Structural flexibility in the biocatalyst-mediated functionalization of ring 'A' in salannin, a tetranortriterpene from Azadirachta indica

Nocardia sp. quantitatively converts salannin 1 and 3-de-O-acetylsalannin 2 (C-seco limonoids) into 3-deacetoxy-1-de[(E)-2-methylbut-2-enoyloxy]salann in-1-en-3-one 10, a novel and potentially bioactive compound with an alpha, beta-unsaturated ketone moiety in ring 'A'. In order to establish the seque nce of events involved in this transformation and the structural specificit y of this bacterial system, several new derivatives of salannin 1 have been prepared. These studies have indicated that the transformation is initiate d by deacetylation at C-3, followed by oxidation of the secondary hydroxy g roup to 3-keto, which appears to facilitate the elimination of the tigloylo xy/acetoxy group at C-1 with the formation of an olefinic linkage between C -1 and C-2. The organism very efficiently transforms some of the derivatives of salanni n into their corresponding compounds possessing an enone system in ring 'A' , an essential pre-requisite for various biological activities. Some of the C-seco limonoids prepared in the present study, viz. 10, 1,2-didehydro-1,3 -dideoxy-3-oxosalannic acid 18, 3-deacetoxy-1-de[(E)-2-methylbut-2-enoyloxy ]-20,21,22,23-tetrahydrosalannin-1-en-3-one 15 and 1,2-didehydro-1,3-dideox y-3-oxosalannol 23 were hitherto not known.