Synthetic intermediates potentially useful for the synthesis of tetrodotoxin and derivatives. 9. Hydroquinazolines possessing the carbon skeleton of tetrodotoxin

Water: new aspect of hydrogen bonding in the solid state

All water-water contacts in the crystal structures from the Cambridge Structural Database with d OO ≤ 4.0 Å have been found. These contacts were analysed on the basis of their geometries and interaction energies from CCSD(T)/CBS calculations. The results show 6729 attractive water-water contacts, of which 4717 are classical hydrogen bonds (d OH ≤ 3.0 Å and α ≥ 120°) with most being stronger than -3.3 kcal mol-1. Beyond the region of these hydrogen bonds, there is a large number of attractive interactions (2062). The majority are antiparallel dipolar interactions, where the O-H bonds of two water molecules lying in parallel planes are oriented antiparallel to each other. Developing geometric criteria for these antiparallel dipoles (β1, β2 ≥ 160°, 80 ≤ α ≤ 140° and T HOHO > 40°) yielded 1282 attractive contacts. The interaction energies of these antiparallel oriented water molecules are up to -4.7 kcal mol-1, while most of the contacts have interaction energies in the range -0.9 to -2.1 kcal mol-1. This study suggests that the geometric criteria for defining attractive water-water interactions should be broader than the classical hydrogen-bonding criteria, a change that may reveal undiscovered and unappreciated interactions controlling molecular structure and chemistry.

Stereocontrolled Total Synthesis of Tetrodotoxin from myo-Inositol and D-Glucose by Three Routes: Aspects for Constructing Complex Multi-Functionalized Cyclitols with Branched-Chain Structures

This report describes the stereocontrolled total synthesis of the multi-functionalized cyclitol derivative, tetrodotoxin, containing eight asymmetric carbons and different types of branched-chains, from myo-inositol and D-glucose using three different methods. The tetrodotoxin derivatives possess a relatively small molecular weight but unique structural and chemical properties. Selection of the appropriate synthetic method may be useful not only for compounds related to TTX (including related derivatives), but also for other highly complex multi-functionalized cyclitols containing branched-chains.

Stereocontrolled Total Synthesis of Tetrodotoxin from myo-Inositol and D-Glucose by Three Routes: Aspects for Constructing Complex Multi-Functionalized Cyclitols with Branched-Chain Structures

This report describes the stereocontrolled total synthesis of the multi-functionalized cyclitol derivative tetrodotoxin containing eight asymmetric carbons and different types of branched-chains from myo-inositol and D-glucose using three different methods. The tetrodotoxin derivatives possess a relatively small molecular weight but unique structural and chemical properties. Selection of the appropriate synthetic method may be useful not only for compounds related to TTX (including related derivatives) but also for other highly complex multi-functionalized cyclitols containing branched-chains.

The chemical synthesis of tetrodoxin: an ongoing quest

This contribution reviews all the synthetic work on tetrodotoxin that has appeared in the literature through June 2011.

Approach to tetrodotoxin via the oxidative amidation of a phenol

An approach to tetrodotoxin that relies on the oxidative amidation of methyl 4-hydroxyphenylacetate as a key step is described. The stereoselective introduction of a beta-hydroxynitrile functionality on one of the double bonds of the emerging dienone is achieved through an intramolecular nitrile oxide cycloaddition-fragmentation sequence.

Formation of Five- and Six-Membered Carbocycles with Nitrogenated Tetrasubstituted Carbons by Radical Addition-Carbocyclization of Alkynyl Ketoxime Ethers

C3-Ketoxime ethers bearing alkynes with terminal delta-yne or internal gamma-yne functions were prepared in five or six steps and approximately 20% overall yield from commercial glucofuranose derivatives. Their thiyl-, stannyl-, or carbon radical-promoted addition-carbocyclization gave five- or six-membered carbocycles nitrogenated at one of the bridgehead positions. For internal gamma-yne ethers the tandem process was strongly dependent on both the alkyne substituent and the radical-promoting species and could be directed toward either the five- or the six-membered carbocycle. These results are presented and discussed in the context of studies working toward (-)-tetrodotoxin.

Asymmetric total synthesis of complex marine natural products

Among nature's ecosystems, the marine environment has been an extremely rich source of structurally complex and biologically active molecules. This review aims to cover the recent developments in the synthesis of marine natural products, also reflecting the trend of their increased use to address biological questions. The examples chosen should be viewed as representative of the different structural motifs on the one hand and the strategies and stimuli for their synthesis on the other.

Armed/Disarmed Effects and Adamantyl Expansion of Some Caged Tricyclic Acetals en Route to Tetrodotoxin(1)(,)(2)

Transformation of the previously prepared tricyclic ketone 4 into an advanced intermediate, 2a, of the Kishi-Goto synthesis of tetrodotoxin requires, among other things, cleavage of the internal acetal. In our attempts to carry this out, we were confronted by two major obstacles, one resulting from armed/disarmed effects encountered during acid-catalyzed acetolyses. Thus ester protecting groups proximal to the acetal moiety inhibited cleavage, e.g., 4 --> 8a and 7 --> 8b. Although the corresponding ether analogs 9a and 9b did undergo acetolysis, the products obtained, 10a and 10b, respectively, revealed the second obstacle, namely the proclivity of the caged systems to undergo adamantyl expansion. The latter result was found to depend upon the presence of properly positioned nucleophilic substituents. Thus 11b underwent adamantyl expansion to 12b but its C7 epimer 15 experienced facile cleavage to bicyclic product 16. As an alternative to solvolysis for cleavage of the internal acetal, reductive elimination was examined. For example, compound 28a, obtained from 4 by standard procedures, reacted with zinc to give, after protection and saponification, gamma,delta unsaturated carboxylic acid 29a, which underwent smooth iodolactonization. Replacing the iodide of this product with an hydroxyl (32a --> 32b) by a free radical process has succeeded albeit in disappointing yield. Nevertheless the resulting hydroxy lactone is a promising synthon of the advanced Kishi-Goto intermediate.