Unexpected ring C enlargement of the aspidospermane skeleton
摘要:
Treatment of (-)-16-chloro-1-dehydro-5-methoxy-vineadifformine 4 with Ac2O-pyridine led to the rearranged compound 6 through an unexpected ring C expansion. Upon reaction with dimethyl acetylenedicarboxylate. 4 provided the [2+2] cycloaddition derivative 9 but did not undergo the ring C enlargement. (C) 2001 Elsevier Science Ltd. All rights reserved.
The first biomimetic approach to the skeleton of goniomitine (12) from an Aspidosperma alkaloid is described. With 16-chloro-1-dehydro-5-methoxyvincadifforme (4) (easily available from vincadifformine (1) as starting material, the rearrangement is performed in two steps: (a) Oxidation of 4 by m-CPBA followed by methanolysis affords the hemiketal 10 with a tetrahydro-1,2-oxazine structure for ring C, and (b) acid-catalyzed reaction on 10 provides either a mixture of both rearranged compounds 15 and 16 or 15 alone according to reaction time. Mechanisms of formation of 10, 15, and 16 are discussed.