3650-28-0

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Sesquiterpene synthases catalyze with different catalytic fidelity the cyclization of farnesyl pyrophosphate (FPP) into hundreds of known compounds with diverse structures and stereochemistries. Two sesquiterpene synthases, Cop4 and Cop6, were previously isolated from Coprinus cinereus as part of a fungal genome survey. This study investigates the reaction mechanism and catalytic fidelity of the two enzymes. Cyclization of all-trans-FPP ((E,E)-FPP) was compared to the cyclization of the cis-trans isomer of FPP ((Z,E)-FPP) as a surrogate for the secondary cisoid neryl cation intermediate generated by sesquiterpene synthases, which are capable of isomerizing the C2-C3 π bond of all-trans-FPP. Cop6 is a "high-fidelity" α-cuprenene synthase that retains its fidelity under various conditions tested. Cop4 is a catalytically promiscuous enzyme that cyclizes (E,E)-FPP into multiple products, including (-)-germacrene D and cubebol. Changing the pH of the reaction drastically alters the fidelity of Cop4 and makes it a highly selective enzyme. Cyclization of (Z,E)-FPP by Cop4 and Cop6 yields products that are very different from those obtained with (E,E)-FPP. Conversion of (E,E)-FPP proceeds via a (6R)-β-bisabolyl carbocation in the case of Cop6 and an (E,E)-germacradienyl carbocation in the case of Cop4. However, (Z,E)-FPP is cyclized via a (6S)-β-bisabolene carbocation by both enzymes. Structural modeling suggests that differences in the active site and the loop that covers the active site of the two enzymes might explain their different catalytic fidelities.

Sativene, a metabolite of Helminthosporium sativum or Abies magnifica, was synthesized in the racemic form from 2,3-dimethyl-1,3-cyclohexadiene and methyl coumalate via an inversely-electron-demanded high-pressure Diels-Alder reaction in five steps.

The intramolecular Diels-Alder reactions of (E)- and (Z)-trimethylsilyl cyclopentadienyl ethers, (E)-6a and (Z)-6a, proceed with excellent stereo- and regioselectivity.Starting from the tricyclic keto ester 8, available from the former reaction, a stereoselective synthesis of (+/-)-sativene (26) is described.

Starting from 2-cyclopentenoyl chloride ((RS)- or (S)-8), the racemic as well as the enantiomerically pure (+)-sesquiterpenes longifolene ((+/-)- and (+)-1, resp.) and sativene ((+/-)- and (+)-2, resp.) were synthesized efficiently by a sequence of nine and ten steps, respectively.The key sequence 10 -> 16 -> 3 is the first strategic application of an intramolecular photoaddition/retro-aldolization sequence (intramolecular de Mayo reaction) in organic synthesis.

(+/-)-Sativene (3) is stereoselectively synthesised from tricyclic ketoester 1 in 32percent overall yield.

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