Stereoselective Syntheses of 2,6-disubstituted Piperidin-3-oles (alkaloid lipids)

Abstract

Alkaloid lipids comprise a class of natural compounds with significant biological activities of medicinal interest. Their structural framework consists of a 2,6-disubstituted-3-piperidinol moiety (polar head group) and a lipid tail. Representative examples of alkaloid lipids are (+)-Cassine, (-)-Prosophylline, (-)-Prosopinine, (-)-Spectalline. Many synthetic strategies have been developed for the efficient stereoselective synthesis of various natural alkaloid lipids and their synthetic analogues. This mini-review focuses on the most successful approaches published during the new millennium. The synthetic strategies are grouped in two main sections, depending on whether the lipophilic side chain is introduced before (“early”) or after (“late”) piperidine ring formation. These sections are further organised based on the key ring forming reaction/strategy employed. For the strategies that opt to introduce the lipophilic side chain early on the synthetic sequence, the intramolecular reductive amination, either with or without isolation of the intermediate imine/enamine, constitutes one of the most popular methods for constructing the piperidine ring followed closely by approaches based on the intramolecular alkylation of suitable amine derivatives. Olefin ring closing metathesis and intramolecular hetero-Diels-Alder reactions have also been exploited to secure the azaheterocycle. The intramolecular alkylation of an amine or the, retrosynthetically related, intramolecular amine addition to an olefin have also been employed as key ring-forming reactions by syntheses that adopt the more convergent approach of introducing the side chain on a preformed heterocycle. In this regard the introduction of the side chain via alkylation of pyridinium salts, alkylation of a lactam (2-piperidone) or glutarimide derivatives have been investigated. Several groups have also exploited the oxidative cyclization of α-furylamines to prepare 2-dihydropyridones as a convenient scaffold for the late introduction of the lipid tail. In some cases, carbohydrates and aminoacids were used (either as starting materials or as chiral auxiliaries) for the enantioselective synthesis of alkaloid lipids. Enzymatic or chemical (Jacobsens epoxide hydrolysis protocol) resolutions of racemic starting materials and Sharpless asymmetric epoxidation, dihydroxylation or aminohydroxylation of olefins have also been utilized. Finally, it is noteworthy that in several cases, apart from the Witting reaction and the use of Grignard reagents, olefin cross-metathesis has also been successfully employed for the introduction of the lipid side chain.