Asymmetric and enantiospecific syntheses of 1-hydroxymethyl pyrrolizidine alkaloids

Stereodivergent Synthesis of 1-Hydroxymethylpyrrolizidine Alkaloids

A first stereodivergent strategy for the asymmetric synthesis of all stereoisomers of 1-hydroxymethylpyrrolizidine alkaloids is developed using an asymmetric self-Mannich reaction as a key step. An anti-selective self-Mannich reaction of methyl 4-oxobutanoate with the PMP-amine catalyzed by a chiral secondary amine is successfully optimized for the asymmetric synthesis of (+)-isoretronecanol and (-)-isoretronecanol. A syn-selective self-Mannich reaction catalyzed by proline is utilized for the asymmetric synthesis of the diastereomer, (+)-laburnine, and its enantiomer, (-)-trachelanthamidine.

Recent applications of the Wittig reaction in alkaloid synthesis

The Wittig reaction is the chemical reaction of an aldehyde or ketone with a triphenyl phosphonium ylide (the Wittig reagent) to afford an alkene and triphenylphosphine oxide. Noteworthy, this reaction results in the synthesis of alkenes in a selective and predictable fashion. Thus, it became as one of the keystone of synthetic organic chemistry, especially in the total synthesis of natural products, where the selectivity of a reaction is paramount of importance. A literature survey disclosed the existence of vast numbers of related reports and comprehensive reviews on the applications of this important name reaction in the total synthesis of natural products. However, the aim of this chapter is to underscore, the applications of the Wittig reaction in the total synthesis of one the most important and prevalent classes of natural products, the alkaloids, especially those showing important and diverse biological activities.

Stereoselective synthesis of sugar mimetics from simple monosaccharides

A number of bicyclic imino- and carba-sugars (examples are presented in the Scheme) can be obtained from simple monosaccharides (hexoses or pentoses) by various methods.

Formal syntheses of (-)-isoretronecanol, (+)-laburnine, and a concise enantioselective synthesis of (+)-turneforcidine

The synthesis of functionalized pyroglutamates 15 and 16 could be achieved by the application of recently developed diastereodivergent asymmetric Michael addition reaction of iminoglycinate 7 to ethyl γ-silyloxycrotonate with >98:<2 diastereoselectivity followed by hydrolysis and lactamization. Formal syntheses of (-)-isoretronecanol and (+)-laburnine as well as a concise enantioselective synthesis of (+)-turneforcidine could be achieved from functionalized pyroglutamates 15 or 16.

Pyrrolizidine alkaloids: occurrence, biology, and chemical synthesis

Covering: 2013 up to the end of 2015This review covers the isolation and structure of new pyrrolizidines; pyrrolizidine biosynthesis; biological activity, including the occurrence of pyrrolizidines as toxic components or contaminants in foods and beverages; and formal and total syntheses of naturally-occurring pyrrolizidine alkaloids and closely related non-natural analogues.

Hydrogen bond directed epoxidation: diastereoselective olefinic oxidation of allylic alcohols and amines

This article compares the diastereoselective epoxidation of acyclic and cyclic allylic alcohols, with the chemo- and diastereoselective olefinic oxidation of a range of acyclic and cyclic allylic amines. The diastereoselectivity in these systems is compared and a discussion about the origin of this high diastereocontrol is also presented. The ammonium directed epoxidation methodology has been extended to more complex substrates and representative applications of this protocol in natural product synthesis are also summarised.