Kaurene synthesis

In confirmation of earlier indieations that enf-kaurene synthesis is localised in plastids [60-62], Aaeh et al. [44] demonstrated clearly the presence of CPS/EKS activity in the stroma of wheat and pea etioplasts and leucoplasts from C. maxima endosperm. They showed, furthermore, that mature chloroplasts contain no CPS/EKS activity and that the activity is associated with dividing cells in the meristem [45]. These findings, together with the demonstration that CPS is targeted to plastids, provide firm evidence that the first part of GA biosynthesis takes place in plastids. 

Thlaspi arvense mutant EMS-141 (gene not designated) ent-kaurene synthesis bioassay of intermediates GA and precursor content 118... 

We have recently re-examined the effect of CCC and AMO-1618 on the incorporation of (R)-[2- C]mevalonic acid into /-kaurene in a cell-free system from Cucurbita maxima endosperm. The dose-inhibition curves for the two retardants (Fig. 3) confirm that AMO-1618 with an I50 of about 5 X 10 M (inhibitor concentration at which the reaction rate is reduced by 50%) is a much more effective inhibitor than CCC (I50 about 5x lO M). However, the concentration of CCC required to retard growth in wheat seedlings would be sufficient to inhibit ent-kaurene synthesis in the C. maxima system. Thus, the evidence suggests that for growth inhibition the primary site of action of CCC, except at very high concentrations, is the GA biosynthetic pathway. 

EPIC decreased fatty acid synthesis and desaturation (1-3), ent-kaurene synthesis and oxidation (4, 5) and total Isoprenold pigment qul-none, chlorophyll, and contents (6, 7). Commonality of substrates and cofactors In these diversified biosynthetic processes Includes Ac, ... 

Railton, I.D., B. Fellows, and C.A. West E /i -kaurene synthesis in chloro-plasts from higher plants. Phytochem. 23, 1261 (1984). 

Recently, EPTC has been shown to inhibit the conversion of acetate and pyruvate to acetyl-CoA. However, this cannot directly explain the inhibition of enf-kaurene synthesis from MVA. It seems likely that the thiocarbamate effect on gibberellin biosynthesis is a secondary target. This was the conclusion from a study with diallate, which inhibited enf-kaurene synthetase only at high concentrations. Gibberellins may have a role in... 

Since GAs as diterpenes share many intermediates in the biosynthetic steps leading to other terpenoids, eg, cytokinins, ABA, sterols, and carotenoids, inhibitors of the mevalonate (MVA) pathway of terpene synthesis also inhibit GA synthesis (57). Biosynthesis of GAs progresses in three stages, ie, formation of / Akaurene from MVA, oxidation of /-kaurene to GA 2" hyde, and further oxidation of the GA22-aldehyde to form the different GAs more than 70 different GAs have been identified. 

Most of the GA-synthesis inhibitors characterized so far affect two segments of the compHcated pathway from MVA to the many different GAs identified. The cycHzation reactions that produce / Akaurene are inhibited by the onium growth retardants, and the oxidations of /-kaurene to /-kaurenoic acid are sensitive to heterocycHc triazoles such as paclobutrazol and similar compounds. Other enzymes in the pathway are points for pathway dismption by as yet undeveloped GA biosynthesis inhibitors (236). 

Cationic cyclization. A key step in the synthesis of the diterpenes cafestol5 and atractyligenin4 involves a novel cation cyclization of bicyclic cyclopropanes to the tetracyclic systems of the diterpenes (equations I and II). Thus treatment of 1 with a slight excess of triflic anhydride and 2,6-lutidine effects cyclization to the rather unstable pentacycle 2 with the kaurene system. The related conversion of 3 to 4 can be effected with triflic anhydride and 2,6-di-r-butyl-4-methylpyridine in 1-nitropropane. 

Diterpenoid Alkaloids.—The structure and synthesis of the diterpenoid alkaloids have been reviewed. The subject is also covered in detail in the Specialist Periodical Report on the Alkaloids. A number of new alkaloids with the ent-kaurene skeleton have been isolated from Anopterus glandulosus and A. macleayanus. The structure of the major alkaloid anopterine (104) was assigned ... 

Takano et al. 69) exploited the asymmetric aldolization for the synthesis of more functionalized chiral products which possess units suitable for the construction of certain tetracyclic triterpenes, such as gibberellins and kaurenes. They described the enantioselective synthesis of the tricyclic enone (33) from the symmetric triketone (32) and its conversion into the gibbane framework. Again, (S)-proline was used as the catalyst. 

Microbiological Methods. The low substrate specificity of many of the enzymes involved in GA biosynthesis in Gibberella fujikuroi has been utilized for the preparation of higher plant GAs. Suitable analogs of the natural GA-precursors are converted by the fungus to the corresponding GA analogs. It is usual to prevent the synthesis of the natural GAs in order to facilitate purification of the unnatural products. A mutant strain, Bl- la, in which GA biosynthesis is blocked early in the pathway (67) (between ent-kaurenal and ent-kaurenoic acid) has been used. 

It is also possible to block GA synthesis chemically using inhibitors such as AMO-l6l8 (68), or the quaternary ammonium iodide compound in Figure 11 X 9,70), which block ent-kaurene formation. 

Hedden, P., Phinney, B.O. "Comparison of ent-kaurene and ent-isokaurene synthesis in cell-free systems from etiolated shoots of normal and dwarf-5 maize seedlings."... 

Beams and Manders reported (177) the synthesis of the tricyclic dione 349 via two different synthetic routes. The Australian chemists utilized an approach analogous to one developed earlier by Masamune (178) during the synthesis of atisine and kaurene. They prepared tetrahydro-7-methoxy-2-naphthoic acid (350) from 7-methoxytetral-l-one by published procedures. 

The partial synthesis from epicandicandiol of some C- and H-labelled kaurene derivatives has been described.These have possible application in the study of the biosynthesis of the Isodon diterpenoids. The syntheses of [17- C]kaur-16-en-20-ol from enmein and of 3-oxygenated derivatives of [17- C]kaur-16-ene from ent-3jS,19-dihydroxy-kaur-16-ene have also been described. The synthesis of radioactive aphidicolin has also been reported. It has been shown ° that ent-kaur-15-ene is formed by the dwarf mutant (ds) of maize in place of ent-kaur-16-ene. 

Alternatively, some of the natural kaurenes may function as free radical scavengers against oxidative stress. These compounds are amenable to photooxidation in the presence of photosensitizers under moderate UV light to give allylic alcohol derivatives via the corresponding hydroperoxides 50 and 51 (Banerjee et al., 1973). This reaction may explain at least in part the synthesis in vivo of various hydroxyl kaurenes naturally found in Espeletiinae and other plant groups. 

In a biogenetically patterned synthesis, the cyclization of epoxygeranylgeraniol with boron trifluoride etherate has been shown to afford the compounds (103) and the pimaradienols (104). The cyclization of epoxygeranylgeranyl pyrophosphate by kaurene synthetase affords the epimeric 3-hydroxykaurenes. A cyclization of olefinic jS-keto-esters (105) to the bicyclic compound (106) has formedthe basis of a novel synthesis of A -podocarpen-13-one (107), a compound which is an intermediate in several diterpenoid total syntheses. Synthetic approaches to the... 

One further example of quaternary center formation by an AHR has been reported, this being the conversion of the aryl triflate 114 to a 3 1 mixture of the tricycle 115 and its isomer 116, both of which can be converted to the enone 117, a key intermediate in the synthesis of kaurene 118 and abietic acid 119 (Scheme Compound 115 can... 

Plants possessing genes Ih and Is contain very low levels of GA-like activity [ 17] and respond as well as wild-type plants (which were dwarfed with the GA-synthesis inhibitor, AMO 1618) to all GA-precursors examined, including e r-kaurene [6], suggesting that these genes block GA-biosynthesis prior to e kaurene. Neither Is nor Ih plants appear to possess impaired carotenoid production, suggesting that the... 

At first glance, it is difficult to recognize the gibberellins as diter-penes. However, any element of doubt about this has been dispelled by isotope experiments in vivo and in a cell-free system. In the fluid endosperm of the wild cucumber Echinocystis macrocarpa the following synthetic pathway has been demonstrated (Fig. 85) geranylgeranyl pyrophosphate—(—) kaurene—(—) kauren-19-ol —gibberellin A5. Presumably the synthesis then proceeds via Aj to gibberellic acid. 

A series of ionization and proton-induced cycUzations 16 20 catalyzed by the taxadiene synthase convert GGPP 1 into the taxadienes 21 and 22 for en mute synthesis of biological important natural products such as taxol and analogs (Scheme 8.2) [7]. Abietanes andkauranes are important classes of diterpenes. In the presence of abietadiene synthase, 1 undergoes a proton-induced cascade cyclization to produce the copalyl diphosphate intermediate 23 (Scheme 8.2) [8]. The latter is converted to 24 via an ionic cyclization and then to 25 through a methyl 1,2-shift between C13 and C14 [9]. In contrast, diterpene mt-kaur-16-ene 29 is not formed via the same cyclase enzyme. Consequently, it was proposed that two enzymes, mt-copalyl diphosphate and cnt-kaurene... 

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