Analogues of amphibian alkaloids: total synthesis of (5R,8S,8aS)-(−)-8-methyl-5-pentyloctahydroindolizine (8-epi-indolizidine 209B)and[(1S,4R,9aS)-(−)-4-pentyloctahydro-2H-quinolizin-1-yl]methanol

Abstract

Background

A Xylochemically Inspired Synthesis of Lamellarin G Trimethyl Ether via an Enaminone Intermediate

“…The disconnection envisaged in the present approach is of the latter type (Scheme ). The reason for this choice of strategy arises from our long-standing interest in the use of enaminones as intermediates for the synthesis of alkaloids and other nitrogen heterocycles. , We visualized a pivotal role for the enaminone 4 in a late-stage assembly of the pyrrole. By reacting this intermediate with a bifunctional reagent such as ethyl bromoacetate, we believed we could take advantage of the enaminone’s nucleophilicity at nitrogen in displacing the halide and its electrophilicity at the carbonyl substituent by condensation with the α-methylene site of the ester.…”

A Very Efficient Cerium(IV) Ammonium Nitrate Catalyzed, Four‐Component Synthesis of Tetrahydropyridines and Its Application in the Concise Generation of Functionalized Homoquinolizine Frameworks

“…[18,19] Although species related to 12 have been postulated as intermediates in the reduction of 1,4-dihydropyridines in acidic media, to the best of our knowledge they have not been used for carbon-carbon bond formation reactions, which are relevant because many natural quinolizidine and pyridoA C H T U N G T R E N N U N G [2,1-a]azepine alkaloids are substituted at the position adjacent to nitrogen. [20] To test this possibility, we briefly examined the Lewis acid catalyzed reaction between the representative compounds 11 and various nucleophiles, finding that treatment of compounds 11 c, 11 e, and 11 f with allyltrimethylsilane in the presence of boron trifluoride efficiently afforded the corresponding allyl derivatives 13 in good to excellent yields, presumably through species 12 as [a] For greater clarity, the letters used to designate compounds 9 have been retained here, and hence compound 11 a does not exist.…”

Cyclization of enaminones derived from N-phenacylpyrrolidin-2-ones to pyrrolizines under acidic conditions

“…The strategy we planned for preparing the key enaminones 4 is shown in disconnection form in Scheme 1. Eschenmoser sulfide contraction, 4,5 a method we have used with much success in our routes to alkaloids and related nitrogen heterocycles, [6][7][8][9][10] was envisaged between pyrrolidine-2-thiones 7 and appropriately substituted phenacyl halides 8. Intermediates 7 would themselves be made by thionation of the corresponding lactams 9, which would in turn be prepared by N-alkylation of pyrrolidin-2-one (10) with phenacyl halides 8'.…”

“…Scheme 1. Proposed disconnection scheme for the synthesis of enaminones 4 by sulfide contraction Synthesis of the desired N-substituted lactams was effected by alkylation of the cyclic imidate 11, prepared from pyrrolidin-2-one (10) and dimethyl sulfate, 11 with phenacyl bromide (12a), 3,4dimethoxyphenacyl bromide (12b), 12 and 2-hydroxyphenacyl bromide (12c) 13 (Scheme 2). The products 13a-c…”