KD16-U1, a new metabolite of Streptomyces: isolation and structural studies

Steglich esterification: A versatile synthetic approach toward the synthesis of natural products, their analogues/derivatives

The exploitation of natural products and their analogues in the field of pharmacology has been regarded as of great importance. It can be attributed to the fact that these scaffolds exhibit diverse chemical properties, distinct biological activities and zenith specificity in their biochemical processes, enabling them to act as favorable structures for lead compounds. The synthesis of natural products has been a crafty and hard-to-achieve task. Steglich esterification reaction has played a significant role in that area. It is a mild and efficient technique for constructing ester linkages. This technique involves the establishment of ester moiety via a carbodiimide-based condensation of a carboxylic acid with an alcohol, thiol or an amine catalyzed by dimethyl aminopyridine (DMAP). Specifically, labile reagents with multiple reactive sites are esterified efficiently with the classical and modified Steglich esterification conditions, which accounts for their synthetic utility. This review encloses the performance of the Steglich esterification reaction in forging the ester linkage for executing the total synthesis of natural products and their derivatives since 2018.

Chemoselective Nozaki-Hiyama-Takai-Kishi and Grignard reaction: short synthesis of some carbahexopyranoses

A common, divergent, efficient, stereoselective and short approach for the total syntheses of some carbahexopyranoses namely, MK7607, (-)-gabosine A, (-)-conduritol E, (-)-conduritol F, 6a-carba-β-d-fructopyranose and other carbasugars using chemoselective Grignard or Nozaki-Hiyama-Takai-Kishi (NHTK) reactions and RCM. Herein, the Grignard and NHTK reactions are able to differentiate the reactivity difference between lactol or lactolacetate and aldehyde of 2 & 6 under given conditions to give the desired skeleton chemoselectivity.

Enantioselective Synthesis of a New Non-Natural Gabosine

The preparation of a new non-natural gabosine is reported, in which the chirality is transferred from the toluene’s biotransformed metabolite (1R,2S)-3-methylcyclohexa-3.5-diene-1,2-diol. Further chemical transformations to introduce additional functionality and chirality to the molecule were also accomplished.

A divergent strategy to synthesize gabosines featuring a switchable two-way aldol cyclization

Gabosines and their natural analogues, belonging to C7 carbasugars, have attracted great attention in synthesis due to their rich structural variety and promising biological activities. A new diversity-oriented approach for the gabosine-type carbasugars based on a tunable regioselective aldol cyclization of flexible precursor 2 is explored. Two cyclization modes (A and B) of the precursor can be well controlled by switching promoters to selectively produce two resulting cyclohexa(e)nones 3 and 10, both of which are versatile intermediates for various C7 carbasugars. After the conversion of 3 to eight natural carbasugars, the utility of intermediate 10 is illustrated by the first synthesis of (-)-gabosine L, as well as the new synthesis of (-)-gabosine A, (-)-gabosine B, (-)-gabosine N and (-)-gabosine O. The chemical structure and the absolute configuration of (-)-gabosine L are confirmed by its total synthesis.

Synthesis of Natural Products and the Development of Synthetic Methodology: The Case Study of (–)-Atrop-abyssomicin C

During our attempt to follow the planned synthetic route to the naturally occurring antibiotic (–)- atrop-abyssomicin C, we encountered two shortcomings, which forced us to reconsider our tactics and find new methods to overcome the problems. These methods turned out to be of general applicability, as demonstrated later in total syntheses of two other natural products: (+)-allokainic acid and (-)-gabosine H. The paper provides a brief account of these endeavors.

Crystal structure and absolute configuration of (4S,5R,6S)-4,5,6-trihy-droxy-3-methyl-cyclo-hex-2-enone (gabosine H)

The mol-ecule of the title keto carbasugar, C7H10O4, is formed by a cyclo-hexene skeleton with an envelope conformation, substituted by carbonyl, methyl and hydroxyl groups. The crystal structure is controlled mainly by a combination of strong O-H⋯O and weak C-H⋯O hydrogen bonds, forming nearly perpendicular chains running parallel to the [110] and [-110] directions. This perpendicularity is caused by a tetra-gonal pseudosymmetry influenced by the similarity between the a and b axes, the value of 90.9770 (10)° of the β angle and the action of a 21 screw axis, which transform each chain into its corresponding nearly orthogonal one.

Total Syntheses of (+)-Gabosine P, (+)-Gabosine Q, (+)-Gabosine E, (-)-Gabosine G, (-)-Gabosine I, (-)-Gabosine K, (+)-Streptol, and (-)-Uvamalol A by a Diversity-Oriented Approach Featuring Tunable Deprotection Manipulation

A new diversity-oriented approach to C7-cyclitols, which possess a broad spectrum of biological activities, is developed. The key polyoxygenated intermediates with different O-protecting groups were accessed by an intramolecular aldol-cyclization of a diketone derived from δ-d-gluconolactone. The versatile intermediates can be easily transformed into structurally different carbasugars based on control of deprotection manipulation. The utility of the robust approach is illustrated by the first syntheses of (+)-gabosines P and Q, as well as the syntheses of several other gabosines and related analogues viz. (+)-gabosine E, (-)-gabosine G, (-)-gabosine I, (-)-gabosine K, (+)-streptol, and (-)-uvamalol A. In addition, the absolute configuration of (-)-uvamalol A is assigned by its total synthesis.

Carbasugar Synthesis via Vinylogous Ketal: Total Syntheses of (+)-MK7607, (-)-MK7607, (-)-Gabosine A, (-)-Epoxydine B, (-)-Epoxydine C, epi-(+)-Gabosine E and epi-(+)-MK7607

Carbasugars, the carbocyclic analogues of sugars, constitute an important class of natural products with more than 140 members known and have attracted much attention due to their diverse biological activities like anticancer, antibacterial, herbicidal, and various enzyme inhibitory activities. As many carbohydrates are involved in various cellular signaling pathways, there is great interest in synthesis and biological exploration of carbasugars. Herein, we have developed a methodology to install an α,β-unsaturated aldehyde functionality on different inositols and derivatives by vinylogous elimination of the O-protecting group under mildly acidic condition. We have illustrated the versatility and utility of our methodology by the total syntheses of seven carbasugars viz. (-)-MK7607, (-)-gabosine A, (-)-epoxydine B, (-)-epoxydine C, (+)-MK7607, 1-epi-(+)-MK7607 and 1-epi-(+)-gabosine E.

Biosynthesis and Biological Activity of Carbasugars

The first synthesis of carbasugars, compounds in which the ring oxygen of a monosaccharide had been replaced by a methylene moiety, was described in 1966 by Professor G. E. McCasland’s group. Seven years later, the first true natural carbasugar (5a-carba-R-D-galactopyranose) was isolated from a fermentation broth of Streptomyces sp. MA-4145. In the following decades, the chemistry and biology of carbasugars have been extensively studied. Most of these compounds show interesting biological properties, especially enzymatic inhibitory activities, and, in consequence, an important number of analogues have also been prepared in the search for improved biological activities. The aim of this review is to give coverage on the progress made in two important aspects of these compounds: the elucidation of their biosynthesis and the consideration of their biological properties, including the extensively studied carbapyranoses as well as the much less studied carbafuranoses.

How a diversity-oriented approach has inspired a new hypothesis for the gabosine biosynthetic pathway. A new synthesis of (+)-gabosine C

A new synthesis of (+)-gabosine C has been accomplished as part of a general diversity-oriented approach that also delivered the previously unknown (-)-4-epi-gabosine C. The identification of the unexpected intermediate (+)-8, together with the isolation of ketones 9 and 10 in previous investigations, prompted us to formulate a new hypothesis for the biosynthesis of gabosines, based on a keto-enol equilibrium cascade pathway starting from 2-epi-5-epi-valiolone, along which the necessary precursors for all the different types of gabosines are generated.

Toward synthesis of carbasugars (+)-gabosine C, (+)-COTC, (+)-pericosine B, and (+)-pericosine C

Asymmetric total synthesis of (+)-gabosine C, (+)-pericosine B, and (+)-pericosine C has been reported from readily available d-(-)-isoascorbic acid and d-ribose involving Grubbs ring closing metathesis, Morita-Baylis-Hillman (MBH) reaction, and Luche reduction.

Stereoselective synthesis and relative configuration assignment of gabosine J

The first total synthesis of (+)-gabosine J and that of the epimer at C4 of its enantiomer have been accomplished through an enantioselective approach from a common intermediate 1. These syntheses have allowed us to establish the correct relative configuration of the natural metabolite, which was originally misassigned. This work, together with our former syntheses of other gabosines and related compounds, validates enone 1 as a general synthetic precursor for this kind of carbasugars.

Carbocyclization of carbohydrates: diastereoselective synthesis of (+)-gabosine F, (-)-gabosine O, and (+)-4-epi-gabosine O

Exploitation of silica gel/chloramine T mediated intramolecular nitrile oxide-alkene cycloaddition (INOC) of sugar-derived oximes to carbocycles furnished the first synthesis of gabosine F from l-arabinose in 12 steps with 23% overall yield, thereby confirming its absolute configuration. Similarly, efficient syntheses of gabosine O and 4-epi-gabosine O were accomplished from D-mannose in 9 and 11 steps with 41% and 38% overall yields, respectively, involving INOC, regioselective dehydration, and diastereoselective hydrogenation as the key steps.

Facile syntheses of (+)-gabosines A, D, and E

(+)-Gabosines A (12), D (4), and E (5), which share the same trihydroxycyclohexenone skeleton, were synthesized from enone 11 as the common intermediate. The key building block 11 was accessed by an intramolecular aldol cyclization of a diketone derived from D-glucose (8).

Synthetic efforts for stereo structure determination of cytotoxic marine natural product pericosines as metabolites of Periconia sp. from sea hare

Pericosines are unique C₇ cyclohexenoid metabolites of Periconia byssoides OUPS-N133 fungus that was originally isolated from the sea hare, Aplysia kurodai. Pericosines show significant in vitro cytotoxicity against P388 lymphocytic leukemia cells. Pericosine A, in particular, shows the most potent activity and significant in vivo antitumor activity against P388 cells. Thus, pericosines are promising candidates for seed compounds of anticancer drugs. However, before the total syntheses of pericosines were accomplished, their stereo structures could not be determined by spectral analyses because they have multi-functionalized cyclohexenoid structures with torsional strain. In this review, synthetic efforts for pericosines in this decade are surveyed.

Marine natural products

This review covers the literature published in 2005 for marine natural products, with 704 citations (493 for the period January to December 2005) referring to compounds isolated from marine microorganisms and phytoplankton, green algae, brown algae, red algae, sponges, coelenterates, bryozoans, molluscs, tunicates and echinoderms. The emphasis is on new compounds (812 for 2005), together with their relevant biological activities, source organisms and country of origin. Biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.

Enantioselective Synthesis and Absolute Configuration Assignment of Gabosine O. Synthesis of (+)- and (−)-Gabosine N and (+)- and (−)-Epigabosines N and O

[reaction: see text] A rational approach to the synthesis of gabosines and other related carba-sugars starting from a masked p-benzoquinone has been designed. The enantioselective acetylation of the hydroxyketal 2 provides a practical entry to either enantiomer of the target products. The strategy has been applied to the synthesis of (+)- and (-)-gabosines N and O and (+)- and (-)-epigabosines N and O. The absolute configuration of natural gabosine O has been established.