Recovered 46b (0.66 g, 1.3 1 mmol) was dissolved in CH2C12 (1 3 mL), cooled down to -70 'C, and treated with gaseous BF, (165 mL, 6.5 mmol). The mixture was kept at this temperature for 20 min and then quenched with MeOH (absolute, 2 mL). After the usual treatment the mixture 46a,b was isolated (0.56 g, yield ca. 85%, a:b = 3:1) and identified by 'H NMR data comparison of the decomplexed material with the abovementioned sample.Acknowledgment.
Abstract:The total synthesis of naturally occurring (-)-physostigmine is described. The key element for the asymmetric induction is the chirality transfer from optically active 2-(alkylsulfinyl)indoles to indoline butyrolactones bearing two chiral centers. Novel features of this synthesis involve the use of a new class of sulfoxylating agents, N-(alkylsulfinyl)oxazolidinones, to prepare the starting indolyl sulfoxides and the correlation of the size of the alkyl group on the sulfoxide with the degree of asymmetric induction. The overall synthesis requires a dozen steps from commercially available 5-(benzy1oxy)indole.
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