Allenes groups

Allenic groups — Neoxanthin, a xanthophyll found in many foods, has an allenic group at the C-6,7,8 position where the two double bonds are perpendicular to each other, and the C-7,8 double bond coplanar with the polyene chain contributing effectively to the chromophore since the C-6,7 bond is in a different plane, it makes no contribution. Therefore, neoxanthin, despite its 10 conjugated double bonds, has a UV-Vis spectrum similar to that of a conjugated nonaene such as violaxanthin. 

Weinreb86 has reported the Alder-ene cyclization of enallenes under thermal conditions (Equation (85)). Varying the substitution pattern of alkene and allene groups had little effect on the yield of cyclized product. One exception was a,/ -unsaturated ester 130(Equation (86)) cycloisomerization under thermal conditions led to the formation of the Alder-ene product 131 and the unexpected hetero-Diels-Alder product 132 in a 3 1 ratio. 

C-NMR spectrum, only the Ru(3) carbonyl groups are not affected by proton decoupling 102). An intermediate isomeric form of the complex HRu3(CO)9(C6H9) has also been isolated, and the X-ray structure (Fig. 13) establishes the coordination of a ir-allene group 103). This isomer converts to the 7r-allyl derivative on warming. 7r-Allyl derivatives may... 

From the chapters of this book it is evident that the allene group is more versatile than ever before and that the developments in new reactions are still proceeding at a high rate, such that we are not witnessing the decreasing slide after a peak of scientific interest, but rather a continuous development. 

Furthermore, this protocol can be employed for the highly efficient introduction of two (159) and even three allene entities (161) into an aromatic workbench (Scheme 2.51). Thus, by starting with two different halides, e.g. 162 (or with identical halides in different positions of a heteroaromatic substrate), two diverse allenic groups can be introduced by sequential coupling reactions. Furthermore, a structurally different bisallene 166 was also assembled via a twofold coupling of the bispro-pargyl bromide 165 with the functionalized aryl iodide 164 [85],... 

Allene (1) and its alkyl and aryl derivatives have long been used in organic synthesis, especially in cycloaddition reactions, whether these are thermally [5] or photochemi-cally induced and involve metal catalysis or polar reagents [2], Potentially more interesting derivatives arise when the allene group is connected with other unsaturated building blocks as shown in Scheme 5.1. 

A direct connection between the allene group and another unsaturated functional group is no prerequisite for interesting chemical behavior, as illustrated for hydrocarbons 28-31 in Scheme 5.3, in which the allene unit and the above four substituents are connected either by a methylene group or by an ethano bridge, as for allenes 32-35. 

In the second example, one carbon atom of the future allene group is contributed by the sulfoxide 72, which is first converted to the ketone 74 by LDA treatment followed by quenching of the resulting carbanion with methyl 3-phenylpropanoate (73). To convert the now complete carbon framework into a central allene unit, the enol triflate of 74, intermediate 75, is produced under the conditions given in the scheme. Ultimate butyllithium treatment of 75 then furnishes the alkylated allene 76 [23],... 

To prepare the cross-conjugated isomer 3-vinyl-l,2,4-pentatriene (4), the route summarized in Scheme 5.30 was taken [76]. Starting with the protected bishomoal-lyl alcohol 205, its double bond was first extended to an allene group by the DMS method. Hydrolysis and esterification of the resulting diol then provided the bisace-tate 206, which was pyrolyzed at 550 °C under flash vacuum conditions. 

The reaction sequence to the latter hydrocarbons is the most flexible one and starts from the allenic alcohols 212, which are first converted to the l,3-hexadien-5-ynes 213 by an elimination reaction the allene group is then generated by a pro-pargylic rearrangement initiated by the addition of a Grignard reagent. 

As in the other extensions of vinylallene (2), there are also four ways to connect an additional allene group to 2, of which two apparently have been described in the chemical literature. 

In semicydic allenic hydrocarbons, one of the terminal allene carbon atoms is part of an alicyclic ring system, as illustrated by the general structure 37 in Scheme 5.3. Numerous hydrocarbons of this type are known, some of them carrying more than one allene group, such as in the case of the conjugated bisallenes 127 and 129 (see Scheme 5.17), and many of them are described in the review literature [2] and will not be repeated here. However, since Chapter 6 on cycloallenes does not treat these derivatives, some new developments in this area will be briefly presented, limited to the two cases in which cydopropane rings form the end groups of the allene moiety, i.e. 246 and 249. 

The Pd °-catalyzed dimerization of 2 leads to the Diels-Alder adduct 314 as the main product (81%), for whose formation only one allene group has participated in the dimerization process, in addition to the dimer 315 (14%), in which no allene group has survived in addition, several other dimers are produced in trace amounts [131]. 

Although this route to a,cu-diallenes from terminal bisolefmic precursors has often been used in allene hydrocarbon chemistry, the reaction is actually complex and yields product mixtures containing cyclic and bicyclic hydrocarbons when the spacer between the double bond and the allene group is short (between one and three methylene groups), see (a) Th. Miebach, H. Wiister,... 

The isomerization reactions of alkynes via [2,3]- or [3,3]-sigmatropic rearrangements are probably among the most important methods available for the introduction of the allene group into an organic molecule. The usefulness of this synthetic method [61, 62] and the access to new functionalized allenes [63] have been reviewed recently. 

Most of the more recently described allenic steroids bear an allene group at the 17-position, which was usually formed by an SN2 substitution [106] or reduction [86d] process of a suitable propargylic electrophile. Thus, reduction of the pro-pargylic ether 109 with lithium aluminum hydride followed by deprotection of the silyl ether resulted in the formation of the allenic steroid 110, which irreversibly inhibits the biosynthesis of the insect moulting hormone ecdysone (Scheme 18.35) [107]. 

The allene VCD spectra can be interpreted in terms of the ring current mechanism (114). As shown in 46 and 47, for a conformation that maintains a planar orientation due to conjugation with the allene, for both the phenyl and carboxyl groups relative to the adjacent portion of the allene group, an intramolecular hydrogen bond is possible between the carboxyl hydrogen and a x orbital of the central allene carbon atom. The antisymmetric C=C=C stretch can be de-... 

Homoallyl rearrangements occur in the solvolyses of allenic /j-toluenesulfonates 1 (5-tosy-loxypcnta-1,2-dienes, ethenylidene p-toluenesulfonates). Hydrolysis or acetolysis of acyclic and cyclic compounds indicates that the allene group participates strongly to give allyl cations which rearrange to methylenecyclobutanols 2 (see Houben-Weyl, Vol. 4/4, p63). 

The propargylsilane group can participate in intramolecular cyclization reactions to give a five-membered ring with an exocyclic allene group. An example is the cyclization of 2 to a mixture of 3 and 4 when treated with acetic acid. 1... 

Potassium 3-aminopropylamide [56058-00-7] (KAPA), KNHCH2CH2CH2-NH2, pKa = 35, can be prepared by the reaction of 1,3-diaminopropane and potassium metal or potassium hydride [7695-26-7] (57—59). KAPA powder has been known to explode during storage under nitrogen in a drybox, and is therefore made in situ. KAPA is extremely effective in converting an internal acetylene or allene group to a terminal acetylene (60) (see ACETYLENE-DERIVED chemicals). 

Tables 2.6 through 2.10 (see pp. 75-83) list AH° values for a large number of groups (see Section 9.1 for additivity data for radicals). In these tables, Cd refers to a carbon that is forming a carbon-carbon double bond. The notation Cd-(H)2(Cd) is shortened to Cd-(Ha), since all carbon-carbon double bonds are between two sp2 carbons. Similarly, Ct-(X) refers to a carbon triply bonded to another sp carbon and to an X ligand CB-(X) refers to an aromatic ring carbon bonded to two other ring carbons and to a substituent X and Ca refers to the central carbon of the allenic group C=C=C. Other group abbreviations are noted at the end of the appropriate table.

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