Natural quassinoids

In recent years, a number of studies on the synthesis of quassinoids have been reported [126], Among them, total synthesis of quassin (possessing 7 chiral centers) and of castelanolide (possessing 9 chiral centers) both carried out Grieco s group constitute two successful works to synthesize natural quassinoids [1]. Below the main total synthesis of some quassinoids are described. 

ANTI-HIV ACTIVITY OF NATURAL QUASSINOID GLYCOSIDES AND THEIR AGLYCONES... 

Quassinoids are natural products formed by oxidative degradation of triterpene derivatives their biosynthetic precursors are similar to those of limonoids but the biosynthetic pathways of quassinoids have not been established so far [1]. At least 351 different natural quassinoids have been described in the literature (Fig. 125.1), and a large number of semisynthetic and synthetic analogues have been prepared for synthesis or medicinal chemistry purposes, mostly in last 30 years [see, for example, 2-4]. Several base skeletons have been described in the literature, and these can be classified into five distinct groups according to the number of atoms of the main chain (Fig. 125.1). 

The three C22 natural quassinoids known in the literature have a butenolide moiety attached to the A ring, presumably originating from the aldol cyclization of a a-acetoxycarbonyl moiety from the normal C20 skeleton. In this series, sergeolide has attracted much attention due to its very good antimalarial potential (Fig. 125.4) [7]. 

Sergeolide (62) 60) is the first natural quassinoid to possess a butenolide function. It was isolated along with the known isobruceine B (45) 45) from the roots of the French Guyanan Simaroubaceae Picrolemma pseudocoffea. The structure of sergeolide was established by analysis of the 400 MHz- H-n.m.r. data and the C-n.m.r. spectrum. 

Soulameolide (81) and guanepolide (82) are rare examples of natural quassinoids which have a 14,15-double bond. This suggests that the oxygenated functions at these carbon atoms, which are present in several quassinoids, are introduced subsequent to the formation of the 5-lactone. 

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