With Acyclic Substituents

Most other reviews have classified coumarins in an arbitrary manner according to whether, for example, particular compounds were derivatives of linear or angular furanocoumarins or pyranocoumarins. In compiling the Tables included in the present review a biogenetically related approach has been adopted which is based firstly upon the number of nuclear oxygen atoms. Within each Table, the entries are presented in the following order — (i) coumarins with acyclic substituents, (ii) dihydrofuranocoumarins, (iii) furanocoumarins, (iv) dihydro-... 

The rearrangement with ring contraction probably is the most important synthetic application of the Favorskii reaction it is for example used in the synthesis of steroids. Yields can vary from good to moderate. As solvents diethyl ether or alcohols are often used. With acyclic a-halo ketones bearing voluminous substituents in a -position, yields can be low a tcrt-butyl substituent will prevent the rearrangement. 

Kuokkanen evaluated a series of constants. Kpeg for substituted diazonium ions with PEG 1000 and found a reaction constant (p = 1.12) comparable to those for complexation with the three crown ethers investigated by Nakazumi et al. (1983), p = 1.18-1.38). It is therefore likely that the host-guest interaction of diazonium ions with acyclic polyethers is basically similar to that with crown ethers. A dual substituent parameter analysis (DSP, see Sec. 8.3) for (Nakazumi et al., 1987)... 

Disubstituted (cyclobutadiene)Fe(CO)3 complexes in which the two substituents are different may exist as enantiomers. Racemic cyclobutadiene carboxylic acids or cyclobutadiene amine complexes of this type have been separated by classical resolution methodology234. These optically active (cyclobutadiene)Fe(CO)3 complexes are stable with respect to racemization at 120°C for 24 h. This stability contrasts with acyclic... 

Systematically increasing the size and lipophilic character of the tertiary amino group by replacing the dimethylamino substituent with acyclic and cyclic amines failed to produce much increase in activity, and the weakly basic pyrazole (11) and triazole (12) were both inactive. However, the N-imidazolyl compound (13) was unexpectedly found to be at least 10-times more potent (RVN pA2 7.61) than the parent dimethylamino derivative (10) and became a template for further elaboration. 

Similar observations are possible with acyclic Pv compounds if the substituents of phosphorus are different and also rather bulky (and more especially if chelation can occur to stabilise the conformers).(1969,110) In (CH3)2NP(0)F(C4H9-f), the methyl groups are non-equivalent with 37(P-N-CH3) of 8-2 and 0-0 Hz.(1965,8) The phosphorus inversion remains the dominating phenomenon here. (1969>in>... 

Stability of both the parent system and the S- oxide is conferred by the presence of electron-withdrawing substituents on the carbon framework, especially at positions 2, 4 and 6, while electron-donating substituents on sulfur also help. Compounds (48), (49) and (50) illustrate these conclusions, as they are air-stable, isolable species (74CL1101). Exactly analogous factors acting on stability are seen with acyclic sulfonium and sulfoxonium ylide compounds. 

More interesting were the results obtained in reactions of 99 with acyclic dienes catalyzed by Eu(fod)3 (-20°C) and TiCl4 (-78°C). The resulting adducts 100 are unstable and underwent spontaneous sulfinyl elimination at room temperature, affording cyclohexadienes. Reactions with dienes lacking substituents at C-1 (butadiene, 2-methyl butadiene and 2,3-dimethyl butadiene) yielded optically pure compounds 101 (Scheme 49). These results indicate that the regio-selectivity and the 7r-facial selectivity of the cycloadditions are complete (only one adduct is formed) under both catalytic conditions. Desulfinylation of 100 is also completely regioselective. 

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