Abstract
Reaction cascades induced by the addition to diisopropyl squarate of at least one cycloalkenyl anion possessing an oxygenated leaving group have been examined for the purpose of evaluating the level of chirality transferred to several new stereogenic centers as the original one is destroyed through β-elimination. A methoxy group alone necessarily resides too far from the bonding sites involved in conrotatory ring closure of the coiled octatetraene intermediate to have a large impact, viz. greater than 22% ee. The situation is considerably improved if a (tert-butyldimethylsilyl)oxy (OTBS) substituent is positioned nearer to this key helical biasing site. The relative rates of ring closure from the two helices of different pitch now become sufficiently different (>100:1) that mechanistic control by OTBS dominates completely. Finally, an example of overwhelming steric bias is detailed (see 30), showing that electrocyclic control can also be achieved in this manner.
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