Abstract
Natural brassinosteroids are widespread in the plant kingdom and it is known that they play an important role in regulating plant growth. In this study, two new brassinosteroid analogs with shorter side chains but keeping the diol function were synthesized. Thus, the synthesis of 2-deoxybrassinosteroids analogs of the 3α-hydroxy-24-nor, 22,23-dihydroxy-5α-cholestane side chain type is described. The starting material is a derivative from hyodeoxycholic acid (4), which was obtained with an overall yield of 59% following a previously reported five step route. The side chain of this intermediate was modified by oxidative decarboxylation to get a terminal olefin at the C22-C23 position (compound 20) and subsequent dihydroxylation of the olefin. The resulting epimeric mixture of 21a, 21b was separated and the absolute configuration at the C22 carbon for the main product 21a was elucidated by single crystal X-ray diffraction analysis of the benzoylated derivative 22. Finally, lactonization of 21a through a Baeyer-Villiger oxidation of triacetylated derivative 23, using CF3CO3H/CHCl3 as oxidant system, leads to lactones 24 and 25 in 35% and 14% yields, respectively. Deacetylation of these compounds leads to 2-deoxybrassinosteroids 18 and 19 in 86% and 81% yields. Full structural characterization of all synthesized compounds was achieved using their 1D, 2D NMR, and HRMS data.
Highlights
Since the discovery of brassinolide (1), a polyhydroxysteroidal hormone that regulates plant growth and development, other brassinosteroids (BRs) have been found throughout all the plant kingdom and much effort has been dedicated to the synthesis of BR analogs
We describe the synthesis of synthesis applicable to this compound class [18,19,20,21,22,23,24,25,26,27,28,29,30,31]
We describe the synthesis of new new 2-deoxybrassinosteroid analogs bearing a shorter side chain but retaining the diol function, 2-deoxybrassinosteroid analogs bearing a shorter side chain but retaining the diol function, i.e., a 3αi.e., a 3α-hydroxy-24-nor-22,23-dihydroxy-5α-cholestane side chain type
Summary
Since the discovery of brassinolide (1), a polyhydroxysteroidal hormone that regulates plant growth and development, other brassinosteroids (BRs) have been found throughout all the plant kingdom and much effort has been dedicated to the synthesis of BR analogs. Most of this work has been focused on determining the structural requirements that these compounds should possess to elicit strong biological activity [1,2,3]. The latter is a natural 2-deoxybrassinosteroid that may act as important biosynthetic precursors of more active brassinosteroids [4,5,6,7]. Figure (2) and and typhasterol typhasterol (3).
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