Intramolecular hydrogen abstraction. The use of organoselenium reagents for the generation of alkoxy radicals

Two-Stage Syntheses of Clionastatins A and B

A concise and divergent synthesis of the polychlorinated marine steroids clionastatin A and B from inexpensive testosterone has been achieved through a unique two-stage chlorination-oxidation strategy. Key features of the two-stage synthesis include (1) conformationally controlled, highly stereoselective dichlorination at C1 and C2 and C4-OH-directed C19 oxygenation followed by a challenging neopentyl chlorination to install three chlorine atoms; (2) desaturation through one-pot photochemical dibromination-reductive debromination and anti-Markovnikov olefin oxidation by photoredox-metal dual catalysis to enhance the oxidation level of the backbone; and (3) Wharton transposition to furnish the D-ring enone. This synthesis proved that the introduction of the C19 chloride in the early stage of the synthesis secured the stability of the backbone against susceptibility to aromatization during the oxidation stage.

Contemporary advancements in the semi-synthesis of bioactive terpenoids and steroids

Many natural products have intriguing biological properties that arise from their fascinating chemical structures. However, the intrinsic complexity of the structural skeleton and the reactive functional groups on natural products pose tremendous challenges to chemical syntheses. Semi-synthesis uses chemical compounds isolated from natural sources as the starting materials to produce other novel compounds with distinct chemical and medicinal properties. In particular, advancements in various types of sp3 C-H bond functionalization reactions and skeletal rearrangement methods have contributed to the re-emergence of semi-synthesis as an efficient approach for the synthesis of structurally complex bioactive natural products. Here, we begin with a brief discussion of several bioactive natural products that were obtained via a semi-synthetic approach between 2008 and 2015 and we then discuss in-depth contemporary advancements in the semi-synthesis of bioactive terpenoids and steroids reported during 2016-2020.

A biocatalytic hydroxylation-enabled unified approach to C19-hydroxylated steroids

Steroidal C19-hydroxylation is pivotal to the synthesis of naturally occurring bioactive C19-OH steroids and 19-norsteroidal pharmaceuticals. However, realizing this transformation is proved to be challenging through either chemical or biological synthesis. Herein, we report a highly efficient method to synthesize 19-OH-cortexolone in 80% efficiency at the multi-gram scale. The obtained C₁₉-OH-cortexolone can be readily transformed to various synthetically useful intermediates including the industrially valuable 19-OH-androstenedione, which can serve as a basis for synthesis of C19-functionalized steroids as well as 19-nor steroidal drugs. Using this biocatalytic C19-hydroxylation method, the unified synthesis of six C19-hydroxylated pregnanes is achieved in just 4 to 9 steps. In addition, the structure of sclerosteroid B is revised on the basis of our synthesis.

C19 hydroxylation is a unique feature of some bioactive steroids. Here, the authors developed a direct C19 hydroxylation approach to scalably access 19-OH-cortexolone in the host T. cucumeris and then converted the product into various pharmaceutically useful products via chemical synthesis.

Development of a Terpene Feedstock-Based Oxidative Synthetic Approach to the Illicium Sesquiterpenes

The Illicium sesquiterpenes are a family of natural products containing over 100 highly oxidized and structurally complex members, many of which display interesting biological activities. This comprehensive account chronicles the evolution of a semisynthetic strategy toward these molecules from (+)-cedrol, seeking to emulate key aspects of their presumed biosynthesis. An initial route generated lower oxidation state analogs but failed in delivering a crucial hydroxy group in the final step. Insight gathered during these studies, however, ultimately led to a synthesis of the pseudoanisatinoids along with the allo-cedrane natural product 11- O-debenzoyltashironin. A second-generation strategy was then developed to access the more highly oxidized majucinoid compounds including jiadifenolide and majucin itself. Overall, one dozen natural products can be accessed from an abundant and inexpensive terpene feedstock. A multitude of general observations regarding site-selective C(sp³)-H bond functionalization reactions in complex polycyclic architectures are reported.

Contemporary Synthetic Strategies toward seco-Prezizaane Sesquiterpenes from Illicium Species

Since the elucidation of the structure of anisatin in the late 1960s, sesquiterpene lactones from various Illicium species of plants have captivated synthetic chemists worldwide, resulting in a large body of synthetic work. In particular, Illicium sesquiterpenes containing the seco-prezizaane carbon framework have seen immense interest in recent years owing to desirable structural and medicinal attributes. This synopsis will focus on recently developed synthetic strategies to access these compact, highly oxidized terpenoids.

Traceless Silylation of β-C(sp3)-H Bonds of Alcohols via Perfluorinated Acetals

We report the silylation of primary C-H bonds located β to secondary and tertiary alcohols by exploiting perfluorinated esters as traceless directing groups. The conversion of a secondary or tertiary alcohol to a perfluoroalkyl ester and conversion of the ester to the corresponding silyl acetals by hydrosilylation allows for selective β-C(sp3)-H silylation catalyzed by the combination of [Ir(cod)OMe]2 and Me4Phen (3,4,7,8-tetramethyl-1,10-phenanthroline) to form 6-membered dioxasilinane. Tamao-Fleming oxidation of these dioxasilinane leads to 1,2 diols. The developed sequence was applied to a series of natural products containing hydroxyl groups.

Oxidative Entry into the Illicium Sesquiterpenes: Enantiospecific Synthesis of (+)-Pseudoanisatin

Illicium sesquiterpenes have been the subject of numerous synthetic efforts due to their ornate and highly oxidized structures as well as significant biological activities. Herein we report the first chemical synthesis of (+)-pseudoanisatin from the abundant feedstock chemical cedrol (∼$50 USD/kg) in 12 steps using extensive site-selective C(sp³)-H bond functionalization. Significantly, this work represents a novel oxidative strategic template for future approaches to these natural products and their analogs.

Development of a concise synthesis of ouabagenin and hydroxylated corticosteroid analogues

The natural product ouabagenin is a complex cardiotonic steroid with a highly oxygenated skeleton. This full account describes the development of a concise synthesis of ouabagenin, including the evolution of synthetic strategy to access hydroxylation at the C19 position of a steroid skeleton. In addition, approaches to install the requisite butenolide moiety at the C17 position are discussed. Lastly, methodology developed in this synthesis has been applied in the generation of novel analogues of corticosteroid drugs bearing a hydroxyl group at the C19 position.

Evaluation of 'east-to-west' ether-forming strategies for the total synthesis of maoecrystal V

Model systems that evaluated different approaches to construct the central ether ring of maoecrystal V are described. Our first model systems attempted the ether formation using C-H functionalization reactions, which led to interesting rearrangements but none of the desired ether product. An intramolecular conjugate addition strategy was then explored that successfully formed the targeted C-O bond but resulted in the undesired stereochemistry.

A synthesis of the carbocyclic core of maoecrystal V

An approach toward the synthesis of the complex polycyclic diterpene maoecrystal V (1) is described. Construction of the advanced tetracyclic core structure (i.e., 19) was achieved in 13 steps from 3,3-dimethylcyclohexanone (6) by employing a stereoselective Nazarov cyclization followed by a Diels-Alder reaction to forge the two contiguous quaternary stereocenters.

Synthesis of (+)-8-deoxyvernolepin and its 11,13-dihydroderivative. A novel reaction initiated by sulfene elimination leads to the 2-Oxa-cis-decalin skeleton

The title compounds are interesting candidates for antifungal screening. This paper describes the enantiospecific synthesis of these compounds starting from (+)-costunolide isolated from a commercially available extract. We used two novel reactions as key synthetic steps in this work: the acid-induced cyclization of an delta,epsilon-epoxy ester, which stereoselectively gave a hydroxymethyl-substituted delta-lactone, with the hydroxyalkyl group in the desired beta-equatorial disposition, and a reaction cascade, initiated by a base-promoted sulfene elimination, which led to a 10-oxiranyl-2-oxa-cis-decalin from the mesylate of a trans-fused delta-lactone. We also found that the reaction between selenium dioxide and the 1,5-diene system of elemanolides gave selenadecalins analogous to natural eudesmanolides. Our results prove that the synthetic strategy employed, on the basis of biomimetic concepts, is a useful procedure for the enantiospecific preparation of (+)-vernolepin-related compounds from accessible germacrolides.

Synthesis of Chiral Spiroacetals from Carbohydrates

Chiral spiroacetals of the 1,7-dioxaspiro[5.5]undecane, 1,6-dioxaspiro[4.5]decane, and 1,6-dioxaspiro[4.4]nonane types have been prepared from carbohydrates in pyranose or furanose forms. The spirocyclization reaction has been accomplished from a conveniently homologated carbohydrate by an intramolecular hydrogen abstraction reaction promoted by alkoxy radicals. Thus, 2,3,4,6-tetra-O-benzyl-1-deoxy-1-(3'-hydroxypropyl)-alpha-D-glucopyranose (2) was photolyzed with visible light in the presence of (diacetoxyiodo)benzene and iodine to give a mixture of (1R)-(3) and (1S)-2,3,4,6-tetra-O-benzyl-1-deoxy-D-glucopyranose-1-spiro-2'-tetrahydrofuran (4). The photolysis of methyl 6-deoxy-6-(2'-hydroxyethyl)-2,3,4-tri-O-methyl-alpha-D-glucopyranoside (8) gave the isomeric spiroacetals methyl (5S)- (9) and (5R)-6-deoxy-5,2'-epoxy-6-ethyl-2,3,4-tri-O-methyl-alpha-D-glucopyranoside (10) in which the spirocenter is now located at C-5. The spiroacetals of the [5.5]undecane series: methyl (5R)- (19) and (5S)-6-deoxy-5,3'-epoxy-2,3,4-tri-O-methyl-6-propyl-beta-D-glucopyranoside (20) have been prepared starting from methyl 6-deoxy-6-(3'-hydroxypropyl)-2,3,4-tri-O-methyl-beta-D-glucopyranoside (18). The reaction has also been applied to hexofuranoses and 1-deoxy-1-(3'-hydroxypropyl)-2,3:5,6-di-O-isopropylidene-alpha-D-mannofuranose (21) gave rise to (1S)- (22) and (1R)-1-deoxy-2,3:5,6-di-O-isopropylidene-D-mannofuranose-1-spiro-2'-tetrahydrofuran (23); and 1-deoxy-1-(4'-hydroxybutyl)-2,3:5,6-di-O-isopropylidene-alpha-D-mannofuranose (28) to (1R)- (30) and (1S)-1-deoxy-2,3:5,6-di-O-isopropylidene-D-mannofuranose-1-spiro-2'-tetrahydropyran (32). Both spiroacetal enantiomers are formally available from the same carbohydrate.