Cyclopropyl aldehydes rearrangement

In order to determine whether the partial suppression of the free-rotor effect was required for the success of the ODPM rearrangement process, the study was extended to the aldehyde 33 [51], When 33 is irradiated (15 min), under similar conditions to those used for 29, the cyclopropyl aldehyde 34, resulting from an ODPM rearrangement, was obtained, as the tran -diastereoisomer, in 90% isolated yield. This result demonstrated clearly that the ODPM reactivity of (3,y-unsaturated aldehydes is not restricted to cyclic compounds, such as 29, but can also be extended to acyclic derivatives. Therefore, the suppression of the free-rotor effect is not essential for the success of the rearrangement and the reaction is probably controlled by both the excitation of the molecule to the TiCtt, -it )... 

To confirm this hypothesis the photoreactivity of aldehyde 23, previously reported as unreactive in the ODPM mode [47], was reinvestigated. The results obtained show that wi-methoxyacetophenone-sensitized irradiation of 23 (2 hr) brings about the formation of the cyclopropyl aldehyde 11, resulting from an ODPM rearrangement, in 57% isolated yield. Further support for the above postulates was obtained from studies of triplet-sensitized irradiation of aldehydes 22, 35, and 36. These substances undergo the ODPM rearrangement to yield the corresponding cyclopropyl aldehydes 37 (82%), 38 (83%) and 39 (96%), respectively [51]. 

The possible competition between ODPM and DPM processes was also studied. Triplet-sensitized irradiation of 44 for 10 min affords the cyclopropyl aldehyde 45 (19%) resulting from an ODPM rearrangement, exclusively. However, under the same conditions, aldehyde 46 yields the cyclopropyl aldehyde 47 (48%), resulting from a DPM rearrangement [51]. 

Oxa-di-Jt-methane rearrangement leads to cyclopropane derivatives, compounds that are otherwise difficult to synthesize. The diphenylenal 88, for example, is converted to the cyclopropyl aldehyde 89 by triplet sensitization (Scheme 6.35).660 The photoproduct can be further transformed to other compounds, for example a diphenylvinylcyclopropane derivative 90. 

Decarbonylation and rearrangement occurs on irradiation of the cyclopropyl aldehyde (564) producing (565) which, together with its 6a-epimer, has been independently synthesised. Methanolic perchloric acid converted the 6)3-methyl ether into the 1(10),5-diene. 

Condensation of the enol ethers of P-dicarbonyl compounds with dimethylsul-phonium methylide generally takes place by attack on the carbonyl group, leading to furans. However, enol ethers derived from P-keto-aldehydes are attacked first at the double bond to give cyclopropanes. These further react at the carbonyl group, the resulting cyclopropyl epoxides rearranging to dihydropyrans (Scheme 1). 

These electrophilic conjunctive reagents require donor reactants. The cyclopropyl-carbinols as precursors to cyclobutanones arise by simple addition of organometallics. For example, the cyclobutanone 47 derives by addition of vinyllithium to 44 followed by rearrangement with aqueous fluoroboric acid 92). In some cases, this route to cyclopropylcarbinols is preferred. Addition of 41 to aldehydes or ketones... 

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" . 

Carbonyl compounds can also act as the nncleophtles in intermolecular processes with 1,6-enynes. Thus, the gold(I)-catalyzed reaction between enynes and aldehydes or acetone gives stereoselectively tricyclic compounds (equation 71). The transformation is mechanistically intriguing, as it proceeds by a rearrangement of the initially formed cyclopropyl gold carbene (the intermediate in the donble-cleavage mechanism), which is then trapped by the carbonyl compound to form the products. 

The BH.i/BF.vOEt2 reagent reduces not only aryl ketones and aldehydes, but also cyclopropyl aryl or dicyclopropyl ketones to the methylene compounds without opening or rearrangement in the cyclopropyl unit (equation 21). ... 

Due to the higher oxidation level cyclopropylcarbinyl-homoallyl rearrangement (see also Section IV.C) of oxycyclopropanes gives, y-unsaturated carbonyl compounds. This feature has been discovered by Julia and coworkers, who reacted certain esters with Grignard reagents or LiAlH4. The corresponding cyclopropyl carbinol opens under acidic conditions to j5,y-unsaturated aldehydes (equation This conversion also works for... 

Cyclopropane participation occurs in the reaction of (193 equation 81) which rearranges exclusively to the aldehyde (194), although the latter is unstable to the reaction conditions. The endo (cyclopropyl) isomer was also examined in this study,and gave mostly polymeric product. 

An alternative and excellent route to oxaspiropentanes 83 and thus of cyclobutanones 84 proceeds through the condensation of cyclopropyldiphenylsulfonium tetrafluoroborate (82) with aldehydes and ketones. Table 2 presents some illustrative examples. With cyclopropyl methyl ketone and benzophenone as substrates, the oxaspiropentanes cannot be isolated but rapidly rearrange to the corresponding cyclobutanones." Here ring expansion is facilitated by the presence of excellent carbenium ion stabilizing groups. 

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