A Highly Stereoselective One-Pot Tandem Consecutive 1,4-Addition-Intramolecular 1,3-Dipolar Cycloaddition Strategy for the Construction of Functionalized Five- and Six-Membered Carbocycles(,)(1)

The Chemistry of the Defensive Secretions of Three Species of Millipedes in the Genus Brachycybe

Millipedes have long been known to produce a diverse array of chemical defense agents that deter predation. These compounds, or their precursors, are stored in high concentration within glands (ozadenes) and are released upon disturbance. The subterclass Colobognatha contains four orders of millipedes, all of which are known to produce terpenoid alkaloids-spare the Siphonophorida that produce terpenes. Although these compounds represent some of the most structurally-intriguing millipede-derived natural products, they are the least studied class of millipede defensive secretions. Here, we describe the chemistry of millipede defensive secretions from three species of Brachycybe: Brachycybe producta, Brachycybe petasata, and Brachycybe rosea. Chemical investigations using mass spectrometry-based metabolomics, chemical synthesis, and 2D NMR led to the identification of five alkaloids, three of which are new to the literature. All identified compounds are monoterpene alkaloids with the new compounds representing indolizidine (i.e. hydrogosodesmine) and quinolizidine alkaloids (i.e. homogosodesmine and homo-hydrogosodesmine). The chemical diversity of these compounds tracks the known species phylogeny of this genus, rather than the geographical proximity of the species. The indolizidines and quinolizidines are produced by non-sympatric sister species, B. producta and B. petasata, while deoxybuzonamine is produced by another set of non-sympatric sister species, B. rosea and Brachycybe lecontii. The fidelity between the chemical diversity and phylogeny strongly suggests that millipedes generate these complex defensive agents de novo and begins to provide insights into the evolution of their biochemical pathways.

Intramolecular [3 + 2] nitrone cycloaddition reaction: highly regio and diastereoselective synthesis of bicyclo[3.2.1]octane scaffolds

A regio and diastereoselective strategy has been developed for the synthesis of complex bicyclo[3.2.1]octane scaffolds from the reaction of readily accessible vinylogous carbonates with N-substituted hydroxylamine hydrochlorides via intramolecular 1,3-dipolar nitrone cycloaddition reaction. Wide varieties of bicyclic isoxazolidines were synthesized in high yields under catalyst free conditions through a highly efficient and operationally simple protocol.

Total Synthesis of Fawcettimine-Type Alkaloid, Lycojaponicumin A

The efficient total synthesis of lycojaponicumin A (1) has been accomplished for the first time. The remarkable features of this novel strategy include the following: (1) rapid construction of tricyclic intermediate 4 through a regio- and stereoselective semipinacol ring expansion, which simplified the construction of rings A and B of 1; (2) the subsequent regio- and stereoselective formation of the highly strained rings C-E of 1 through a tandem oxa-hetero [3 + 2] cycloaddition/N-cycloalkylation.

Synthesis of Bicyclic Isoxazoles and Isoxazolines via Intramolecular Nitrile Oxide Cycloaddition

An efficient and straight forward procedure for the syntheses of bicyclic isoxazole/isoxazoline derivatives from the corresponding dimethyl-2-(2-nitro-1-aryl/alkyl)-2-(prop-2-yn-1yl)malonates or dimethyl 2-allyl-2-(2-nitro-1-aryl/alkyl ethyl)malonate is described. High yields and simple operations are important features of this methodology.

1,4-Addition of TMSCCl₃ to nitroalkenes: efficient reaction conditions and mechanistic understanding

Improved synthetic conditions allow preparation of TMSCCl3 in good yield (70%) and excellent purity. Compounds of the type NBu4X [X=Ph3SiF2 (TBAT), F (tetrabutylammonium fluoride, TBAF), OAc, Cl and Br] act as catalytic promoters for 1,4-additions to a range of cyclic and acyclic nitroalkenes, in THF at 0-25 °C, typically in moderate to excellent yields (37-95%). TBAT is the most effective promoter and bromide the least effective. Multinuclear NMR studies ((1)H, (19)F, (13)C and (29)Si) under anaerobic conditions indicate that addition of TMSCCl3 to TBAT (both 0.13 M) at -20 °C, in the absence of nitroalkene, leads immediately to mixtures of Me3SiF, Ph3SiF and NBu4CCl3. The latter is stable to at least 0 °C and does not add nitroalkene from -20 to 0 °C, even after extended periods. Nitroalkene, in the presence of TMSCCl3 (both 0.13 M at -20 °C), when treated with TBAT, leads to immediate formation of the 1,4-addition product, suggesting the reaction proceeds via a transient [Me3Si(alkene)CCl3] species, in which (alkene) indicates an Si⋅⋅⋅O coordinated nitroalkene. The anaerobic catalytic chain is propagated through the kinetic nitronate anion resulting from 1,4 CCl3(-) addition to the nitroalkene. This is demonstrated by the fact that isolated NBu4[CH2=NO2] is an efficient promoter. Use of H2C=CH(CH2)2CH=CHNO2 in air affords radical-derived bicyclic products arising from aerobic oxidation.

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.

Efficient conversion of nitronate into nitrile oxide using cyanuric chloride. One-pot synthesis of bicyclic isoxazolines and isoxazoles from nitroalkenes

Bicyclic isoxazolines and isoxazoles are obtained in good yields by proceeding through a convenient one-pot, two-step procedure utilizing 2,4,6-trichloro-1,3,5-triazine (TCT) as a dehydrating agent.

A Stereoselective and Atom-Efficient Approach to Multifunctionalized Five- and Six-Membered Rings via a Novel Michael-Initiated Intramolecular Diels−Alder Furan Reaction

[reaction: see text] A variety of key precursors to the intramolecular Diels-Alder reaction of furan diene (IMDAF) have been prepared via a very facile 1,4-addition of O-, S-, N-, and C-centered nucleophiles possessing unsaturated tether to beta-furyl nitroethylene. Subsequent IMDAF reaction of the 1,4-adducts, carried out under thermal conditions, provided five- and six-membered carbocycles and heterocycles fused to an easily cleavable oxabicycloheptene moiety. The structure and stereochemistry of the cycloadducts were determined by 2D-NMR experiments and further confirmed by X-ray crystallography. The salient features of the strategy include high degree of stereoselectivity (>80:20) in the cycloaddition, atom and step economy, and generation of multiple chiral centers and functionalities. The feasibility of the cleavage of the oxa bridge in the cycloadducts to afford novel multifunctional molecules has also been demonstrated.

Asymmetric Synthesis of 3-Substituted Proline Chimeras Bearing Polar Side Chains of Proteinogenic Amino Acids

The amino-zinc-ene-enolate cyclization reaction is a straightforward route to the synthesis of 3-substituted prolines. Herein we report the application of this reaction to the syntheses of proline chimeras of lysine, glutamic acid, glutamine, arginine, and serine. All these compounds were obtained in enantiomerically pure form and suitably protected for peptide synthesis.

Novel transformations of gamma-silyl nitro compounds

Introduction of a gamma-silyl group into nitro compounds of dihydrobenzofuran, dihydrobenzo[b]thiophene, and dihydrofuran allowed new transformations to take place in the presence of a Lewis acid to give the corresponding alpha,beta-unsaturated oximes or multisubstituted dihydrofurans, respectively, in good to excellent yields.[reaction: see text]

Sequential 1,3-dipolar cycloadditions in the synthesis of bis-isoxazolo substituted piperidinones

A strategy for the efficient synthesis of novel bis-isoxazolo substituted piperidinones has been developed. The protocol consists of the Michael addition of an unsaturated alkoxide to beta-nitrostyrene followed by an intramolecular nitrile oxide cycloaddition (INOC) or an intramolecular silyl nitronate olefin cycloaddition (ISOC) to give III. Grignard addition to this isoxazoline intermediate followed by DCC coupling of the resulting isoxazolidine with nitroacetic acid gave II, and a second intramolecular cycloaddition via 1,3-dipoles result in the formation of the targeted novel tetracycles (I). A solid-supported scavenger was employed to increase the efficiency and yield of the Michael addition step.