyclization will take place in the conformation which minimizes the non-bonded repulsions. 

Buxpsiine, isolated from B. sempervirens, is an a, -unsaturated ketone the NMR-spectrum of which disclosed the presence of three olefinic protons. A positive multiple Cotton effect is in conformity with the presence of a Ji -20-keto moiety. These data together with the UV-evidence are consistent with the structure CCCLXVII which was confirmed by the occurrence of an intense peak at m/e 84 in the mass spectrum. 

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