Sultam complexes, cycloaddition

Cycloadditions (Athene Five-Membered Cyclo-adduct) The levels of selectivity found for 1,3-dipolar cycloaddition reactions are not as high as those obtained for Lewis acid-catalyzed Diels-Alder reactions. However, the 10,2-camphorsultam auxiliary can achieve synthetically useful levels of induction in these reactions, and this has been attributed to efficient enoyl conformational control by the sultam moiety leading to preferred C(a)-re face attack even in the absence of metal complexation. ... 

Highly selective Diels-Alder reactions with other chiral auxiliaries attached to the dienophile have been documented. For example, chiral 2-oxazolidinones or the camphor sultam auxiliaries have proven particularly useful. Such cycloaddition reactions, catalysed by an alkylaluminium chloride, occur with a variety of dienes to give adducts in high yield and with very high diastereoselectivity. In many cases these adducts can be obtained diastereomerically pure by crystallization. The reactions are thought to occur by way of complexed ion pairs (e.g. 129), in which the substituent on the auxiliary shields one face of the dienophile from attack by the diene. For example, 2-methylbutadiene (isoprene) gave the adduct 127, which was converted into (i )-(+)-a-terpineol 128 (3.92). ... 

Chelation is also thought to play an important part in directing the facial selectivity of cycloadditions with the camphor sultam auxiliary. A variety of dienes can be used and adducts are obtained with very high diastereomeiic excesses. Both inter-and intramolecular cycloaddition reactions are amenable to the use of a chiral auxiliary. An intramolecular example is illustrated in Scheme 3.93, in which the diene and dienophile are tethered and in which cycloaddition leads to predominantly one of the two diastereomeric trans-fusQd bicyclic (endo) products. The dienophile is thought to adopt the s-cis conformation, with the aluminium atom complexed to the carbonyl and one of the two sulfone oxygen atoms. 

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