Cyclopropanone special

Cyclopropanones deserve special comment, not because of their practical importance (they have no commercial value at this time), but because of their novel behavior and reactivity. No unambiguous synthesis of cyclopropanones was known prior to 1965, and the older textbooks usually contained statements such as cyclopropanones apparently cannot exist. However, they had been postulated as intermediates in various reactions (see, for example, the Favorskii rearrangement, Section 17-2C and Exercise 17-15), but until recently had defied isolation and identification. The problem is that the three-ring ketone is remarkably reactive, especially towards nucleophiles. Because of the associated relief of angle strain, nucleophiles readily add to the carbonyl group without the aid of a catalyst and give good yields of adducts from which the cyclopropanone is not easily recovered ... 

Cyclopropanon.es are highly reactive organic systems containing a number of labile sites on a small carbon skeleton. They represent valuable substrates for studying theoretical aspects of the chemistry of small strained ring systems and are of special interest in synthesis because of the variety of transformations in which they take part. 

In other special cases, cyclopropanones have been prepared by photochemical rearrangements. For example, 9,9 -dianthracyl ketone (27) 32> and carbinol (28) 33> undergo ring closure to their respective... 

In connection with the above discussion, formation of 3,3-disubsti-tuted 2 (3 H)-oxepinones (73) in the dye-sensitized photooxygenation of 6,6-disubstituted fulvenes is of special interest. 57>58> The reaction may be pictured in terms of an allene oxide intermediate which, as shown in Scheme 11, isomerizes to a cyclopropanone, followed by intramolecular rearrangement. 

Reactions of cyclopropanones with nucleophiles frequently lead to ring enlargement reactions since the formation of four-membered rings from the reactive intermediates is accompanied by a considerable reduction in strain energy. Thus, 2 reacts with diazomethane to form cyclobutanone96>, with hydrazoic acid to form (3-lactam 76,89) and, under special conditions, with amines and hydroxyl amine derivatives to form N-sub-stituted (3-lactams 87> (Scheme 24). 

The cyclopropanone ring is susceptible to attack by electrophilic reagents other than acids, e.g. bromine, phenol, acid chlorides. Most of these reactions have been observed in cyclopropanone acetals and all proceed by attack at C2 or Cg in a manner analogous to path b, Scheme 28. As shown in Table 18, esters are usually obtained but, under special bro-mination conditions, 1,1-dialkoxycarbonium halides are formed.109) These salts lose ROBr upon warming to — 30 °C to give the expected esters. 

Cyclopropanones act as three-carbon sources in [4 + 3] cycloadditions. However, these small-ring compounds are impractical for preparative scale experiments, because they are generally inaccessible and also must be handled with special precautions, in contrast to other oxyallyl species. One of the early descriptions of the synthesis of cyclopropanones appeared in 1932. NeverAeless, the development of the chemistry of cyclopropanones proceeded at quite a slow pace until the late 1960s when Turro reported their utilization in [4 -i- 3] cycloaddition reactions. 2,2-Dimethylcyclopropanone (14) adds to furan, cyclopentadiene, 6,6-dimethylfulvene and the relatively nucleophilic N-methylpyrrole to give the corresponding cycloproducts, but fails to react with anthracene and 1,3-butadiene. Parent cyclopropan-one cannot be used. 

Dehalogenations involving substrates with an sp carbon atom at C2 deserve special attention. 1,3-Dihalo ketones can, in principle, give extremely unstable cyclopropanones. The corresponding acetals may, however, be stable, therefore acetals of 1,3-dihalo ketones will also be discussed here. 

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