The pigments of “dragon's blood” resin. Part V. Some flavans

Abstract

developed in Part I11 (J., 1960, 3117) has been extended to the synthesis of flavani and their derivatives from a series of polyhydroxyphenols. From some a number of a-and y-pyronoflavans have been prepared.IN reviewing possible routes for the synthesis of dracorubin-type compounds we were attracted by the idea of employing a residue of type (I), containing the flavan system already formed, rather than to attempt the construction of this unit on to a flavylium compound, more especially because of the experimental limitations encountered in the synthesis of the flavan system (Part 111, J., 1950, 3117). As a preliminary to exploring this route to the dracorubin system the general procedure developed in Part I11 (Zoc. cit.) has been extended to the svnthesis of a series of flavans from resorcinol, orcinol, quinol, hydroxyquinol, and keto-derivatives it (111), and (IV). R phloroglucinil, and pyrogallol. Further, from some of the formylhas been possible to obtain, e.g., cc-and y-pyronoflavans, types (11),In these studies there emerged several experimental features of general interest which may be noted. (a) Most parent flavans are extremely dficult to crystallise and several of the hydroxy-compounds are readily susceptible to aerial oxidation. (b) The flavan oxygen system is sensitive to acidic reagents above room temperature and attempts to demethylate O-methylflavans by the standard methods are uniformly unsuccessful. Further, whilst, e.g., 8-formyl-7-hydroxyflavan (I ; R = CHO) readily reacted with acetophenone in ethereal hydrogen chloride, the resulting flavylium salt was resinous and could not be purified. (c) With chromic anhydride O-methylflavans furnish low yields of the corresponding flavanones, thus providing a method of detecting the flavan system. EXPERIMENTAL 7-HydroxyfEavan (I ; R = H).-A solution of 7-hydroxyflavanone (Ellison, J. , 1927, 1722) ( 1 g.) in acetic acid (40 ml.), containing concentrated hydrochloric acid ( 5 ml.), was reduced with zinc amalgam (from 10 g . of zinc dust) at room temperature and 24 hr. later the solution was decanted and diluted with water. Purification of the resulting precipitate from light petroleum (b. p. 6O--SO0), containing a small amount of methanol, yielded 7-hydroxyfavan (0.7 g.) in prisms, m. p. 71", readily soluble in 2~-aqueous sodium hydroxide and having a negative ferric reaction (Found : C, 79.6; H, 6.3. Cl5Hl4O2 requires C, 79.6; H, 6.2%). 7-Hydroxy-8-rnetlzyZ~7avan (I ; R = Me) .-(a) Prepared by Gattermann's method from 7-hydroxyflavan (1 g.), hydrogen cyanide ( 5 ml.), and zinc chloride (0.5 g.) in ethereal hydrogen *

Isolement, Structure et Synthèse de la Vochysine, Pyrrolidinoflavanne de Vochysia guianensis

“…Le traitement de cette derniére par l'acide chlorhydrique dans le méthanol (16) permet d'accéder á la benzyloxy-7 hydroxy-4' méthoxy-5 flavanone (18). La réduction de cette molécule par l'amalgame de zinc en milieu acide (17) fournit la benzyloxy-7 hydroxy-4' méthoxy-5 flavanne (19) qui, aprés hydrogénolyse, conduit á la dihydroxy-7,4' méthoxy-5 flavanne (7).…”

“…L'accés á la dihydroxy-5,4' méthoxy-7 flavanne (8) est beaucoup plus aisé du fait de la forte réactivité de l'hydroxyle en 7 de la naringénine (9) (17). La méthylation de la naringénine 9 par un équivalent d'iodure de méthyle fournit la dihydroxy-5,4' méthoxy-7 flavanone (18) (-sakuranétíne) (20).…”

“…Le milieu réactionnel est ensuite versé dans l'eau glacée. Le précipité alors obtenu fournit, aprés chromatographic sur colonne de sílice (solvant: CHCl3-MeOH 95:5), 108 mg de triacétylvochysine (4), non obtenue á l'état cristallisé (Rdt: 75%); ir: 1775, 1640, 1200; sm: 467(M + ) (17), 425( 17), 424 (14), 408(42), 314 (8), 220 (13), 204(33), 191(22), 133 (8), 120 (19), 112(100). 'H-rmn (CDC13) (ppm): 7,36 (2H, dj=9 Hz, H-2', H-6'); 7,02 (2H, d, 7=9 Hz, H-3', H-5'); 6,38 (1H, s, H-6); 4,99 (1H, m, H-2); 4,85 (1H, m, H-2"); 3,61 (3H, s, OCH3); 2,30 (3H, s, OAc); 2,26 (3H, s, OAc); 1,79 (3H, s, NAc).…”

“…Degradation de la vochysine: dihydroxy-s ,4 methoxy-7 flavanne (8).-Une solution de vochysine (1) (377 mg, 1,1 mM) dans 75 ml d'EtOH est agitée á 20°C pendant 72 h. La solution est alors évaporée sous pression réduite. Le résidu fournit, aprés chromatographie sur colonne de sílice (solvant: CHCl3-MeOH 90:10) puis cristallisation dans EtOAc, 98 mg de dihydroxy-5,4' méthoxy-7 flavanne (8) (Rdt: 40%); F= 190-191°; uv: 235, 276; ir: 3370, 1625, 1600, 1140; sm: 272(M+) (71), 166 (18), 16500), 153000), 133(25), 120(48), 107 (17). 'H-rmn (CDC13) (ppm): 7,24 (2H, d, J=9 Hz, H-2', H-6'); 6,78 (2H, d,J=9 Hz, H-3', H-5'); 6,05 (1H, dj = 3 Hz, H-8); 5,94 (1H, d,J=3 Hz, H-6); 4,87 (1H, ddj= 10Hz,/ = 2 Hz, H-2); 3,67 (3H, s, OCH3); 2,65 (2H, m, CH2-4); 2,20(2H, m, CH2-3).…”

“…D20, OH-5); 7,68 (2H. d,/=9 Hz, H-2', H-6'); 7,27 (2H, d J=9 Hz, H-3', H-5'); 6,41 (2H, s, H-6, H-8); 5,79 (1H, ddj= 12 Hz, J' = 3 Hz, H-2); 3,40 (1H, dd, J= 17 Hz,/= 12 Hz, H-3a); 2,90 (1H, dd,/= 17 Hz, J' = 3 Hz, H-3b); 2,30 et 2,27 (2 x 3H, 2s, 20Ac). -Une solution de diacétoxy-7,4' hydroxy-5 flavanone (13X356 mg, 1 mM) dans Me2CO(2,5 ml) est additionnée de CH3I (0,2 ml) et de K2C03 (0,2 g) puis chauffée á reflux pendant 2 h. Le milieu réactionnel est ensuite filtré puis évaporé sous pression réduite.…”