Oxidation of squalene

Figure 27.13 Proposed mechanism of the oxidation of squalene by flavin hydroperoxide.

More recently it has been shown (6, 7) that zinc dialkyl dithiophosphates also act as chain-breaking inhibitors. Colclough and Cunneen (7) reported that zinc isopropyl xanthate, zinc dibutyl dithiocarbamate, and zinc diisopropyl dithiophosphate all substantially lowered the rate of azobisisobutyronitrile-initiated oxidation of squalene at 60°C. Under these conditions, hydroperoxide chain initiation is negligible, and it was therefore concluded that inhibition resulted from removal of chain-propagating peroxy radicals. Also, consideration of the structure of these zinc dithioates led to the conclusion that no suitably activated hydrogen atom was available, and it was suggested that inhibition could be accounted for by an electron-transfer process as follows ... 

FIGURE 6.28 Oxidation of squalene to 6-methyl-5-hepten-2-one, acetone, geranyl acetone, and 4-oxopentanal (adapted from Fruekilde et al., f998). 

Oxidative polycyclizations with, for example, RuOa catalysts can be carried out with polyene substrates as complex as farnesyl acetate, geranylgeranyl acetate, and squalene. The f , f , /ra j,/ra r,/ra r-configuration of the penta-tetrahydrofuranyl diol product resulting from the oxidation of squalene (Scheme 57) has been determined by nuclear magnetic resonance (NMR) spectroscopy <2005T927>. 

Formation of steroids results from enzymatic oxidation of squalene followed by cyclization. This produces... 

Barnard et al.20 presented a somewhat different picture of the involvement of sulfur compounds in oxidation inhibition involving olefinic hydrocarbons. They studied the oxidation of squalene (an olefin) in the presence of sulfur compounds and concluded by careful measurement of oxygen uptake that it was not the sulfide that inhibited oxidation but the initially formed sulfoxide, and that inhibition was very dependent on the chemical structure of the sulfide. However, they did not suggest any specific mechanism for the inhibition. 

Squalene 2,3-epoxide has been isolated from the green alga Caulerpa prolifera. Oxidation of squalene with t-butyl hydroperoxide in the presence of Mo02(acac)2 and di-isopropyl (+)-tartrate gave the 2,3-epoxide (31%) with an induced asymmetry of about 14% in favour of the (35)-isomer. The ability of oxidosqualene cyclases to accept unnatural precursors has been further extended by the observation that lanosterol cyclase from rabbit liver converts the synthetic epoxide (1) into the jS-onocerin derivative (2). An authentic sample of (2) was prepared by sodium cyanoborohydride reduction of /3-onoceradione... 

Figure 4 shows the effectiveness of the cathodic solution in protection of squalene (5). When the oxidation of squalene dispersed in the cathodic and NaCl solutions was induced by 1.0 mM of AAPH Figure 4, (A) and (B)), the rate of formation of total peroxides in the NaCI solution was much higher than that in the cathodic solution (Figure 4, (A)). The same result for the antioxidant effect of the cathodic solution was obtained by analyzing the decrease in the unoxidized squalene upon oxidation (Figure 4, (B)). The cathodic solution also inhibited the aqueous oxidation of squalene induced by 5.0 mM 2,2 -azobis (2,4-dimethyl-valeronitrile) (AMVN) (Figure 4, (C) and (D)), but the effect of the cathodic solution on AMVN-induced oxidation was weaker than that on AAPH-induced oxidation.

Cyclization of squalene in eukaryotes is initiated by protonation of the epoxide derivative, squalene oxide 96, formed by flavin-mediated oxidation of squalene with molecular O2. A wide variety of polycychc structures with different ring sizes and stereochemistries... 

Open-chain 1,5-polyenes (e.g. squalene) and some oxygenated derivatives are the biochemical precursors of cyclic terpenoids (e.g. steroids, carotenoids). The enzymic cyclization of squalene 2,3-oxide, which has one chiral carbon atom, to produce lanosterol introduces seven chiral centres in one totally stereoselective reaction. As a result, organic chemists have tried to ascertain, whether squalene or related olefinic systems could be induced to undergo similar stereoselective cyclizations in the absence of enzymes (W.S. Johnson, 1968, 1976). 

The achiral triene chain of (a//-rrans-)-3-demethyl-famesic ester as well as its (6-cis-)-isoiner cyclize in the presence of acids to give the decalol derivative with four chirai centres whose relative configuration is well defined (P.A. Stadler, 1957 A. Escherunoser, 1959 W.S. Johnson, 1968, 1976). A monocyclic diene is formed as an intermediate (G. Stork, 1955). With more complicated 1,5-polyenes, such as squalene, oily mixtures of various cycliz-ation products are obtained. The 18,19-glycol of squalene 2,3-oxide, however, cyclized in modest yield with picric acid catalysis to give a complex tetracyclic natural product with nine chiral centres. Picric acid acts as a protic acid of medium strength whose conjugated base is non-nucleophilic. Such acids activate oxygen functions selectively (K.B. Sharpless, 1970). 

Tsai et al. have also used RAIR to investigate reactions occurring between rubber compounds and plasma polymerized acetylene primers deposited onto steel substrates [12J. Because of the complexities involved in using actual rubber formulations, RAIR was used to examine primed steel substrates after reaction with a model rubber compound consisting of squalene (100 parts per hundred or phr), zinc oxide (10 phr), carbon black (10 phr), sulfur (5 phr), stearic acid (2 phr). 

The biomimetic approach to total synthesis draws inspiration from the enzyme-catalyzed conversion of squalene oxide (2) to lanosterol (3) (through polyolefinic cyclization and subsequent rearrangement), a biosynthetic precursor of cholesterol, and the related conversion of squalene oxide (2) to the plant triterpenoid dammaradienol (4) (see Scheme la).3 The dramatic productivity of these enzyme-mediated transformations is obvious in one impressive step, squalene oxide (2), a molecule harboring only a single asymmetric carbon atom, is converted into a stereochemically complex polycyclic framework in a manner that is stereospecific. In both cases, four carbocyclic rings are created at the expense of a single oxirane ring. 

Scheme 1, Enzyme-induced cyclizations of squalene oxide (2) (a) and the Stork-Eschenmoser hypothesis (b).

The degradation of squalene by Marinobacter sp. strain 2sq31 (Rontani et al. 2002) is initiated by oxidative fission, although the subsequent steps are carried out by ()-oxidation and carboxylation that are comparable to those used for branched alkanes. 

Polyene cyclizations are of substantial value in the synthesis of polycyclic terpene natural products. These syntheses resemble the processes by which the polycyclic compounds are assembled in nature. The most dramatic example of biosynthesis of a polycyclic skeleton from a polyene intermediate is the conversion of squalene oxide to the steroid lanosterol. In the biological reaction, an enzyme not only to induces the cationic cyclization but also holds the substrate in a conformation corresponding to stereochemistry of the polycyclic product.17 In this case, the cyclization is terminated by a series of rearrangements. 

The sequence of transformations from squalene to lanosterol begins by the enzymatic oxidation of the 2,3-double bond of squalene to form (3S)-2,3-oxidosqualene [also called squalene 2,3-epoxide]. 

Until recently, the only marine example of cycloartenol (32) production was in the chrysophyte Ochromonas sp. [20], A survey, documenting the products of squalene oxide (37) cyclization (see Scheme 3) using crude enzyme preparations of various algal phyla has recently been reported [21]. Interestingly, while all... 

Sea cucumbers (Holothuroidea, Echinodermata) appear to be unique in their mode of squalene oxide (37) cyclization. Tritium-labeled lanosterol (33), cycloartenol (32) and parkeol (38) were individually administered to the sea cucumber Holothuria arenicola. While the former two triterpenes were not metabolized [22], parkeol was efficiently transformed into 14x-methyl-5a-cho-lest-9(l l)-en-3/ -ol (39) (Scheme 3). Other A1 sterols present in H. arenicola were not found to be radioactive and were thus assumed to be of dietary origin. The intermediacy of parkeol was confirmed by the feeding of labeled mevalonate (23) and squalene (26) to the sea cucumber Stichopus californicus [15]. Both precursors were transformed into parkeol, but not lanosterol nor cycloartenol, aqd to 4,14a-dimethyl-5a-cholest-9(ll)-en-3/J-ol (40) and 14a-methyl-5a-cholest-9(ll)-en-3/ -ol. Thus, while all other eukaryotes produce either cycloartenol or lanosterol, parkeol is the intermediate between triterpenes and the 14-methyl sterols in sea cucumbers. 

As described above, the cyclization of squalene oxide is a biosynthetic branching point not only for phytosterols and triterpenes but also for dammarane- and oleanane-type ginsenosides. In ginseng, the enzyme... 

The isoprenoid polyenes famesyl acetate, geranyl acetate and squalene underwent oxidative poly cyclisation to bis-, tris- and penta-tetrahydrofurans with RuO /aq. Na(IO )/CH3CN-EtOAc [185]-[188]. This oxidative polycyclisation of squalene with RuO was shown to lead to the cis-threo-cis-threo-trans-threo-trans-threo-trans penta-tetrahydrofuranyl diol product, this configuration being determined by 2D-NMR (Fig. 3.14) [185]-[188] cf mech. Fig. 1.8 [185]. 

NT325 Reid, W. W. Accumulation of squalens-2,3-oxide during inhibition of phytosterol biosynthesis in Nicotiana tabacum. Phytochemistry 1968 7(3) NT338... 

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