Dienes into carbonyl compounds

Bivalent Organo-tin and -lead Derivatives. Photolysis of dialkyltin(iv) oligomers at room temperature produces, via intermediate oligomeric diradicals, dialkyl-stannylenes, which insert into 8n—H, 8n—C, and 8n—8n bonds and react with dienes and carbonyl compounds. Dialkylstannylenes are also produced by the thermolysis of XR2Sn8nR2X (X = C1 or H) at 120—130 C. The stannylenes so... 

Diene carboxylates can be prepared by the reaction of alkenyl halides with acrylates[34]. For example, pellitorine (30) is prepared by the reaction of I-heptenyl iodide (29) with an acrylate[35]. Enol triflates are reactive pseudo-halides derived from carbonyl compounds, and are utilized extensively for novel transformations. The 3,5-dien-3-ol triflate 31 derived from a 4,5-unsaturated 3-keto steroid is converted into the triene 32 by the reaction of methyl acrylate[36]. 

To achieve catalytic enantioselective cycloaddition reactions of carbonyl compounds, coordination of a chiral Lewis acid to the carbonyl functionality is necessary. This coordination activates the substrate and provides the chiral environment that forces the approach of a diene to the substrate from the less sterically hindered face, introducing enantioselectivity into the reaction. 

For the reaction of carbonyl compounds with conjugated dienes two mechanistic pathways have generally been taken into account when Lewis acid-catalyzed reactions are considered ... 

Titanium silicate molecular sieves not only catalyze the oxidation of C=C double bonds but can be successfully employed for the oxidative cleavage of carbon-nitrogen double bonds as well. Tosylhydrazones and imines are oxidized to their corresponding carbonyl compounds (243) (Scheme 19). Similarly, oximes can be cleaved to their corresponding carbonyl compounds (165). The conversion of cyclic dienes into hydroxyl ketones or lactones is a novel reaction reported by Kumar et al. (165) (Scheme 20). Thus, when cyclopentadienes, 1,3-cyclohexadiene, or furan is treated with aqueous H202 in acetone at reflux temperatures for 6 h in the presence of TS-1, the corresponding hydroxyl ketone or lactone is obtained in moderate to good yields (208). 

The mode of reaction of titanacydobutenes with carbonyl compounds is largely dependent on steric factors (Scheme 14.31) [72]. Ketones and aldehydes tend to insert into the titanium—alkyl bond of 2,3-diphenyltitanacydobutene, and homoallylic alcohols 70 are obtained by hydrolysis of the adducts 71 [65a,73]. On the contrary, when dialkyl-substi-tuted titanacydobutenes are employed, the reaction with aldehydes preferentially proceeds through insertion into the titanium—vinyl bond. Thermal decomposition of the adducts 72 affords conjugated dienes 73 with E-stereoselectivity as a result of a concerted retro [4+2] cycloaddition [72]. 

The synthesis of fenozan BO-7 4 involves two key steps, the first of which employs a 4 + 2 cycloaddition of singlet oxygen to the diene 83a122-20 123. This provides the endoper-oxide 83b that can be transformed into the target cis-fused 1,2,4-trioxane by treatment with the Lewis acid, TMSOTf, in the presence of a carbonyl compound. The reaction proceeds by Lewis acid promoted heterolysis of the C—O bond to give an intermediate peroxy allyl cation 83c that is captured by the carbonyl compound (in this case, cyclopen-tanone) to give the product (Scheme 30). A number of different carbonyls have been used in this reaction along with a number of different endoperoxide templates and detailed SAR have been developed (Scheme 30). 

Cyclopropane formation occurs from reactions between diazo compounds and alkenes, catalyzed by a wide variety of transition-metal compounds [7-9], that involve the addition of a carbene entity to a C-C double bond. This transformation is stereospecific and generally occurs with electron-rich alkenes, including substituted olefins, dienes, and vinyl ethers, but not a,(J-unsaturated carbonyl compounds or nitriles [23,24], Relative reactivities portray a highly electrophilic intermediate and an early transition state for cyclopropanation reactions [15,25], accounting in part for the relative difficulty in controlling selectivity. For intermolecular reactions, the formation of geometrical isomers, regioisomers from reactions with dienes, and enantiomers must all be taken into account. 

An alternative procedure is to convert the carbonyl compound into the toluene-p-sulphonylhydrazone,12 followed by reduction with either sodium borohydride in acetic acid,13 or with catecholborane, followed by decomposition of the intermediate with sodium acetate or tetrabutylammonium acetate.14 The former method is illustrated by the conversion of undecan-6-one into undecane (Expt 5.6), and the latter by the conversion of acetophenone into ethylbenzene (Expt 6.4, Method B). A feature of these methods is that with a, / -unsaturated ketones, migration of the carbon-carbon multiple bond occurs thus the tosylhydrazone of isophorone gives 3,3,5-trimethylcyclohex-l-ene, and the tosylhydrazone of oct-3-yn-2-one gives octa-2,3-diene. 

The dialkenylchloroboranes undergo the usual reactions of vinylic boranes, e.g., protonolysis with acetic acid gives olefins, oxidation with alkaline hydrogen peroxide provides the corresponding carbonyl compounds. However, the most useful reactions of these compounds are their ready conversion to the stereochemically pure (E, Z)-1,3-dienes by the Zweifel reaction with I2-NaOH 37>107.108> and into the symmetrical (E,E)- 1,3-dienes, 09 0), mono-olefins 1U) and 1,4-dienes (Chart 10). 

Metallated 1-ethoxy-1,3-dienes 697 and 712, obtained from the corresponding acetals by means of the LICKOR base, have been treated with alkyl halides, epoxides, carbonyl compounds, carbon dioxide and carboxylic esters affording ( )-l-substituted 1-ethoxy-1,3-dienes and, after hydrolysis, a,P-unsaturated carbonyl compounds1007-1010 (Scheme 186). Intermediates 697 and 712 have been transformed into the corresponding vinyl stan-nanes, which were submitted to Stille couplings with iodobenzene and benzoyl chloride823. 

A number of new Ni-catalyzed protocols have been developed recently for the coupling of carbonyl compounds and unsaturated substrates such as alkynes or 1,3-dienes. For instance, a three-component intramolecular coupling (alkyne/aldehyde/hydrosUane) has led to the formation of bicyclic N-heterocycles. A number of similar inter- and intramolecular coupling reactions have been developed into useful synthetic routes (Scheme 27). 

Another isohypsic transformation of special significance involves elimination of H-X elements from allylic derivatives to form 1,3-dienes. Besides being extremely important compounds as monomers, 1,3-dienes occupy a unique position in synthetic practice as components in the Diels-Alder reaction. One of the common routes of synthesis of 1,3-dienes also employs a vinyl Grignard addition to carbonyl compounds as the initial step (Scheme 2.55). Allylic alcohols thus formed can easily undergo 1,2-elimination (in some cases it is preferable first to transform the alcohols into their respective acetates). 

Lewis acid-mediated reactions can be classified into two groups (Fig. 4). In the first (type 1) the complex between substrate and Lewis-acid reagent produces the product. Claisen rearrangement promoted by a Lewis-acid catalyst is a typical example of this type. Some complexes formed between Lewis acids and substrates are, however, stable enough to react with a variety of reagents from outside the system to generate the product (type 2). The Diels-Alder reaction between Lewis acid-activated unsaturated carbonyl compounds and dienes is an example of type 2 reactions. 

Isomerization of perfluoroalkenes can be realized by use of SbFs catalysis [37]. The terminal carbon-carbon bonds of these alkenes are usually moved to the 2-position under the influence of this catalyst (Eq. 19). A further inward shift generally occurs only if H or Cl atoms are present at the 4-position of the alkenes. As a rule, isomerization leads to the predominant formation of trans isomers. Terminal fluorodienes also isomerize exothermally into dienes containing internal double bonds in the presence of SbFj. With a catalytic amount of SbFs, perfluoro-l,4-cyclohexadiene disproportion-ates to hexafluorobenzene and perfluorocyclohexene. SbFs promotes the rearrangement of perfluoroepoxides to carbonyl compounds (Eq. 20) [38]. 

Some hydrogenations can be also carried out under so-called inverse PTC (IPTC) conditions where the function of a PT agent (e.g, cyclodextrin, CD) comprises the transfer of an organic substrate into aqueous phase [44]. Conjugated dienes are reduced with hydrogen to monoolefins in the presence of y9-CD and hydridopentacyanocobaltate anion, generated in situ, in alkaline aqueous solution [45], The same catalytic system is also highly effective for the IPTC reduction of the C=C bond in a,)ff-unsamrated carbonyl compounds [46]. 

Where, as illustrated in the above described example, the Diels-Alder reaction provides avenues into the formation of C-glycosides from furan-derived dienes, the hetero Diels-Alder reaction allows for the direct formation of sugar rings from carbonyl groups. As shown in Scheme 7.5.2, Schmidt, et al.,25 effected reactions between conjugated carbonyl compounds and olefins. The illustrated reaction proceeded in 81% yield giving an adduct which, after further manipulations, was converted to a C-aryl glycoside. 

The catalytic hetero-Diels—Alder reaction is also of particular interest, since it allows a convenient access to prepare six-membered heterocycles. The hetero-Diels—Alder reaction is classified into two groups, as shown in Scheme 136 (a) [4 + 2]-cycloaddition of 1,3-dienes with a carbon-heteroatom or heteroatom-heteroatom double bond198 and (b) [4 + 2]-cycloaddition of a,/Tunsaturated carbonyl compounds with olefins.199... 

The [4 + 2] cycloaddition of chiral dienes to carbonyl dienophiles has been studied extensively. 1-Alkoxydienes l23 were chosen as model compounds, and as the chiral auxiliaries derivatives of D-hexoses D-glucose and D-galactose. Adducts thus obtained are, in fact, disaccharide precursors and can be converted into naturally occurring disaccharides24. 

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