Allenal, rearrangements with

Petrov and coworkers [41] showed that the reaction of dibromides of alkenephos-phonic acids with acetylenic alcohols involved an acetylene-allene rearrangement. The products so formed hydrolyzed easily to the corresponding phosphinic acids. The latter on heterocyclization afforded 2,5-dihydro-l,2-oxaphosphole-2-oxide derivatives (Scheme 11). 

Boisselle, A.P. and Meinhardt, N.A., Acetylene-allene rearrangement reactions of trivalent phosphorus chlorides with a-acetylenic alcohols and glycols,. Org. Chem., 27, 1828, 1962. 

In the case of carboxylic acids [33], ketones [47] or sulfones [49], it was possible to prove a further prototropic rearrangement of allenes 19 with R2 = H yielding the alkynes 20. In other examples, the prototropic isomerization of the triple bond leads to conjugated dienes directly thus the allene of type 19 could only be postulated as an intermediate [50]. Braverman et al. showed that the allenes generated by prototropic isomerization of dipropargylic sulfones or sulfoxides, for example 24, are unstable. They are transferred rapidly via diradical intermediates to polycycles such as 27 [51-53],... 

The Doering-Moore-Skattebol method including a cyclopropylidene-allene rearrangement is often used for the synthesis of allenes. However, the reaction conditions applied are often not compatible with acceptor substituents. One of the rare exceptions is the transformation 76 —> 77 (Scheme 7.11) [122]. The oximes 77 are not accessible by the classical route starting from allenyl ketone and hydroxylamine (see Section 7.3.2). 

Cutting and Parsons described the transformation of acetylenic alcohols 314 into allenyl phenyl thioethers 316 by a two-step procedure (Scheme 8.85) [174], Deprotonation of alkynes 314 with n-butyllithium is followed by addition of phenylsulfenyl chloride, forming sulfenyloxy intermediates which subsequently rearrange to allenic sulfoxides 315. Treatment of allenes 315 with methyllithium results in loss of the sulfoxide moiety to form allenyl sulfides 316 in reasonable yields. 

The oxidative cyclization of vinylallenes need not be directed by a pendant hydroxyl group in order to succeed. The higher reactivity of the allene compared with the exocyclic methylene group in 73 (Eq. 13.23) with monoperphthalic acid leads primarily to the allene oxide which rearranges to cydopentenone 74 [27]. Inevitably some epoxidation of the alkene also takes place during the reaction. When m-CPBA is used as the oxidant, another side reaction is associated with m-chlorobenzoic add-mediated decomposition of the intermediate epoxide. It is possible to overcome this problem by performing the epoxidation in dichloromethane in a two-phase system with aqueous bicarbonate so as to buffer the add [28]. 

In 1998, Saigo et al. [150] discovered the Rh(I)-catalyzed allenic version of the vinylcyclopropane rearrangement. With a subsituent on the cyclopropane ring two different isomers can be formed depending on which of the different allylic C-C bonds in the three-membered ring is broken. Depending on R2, different regioselec-tivites were obtained with 250. Alkyl-substituted substrates lead to 251 and aryl-substituted substrates to 252 (Scheme 15.79) [150]. 

Diastereoselective [2,3] Wittig rearrangement of propynyloxyacetic acids and esters has recently been reported95 and leads to allenic esters with d.r. in excess of 90 10 [(S,M)/(7t,Af)] for the examples shown. 

Further study by Katsuki, McMillen, Hashimoto, and Wang improved the enantioselectivity up to a moderately high level.Wang and co-workers further extended the asymmetric catalysis to the [2,3]-sigmatropic rearrangement of propargyl sulfonium ylide to give allenic products with up to 81% ee (Equation (18))." ... 

Treatment of 3-allyl-3-(2-propynyl)-2,4-pentanedione with methyl-hydrazine to give condensation and propargyl-allene rearrangement [118]. 

Reaction of the optically active allene (205) with bromine leads to the inactive 3-bromodihydropyran (208 X = Br), whereas activity is retained in the reaction with 2,4-dinitrobenzenesulfenyl chloride, giving (208 X = SC6H3(N02)2 Scheme 39) (67JA7001). The different behaviour has been attributed to the varying stabilities of the cyclic intermediates (206 X = Br or SC6H3(N02)2), the latter showing less tendency to rearrange to the achiral allylic cation (207). 

In the course of investigations on allylic and acetylene-allene rearrangements of 3-substituted quinuclidines, it was found that by oxidation and ozonolysis of compounds with functional groups at positions allylic to the double bond, not only the double bonds but also the adjacent carbon-carbon bonds are broken. For example, in the oxidation of 3-hydroxy-3-vinylquinuclidine (119), with potassium permanganate under mild conditions, and in its ozonolysis, qui-nuclidin-3-one (2) is formed along with 3-hydroxy quinuclidine-3-carboxylic acid.161 The positions of double bonds in such systems can be firmly established by NMR spectroscopy, but not by oxidative methods.101... 

Alkenes, perfluoro-cycloadditions to benzyl azide, 60, 35 pyridine imines and ylids, 60, 36 Alkenes, perfluoro-, as heterocyclic precursors, 59, 10 Alkylation, free-radical, of 1,3-dimethyluracil, 55, 227 A-Alkylation, [ 1,2,4]triazolo[ 1,5-u)-pyrimidines, 57, 110 Alkynes, photocycloaddition to uracils, rearrangement with HCNO elimination, 55, 149 Alkynes, ethoxy-, cyclaoddition to hexafluoroacetone azine, 60, 32 Alkynes, perfluoro-, as heterocyclic precursors, 59, 10 Allenes... 

Similarly, 1-halo-l-sulfonylallenes (139) have been prepared by heating in toluene at 80 °C of propargyl esters (138) via [2,3]sigmatropic rearrangement of the latter (Scheme 32) [51]. 1-Bromo-l-sulfonylallenes 139, when treated with bromine, undergo attack on central allenic carbon with formation of intermediate carbenium bromide followed by hydrogen bromide eUmination, and afford stereospecifically the 2,3-dibromo-l-sulfonyl-l,3-dienes 140. 

Interaction of a strong base with nitrosoureas 601 led to the formation of di-azocyclopropanes 603, which proved to be less stable and to decompose via a carbene intermediate (622) (for trapping of 603, see Refs 43,101,174, 606), rearranging to allenes (624)39 3-85,88.141,174,606.607 (Scheme 16). Allene rearrangement was also reported for a cyclopropyl-l,2-bisnitrosourea 5. By the sequence 623 -4 624 allenes could be synthesized in good yields. It was shown that optical activity was transferred from the nitrosourea precursor 601 (central chirality) to the allene product 624 (axial chirality) 5o. 6s. ss... 

Vinji allenes (44) are rearranged with heat or metal catalysis and photosensitized isomerization to produce the vitamin D triene (156—160). 

In the following steps (eq 7), we observed two HCl-catalyzed processes an acetylenic-allene rearrangement (to 10) and an allenic-acetylene rearrangement (to 11). While both 10 and 11 were easily identified by their characteristic NMR signals and could be distilled as a mixture without decomposition, only the final acetylene derivative (11) could be isolated in pure form after fractional distillation. Propargyl bromide reacted similarly with the three-coordinate phosphine, but in this case all of the chemical transformations occurred very rapidly and only the final acetylene phosphoranimine (11) could be isolated. 

We also obtained interesting results in studying the reactivity of acetylenic alcohols with the two-coordinate phosphines (eq 9). Depending on the position of the OH group relative to the triple bond, different types of products are obtained. In all cases, however, the first step of the reaction is addition of the 0-H group to the P=N center to yield the three-coordinate phosphine. In the case of a-acetylenic alcohols, this addition is followed by a rapid acetylene-allene rearrangement and a... 

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