Conformational Analysis of the Oxazolidine and Tetrahydro-1,3-oxazine Ring of C20-Diterpenoid Alkaloid Derivatives

Article abstract of DOI:10.1021/jo00159a001

Chemical and spectral data about the behavior of the oxazolidine ring of atisine and veatchine in hydrogen-bonding and non-hydrogen-bonding solvents are presented.The steric effect of the C(16) methyl group on closure of the normal- and iso-type oxazolidine ring in various derivatives of atisine, veatchine, cuauchichicine, and garryfoline has been studied by 13C NMR spectroscopy.The normal-type oxazolidine ring of veatchine-related derivatives closes from one side of the trigonal C(20) carbon to give a single C(20) epimer when the C(16) methyl group is in a β configuration.By contrast, the presence of the C(16) methyl group in an α configuration affords both C(20) epimers on ring closure.The iso-type oxazolidine ring closes from both sides of the trigonal C(19) carbon to give both epimers regardless of the conformation of the C(16) methyl group.Treatment of various imine derivatives with glycidol afforded the favored six-membered tetrahydro-1,3-oxazine derivatives exclusively, instead of the possible five-membered oxazolidine-ring-containing derivatives.The structure of one of the isomers of 22-hydroxyhomoveatchine acetate (42) was confirmed by a single-crystal X-ray analysis.Modes of formation and stereochemistry at C(20) of the oxazolidine and tetrahydro-1,3-oxazine derivatives are discussed.