Pumiliotoxin derivatives reduction

The pyrrolobenzodiazepine-5,11 -diones II have been utilized in asymmetric syntheses of both the cis- and tra i-decahydro-quinoline alkaloids (Schemes 21 and 22). For example, reduction of 100 with 4.4 equiv. of potassium in the presence of 2 equiv. of t-BuOH, followed by protonation of the resulting enolate with NH4CI at —78 °C gave the cA-fused tetra-hydrobenzene derivative 101.Amide-directed hydrogenation of 101 gave the hexahydrobenzene derivative with diastereo-selectivity greater than 99 1. Removal of the chiral auxiliary and adjustment of the oxidation state provided aldehyde 103 which was efficiently converted to the poison frog alkaloid (+)-pumiliotoxin C. 

Alkali metal in ammonia reductions of pyrrolobenzodiazepine-5,11-diones give trans-2-aminocyclohexanecarboxylic acid derivatives (e.g., 4) in enantiomerically pure form.2 3 A method for preparation of cis-2-aminocyclohexanecarboxylic acids related to 4 is based on the enantioselective hydrolysis of symmetrical diesters with pig liver esterase.4 cis-2-Aminocyclohexane derivatives have been used for syntheses of aminocyclitol antibiotics.4 5 6-Alkyl-cis-2-aminocyclohexanecarboxylic acids can be prepared by alkali metal in ammonia reduction of pyrrolobenzodiazepine-5,11-diones followed by olefin hydrogenation the cis-decahydroquinoline alkaloid (+)-pumiliotoxin C has been prepared by this methodology.2... 

The usefulness of nucleophile-promoted Iminium Ion-alkyne cyclizations derives from the ready availability of alkynylamines and the subsequent transformations of the cyclization products made possible because of their vinylic functionality (e.g., equations 1 and 2). Equation 2 illustrates use of this chemistry to elaborate an exocyclic tetrasubstituted double bond with complete stereooontrol. Net "reductive iminium ion alkyne cyclizations can be accomplished by dehalogenation of vinyl halide cyclization products. The conversion illustrated in equation 3 is a key step in an efficient, practical synthesis of the cardiotonic frog alkaloid pumiliotoxin A. ... 

The next synthesis uses a chiral auxiliary approach in which the auxiliary is not sacrificed. The synthesis begins with anthranihc acid derivative 157. A potassium-ammonia reduction provided a mixture of diastereomers 158 and 159. A directed hydrogenation of the major diastereomer (159), using Crabtree s cationic iridium catalyst, provided 160. This material was moved forward to pumiliotoxin-C through intermediates we encountered in the Overman approach (compare the substitution pattern and relative stereochemistry of 160 with 131 on Pumihotoxin-3). 

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