The Complete Assignment of 1 Hand 13 C-NMR of Prenylated Xanthones from Tovomita spp . ( Guttiferae )

A 1,6-dihidroxy-5-(3-metilbut-2-enil)-6’,6’-dimetil-pirano(2’,3’;7,8)6”,6”dimetilpirano (2”,3”;3,2)xantona e a 1,3,6-trihidroxi-6’,6’-dimetil-pirano(2’,3’;7,8)-2,5di-(3-metilbut-2-enil)xantona tiveram seus deslocamentos químicos de hidrogênio e carbono-13 atribuídos de forma inequívoca, com base nas informações obtidas através dos espectros de RMN de H[1D e 2D(H-H-COSY) e de correlação heteronuclear H-C-COSY[JCH(n = 1, 2 e 3)]. Estes da dos ainda foram confirmados utilizando-se a técnica de NOE diferencial.


Ex per i men tal
1D and 2D NMR spec tra were re corded at 200 MHz for 1 H, and at 50.3 MHz for 13 C, us ing a Bruker AC-200 spectrom e ter.The pulse se quences used are con tained in the XHCORR > AU Bruker pro gram for heteronuclear cor rela tion ( 1 H-13 C-COSY), with D 3 = 0.5/J CH and D 4 = 0.25/J CH , mod u lated for 1 J CH = 135 Hz and n J CH = 8.0 Hz (long range, n = 2 and 3).The COSY.AU pro gram was ap - Po si tion δC a δH (J,Hz) and 1 H -13 C-COSY( 1 JCH )

Re sults and Dis cus sion
The ob served sig nals in the NMR 1 H-spectra [1D and 2D ( 1 H-1 H-COSY)], in con junc tion with the ob served inter ac tion in the heteronuclear cor re la tion spec tra 1 H-13 C-COSY[ n JCH (n = 1,2 and 3)] (Ta bles 1 and 2, Fig. 1), were com pat i ble with the pres ence of two pyrano and one prenyl unit in Struc ture 1, and one pyrano and two prenyl units in Struc ture 2.
The car bon chem i cal shift data for the xanthone skel eton ob tained by 13 C-NMR (PND) from 1 and 2 (Ta bles 1 and 2) when com pared to mod els 3 3 and 4 4 , showed the same sub sti tu tion pat tern on ring A of 1 and 2 re spec tively.The lo ca tion of the pyrano units in 1 and the prenyl units in 2 was con firmed by long-range heteronuclear chem i cal shift cor re la tion (Fig. 1).The long-range cou pling be tween H-1" ( δ 3.18), 1-OH( δ 13.58), H-4( δ 6.15), and C-2( δ 109.6) in Struc ture 2 are in ac cor dance with ring A non-substituted at C-4 po si tion.Con se quently, the long-range cou pling be tween H-5"( δ 5.55) and C-2( δ 103.7) in struc ture 1 (Fig. 1) con firms the same hy dro ge nation pat tern of ring A.
The other 13 C sig nals of the xanthone skel e ton were non-hydrogenated.Two of them are ox y gen ated car bons ( δ 150.3 and 137.6) of 1 and ( δ 149.9 and 137.3) of 2, and two The hydrogentaion pat terns was de duced by DEPT ( Θ = 90° and 135°).
did not have any heteroatom at tached to them ( δ 115.1 and 116.9) for 1 and ( δ 115.4 and 116.9) for 2, which are in accor dance with a fully sub sti tuted ring B.
The long-range cou pling be tween H-1"' ( δ 3.40) and C-10a ( δ 150.6), and H-1"' ( δ 3.40) and C-6 ( δ 150.3) of 1, and H-1''' ( δ 3.42) and C-6 ( δ 149.9) of 2 are in ac cor dance with the prenyl unit at tached to C-5.The same kind of obser va tion was made in re la tion to H-4' ( δ 7.80) and C-7 (δ 137.6) of 1 and H-4' ( δ 7.84) and C-7 ( δ 137.3) of 2 which was in ac cor dance with the pres ence of the pyrano unit at C-7/C-8.In ad di tion the hydroxyl group should be at tached at C-6.This ex per i men tal in for ma tion sug gests that the attri bu tion of the chem i cal shift for C-4' and C-5' re ported in the lit er a ture for 3 3,5 an 5 5 must be ex changed.
The ob served NOE for 1, with dou ble ir ra di a tion, defined the at tri bu tion of methyl groups from pyrano and prenyl units of 1 and 2 (Ta bles 1 and 2).In the prenyl units, NOE ex per i ments were very im por tant for es tab lish ing the stereochemistry of the methyl groups.In Struc ture 1, the irra di a tion in the H-1"' fre quency gen er ated NOE of 10.0 and 6.0 % in H-2"' and H-4"', re spec tively.On the other hand, ir ra di a tion in the fre quency of H-2''' gives a NOE of 7.0% in H-5"', while the ir ra di a tion of the H-4"' sig nal gives a NOE of 3.0 % in H-1"' and H-5"'.Again, ir ra di a -tion at the fre quency of H-5"' pro duces NOE of 7.0 and 4.0 % in H-2"' and H-4"', re spec tively (Ta ble 1).

Fig ure 1 .
Fig ure1.Long-range heteronuclear cor re la tions ob served for 1 and 2.