Ormosia Alkaloids: Iv. Stereochemistry of Ormojanine; Structure and Stereochemistry of Piptanthine, Dasycarpine, and Ormosinine

Abstract

By using the known structure and stereochemistry of the alkaloid ormosanine, C20H35N3 (I), as reference, the structure and stereochemistry of the isomeric compounds piptanthine (II), tetrahydro-ormojanine (XIV), isotetrahydro-ormojanine (XIII), and dasycarpine (XX) have been elucidated by isomerization and reduction experiments. Three additional C20H35N3 bases possessing the ormosanine skeleton have been prepared by the reduction of pyridine VII. Thus, of the 15 possible stereoisomers of ormosanine, 7 are now known.The study leads to the stereo-formula XIX for the alkaloid ormojanine, C20H31N3. The alkaloid ormosinine has been found to be a dimer of panamine.Stereochemical aspects of the catalytic and chemical reductions of the double bonds and pyridine rings contained in the described compounds are discussed. A possible biogenetic intermediate is proposed which explains the presence of both antipodal series of Ormosia alkaloids in nature.

Ormosia alkaloids. VII. Total synthesis of ormosanine, piptanthine, 18-epiormosanine, and d,l-panamine

“…A similar stability order at C16 had previously been observed for the epimeric pair tetrahydro-ormojanine -isotetrahydro-or~nojanine (10). The only other possible isomer with a cisoid attachment of ring A, 18, could clearly be expected to epimerize to 19 (10). Finally, a mild reduction sequence in which the keto group of dilacta~n 17 was removed by treatment with sodiunl borohydride, dehydration of the resulting alcohol and hydrogenation did not lead to ormosanine (I), piptanthine (2), or any other known Cz0H35N3 base (10) containing the ClI-H atom cis to the C7-C9 bridge.8…”

“…This is expected for stereochemistry 17 (see stereofor~nula 170). A similar stability order at C16 had previously been observed for the epimeric pair tetrahydro-ormojanine -isotetrahydro-or~nojanine (10). The only other possible isomer with a cisoid attachment of ring A, 18, could clearly be expected to epimerize to 19 (10).…”

“…both orrnosanine (1) and piptanthine (2), while been converted in our laboratory into five of the the less favorable exclusive formatioil of corn-eight theoretically possible diastereoisorners of pounds with C6-H cis to the C7-C9 bridge could the general formula 5 (10). This reduction study at least allow the synthesis of piptanthine (2).…”

“…(8) and in all respects with natural racenlic piptanthine (14). The structure of the two synthetic bases was further confiriiied by their coilversion to the corresponding cyclic ureas, hotnoxyormosanine, and honloxypiptanthine (10).…”

“…The stereochemical problem is, however, significantly simplified by previous work on the structure, interconversion, and configuration of Ormosia alkaloids. S o~n e time ago, we have been able to achieve a catalytic epimerization at C6 which converted ormosanine into the alkaloid piptanthine (2) (10). Thus, the preparation of compounds with C6-H trans t o the C7-C9 bridge would lead to the synthesis of Can.…”