Carbocycle formation aldehydes

As demonstrated by Hoffmann and coworkers, hydroformylation can also be combined with an allylboration and a second hydroformylation, which allows the formation of carbocycles and also heterocycles [213]. A good regioselectivity in favor of the linear aldehyde was obtained by use of the biphephos ligand [214]. Reaction of the allylboronate 6/2-76 having an B-configuration with CO/H2 in the presence of catalytic amounts of Rh(CO)2(acac) and biphephos led to the lactol 6/2-80 via 6/2-77-79 (Scheme 6/2.17). In a separate operation, 6/2-80 was oxidized to give the lactone 6/2-81 using tetrabutyl ammonium perruthenate/N-methylmorpholine N-oxide. 

A proposed mechanism of the bis(allene) cyclization involves the formation of the allyl(stannyl)palladium species 6, which undergoes carbocyclization to give vinyl(stannyl)palladium intermediate 7 (Scheme 36). Reductive elimination and cr-bond metathesis may lead to the formation of the m-pentane derivative and the bicyclic product, respectively. The cyclization of allenic aldehydes catalyzed by a palladium complex was also reported.163... 

Zr-catalyzed enantio-selective intramolecular diene cyclizations with allylic alcohol and ether substrates afford carbocycles bearing quaternary carbon stereogenic centers the unexpected formation of the aldehyde product 19 is noteworthy. 

Another interesting example of a photochemi-cally induced domino process is the combination of the photocyclization of aryl vinyl sulfides with an intramolecular addition as described by Dittami et al. [901 as intermediate a thiocarbonyl ylide can be assumed. The domino-Norrish I-Knoevenagel-allyl-silane cyclization developed by us allows the efficient stereoselective formation of 1,2-trans-subsituted five- and six-membered carbocycles.1911 A photochemical cycloaddition of enamino-aldehydes and enamino-ketones with the intermediate formation of an iminium salt followed by addition to allylsilanes gives access to novel bicyclic heterocy-des. New examples of photochemically induced... 

However, these latter phosphines Id - k also suffer photofragmentations. a-Cleavage occurs under formation of either aldehydes 2d - h, via subsequent H-abstraction of aroyl radicals from solvent (Scheme 3), or hetero- and carbocycle 7 and 8, via cyclization of the corresponding aroyl radicals under neighbouring group participation (Schemes 4 and 5). 

Dehydrogenations, which involve the elimination of hydrogen Ifom organic molecules, lead to compounds containing double bonds, multiple bonds, or aromatic rings. For practical reasons, only the formation of carbon-carbon double bonds, of carbon-nitrogen double bonds in cyclic amines, and of aromatic rings (both carbocyclic and heterocyclic) will be discussed in this chapter. The conversion of alcohols into aldehydes and ketones and of amines into imines and nitriles will be discussed in the chapter Oxidations (Chapter 3). 

Initially, application of this method to the formation of 5-membered carbocycles was reported by Sinay et al. [222]. In their case, a dialdehyde was used as the substrate, and cyclization occurred with a comparable overall yield of carbocyclic cw-diols but with much lower diastere-oselectivity. Later, ladonisi et al. reported the results of the application using keto-aldehydes as the substrates [223]. Compared to Sinay s results, cyclization of the keto-aldehydes occurred with a relatively high stereoselectivity, and it should be suggested that, in particular, electrostatic interactions play an important role in the stereocontrol of this reaction. 

In the field of synthesis of natural macromolecular carbocycles, macrocyclization through formation of a carbon-carbon bond has been intensively investigated. We will give just a few examples the alkylation of carbanions (Fig. 1 la, I lb) the addition of allylstannane to aldehyde (Fig. 11c) and the intramolecular Wittig reaction (Fig. 1 Id). ... 

In 2010, the Sato group disclosed the synthesis of tetrodotoxin 86 starting from d-glucose, in which the Perrier carbocyclization of enol acetate 92 was employed for the construction of the cyclohexane core of tetrodotoxin. 4,6-O-Benzylidene derivative 99 was converted into pyranoside 93 possessing an exo-methylene group at C-3 (Scheme 12.47). m-CPBA oxidation of 93 stereoselectively provided epoxide 183. Alkaline hydrolysis of the epoxide in 183 followed by acetonide formation gave primary alcohol 184. Oxidation of 184 afforded aldehyde 185, which was converted into (Z)-enol acetate 92 by the action of acetic anhydride and potassium carbonate. When enol acetate 92 reacted with Hg(OAc)2, followed by NaCl treatment, the Perrier carbocyclization reaction successfully took place to afford a mixture of... 

Oxime ethers derived from hydroxy aldehydes, upon conversion to their phenyl thionocarbonate esters, undergo radical cyclizations resulting in the formation of carbocycles. For example, an oxime ether obtained from D-glucose is converted into its phenyl thionocarbonate ester at C-5 and, upon heating in benzene in the presence of tributyltin hydride, affords cyclopentanes in 93% yield as a 62 38 mixture of two diastereomers (eq 7). In general, only low to modest stereoselectivity between the newly formed stereocenters is observed in a number of substrates examined. 

The alkaloids imder discussion share a common Corynanthe-type nucleus derived from secologanin. Yohimbine is a carbocyclic variant of ajmalicine, and the enzymes that convert deglycosylated strictosidine to yohimbine have not been identified [124, 125]. Strychnine and bmcine are synthesized from the preakuammicine structure by hydrolysis, decarboxylation, and condensation reactions to aldehyde (Wieland-Gumlich), and subsequently reaction with acetyl-CoA to make a hemiacetal form of aldehyde (Wieland-Gumlich) and strychnine and brucine. Brucine is a dimethoxy form of strychnine. Ellipticine is a representative member of pyrido[4,3-b] carbozole alkaloid, and the formation of ellipticine is from ajmalicine (corynanthean skeleton) [126] (Fig. 20.8). 

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