A double alkylation—ring closing metathesis approach to spiroimines

Synthesis of the C4-C16 Polyketide Fragment of Portimines A and B

Stereoselective synthesis of the C4-C16 polyketide fragment of portimines A and B is reported, enabled by our previously established method for the stereoselective synthesis of syn-α,α'-dihydroxyketones. The preparation of this advanced fragment provides insights useful for the total synthesis of portimines A and B. An asymmetric Evans aldol reaction was used to install the C10-C11 adjacent stereogenic centers before incorporation of indantrione, followed by epoxidation and epoxide opening to forge the challenging syn-α,α'-dihydroxyketone functionality.

Synthesis and biology of cyclic imine toxins, an emerging class of potent, globally distributed marine toxins

From a small group of exotic compounds isolated only two decades ago, Cyclic Imine (CI) toxins have become a major class of marine toxins with global distribution. Their distinct chemical structure, biological mechanism of action, and intricate chemistry ensures that CI toxins will continue to be the subject of fascinating fundamental studies in the broad fields of chemistry, chemical biology, and toxicology. The worldwide occurrence of potent CI toxins in marine environments, their accumulation in shellfish, and chemical stability are important considerations in assessing risk factors for human health. This review article aims to provide an account of chemistry, biology, and toxicology of CI toxins from their discovery to the present day.

Synthesis of the C10-C24-bis-spiroacetal core of 13-desmethyl spirolide C based on a sila-Stetter-acetalization process

Synthesis of the bis-spiroacetal core of 13-desmethyl spirolide C has been completed based on a sila-Stetter-acetalization process. The acylsilane and enone partners in the Stetter reaction were prepared in 7 and 11 steps, respectively, from (S) and (R)-aspartic acid. The quaternary center at C19 in the enone moiety was controlled by relying on the Seebach's chiral self-reproduction method using an enantiopure (S)-lactic acid based dioxolanone. The final acid-catalyzed spiroacetalization provided the desired spiroacetal as a mixture of diastereoisomers in 13 linear steps. Whatever the conditions used, the non-natural transoid isomer was formed preferentially. However, both cisoid and transoid isomers were isolated pure and their structure assigned unambiguously through NMR spectroscopic studies.

Total synthesis of the spirocyclic imine marine toxin (-)-gymnodimine and an unnatural C4-epimer

The first total synthesis of the marine toxin (-)-gymnodimine (1) has been accomplished in a convergent manner. A highly diastereo- and enantioselective exo-Diels-Alder reaction catalyzed by a bis-oxazoline Cu(II) catalyst enabled rapid assembly of the spirocyclic core of gymnodimine. The preparation of the tetrahydrofuran fragment utilized a chiral auxiliary based anti-aldol reaction. Two major fragments, spirolactam 56 and tetrahydrofuran 55, were then coupled through an efficient Nozaki-Hiyama-Kishi reaction. An unconventional, ambient temperature t-BuLi-initiated intramolecular Barbier reaction of alkyl iodide 64 was employed to form the macrocycle. A late stage vinylogous Mukaiyama aldol addition of a silyloxyfuran to a complex cyclohexanone 83 appended the butenolide, and a few additional steps provided (-)-gymnodimine (1). A diastereomer of the natural product was also synthesized, C4-epi-gymnodimine (90), derived from the vinylogous Mukaiyama aldol addition.

Studies toward the synthesis of spirolides: assembly of the elaborated E-ring fragment

A stereoselective synthesis of the spiroimine fragment of spirolide C is described. The congested C7 and C29 tertiary and quaternary centers are constructed by a diastereoselective Ireland-Claisen rearrangement. The E ring is completed by means of an aldol cyclocondensation. Additional studies were preformed on the advanced intermediate to probe a future coupling strategy.

(±)-2'-Phenyl-cyclo-hexa-nespiro-4'-(aze-pano[1,2-b]isoxazolidine)

In the crystal structure of the racemic title isoxazolidine, C₁₉H₂₇NO, the relative stereochemistry between the phenyl group and the bridgehead H atom is shown to be syn. There are two mol­ecules in the asymmetric unit, one of which is the 7R*,13R* enanti­omer, and one of which is the 7S*,13S* enanti­omer. These enanti­omers adopt different orientations of the phenyl ring with respect to the isoxazolidine ring, with C—C—C—C torsion angles of 63.6 (4) and 86.8 (4)°, respectively. In both enanti­omers, the six-membered ring adopts a chair conformation, while the seven-membered ring adopts a twist-chair conformation.

(1R,6R)-1-Methyl-8-aza-spiro-[5.6]dodecan-7-one

The crystal structure of the title compound, C₁₂H₂₁NO, has been investigated to establish the absolute stereochemistry at position 1. The absolute stereochemistry at the quaternary centre at position 6 is established to be R using an asymmetric Birch reductive alkyl­ation reaction for which the stereochemical outcome is known. The crystal structure indicates the presence of two conformers of the bicyclic (1R,6R)-spiro­lactam ring system that differ in the conformation adopted by the six-membered ring. In one conformer, the meth­yl group adopts an axial position whereas in the other conformer, the same methyl group adopts an equatorial position. In both conformers, the seven-membered ring adopts a chair conformation. The two conformers of the bicyclic spiro­lactam are connected to each other via inter­molecular N—H⋯O hydrogen bonds forming a heterodimer. The asymmetric unit contains two such dimers.

Synthesis of the bis-spiroacetal moiety of the shellfish toxins spirolides B and D using an iterative oxidative radical cyclization strategy

The enantioselective synthesis of the bis-spiroacetal fragment of the shellfish toxins, spirolides B 1 and D 2, is reported. The carbon framework was constructed via a Barbier reaction of dihydropyran 10 with aldehyde 11, followed by two oxidative radical cyclizations to construct the bis-spiroacetal ring system. A silyl-modified Prins cyclization and enantioselective crotylation successfully installed the stereocenters in the cyclization precursor 21. The initial unsaturated bis-spiroacetals 9a-d underwent equilibration during epoxidation to trans-epoxide 24 that was converted to tertiary alcohol 7.

Studies toward a Marine Toxin Immunogen: Enantioselective Synthesis of the Spirocyclic Imine of (−)-Gymnodimine

[reaction: see text] An enantioselective Diels-Alder reaction catalyzed by an Evans' copper-bis(oxazoline) complex was utilized to construct a highly functionalized spirolactam, a key intermediate in our projected total synthesis of the marine toxin, gymnodimine. Additional transformations, including a mild N-tosyl group deprotection, afforded a keto spirocyclic imine moiety, the proposed pharmacophore of gymnodimine. Thus, the prepared ketone is a potentially useful intermediate for conjugation to provide an immunogen for eventual monitoring of gymnodimine and congeners.

Synthesis of the Bis-spiroacetal Moiety of Spirolides B and D

An enantioselective synthesis of the bis-spiroacetal fragment of spirolides B and D is reported. The carbon framework was constructed via Barbier reaction of dihydropyran 3 with aldehyde 4, followed by a double oxidative radical cyclization to construct the bis-spiroacetal. A silyl-modified Prins cyclization and enantioselective crotylation successfully installed the stereocenters in the cyclization precursor. The initial unsaturated bis-spiroacetals 2a-d underwent equilibration during epoxidation to trans-epoxide 14 that was converted to a tertiary alcohol. [reaction: see text]