Protium–deuterium exchange of 17-oxosparteine: Homoenolization of a lactam

Abstract

, 565 (1989). The H-D exchange of 17-oxosparteine (16) has been studied under homoenolization conditions. We find that l b undergoes H-D exchange at the bridgehead carbon C7 at 130°C, but forcing conditions (240°C) using the medium (CD3)3COK/ (CD3)3COD are required to induce exchange at C11, CIS, and other sites. CIS-Ha exchanges only marginally faster than C15-He, indicating that the p-lactam carbanion is stabilized primarily by the dipolar effect of the amide group.

Indolizidine and quinolizidine alkaloids

“…Protonation constants of several sulphur-containing C,, 12 Lythraceae Alkaloids Publications dealing with this group of alkaloids over the period 1979-1987 have been reviewed.l12 Four synthetic approaches to Lythraceae arylquinolizidines are summarized in Scheme 24. The point of novelty in a new formal non-stereoselective synthesis of the alkaloids lasubine-I (177) and lasubine-I1 (178) is the cycloaddition of a nitrone to a vinylcyclopropane (1 79) followed by thermal rearrangement of the isoxazolidine adducts to the separable ketones ( 180) and (lsl), formed in 26 % and 38 % yields re~pective1y.l'~ The conversion of these ketones into the alkaloids has been described elsewhere in this journal (c$ ref. 26f).…”

“…Another formal synthesis of the lasubines exploits the Wittig reaction with cyclic 2hydroxycarbamates such as (1 82), and converges with ketones (180) and ( 181) by way of pelletierine (183).l14 The problem of stereoselectivity was more satisfactorily addressed by Comins and co-w~rkers,~'~ who prepared the pivotal 2,6-cis-disubstituted piperidine (184) by sequential addition of organometallic reagents to electrophiles derived from a pyridinium substrate. In this work, hydride was delivered equatorially to ketone (1 8 1) by lithium trisiamylborohydride at -78 "C with 96 YO stereoselectivity to give lasubine-I1 (178) in 81 % yield. With sodium borohydride, the yields of lasubine-I1 and the 2-epimer were 19 YO and 70 YO respectively.…”