Cyclopropyl ketones rearrangement

Cycloadduct 212 is converted into the dihydrobenzofuran 213 when treated with boron trifluoride diethyl etherate in good yield <2000JA8155>. The formation of the dihydrobenzofuran proceeds by an initial ring opening followed by a subsequent dehydration and acid-catalyzed cyclopropyl ketone rearrangement (Equation 142). 

Lewis Acid-Catalysed Cyclopropyl Ketone Rearrangement... 

CLOKE - WILSON Cyclopropylkelone Rearrangement Rearrangement o( cyclopropyl ketones or imines to dihydnofurans or dihydropyrroles, thermally, photochemically, or by Lewis adds... 

Photochemical cyclohex-2-enone bicyclo (3.1.0)hexan-2-one rearrangement, 320 Photochemical a-cyclopropyl ketone isomerization, 313... 

Formation of rearranged products in the solvolysis of homopropargyl systems need not involve triple-bond participation and vinyl cations in all instances. Ward and Sherman investigated the formolysis of 4-phenyl-1-butyn-l-yl brosylate, 57 (80). At 80°C in the presence of one equivalent of pyridine, they observed formation of phenyl cyclopropyl ketone, 58, and... 

When two equivalents of pyridine were added to the nmr sample and the probe heated to 80° C, the enol formate 61 decreased and phenyl cyclopropyl ketone 58 appeared at a rate approximately ten times faster than in the previous buffered system. The observation of intermediate 61 and the kinetic results, together with the observed induction periods, are consistent with the idea that some and perhaps all of the rearranged product ketone in the solvolysis of this system arises via double-bond participation in 61 rather than triple-bond participation and a vinyl cation (80). 

Zimmerman(24,a6> has provided strong circumstantial evidence that zwitterionic intermediates can be involved in formation of cyclopropyl ketones from dienones. His approach was to generate the dipolar species via ground state chemistry and show that these rearrange to the photoproduct ... 

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). 

Enones undergo a variety of photorearrangements 333). In contrast to a,P-unsaturated ketones which react in many different ways, p,y-unsaturated ketones undergo the oxadi-7c-methane rearrangement in sensitized irradiations in complete analogy the products formed are cyclopropyl ketones... 

As mentioned earlier (cf. chapter 2.1.1), p,y-enones undergo an 1,3 acyl shift from the Si-state. In sensitized reactions they rearrange to cyclopropyl ketones in the so termed oxadi-pi-methane rearrangement, as illustrated for... 

Direct interception of the initially formed bicyclic oxyallyl zwitterion derived from cyclohexadienones is difficult, due to its facile rearrangement to cyclopropyl ketone. One notable exception is the observation that 4-trichloromethyl-substituted dienone 94 gave solvent adduct 97 when irradiated in methanol, and both 97 and 98 upon irradiation in acidic methanol (Scheme 24)48. It was proposed that 97 arises either from a facile... 

The analogous 2,6-cycloheptadien-l-ones display zwitterion-derived reactivity as well52,53. For example, the parent compound 113, upon irradiation in either acetic acid or t-butanol, gave diastereomeric solvent adducts 114 and 115 (Scheme 28)52a. On the other hand, tetramethoxy derivatives 116 furnished rearranged products 117, which are equivalent to the cyclopropyl ketone 1,4-shift products seen with cyclohexadienones52b. 

Di-TT-methane reactions are not restricted to 1,4-dienes. Other 1,4-unsatu-rated systems, such as p,y-unsaturated ketones, undergo similar rearrangements yielding cyclopropyl ketones in the oxa-di-TT-methane (ODPM) version of the reaction. The first example of the ODPM reaction was reported in 1966, in which irradiation of ketone 3 affords the cyclopropyl ketone 4 [13] (Scheme 3). 

This type of cyclobutanone annelation is feasible with various dibromocyclopropanes. When diaryl ketones are used as electrophiles, the oxaspiropentane-cyclobutanone rearrangement occurs spontaneously, so that the cyclobutanone is obtained directly (equation 63)"° . When 1-bromo-l-lithiocyclopropanes are allowed to react with aldehydes, the formation of cyclopropyl ketones results" . 

Donor-substituted 1-aminomethylcyclopropanes 108 110 and tosylhydrazones of 1-donor-substituted cyclopropyl ketones 111 can undergo ring enlargement to cyclobutanones through deamination. To this purpose, aminomethylcyclopropanes were diazotized with sodium nitrite 108-110 or isopentyl nitrite 109 in acidic medium and tosylhydrazones were decomposed in basic medium.111 The rearrangements proceed via diazonium ions and are especially useful for the construction of bicyclic systems. For examples of these rearrangements see 1,108 2,109 3,109 4,110 5,111 6 and 7.1 1... 

It is beneficial in terms of yield to convert an a,/3-unsaturated ketone into the cyclopropyl ketone using dimethyloxosulfonium methylide. The epoxide, formed as above, is rearranged during chromatography on silica gel. The corresponding pent-2-ene-l,5-diols are also formed, presumably through hydration of the cation, and these may be dehydrated to the pyran with p-toluenesulfonic acid (74JCS(P1)1674). 

The photoequilibrium (114) (118) (see section III-A) and the irreversible rearrangement (118) - (120) contribute additional examples of photochemical isomerizations of cyclopropyl ketones to enones. The triplet nature of these isomerizations of (118) has been demonstrated by complete... 

The TiCU-induced three-component coupling reaction of an a-haloacylsilane, allylsilane and another carbonyl compound gives 48 in good yield. A silyl enol ether intermediate is suggested (equation 31)82. The reaction of a cyclopropyl ketone with allylsilane yields a mixture of skeletal rearranged products83. 

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