Bis-allylic systems

The nature of this species may be different in each case. In addition to the o—allyl— w-allyl equilibrium, the various possible configurations of the bis(allyl) system need to be considered (see Section IV,B). 

In 1959, Cope rearrangement was reported for the first time by A. C. Cope, which refers to [3,3]-sigmatropic rearrangement reaction of bis(allyl) system in 1,5-diene under thermal condition. In many cases, Cope rearrangement features high yield and good selectivity and is thus widely applied in organic synthesis (Scheme 4.1) [21-23]. 

Interestingly, the allylation of a stabilized carbon nucleophile has been found to be reversible. Complete isomerization of dimethyl methylmalonate, involving bis-allylic C—C bond cleavage, from a secondary carbon 38 to a primary carbon 39 was observed by treatment with a Pd catalyst for 24 h. The C—C bond cleavage of a monoaliylic system proceeds slowly[40]. 

Similarly, efficient tetracyclization (MeAlCl2-promoted) of the bis-allylic silane/ bis-epoxide 97 constitutes the key step in the synthesis of (+)-a-onocerin. In this case, because of the presence of the bis-allylic silane group, a double bis-annula-tion occurs, with the formation of the ethylene-bridge linked bis-decalin system present in the target compound (Scheme 8.26) [46],... 

Trost and Hachiya [140] studied asymmetric molybdenum-catalyzed alkylations. Interestingly, they noticed that the regioselectivity of this transformation performed with a non-symmetric allylic substrate varied according to the nature of the metal Pd-catalyzed substitutions on aryl-substituted allyl systems led to attack at the less substituted carbon, whereas molybdenum catalysis afforded the more substituted product. They prepared the bis(pyridylamide) ligand 105 (Scheme 55) and synthesized the corresponding Mo-complex from (C2H5 - CN)3Mo(CO)3. With such a catalyst, the allylic... 

Tetramethylene-ethane (TME), or 2,2/-bis-allyl diradical 81, was suggested as an intermediate in the thermal dimerization of allene, as well as in the interconversions of 1,2-dimethylenecyclobutane 82, methylenespiropentane 83, bis-cyclopropylidene 84 and other bicyclic systems (equation 30)45. The isolation of two different isomeric dimethylene cyclobutanes 87 and 88 (in a ca 2 I ratio) after the thermal rearrangement of the deuteriated 1,2-dimethylene cyclobutane 85 suggests that the rearrangement proceeds via a perpendicular tetramethyleneethane diradical (2,2/-bisallyl) 86 (equation 31)45. 

The Corey allylation system based on a chiral bis(sulfonamide) auxiliary was put to use with success in a number of synthetic efforts, including the total synthesis of the anticancer agent leucascandrolide (Scheme 13). Chiral reagent 152 is added to an achiral aldehyde, 3-(/ -methoxybenzyloxy)propanal, affording intermediate 153 in high stereoselectivity. The latter is transformed into a pyranyl aldehyde, which is subjected to a second allylation (this time, a doubly diastereoselective addition) en route to the completion of leucascandrolide. 

The requisite hydroxylamine function for such cyclizations can also be generated from a precursor having a nitro group. This novel route has provided access to hitherto unknown l-hydroxy-6-allyl-, and -6,6-bisallyl-piperazine-2,5-diones (91UP1). The starting material is an W-nitroacetyl amino acid ester that can be either mono-or bis-allylated at the methylene adjacent to the nitro group. Reduction of the N02 to NHOH using zinc/ ammonium chloride, followed by cyclization, leads to the desired products (Scheme 76). Compound (215) is unique in that it possesses a chiral center at C-3 and a quaternary carbon at C-6 on a l-hydroxypiperazine-2,5-dione system. 

Replacement of ligands in C3H5MoCl(CO)2(NCMe)2 by isocyanides has given the substituted products C3H5MoC1(CO)2(CNR)2 (R = alkyl) and C3H5MoC1(CO)(CNBu )3, and the reduced products [MoC1(CNBu )4]2 and m-Mo(CO)2(CNR)4 (R = Me, Et). No rationale for the loss of allyl and allyl chloride in the latter two cases was proposed (206). These reactions are rare examples of the formation of low-oxidation state metal-isocyanide complexes via reductive elimination of allyl or allyl chloride from metal-allyl species. The potential applications of mono-, bis-, and tris-n-allylic systems as precursors to low-oxidation state compounds remain to be explored. Substitution and simultaneous reduction of Mo(SBu )4 also occurred on reaction with CNBu to give Mo(SBu )2(CNBu )4 (207) (see Section IV,D,2). 

As shown in Scheme 5.1, formic acid behaves differently to other carboxylic acids. Expected octadienyl formate is not formed. The reaction with formic acid in the presence of EtjN affords 1,7-octadiene (140) [62 64], The first step is the protonation of bis-Ji-allylpalladium 68 with formic acid to generate 167. Formic acid is a hydride source, ft is known that the Pd hydride, formed by the decarboxylation of palladium formate, attacks the substituted side of 7i-allylpalladium as shown by 167 to form the terminal alkene 140 [65], The regioselective attack of Pd—H at the more substituted side of 7r-allyl systems is covered in Section 4.3.7. 

Another example of a reaction leading to exclusive formation of one metal configuration is (+)-bis(7r-pinenyl)nickel (163), in which the Ni atom is bonded to two 7r-allyl systems. The combination of one 7r-pinenyl system with a Ni atom is a chiral entity, in which the metal is an asymmetric center. The opposite configuration at Ni would result should the... 

Deprotonation of BSMA imines followed by trapping with an electrophile of the intermediate anion has been described (see Section IV.C.4). However, when these imines derive from conjugated carbonyl compounds, attack of the base takes place at the end of the conjugated system to give delocalized bis(allyl) anion that could be alkylated or silylated in the (3-position from the nitrogen.227... 

The scheme shown in Eq. (46) summarizes and explains mechanistically the results so far obtained. Two butadiene molecules may be supposed to react together to form a bis(7r-allyl) C8 chain bonded to the Ni-ligand moiety. This bis(7r-allyl) system exists in two configurations, (XLIII) and (XLIV), from which DVCB and COD are formed. The nature of the ligand in these intermediates seems to be relatively unimportant. 

The interesting behavior own mainly by (V)-EPTM was not confirmed by terpolymers containing 5,6-dimethylene-24ioibomene (X) or 5- 2 -vinyl-3 -butenyl)-2-norbomene (XI), both being model compounds of triene (V). In fact, the first EPTM contains two conju ted double bonds in obligatory dsoid conformation which were found very reactive toward methyl free radicals. The second EPTM has one bis-allylic hydrogen atom in the unsaturated system ... 

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