Rearrangement, of aldehydes

In an attempt to establish the limits for ODPM reactivity of (B.y-unsaturated aldehydes, we have extended our studies to a series of aldehydes 65, (Scheme 10) which are monosubstituted at C-2. Triplet-sensitized irradiation of 65 leads to the formation of the corresponding cyclopropanecarbaldehydes 66 [59] (Scheme 10). The diphenyl-substituted aldehydes 65b and 65d yield, in addition to the ODPM products, the corresponding alkenes 67a and 67b, resulting from photodecarbonylation. The formation of these alkenes is probably due to stabilization of the radical, formed by allylic cleavage, by diphenyl conjugation. The ODPM rearrangement of aldehydes 65 is diastereoselective, yielding only one diastereoisomer of 66 (Scheme 10). 

Acid-Catalyzed Rearrangements of Aldehydes and Ketones 1 /Alkyl,2/alkyl-interchange, etc. 

A-acyliminium ions (equations 41, 43, 55 and 56) Danilov rearrangement of aldehydes 347 into ketones 349149 (equation 95), the acyloin rearrangement 350 -> 35290 (equation 96) and dienone-phenol rearrangement 353-355150 (equation 97). 

Dehydrogenations, racemizations, exchange of a-protons and rearrangements of aldehydes catalysed by enoate reductase. Depending on the rate of reaction different amounts of enzyme have been used. The racemizations have been studied at pH 7.0, the H/ H-exchange and the dehydrogenations at pH 8.0 (18). 

Some interesting and potentially synthetically useful diastereoselectivities have been observed in the ODPM rearrangement of 2,4-cyclohexadienones incorporating chiral auxiliaries. For example, direct irradiation of coirpound 92 in the presence of NaY zeolite gave a mixture of photoproducts 93 and 94 in a ca. 4 1 ratio (predominant diastereoisomer undefined). The sensitized rearrangement of aldehyde 95 also appears to be diastereoselective in that only the endo-configured photoproduct 96 is obtained, albeit in just 25% yield because of the intervention of other isomerization processes. In related systems, mixtures of exo- and endo-products are typically observed." ... 

The Claisen rearrangement of aldehydes into substituted-2-oxohex-5-enoic acids in a tandem three step, one pot method, water, K COj and under microwave irradiation (50 W) was reported (Quesada and Taylor, 2005). The reaction was competed in 10 min and good to excellent yield (55-93%) was obtained. 

The rearrangements of aldehydes 31 and 32 demonstrated that the ODPM reactivity of P,y-unsaturated aldehydes is not restricted to phenyl-substituted compounds, but can also be extended to systems in which the intermediate biradicals are stabilized by conjugation with a vinyl group. Furthermore, the efficient synthesis of compounds 33 and 34 is of importance because it opens a novel photochemical route to chrysanthemic acid and other cyclopropyl components present in pyrethrins and pyrethroids. In fact, aldehyde 33 can be transformed to irans-chrysanthemic add by simple oxidation. This new s)mthetic route to ecologically benign insectiddes competes with the one previously described by us using the 1-ADPM rearrangement of P,y-unsaturated oxime acetates. ... 

Apart from the thoroughly studied aqueous Diels-Alder reaction, a limited number of other transformations have been reported to benefit considerably from the use of water. These include the aldol condensation , the benzoin condensation , the Baylis-Hillman reaction (tertiary-amine catalysed coupling of aldehydes with acrylic acid derivatives) and pericyclic reactions like the 1,3-dipolar cycloaddition and the Qaisen rearrangement (see below). These reactions have one thing in common a negative volume of activation. This observation has tempted many authors to propose hydrophobic effects as primary cause of ftie observed rate enhancements. 

The 5-oxohexanal 27 is prepared by the following three-step procedure (1) 1,2-addition of allylmagnesium bromide to an a, / -unsaturated aldehyde to give the 3-hydroxy-1,5-diene 25, (2) oxy-Cope rearrangement of 25 to give 26, and (3) palladium catalyzed oxidation to afford 27. The method was applied to the synthesis of A -2-octalone (28), which is difficult to prepare by the Robinson annulation[25]. 

The Pd(0)-catalyzed rearrangement of the iV-allylenamine 800 in CF3CO2H affords the (5, -unsaturated imine 801, which is hydrolyzed to give the 7, 8-unsaturated aldehyde 802[498]. The vinyloxaspirohexane 803 undergoes rearrangement-ring expansion to give the cyclopentanone 804 in the presence of 1 equiv. of p-nitrophenol[499]. 

The PdCli-catalyzed instantaneous rearrangement of A -carbethoxy-S-azabi-cyclo[5.1.0]oct-3-ene (60) takes place at room temperature to give A -car-bethoxy-8-azabicyclo[3.2.1]oct-2-ene (61)[50], The azepine 62 undergoes a smooth skeletal rearrangement to give 63, and the diazepine 64 is converted into the open-chain product[51]. Beckmann fission of the oxime 65 of ketones and aldehydes to give the nitrile 66 is induced by a Pd(0) complex and oxygen [52,53]. 

Retrosynthesis a in Scheme 7,1 corresponds to the Fischer indole synthesis which is the most widely used of all indole syntheses. The Fischer cyclization converts arylhydrazones of aldehydes or ketones into indoles by a process which involves orf/io-substitution via a sigmatropic rearrangement. The rearrangement generates an imine of an o-aminobenzyl ketone which cyclizes and aromatizes by loss of ammonia. 

Cyclopropanes can also be obtained by the reaction of vinyltrialkylborates with aldehydes followed by treatment with phosphoms pentachloride and base (300), and by the rearrangement of 5-substituted alkynyltrialkylborates (308). It is also possible to utilize this approach for the synthesis of five- and six-membered rings (3). Trans-1,4-elimination ia cycHc systems leads to the formation of stereodefined acycHc 1,5-dienes or medium-ring dienes, depending on the starting compound (309). 

Ring expansions of suitable /3-lactams can also be achieved on treatment with base rearrangement of the Af-substituted azetidin-2-ones (133) occurs in the presence of LDA to give (134) (72JA9261). Aminolysis of the /3-lactam epoxide (135) and the aldehyde (137) affords (136) and (138) respectively (81JHC1239). 

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