Dienes intramolecular alkyl-substituted diene

The intramolecular coupKng of enolethers with enolethers, styrenes, dienes, alkyl-substituted olefins, allylshanes, and vinylshanes was systematically studied by Moeller [90,198-200]. These couplings allow the smooth formation of quaternary carbon atoms in fused bicycKc rings having a cis stereochemistry (Fig. 43) [201, 202]. 

Recent efforts in the area of Friedel-Crafts alkylations have focused on the direct introduction of alkyl groups bearing functionalities, and on intramolecular alkylative cycliz-ations. The cyclization of the 2-substituted thiophene (55) to (56) has been achieved in 65% yield (80JOC3159). Similarly, the 3-substituted thiophene (57) has been cyclized to the isomeric (58). In both cases, detosylation of the product was achieved by treatment with diisobutylaluminum hydride (DIBAL). An elegant bis-cyclization of the diene (59) to the octahydrodibenzothiophene (60) has been reported (70CJC2587). 

Erom a formal point of view the reaction of kinetically stabilized phosphaalkynes with 1,3-dienes should furnish 1-phosphacyclohexa-l,4-dienes. However, at 90°C with alkyl-substituted phosphaalkynes the primarly formed Diels-Alder adducts have not been detected, because the initial [4-f 2] cycloaddition was followed by an ene-reaction and a final intramolecular [4-1-2] cycloaddition to give diphosphatricyclooctenes. 

This method provides the basis for a general and efficient route to alkyl-substituted and heterofunctionalized bicyclo[6.4.0]dodecanes and to bicyclo[5.3.1]undecanes. In several model studies, the nickel-catalyzed intramolecular [4 + 4] cycloaddition of bis-dienes has been applied to the construction of diterpene systems, such as taxol (7), crispolide, and vulgarolide13 1S. Thus, in the case of taxol. both AB- and BC-ring fusion is achieved with high yield and high diastereoselectivity13. 

B.ii. Intramolecular Diene Coupling of Alkyl-Substituted Dienes ... 

The construction of spiro[n,5]dienones by base-catalysed intramolecular alkylation of suitably substituted phenols has now been applied to the synthesis of jS-vetivone (174)," and a novel approach to spiro[4,5]deca-6,9-dien-8-one systems such as (173) and (176) has been found to lie in the copper(i)-halide-catalysed decomposition of phenolic a-diazo-ketones, e.g. (175). ... 

The cationic allene complexes (32) (R, R , R , R = H or Me) undergo rotation about the n -bond with barriers that increase with increasing alkyl substitution of the allene - particularly at the syn-C-3 position. Intramolecular syn-anti isomerization of (32) is a higher-energy process but becomes easier with increasing substitution e.g. AC = 23.1 kcalmol-i (R = R = R = H, R =Me) and AG = 16.3 kcal mol" (R = R = R = R = Me). The mechanism for this isomerization involves a series of 1,2-shifts and a planar intermediate cannot be formed since chirality is retained in optically active j -cyclopentadienyl-( -cyclonona-l,2-diene)dicarbonyliron cation. 

Copper(I)-catalyzed intramolecular [2 + 2]-photocycloaddition of diallyl ethers and homoallyl vinyl ethers provides a new route to 3-oxabicyclo- and 2-oxabicyclo[3.2.0]heptanes, respectively.Different structural variants of diallyl ethers have been investigated. UV irradiation of diaUyl ethers 77a-f in the presence of CuOTf catalyst produces 3-oxabicyclo[3.2.0]heptanes 78a-f in moderate to excellent yields (Scheme 22). The diallyl ethers 77c and 77d having alkyl substitution at the aUyhc position produce exclusively the 2-exo-alkyl-3-oxabicyclo [3.2.0] heptanes 78c and 78d, respectively. The observed stereoselectivity arises through photocycloaddition of the Cu(l)-diene complex 80, which is stericaUy less crowded than the complex 81 with an axial alkyl group. The bicychc ethers 78 can be oxidized smoothly to the lactones 79 with RuO. Cu(l)-catalyzed photocycloaddition of homoallyl vinyl ethers 82 also proceeds smoothly, producing 2-oxabicyclo[3.2.0]heptanes 83 (Scheme 22). 

Nucleophilic attack occurs at C(2) of the diene. The 1,3-cyclohexadiene complex 66 is converted to the homoallyl anionic complex 67 by nucleophilic attack, and the 3-alkyl-1-cyclohexene 68 is obtained by protonation. Insertion of CO to 67 generates the acyl complex 69, and its protonation and reductive elimination afford the aldehyde 70 [20]. Reaction of the butadiene complex 56 with an anion derived from ester 71 under CO atmosphere generates the homoallyl complex 72 and then the acyl complex 73 by CO insertion. The cyclopentanone complex 74 is formed by intramolecular insertion of alkene, and the 3-substituted cyclopentanone 75 is obtained by reductive elimination. The intramolecular version, when applied to the 1,3-cyclohexadiene complex 76 bearing an ester chain at C(5), offers a good synthetic route to the bicyclo[3.3.1]nonane system 78 via intermediate 77 [21]. 

Substituted cyclopentanes.1 The central methylene groups of this diene can undergo substitution by an electrophile followed by intramolecular cyclization to give substituted pentanes. In particular, the reaction with acyl chlorides provide a useful route to l-alkyl-2,5-divinylcyclopentanols. The reaction usually results in dl-and meso-isomers, but it can be stereoselective, as shown in the second example. 

The theimolysis of 2-substituted 2,5-dihydrothiophene 1,1-dioxides leads to ( ) conjugated dienes via cycloieversion followed by the concerted cheletropic extrusion of sulfur dioxide. The thermolysis of the a, -alkylated sulfone (96) gives the intennediate, which loses SO2 to give 1-dodecadien-l-yl acetate (98), a component of the sex pheromone of the red bollwoim moth. Tliis procediue has been extended to the thermolysis of a,3-dialkylated sulfones in order to obtain ( , )-1,4-disubstituted-1,3-dienes (equation 46). Similar processes have been used for the syntheses of alkaloids. The synthesis of an Elaeocarpus alkaloid, elaeokwine A (100), makes use of the retrodiene extrusion of sulfur dioxide to give the 1,3-diene intermediate (99) that is subsequently consumed by an intramolecular imino DA reaction (equation 47). o-Xylylene (102) has been generated by rDA expulsion of SO2 from benzo-fiised 3,6-dihy(ho-l,2-oxathiin 2-oxide (101). ... 

In a similar case, hydrosilylation of a 1,6-diene or 5-en-l-one with tris(trimethylsilyl)silane gives the cyclized products with a preferred as orientation of the alkyl groups7. The intermediate silylalkyl or siloxyalkyl radical, respectively, is trapped intramolecularly to give substituted cyclopentanes. 

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