Natural product synthesis via cyclobutanes : part I, Asymmetric synthesis of (+)-byssochlamic acid, part II, An approach to the nootropic agent huperzine A Public Deposited

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  • PART I. Asymmetric syntheses of both natural (+)- and nonnatural (-)- byssochlamic acid via a [2+2] photoaddition-cycloreversion strategy are described. X-ray crystallographic analysis of the cyclohexylamine salt 99 showed that the structure of the monomethyl ester 100 from esterase hydrolysis of 44 was originally misassigned as 56. The enantiomeric relationship of the two diolides 106 and 70 permitted syntheses of nonnatural byssochlamic acid (-)-3 and natural byssochiamic acid (+)-3 from enantiopure alcohol (+)-64 and from its enantiomer (-)-110, respectively. Through the use of (±)-103 to reach both enantiomers of byssochlamic acid (3) and subsequent epimerization of the npropyl chain, it was proved that the cis configuration of the two alkyl substituents is strongly preferred in the natural product. PART II. An asymmetric approach towards the nootropic agent huperzine A is described. Formation of cyclobutane 122 with the desired stereochemistry was accomplished using intramolecular [2+2] photoaddition of the enantiopure enone 121. Attempts to prepare the methoxypyridine system via an azadiene Diels- Alder reaction were unsuccessful. However, intramolecular Michael addition of 181 produced silyl ether 182 which was converted into the pyridone 187 by treatment with hydrogen fluoride followed by selenoxide elimination. Attempts to effect the key sigmatropic rearrangement of ketone 197 into a direct precursor of huperzine A were unsuccessful.
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