CRYSTAL-STRUCTURE OF SALOGRAVIOLIDE-A, A GUAIANE-TYPE SESQUITERPENE LACTONE

A new naturally occurring sesquiterpene lactone, 3beta-acetoxy-8alpha, 9beta-dihydroxy-1alphaH, 5alphaH, 6betaH, 7alphaH-guaian-4(15), 10(14), 11(13)-trien-6,12-olide (Scheme 1) bas been structurally characterized by X-ray diffraction methods. The compound crystallizes in the monoclinic space group P2(1) with two molecules in the unit cell of dimensions a = 8.485(2), b = 7.398(1), c = 12.919(3)angstrom, beta = 98.17(3)-degrees. The structure has been refined to a final value of R of 0.039 for 2091 observed reflections. The molecule possesses a polyunsaturated guaianolide system which contains a twist chair exomethylenecycloheptane moiety, cis-fused with the exomethylene-cyclopentane [at C(1) and C(5)] and trans-annelated with the alpha-exomethylene-gamma-lactone at C(6) and C(7). The acetoxy substituent at C(3) is beta-oriented and trans to the C(8) hydroxyl which, in tum, is trans to the neighboring C(9) hydroxyl. Substitution at C(9) is unprecedented among guaianolides isolated from Centaureinae subtribe.