Poison Ivy and Poison

Poison ivy (Rhus radicans or Toxicodendron radicans; Anacardiaceae) is a woody vine with three-lobed leaves that is common in the USA. The plant may be climbing, shrubby, or may trail over the ground. It presents a considerable hazard to humans should the sap, which exudes from damaged leaves or stems, come into contact with the skin. The sap sensitizes most individuals, producing delayed contact dermatitis after a subsequent encounter. This results in watery blisters that break open, the fluid quickly infecting other parts of the skin. The allergens may be transmitted from one person to another on the hands, on clothing, or by animals. The active principles are urushiols, a mixture of alkenyl polyphenols. In poison ivy, these are mainly pentadecylcatechols with varying degrees of unsaturation (A8, A8-11, A8'11'14) in the side-chain. Small amounts of C17 side-chain analogues are present. These catechols become oxidized to an ortho-quinone, which is then attacked by nucleophilic groups in proteins to yield an antigenic complex.

Poison oak (Rhus toxicodendron or Toxicodendron toxicaria: Anacardiaceae) is nearly always found as a low-growing shrub, and has lobed leaflets similar to those of oak. It is also common throughout North America. There appears considerable confusion over nomenclature, and Rhus radicans may also be termed poison oak, and R. toxicodendron oakleaf poison ivy. Poison oak contains similar urushiol structures in its sap as poison ivy, though heptadecylcatechols (i.e. C17 side-chains) predominate over pentadecylcatechols (C15 side-chains).

Related species of Rhus, e.g. R. diversiloba (Pacific poison oak) and R. vernix (poison sumach, poison alder, poison dogwood) are also allergenic with similar active constituents. The allergen-containing species of Rhus have been reclassified under the genus Toxicodendron, though this nomenclature is not commonly employed. Dilute purified extracts containing urushiols may be employed to stimulate antibody production and thus build up immunity to the allergens.

in a C17 side-chain. Large quantities of anacardic acids containing C15 side-chains with one, two, and three double bonds are also found in the shells of cashew nuts (Anacardium occidentale; Anacardiaceae).

A saturated C6 hexanoate starter unit is used in the formation of the aflatoxins*, a group of highly toxic metabolites produced by Aspergillus flavus, and probably responsible for the high incidence of liver cancer in some parts of Africa. These compounds were first detected following the deaths of young turkeys fed on mould-contaminated peanuts (Arachis hypogaea; Legu-minosae/Fabaceae). Peanuts still remain one of the crops most likely to represent a potential risk to human health because of contamination with fungal toxins. These and other food materials must be routinely screened to ensure levels of aflatoxins do not exceed certain set limits. The aflatoxin structures contain a bisfuran unit fused to an aromatic ring, e.g. aflatoxin B1 and aflatoxin G1, and their remarkably complex biosynthetic origin begins with a poly-^-keto chain derived from a hexanoyl-CoA starter and seven malonyl-CoA extender units (Figure 3.49). This gives an anthraquinone norsolorinic acid by now-familiar condensation reactions, but the folding of the chain is rather different from that seen with simpler anthraquinones (see page 64). The six-carbon side-chain of nor-solorinic acid is cyclized to give, in several steps, the ketal averufin. Versiconal acetate is another known intermediate, and its formation involves a Baeyer-Villiger oxidation (see page 28), resulting principally in transfer of a two-carbon fragment (the terminal ethyl of hexanoate) to become an ester function. These two carbons can then be lost in formation of versicolorin B, now containing the tetrahydrobisfuran moiety, oxidized in versi-colorin A to a dihydrobisfuran system. Sterig-matocystin is derived from versicolorin A by

CoAS

7 x malonyl-CoA

aldol and Claisen reactions; O O SEnz q aromatizations; oxidation oh O OH O

AAA, hexanoyl-CoA

aldol and Claisen reactions; O O SEnz q aromatizations; oxidation oh O OH O

AAA,

7 x malonyl-CoA

hemiacetal formation

Baeyer-Villiger oxidation

rearrangement - details not known; ketal substrate shown hydrolysed back to ketone/alcohols rVcpHl p ^O^f

O norsolorinic acid several steps: cyclizations to achieve bondings indicated gives ketal, probably via

OH O OH

OH O OH

versiconal acetate

acetyl carbons lost by hydrolysis; formation of new furan ring

(ester)

f&To several steps including Baeyer-Villiger oxidation ketone ^ ester

(ketal)

averufin

(ketal)

OH O OH

versicolorin B

OH O OH

versicolorin B

tetrahydro-bisfuran

versicolorin A

dihydro-bisfuran

OH O OH

OH O OH

versicolorin A

\ P /(Y methylation reduction

aflatoxin Gi

Baeyer-Villiger oxidation

Baeyer-Villiger oxidation ketone ^ ester allows hydrolysis and ring cleavage oxidative cleavage of aromatic ring with loss of one carbon; recyclization as indicated it decarboxylation

I several steps: rotation * of left-hand ring system; I formation of ether linkage to give xanthone

aflatoxin Gi

Baeyer-Villiger oxidation

aflatoxin Bi

sterigmatocystin (xanthone)

Figure 3.49

oxidative cleavage of the anthraquinone system involving a second Baeyer-Villiger oxidation, and recyclization through phenol groups to give a xanthone skeleton. Rotation of an intermediate leads to the angular product as opposed to a linear product. One phenol group is methylated, and, quite unusually, another phenol group is lost (contrast loss of oxygen functions via reduction/dehydration prior to cyclization, see page 62). Aflatoxin B1 formation requires oxidative cleavage of an aromatic ring in sterigmatocystin, loss of one carbon and recyclization exploiting the

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