Polyenes, diastereomers

Similar schemes can be developed easily for analogous reactions of acceptor-substituted polyenes. For example, a triene with an acceptor group in 1-position can form six regioi-someric products of Michael addition and electrophilic capture, and each of these exists as E/Z stereoisomers, diastereomers and/or enantiomers. Thus, reactions of this type are only useful if both the regio- and stereoselectivity can be controlled fortunately, only one isomeric Michael adduct is formed in many cases. This is true in particular for polyunsaturated Michael acceptors which bear at least one triple bond besides one or more double bonds. An additional feature of the latter substrate type is that nucleophilic additions can... 

In a cycloaddition reaction, the two active n systems may approach each other in either of two orientations, for example, head to head or head to tail. If one combination dominates, the reaction is said to be regioselective. In the course of the reaction, 4 new saturated centers are formed. With maximum labeling, a total of 16 (=24) stereo-isomeric forms, consisting of 8 enantiomeric pairs of diastereomers if neither polyene is chiral, may be formed. In pericyclic reactions, the stereochemistry is determined by specifying the stereochemical mode in which each component reacts. Each of the two... 

Fluorosulfuric acid is a widely used, highly effective reagent808 in electrophilic polyene cyclizations to synthesize fully cyclized compounds in a selective and stereospecific way. Furthermore, an internal nucleophile, most often oxygen, allows the construction of polycyclic skeletons with a heteroatom (vide supra). Snowden and co-workers809 transformed trienone 202 (a mixture of 4 diastereomers) into three isomeric irone derivatives [Eq. (5.293)]. Other acids gave inferior results. 

The feasibility of cyclizing polyenes with iminium ion initiators has received only scant attention. 72 Cyclization of imine (77) under aprotic conditions with SnCU affords predominantly rran -decalins containing endocyclic unsaturation. This mixture was deduced by H NMR analysis to contain diastereomers (78) to (81) in the indicated abundances as depicted in equation (7). The extent of asymmetric induction in forming the decalin ring system (9S,10iS 9/ ,10/ = 61 39) is significantly lower than that of related cyclizations of chiral acetal substrates. ... 

D. Catalytic hydrogenation in this case reduces the extended polyene in a single step to afford estrone 3-methyl ether (13-2) as a single diastereomer. 

Isomerization of ll-cw-retinal to the all-trans diastereomer is sterically favored because of the sterical repulsion between the 13-methyl and 10-H substituents in a planar polyene. In solutions of retinal or retinyl ester in which all stereoisomers are equilibrated by the reversible addition of iodine or trifluo-roacetic acid one finds only 0.1% of the W-cis diastereomer. The energy difference between all-trans- and ll-cis- retinal is 4.1 kcal/mol. The crystal structure of 11-cw-retinal shows a highly twisted polyene at C12-C13. The methyl group... 

In this process, the Pd-catalyzed Stille cross-coupling reaction of enantiopure iodide 259 and stannane 258 afforded polyene 262, which underwent a conrotatory 87t-electrocyclization resulting in the cyclooctatrienes 263 and 264. These compounds reacted further in a disrotatory 67t-electrocyclization to a 5 1 mixture of the diastereomers shimalactone A (260) and B (261) in 55% and 11% yield, respectively (Scheme 14.40). 

After total synthesis of rhododaurichromanic acid by photochemical [2+2] cycloaddition, inspired by the precedents in polyene cyclizations, especially the work of Omura and coworkers [74], we attempted to treat daurichromenic ester 118a with various Lewis acids and mercury reagents but failed, as shown in Scheme 12.28. Fortunately, the outcome changed when using Brpnsted acids such as TFA, especially when it was used at 30 equiv or as a cosolvent with CH2CI2, we isolated two products 121 and 122 with each as a single diastereomer, the formation of 122 likely proceeds through the classic polyene cyclization via 123 [75]. 

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