Triterpenoid intermediate

Cycloartenol (94) rather than lanosterol is thought to be the crucial triterpenoid intermediate in the biosynthesis of plant sterols, although in a chlorophyll-containing phylum there appears to be no direct correlation between ability to photosynthesize and the operation of the cycloartenol pathway.163 The cleavage of the cyclopropane ring of (94) between C-9 and C-19 should be accompanied by the incorporation of a proton at C-19, possibly from the medium. That this is the case in pea was shown164 by the incorporation of deuterium specifically at C-19 of cycloartenol obtained from... 

Farnesol pyrophosphate is an immediate precursor of squalene, the key intermediate in steroid and triterpenoid biogenesis, which arises from the coupling of two farnesol pyrophosphate molecules or of C,s units derived therefrom. The numerous types of sesquiter-penoid carbon skeletons represent various modes of cyclization of farnesol (sometimes with rearrangement) and it is probable that farnesol pyrophosphate is also the source of these compounds. 

Saponins are glycosylated secondary metabolites that are widely distributed in the Plant Kingdom.3,4 They are a diverse and chemically complex family of compounds that can be divided into three major groups depending on the structure of the aglycone, which may be a steroid, a steroidal alkaloid, or a triterpenoid. These molecules have been proposed to contribute to plant defense.3 6 Saponins are also exploited as drugs and medicines and for a variety of other purposes.4 Despite the considerable commercial interest in this important group of natural products, little is known about their biosynthesis. This is due in part to the complexity of the molecules, and also to the lack of pathway intermediates for biochemical studies. 

In an approach to the stereocontrolled creation of the acyclic side-chain of tetracyclic triterpenoids and other natural products, Trost and his colleagues have converted the acyclic starting compound (29) into the cyclopropanoid intermediate (31) via the diazo-ketone (30). The key step in the scheme is the cleavage of the cyclopropane with lithium dimethylcuprate to give (32). The stereochemistry at C-7 is determined by the configuration of the double bond in (29). The c.d. and u.v. spectra of a series of triterpenoid olefins have been measured. The Scott-Wrixon rules can be used to correlate the sign of the c.d. curves with molecular structure. A... 

Figure 25-13 shows that cholesterol is a triterpenoid, formed from six isoprene units with loss of three carbon atoms. The six isoprene units are bonded head to tail, with the exception of one tail-to-tail linkage. The triterpene precursor of cholesterol is believed to be squalene. We can envision an acid-catalyzed cyclization of squalene to give an intermediate that is later converted to cholesterol with loss of three carbon atoms. Possible mechanisms are outlined in Figures 14-6 and 14-7 (pages 651-652). 

In recent years, Ireland and his colleagues have made a major contribution to the total synthesis of tetra- and penta-cyclic triterpenoids. The details of some of this work have been published in a series of papers.55-58 The tetracyclic ketone (75) was conceived as a key intermediate in the synthesis of shionone (76). Three routes to (75) were undertaken.55 The most efficient involved a triethylaluminium-catalysed conjugate addition of cyanide to the enone (77) (see Vol. 5, p. 135). The second... 

Attempted catalytic hydrogenation of the triterpenoid synthetic intermediate (136) resulted in methyl group migration with formation of (137). ... 

Saponins maybe classified into two groups, the triterpenoid saponins, which will not be considered here (for reviews, see Mahato et al, 1992 Conolly et al, 2008 Abe, 2007 Vincken et al, 2007 Liang and Zhao, 2008), and the steroid saponins. Steroid alkaloids behave like saponins but are sometimes treated as alkaloids, although these compounds are formed from intermediates of the steroid saponin pathway. Steroid saponins constitute a vast group of plant-borne glycosides present almost exclusively in the monocotyledonous angiosperms and occurring in only a few dicotyledonous families, such as the... 

The Cram modification has not been greatly exploited in multistep synthetic endeavors, possibly because of the above-mentioned need to preform and isolate the hydrazone intermediate coupled with very slow addition of the hydrazone. This latter requirement is particularly inconvenient when milligram quantities are involved. The aldehyde in a triterpenoid (partial structure 18) was successfully reduced to the corresponding methyl group using the Cram method, albeit in low yield (32%). Notably, the hydrazone (lO mg) was not added slowly. The Cram process was not successful for the reduction of the cyclo-butanone (19) (the Huang-Minlon process succeeded), or the pyrrole aldehydes (20) and (21). ... 

Two notable total syntheses of pentacyclic triterpenoids have been reported. In the synthesis of ( )-germanicol (115), the tricyclic ketone (116) was conceived as a general intermediate which embodies the features of rings A, B, and c of many triterpenoids. Conjugate addition of the benzyl Grignard reagent to the... 

Although cycloartenol (44) belongs to the tetracyclic triterpenoids as the parent compound, it is implicated as an important biosynthetic intermediate both in that class and in plant steroids and is thus a relevant introduction to the latter group which have been so significant in semi-synthetic applications. Following the enzyme-catalysed cyclisation of squalene epoxide, it is the first intermediary compound encountered and in the plant series it has a comparable position to lanosterol (45) in the animal... 

The compounds (160),114 (161),115 and (162)116 have been prepared as possible intermediates for pentacyclic triterpenoid synthesis. Treatment of oleana-12,15-diene-3p,28-diol 3-acetate with toluene-p-sulphonyl chloride in pyridine affords the D(16a)-homo compound (163). The mechanisms of this and related reactions are discussed.117 2a,3[3-Diacetoxy-28-noroleana-12,17-diene has been synthesized.118... 

Terpenes are flammable unsaturated hydrocarbons, existing in liquid form. They are found in essential oils, resins or oleoresins. They are used as intermediate for the synthesis of sesquitrepenes and terpenoids. They are classified as mono, di or triterpenoids. 

Synthesis of acetylenes (2, 240). Ireland et al. applied the lithio-l-trimethyl-silylpropyne procedure of Corey and Kirst2 to a synthesis of the enediyne (3), desired as an intermediate in a triterpenoid synthesis. But application of this coupling procedure to the /ram -dibromide (1) afforded less than 5% of the desired disilyl derivative (2). They then explored the coupling of (1) with propargyl... 

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