Cyclopropyl ketones conformation

The second salt studied for which conformational bias in the enantiodiffer-entiating step was expected to be small was the benzocyclohexadienone derivative 55. Compounds of this type are known to undergo what is formally an oxadi-7i-methane photorearrangement [44], and salt 55 was no exception, affording cyclopropyl ketone 56 upon irradiation in the crystalline state and diazomethane workup [45]. The ee in which this photoproduct was formed at room temperature was 81% at 25% conversion and 71% at 80% conversion. As usual, lowering the temperature raised the ee (at -78 °C, the ee was 87% at 30%... 

Norin (71) and Dauben and collaborators (72-74) have shown that the bond of the cyclopropane ring which is reductively cleaved corresponds to the bond which better overlaps with the n system of the adjacent carbonyl group. Thus, when the cyclopropyl ketone exists preferentially in the cisofd and transold conformations described by the Newman projections 238 and 239, the Cq — Cg bond is always cleaved in preference to the Cq— C3 bond. 

To what aspect of the solid-state environment can the high ees be attributed In general, one can imagine two scenarios a topochemical effect, in which the course of the reaction is governed by steric effects between the reactant and its lattice neighbors, and a conformational effect, in which reactivity is controlled by the conformation of the individual reacting molecules in the crystal. Zimmerman and Flechtner showed that the photoisomerization of cyclopropyl ketone 5 most likely involves rupture of the C1-C2 (or C1-C3) bond followed by C2-C3... 

The above discussions about octant and antioctant behavior of cyclopropyl ketones has revealed that often there exists ambiguities in the interpretation of the CD spectra of these compounds. Therefore, sometimes the discussions of CD spectra of cyclopropyl ketones are a posteriori explanations of the experimental findings rather than a priori predictions of the outcomes of the experiments. This means that, in particular, the antioctant rule is viewed as a regularity and discrepancies have to be explained in terms of deviations from the foundations of that rule (such as conformer populations and/or special electronic situations of the chromophore). 

Apart from the (n, n ) Cotton effect for the cyclopropyl ketones also the (tt, n ) CE can be utilized for the deductions of (absolute) molecular conformations. It has been suggested that there exists a correlation between the sign of the (tc, n ) CE and the molecular structure in a way which is formally dictated by the normal octant rule (Figure 10). Accordingly, 139,140,144-148 should exhibit negative (n, n ) CEs (139,140,147,148) or positive (tc, n ) CEs (144-146), respectively. This, indeed, is observed Kuriyama... 

It has been found that cyclopropyl ketone exhibits a conformer mixture The preferred component has been identified as the bisected s-c/s conformer At the present time, however, it is not clear whether the second conformer (less stable) corresponds to the bisected s-trans form or anti-gauche form. On the other hand, on the basis of a theoretical analysis (on the semiempirical INDO-level) and a proton NMR investigation for the cis-trans isomeric 2-methylcyclopropyl-l (methyl) ketones 15 and 16 it was suggested that the trans compound 15 exists as an equilibrium of the bisected s-cis and s-trans conformers. The s-c/s and s-trans conformers are separated by a rotational barrier of 5.7 kcal mol ... 

Photocyclisation of 8-alkoxy-l,2,3,4-tetrahydro-l-naphthalenones and 4-alkoxy-6,7,8,9-tetrahydro-5H-benzocyclohepten-5-ones gives naphtho[l, 8-bc]furans and cyclohepta[cd]benzofurans respectively, and conformational and substituent effects of 1,5-biradicals in the cyclisation process are discussed." " The same authors also describe substituent effects on the photocyclisation of ethyl 2-(8-oxo-5,6,7,8-tetrahydro-l-naphthyloxy)acetates and ethyl 2-(5-oxo-6,7,8,9-tet-rahydro-5H-benzocyclohepten-4-yloxy)acetates to give naphtho[l,8-bc]furans and cyclohepta[c,d]benzofurans respectively." Also reported are cyclisations involving photogenerated radical cations of unsaturated silyl enol ethers, fragmentation cyclisations of unsaturated ot-cyclopropyl ketones which occur by photoelectron transfer and give polycyclics, and kinetic and theoretical studies of [2+3] cycloadditions of nitrile ylids. These reactions have been studied mechanistically and their synthetic potential investigated. 

A MO study of hydrogen bonding has led to the prediction that a water molecule approaches cyclopropenone in the direction of a non-bonding electron pair on the carbonyl oxygen atom. The conformational analysis and electronic structures of the t-butyl cyclopropyl ketones (1)—(3) have been examined in the framework of the... 

There is already substantial experimental evidence and theoretical justification for electronic interaction between cyclopropyl and carbonyl groups. A charge-transfer band in the vapour-phase u.v. spectra of several cyclopropyl ketones has been detected at ca. 180 nm (s ca. 4000). The circular dichroism of several optically active cyclopropyl ketones, together with conformational analysis based upon n.m.r. spectroscopy results and INDO theory, has been used to extend the octant rule for such compounds. ... 

Chalcone analogs of pyridine ketones (33) and the corresponding cyclopropyl derivatives 34, which are molecules of biological interest, have been studied (74BSF1427 74BSF1442) by NMR, IR, and UV spectroscopy in their isomeric forms. The N,0-trans conformation prevails in the 2- and 3-substituted pyridines and, in derivative 34, coplanarity of the pyridine ring and carbonyl plane is reported (74BSF1442). 

Electron dilfraction (64TL705) and microwave spectroscopy (65JCP647) of cyclopropanecarboxaldehyde exhibit a twofold barrier for internal rotation, and the two conformers possess almost the same energy content in the gas phase. Ab initio MO calculations (83JST(104)1 IS) in the extended b-BlG basis set predict the s-cis form to be more stable than the s-trans, with the transition state located at 0 a 100°. A twofold barrier was also found (65JCP3043) by electron diffraction for cyclopropyl methyl ketone, with the s-trans isomer destabilized by steric interactions with respect to the aldehyde. 

It is clear from the above discussion that each individual base defines its own acidity function. Experience has shown that 0-values for compounds containing the same functional group lie in most cases in a narrow range, possibly within experimental error. There is a recent analysis of the behavior of ketones, which shows, however, a variety of 0-values (see Table 4). The trends in Table 4 are in conformity with these principles since, obviously, an alkyl group has less ability to disperse a positive charge than a cyclopropyl, vinyl or phenyl group. Differences in 0-values between alkyl and phenyl esters or amides are also found (see Table 4), although of smaller... 

---