Ketones, cyclopropyl => conjugated

The initially formed cyclopropyl ketones from the photolysis of cross-conjugated cyclohexadienones are also photolabile. Usually the photochemical rearrangements of cyclopropyl ketones involve cleavage of the cyclopropyl bond which forms part of the cyclopentenone ring followed by either (a) a substituent migration or (b) rearrangement through a spiro intermediate. While the literature in this area is too voluminous to review in... 

The photorearrangement of a dienone was noted<4) as early as 1830 in a study of the sesquiterpene a-santonin (1). However, the structure and stereochemistry of the various photoproducts were not conclusively established until 1965.(6) Upon irradiation in neutral media, a-santonin (1) undergoes rapid rearrangement to the cyclopropyl ketone, lumisantonin (2). However, if the irradiation is not terminated after a short period of time the lumisantonin itself rearranges into a linearly conjugated dienone (3). The dienone (3) can be isolated from the photolysis of either (1) or (2) in benzene or ether. In nucleophilic solvents (alcohol or water) the dienone (3) is also photo-chemically active and is further converted into an ester or an acid (photo-santonic acid) (4). 

The reductive opening of a cyclopropane ring of a conjugated cyclopropyl ketone with lithium in liquid ammonia can be viewed as an overall two electrons reduction which yields the equivalent of a carbanion and an enolate ion (cf. 237). Successive protonations of 237 then gives the reduced ketone. 

The 1-alkoxycyclopropyllithiums 136 and 140 have been added to a variety of conjugated aldehydes and ketones to produce cyclopropyl carbinols such as 141,... 

Cyclopropyl ketones (29) behave as weakly conjugated compounds, with significant orbital overlap between one ty-bond (or sometimes two) of the cyclopropane ring and the jr-orbital of the carbonyl group (p. 242). The three-membered ring is reduced by lithium in liquid ammonia, with rupture of whichever cr-bond most effectively overlaps the carbonyl tt-orbital [280]. The reduction probably involves transfer of an electron to the jS -carbon atom of this cr-bond, so that conjugative overlap can be maintained during a smooth transition to the species (30) which is both a carbanion and a resonance-stabilised enolate radical. A second electron trans-... 

Diazomethane adds across the C=C bonds of suitable conjugated ketones to give pyrazolines [28g], Initial attack of the nucleophilic reagent upon the jS -carbon gives an intermediate 36) which cyclises to form a five-membered ring, in preference to a cyclopropyl ketone (with loss of Ng), presumably because of the greater strain which would be introduced in the latter system. 

Most p.-y-unsaturated ketones rearrange upon triplet sensitization to cyclopropyl ketones via the so-called oxa-di-TT-methane (ODPM) rearrangement, the result of which is an overall 1,2-acyl shift as schematically shown for (1) -+ (2). p,y-Enone arrangements in which the alkene moiety is further conjugated to either phenyl or C—O sites also undergo this transformation upon direct excitation, i.e. via the singlet excited state. 

From the very beginning the octant rule ran into difficulties because several (mostly well-defined) observations did not agree with its predictions. Striking examples where the octant rule is in serious conflict with the observations for a whole class of compounds are provided by the conjugated (a, )5)-cyclopropyl ketones. Here, the predicted signs of the (n, 7T ) CE were often opposite to the experimental findings. Therefore, for these types of molecules as an empirical finding an antioctant rule was assumed to be applicable. 

A different type of homoconjugation is found for the couple 155 and 158. It is visualized by XXXII. Therefore, the treatment of these compounds under the subheading non-conjugated cyclopropyl ketones is only justified by the formal criterion that the carbonyl and cyclopropane moieties are not in a, J -position. 

Conjugative and substituent properties of the cyclopropyl group TABLE 17. Heats of protonation of ketones... 

This concerted ionization and disrotatory ring-opening of cyclopropyl substrates has been an area of continuing interest . It can be prohibited by steric or conjugative interactions . For instance, 1-cyclopropylcyclopropyl chloride (3a) or tosylate (3b) lead to unrearranged solvolysis products, i.e. 1-cyclopropylcyclopropanol (4), and to cyclopropylethyl ketone (5) arising from homoketonization (equation 4). 

The reaction of LiCuMe2 with ) -cyclopropyl a,) -unsaturated ketones (81) gave a 48 52 mixture of the normal conjugate addition product (82) and of the cyclopropane ring-opened product (83) (equation 51) . 

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