Electrocyclic reactions diastereoselectivity

Gold-catalysed cycloisomerization of l,7-diyn-3,6-bis(propargyl carbonate)s provide for a diastereoselective synthesis of naphtho[ ]cyclobutenes by a cascade sequence involving gold-catalysed double [3,3]-rearrangement, 6 r-electrocyclic reaction, and a decarbonylative cyclization (Scheme 15). °... 

A review which covers sigmatropic rearrangements in addition to Diels-Alder reactions, 1,3-dipolar cycloadditions, electrocyclic reactions, and ene reactions has appeared. The stereochemistry of [3,3]-sigmatropic reactions of chiral carbon compounds has been reviewed, as have diastereoselective Claisen rearrangements of substrates bearing chiral auxiliary and enantioselective variants of achiral substrates. Examples of [3,3]-sigmatropic rearrangements used in the synthesis of various types of 3-chromene derivatives have been reviewed. ... 

AB ABCE ABCDE - ABCDEF W-G aldehyde (-)-Strychnine] (27) After his racemic synthesis of strychnine (26), Kuehne also achieved an enantioselective synthesis of (—)-strychnine (Scheme 9). To avoid the low yield conversion of isostrychnine to strychnine, the second approach was directed to the W-G aldehyde. Starting from L-tryptophan methyl ester (86), the cyclization precursor 87 was prepared in seven steps in a similar way as in the previous racemic synthesis. The domino condensation-electrocyclization reaction of 87 with dienal 88 proceeded with quite high diastereoselectivity (>95% de) [AB ABCE, C7 quaternary center] (85). After conversion of the tetracyclic compound 89 to tosylate 92, removal of the benzyl group resulted in the clean formation of the D ring [ABCE ABCDE ]. Unlike in the first synthesis, introduction of the hydroxyethylidene side chain by a Horner-Wadsworth-Emmons reaction of ketone 93 proceeded with high stereoselectivity (E Z = 17 1). Einally, the E isomer 94E was converted to (-)-strychnine via the W-G aldehyde (50). 

Reversibility of 67t-Electron Electrocyclic Ring-Closure. The high diastereoselectivity obtained in these reactions here is likely a result of the reversible 67t-electron electrocyclic ring-closure.20,37 41 The best evidence for the reversibility of this ring-closure is described in Scheme 12. We were able to isolate both the desired major isomer 10 and the minor isomer 44 from the formal cycloaddition reaction of the iminium salt 56 with pyrone 12. 

The unselective 6jt-electrocyclization can be avoided with substrates that lack a [3-olefin substituent (Scheme 7.41).116 When chiral, nonracemic versions of these substrates are employed, Woerpel and Calad demonstrated that the cascade reaction efficiently transfers the chiral information. While silver salts were competent catalysts for the diastereoselective formation of silalactone 144 from 1301, on scale-up, copper(II) triflate proved to be more efficient. The authors attribute the poorer performance by silver to product inhibition. In eight steps, silalactone 144 was transformed into the antibiotic (+ Hyu-acetomycin, to further highlight the synthetic utility of the cascade process. 

The diastereoselective synthesis of spirofurooxindoles has been described based on the three-component reaction involving intermolecular carbonyl ylide dipole 23 [69]. The reaction of 3-diazoindol-2-one 21, benzaldehyde and DMAD in the presence of 0.5 mol % of rhodium(II) acetate catalyst furnished the spirofurooxindole 22 (Scheme 7). On the basis of the diastereoselectivity of these reactions, the conformation of the intermolecular carbonyl ylide 23 is provided. In the absence of dipolarophiles, the carbonyl yhde underwent electrocyclization to yield oxiranes 24 [70]. 

This method in the content of microwave-assisted cycloaddition reactions with relevant dienophiles via electrocyclic ring-opening which facilitates generation of a reactive diene 31 for an inter-molecular Diels-Alder [4 + 2] cycloaddition (Scheme 3.29) has also been demonstrated. The overall scope of this method is broad and amenable to a variety of functional group substitutions on the aromatic as well as the cyclobutane moiety. The reported yields range from modest to excellent and the overall diastereoselectivity is impressive. 

The high diastereoselectivity obtained in these reactions is likely a result of the reversible 67t-electron electrocyclic... 

The reaction of nitrones with vinyldiazoacetale catalysed by Rh(ll) has been shown as an efficient and diastereoselective access to multifunctionalized tricyclic heterocycles (Scheme 6). The reaction is a domino process involving dipolar cycloaddition 0 of the nitrone to the double bond followed by an intramolecular cyclopropanation, an electrocyclic ring enlargement and finally a [l,7]-oxygen migration. 

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