Synthesis of the ester side chains of some potently antileukemic harringtonia alkaloids from chiral citrates

Can a Ketone Be More Reactive than an Aldehyde? Catalytic Asymmetric Synthesis of Substituted Tetrahydrofurans

O-heterocycles bearing tetrasubstituted stereogenic centers are prepared via catalytic chemo- and enantioselective nucleophilic additions to ketoaldehydes, in which the ketone reacts preferentially over the aldehyde. Five- and six-membered rings with both aromatic and aliphatic substituents, as well as an alkynyl substituent, are obtained. Moreover, 2,2,5-trisubstituted and 2,2,5,5-tetrasubstituted tetrahydrofurans are synthesized with excellent stereoselectivities. Additionally, the synthetic utility of the described method is demonstrated with a three-step synthesis of the side chain of anhydroharringtonine.

Cephalotaxus Alkaloids

Cephalotaxus alkaloids represent a family of plant secondary metabolites known for 60 years. Significant activity against leukemia in mice was demonstrated for extracts of Cephalotaxus. Cephalotaxine (CET) (1), the major alkaloid of this series was isolated from Cephalotaxus drupacea species by Paudler in 1963. The subsequent discovery of promising antitumor activity among new Cephalotaxus derivatives reported by Chinese, Japanese, and American teams triggered extensive structure elucidation and biological studies in this family. The structural feature of this cephalotaxane family relies mainly on its tetracyclic alkaloid backbone, which comprises an azaspiranic 1-azaspiro[4.4]nonane unit (rings C and D) and a benzazepine ring system (rings A and B), which is linked by its C3 alcohol function to a chiral oxygenated side chain by a carboxylic function alpha to a tetrasubstituted carbon center. The botanical distribution of these alkaloids is limited to the Cephalotaxus genus (Cephalotaxaceae). The scope of biological activities of the Cephalotaxus alkaloids is mainly centered on the antileukemic activity of homoharringtonine (HHT) (2), which in particular demonstrated marked benefits in the treatment of orphan myeloid leukemia and was approved as soon as 2009 by European Medicine Agency and by US Food and Drug Administration in 2012. Its exact mechanism of action was partly elucidated and it was early recognized that HHT (2) inhibited protein synthesis at the level of the ribosome machinery. Interestingly, after a latency period of two decades, the topic of Cephalotaxus alkaloids reemerged as a prolific source of new natural structures. To date, more than 70 compounds have been identified and characterized. Synthetic studies also regained attention during the past two decades, and numerous methodologies were developed to access the first semisynthetic HHT (2) of high purity suitable for clinical studies, and then high grade enantiomerically pure CET (1), HHT (2), and analogs.

A Concise and Scalable Formal Synthesis of the Enantiopure Side-Chain Diester of Homoharringtonine

The enantiopure side-chain diester of homoharringtonine (HHT) has been prepared in 38% overall yield, from cheap L-aspartic acid by a linear sequence of 10 chemical operations (mean yield per step: 91%). The experimental operation in each step is convenient, economic and prone to scale up. About 7–10 g of the side-chain diester of HHT is easily prepared within one week from 5 g of L-aspartic acid.

Enantiospecific synthesis of the diacid side-chain of deoxyharringtonine and homodeoxyharringtonine

The enantiospecific synthesis of the diacid side-chain of deoxyharringtonine (2) and homodeoxyharringtonine (3) was accomplished from (L)-N-Boc-α-amino alcohol 10 in high yield. A key feature of the synthesis is the construction of the chiral tertiary alcohol by a three-step sequence (i.e., Wittig reaction, Meisenheimer rearrangement, and catalytic hydrogenation).

Construction of chiral tertiary alcohol stereocenters via the [2,3]-Meisenheimer rearrangement: enantioselective synthesis of the side-chain acids of homoharringtonine and harringtonine

For the first time, the [2,3]-Meisenheimer rearrangement has been developed into a general strategy for the construction of chiral tertiary alcohols. The effectiveness and practicality of this methodology are illustrated by the successful synthesis of (R)-20 and (R)-30, the side chain acids of homoharringtonine and harringtonine, respectively.