Autoxidation of salvinorin A under basic conditions

Neoclerodanes as atypical opioid receptor ligands

The neoclerodane diterpene salvinorin A is the major active component of the hallucinogenic mint plant Salvia divinorum Epling and Játiva (Lamiaceae). Since the finding that salvinorin A exerts its potent psychotropic actions through the activation of opioid receptors, the site of action of morphine and related analogues, there has been much interest in elucidating the underlying mechanisms behind its effects. These effects are particularly remarkable because (1) salvinorin A is the first reported non-nitrogenous opioid receptor agonist and (2) its effects are not mediated through the previously investigated targets of psychotomimetics. This Perspective outlines our research program, illustrating a new direction to the development of tools to further elucidate the biological mechanisms of drug tolerance and dependence. The information gained from these efforts is expected to facilitate the design of novel agents to treat pain, drug abuse, and other central nervous system disorders.

Synthesis of neoclerodane diterpenes and their pharmacological effects

Salvinorin A is a neoclerodane diterpene that has been shown to be an agonist at kappa opioid receptors. Its unique structure makes it an attractive target for synthetic organic chemists due to its seven chiral centers and diterpene scaffold. This molecule is also interesting to pharmacologists because it is a non-serotonergic hallucinogen, and the first opioid ligand discovered that lacks a basic nitrogen. There have been several total synthesis approaches to salvinorin A, and these will be detailed within this chapter. Additionally, research efforts have concentrated on structure modification of the salvinorin A scaffold through semi-synthetic methods. Most modifications have focused on the manipulation of the acetate at C-2 and the furan ring. However, chemistry has also been developed to generate analogs at the C-1 ketone, the C-4 methyl ester, and the C-17 lactone. The synthetic methodologies developed for the salvinorin A scaffold will be described, as well as specific analogs with interesting biological activities.

Neuropharmacology of the naturally occurring kappa-opioid hallucinogen salvinorin A

Salvia divinorum is a perennial sage native to Oaxaca, Mexico, that has been used traditionally in divination rituals and as a treatment for the "semimagical" disease panzón de borrego. Because of the intense "out-of-body" experiences reported after inhalation of the pyrolized smoke, S. divinorum has been gaining popularity as a recreational hallucinogen, and the United States and several other countries have regulated its use. Early studies isolated the neoclerodane diterpene salvinorin A as the principal psychoactive constituent responsible for these hallucinogenic effects. Since the finding that salvinorin A exerts its potent psychotropic actions through the activation of KOP receptors, there has been much interest in elucidating the underlying mechanisms behind its effects. These effects are particularly remarkable, because 1) salvinorin A is the first reported non-nitrogenous opioid receptor agonist, and 2) its effects are not mediated by the 5-HT(2A) receptor, the classic target of hallucinogens such as lysergic acid diethylamide and mescaline. Rigorous investigation into the structural features of salvinorin A responsible for opioid receptor affinity and selectivity has produced numerous receptor probes, affinity labels, and tools for evaluating the biological processes responsible for its observed psychological effects. Salvinorin A has therapeutic potential as a treatment for pain, mood and personality disorders, substance abuse, and gastrointestinal disturbances, and suggests that nonalkaloids are potential scaffolds for drug development for aminergic G-protein coupled receptors.

Natural products from the hallucinogenic sage

The isolation, structures and biological activity of the neoclerodane and other natural products obtained from the Mexican hallucinogenic sage, Salvia divinorum are reviewed.

Chemical methods for the synthesis and modification of neoclerodane diterpenes

Diterpenes are a structural class of molecules that are derived from four isoprene subunits and are widespread throughout nature. A number of neoclerodane diterpenes have been found to have biological activity but a limited number of chemical investigations have been conducted. Recently, the neoclerodane diterpene, salvinorin A (12) has been investigated due to its unique pharmacological profile. This review will discuss the chemical methods used to chemically modify and synthesize 12.

Synthesis of deacetyl-1,10-didehydrosalvinorin G

To unambiguously confirm the actual product in autoxidation of salvinorin A under basic conditions, deacetyl-1,10-didehydrosalvinorin G was synthesized from salvinorin C via intermediate salvinorin H. Furthermore, oxidation of salvinorin D with manganese dioxide gave salvinorin G in good yield.

Revised structure of deacetyl-1,10-didehydrosalvinorin G

In comparison with the NMR data of salvinorin A and its 8-epimer, the published structure of deacetyl-1,10-didehydrosalvinorin G was revised to its 8-epimer. The stereochemistry of 8-epi-deacetyl-1,10-didehydrosalvinorin G was further confirmed by NOESY and chemical synthesis.

8-epi-Salvinorin B: crystal structure and affinity at the kappa opioid receptor

There have been many reports of epimerization of salvinorins at C-8 under basic conditions, but little evidence has been presented to establish the structure of these compounds. We report here the first crystal structure of an 8-epi-salvinorin or derivative: the title compound, 2b. The lactone adopts a boat conformation with the furan equatorial. Several lines of evidence suggest that epimerization proceeds via enolization of the lactone rather than a previously proposed indirect mechanism. Consistent with the general trend in related compounds, the title compound showed lower affinity at the kappa opioid receptor than the natural epimer salvinorin B (2a). The related 8-epi-acid 4b showed no affinity.

Neo-clerodane diterpenes from the hallucinogenic sage Salvia divinorum

Seven new neo-clerodane diterpenes, salvidivins A (2), B, (3), C (4), and D (5), salvinorins H (6) and I (7), and divinatorin [corrected] F (8), along with eight known neo-clerodane diterpenes, salvinorins A (1)-F, divinatorins A and B, and seven other constituents, were isolated from the hallucinogenic sage Salvia divinorum. The structures of 1-7 were elucidated on the basis of 2D NMR spectroscopic studies.

Synthetic studies of neoclerodane diterpenes from Salvia divinorum: selective modification of the furan ring

A synthetic sequence has been developed to selectively functionalize the furan ring of the natural product salvinorin A (2a). The synthetic routes described convert the furan ring in 2a into an N-sulfonylpyrrole, oxazole or an oxadiazole. In addition, a procedure has been found to remove the furan skeleton completely. Biological results indicate that replacement of the furan ring with an N-sulfonylpyrrole leads to reduced affinity and efficacy at kappa opioid receptors.

Studies Towards the Synthesis of Salvinorin A

Salvinorin A 1, a psychoactive neoclerodane diterpenoid from the Mexican sage S. divinorum, has gained interest as a selective κ-opioid receptor agonist. Non-racemic 3-furylamines 9a and 9b have been prepared from (+)-pseudoephedrine and (–)-ephedrine for application in the stereoselective synthesis of the ketone ring of 1. Diels–Alder reaction of 9b with methyl acrylate in aqueous media, followed by selective ether bridge cleavage, has allowed access to the cyclohexenone 17 with preservation of stereochemistry at C2. A model route to the lactone ring has also been achieved through a one-pot deconjugation/esterification procedure of 2-bromocrotonyl chloride 20 to the furyl alcohol 19 followed by Reformatski-mediated ring closure.

Diterpenoids

This review covers the isolation and structures of diterpenoids, including labdanes, clerodanes,pimaranes, abietanes, kauranes, cembranolides, taxanes and marine diterpenoids. The literature from January to December 2005 is reviewed, and 195 references are cited.