Brassinolide synthesis

The discovery of brassinolide by scientists at USDA led to the development of a new group of plant growth substances, termed brassinosteroids, which have the potential for increasing crop yields and biomass. The chapter summarizes the USDA s contributions to brassinosteroids, which include isolation and identification of brassinolide, synthesis of brassinolide and its analogs, and limited biological and field studies on brassinosteroids. An overview of the current data requirements for the registration of brassinosteroids as biochemical pesticides under the Federal Insecticide, Fungicide, and Rodenticide Act is presented. 

John Siddall was a prolific scientist who published a large number of articles and patents. He always maintained a complementary interest in insect physiology, as well as in chemical synthesis, particularly of chiral compounds. In fact, just before he entered the hospital, he and Steven Fung completed a total synthesis of the plant growth regulator Brassinolide, a complex steroid with many asymmetric centers. 

Baeyer-Villiger oxidation of the 5a-6-keto steroid 1 with trifluoroperacedc acid is 1000 times faster than oxidation with m-chloroperbenzoic acid and also is more regioselective. This oxidation was used in the last step in a synthesis of brassinolide (2), a natural steroid that promotes plant growth.2... 

Schmittberger, T. Uguen, D. A formal synthesis of brassinolide. Tetrahedron Lett. 1997, 38, 2837-2840. 

Among the compounds that have been isolated from the brassins were a new group of glucosyl esters of fatty acids, the structures of which were established by spectral methods (254, 255) as well as by chemical and biochemical synthesis (256, 257, 258). Although these esters were much less active than brassinolide in the bean second internode bioassay, their presence may be essential for seed germination since the enzymatic synthesis of these esters was correlated to germination (257, 258). 

Fraser-Reid and co-workers have examined serial radical cyclization of pyranose-derivatives [95AJC333] in the stereocontrolled synthesis of Woodward s reserpine precursor [95JOC3859]. Treatment of the bromosilane 188 under reductive conditions resulted in a 5-exo followed by a 6-exo cyclization. The intermediate radical eliminates phenylsulfinyl radical to provide the alkene 189 as the product. The intermediate has been converted to the reserpine precursor 190. The temporary silicon method has been utilized for the synthesis of brassinolide side chain [95SL850]. 

Allylic mesyloxy sulfinyl steriod 46 also underwent highly stereoselective SN2 displacement when treated with a cyanocuprate to yield 47 (Scheme 11). In this manner, a formal synthesis of the plant growth regulator brassinolide was achieved.34... 

The original discovery of brassinolide, a natural plant-growth promoter extracted from bee-collected pollen, was made at the USDA Agricultural Research Center in Beltsville, Maryland. Although the USDA terminated brassinosteroid research, others around the world have carried on the work. This volume addresses the history, biochemistry, physiology, practical applications, production, synthesis, and entomological effects of these compounds. 

During the decade since publication in 1979 of the structure of brassinolide (I), and the synthesis of the first brassinosteroid (2), also in 1979, events related to the "brassins project have taken some interesting turns. At this point (October 1990), there seems to be little interest in developing brassinosteroids for agricultural... 

We have developed a synthesis of antheridiol in which the key step is an aldol condensation of a C-22 aldehyde with the anion derived from 3-isopropyl-but-2-enolide (4) which gives directly the sidechain of antheridiol as illustrated by structures 2 and 3. In this reaction, chiral centers are created at C-22 and C-23. The stereochemistry at C-22 in the major product is that predicted by the Cram rule (i.e. R) and careful study of the reaction showed that the stereochemistry at C-23 is determined by the temperature at which the aldol reaction is carried out. If the temperature is maintained below -70 °C, the major product has the R configuration at C-23. Thus, this method could be used to construct the sidechain of brassinolide with correct stereochemistry at C-22 and C-23. 

Before discussing our synthetic studies, mention should be made of different approaches employed in the synthesis of brassinolide by other investigators. The most practical method would be to start with a readily available steroid having the same carbon skeleton as brassinolide, and then introduce the required functional groups in rings A and B and the sidechain. In the case of 28-homobrassinolide (5), tiie ideal starting compound would be the abundant sterol stigmasterol (4). 

The required starting compound for the synthesis of brassinolide by hydroxylation of a A22 double bond is 22-dehydrocampesterol, which is not readily available (7). It can be isolated as a mixture with the C-24 epimer, brassicasterol, from oysters (Crassostrea virginica), but the mixture is difficult to separate. It is more readily prepared by synthesis (6). For the synthesis of 24-epibrassinolide, brassicasterol, which can be obtained from rapeseed oil, is an obvious starting compound (15). Ergosterol, which is readily available, has also been employed (16). 

A more widely used approach to brassinolide is reaction of a C-22 aldehyde, derived from stigmasterol, with a carbanion containing a double bond (or potential double bond). In nearly all cases, epoxidadon of die double bond was employed to introduce one or both oxygen functions (17-24). A synthesis of the brassinolide sidechain starting from pregnenolone has also been reported (25). 

Our first synthesis of brassinolide started from stigmasterol which was converted to (20S)-6p-methoxy-3a,5-cyclo-5a-pregnane-20-carboxaldehyde (2). This pathway involved formation of the mesylate with methanesulfonyl chloride in pyridine and tetrahydrofuran, treatment of the mesylate with potassium acetate in methanol, and ozonolysis of die i-sterol with reductive work-up. 

Subsequent to our use of butenolide anions in aldol reactions to construct sidechains of steroids, two groups have reported a similar method for synthesis of brassinolide or its analogs. Kametani and co-workers employed the dianion of 3-isopropyltetronic acid which was reacted with a 20-keto steroid. The aldol product was dehydrated and the product hydrogenated to furnish the brassinolide sidechain (28,29). Zhou and Tian have used the reaction of a C-22 aldehyde derived from... 

This brief review covers the period of the last ten years in the development of brassinosteroid chemistry in our laboratory. Our interest has been focused on the synthesis of natural brassinosteroids (brassinolide, homobrassinoli-de, epibrassinolide, norbrassinolide, etc.), their analogues and intermediates starting from stigmasterol, ergosterol, or pregnenolone. Some aspects of biological activity are discussed. 

It has been assumed (21) that the fresh-weight gain of radish cotyledons by kinetin is due to the activation of de novo synthesis of the hydrolytic enzyme invertase. Seeking a similar influence of brassinosteroids on this enzyme we also found, that brassinolide 1 and (22S,23S)-homobrassinolide 3 treatment resulted in an increased activity of the soluble invertase even to a remarkably higher extent than kinetin in the case of brassinolide 1 (Figure 2). This increase... 

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