Concerted rearrangement asymmetric reactions

In particular the synthetic approach to dihydrofurans (first equation in Figure 4.23) represents a useful alternative to other syntheses of these valuable intermediates, and has been used for the preparation of substituted pyrroles [1417], aflatoxin derivatives [1418], and other natural products [1419]. The reaction of vinylcarbene complexes with dienes can lead to the formation of cycloheptadienes by a formal [3 + 4] cycloaddition [1367] (Entries 9-12, Table 4.25). High asymmetric induction (up to 98% ee [1420]) can be attained using enantiomerically pure rhodium(II) carboxylates as catalysts. This observation suggests the reaction to proceed via divinylcyclopropanes, which undergo (concerted) Cope rearrangement to yield cycloheptadienes. 

The use of a cationic aza-Cope rearrangement in concert with a Mannich cyclization has also been applied to the total synthesis of enantiomerically pure (—)-crinine (359) (205). In the event, nucleophilic opening of cyclopentenoxide with the aluminum amide that was formed on reaction of (/ )-a-methylbenzyl-amine and trimethylaluminum gave the amino alcohol 485 together with its (15,25) diastereomer. Although there was essentially no asymmetric induction in this process, the diastereomeric amino alcohols were readily separated by chromatography, and the overall procedure therefore constitutes an efficient means for the preparation of enantiomerically pure 2-amino alcohols from epoxides. When the hydrochloride salt derived from 485 was treated with paraformaldehyde and potassium cyanide, the amino nitrile 486 was formed. Subsequent Swem oxida-... 

Due to the very limited amount of experimental data available, a choice between the two mechanisms is even more difficult for reaction III than for reaction II. Orban et investigated the photochemical rearrangement, in pentane, of an aliphatic, optically active ketone with a single asymmetric carbon atom in the y position. Their results demonstrate a partial retention of configuration during photochemical cyclobutanol formation, which can be explained neither by a biradical mechanism alone, if this involves a long-lived radical, nor by a concerted mechanism alone. The results are reconcilable with the competitive participation of both mechanisms, but they are just as compatible with the assumption of the production of a short-lived biradical whose rates of racemization and of cyclization are comparable. 

This chapter examines reactions that involve molecular rearrangements and cycloadditions. The use of these terms will not be restricted to concerted, pericyclic reactions, however. Often, stepwise processes that involve a net transformation equivalent to a pericyclic reaction are catalyzed by transition metals. The incorporation of chiral ligands into these metal catalysts introduces the possibility of asymmetric induction by inter-ligand chirality transfer. The chapter is divided into two main parts (rearrangements and cycloadditions), and subdivided by the standard classifications for pericyclic reactions e.g., [1,3], [2,3], [4-1-2], etc.). The latter classification is for convenience only, and does not imply adherence to the pericyclic selection rules. Indeed, the first reaction to be described is a net [1,3]-suprafacial hydrogen shift, which is symmetry forbidden if concerted. 

The Claisen rearrangement [5], the intramolecular reaction of allyl enol ethers 1 to y,<5-unsaturated carbonyl compounds 3, has become a valuable tool for organic synthesis [6]. The sigmatropic process allows a significant alteration of the molecular framework within a single step. The concerted mechanism involves a highly organized transition state 2 that often directs the stereochemical course in the reaction of substituted derivatives and enables the simultaneous formation of two asymmetric centers (Scheme 1). 

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