Allene, cyclization

PtCl2 was shown to catalyze a similar Alder-ene transformation, as in the cycloisomerization of allenyne 117 to triene 118 (Equation (76)).78 In the same study, it was noticed that tetrasubstituted allenes cyclized to bicyclic compounds, such as 120 (Scheme 23), under identical PtCl2 conditions, presumably due to A(1,3) strain in intermediate 119. 

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... 

Scheme 4.42 Total synthesis of furanomycin usingthe SN2 -reduction and allene cyclization.

That the thermal isomerizations of these substrates lie completely on the side of the cyclic structures 324 comes as no surprise after what has been said about the energetics of the allene cyclization reactions above [125] the cydoisomerization may also be induced by metal salts, such as cuprous chloride, incidentally [136]. 

In view of this background, it is the aim of this chapter to organize the fundamentals of radical additions to 1,2-dienes and to present its state of the art in organic synthesis. All aspects of enyne allene cyclizations [19, 20] have been omitted since this topic is addressed in Chapter 20. In order to simplify the mechanistic discussion, the positions and Jt-bonds of allenes have been consistently numbered using the nomenclature outlined in Figure 11.1. 

Allene Cyclizations Leading to Dihydrofurans, Furans, Pyrrolines and Pyrroles... 

A rather different allene cyclization that takes place in tandem with an alkylative process has been described by Trost and Urabe [66] (Eq. 13.52). Exposure of ketoal-lene 169 to divinylmethylaluminum 170 in dichloromethane solution leads to 171 in 60% yield. The mechanism of this alkylative cyclization involves complexation of the Lewis acidic aluminum to the carbonyl oxygen atom, followed by ring closure, leading to zwitterion 172. Intramolecular transfer of a vinyl group from aluminum to carbon completes the process. 

Ether was a better solvent in cases where the propargylic position was unsubstituted (methylene group) since, in THF, a competitive [1,2] -Wittig rearrangement took place and led to diminished yields. When applied to the secondary homoallylic propargyl ethers 388, the zinc-ene-allene cyclization afforded the c -2,3,5-trisubstituted tetrahydrofurans 389 with moderate to satisfactory levels of diastereoselectivity, which could be rationalized by the preferential pseudo-equatorial positioning of the homoallylic substituent in the cyclic... 

The synthesis of polysubstituted pyrrolidines has been achieved344 in a diastereo-selective and enantioselective manner via the zinca-ene-allene cyclization. In an extension of this work,345 the zinca-ene-allene reaction of polysubstituted enynes lithiated on the propargylic position has been used to prepare polysubstituted... 

Scheme 61 Preparation of tricyclic P-lactam 175 using amino-allene cyclization (i) TsNCO, THF, RT (ii) Pd (OAc)2 10 mol%, LiBr, Cu (OAc)2, k2co3, o2,...

Scheme 65 Preparation of bicyclic P-lactams 186 using Au-catalyzed oxy-allene cyclization...

Aluminum—tetradentate ligand catalyst system, in epoxide homopolymerization, 11, 601 Aluminum(I) tetrahedra, synthesis, 9, 262 Aluminum(III)-tin exchange, process, 9, 265 Aluminum-transition metal bonds, characteristics, 9, 264 Amavadine, for alkane carboxylations, 10, 234—235 Ambruticin S, via ring-closing diene metathesis, 11, 218 Amide-allenes, cyclizations, 10, 718 Amide ether complexes, with Zr(IV) and Hf(IV), 4, 783 Amide hybrid ligands, in organometallic synthesis, 1, 64 Amides... 

It might, at first glance, come as a surprise that, despite the ban on 5-endo-trig cyclizations, 5-endo-dig cyclizations are allowed. A possible reason for this observation is that intramolecular nucleophilic or electrophilic attack at an alkyne can proceed with the substrate in a flat conformation, because two orthogonal jt (or jt) orbitals are available. Similarly, hydroxymethyl allenes cyclize on treatment with a base (Scheme 9.7), because the terminal Jt orbital of the allene and the hydroxymethyl group can readily achieve a synperiplanar orientation. Intramolecular... 

Gange, D. Magnus, P. An unusual new allene cyclization reaction. Synthesis of dihydro-furan-3(2H)-ones. J. Am. Chem. Soc. 1978, 100, 7746-7747. 

To improve the thermal tandem [3,3]-sigmatropic rearrangement-enyne allene cyclization, Grissom et al.65 focused their attention on the transformation of 2-propynylvinyl ethers to allenes using silver salts as catalysts. The tetrafluoroborate proved to be the most effective, quantitatively yielding the expected aflenyl aldehyde (Scheme 3.42). However, isomerization problems appeared in some cases (see Scheme 3.33). 

A recent paper explored the 1,2-functionalization of the allene moiety in monocyclic P-lactam allenynol derivatives under palladium-catalyzed reaction conditions <02CEJ1719>. Remarkably, a novel domino process, namely allene cyclization/intramolecular Heck reaction... 

PraU, M. Wittkopp, A. Schreiner, P. R. Can fulvenes form from enediynes A systematic high-level computational study on parent and benzannelated enediyne and enyne—allene cyclizations, 7. Phys. Chem. A 2001,105, 9265-9274. 

Singleton carried out a combined experimental/computational/MD study of the enyne-allene cyclization problem. First, he determined that the experimental KIE for the cyclization of 60 is 1.43. This value is smaller than that usually found for concerted ene reactions, where kg/kjj is often greater than... 

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