Allylic compounds transposition

PhS stabilise carbanions, these allyl compounds 12,13, and 15 form anions which can be used in synthesis. This article describes further developments in our work with these two groups, involving both migration and transposition, leading to regiospecific reagents for synthons with normal reactivity or umpolung and their application to natural product synthesis. 

En route to a total synthesis of the anticancer compound FR901464, Koide and coworkers carried out a diastereoselective allylic selenoxide rearrangement upon oxidation of either allyl selenide 251 [Scheme 18.641. Optimization studies using the preformed allyl selenide 251 identified the o-nitrophenyl selenide as an effective aryl substituent and N,N-dimethylaminopyridine as the best selenophilic base additive in the formation of rearrangement product 252 (see top of Scheme 18.641. Reactions were slower and diastereoselectivity, a crucial parameter here, was lower using other bases or with less than 3 equiv of DMAP. Using the optimized conditions, a one-pot method for overall 1,3-allylic alcohol transposition was... 

Allylsilanes react with carbonyl compounds to transfer the allyl group with 1,3-transposition, in the presence of Lewis acids, typically titanium(IV) chloride47. Recently this reaction has been carried out under super-acid catalysis48. Transfer of the allyl group is also induced by tetrabutylammonium fluoride, but in this case reaction takes place regioselectively at the less substituted end of the allyl fragment49. 

Both the palladium- and the nickel-catalysis enables the use of allylic acetate as starting reagent. In the two approaches a zinc compound is evoked as key intermediate, though its formation has been demonstrated only indirectly. In the two methods allylic transposition is observed. The authors have then concluded that these electrochemical allylation reactions closely parallel the chemical allylation reactions involving allylzinc intermediates. 

The authors of this book are not aware of any case, in which a primary allylic alcohol suffers an oxidative transposition with PCC. Such case would be most unlikely, because it would involve an equilibrating pair of allylic chromate ester, in which the less stable minor one would evolve to a carbonyl compound. 

Alkylation of allylnitroalkanes.9 Cyclic allylic nitro compounds react with stabilized carbanions or amines in the presence of Pd(0) at the allylic position without allylic transposition. The reactive intermediate is presumably an allylpalladium(Il) species. 

Scheme 12.6). Stereoselective reduction and chromatographic separation afforded diastereomerically pure derivative 32 in 94% ee. Removal of the silicon protecting group, followed by acetylation of the two secondary alcohols, set the stage for an elegant palladium-catalyzed allylic transposition that provided compound 33 with...

Allylboranes, which are available from organometallics or l droboration, undergo oxidation without aUylic transposition and with retention of stoeochemistry. This qrplies both to regio- and stereo-chemically deHned allylic dialkylboryP and allylic dialkoxyb P compounds (equations 14 and IS). 

The corresponding ferrilactams (156), which have also been the subject of much attention, are available by the nucleophilic substitution of (155) by amines in the presence of a Lewis acid, usually ZnCb. The substitution occurs with allylic transposition,that is, attack at C-3 of (155). In selected cases, compounds of the type (156) have been prepared by oxidative addition of vinylaziridines or m-4-amino-l-butenols with (2). A bridged ferrilactam bonded through C-2 of the allyl (157) unit has been reported recently and was prepared by way of oxidative addition of a cyclic allylic... 

The oxidative rearrangement of allylic alcohols to a -unsaturated ketones or aldehydes is one of the most widely used synthetic reactions in this group, and forms part of a 1,3-carbonyl transposition sequence. Scheme 7 shows this reaction and the related conversion of the allylic alcohol to an a, -epoxy carbonyl compound. Chromate reagents induce some allylic alcohol substrates to undergo a dirMted epoxidation of the alkene without rearrangement, but this reaction is beyond the scope of the present discussion. 

With related compounds, such as allylic acetals, they react with total allylic transposition in good yields to afford the corresponding enol ethers [57 Eq. (41)]. 

The rearrangement can be catalytically brought about with silver and copper salts or mote recently with Pd compounds. a, -Unsaturated carbonyl compounds are also often formed under rearrangement on oxidation of allylic alcohols, especially when they are tertiary. This process can serve to perform a 1,3-transposition of carbonyl groups in a, -unsaturated ketones (Scheme 9). ... 

Heteroatom to hydrogen transpositions in allylic systems (equation 43) can be observed in different reactions, namely Ae protonation of allyl metal compounds, in reductions of allyl heterocompounds and in retro-ene reactions. Except for the last reaction, which proceeds by a cyclic transition state, the problem of regioselectivity restricts the synthetic value of such reactions and it is only in specific cases that this can be overcome. 

Reactions of Enols and Enolate Anions.—Several methods are described for transposition of an oxo-function to the adjacent site. They involve formation of a suitable a-substituted derivative (hydroxymethylene ° or benzylidene ) and subsequent steps which transform the substituent into an isolated oxo-group. Condensations leading to both the 2-hydroxymethylene- and the 2-arylidene-3-oxo-steroids are described for 3-ketones of the 5jS-series, and also of the 5j8,9j5,10a-( retro ) series.Condensations of aromatic aldehydes at C-2 in the 5 -series are unusually slow enolisation towards C-4 is preferred, but steric compression between C-4 and C-6 in 5/3-compounds severely hinders the condensation reaction at C-4, allowing reaction at C-2 via the 2-enol. Reduction of a 21-hydroxymethylene-pregnan-20-one (337) with sodium borohydride afforded the homopregnanediol (338), although reduction of enolised P-dicarbonyl compounds frequently proceeds via elimination to give enones, and thence allylic alcohols. 

Transposition of Allylic Alcohols into Carbonyl Compounds Mediated by Transition Metal Complexes Uma, R. Crevisy, C. Gree, R. Chem. Rev. 2003,103,27. 

Hydrogenolysis. Deoxygenation of l-alkene-3,4-diols with transposition of the double bond is accomplished via the cyclic carbonates. Heating the latter compounds with RuH2(PPhj)4 and HCO NH in THF leads mainly to the ( )-allylic alcohols. Some saturated alcohols are also formed. 

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